Renewable and Sustainable Energy Reviews 69 (2017) 415–428
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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
Potential of nanorefrigerant and nanolubricant on energy saving in
refrigeration system – A review
MARK
⁎
W.H. Azmia,b, , M.Z. Sharifa, T.M. Yusofa,b, Rizalman Mamata,b, A.A.M. Redhwana,c
a
b
c
Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600 Pekan, Pahang, Malaysia
Automotive Engineering Centre, Universiti Malaysia Pahang, 26600 Pekan, Pahang, Malaysia
Faculty of Manufacturing Engineering Technology, TATI University College, 24000 Kemaman, Terengganu, Malaysia
A R T I C L E I N F O
A BS T RAC T
Keywords:
Nanorefrigerant
Nanolubricant
Refrigeration system
Energy saving
Refrigeration system is one of the biggest reason of the expanding pattern of energy consumption, thus, energy
saving is one of the best approach to overcome this issue. Nanofluids show extraordinary potential in upgrading
the thermodynamic and mechanical performance of the refrigeration system. In the refrigeration system, the
effort to improve the efficiency of the system is by introducing nanoparticles in refrigerant (nanorefrigerant) and
in lubricant (nanolubricant). In this paper, a comprehensive review is carried out to investigate the impact of
nanorefrigerant and nanolubricant on energy saving. The overview consists of properties enhancement of
nanorefrigerant and nanolubricant, tribological performance, heat transfer enhancement, performance in heat
exchanger, improvement in refrigeration system and pressure drop characteristic. The previous results showed
that the best energy saving with 21% less energy used was with the use of 0.5% volume ZnO-R152a refrigerant
nanolubricant. Both the suction pressure and discharge pressure were brought down by 10.5% when
nanorefrigerant was utilized. The evaporator temperature was lessened by 6% with the utilization of
nanorefrigerant. The replacement of R134a with R152a gives a green and clean environment, with zero ozone
depleting potential (ODP) and less global warming potential (GWP). The performance of refrigeration system
was significantly enhanced.
1. Introduction
In the last few decades, the world electricity generation and
consumption became a main issue of discussion among researchers,
politicians and environmental activists because of its increase in
generation and consumption. Consequently, it affected the cost of
electric generation paid by the government in terms of fuel subsidization and also the use of fuel in high quantities would contribute to
global warming. The world energy consumption is roughly
153,071×1012 kW of energy in 2007 [1]. The combined worldwide
energy utilization from 2007 to 2035 is estimated to be expanded by
1.4% every year, thus in 2035 the aggregate vitality expended will be
roughly 216,500×1012 kW [1]. In spite of the high interest for energy,
the measure of fossil fuel vitality resource is declining and the oil fuel
store is in critical condition. By 2035, the cost of oil will be required to
move up to USD210 per barrel. With this, the effect of high fuel cost is
huge in heat engine conversions. Basically, the energy saving management must be implemented and improved in order for long term
sustainability [2].
There are three main consumers of electricity worldwide; industry,
residential and commercial. In the United States for example, electric
consumption in residential sector accounted for 38% of the total
generation in 2009 [3]. In European countries, the residential sector
consumed 35% of the total electric generation in 2009 [4]. Meanwhile,
in Malaysia, the residential sector uses about 21% of the total country's
electricity generation in 2009 [5]. Fig. 1(a) and (b) show electricity
consumption in US and Malaysia for the three main consumers in
2009. Nowadays, the world population is growing, with the demand for
food will also increase. Our food production is dependent on three
critical resources: water, land, and energy. For example, countries such
as Africa, the cultivated area (AI) are expanding by 0.3–49.5% from
2011 to 2035, and 16.5–83.2% from 2011 to 2060, respectively [6].
These areas are potentially used as agricultural land. The increase in
cultivated area (AI) and also agricultural areas will lead the increment
of the demand for water. Various researches had been undertaken to
improve and renew the irrigation system [7–10]. Increased activity of
agriculture and modern irrigation system will lead to more demand for
electricity in the future.
⁎
Corresponding author at: Faculty of Mechanical Engineering, Universiti Malaysia Pahang, 26600 Pekan, Pahang, Malaysia.
E-mail addresses: wanazmi2010@gmail.com (W.H. Azmi), sharif5865@yahoo.com (M.Z. Sharif), myusof@ump.edu.my (T.M. Yusof), rizalman@ump.edu.my (R. Mamat),
redhwan323@gmail.com (A.A.M. Redhwan).
http://dx.doi.org/10.1016/j.rser.2016.11.207
Received 2 December 2015; Received in revised form 9 October 2016; Accepted 14 November 2016
Available online 20 November 2016
1364-0321/ © 2016 Elsevier Ltd. All rights reserved.
Renewable and Sustainable Energy Reviews 69 (2017) 415–428
W.H. Azmi et al.
