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Progress in ejector-expansion vapor compression refrigeration and heat
pump systems
Article in Energy Conversion and Management · March 2020
DOI: 10.1016/j.enconman.2020.112529
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Energy Conversion and Management 207 (2020) 112529
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Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Progress in ejector-expansion vapor compression refrigeration and heat
pump systems
T
⁎
Zhenying Zhang , Xu Feng, Dingzhu Tian, Jianjun Yang, Li Chang
Department of Architecture and Civil Engineering, North China University of Science and Technology, Tangshan 063210, China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Ejector
Refrigeration
Expansion energy recovery
Heat pump
Review
The use of an expansion energy recovery ejector has been given great concern owing to virtues of no moving
elements, cost-effectiveness, high reliability and comparable efficiency. This paper is to explore the advancements in ejector-expansion refrigeration and heat pump systems. Firstly, the historical background and operating
principles of ejector are described. In the second part, the theoretical and experimental progresses of the
standard ejector-expansion systems as well as some special issues are presented. The third part of the paper
focuses on the other novel ejector-expansion systems, including liquid recirculation ejector-expansion systems,
dual evaporator ejector-expansion systems, cascade ejector-expansion systems and parallel multi-ejector expansion systems. Finally, conclusions and prospects are drawn, suggesting the inherent mechanisms and the
future striving of the ejector-expansion refrigeration and heat pump systems.
1. Introduction
The refrigeration and heat pump industries are striving to improve
energy efficiency owing to the rising energy prices, generalized environmental consciousness and government policy orientation. Fig. 1
shows the schematic of the conventional vapor compression systems
(CVCSs). The CVCSs use a capillary tube or a throttle valve to expand
the working fluid leaving from the high-pressure heat exchanger.
Compared with the ideal refrigeration cycle, also known as the reverse
Carnot cycle, the aforementioned expansion process causes thermodynamic loss called throttling loss, where expansion energy is entirely
dissipated through friction. Fig. 2 illustrates the specific throttling loss
of R744 and that of R134a in a temperature-entropy (T-s) diagram
[1,2]. Remarkably, the throttling loss of R744 is considerably larger
than that of R134a, particularly under higher outdoor temperatures
where R744 systems usually work under transcritical conditions. To
lower this thermodynamic loss, several improved options, such as
subcooling methods [3–7], expansion energy recovery methods [8],
and multi-stage or parallel compression methods [9–11] have been
proposed. The concept behind expansion energy recovery methods is to
convert the expansion energy into utilizable work. Thus it has a dual
effect on energy efficiency improvement of cycles: cooling capacity
increasing and system power requirement decreasing.
The expansion energy recovery devices are classified into two types:
expander and ejector. The investigation of alternative two-phase
⁎
expanders is continuing along many parallel routes, such as reciprocating piston [12,13], rolling/swing piston [14–16], vane [17,18],
scroll [19–21], screw [22,23] and turbo [24,25], each of which has
indicated the feasibility of this concept and yielded surprising results.
However, none of the expanders seems to have been commercialized so
far. The use of an expander not only faces many technical obstacles but
also is not economical for refrigeration and heat pump systems, especially for low capacity ones. The expansion energy recovery ejector has
the virtues of no moving elements, cost-effectiveness, high reliability
and comparable efficiency compared with the expanders. However, the
two-phase characteristic of the ejector brings modeling obstacles and
manufacturing challenges. Great efforts have been made to understand
their operating characteristics and to facilitate their application.
Although different reviews on ejector refrigeration technologies
have been published recently [26–30], most of them are focused on
steam jet refrigeration systems but not on ejector-expansion vapor
compression systems (EVCSs). The papers of Sumeru et al. [31], Sarkar
[32] and Elbel and Lawrence [2] have provided reviews about refrigeration or heat pump EVCSs. These reviews were focused on ejectorexpansion systems 4–7 years ago, and few EVCS special issues as well as
novel EVCSs that have emerged in recent years are involved. The goal
of this paper is to thoroughly review the advancements in ejector-expansion vapor compression refrigeration and heat pump systems. It
endeavors to cover the entire technologies in EVCSs presented in the
literature. Firstly, the historical background and operating principles of
Corresponding author.
E-mail address: zzying@tju.edu.cn (Z. Zhang).
https://doi.org/10.1016/j.enconman.2020.112529
Received 18 September 2019; Received in revised form 19 January 2020; Accepted 21 January 2020
0196-8904/ © 2020 Elsevier Ltd. All rights reserved.
Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Nomenclature
Greek
Acronyms
η
CAM
CEVCS
COP
CPM
CSPF
CVCS
DeEVCS
Variables
Constant-area mixing
Cascade ejector-expansion vapor compression system
Coefficient of performance
Constant-pressure mixing
Cooling seasonal performance factor
Conventional vapor compression system
Dual evaporator ejector-expansion vapor compression
system
EVCS
Ejector-expansion vapor compression system
HEM
Homogeneous equilibrium model
IHE
Isentropic homogeneous equilibrium
IHX
Internal heat exchanger
LPM
Lumped parameter model
LrEVCS Liquid recirculation ejector-expansion vapor compression
system
MER
Mass entrainment ratio
MFR
Mass flow rate
ORC
Organic Rankine cycle
PD
Pressure difference
PLR
Pressure lift ratio
PmEVCS Parallel multi-ejector expansion vapor compression
system
RF
Refrigerator-freezer
SEVCS
Standard ejector-expansion vapor compression system
Efficiency
h
L
p
Q
t
x
Specific enthalpy, kJ/kg
Length, m
Pressure, Pa
Cooling/Heating capacity, kW
Temperature, °C
Quality
Subscripts
c
com
con
diff
eje
eva
gc
h
in
mix
opt
out
pn
sn
t
Cooling
Compressor
Condenser
Diffuser
Ejector
Evaporator
Gas cooler
Heating
Inlet
Mixing section
Optimum
Outlet
Primary nozzle
Secondary nozzle
Throat
ejectors are described. Secondly, the progresses of the standard ejectorexpansion vapor compression systems (SEVCSs) are presented. It
mainly contains three subparts, i.e., theoretical analysis of SEVCSs,
experimental analysis of SEVCSs and special issues of SEVCSs (Exploratory researches of adjustable measures, geometry optimization of
the ejector, optimization of the liquid–vapor separating process, effect
of IHX in SEVCSs, and SEVCSs using zeotropic mixture). The third part
focuses on the other novel EVCSs, including liquid recirculation ejectorexpansion vapor compression systems (LrEVCSs), dual evaporator
ejector-expansion vapor compression systems (DeEVCSs), cascade
ejector-expansion vapor compression systems (CEVCSs), and parallel
multi-ejector expansion vapor compression systems (PmEVCSs).
Fig. 2. Throttling loss for R134a and R744 in T-s diagram [1,2].
Fig. 1. Schematic of conventional vapor compression system (CVCS).
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
The basic scheme of an ejector is shown in Fig. 3. With the static
pressure decreasing, the isentropic acceleration of the high-pressure
primary stream occurred in the primary nozzle. Ultimately, the primary
stream becomes high-speed flow and jets from the nozzle outlet. The
pressure inside the secondary chamber lowers owing to a Venturi effect.
Thus a profitable pressure gradient is produced between the secondary
nozzle inlet and the secondary chamber. Then, the low pressure secondary stream enters the secondary chamber and mixes with the highspeed primary stream. Momentum is further exchanged between the
two streams in the mixed part. The isentropic deceleration of mixed
stream happens in the diffuser converting the kinetic energy of the
mixed stream into pressure flow work. Thus the pressure is lifted at the
diffuser outlet compared with the secondary stream pressure. Table 1
shows the ejector classification.
Mass entrainment ratio (MER), pressure lift ratio (PLR), and ejector
efficiency are usually applied to depict the ejector performance. The
MER, expressed in Eq (1), evaluates the ejector capability of entraining
or pumping mass. The PLR, expressed in Eq (2), evaluates the pressure
lift quantity provided to the secondary fluid by the ejector. For an
ejector-expansion system, it is fulfilling to have both high MER and high
PLR. Nevertheless, there is a tradeoff between MER and PLR in an
ejector-expansion system, signifying that the two arguments should be
considered synchronously for evaluating ejector performance.
Finally, conclusions and prospects are drawn, suggesting the inherent
mechanisms and the future striving of EVCSs.
2. Historical background and ejector working principle
In 1858, the French inventor Henry Giffard patented a condensing
ejector for pumping liquid water into a tank in a steam engine boiler
[33,34]. But a convergent primary nozzle was used due to confusing
with expansion characteristics of steam on that era. A needle was integrated to control the primary flow rate through changing the throat
area of the primary nozzle. Since then, ejectors have been widely used
in many different fields. By 1860, the ejectors have been used by the
locomotives and navy in French. In 1869, a convergent-divergent primary nozzle was first used in the ejector design. In 1901, Parsons
successfully employed an ejector to evacuate incondensable gases from
the condensers in steam power plants [35]. In 1910, Leblanc proposed
the first ejector jet refrigeration system that used an ejector to create a
low-pressure vessel where water vaporized and produced a cooling
effect by utilizing low-grade energy sources. This system was initially
successful in ships owing to the availability of cool seawater, simplicity
and ruggedness. And it was widely used as the air conditioning of
factories, large buildings and trains during the early 1930s. However, it
was later replaced by refrigeration systems driven by mechanical
compressors. Such systems are now usually used in utilization of solar
energy or low-grade heat sources. In addition, the ejector can also be
employed in various energy conversion systems, such as emergency
cool water provision for nuclear reactors, thrust augmentation for aircraft propulsion systems, recirculation system of fuel cell systems,
ejector-organic Rankine cycle systems. The refrigeration EVCS was first
patented by Gay in 1931[36]. Afterwards, Kemper et al. [37] and
Newton et al. [38,39] suggested their modified schemes in 1966 and
1972 respectively. However, this idea has gained extensive attention
since the transcritical R744 vapor compression refrigeration systems
was concerned in the 1990s for expansion energy recovery.
MER =
PLR =
Secondary MFR
Primary MFR
Static pressure at diffuser exit
Static pressure of secondary flow
(1)
(2)
The ejector efficiency is usually described as the ratio between the
actual recovery energy and the maximal acquirable power in the primary stream. Several ejector efficiencies have been defined to evaluate
the performance of ejectors in the literature, such as [40–43]. However,
each efficiency definition usually results in a different value under the
identical working conditions owing to different assumptions, i.e., the
Fig. 3. Schematic of ejector [30,31].
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Table 1
Ejector classifications [27].
Parameters
Condition
Classification
Remarks
Nozzle position
Inside suction chamber
Inside constant-area section
Convergent
Convergent-divergent
Primary flow
Second flow
Vapor
Vapor
Liquid
Liquid
Vapor
Liquid
Liquid
Vapor
CPM ejector
CAM ejector
Subsonic ejector
Subersonic ejector
Better performance if compared with CAM ejector
–
–
–
Vapor jet ejector
Liquid jet ejector
Condensing ejector
Two-phase ejector
Possible two-phase flow Possible shock waves
No shock waves, single-phase flow
Two-phase flow with primary flow condensation Strong shock waves
Two-phase flow Shock waves possible
Nozzle design
Number of phases
Exit flow
Vapor
Liquid
Liquid
Two-phase
3.1. Theoretical analysis of SEVCSs
ejector performances from various researches cannot be compared directly if not evaluated with the identical efficiency. In addition, the
ejector efficiency cannot thoroughly represent the operation and performance of the ejector [2]. The most commonly used efficiency definition for expansion energy recovery ejector is shown in Eq. (3), which
was proposed by Elbel and Hrnjak [42]. This efficiency definition can
be computed by measuring the global ejector variables without
knowing the mixed pressure. Lawrence and Elbel [44] found that the
ejector efficiency usually ranges from 20% to 30% for transcritical
systems using R744 as refrigerant, whereas this figure is usually less
than 20% for subcritical systems using low-pressure refrigerants
(R410A, R134a). Besides, it was also found that the best system performance may not occur when the efficiency is up to optimum for a
given ejector geometry.
ηeje = MER
For the theoretical analysis of the SEVCSs, the lumped parameter
model (LPM) based on homogeneous equilibrium model (HEM) is often
used. The model assigns efficiency and employs related conservation
equation of each ejector section to evaluate system performance. The
methods to calculate the mixed process fall into the constant-pressure
mixed (CPM) model [45,46] and the constant-area mixed (CAM) model
[47]. The primary and secondary streams are supposed to interact
firstly and mix with identical pressure in the CPM model. However, the
two streams are supposed to interact firstly in the CAM section of
ejector in the CAM model, and the pressure of the two streams is inequable.
3.1.1. Theoretical analysis of subcritical SEVCSs
In 1990, Kornhauser [48] developed an iterative LPM for ejectors
based on HEM and CPM model, assuming constant for the isentropic
efficiency of each part. The integrated model comprises four submodels, each representing a section of the ejector. The vapor fraction at
the ejector exit (xeje, out) and the MER are connected as seen in Eq. (4).
Due to this dependency, the pressure at the primary and secondary
nozzle outlet can be computed iteratively. It was found that the theoretical COP increased by 13%, 21%, 20%, 30% and 12% respectively,
compared with the corresponding CVCS of R11, R12, R22, R502 and
R717. But the COP improvement by ejector is limited owing to a majority of the loss produced by heat transfer from the superheated vapor
for R717.
h (pdiff, out , ssn,in ) − hsn,in
h pn,in − h (pdiff,out , spn,in )
(3)
3. Standard ejector-expansion vapor compression systems
(SEVCSs)
In 1931, Gay [36] first applied the ejector to recover the expansion
work in vapor compression refrigeration system. The schematic of the
SEVCS is depicted in Fig. 4. The ejector converts the expanding work
into kinetic energy, which is transformed into increased compressor
inlet pressure afterwards, thereby reducing compression work in comparison with the CVCS. The ejector operates as a “free pump” cycling
liquid refrigerant through the evaporator in the individual circuit simultaneously. The performance of SEVCS is usually studied in comparison with CVCS.
x eje,out = 1/(1+MER)
(4)
In 1995, Domanski [49] found that a COP improvement of
10%–30% was gained by using an ejector for various working fluids
Fig. 4. Schematic and p-h diagram of SEVCS [36].
