A Review on Performance Analysis of Hybrid Solar-Geothermal CO2
Heat Pump System for Residential Heating
B. B Samantaray & A. Rout
School of Mechanical Engineering, KIIT University, Bhubaneswar - 24, Odisha, India
E-mail: bbsr .comofficial@gmail.com
Abstract – A simulation study of hybrid solar-geothermal
heat pump system for residential applications using carbon
dioxide was carried out under different operating conditions.
The system consists of a solar unit (concentric evacuated
tube solar collector and heat storage tank) and a CO2 heat
pump unit (three double-pipe heat exchangers, electric
expansion valve, and compressor). As a result, the
differential of pressure ratio between the inlet and the outlet
of the compressor increases by 19.9%, and the compressor
work increases from 4.5 to 5.3 kW when the operating
temperature of the heat pump rises from 40 Cto 48 C.
Besides, the pressure ratio of the compressor decreases from
3 to 2.5 when the ground temperature increases from 11 C to
19 C. The operating time of the heat pump is reduced by 5 h
as the daily solar radiation increases.As the solar radiation
increases from 1 to 20 MJ/m2, the collector heat rises by
48% and the maximum collector heat becomes 47.8 kWh.
The heating load increases by 70% as the indoor design
temperature increases from 18 C to 26 C. However, the solar
fraction is reduced from 11.4% to 5.8% because of the
increases of the heating load.
I.
pump using carbon dioxide is lower than that of a
system using a subcritical cycle refrigerant because of
large irreversibility during compression and cooling.
Moreover, system reliability is very low due to large
performance variations with operating conditions.
II. SYSTEM MODELING
The hybrid solar-geothermal CO2 heat pump system
(HSG-CHPS) consists of a solar heat unit and a CO2
heat pump unit. The solar heat unit has a concentric
evacuated tube collector, and a heat storage tank. The
CO2 heat pump unit consists of two double-pipe heat
exchangers (high- and low-temperature), a double pipe
type evaporator, an electric expansion valve (EEV) and
a semi-hermetic type reciprocating compressor
(Q_cooling ¼ 10.55 kW). The heat source can divide
into two parts; one is from solar and the other is from
the heat pump. The heat source can divide into two
parts; one is from solar and the other is from the heat
pump. The heat is acquired by the collector is
transferred to a thermal heat storage tank. A heat storage
tank is controlled to maintain the designed temperature
to supply for heating and hot water.
INTRODUCTION
Recently, hydrocarbon shortage and global energy
crisis have aroused great interest in alternative energy
supplies. This is especially true for South Korea that
badly depends on imported energy resources. However,
most alternative energy technologies are faced with
difficulties when it comes to application for community
facilities because of regional restrictions and operating
cost. Therefore, researches on energy saving and
optimal operation of residential heat pump systems are
urgently required. To this end, using renewable energy
(e.g. solar or geothermal) for refrigeration and air
conditioning applications
becomes increasingly
important and draws considerable attention. As for
working fluid, carbon dioxide is a natural climatefriendly refrigerant as it does not deplete ozone layer
and has a low direct global warming potential with
reference value 1. Generally, the performance of a heat
2.1. Solar collector and heat storage tank modeling
The solar collector has eight concentric evacuated
tube collectors, which can be reliably operated for
getting heat for residential applications. As a working
fluid water-propylene glycol mixture (water to
propylene glycol ratio of 80:20) is used. The solar
collector model was developed based on test results of
the concentric evacuated tube collector by Korean
Institute of Energy Research [13].In this study, the
efficiency of the solar collector is calculated by ƞcollector
= FRƮα - FRUL(Tin - TAir)/lt.. Where, IT, FR, FRsa, and
UL are daily solar radiation, collector heat removal
factor, intercept of efficiency curve, and collector
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overall heat loss coefficient. Also, tin tair is the
temperature difference between inlet temperature at the
collector and ambient temperature.