882, 25%
1363, 38%
1323, 37%
Residential
Commercial
Industry
(a) Electricity Consumption in US (GWh)
Fig. 2. Energy label use in Malaysia [5].
conditioners contribute large energy use domestically, these types of
home appliances should be more efficient in energy use. In order to
achieve energy efficiency in Malaysian refrigeration system, Masjuki
et al. [13] reported that it can be achieved by implementing minimum
energy efficiency standards as procedure and regulations that prescribe
the energy performance of manufactured products, and sometimes
prohibiting the sales of products that are less efficient than a minimum
level. Prediction of CO2 reduction was also estimated with
30,336,719 kg CO2 produced in 2012 compared with 72,357,910 kg
CO2 in 2003 [14]. Energy labels are informative labels affixed to
manufactured products to describe the product's energy performance.
The importance of energy labels is that it will enable consumers to
compare the energy efficiency of appliances on a fair and equitable
basis. They also encourage manufacturers to improve the energy
performance of appliances [15,16]. The implementation of energy
labels in Malaysia started since 2005 for domestic refrigerators and
opened to all manufacturers on a voluntary basis. The label is similar to
type B in Fig. 2 which was investigated by Mahlia et al. [17]. The same
author also conducted research for projected electricity saving form
implementing minimum energy standards for household refrigerators
in Malaysia [18,19].
The minimum energy efficiency standards for appliances have been
enacted in other countries such as Australia, Brazil, Canada, China,
Europe, Japan, Korea, Philippine, Russia and US. The program can be
mandatory or voluntary. However, most countries have adopted
mandatory standards while several countries such as Brazil, Japan
and Korea have successfully implemented voluntary standards. China
contributes the largest refrigerator sales worldwide with household
refrigerators reaching 15.99 million in 2002. Refrigerators accounted
for more than 32% of the total residential electricity used in China with
the total consumption of 1620 TWh a year [20,21]. The Chinese has
established their energy efficiency standards for household refrigerators since 2003 and replaced by 2008. Replacement or review of the
energy efficiency improvement per year is under consideration for all
manufacturers.
There are many methods that have been implemented in home
appliances in order to achieve energy efficient standards. Some
methods focus on the controller which is more intelligent in its
operation and other methods focus on the efficient electric motor. On
the other hand, the application of light emitting diode (LED) also give
significant savings in electricity consumption and achieved high energy
rating in their standard. The diversification of methods to achieve an
energy efficient product gives a lot of advantages to engineers in
multiple disciplines either mechanical, electrical or controller. Since
17124, 21%
36609, 45%
28285, 34%
Residential
Commercial
Industry
(b) Electricity Consumption in Malaysia (kWh)
Fig. 1. Electricity consumption for three main consumer [5,134].
Government policies on energy saving have been implemented in
many countries worldwide with the purpose to reduce energy consumption, consequently reducing the cost and also greenhouse gas
emissions. The European Commission of Energy leads the energy
policies and activities in the European countries. In the United States,
the US Department of Energy leads the policies in energy saving.
Meanwhile in Malaysia, the Malaysia Energy Commissioner is the
responsible agency developing and enforcing energy saving policies. In
Malaysia, refrigerators and air conditioners use 22% and 14% of
energy, respectively [11]. Many countries worldwide agreed that energy
consumption can be reduced by implementing energy efficient products
in the residential and product sectors.
Energy consumption increases exponentially with the rise of the
numbers of refrigerators and air conditioners worldwide. The issue of
increment in energy consumption has been discussed by researchers,
politicians and also government leaders. This is because energy
generation is directly related to the use of fuel and also contributes
to the greenhouse effect as a result of high levels of combustion.
Various literatures are focused on energy in refrigerator with the
purpose to reduce energy consumption. The effort became popular
after the oil price shock in the 1970s and in the past few decades. The
oil price remains in a fluctuating condition till today.
With regards to reducing greenhouse gas emissions by reducing
CO2 content in the atmosphere, the quantity of fuel combustion to
produce energy should be reduced [12]. Since refrigerators and air
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W.H. Azmi et al.
increase in sales each year and orders in 1924 were 350% higher than
the year earlier. Currently, there are about 1000 million units of
refrigerators worldwide. There have been many researches and development since a new domestic refrigerator concentrated on insulation
methods using balsa wood and quality in sealing methods. In the 1950s
urethane foam had been developed and in the early 1960s the rigid
urethane foam was produced with fluorinated hydrocarbon expanding
agents such as R-11 and R-12 and because of environmental concerns
under the Montreal Protocol for ozone depletion by CFCs, the development of nonfluorocarbon of blowing agents was discussed. Vacuum
insulation was also seen as an option with commercial potential.
However, the production of vacuum insulation was more concerned
toward the cost of production.