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
R134a system. In 2014, Molés et al. [48] found that the configuration
using ejector increased the system COP by 9%–15% and 11%–20% for
R1234yf and R1234ze respectively than the corresponding CVCS using
R134a. In 2019, Rostamnejad and Zare [61] proposed a booster added
in the SEVCS to improve the performance, whose schematic was shown
in Fig. 5. It was indicated that among the six working fluids (R1234ze,
R500, R134a, R346fa, 1234yf and R227ea), system with R1234ze had
the optimal performance, with 15.5% and 5.7% higher exergetic efficiency than CVCS and SEVCS respectively at tcon of 40 °C and teva of
5 °C.
In 2012, an exergetic analysis of Ejemni et al. [62] showed that the
introduction of ejector in R744/R152a cascade system could increase
the exergetic efficiency by 27.3% at teva of −30 °C. In 2014, Aghazadeh
Dokandari et al. [63] investigated the performance of the R744/R717
cascade refrigeration SEVCS according to the first and second laws of
the thermodynamics. In comparison with the R744/R717 cascade refrigeration CVCS, the optimum COP and exergetic efficiency of the
cascade SEVCS was increased by 7% and 5% respectively, and the exergy destruction rates could be decreased by approximately 8%.
(R12, R22, R32, R134a, R290, R600a and R717). The COP of the SEVCS
was found to be comparable to that of the economizer system if the twophase ejector efficiency was assigned to be 80%. In 2007, Nehdi et al.
[50] found that a maximal COP improvement of 22% can be gained by
using an ejector in a system using R141b as working fluid. In 2008, Yari
[51] revealed that the overall exergy loss of the CVCS was approximately 24% higher than that of the refrigeration SEVCS and the exergetic efficiency of the refrigeration SEVCS was approximately 16%
higher than that of the corresponding CVCS under typical air conditioning application using R134a. In 2009, Bilir and Ersoy [52] theoretically revealed that the substitution ejector for the throttle valve in
the refrigeration system using R134a could improve COP by 22.3%. In
2010, Sarkar [53] investigated the performance of the refrigeration
SEVCS using natural refrigerants and revealed that the optimum MER
increased with evaporation temperature and decreased with condensation temperature. In 2014, Sumeru et al. [54] proposed a new
refrigeration EVCS through modifying refrigeration SEVCS. Through
the experiment for a split air conditioning system using R22, it was
found that compared with the SEVCS, the modified EVCS could gain
4.17%–13.78% COP improvement when the outdoor temperature
varied from 30 °C to 40 °C. In 2016, Wang et al. [55] evaluated the
performance of the refrigeration SEVCS using R141b based on CPM
ejector model. It was found that an optimal ejector mixing pressure
exists where the system COP, MER and outlet pressure reach a maximum. The value of optimal mixed pressure was just below the secondary stream pressure, but far above the critical pressure. In 2015,
Hassanain et al. [56] developed an ejector 1-D model based on HEM
and CAM model, but the diameters of the several sections were considered. The Dpn,t was calculated according to Henry and Fauske model
[57]. Thus the Qc can be calculated. It was found that the system COP
could be predicted with a maximal deviation of 2.3% in comparison
with the experimental data of [58]. In 2015, Zhang et al. [59] found the
substitution ejector for throttle valve in refrigeration system using R32
could increase the system COP by 5.22%–13.77% and exergetic efficiency by 5.13%–13.83% respectively through optimizing the value of
the pressure difference between mixing section and evaporator (PDME).
The reduction of overall exergy destruction was ranged from 8.84% to
15.84%.
The low GWP working fluids (R1234yf, R1234ze and so on) that
have been suggested as substitutions for HFCs usually lead to the decline of system performance. In 2014, Li et al. [60] theoretically found
that the refrigeration SEVCS using R1234yf was superior to the corresponding CVCS, especially in extreme operating environments. It was
also found that R1234yf refrigeration SEVCS has a lower COP, but it
possesses a higher COP improving potential than the corresponding
3.1.2. Theoretical analysis of transcritical SEVCSs
Owing to the high throttle losses and large COP enhancement potential, many studies investigated ejector utilization in the transcritical
R744 refrigeration system. In 2005, Li and Groll [45] investigated a
transcritical R744 refrigeration SEVCS based on the HEM and CPM and
reported that through ejector replacement, the COP was increased by
over 16% compared with the corresponding CVCS. This configuration
brings some difficulties in controlling the working conditions owing to
the close relation between xeje,out and MER. Therefore, they proposed a
novel configuration shown as Fig. 6, where a proportion of gas in the
separator is fed back to the evaporator through the throttle. This novel
cycle was reported to increase COP by about 18%. In 2007, Deng et al.
[64] also established a thermodynamic model for the R744 refrigeration SEVCS according to CPM model, and concluded that the COP improvement was 22% compared with the corresponding CVCS. In 2008,
Sarkar [65] presented optimization analysis of high-side pressure along
with MER and PLR according to the optimum COP for the transcritical
R744 SEVCS and the system proposed by Li and Groll [45] by using
CAM model. The maximum improvement of exergetic efficiency was
found to be 9% by applying ejector over throttle valve system. Based on
the first and second laws of thermodynamics, Fangtian and Yitai [66]
showed that ejector substitute could reduce the exergy destruction by
more than 25% and enhance the COP by more than 30% in the transcritical R744 refrigeration system. In 2012, Liu et al. [67] developed
and validated a model for air-to-air transcritical R744 air conditioners
Fig. 5. Schematic and p-h diagram of SEVCS with booster compressor [61].
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Fig. 6. Transcritical SEVCS with gas fed back [45].
real fluid properties instead of the ideal gas hypothesis. The irreversibility caused by the friction in the different parts of the ejector was
expressed by polytropic efficiencies. It was revealed that the model
could not only predict the performance for a definite geometry of
ejector and given entrance working parameters under off-design conditions, but also evaluate ejector dimension parameters for given entrance and exit working conditions. In 2019, Taslimi et al. [72] investigated the transcritical R744 SEVCS performance by combining an
experimental verification R744 heat pump system model with an
ejector model. It was revealed that with the increasing of the heat
transfer area ratio (AR), system COP and Qh grew up, but the optimum
gas cooler pressure (pgc,opt) declined. Under given ejector geometries
and working conditions, the improvements of COP and Qh of the EVCS
could attain about 17% and 20% respectively through the increase of
the heat transfer AR. In 2017, Choudhary et al. [73] presented a transcritical R744a refrigeration SEVCS, and thermodynamically revealed
that the maximal COP of R744a SEVCS was around 10% higher than
maximal COP of R744 SEVCS.
In 2015, Bai et al. [74] proposed an vapor injection transcritical
R744 EVCS with subcooler, shown as Fig. 7. The simulated results revealed that the proposed system increased the COP and volumetric Qh
by 7.7% and 9.5%, respectively compared with the traditional vapor
injection system. The exergy efficiencies of gas cooler and ejector were
with a needle adjustable ejector. The component efficiencies of ejector
were expressed by empirical correlations rather than simply assume to
be constant as in other references. It was found that at an ambient
temperature of 37.8 °C, COP and Qc of EVCS were increased by about
30.7% and 32.1% respectively in comparison with a transcritical R744
CVCS. In 2014, Zhang et al. [68] studied the impact of PDME on the
transcritical R744 refrigeration SEVCS performance and revealed that
the optimal PDME was primarily concerned with the efficiencies of the
ejector parts, but it was almost non-correlated with the working temperatures. It was ultimately found that in comparison with the CVCS,
the maximum COP improvement of the SEVCS could be attained by
more than 45.1% through optimizing PDME, and the exergy destruction
of the SEVCS could be decreased by approximately 43.0%.
In 2016, Minetto et al. [69] presented an analysis of a water-towater transcritical R744 multifunctional SEVCS, which was used for
winter heating, summer cooling, and tap water production. The ejector
MER was based on the correlation suggested by Banasiak et al. [70]. It
was found that the R744 system with ejector could achieve identical
energy consumption with the R410A unit. The seasonal COP improvement of 27.59% over the corresponding basic R744 layout was reported. In 2018, Taslimi et al. [71] developed a thermodynamic model
for both single and dual choking conditions to design R744 ejectors.
Based on HEM, the conservation equations were solved by applying the
Fig. 7. Transcritical SEVCS with subcooler [74].
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
intercoolers, one with exterior coolant and the other one with system
working fluid. Fig. 9 depicts the schematic and P-h diagram of the
system. The simulated results revealed that the maximum COP of the
proposed SEVCS were 19.6% and 15.3% higher than those of one-stage
refrigeration SEVCS with and without IHX respectively. The effects of
the arguments such as pgc, interstage pressure, tgc,out and teva on system
COP were also investigated. In 2015, Goodarzi et al. [82] found that an
IHX can improve the cycle COP at low gas cooler pressures.
In 2017, Megdouli et al. [83] tried to recover the exhaust heat from
the gas cooler of the transcritical R744 SEVCS through a transcritical
R744 ORC. The power generated by ORC was applied to drive the
compressor and the feed pump, thus lowering the input power consumption and enhancing the performance of the whole system. Fig. 10
shows the schematic and P-h diagram of the system. The simulated
results revealed that the application of ORC resulted in a COP improvement of 12% compared with the refrigeration SEVCS under the
same working conditions. In 2018, Nemati et al. [80] evaluated the
application potential of the ORC for rejected heat recovery of the gas
cooler in a transcritical double-stage refrigeration SEVCS. The identical
refrigerant was used in the ORC as the refrigeration system. Based on
the results, it was found that the ORC system utilization could improve
COP by 10.75% and 8.37% respectively for R744 and ethane systems.
In 2011, Yari and Megdouli [77,78] proposed a transcritical R744
cascade refrigeration SEVCS with ORC and found that the cycle could
improve system COP by 18%–31.5% in comparison with that of the
cascade CVCS. In 2017, Megdouli et al. [84] studied the influence of
ORC on the performance of the transcritical R744/R744a cascade
SEVCS for cryogenic applications. R744a was employed for the lowtemperature circuit (LTC), and R744 was employed for the high-temperature circuit (HTC). Compared with cascade SEVCS, the system COP
and exergetic efficiency were increased by more than 9% owing to the
ORC application.
In 2019, Liu et al. [85] presented a transcritical R744 refrigeration
SEVCS with a thermoelectric subcooler, which is shown in Fig. 11. The
approximately 57.9% and 69.7% respectively, which were critical parts
for improving system energy efficiency. In 2016, Bai et al. [75] investigated a transcritical R744 refrigeration SEVCS with exergy analysis
and found that the compressor with the highest exergy loss should be
given improving priority, the ejector second, the evaporator third. It
was found that 43.44% of the system exergy loss could be prevented
with components improvement.
In 2008, Yari and Sirousazar [76] proposed a transcritical R744
double-stage compression refrigeration SEVCS including IHX, ejector
and intercooler. The schematic and P-h diagram of the proposed SEVCS
is depicted in Fig. 8. The simulated results showed that in comparison
with the corresponding CVCS and SEVCS, the COP and the exergetic
efficiency of the proposed system were increased by around 55.5% and
26% respectively when teva and tgc,out were 10 °C and 40 °C respectively.
Subsequently, Yari [77,78] performed an optimizing analysis of the
system by the first and the second laws of thermodynamics. The correlated formulas for predicting the designing specifications of the
system were proposed. The maximal COP and exergetic efficiency of the
system were increased by around 12.5%–21% compared with the
double-stage system without ejector. In 2017, Nemati et al. [79] performed a comparison among R744, R170 and R744a as the working
fluids of the aforementioned system. It revealed that R170 was the
optimum working fluid from the viewpoint of energy and exergy, and
R744a was most suitable in terms of the exergoeconomic aspects. In
2018, Nemati et al. [80] studied the aforementioned system using R170
and R744 as working fluid. It revealed that in comparison with the
R744 system, the COP and the exergetic efficiency of the R170 system
were increased by 9.37% and 9.43% respectively, and the compressor
exhaust temperature of the R170 system was decreased by about
17%–25%. Moreover, the low pressure level of the ethane cycle is a
remarkable advantage compared with the R744 system owing to the
decrease of the system sealing cost.
In 2012, Manjili and Yavari [81] proposed a double-stage compression transcritical R744 refrigeration SEVCS, including both
Fig. 8. Transcritical SEVCS with two-stage compression [76].
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Fig. 9. Transcritical SEVCS with two-stage compression and multi-inercooling [81].
Fig. 10. Transcritical SEVCS with ORC [83].
system performance was studied in comparison with the CVCS, the
system with a thermoelectric subcooler, and the SEVCS. The novel
system was found to exhibit higher COP and lower pgc in comparison
with the other three systems. The optimum COP of the novel system
was 39.34% higher than the corresponding CVCS and the pgc,opt was
decreased by 8.01% under working conditions of teva = 5 °C and
tgc,out = 40 °C.
3.2. Experimental analysis of SEVCSs
3.2.1. Experimental analysis of subcritical SEVCSs
In 1991, Menegay [86] built an experimental SEVCS that was
modified from air-to-air refrigeration system using R12. The experimental apparatus schematic is shown in Fig. 12, where a hot gas bypass
was used to control the condenser and evaporator MFR. The experiment
was carried on through changing the amounts of hot gas bypass flow.
The experimental results showed that COP improvement owing to the
ejector was only a few percent. The pressure lift owing to the ejector
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Fig. 11. Transcritical SEVCS with thermoelectric subcooler [85].
Fig. 12. Schematic of the air-to-air refrigeration SEVCS test setup [86].
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system under the conditions that the brine temperature entering the
evaporator was 13.7 °C and the water temperature entering the condenser ranged from 25.4 °C to 32.7 °C. The pressure drop through the
evaporator decreased from 96 to 133 kPa of the CVCS to 2–4 kPa of the
SEVCS. The studies indicated that the application of the ejector could
not only realize the unloading of the compressor but also enhance the
evaporator performance to further improve COP. In 2016, Bilir and
Ersoy [90] experimentally found that the COP of the SEVCS was
5%–13% higher than the CVCS through optimization of Dpn,t when the
brine water temperature entering the evaporator was 13.9 °C and the
water temperature entering the condenser ranged from 30 °C to 46 °C. It
was also found that the COP was decreased by 7.5%–12.9% due to
deviation of Dpn,t from its optimum value.