2.3. Heat exchanger modeling
The CO2 heat pump system consists of three
double-pipe heat exchangers (high- and low-temperature
gas cooler, evaporator), an EEV, and a reciprocating
compressor of semi-hermetic type. In the heating mode,
the double-pipe heat exchanger acts as an evaporator to
exchange the heat between the refrigerant and the water
supplied from the ground heat exchanger (GHX). Two
double-pipe heat exchangers play the role of the gas
cooler. The high temperature heat exchanger is placed at
the compressor outlet. It exchanges the heat from hightemperature water to the middle of the heat storage tank
so as to supply the hot water to the user. The low
temperature double-pipe heat exchanger exchanges the
heat from low-temperature water at the bottom of the
heat storage tank to supply the mean temperature water
for heating. From Table the specification of three kinds
of double tube type heat exchanger. A tube-by-tube
method was used to calculate the performance of the
heat exchangers.
2.2. Compressor modeling
Since the CO2 compressor is operated under high
pressure, the semi-hermetic reciprocating compressor is
selected as an analytical model. Generally, it shows high
efficiency and reliability under high pressure conditions.
The compression process was calculated by using
volumetric efficiency, compression efficiency, and
discharge temperature, which had been obtained and
adjusted using Sanchez et al. [7] correlations. The
compressor model of semi-hermetic reciprocating
compressor has a displacement ð GÞ of 3.48 m3/h at
1450 rpm(N). The compressor work is obtained as a
function of mass flow rate, compression efficiency(hc),
the rate of heat transfer from the motor to the refrigerant
ð QmotÞ, and enthalpy difference between inlet and
outlet through the compressor. The compressor
efficiency and mass flow rate are represented.
.
.
2.4. Simulation conditions
To simulate the heating load for residential
application, the indoor space was designed to have
66.25 m2. The heat loss through inner walls and floor is
ignored, but losses through the roof and outside walls
with windows are taken into account. Besides, the daily
usage of the hot water was assumed to be 280 L for
four-member family. The design parameters of heating
load and hot water load are shown in Tables. Solar
radiation was set by the average value in Korea during
winter season and ground temperature was also set
based on the average temperature on 5 m underground.
The heat pump operating temperature means the heat
pump start to supply the heat to a heat storage tank when
the water temperature in the storage tank is reduced
below the target temperature. Simulation conditions for
this study are shown in Table. The simulation was
performed and simulation results were obtained every
15 min. The EEV with a diameter of 1.6 mm is used as
an expansion device for calculating the mass flow rate
(Kim et al. [1]). In this study, the expansion process is
assumed to be isenthalpic. The mass flow rate of the
CO2 in EEV is calculated by using 6-physical and 4geometrical variables based on Buckingham p-theory.
III. RESULTS AND DISCUSSION
Since carbon dioxide is used as a transcritical cycle
because of its low critical temperature, under normal
operating conditions the gas cooling process occurs
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above the critical temperature and pressure. Besides,
system performance of a CO2 heat pump varies
significantly according to operating conditions.
Especially, it is strongly affected by gas cooling and
evaporating conditions. Also, EEV opening of the CO 2
heat pump is one of the most important control factors
to determine system performance because it affects the
mass flow rate of the refrigerant and the compressor
discharge pressure. Therefore, the optimal EEV opening
is firstly investigated for the given operating conditions.
Hence, the heating time is reduced by 2.7 h from 9.5 to
6.8 h as the heat pump operating temperature rises from
40 C to 48 C.
3.2. The effect of ground temperature on system
performance
The temperature and pressure at the inlet of the
compressor increases as the ground temperature
elevates. The compressor pressure ratio decreases and
mass flow rate of the refrigerant increases because of
high temperature and pressure of the evaporator. Hence,
optimal EEV opening gets larger with an increase of
ground temperature. Besides, the compressor outlet
temperature decreases gradually as the compressor
pressure ratio goes down. In this study, the compressor
pressure ratio decreased from 3 down to 2.5 when
ground temperature increased from 11 C to 19 C. The
mass flow rate of the refrigerant increases from 62.5 to
81.4 g/s because the compressor pressure ratio
decreases. In addition, the outlet temperature of the
compressor decreased by 23 C. The elevation of the
ground temperature can significantly reduce the
refrigerant temperature at the outlet of the compressor
that can improve system performance and reliability.