In 2004, Johnson [22] reported the comparison effect of blowing
agent selection on energy consumption and the life cycle climate
performance (LCCP) of typical European refrigerators. Two types of
blowing agents were analyzed which is the HFC-245fa and pentane for
the same product specifications is considering 10% of form to go to
landfills instead of the current practice of 60%. The author found that
with current foam formulations, the use of HFC-245fa as a blowing
agent, instead of the cyclopentane and n-pentane blend, offers a
significant advantage in energy consumption for refrigerator/freezers
although the global warming potential (GWP) for HFC-245fa is high
compared with the pentane blend [22]. Besides the insulation of the
refrigerator, component efficiency also became the main discussion
among researchers. Generally, a refrigerator consists of four main
components connected in sequence order to form a closed system. The
components are compressor, condenser, capillary tube and evaporator.
Among these four components, the compressor is considered the main
part of the system that works to compress the refrigerant and produce
primary force to circulate the refrigerant throughout the system. The
compressor consumes electricity and the internal design is moderately
complex. The measure of compressor performance in domestic refrigerators is the energy efficiency ratio (EER). The Energy Efficiency Ratio
is the ratio of the cooling capacity of a refrigerator in British Thermal
Unit (BTU) per hour, to the total electrical input (in watts) under
certain specified tests.
The most concentration of research on the compressor domestic
refrigerator in the past few decades was in the compressor valve
dynamics, lubrication, noise emissions and vibration and compressor
efficiency. Various literatures can be obtained for such fields of study in
compressor for domestic refrigerators. Shaffer and Lee [23] reported
on the energy consumption in quarter horsepower hermetic refrigerator compressor by analyzing several factors of losses. The author was
recommended to increase the motor efficiency, improve the compressor geometry and reduce the suction gas heating [23]. Karll [24] from
Danfos, one of the main compressor manufacturers reported that there
are three main areas to be tackled such as mechanical losses, electrical
losses and gas circulation losses in order to improve a company's
annual production. Riffe [25] improved the efficiency of reciprocating
refrigerator compressor up to 40% by introducing unitary connecting
rod wrist pins and notched piston and cylinder design. Schroeder [26]
introduced design improvement of electric motor for reciprocating
refrigerator compressor from 73% to 80% efficiency using positive
temperature coefficient resistors and coupled to run capacitors. Nelson
and Middleton [27] developed an energy efficient compressor for
refrigerators and freezers by introducing four-pole electric motors
combined with positive temperature coefficient resistors and also
improved compressor geometry. The four-pole electric motor was also
implemented with the effort to improve EER for a hermetic reciprocating compressor. The efforts on improving the efficiency of electric
motors, compressor geometry and other compressor parts were
continued by other researchers in the following years with similar
improvement methods of the previous researchers [28–30].
Since the condenser and evaporator are both heat exchanger
devices, their performance are very significant as a performance
Fig. 3. Refrigeration system one loop system with direct expansion.
refrigerator and air conditioner are appliances which contribute the
most significant energy consumption in the domestic sector, trivial
savings are able to contribute a huge impact in the world energy
scenario. In this paper, a comprehensive review was carried out to
investigate the impact of special types of nanofluids (nanorefrigerant
and nanolubricant) on energy saving in the system. The overview
consist of properties enhancement of nanorefrigerant and nanolubricant, tribological performance, heat transfer enhancement, performance in heat exchanger, improvement in refrigeration system performance and pressure drop characteristic.
2. Refrigeration system
The basic refrigeration system consists of four main components: a
compressor, a condenser, a capillary tube and an evaporator. The four
components of the system are useful for the application of refrigerators
and air conditioners. Assembled in sequence of its order, the compressor will compress the refrigerant in vapour form to high pressure
and temperature, then this refrigerant is fed into the condenser as
shown in Fig. 3. In the condenser, high pressure and temperature
refrigerant will be cooled by means of free convection heat transfer and
then fed into a capillary tube. A capillary tube is a metering device
which reduces condenser pressure to evaporator pressure. In the
meantime, the temperature of the refrigerant also decreases and it will
change the phase of refrigerant from sub-cooled liquid into mixture.
Then the refrigerant is fed into the evaporator. Evaporators are heat
exchanger devices to absorb available heat in a refrigerated space and
the heat is then carried by the refrigerant into the compressor. These
processes occurred continuously in all components as detailed in Fig. 4.
2.1. Development in refrigeration system
Since the domestic refrigerators were introduced in US in the early
20th century, the demand of such appliances increased from year to
year. It was recorded that from 1919 to 1924 there was over a 100%
Fig. 4. Basic refrigeration cycle layout.