In 2015, Pottker and Hrnjak [91] quantified the gain of the ejector
owing to energy recovery and the liquid-feed evaporating respectively
through comparing corresponding CVCS and flash gas bypass systems
(shown in Fig. 14). The refrigerant of the apparatus was R410A. The
test setup was composed of two closed-loop wind tunnels designed to
accommodate the heat exchangers. A micro-channel condenser and a
fined tube evaporator were used. A subcooling valve was installed upstream of primary nozzle to adjust the MFR of the primary stream and a
metering valve was installed upstream of evaporator to control the MFR
of the secondary stream. Fig. 15 shows the ejector prototype. During the
experiment, the air temperature entering the condenser and the
was decreased for higher MFRs. He attributed this trend to excessive
friction pressure drop and delayed flashing of the primary stream. In
1996, Menegay and Kornhauser [87] attempted to enhance the performance of a SEVCS by seeding the primary nozzle with bubbly flow in
the tests of an apparatus using R12 under typical air conditioning operation conditions, and ultimately the COP improvement was found to
be ranging from 2.3% to 3.8%. In 1997, Harrell [88] conducted an
experiment on a refrigeration EVCS using the R134a. The ejector was
designed based on single-phase and wet steam ejector design methods.
The COP improvement over the CVCS was found to be between 3.9%
and 7.6%.
In 2014, Ersoy and Bilir Sag [58] developed an ejector prototype
based on CAM model and tested the performance of the SEVCS in a
water-to-water refrigeration apparatus using R134a. The schematic of
the test setup is shown in Fig. 13. The condenser and the evaporator
were brazed-plate types. Four valves were used to switch between
SEVCS mode and CVCS mode. It was found that COP improvement
ranged from 6.2% to 14.5% at identical capacity and ejector recovered
14%–17% of the expansion work under the condition that the brine
temperature entering the evaporator was 20 °C and the cool water
temperature entering the condenser ranged from 40.3 °C to 46.2 °C. In
2015, through the experiment, Bilir Sag et al. [89] indicated that the
employment of an ejector increased the COP and exergetic efficiency by
7.3%–12.9% and 6.6%–11.24% respectively for an R134a refrigeration
Fig. 13. Schematic of water-to-water refrigeration SEVCS test setup [58].
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3.2.2. Experimental analysis of transcritical SEVCSs
Many literatures have developed ejectors with fixed geometry for
transcritical R744 refrigeration systems and investigated the system
performance as well as gas cooler pressure control measures.
In 2011, Lee et al. [93] developed a fixed geometry ejector and
tested in a transcritical R744 water-to-water refrigeration SEVCS with
an IHX. The gas cooler, evaporator, and IHX were all counter-flow
copper co-axial double pipe heat exchangers with high-pressure fluid
flowing in the interior tubes and low-pressure fluid flowing in the annulus gap. A semi-hermetic reciprocal compressor was used. During the
test, the temperature of water entering the gas cooler and the evaporator was 30 °C and 27 °C respectively. It was experimentally found
that the COP was improved by about more than 15% in comparison
with the CVCS at matched capacity owing to the ejector. A maximum
MER was found to be about 0.9. In 2014, Lee et al. [94] continued to
investigate the variations of the maximum MER under various compressor speeds and outdoor temperatures and found that the maximum
MER was generally between 0.7 and 0.9. The improvement of both COP
and QC were expected to be about 6%–9% and 2%–5% respectively
compared with the corresponding CVCS. Additionally, the control
strategy was discussed to ensure that the SEVCS was superior to the
corresponding CVCS. In 2012, Lucas and Koehler [95] built a water-towater SEVCS refrigeration test rig, where the heat exchangers were all
plate types. Water was used as coolant of the gas cooler and the waterglycol mixture was served as the heat source fluid of the evaporator.
They showed how to regulate gas cooler pressure through changing
compressor speed to optimize COP of a system with a fixed geometry
ejector under specific working conditions, but a large capacity change
would happen simultaneously. At identical capacity, the maximum COP
enhancement was found to be 17% and the ejector expansion energy
recovery efficiency was up to 22%. But they also highlighted that the
use of the fixed geometry ejector lowered the system performance over
5% at some particular operating conditions.
In 2010, Chen et al. [96] designed an ejector prototype based on
CPM model and gas dynamic methods for a transcritical R744 water-towater heat pump, where a bypass valve was used to control the primary
MFR and the pressure of the primary stream. It was found that the MER
increased as primary pressure and secondary pressure increased. The
decrease of the mixing pressure resulted in an enhancement of the MER.
When the MER attained the maximum value, it was nearly invariant in
off-design operations if the mixing pressure was decreased further. In
2012, Banasiak et al. [97] designed an ejector prototype to study the
feasibility of SEVCS in domestic R744 water-to-water heat pumps. The
gas cooler and the evaporator were all brazed-plate types. They
Fig. 14. Flash gas bypass refrigeration systems [91].
evaporator ranged from 38 °C to 52 °C and from 10 °C to 27 °C respectively. It was found that COP improvements solely owing to the
application of the ejector were in the range of 1.9%–8.4% compared
with liquid-feed evaporating system at the identical Qc. The COP improvement of the SEVCS over the CVCS ranged from 8.2% to 14.8%
owing to the synthetic gains of liquid-feed evaporating and energy recovery. The total efficiencies of ejector were reported in the range of
12.2%–19.2%.
In 2019, Li and Yu [92] presented an experimental study on the
factors of evaporation temperature variations in a SEVCS using R290 as
the working fluid, and found that the value of Dpn,t directly influenced
the evaporation temperature. When Dpn,t varied from 0.7 mm to
1.0 mm, the corresponding evaporation temperature ranged from
−34 °C to −26 °C. The evaporation temperature could be adjusted
through appropriate matching between refrigerant charge quantity and
Dpn,t or the capillary length while the fluid MFR through the evaporator
was maintained within a certain extent.
Fig. 15. Ejector prototype developed by Pottker and Hrnjak [91].
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presented that ejector efficiency could be optimized by changing discharge pressure (varying compressor speed), and COP improvement of
6.6%–8.3% was found at matched capacity. The gained ejector efficiency was reported to be 24%–31%. In 2013, Minetto et al. [98] presented an experimental investigation of an R744 water-to-water SEVCS
heat pump prototype, where the heat exchangers were all plate types. It
was found that the MER was ranged from 0.8 to 1.6 and a maximum of
0.55 MPa for the secondary pressure lift was achieved at the chosen
testing conditions (Qh = 5 kW, teva = 0 °C, pgc = 10 MPa,
tgc,out = 35 °C). If the UA values of the two systems were identical, the
SEVCS was found to attain 25% COP enhancement. However, they also
presented that the enhancement declined remarkably owing to the
lower evaporator UA value of the SEVCS caused by improper oil return.
This means that the proper management of oil is also a challenge for
practical application of SEVCS. The proposed solution was a novel liquid–vapor separator, which will be discussed in the following part.
In 2018, Zhu et al. [99] developed a transcritical R744 air source
SEVCS water heater, shown as Fig. 16. A convergent primary nozzle
was used for the ejector prototype. The gas cooler was a concentric
double pipe type with cooling water flowing in the interior pipe and
R744 flowing through the ring-shaped gap. The evaporator was a
micro-channel type with a variable-speed fan controlling the evaporator
exiting superheat. The outlet temperature of tap water ranged from
50 °C to 90 °C. The system COP was found to achieve 4.6, which was
10.3% higher than the CVCS under the condition that the tap water
exiting temperature was 70 °C. They also revealed that the use of an
ejector was more profitable for the condition of high-temperature hot
water production.
3.3. Special issues of SEVCSs
Recently, some studies have focused on special issues of SEVCSs.
These include exploratory researches of adjustable measures, geometry
optimization of the ejector, optimization of the liquid–vapor separating
process, effect of IHX in SEVCSs and SEVCSs using zeotropic mixture.
This section will review these studies. However, as can be seen, the
material is limited for most of these topics.
Fig. 17. Schematic of the adjustable measures for ejector [42,100–104].
Fig. 16. Schematic of the transcritical R744 air source SEVCS water heater test setup [99].
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less the nozzle opening, the lower the ejector efficiency. COP and Qc of
the transcritical R744 SEVCS were found to be increased by 7% and 8%
respectively compared with CVCS, and they predicted that the COP
enhancement could be up to 18% at the identical capacity. The ejector
work recovery efficiency was reported to be up to 15%. Additionally,
Elbel and Hrnjak [42] suggested a pgc,opt correlation for the transcritical
R744 refrigeration SEVCS (pgc,opt = 0.16tgc,out + 3.61 MPa).
In 2012, Liu et al. [100] built a transcritical R744 refrigeration
SEVCS without IHX experimental setup shown as Fig. 19. The test was
carried out in an environmental control unit with a cooling capacity of
10.3 kW. A semi-hermetic reciprocal compressor was used. The gas
cooler and the evaporator were all microchannel types. They developed
a needle adjustable ejector with a converged primary nozzle shown as
Fig. 20. They proposed an approach for evaluating the efficiencies of
ejector components. The results revealed that the efficiencies of the
primary and secondary nozzle were generally 50%–90%, and the efficiency of the mixing chamber was 50%–100%. The values were smaller
than that usually supposed in the aforementioned theoretical investigations. The experimental data of Liu et al. [106] revealed that
compared with the CVCS of a fixed-speed compressor, the improvement
of COP and Qc of the SEVCS with variable-speed compressor attained
about 147% and 25% respectively for transcritical R744 SEVCS test
setup. In 2016, Liu et al. [107] experimentally investigated the simultaneous cooling and heating performance of the transcritical R744
SEVCS test setup with a needle adjustable ejector and a variable-speed
compressor. It was revealed that the maximum total COPimp of the
system was about 71.4%, but the total capacity was reduced by 21.3%.
In addition, the empirical relationships between COP and ejector geometrical parameters, compressor efficiency, and working conditions
were established.
In 2009, Chen et al. [108] also developed a needle adjustable ejector
for an R744 water heating SEVCS without IHX. A laterally tapered
needle stretching to the throat of primary nozzle was installed to adjust
the throat area. The needle was connected to a step by step motor
through an electromagnetism loop in the end and was controlled by a
programmed driver. The experimental results revealed that the needle
adjustable ejector performed excellently and worked well in extensive
working conditions. Through the experiment of a transcritical R744
heat pump SEVCS with a needle adjustable ejector, Xu et al. [109] revealed that there was a pgc,opt for maximum COPh and Qh. A higher pgc
was favorable to the system COP and outweighed the decrease of
ejector efficiencies. The efficiency of ejector was primarily between
20% and 30%. A linear correlation between the pgc,opt and the tgc, out
3.3.1. Exploratory researches of adjustable measures
Since the fixed geometry ejector is hypersensitive to load variation
and to operating conditions, the ejector has to operate in limited
working conditions to ensure efficient operation owing to its innate
working characteristics. However, the requirement of practical refrigeration and heat pump systems is to adapt for variations of load and
working conditions. Also, a fixed geometry ejector cannot actively
control high-side pressure on its own for transcrtical R744 EVCS. As
previously mentioned, Menegay [86] and Chen et al. [96] adjusted the
primary MFR through changing the amounts of hot gas bypass flow
between compressor outlet and primary nozzle inlet. But the system
performance decreases markedly by this scheme. Thus it is significant
to exploit new adjustable measures that can be adapted to the variation
of the operation conditions. Fig. 17 summarized four adjustable measures for ejector in the literature. Fig. 17(a) shows a schematic of the
needle adjustable measure [42,100], in which a needle is inserted into
the primary nozzle throat and used to control the effective area of
primary nozzle by changing the needle position. Fig. 17(b) shows
parallel multi-ejectors adjustable measure [101], in which usually
various-sized, fixed ejectors are assembled and turned on or off independently for obtaining the appropriate effective nozzle size under
certain condition. This measure will be reviewed detailed in the Section
4.4 in this paper. Fig. 17(c) shows a series-parallel valve adjustable
measure [102], which uses a throttle valve upstream of the primary
nozzle (to adjust the MFR of primary stream or raise pgc for transcritical
R744 systems) or in parallel with the ejector (to reduce pgc for transcritcial R744 systems). Fig. 17(d) shows a primary inlet vortex adjustable measure [103,104], where the ratio of MFR through the two
inlets is adjusted by valves installed at the primary stream axial and
tangential inlets, thereby changing the vortex strength, thus primary
stream can be adjusted.
Many documents have developed the needle adjustable ejector for
transcritical R744 systems. Elbel and Hrnjak [42,105] proposed a
needle adjustable ejector to adjust gas cooler pressure for the transcritical R744 refrigeration system with the IHX. The test setup was
composed of two closed-loop wind tunnels to accommodate the microchannel gas cooler and evaporator. The test ejector prototype is
shown in Fig. 18. They experimentally found that the maximum system
COP could be reached through optimizing the compressor discharge
pressure by adjusting the needle. Nevertheless, they also found that the
addition of the needle in the nozzle lowered ejector efficiency owing to
more friction occurred between the refrigerant and the needle in the
process of the refrigerant expanding through the primary nozzle. The
Fig. 18. Ejector prototype developed by Elbel and Hrnjak [42,105].
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Fig. 19. Schematic of transcritical R744 SEVCS test setup [100].
Fig. 20. Photograph and schematic of adjustable ejector developed by Liu et al. [100].
dynamic model of the test system. It was experimentally found that it
has a good tracking performance for pgc and cooled water exit temperature. Moreover, the ejector could be driven from the subcritical
condition to the critical condition by the controller, ensuring the excellent performance of ejector and system. In 2018, Suo [112] developed a needle adjustable ejector with a two-stage primary nozzle in the
transcritical R744 SEVCSs. The results revealed that Qc and COP of the
SEVCS were 33% and 37.8% higher than the corresponding CVCS,
was also deduced (pgc,opt = 0.118 tgc,out + 5.6 MPa). It was larger than
that of Elbel and Hrnjak [42] under the identical tgc,out. It illustrates
that the application of IHX results in a decreasing of the pgc,opt.
In 2017, He et al. [110,111] built a test setup for transcritical R744
SEVCS with a needle adjustable ejector. The copper concentric double
pipe heat exchanger was used for the gas cooler and the evaporator
with R744 in the interior tube and the water in the ring-shaped gap.
They developed an optimum multivariable controller according to the
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et al. [118] found that NXPopt was 2.8 mm for the highest MER through
CFD simulation of an R134a ejector. It was also revealed that NXP had a
significant effect on the size of recirculating bubbles in the secondary
nozzle. Recirculating bubbles prevent the propagation of pressure
waves from the secondary nozzle to secondary channel, which was
believed to be one of the reasons for the significant entrained performance. In 2018, Suo [112] assessed three cases of NXP for R744
SEVCSs: 0 mm, 9 mm and 15 mm. It was found that NXPopt was 9 mm.