3.1. The effect of heat pump operating temperature on
system performance
The fig. shows variations of the compressor
pressure ratio, optimal EEV opening, compressor work,
and COP according to heat pump operating temperature.
Since the heat pump operating temperature is one of big
factors to system performance, the system performance
was analyzed and investigated according to heat pump
operating temperature. The optimal EEV opening
decreases from 52% to 40% as the heat pump operating
temperature increases from 40 C to 48 C. Besides, the
heating COP decreases by 23.5% from 2.77.Fig. 3
shows variations of the heating time, heat pump
operating time, collector heat, and solar fraction
according to heat pump operating temperature. As the
heat pump operating temperature increases, the
temperature of the heat storage tank and gas cooler in
the CO2 heat pump increases, simultaneously. For the
given simulation condition, the hot water load is
proportionally increased with heat pump operating
temperature because the water with high temperature is
used for hot water load. In addition, the supplying heat
rate from the CO2 heat pump. However, the daily
supplied heat is almost the same regardless of heat pump
operating temperature because indoor heating load is
designed with same heat load. In addition, the
temperature of heating water supplied to the indoor
heating unit (FCU, fan coil unit) is also increased with
the increase of temperature in the storage tank.
3.3. The effect of daily solar radiation on system
performance
The collector operating time and collector
efficiency simultaneously increases with increase of
daily solar radiation. The collector efficiency increases
up to 47.8% when daily solar radiation is 20 MJ/m2
compared to 0 MJ/m2. Especially, it increases
dramatically when the daily solar radiation elevates
from 1 to 5 MJ/m2. The rise of the collector operating
time is directly related to the increase of the collecting
heat gain at the solar collector. Moreover, it can increase
heat supply to the storage tank. Otherwise, the pump
operating time and heat pump heat are reduced.
.
Fig. 2: Schematics of analytical scheme for double tube heat exchange
.
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Special Issue of International Journal on Advanced Computer Theory and Engineering (IJACTE)
3.4. The effect of indoor design temperature on system
performance
V. REFERENCES
The Figure shows variations of solar fraction,
collector heat, heating heat, and hot water heat
according to indoor design temperature. As the indoor
temperature increases, the heat loss of the indoor space
increases due to the large temperature difference
between the outdoor and indoor, resulting that the
indoor heating load increases. However, the collector
heat and the hot water load remain the same regardless
of the variation of the indoor design temperature. This is
because daily solar radiation and heat pump operating
temperature are designed to be constant except the
indoor design temperature for this analytic case.
Therefore, the design of proper indoor temperature for
variable outdoor conditions is very important in order to
maintain high system performance and reliability in the
hybrid solar-geothermal CO2 heat pump system.
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W. Kim et al. / Renewable Energy 50 (2013) 596604,Published on 31.8.2012 in Sciencedirect.com.
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IV. CONCLUSION
Performance characteristics of the hybrid solargeothermal CO2 heat pump system (HSG-CHPS) have
been analyzed under varying operating conditions using
a simulation model. The system consists of a solar unit
(concentric evacuated tube solar collector and heat
storage tank) and a CO2 heat pump unit (three doublepipe heat exchangers, electric expansion valve, and
compressor .With solar radiation increase from 1 to 20
MJ/m2, the collector heat and the solar heat fraction rise
by 48% and 22%, respectively. The maximum collector
heat at daily solar radiation of 20 MJ/m2 is 47.8 kWh.
The solar fraction increases by 5.7% on average per 5
MJ/ m2 rise of daily solar radiation. As the indoor
design temperature rises from 18 C to 26 C, the heating
load increases by 70%.However, the solar fraction
reduces by 5.9% due to the rise of the heating load.
Besides, the heat pump operating time rapidly increases.
Therefore, the design of proper indoor temperature for
variable outdoor conditions is very important in order to
maintain high system performance and reliability in the
hybrid solar-geothermal CO2 heat pump system. It was
found that the performance of a CO2 heat pump can
improve significantly by using solar and geothermal
system and it can supply sufficient heat to the space
during winter season.
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