This value was about four times of Dpn,t, and 0.7 times of Dmix. In 2017,
Huang et al. [116] experimentally found that the smaller NXP was
helpful to the improvement of a two-stage primary nozzle ejector performance in an R134a SEVCS. Under the condition of teva = −10 °C
and tcon = 40 °C, the NXPopt was found to be 0 mm where the MER, PLR
and system COP were all maximum. Conversely, Bilir and Ersoy [90]
presented that the variations of COP based on NXP were lower than 1%,
which means there is no NXPopt in R134a SEVCS at the test conditions.
The two-stage expansion was sometimes used to decrease the thermodynamic nonequilibrium and improve the primary nozzle efficiency.
In 2004, Takeuchi et al. [119] introduced a two-stage primary nozzle
(Fig. 22) in which the first throttle provides the functions of boiling
nucleus generation and flow rate control, the second throttle (fixed in
geometry) recovering expansion energy that would otherwise be lost.
The experiment revealed that the efficiency of the two-stage nozzle
could attain 90%, which was about 50% better than that of the traditional one-stage C-D nozzle. In 2014, Ren et al. [120] also developed a
two-stage primary nozzle in an R134a SEVCS. The experimental results
indicated that the maximum increment of MER and COP with new
nozzle was about 18% and 12% respectively compared with the conventional SEVCS. In 2016, Zhang et al. [121] experimentally found that
MER and system COP increased with the first-stage Dt increasing. With
the increasing of second-stage Dt, the MER was improved, and the COP
firstly increased then decreased. In addition, the smaller divergent
angle of the first-stage nozzle was found to be helpful to improve the
MER. In 2017, Huai [122] also found the similar phenomenon and
pointed out that the first and second nozzle throat equivalent diameters
could not be less than 1.8 mm and 1.4 mm respectively for the stable
operation of the system. In 2017, Huang et al. [116] experimentally
found that the system gained the highest COP and Qc when the divergence angle of first-stage nozzle was 8°under the conditions of
tcon = 40 °C and teva = −10 °C in R134a SEVCSs. The reason was
explained that too large divergent angle of the first-stage nozzle was
against the atomization effect of the liquid refrigerant in the nozzle
throat, but too small divergence angle resulted in the too small crosssectional area of the divergence to be conducive to the bubble nuclei
formation.
The mixed process of the two streams greatly influences the ejector
respectively. In 2019, He et al. [113] tried to perform a comparison
between the needle adjustable measure and fixed ejector through a
homogeneous CFD model for transcritical R744 SEVCSs. It was found
that the needle adjustable ejector could reach similar exergetic efficiency as the fixed one, but the MER of the needle adjustable ejector
decreased by 5%–11% compared with the fixed one owing to the high
exergetic destruction induced by the needle and oblique shock wave in
the secondary nozzle.
In 2014, Hu et al. [114] presented that the optimum Qc and COP can
be obtained by the needle adjustable ejector for a R410A air conditioning SEVCS through optimizing the needle position. The experimental results showed that a maximum of COP improvement was up to
9.1% for the SEVCS. But they also presented that the SEVCS was likely
to have lower performance than corresponding CVCS owing to sample
manufacture limitation and no optimization of ejector. In 2017, Jeon
et al. [115] built a R410A air conditioner SEVCS experimental setup.
The compressor was a twin-rotary type. The condenser was a water
cooled plate type. The evaporator capacity was regulated through
changing the power of a heater. They studied the performance of the air
conditioner for various operating parameters considering the CSPF. A
maximal COP improvement of 7.5% was found for the SEVCS over the
CVCS. The CSPF improvement of the SEVCS over the CVCS was found
to be from 16.0% to 20.3%. In 2017, Huang et al. [116] developed an
adjustable two-stage primary nozzle ejector. It was experimentally
found that the system Qc could be regulated through varying the needle
position in the R134a system, and the optimum effective area ratio of
the primary nozzle throat was 90%. It was found that the highest
system COP was 2.05 when the tcon and teva were 40 °C and −10 °C
respectively.
In 2019, Lawrence and Elbel [102] found that the ejector efficiency
and system COP of the series-parallel valve measure were marginally
lower than that of the needle adjustable measure through the experiment for the transcritical R744 SEVCS. However, the series-parallel
valve measure was suggested by the authors owing to the virtues of
simplicity and cheapness.
The schematic of transcritical SEVCS with primary inlet vortex is
shown in Fig. 21. In 2020, Zhu and Elbel [103] found that the total
ejector efficiency of this measure was higher than that of series expansion valve adjustable measure and close to needle adjustable measure. The pgc could be controlled by this measure to enhance the
transcritical R744 SEVCS performance. Through using this measure, the
maximum improvements of system capacity and COP were reported to
be 11.0% and 8.1% respectively under non-design conditions.
3.3.2. Geometry optimization of ejector
The primary nozzle exiting position (NXP) is usually measured according to the distance between the primary nozzle outlet and the inlet
of the constant area mixed section. In 2011, Lee et al. [93] observed
that the optimum NXP was 1.4 times of Dpn,t for the transcritical R744
air conditioning SEVCS. It was because choking occured at the entrance
of the mixing chamber and evaporator MFR reached a maximum under
this value. It was also found that the system COP was insensitive to
slight NXP change from the optimal value, but COP decreased rapidly
for significant deviation of NXP from optimal value. In 2016, Liu et al.
[107] found that Qc, cooling COP and total COP (Cooling
COP + Heating COP) attained the maximum when NXP was adjusted at
three times of Dmix through the test of a transcritical R744 SEVCS that
cools and heats simultaneously. In 2017, Zheng and Deng [117] found
that primary nozzle isentropic efficiency decreased with increasing
NXP. In 2014, Hu et al. [114] found that the MER was significantly
influenced by NXP for air conditioning SEVCS using R410A as the refrigerant. It was found that Qc increased firstly and then decreased with
increasing NXP. This is because when NXP is too long, MFR of the
secondary stream decreases due to backflow near the primary nozzle
exit. They concluded that there was a NXPopt to ensure the highest efficiency of the ejector. The NXPopt was found to be 3 mm. In 2018, Baek
Fig. 21. Transcritical SEVCS with primary inlet vortex [103].
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Fig. 22. Two-stage primary nozzle proposed by Takeuchi et al. [119].
the mixing pipe, significantly reducing the pressure rising generated by
the ejector. Lmix,opt was predicted to be 20 ~ 25 mm depending on the
primary nozzle inlet pressure. In 2018, Baek et al. [118] investigated
effects of Lmix on ejector entrained performance by numerical methods
for R134a SEVCS. It was found that the optimum MER was obtained
with Lmix = 20 mm among six values of Lmix (10 mm-75 mm). Suo
[112] observed that Lmix,opt was about 124 mm where Lmix/Dmix was
about 10 during the test of an ejector with a two-stage primary nozzle.
In 2012, Liu et al. [67] found that the maximal cooling COP and Qc
emerged at Dmix of 4.1–4.2 mm, which was about 1.5 times of Dpn,t.
Banasiak et al. [97] experimentally demonstrated that the Dmix,opt was
5 mm. For smaller diameters, the ejector was found to be incapable of
entraining sufficient amount of superheated vapor from the evaporator.
The phenomenon was attributed to the secondary stream throttling in
the mixing section and the continuous oblique shock wave flow regime.
For larger diameters, high values of pressure lift or efficiency were
found to be failed to attain owing to the inferior momentum exchange,
which was probably as a result of enhanced recirculation. In 2014, Hu
performance, so the mixing section geometric characteristics is of significance to the design of an ejector. The mixed section is usually designed to have a straight-edged cone to enable good manufacturability.
In 2011, Nakagawa et al. [123] experimentally studied three different
values of Lmix (5 mm, 15 mm, and 25 mm) for a constant rectangular
mixing chamber in an R744 SEVCS. It was found that the highest
pressure recovery, MER, and COP were all gained at Lmix = 15 mm for
the SEVCSs both with and without an IHX. The improper Lmix was found
to decrease system COP by as much as 10%. Elbel [105] used a doubleconed entrance mixed section suggested by [124]. It was found the
ejector efficiency performed an optimum value at the shortest
Lmix = 7.5 mm among four different variants (7.5–82.5 mm) for R744
SEVCS. It was predicted that this phenomenon was due to the decrease
of friction pressure drop and the more favorable shock mode in the
mixed section. In 2012, Banasiak et al. [97] found that the result of
Lmix = 30 mm was the best in the experimental test. For shorter options, the momentum exchange potential was only partially utilized.
For longer options, the friction-induced pressure drop occurred along
Fig. 23. Ejector with a bypass duct of a secondary nozzle [126].
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at the secondary chamber entrance was found to not affect the entrainment performance. In 2018, Bodys et al. [126] proposed a bypass
duct of a secondary section for an ejector in the transcritical R744
SEVCS. The proposed concept is shown in Fig. 23, where the bypass
duct labeled by the blue on the secondary duct in front of the secondary
section. The numerical analysis showed that the secondary MFR was
increased by 36.9% for the bypass angle of 19° compared with conventional ejector with no bypass. An efficiency improvement of ranging
from 22.2% to 30.4% was predicted owing to the bypass implementation. Concerning a well-designed fixed geometry ejector, the bypass
position was suggested to be placed at about 40% of Lmix after the initial
section of the diffuser.
The diffuser geometrical parameters also influence the ejector performance. In 2011, Elbel [105] found that the maximum ejector efficiency was obtained for the minimum diffuser angle, 5° for the R744
SEVCS. Under the test conditions, the ejector efficiency was reported to
be less than 10% at the diffuser angles of 10° and 15°. In 2012, Banasiak
et al. [97] revealed that the optimal diffuser angle was about 3° through
the simulation. But the highest ejector efficiency was yielded with 5°
diffuser angle based on the experimental tests.
3.3.3. Optimization of the liquid–vapor separating process
The process of the liquid-vapor separator was generally assumed to
be perfect in the theoretical analysis of the SEVCS. It means that the
entire vapor that goes into the separator goes out from the vapor outlet,
and all the liquid that goes into the separator goes out from the liquid
outlet. Nonetheless, there is always a certain amount of vapor at the
liquid outlet, and a certain amount of liquid at the vapor outlet due to
the separator inefficiency. The vapor leaving from the liquid outlet will
cause extra mass flow that is pumped by ejector with no increase of
cooling effect. In addition, if an IHX is not included in the system, the
compressor will be damaged due to the excessive liquid entering.
In 2011, Nakagawa et al. [127] presented that the working fluid
quality exiting from the vapor outlet was about 0.9. For the purpose of
maintaining a thorough vapor phase of the working fluid entering the
compressor, Reddick et al. [128] proposed to add three electrothermal
elements between the vapor exit and the compressor entrance. The
system COPimp was found to be 11% owing to ejector replacement using
R134a if the overall heating power of the elements was included in
evaporator Qc. But if not, the COP of the SEVCS was lower than that of
Fig. 24. Separator proposed by Minetto et al. [98].
et al. [114] numerically found that Dmix,opt was 9 mm for the highest
pressure lift. In 2017, Jeon et al. [115] also represented that Dmix,opt
was 9 mm based on the CSPF for a R410A air conditioner SEVCS.
Furthermore, Dmix,opt was found to become larger with the increasing of
the annual average outdoor temperature. In 2018, Baek et al. [118]
found that Dmix,opt was 6 mm through optimizing the ejector MER by
numerical methods for R134a SEVCSs with Dmix ranging from 4 mm to
7 mm.
In 2016, Bodys [125] investigated the effect of swirl flow at the
entrance of the primary and secondary chambers on the performance of
ejectors installed in an R744 supermarket refrigeration system by numerical simulation using a validated HEM model. The results indicated
that the MER was increased with the increasing of the inlet diameter of
primary nozzle owing to a larger tangential velocity component. The
swirl generator was suggested to be installed at the entrance of the
primary nozzle for optimizing entrainment performance. The swirl flow
Fig. 25. Structures of the separator proposed by Huai [122].
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in Fig. 25(a) and (b) respectively. It was found from the experiment that
ejector MER was increased from 0.2~0.46 to 0.56~0.64 according to
the different working conditions owing to the new structure of separator. In 2019, Chen et al. [131] proposed four different separators.
For Separator A, the inlet and gas outlet are set vertically on the top of
the separator, and a baffle is arranged vertically between the inlet and
the gas outlet to prevent the liquid from flowing directly from the gas
outlet. For Separator B, the inlet is obliquely tangential to the tank and
the gas outlet is set at the center of the top cover. Separator C is almost
identical with Separator B except that there is an O-ring under the top
gas outlet. Separator D is almost identical with Separator B except that
the top gas outlet pipe is inserted inside the separation tank. It was
experimentally concluded that separator D had the optimum performance. The COP improvement of SEVCS with separator D was up to
16.7% at teva = −30 °C in comparison with the CVCS.
the CVCS. Then they suggested a DeEVCS, where the primary evaporator plays its traditional role and the secondary evaporator is used to
control the superheat of the flow leaving the separator. In 2012,
Lawrence and Elbel [129] defined liquid and vapor separating efficiencies to analyze the influence of separator on system performance.
These efficiencies are given in Eqs. (5) and (6). It is found that ejector
was no longer profitable in R134a or R1234yf systems at separation
inefficiency of 15% or higher.
ηliquid =
ηvapor =
MFRliquid - at - liquid - port
MFR diff,out (1 − x diff,out )
(5)
MFRliquid - at - liquid - port
MFR diff,out x diff,out
(6)
In 2017, Zhu et al. [130] introduced a mass balance coefficient
(MBC) according to the liquid mass balance between separator inlet and
outlet. It can evaluate the discrepancy between present running status
and a stable condition. It was experimentally showed that the COP
improvement of the SEVCS over the CVCS was decreased from 18.9% to
−11% when the MBC was increased from −0.1 to 0.1. Both the MER
and COP decreased with increasing the MBC.
In 2013, Minetto et al. [98] found that the oil was apt to gather with
the liquid at the separator bottom and flows to the evaporator. The UA
value of the evaporator in the SEVCS was almost 50% lower than that in
the CVCS for the same range of evaporator MFRs owing to the effect of
excess oil circulating the evaporator without returning to the compressor. Additionally, the compressor will be damaged due to insufficient supplied oil over time. Thus they suggested a low-pressure
separator with three exits at different heights based on PAG/R744 behavior, shown in Fig. 24. The main concern of this solution was related
to the liquid outlet position, which must be low enough to prevent
vapor entry and control the oil outlet. The size of the oil outlet needs to
be adjusted to prevent overflow.
In 2017, Huai [122] investigated the effect of the liquid–vapor separator structure on the property of SEVCS with R134a as refrigerant.
The structures of the original one and the redesigned one are displayed
3.3.4. Effect of IHX in SEVCSs
An IHX is often used to reduce the throttling loss and enhance the
system performance in conventional refrigeration systems with isenthalpic throttle valves. The SEVCS with IHX is shown in Fig. 26. The
influence of the IHX on the performance of SEVCS is also a research
hotspot.
For transcritical R744 refrigeration SEVCS, with assuming ideal
efficiency of each part, Kanamaru and Nakagawa [132] thermodynamically discovered that the IHX use in the transcritical R744 refrigeration SEVCS was unprofitable. But Nakagawa et al. [127] experimentally revealed that compared with CVCS, the COP enhancement
was found to be up to 27% owing to the use of ejector and IHX. Additionally, they found that a more COP will be obtained with higher
efficiency of IHX. The maximum energy recovery efficiency was found
to be 22% with the most efficient IHX. Xu et al. [133] revealed that IHX
lessens the gain of the ejector on the account that the compressor
pressure ratio reduction owing to ejector were 5.6%–6.7% and
10%–12.1% for the systems with and without IHX respectively. In
2013, Zhang et al. [134] thermodynamically found that using IHX in
transcritical R744 SEVCS does not always improve system performance.
Fig. 26. Conventional SEVCS with IHX [134].
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Energy Conversion and Management 207 (2020) 112529
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CVCS respectively. The system COP using mixture can be increased by
50% compared with pure R744 system. In 2019, Liu and Yu [141] investigated the performance of SEVCS using zeotropic mixture R290/
R170 for cryogenic freezing applications with teva ranging from −65 °C
to 45 °C. The results showed that COP of SEVCS decreased firstly and
then increased significantly with increasing the MF of R290, and the
minimum COP of 0.65 existed at R290 MF of 0.2. It also revealed that
when the R290 MF was lower than 0.51, the COP of SEVCS was
1.5–12.2% lower than that of CVCS. With the R290 MF increasing, the
COP of SEVCS surpassed that of CVCS. In 2019, Brodal and Eiksund
[142] investigated the performance of SEVCS using mixture R744/
R290, and found that it was inefficient to use mixture R744/R290 in
SEVCS. The blend-based system with IHX outperformed the R744-based
SEVCS under the condition that the tap water was higher than 25 °C,
the ejector efficiency was less than 0.17, or the heat source temperature
drop through the evaporator surpassed 10 °C.
The COP can only be improved through IHX addition under low isentropic efficiency of ejector.
For subcritical refrigeration SEVCS, Sarkar [135] indicated that the
addition of IHX decreases the COP for cycles using R717, R600a and
R290 as refrigerants. In 2014, Molés et al. [136] revealed that using an
IHX in the SEVCS generate a harmful influence on COP, but led to a
remarkable rise in QC. It was concluded that whether COP improves
owing to IHX depends on the efficiency of the ejector. In 2017, Garcia
and Berana [137] also theoretically found that the IHX addition does
not necessarily increase the system COP for the refrigerants R717, R22,
R134a, and R290. In 2018, Rodríguez-Muñoz et al. [138] presented a
new scheme for the SEVCS with IHX. Fig. 27 illustrates the schematic of
the proposed system where the IHX is placed to facilitate superheating
in the primary nozzle. The scheme was investigated for the subcritical
working fluids (R134a, R1234ze and R290). The theoretical analysis
showed that the exergetic efficiency could be increased for the new
scheme compared with the traditional scheme (Fig. 26) if an IHX effectiveness was below 60%. In both cases, IHX addition was found to
promote a decline in the COP of the SEVCS.
The literatures about IHX effect on the SEVCS is summarized in
Table 2. It can be concluded that the IHX addition in the SEVCS does
not necessarily result in an increment of the COP. However, the IHX
addition is usually profitable in the actual systems owing to the fact that
the practical ejector efficiency is usually very low. Furthermore, an IHX
has a secondary advantage that ensures no entrained liquid entering the
compressor. So the IHX is an indispensable component for the actual
SEVCSs.
3.4. Summary
The literatures about theoretical analysis of subcritical and transcritical SEVCSs discussed in this section are summarized in Tables 3
and 4 respectively, indicating the working fluids, operating conditions,
COPimp, and system features. The literatures about experimental analysis of subcritical and transcritical SEVCSs discussed in this section are
summarized in Table 5 and 6 respectively, indicating the working
fluids, operating conditions, Qc, MER, COPimp, and system features. It
can be seen that the COPimp owing to ejector in the subcritical and
transcritical systems are typically in the range of 5–20% and 7–40%
respectively. The gain is attractive to improve the system energy efficiency, particularly for large-capacity systems, which are evaluated not
only by relative values, but also by absolute values. The total gain is
also a very considerable figure due to the wide application of the refrigeration and heat pump systems all over the world today. In the
transcritical refrigeration systems, the supercritical fluid is throttled to
two-phase region, introducing relatively higher throttling loss than that
of the subcritical system. The gain is relatively larger than that of
subcritical system.
In general, most theoretical system studies are based on the LPMs
where the ejector is divided into the typical four sections with appropriate mass, energy, and momentum balances. Although the LPMs are
advantageous to predict flow and thermodynamic characteristics inside
an ejector easily, choosing the right value of the component efficiency
for the thermodynamic model is a challenge, especially for two-phase
3.3.5. SEVCSs using zeotropic mixture
Zeotropic mixture has good temperature glide matching with the
heat/cold source, which can be used to improve the system energy efficiency. In 2015, Zhao et al. [139] studied the performance of SEVCS
using zeotropic mixture R134a/R143a. The simulated results revealed
that the COP get an optimum of 4.18 with the mass fraction (MF) of
R134a being 0.9, where the COP improvement attained 3.06% in
comparison with the corresponding system with pure R134a. The
compressor and ejector were found to account for biggest share of the
exergy loss, and the exergetic efficiency of the SEVCS achieved an optimum value of 23.95% with the R134a MF being 0.7. In 2017, Li et al.
[140] presented a thermodynamic investigation of the refrigeration
SEVCS using zeotropic mixture R744/R32 (MF 0.4/0.6) or R744/R41
(MF 0.2/0.8) as working medium. It was shown that the system COP of
the two mixtures were 30% and 20% higher than the corresponding
Fig. 27. SEVCS with IHX proposed by Rodríguez-Muñoz et al. [138].
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Table 2
IHX effect on the SEVCSs (T-Theoretical, E-Experimental).
Reference
Year
Evaluation
Working fluid
Findings
Kanamaru and Nakagawa [132]
Nakagawa et al. [127]
Xu et al. [133]
Zhang et al. [134]
Sarkar [135]
Molés et al. [136]
2003
2011
2011
2013
2009
2014
T
E
E
T
T
T
R744
R744
R744
R744
R717, R600a and R290
R1234yf, R1234ze
Garcia and Berana [137]
2017
T
Rodríguez-Muñoz et al. [138]
2018
T
R717, R22, R134a and
R290
R134a, R1234ze (E) and
R290
Addition of IHX in SEVCS was not profitable with ideal components assumption.
The maximum COP improvement is up to 27% for SEVCS with 60 cm IHX.
IHX weakens the contribution of the ejector to the system performance.
The addition of an IHX is only applicable at lower ejector isentropic efficiencies.
IHX addition reduces the cooling COP of SEVCS.
The effect of the IHX on the COP not only depends on the IHX effectiveness and the working
conditions, but also depends on ejector efficiencies.
IHX addition does not necessarily increase the system COP.
A new configuration for SEVCS with IHX was proposed.
IHX addition promoted a decline in the COP of SEVCS.
Table 3
Theoretical analysis about subcritical SEVCSs.
Authors
Year
Working fluid
teva/°C
tcon/°C
Baseline
COPimp
System features
Kornhauser [48]
Domanski [49]
Nehdi et al. [50]
Yari [51]
Bilir and Ersoy [52]
Sarkar [53]
1990
1995
2007
2008
2009
2010
−15
8
−15
5
−25–5
−15 to −5
30
46
30
40
35–50
35–55
CVCS
CVCS
CVCS
CVCS
CVCS
CVCS
Li et al. [60]
Molés et al. [136]
2014
2014
−10–10
−13–7
30–55
37–57
CVCS
R134a CVCS
Zhang et al. [59]
2015
−10–10
30–55
CVCS
Wang et al. [55]
Rostamnejad and Zare [61]
Ejemni et al. [62]
Aghazadeh et al. [63]
2016
2019
2012
2014
R11, R12, R22, R502, and R717
R12, R22, R32, R134a, R290, R600a, and R717
R141b
R134a
R134a
R717
R290
R600a
R1234yf
R1234yf
R1234ze
R32
R134a
R141b
R1234ze
R744/R152a
R744/R717
−5–10
5
−30
−55 to −45
35–50
40
–
30–40
CVCS
CVCS
Cascade CVCS
Cascade CVCS
12%–30%
10%–30%
22%
16%
22.3%
11.9%
17.9%
21.6%
8.47%–23.29%
9–15%
11%–20%
5.22%–13.77%
6.63%–17.83%
5%–10%
15.5%
27.3%
7%
Booster
Cascade system
Cascade system
flow [49]. According to some experiments, many investigators set a less
value than that generally in single-phase for the efficiency coefficients.
But the constant efficiency assumption is not consistent with the real
situations in each section because these efficiencies depend on the flow
conditions inside the ejector. Furthermore, the LPM cannot optimize
the geometry of ejectors, since the primary nozzle choked flow and
shock formation in the mixing section was not taken into account
[143]. Fortunately, several researchers have tried to establish the correlations of ejector or each section. Liu and Groll [67,144] investigated
the effect of geometry and operation conditions on section efficiencies
for a transcritical R744 ejector with a converging-only primary nozzle.
The efficiencies of the ejector primary nozzle, secondary nozzle and
mixing chamber were found to be 0.50–0.93, 0.37–0.90, and 0.50–1.00,
respectively. The empirical correlation for the efficiency of each ejector
part was obtained based on their test data. Each correlation was a
function of the ratio between the primary pressure and the secondary
pressure, the ratio between Dpn,t and Dmix, and MER. Similarly, Zheng
and Deng [117] also investigated the empirical correlation of efficiency
for each ejector component combined with the tested data. These empirical correlations for the efficiency of each ejector component are a
significant improvement since realistic efficiency of each section used
over a range of operation conditions was proposed. But the accuracy
and applicability of the correlation are needed to be further verified.
For the experimental studies, the MER of the subcritical and transcritical systems is usually 0.6–0.85 and 0.4–0.8 respectively. The Qc or
Qh of the experimental system usually ranges from 2 kW to 15 kW.
Fewer studies focus on experimentally subcritical SEVCSs compared
with transcritical SEVCSs. Although the system performance of low
GWP refrigerants such as R1234yf, R1234ze, R290 and R600a, has been
analyzed theoretically, the experimental SEVCS using these refrigerants
is limited. The experimental gain is encouraging, but the COP is much
less than the theoretically predicted values. The reported COP improvements are usually the maximum gained in each investigation and
only happen in cases of optimum ejector efficiency and designed condition. The coming efforts should focus on how these COP improvements can be achieved not only under system designing conditions but
also at off-designing conditions as well. Furthermore, the system regulation strategies coupling the ejector behavior with the other components (i.e., throttle valves, compressors, evaporators, and so on) as well
as the influence of the refrigerant properties and charge should be developed.
4. Other novel ejector-expansion vapor compression systems
4.1. Liquid recirculation ejector-expansion vapor compression systems
(LrEVCSs)
Overfeed evaporators have gained increasing attention owing to the
high heat transfer performance. The recirculating of supplementary liquid to overfeed evaporator is always achieved through a mechanical
pump. But the pump increases the equipment investment, operational
complexity and maintaining expense. In 1983, Lorentzen [145] proposed that the liquid recirculation could be driven by the recovered
expansion work of the ejector. The MFR through the evaporator was
increased and the evaporator dryness was eliminated owing to the recirculating effect. Hence, the evaporator performance was improved
owing to the ejector recovery work, which indirectly unloaded the
compressor. Compared with the pump scheme, this scenario is simple,
economical, and avoidance of mobile components.
In 2004, Disawas and Wongwises [146,147] proposed an R134a
LrEVCS (Fig. 28) where no throttling valve was installed at the evaporator upstream to flood the evaporator with the working fluid. The
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Table 4
Theoretical analysis of transcritical SEVCSs.
Authors
Year
Working fluid
teva/°C
tgc,out/°C (pgc/
MPa)
Baseline
COPimp
System feathures
Li and Groll [45]
Deng et al. [64]
Sarkar [65]
Fangtian and Yitai [66]
Liu et al. [67]
2005
2007
2008
2011
2012
R744
R744
R744
R744
R744
7%–18%
22%
9%
30%
30.7%
Vapor feed back
2014
2015
R744
R744
36–48 (8–12)
36–48 (8–12)
30–60
40–45 (9)
27.8–37.8 (10)
(Outdoor)
40–50 (8.5–11)
30–50
CVCS
CVCS
CVCS
CVCS
CVCS
Zhang et al. [68]
Bai et al. [74]
0–10
0–10
−45–5
−5–17
22.3
(Indoor)
0–10
−25 to −5
CVCS
Vapor injection system
45.1%
7.7%
Bai et al. [75]
Minetto et al. [69]
Choudhary et al. [73]
Li et al. [140]
2016
2016
2017
2017
−5–5
−6.2–9.2
−20–5
−10
35 (8–10)
35–42
35–45 (7–12.5)
25–45
–
CVCS
R744 SEVCS
CVCS
Yari and Sirousazar [76]
2008
R744
R744
R744a
R744/R32
R744/R41
R744
10
40
CVCS
43.44%
20%–34%
10%
30%
20%
55.5%
Yari [77,78]
2009
R744
−30–5
35–55 (8–12)
Two-stage system
12.5%–21%
Nemati et al. [80]
2018
R170
−30–0
35–55
9.37%
Manjili and Yavari [81]
2012
R744
−15–5
36–55 (8–12.5)
Megdouli et al. [83]
Nemati et al. [80]
2017
2018
−25–20
−30–0
32–50 (8–14)
35–55
Yari and Megdouli
[77,78]
Megdouli et al. [84]
Taslimi et al. [72]
Liu et al. [85]
2011
R744
R744
R170
R744
−40–0
35–55 (8–14.5)
Corresponding R744
system
One-stage SEVCS with
IHX
One-stage SEVCS without
IHX
SEVCS
Two-stage transcritical
SEVCS
Cascade CVCS
2017
2019
2019
R744/ R744a
R744
R744
−65 to −35
35–55 (8.8–14)
Cascade SEVCS
5
40
CVCS
19.6%
15.3%
12%
10.75%
8.37%
18%–31.5%
9%
17%
39.34%
Component efficiencies of ejector were estimated
using empirical correlations
PDME is optimiazed
Ejector enhanced vapor injection heat pump
system with subcooler
–
Zeotropic mixture
Two-stage SEVCS
intercooler
Two-stage SEVCS
intercooler
Two-stage SEVCS
intercooler
Two-stage SEVCS
intercoolers
including IHX, ejector, and
including IHX, ejector, and
including IHX, ejector, and
which includes two
Transcritical SEVCS with ORC
Two-stage SEVCS with ORC
Cascade SEVCS with ORC
Cascade SEVCS with ORC
Transcritical SEVCS with thermoelectric
subcooler
Table 5
Experimental analysis of subcritical SEVCSs.
Authors
Year
Working fluid
teva/°C
tcon/°C
Q/kW
MER
COPimp
Menegay [86]
Menegay and Kornhauser [87]
Harrell [88]
Ersoy and Bilir Sag [58]
Bilir Sag et al. [89]
Hu et al. [114]
Pottker and Hrnjak [91]
Bilir and Ersoy [90]
Jeon et al. [115]
Huang et al. [116]
Chen et al. [131]
1991
1996
1997
2014
2015
2014
2015
2016
2017
2017
2019
R12
R12
R134a
R134a
R134a
R410A
R410A
R134a
R410A
R134a
R290
−7.2–9.4
–
−7.2–9.4
10
5
26.7 /19.6
0–15
5
–
−10
−30
34–45
–
34–45
55
40
30.6/16.6
40–60
40
–
40
54.4
–
–
–
4.2–4.47
3.9–4.55
3.5–4.3
1.77
4.3
7–7.5
4.4–5.3
0.45
–
–
–
0.63–0.67
0.73–0.83
0.581–0.866
0.615
0.76–0.8
0.6–0.94
0.17–0.3
–
3.8%
2.3%–3.8%
3.9%–7.6%
6%–14%
7.3%–12.9%
9%
8.2%–14.8%
5%–13%
7.5%
–
16.7%
System feathures
Hot gas bypass
Adjustable ejector
Adjustable ejector
Two-stage primary nozzle, Adjustable ejector
Optimizing separator
through bypassing the flash vapor. The COP improvement was about
13% owing to ejector recirculation. In 2015, Lawrence and Elbel [153]
numerically compared the benefit gained by the LrEVCS and the SEVCS
with the microchannel evaporator. SEVCS was suggested for refrigerants which possess high throttle loss, such as R744, and LrEVCS
was suggested for refrigerants which possess low throttle loss, such as
R134a. It was found that R134a system could gain up to 7% COP improvement by using the LrEVCS. Both LrEVCS and SEVCS could gain
about 6% COP improvement for R410A. In 2016, for an R410A system
with a microchannel evaporator, Lawrence and Elbel [154] experimentally found that COPimp were up to 16% and 9% with the LrEVCS
and SEVCS respectively, but the evaporator design had a great influence
on COP of each system. In 2018, Lawrence and Elbel [155] investigated
the effect of evaporator dimensions on the system performance through
numerical modeling a micro-channel air evaporator. Lower circulation
ratio and more refrigerant passes were suggested to enhance the evaporator performance for the SEVCS. For the LrEVCS, the evaporator was
apparatus operated in typical air-conditioning conditions. The experimental results showed that the COP improvement over the CVCS was
about 5%, which was found to rise with the decrease of heat sink
temperature. Compared with the CVCS, the compressor pressure ratio
and the exhaust temperature of the LrEVCS decreased. In 2007 and
2008, Chaiwongsa and Wongwises [148,149] investigated the effects of
Dpn,t, heat sink and heat source temperatures, and primary nozzle outlet
diameters on the performance of the experimental system.
Fig. 29 shows a lay-out of the LrEVCS where the evaporator is located in the outlet of the ejector. In 2011, Dopazo and Fernández-Seara
[150] experimentally observed that recirculation ratios were about 2–4
for a LrEVCS using R717 with a plate evaporator. Dopazo and Fernández-Seara [151] also suggested an R717/R744 cascade refrigeration
LrEVCS that employed liquid recirculation for both HTC and LTC. In
2014, Minetto et al. [152] proposed a transcritical R744 LrEVCS with a
finned-tube evaporator. A liquid–vapor separator was set in the back of
the ejector in the proposed system to feed liquid in the evaporator
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Table 6
Experimental analysis of transcritical R744 SEVCSs.
Authors
Year
Secondary stream conditions
t/°C (p/MPa)
Primary stream conditions
t/°C (p/MPa)
Q/kW
MER
COPimp
System feathures
Elbel and Hrnjak [42,105]
Chen et al. [96]
Liu et al. [106]
2008,2011
2010
2012
0.45–0.6
0.1–1
0.3–0.55
7%
–
36%
Adjustable ejector
Converging primary nozzle
Adjustable ejector
Converging primary nozzle
2011
2014
3.28–6.38
3–6
0.9
0.7–0.9
15%
6%–9%
Banasiak et al. [97]
Lucas and Koehler [95]
Liu et al. [107]
2012
2012
2016
6%–8%
17%
71.4%
Converging primary nozzle
Simultaneous cooling and heating
2013
–
–
24.8–31.5
(Total)
4–5.5
0.55–0.69
0.38–0.65
0.3–0.6
Minetto et al. [98]
0.8–1.6
40.6%
Water heater
Zhu et al. [130]
Zhu et al. [99]
2017
2018
5
5
0.4–0.8
0.55–0.95
−11%–18.9%
10.3%
Chen et al [108]
Suo [112]
2009
2018
26–40 (8–12.4)
35 (7.4–9.8)
27.5–37.5
Outdoor
(8–10)
27
Water inlet
30.5(8–11.5)
30–40 (7–10.5)
35–41.1 Outdoor
(9.7–13.7)
40–60
Water outlet
35 (8–10)
50–90
Water outlet
(9.5)
35 (7.5–9.5)
4.3–5.1
12
10.8–16
Lee et al. [93]
Lee et al. [94]
(3–3.8)
10 (3.3–4.5)
15.5–26.5
Indoor
(4.3)
30–40
Water inlet
(3.55)
−10,-1 (2.6, 3.4)
2.8–26.7 Indoor
(2.8–5.1)
12.1–24.2
Water inlet
21 (3–3.7)
22
Ambient
(3.4)
−3–5
10
2–6
0.2–1.1
0.2–0.45
–
37.8%
Zhu and Elbel [103]
2020
6.5–10.6
35 Outdoor
3.2–5.5
–
8.1%
Converging primary nozzle
Water heater
Converging primary nozzle
Adjustable ejector
Two-stage primary nozzle
Adjustable ejector
Vortex control
Fig. 28. LrEVCS proposed by Disawas and Wongwises [146,147].
Fig. 30. Falling-film water chiller proposed by Li et al. [156].
tube falling film water chiller prototype whose schematic and P–h
diagram is shown in Fig. 30. A screw compressor was used and a converging nozzle was employed for the ejector. The schematic of the
falling-film evaporator with LrEVCS is shown in Fig. 31. It was indicated that the primary MFR had little effect on the MER of the ejector,
and the mean value of MER was about 2.03. The evaporating capacity
was increased from 940.2 kW to 985.5 kW with the primary MFR increasing from zero to 0.43 kg s−1. An optimal COP and capacity improvements of 2.4% and 4.8% were yielded owing to the new system. The COP of the chiller was found to reach a peak value and then
decrease with increasing the recirculation ratio, and the optimum recirculation ratio was found to be 1.135–1.2.
Table 7 summarizes the information available in the public
Fig. 29. LrEVCS proposed by Lorentzen [145].
suggested to overfeed through ejector and the heat transfer and pressure drop should be balanced through choosing appropriate refrigerant
pass numbers.
In 2014 and 2017, Li et al. [156–158] proposed a LrEVCS using
R134a as refrigerant. The experiment was carried out by a horizontal22
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Fig. 31. Falling-film evaporator with liquid refrigerant recirculating [156].
Table 7
Findings of LrEVCSs. (T-Theoretical, E-Experimental).
Reference
Year
Evaluation
Refrigerant
Evaporator
Findings
Disawas and Wongwises [146,147]
Dopazo and Fernández-Seara [150]
Minetto et al. [152]
Lawrence and Elbel [153]
2004/2005
2011
2014
2015
E
E
E
T
Plate
Plate
Finned-tube
Microchannel
Lawrence and Elbel [154]
Li et al. [156]
2016
2014
E
E
R134a
R717
R744
R134a
R410A
R744
R410A
R134a
Li et al. [157,158]
2017
E/T
R134a
Horizontal-tube falling film
COPimp = 5%
Recirculation ratio between 2 and 4
COPimp = 13%
COPimp = 7% (R134a)
COPimp = 6% (R410A)
Little COPimp (R744)
COPimp = 16%
COPimp = 2.4%
Optimal recirculation ratio was 1.135.
Evaporating capacities improvement was 9.5%.
Optimum recirculation ratio was about 1.2.
Microchannel
Horizontal-tube falling film
Fig. 32. Hos-DeEVCS proposed by Oshitani et al. [159].
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both R134a and R1234yf at tcon of 40 °C. Particularly under high condensation temperature, the COP improvement becomes more remarkable for R1234yf. But COP values of the R134a systems were slightly
higher than R1234yf systems for both the CVCS and the new system. In
2015, Ünal and Yilmaz [165] performed thermodynamic analysis of a
bus air conditioning Hos-DeEVCS. A system COP improvement of 15%
was reached. Subsequently, Ünal [166] designed the ejector for bus air
conditioning systems with R134a. The Qc was set as 14 kW and 32 kW
for minibus and bus respectively. The COPimp of the new system was
experimentally found to be about 8%. In 2017, Ünal et al. [167] experimentally revealed that in comparison with the CVCS, the heating
transfer area of evaporator and condenser for the bus was reduced by
55% and 4% respectively owing to Hos-DeEVCS application. It means
that the Hos-DeEVCS not only enhances the system performance but
also lowers the overall bus weight. In 2014, Geng et al. [168] conducted
an experimental investigation of Hos-DeEVCS using R134a as refrigerant. It was found that compared with the CVCS, the improvements
of COP and the exergetic efficiency of Hos-DeEVCS with an invariable
frequency compressor were 16.94–30.59% and 7.57–28.29% respectively. The improvements of COP and exergetic efficiency of the HosDeEVCS with an inverter compressor were about 32.64% and 23.32%
respectively.
In 2019, Jin et al. [169] presented a transcritical R744 Hos-DeEVCS
for independent control of indoor temperature and humidity. The
schematic of the hybrid ground coupled Hos-DeEVCS heat pump system
is shown in Fig. 34. The LTE was used to cool and dehumidify the fresh
air, while the HTE was used to cool down the chilled water to 17.5 °C
for controlling the indoor air temperature. It was shown that the COP of
the EVCS was 12%–60% higher than that of the conventional system
when humidity load accounts for 10–50% of the total thermal load. The
COP values of the suggested system ranged from 3.0 to 6.1 in the
summer climate conditions of Shanghai, China.
The DeEVCS is also attractive for the applications in domestic refrigerator-freezers (RFs). In 2018, Jeon et al. [170] proposed an R600a
domestic RF using Hos-DeEVCS, shown in Fig. 32. In the whole cycle
operation with same Qc, the COP enhancement of the new scheme over
the CVCS was experimentally found to be 11.4% at the MER of 0.18.
Additionally, the new domestic RF revealed a similar temperature curve
in the freezing room with that of the base domestic RF. At identical
cooling load, the power consumption of the new scheme decreased by
10.9% compared with the base domestic RF due to the pressure lifting
effect. In 2014 and 2015, Wang et al. [171,172] proposed a domestic RF
DeEVCS using R600a (Fig. 35). The results indicated that the proposed
prototype reduced the power consumption by 5.45% in comparison
with the traditional domestic RFs.
In 2016, Zheng et al. [173] proposed a transcritical R744 refrigeration DeEVCS, where an evaporator is added at the ejector outlet
compared with the SEVCS. This system was called ejector outlet added
literature on the LrEVCSs. It can be seen that the COP improvement of
the LrEVCS was ranged from 5% to 16%. LrEVCS was appropriate for
refrigerants which possess low throttle loss, and SEVCS was appropriate
for refrigerants which possess high throttle loss [153]. The performance
investigations of LrEVCSs using various refrigerants are still needed.
The ejector design in the LrEVCSs is not as crucial as that in the SEVCSs,
because the liquid pumped by the recovered expansion work is usually
sufficient for overfeed the evaporator. Thus the LrEVCS is easy to operate successfully in the actual systems. Almost all the investigations of
the LrEVCSs are the experiments for subcritical working fluid. Further
efforts are needed to establish the theory of optimization and operation
characteristics of the LrEVCSs for its market spread. Most studies have
shown that LrEVCSs could enhance the evaporator performance. The
evaporator structure is also a significant element affecting the COP
improvement of the LrEVCSs. The interaction effect between the ejector
and the evaporator needs to be addressed in future research. Furthermore, Lawrence and Elbel [153] presented that the ejector was only a
throttle valve if the ejector in the LrEVCS cannot pump any liquid from
the separator, and the system will become a CVCS. Unlike the SEVCS,
the Qc or COP of the LrEVCS is impossible to fall below that of the
CVCS. This implies that the LrEVCS is more appropriate for non-design
conditions or systems using subcritical refrigerants compared with the
SEVCS.
4.2. Dual evaporator ejector-expansion vapor compression systems
(DeEVCSs)
In 2007, Oshitani et al. [159] proposed a system where the liquid
exiting from the high pressure heat exchanger (condenser or gas cooler)
is divided into double streams. One stream enters the primary nozzle
and goes through a nearly isentropic expansion. The other stream goes
through an isenthalpic throttling process and then goes into a lowtemperature evaporator (LTE). The two streams mix and lift the pressure in the ejector and then flow into a high-temperature evaporator
(HTE), where they evaporate before going back to the compressor. The
schematic of this EVCS is shown in Fig. 32. In this system, because the
flow is split at the outlet of the high pressure heat exchangert, this cycle
will be referred to as the high pressure heat exchanger outlet split dual
evaporator ejector-expansion vapor compression system or HosDeEVCS. In the Hos-DeEVCS, both evaporators are usually applied to
cool a single airflow with the purpose of better matching the temperature slip of airflow with the double different evaporating temperatures.
The Hos-DeEVCS was initially marketed in May 2009 to provide
cabin air-conditioning for a commercial bus. The two evaporators and
the ejector were assembled into an integral unit that has the identical
size and shape as a traditional automotive evaporator without ejector.
An image of this unit is shown in Fig. 33. In 2012, Brodie et al. [160]
reported that the COP improvement owing to Hos-DeEVCS was
10%–25% in this application without disclosing which refrigerant was
used. In 2012 and 2014, Lawrence and Elbel [161–163] built an experimental setup to investigate the performance of Hos-DeEVCS. The
two evaporators were arranged in series in the wind tunnel with the
HTE being placed upstream in the air flow. It was experimentally revealed that the Hos-DeEVCS gained relatively higher COP enhancement
when the LTE capacity was small but the temperature differential between the double evaporators was large for the systems using refrigerants R134a and R1234yf. They also indicated that the pressure
drop in the HTE significantly affected COP value of the system. Compared with the CVCS with double evaporating temperature, the maximum COP improvement of the new scheme using R1234yf and R134a
were 12% and 8% respectively. Compared with the CVCS with a single
evaporating temperature, the maximum COPimp of the new scheme
using R1234yf and R134a were 6% and 5% respectively. In 2014,
Boumaraf et al. [164] studied the performance of Hos-DeEVCS using
CAM model. The system was found that the COPimp exceeded 17% for
Fig. 33. Dual evaporator and ejector assembly [160].
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Fig. 34. Schematic of the hybrid ground coupled Hos-DeEVCS heat pump system [169].
was 2.58 times than that of SEVCS. In 2017, Zheng and Deng [174]
studied the Eoa-DeEVCS experimentally. It was shown that the HTE was
critical for enhancing the system performance, and the enhancement
was greater for lower MER. The smaller ejector area ratio was beneficial
evaporator (Eoa) DeEVCS. Its schematic is shown in Fig. 36. The factors
of the system transient responses were studied. The COP of the proposed system was found to be 2.40–3.58 while the COP of SEVCS was
found to be 2.10–2.50. The COP improvement of the proposed system
Fig. 35. Ejector enhanced domestic RFs proposed by Wang et al. [171,172].
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Energy Conversion and Management 207 (2020) 112529
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Fig. 36. Eoa-DeEVCS proposed by Zheng et al. [173].
only a limited amount of investigations openly available today. It
should deserve more attention in future investigations owing to the
prospective practical advantages. This system can also be used for
providing cooling effect with both refrigerating and freezing temperatures in domestic RFs, or separation of latent and sensible loads in independent temperature and humidity control air conditioning systems.
However, the evaporating temperature difference of this system depends on the ejector PLR, while the two evaporator capacity ratio is up
to the ejector MER. Thus there is a trade-off between the evaporating
temperature difference and the LTE capacity owing to the coupling of
PLR and MER.
to higher pgc, MER, Qc and COP.
The studies about DeEVCSs are summarized in Table 8, indicating
the working fluids, system features and contributions from each study.
The COP improvements of the DeEVCSs ranged from about 5%–30% for
subcritical fluids to upwards of 15%–60% for R744. The Hos-DeEVCS is
advantageous because it offers more than one evaporating temperatures
and removes a liquid–vapor separator. The constraint between the MER
and the exit flow quality of the ejector can be avoided. The heat exchange efficiency of the evaporator in the DeEVCS is improved compared with that of the single evaporating temperature owing to the
decrease of the mean temperature difference between the two heat
transfer streams. The DeEVCS can theoretically always possess higher
COP than that of the CVCS. Lawrence and Elbel [175] presented that
this scheme also contributed to oil returning of compressor. The
DeEVCS has been introduced to the automotive market, but there is still
4.3. Cascade ejector-expansion vapor compression systems (CEVCSs)
The purpose of using cascade ejectors is to recover the expansion
Table 8
Findings of DeEVCSs. (T-Theoretical, E-Experimental).
Reference
Year
Evaluation
Refrigerant
System feathures
Findings
Boumaraf et al. [164]
Lawrence and Elbel
[162,163]
2014
2012/2014
T
E
R134a/R1234yf
R134a
R1234yf
Hos
Hos
Brodie et al. [160]
Geng et al. [168]
Ünal and Yilmaz [165]
Ünal [166]
Ünal et al. [167]
2012
2016
2015
2015
2017
E
E
T
E
E
–
R134a
R134a
R134a
R134a
Hos
Hos
Hos
Hos
Hos
Jin et al. [169]
Zheng et al. [173]
Zheng and Deng [174]
Wang et al. [171,172]
2019
2016
2017
2014/2015
T
T
E
E
R744
R744
R744
R600a
Hos
Eoa
Eoa
Domestic RF
Jeon et al. [170]
2018
E
R600a
Domestic RF
COPimp was more than 17%.
COPimp,max was 12% with R1234yf and 8% with R134a compared with a two
evaporator CVCS.
COPimp,max of 6% with R1234yf and 5% with R134a compared with CVCS.
COPimp was 10%–25%.
COPimp was 16.94%–30.59%
COPimp was 15% for the existing bus AC system.
COPimp was 8%.
Heat transfer surface areas of condenser and evaporators were decreased by 4%
and 55% respectively.
COPimp was 12%–60% higher than that of the conventional system
COPimp was 14.3%–43.2%.
HTE played an important role in enhancing the system performance.
Energy consumption reduction was 5.45% compared with the conventional
domestic RFs.
COPimp of the domestic RF was 11.4% at the MER of 0.18.
The energy consumption of the domestic RF decreased by 10.9% compared with
the baseline domestic RF.
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tgc,out = 35 °C. Besides, the influences of pgc on the optimum COPh were
also investigated. In 2019, Manjili and Cheraghi [178] proposed a similar system as Fig. 38, and the only difference was that a proportion of
gas in the separator was bypassed before entering the compressor.
Through the thermodynamic and exergetic analysis, it was found that
the COP improvement of the novel system ranged from 20% to 80%
compared with the CVCS.
In 2015, Bai et al. [179] presented a refrigeration CEVCS with dual
evaporator, shown in Fig. 39. The simulated results revealed that
compared with the traditional dual-evaporator system, the optimum
COP and exergetic efficiency of the new system was increased by
37.61% and 31.9% respectively. The exergy loss of the two ejectors
accounted for about 16.91% of the total exergy input. In 2017, Bai et al.
[180] proposed another transcritical R744 dual-evaporator CEVCS,
shown in Fig. 40. It was found that the COP and exergetic efficiency
improvements of new system were 5.26–25.5% and 9.0–28.7% respectively compared with the single EVCS. Gas coolers accounted for
the highest exergy loss of the system, followed by ejectors, which accounted for 28.9% of the total exergy losses. In 2017, Sarkar [181]
proposed four schemes of two-stage compressed CEVCS with three
evaporators using R32. Results indicated that the highest COP was attained by EECRS4 (Fig. 41) among the four schemes. For the integration
of air-conditioning (5 °C), refrigeration (−20 °C) and freezing (−40 °C)
applications, EECRS4 was found to gain the COP improvement of approximately 20%, 67% and 117% in comparison with double-stage
CVCS, single-stage EVCS and CVCS respectively. It was also found that
the proposed scheme not only enhanced the system performance notably but also decreased the entire compressor size.
The studies about CEVCSs are summarized in Table 9, indicating the
working fluids, system features and key contributions from each study.
All of the investigations concerning CEVCSs are theoretical studies and
focus on transcritical R744 systems showing 10%–80% COP improvement. But it is particularly challenging for practical realization owing to
the system complexity. The verification of the claimed COP enhancements by experiments will be a long way to go.
Fig. 37. Transcritical R744 CEVCS [176].
energy as much as possible. In 2012, Cen et al. [176] proposed a
transcritical R744 CEVCS, shown in Fig. 37. The system COP was reported to be between 2.75 and 7. But the efficiency of the ejector
component in the paper was overvalued, making the obtained COP too
high to be implemented in practical application. In 2014, Xing et al.
[177] suggested a transcritical two-stage compression R744 heat pump
CEVCS, shown in Fig. 38. It was thermodynamically found that the
proposed system provided higher COP and volumetric capacity in
comparison with the conventional two-stage system. Through adding
an IHX, the heat COP improvement attained by 10.5%–30.6% compared with CVCS with the subcooled temperature ranging from 0 °C to
15 °C under the conditions of teva = −15 °C, pgc = 10 MPa and
Fig. 38. Two-stage compression R744 CEVCS [177].
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Fig. 39. CEVCS with dual evaporators [179].
ejectors as shown Fig. 17(b). The ejectors are elaborately designed to
get the optimum system performance under the most common and
expected working conditions.
In 2012, Hafner et al. [101] presented a PmEVCS for R744 supermarket applications. It was found that electricity consumption could be
decreased by about 10% due to the use of PmEVCS in Southern Europe.
In 2014, Banasiak et al. [182] found in the experiment that the efficiency of the individual ejector working in a PmEVCS was about 30%.
Minetto et al. [183] found that the energy saving can reach 22.5%
compared with the conventional basic R744 refrigerating plant due to
the adoption of the PmEVCS in a supermarket in Bari. Hafner et al.
[184] reported that the application of the PmEVCS in a parallel compression configuration decreased the power consumption by 12% for a
supermarket in Fribourg (Switzerland). Hafner et al. [185] also analyzed the simulation model of the PmEVCS for R744 supermarket
4.4. Parallel multi-ejector-expansion vapor compression systems
(PmEVCSs)
The refrigeration and heat pump systems usually operate at various
loads and ambient temperatures. Accordingly, the ejector should be
designed to operate with optimum efficiency at broad working conditions. Although the system MFR can be possibly adjusted through altering the area of primary nozzle throat, there are few attempts to
synchronously adjust the mixing section or diffuser geometries. The
shape factor among sections may be inconsistent with the optimized
one for the whole operating conditions, resulting in decreasing of
ejector efficiency. Moreover, the system with moving elements possibly
results in low reliability. To avoid the aforementioned disadvantages,
PmEVCS with a multi-ejector pack was proposed. The multi-ejector
pack is composed of a series of parallel assembling fixed geometric
Fig. 40. CEVCS with dual evaporators and IHX [180].
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Fig. 41. CEVCS with three-evaporators two-stage compression [181].
heater with a multi-ejectors pack, shown in Fig. 43. The control system
could enable each ejector individually and form 15 different schemes
according to the working conditions. The results showed the probability
to obtain a maximum COP by changing the ejector area when other
dimensions remain unchanged, owing to the ejector regulating effect on
the entrance and exit pressure of the compressor. It experimentally
showed that there was an optimum multi-ejectors configuration. When
the water was heated from 40 °C to 60 °C at an outdoor temperature of
12 °C, and the multi-ejector throat area was 46.5% of the overall intersecting surface, COP and Qh improvements were reported to be
13.8% and 20% respectively compared with the worst case.
In 2017, Bodys et al. [191] suggested the potential areas of pack
shape optimization for a multi-ejector pack in the R744 systems. In
2018, Haida et al. [192] studied performance mapping of the fourejector pack for R744. It was indicated that acceptable ejector efficiency
from 20% to 30% was reached in the pressure ranging from 5.0 MPa to
approximately 10.0 MPa and above 30% in the subcritical region. The
zone where the MER above 0.3 was suggested when the outdoor temperature was higher than 15 °C and pressure lift was lower than
1.0 MPa. In 2018, Gullo et al. [193] theoretically revealed that “R744
only” solutions with multi-ejectors (Fig. 44) has better performance
than subcritical refrigerant systems in medium-sized supermarkets. It
was estimated that the R744 solution with multi-ejectors pack enabled
to lower the energy consumption by 50.3% in comparison with HFCbased units at ambient temperatures of 5 °C-10 °C. Gullo et al. [194]
reported that a multi-ejectors pack was installed in food retail in the
USA. It was claimed that compared with a conventional booster unit,
this solution could raise maximum power savings by 11.3% and
15–23% under un-optimized and optimized working modes, respectively.
Table 10 shows a summary of the main findings about PmEVCSs.
The PmEVCS was proposed in recent years and have been investigated
theoretically and experimentally for R744 systems, showing 15%–26%
theoretical COP improvement and 8%–16% experimental COP improvement. It is revealed that this solution is a promising way to enhance the efficiency of R744 systems and great energy-saving potential
is expected, especially in supermarket applications. Such a system may
experience a wave of growth in the next few years owing to the popularity of R744 refrigeration systems in supermarkets and other applications. An appropriate design of a multi-ejectors pack system
applications. It was found that the COP of cooling mode and heating
mode increased by 5%–17% and 20%–30% respectively compared with
the reference system due to the use of the PmEVCS. In 2015, Banasiak
et al. [70] developed a performance mapping of a multi-ejectors pack
used in an R744 supermarket refrigeration system. The image of the
designed multi-ejectors pack is shown in Fig. 42, where six parallel
ejectors are placed into a casing. It is usually fulfilled with four ejectors
for vapor compression and two ejectors for liquid pumping. The ejector
efficiency was found to be greater than 0.3 over extensive working
conditions. In 2016, Schonenberger [186] estimated energy consumption could be decreased by 15%–25% through applying the PmEVCS
than an R744 refrigeration plant with parallel compression in the stores
of Switzerland. Thus, the R744 refrigeration PmEVCS is a more competitive solution in all climates due to its high efficiency.
In 2016, Smolka et al. [187] performed a comparison between fixed
multi-ejector pack and needle adjustable ejector using a proven HEM
for the transcritical R744 refrigeration system. It was found that each
fixed ejector was highly efficient under various working conditions for
the multi-ejectors pack. The efficiency of adjustable ejector was found
to be usually 25% more than that of the fixed one when the primary
nozzle throat was decreased by about 35%, but a further decrease of the
throat area resulted in an abruptly falling of efficiency for adjustable
ejectors. Additionally, the predictive position of the needle was a
challenge to achieve reasonable efficiencies under all operating conditions. Haida et al. [188] experimentally indicated that the COP and the
exergetic efficiency of the R744 PmEVCS were increased by 7% and
13.7% respectively in comparison with the reference R744 parallel
compressed booster system. The experimental results revealed that
multi-ejectors pack efficiency could reach 33% according to the primary and secondary specifications and the pressure lift. The performance of the R744 PmEVCS can be further improved through properly
designing and operating of the refrigeration assemblies for the optimal
integrating of the multi-ejectors pack.
In 2017, Gullo et al. [189] theoretically revealed that the energy
consumption of the R744 PmEVCS could be decreased by 15.6%–27.3%
in comparison with the R404A throttle valve system in Southern
Europe. Furthermore, an additional 2.4% −5.2% less energy consumption over the traditional system was attained depending on the
scale of the supermarket and the ambient conditions. In 2017, Boccardi
et al. [190] experimentally investigated an air source R744 water
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Energy Conversion and Management 207 (2020) 112529
COPimp is about 20%, 67% and 117% compared with valve expansion two-stage compression, ejector enhanced one-stage compression and
valve expansion one-stage compression system respectively.
COP and exergetic efficiency was increased by 5.26%–25.5% and 9.0%–28.7% over the single ejector system.
Maximum COP and exergetic efficiency was increased by 37.61% and 31.9% respectively over those of dual-evaporator CVCS.
COPh was increased by 10.5%–30.6% compared with that of CVCS.
Fig. 43. R744 water heater PmEVCS [190].
considering the system characteristics, installation load characteristics
and climatic data of the prospective location is crucial to spreading this
solution in the commercial refrigeration, heat pump, and air conditioning units. Also, this solution has the obstacles of large multiejectors pack size in small systems.
R32
R744
R744
The system COP was between 2.75 and 7.
One-stage compression oneevaporator
Two-stage compression oneevaporator
One-stage compression twoevaporator
One-stage compression twoevaporator
Two-stage compression threeevaporator
R744
R744
Findings
System features
Working fluid
All the data from the cited references were gathered in the form of
graphs to give a summary and comparison of the various systems
concerning the evolution of history. The data of the graphs present the
main reported results of the original references.
Fig. 45 shows the historical trend of the COP improvements of different EVCSs over the corresponding CVCS. The theoretical COP improvements ranged from 10% to 30% for different subcritical SEVCSs
and from 10% to 55% for transcritical R744 SEVCSs. The increasing
trend of the theoretical COP improvement was not clear with the year.
This is because it is determined by the inherent nature of the system and
2017
Sarkar [181]
T
2017
Bai et al. [180]
T
2015
Bai et al. [179]
T
2014
Xing et al. [177]
T
2012
Cen et al. [176]
T
Year
Evaluation
Fig. 42. Image of the designed multi-ejectors pack [70].
5. Comparisons and discussions
Reference
Table 9
Findings of CEVCSs. (T-Theoretical, E-Experimental).
Z. Zhang, et al.
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Energy Conversion and Management 207 (2020) 112529
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Fig. 44. “R744 only” parallel compression PmEVCS [193].
thermodynamic properties of the working fluid. The experimental COP
improvement of subcritical SEVCSs exhibited an increasing trend in the
last years, changing from 5% in 1990 s to 16% in recent years. For the
transcritical SEVCSs, the experimental COP improvement ranged from
10% to 40%. Particularly interesting, several experimental values have
reached or even exceeded the theoretical values, which implies that it
may show better performance in actual operation. Almost all of the
LrEVCS have been verified by the experiments for subcritical working
fluid, but there are only a few studies on the mechanism investigation of
this system owing to easy implement. The performance of LrEVCS
showed a growth in last 15 years, passing from 5% in 2004 to 15% in
recent years. It is a potential development direction to improve the
performance of evaporator. The DeEVCS have been verified by the
experiments and commercialized for the compartment air conditioner
in passenger vehicles, with COP improvements ranging from about
5%–30% for subcritical fluids to upwards of 15%–60% for R744. This
system has lower theoretical COP than SEVCS owing to the fact that
only some of the work can be recovered. However, the difference is not
obvious for the experimental results, especially for subcritical working
fluids. All of the investigations concerning CEVCS are theoretical studies and focus on transcritical R744 systems showing 10%–80% COP
improvement, but this value was not much advantageous in comparison
with the other EVCSs. The PmEVCS was proposed in recent years and
have been investigated theoretically and experimentally for R744 systems, showing 15%–26% theoretical COP improvement and 8%–16%
experimental COP improvement. It is noted that most of the experimental COP improvements for EVCSs were ranged from 7% to 20%,
which is much lower than the theoretical values, especially for transcritical R744 EVCSs. The instability of actual EVCS should also be
noted, and the EVCS may have lower COP than the CVCS.
Fig. 46 shows the historical trend of the cooling/heating capacity of
different EVCSs. Before 2005, there were mainly experiments in the
systems with capacity of 2–3 kW, but the capacity scope has been
gradually widened in the last decade, mainly ranging from 1 kW to
35 kW. Particularly interesting, the LrEVCS have been successfully used
in large-capacity systems with about 1000 kW to improve the system
performance through improving the performance of falling-film evaporator. Further research should be considered for the exploitation of
EVCS applied to large-capacity R744 refrigeration systems owing to the
popularity of R744 supermarket refrigeration systems. Additionally, the
efforts of exploiting EVCS used in small scale systems like domestic RF
or air conditioning, light commercial refrigeration and car air conditioning are still needed. Very small systems will face additional
challenges in fabricating the extremely small sized nozzle and ensuring
it unclogged.
Fig. 47 shows the historical trend of the MER of different EVCSs. It
can be seen that the MER is generally in the range of 0.25 to 0.8 for the
investigated five EVCSs. The increasing trend of the MER with the year
was also not clear. For the SEVCSs, including subcritical SEVCSs and
transcritical SEVCSs, there does not seem to be much noticeable difference between the theoretical and the experimental values. It implies
that most of the tested ejectors have attained the expected MER theoretical values for the SEVCSs. According to the literature, the MER is
expressed by the recirculation ratio for the LrEVCSs in this paper. Thus,
the value is significantly larger than the other systems. However, the
definition of the recirculation ratio is various for different literature, for
example, the ratio between evaporator MFR and the condenser/gas
cooler MFR [153] or the ratio between the MFR into the evaporator and
31
Energy Conversion and Management 207 (2020) 112529
the MFR of the vaporized refrigerant [146,147]. Thus, the ejector entrainment performances from various studies cannot be compared directly for the LrEVCSs. For the DeEVCSs, the MER is related to the two
evaporator capacity ratio. The matching between the ejector MER and
the capacity ratio of the two evaporators is a critical issue. All of the
MER values for the CEVCSs are theoretical results. Thus, the experimental verification is needed. The MER of the PmEVCSs is slightly
lower than the SEVCSs, which may be attributed to the mutual effect of
the ejectors in the multi-ejector pack. The coupling between the ejectors
is a critical concern for the PmEVCSs. Furthermore, the relations between the MER and the ejector geometry parameters are still indistinct.
Further research is still needed to understand the variations of MER
under the off-design conditions.
Fig. 48 shows the historical trend of the ejector efficiency of different EVCSs. It can be seen that the papers concerning the ejector efficiency were almost experimental results. There does not seem to be
much noticeable difference for the ejector efficiency of the five EVCSs.
The experimental values of the ejector efficiency showed an increasing
trend over the years, passing from 10% in 1990 s to around 20% in
recent years for subcritical fluids and from 10% in 2000 s to 40% in
recent years for transcritical fluids. The ejector efficiency of transcritical cycles is slightly higher than that of the subcritical systems. But
the values of the efficiency were still very low. The improvement of the
ejector efficiency lies on the insight into the flow characteristics inside
the ejector, which can be realized by numerical simulations [195–202]
or visualization experiment [203–205]. The relations between the
ejector efficiency and, the ejector geometry parameters and operating
conditions, are needed to be studied.
Energy savings ranged from 15.6% to 27.3% in Southern Europe.
When the multi-ejectors throat section was 46.5% of the total cross section, COP and Qh improvements was reported to be 13.8% and 20%
respectively compared with the worst case.
Ejector efficiency from 0.2 to 0.3 was achieved. An area of the MER greater than 0.3 was obtained.
The power input was up to 50.3% at outdoor temperatures from −10 °C to 5 °C.
Peak energy savings was by 11.3% and between 15% and 23% in non- and optimized working conditions respectively.
6. Conclusions and prospects
R744
R744
R744
–
HFC-based units
Two stage R744 booster system
Ejectors can decrease the compression work by reducing the throttling losses and the liquid overfeeding and lifting the compressor inlet
pressure. This review has identified a range of potential technological
updates that could augment the current EVCSs. Conclusions from
practical research reveal that the EVCS is a worth method to improve
energy efficiency of refrigeration and heat pump systems. At present,
most of the experimental COP improvements for the EVCSs over the
corresponding CVCSs are ranged from 7% to 20%, which is much lower
than the theoretical values, especially for the transcritical R744 EVCSs.
However, particularly interesting, several experimental values have
reached or even exceeded the theoretical values, which implies that it
may show better performance in actual operation. The highest experimental value of the ejector efficiency is around 20% for subcritical
fluids and 40% for transcritical fluids. The capacity scope of the investigated EVCSs mainly ranges from 1 kW to 35 kW. The ejector MER
of the experimental EVCSs generally ranges from 0.25 to 0.8. The
system performance of the EVCSs showed an increasing trend over the
years, and novel EVCSs that offer practical advantages for real applications are constantly evolving. The SEVCS is more commonly studied
among the reviewed five EVCSs discussed above. The performance of
the SEVCS can be further improved by using adjustable measures, optimizing the ejector geometry, optimizing the liquid–vapor separating
process, addition of IHX, using two-stage compression, use of ORC, and
so on. Although small quantity of investigations has studied the performance of the other EVCSs, most of which were proposed in the last
five years, and these systems offer their unique advantages in real applications. These new EVCSs include LrEVCS, DeEVCS, CEVCS and
PmEVCS. LrEVCSs have gained increasing attention owing to the virtues of simpleness, economy and avoidance of mobile components. The
MFR through the evaporator was increased and the evaporator dryness
was eliminated owing to the liquid recirculating effect. Hence the
evaporator performance is improved owing to the ejector recovery
work, which indirectly unloads the compressor. More mechanism investigations of the LrEVCSs are needed in the future, especially the
interaction effect between the ejector and the evaporator. The DeEVCS
2018
2018
2018
Haida et al. [192]
Gullo et al. [193]
Gullo et al. [194]
T
T
T
R744
R744
2017
2017
Gullo et al. [189]
Boccardi et al. [190]
T
E
R744
2016
Haida et al. [188]
E
R744
2016
Smolka et al. [187]
T
R744
2016
Schonenberger [186]
T
R744
R744
2014
2015
Hafner et al. [185]
Banasiak et al. [70]
T
E
R744
R744
2014
2014
Minetto et al. [183]
Hafner et al. [184]
T
E
R744
2014
Banasiak et al. [182]
E
R744 system with parallel
compression
R404A CVCS
–
Efficiency of controllable-geometry ejectors was 25% higher than that of the fixed-geometry case when the primary nozzle throat area was
reduced by approximately 35%.
COP and exergetic efficiency improvements was up to 7% and 13.7% respectively. Multi-ejectors pack efficiency wass up to 33%.
COPc and COPh increased by 5%–17% and 20%–30% respectively.
Ejector efficiency was above 0.3 over a wide range of the operating conditions.
The largest COP and exergetic efficiency improvements were 9.8% and 13.1% respectively
Energy savings ranged from 15% to 25% in two Swiss stores.
Energy consumption decreased by 22.5% in a supermarket in Bari (Italy).
Energy consumption decreased by 12% in a supermarket in Fribourg (Switzerland).
Efficiency of the individual ejector was about 30%.
Electricity consumption decreased by about 10% in Southern Europe.
Traditional R744 direct expansion
system
R744 system with parallel
compression
Basic R744 system
R744 system with parallel
compression
Two stage R744 booster system
R744 system with parallel
compression
R744 system with parallel
compression
–
R744
2012
Hafner et al. [101]
T
Refrigerant
Evaluation
Year
Reference
Table 10
Findings of PmEVCSs. (T-Theoretical, E-Experimental).
Baseline
Findings
Z. Zhang, et al.
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Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Fig. 45. COP improvement variations of different EVCSs over the years.
Fig. 46. Cooling/heating capacity variations of different EVCSs over the years.
techniques have been limited to theoretical analysis and laboratory
tests. There are still struggles in developing EVCSs for commercial applications and markets. Firstly, the COP enhancements presented in the
above studies, though encouraging, usually occur under certain conditions of high ejector efficiencies. Future research is required to study
how to attain COP improvements not only under designing conditions
but also under off-designing conditions. Secondly, few unsteady characteristics of the system, such as the start-up procedure, the dynamic
response to the variations of the working parameters and the equilibration time, have been published. Thirdly, the above studies also
highlighted some practical challenges, such as the system adjustment
schemes coupling the ejector behavior with the other parts, refrigeration/heating mode conversion, compressor oil returning and discharge
pressure controlling, and liquid–vapor separator designing. Finally, it
should deserve more attention in future investigations for novel EVCS
is advantageous because it offers more than one evaporating temperatures and removes a liquid–vapor separator. It has been successful to
the market automotive cabin cooling and should deserve special attention in the future owing to the prospective practical virtues. The idea
of using CEVCS can recover expansion work to the utmost extent. But it
is particularly challenging for practical realization owing to the system
complexity. One of the most prospective and challenging concepts is the
PmEVCSs in R744 commercial refrigeration. A large number of papers
have proposed different system layouts and demonstrated the feasibility
of these schemes at various ambient conditions. The attempts of this
system in supermarket R744 refrigeration systems have also been
commercialized successfully. Such a system may experience a wave of
growth in the next few years owing to the popularity of R744 refrigeration systems in supermarkets and other applications.
However, the technology of EVCS isn't mature yet. Most of EVCSs
33
Energy Conversion and Management 207 (2020) 112529
Z. Zhang, et al.
Fig. 47. MER variations of different EVCSs over the years.
Fig. 48. Ejector efficiency variations of different EVCSs over the years.
Technology Research Project of Hebei Province (grant number 2017131).
solutions. Future research should focus not only on improving the energy efficiency of the EVCS but also on effectively overcoming those
troubles in actual application.
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Acknowledgments
This work was supported by the University Scientific Research Key
Project of Hebei Province (grant number ZD2017061); Construction
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