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W.H. Azmi et al.
used as refrigerant fluids in the refrigerator due to their flammability
and toxicity, the use of HCs was discarded and chlorofluorocarbons
(HCFCs) dominated the second generation of refrigerants with applications that were more on residential and small commercial air conditioners and heat pumps. However, ammonia remains in application for
large scaled systems. Molina and Rowland [39] advanced the hypothesis that anthropogenic emissions of certain chlorinated and bromated
compounds, particularly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) can accumulate in the stratosphere (the part of
the atmosphere located at an altitude of roughly 12–50 km) and
substantially deplete the ozone layer that shields the earth.
Therefore, research during the early 1970s linking CFCs to stratospheric ozone depletion, was particularly striking because of their
previous characterization as the ideal refrigerant [39,40].
The discovery of the ozone deflecting properties of CFCs and HCFCs
refrigerants, and their GWP lead to the Vienna Convention (1985), the
Montreal Protocol (1987), London Amendment (1990) and the
Copenhagen Amendment (1992), the Vienna Adjustment (1995) and
the Montreal Amendments (1997), which scheduled the end of
production and use of these refrigerants by 1995 and 2030 respectively
[41].
Development of a new refrigerant becomes crucial as the effect of
the end of the production of CFCs and HCFCs refrigerant. Radermaker
and Kim [38] reported that the effort to explore for new refrigeration
started since the 1960s with two objectives: (1) achieving a low
operator temperature with a moderated pressure ratio during singlestage compression and (2) conserving energy when the refrigeration
duty consists of cooling a fluid stream through a large temperature
range. The most preferred refrigerant was HFC134a and expected to be
used for long term. However, the observation found that HFC134a
systems tolerated fewer contaminants than CFC12 systems, thus,
expected to be used for long term. Therefore, another potential
refrigerant was identified to replace CFC12. It was HFC152a.
However, fluorochemicals are still the primary focus and the
attention was more on the use of hydrofluorocarbons (HFCs) for the
long term. The production of HFCs R-134a is very attractive as a
refrigerant because it has zero ozone depleting potential as well as low
direct GWP. For the sake of the EU's Kyoto Protocol obligations, the
European Commission issued a directive in 2006 mandating the phaseout of R-134a in mobile air conditioning systems and its replacement
by refrigerants with a GWP less than 150. As of 2011, R-134a systems
were banned and applied to all new models, and as of 2017, to all cars
in European countries because the indirect radioactive effect for GWP
is 1300 as tabulated in Table 1.
Pipes, tubing and other components in the refrigeration system
were specially designed by engineers to manage the lubricant returning
back to the compressor. The interaction between the refrigerantlubricant mixture can influence film properties and characteristic on
heat transfer surfaces and system efficiency [42]. The earliest lubricant
used in the refrigeration system was sulphur dioxide. In 1875, Raoul
Pictet used sulphur dioxide as both refrigerant and lubricant in his
indicator in any domestic refrigerator. Early work to investigate
performance of the condenser and evaporator in a domestic refrigerator was conducted by Ding and Chen [31]. The authors reported that
the weight of the evaporator influenced the energy consumption of the
refrigerator. Lee et al. [32] conducted an experimental study for the
effect of frost formation in a flat plate finned tube evaporator. They
analyzed the effects of various factors such as fin spacing, fin arrangement, air temperature, humidity and air velocity on the frost growth
and thermal performance of the evaporator. Karatas et al. [33]
conducted a study on domestic refrigerator finned tube evaporator
coils in order to determine heat transfer coefficient and friction factor.
These two parameters were correlated in terms of Reynolds number
and fin factor, and the established correlation was used for the
calculation of the heat transfer coefficient for the non-uniform flow.
A tri-tube type evaporator was introduced by Lee et al. [34] to be used
in domestic refrigerators. This kind of evaporator was designed to
enhance the defrosting efficiency and basic performance of an indirect
cooling household refrigerator. The evaporator was able to enhance
energy efficiency by reducing the energy consumption about 4% in
Korea. The evaporator has been patented under US patent with
Application No. 10/368 343 in 2003. However it was abandoned by
patent US7726025 [35].
Cho et al. [36] conducted an experiment of mass and heat transfer
coefficient for finned tube evaporators under frost condition. They
tested a single stage and a two-stage evaporator with a variation of
operating parameters and fin geometries. The airside heat and mass
transfer coefficients were calculated from the measured data and the
effects of the tube space and fin alignment in the heat transfer
performance were also investigated. Parallel with the technology
development in recent years, and advanced research approaches,
energy efficiency has been improved in many fields for engineering
with different methods. Increasing the rate of heat transfer is a method
to improve energy efficiency in the field for cooling systems in the
transportation industry, hydronic heating and cooling systems in
buildings. It is also applied in industrial process heating and cooling
systems in petrochemical, textile, pulp, and paper, chemical, food, and
other processing plants.
2.2. Types of refrigerant and lubricant
The working fluid in the refrigerator is called refrigerant which is
employed as the heat absorber or cooling agent. The refrigerant
absorbs heat by evaporating at low temperature and pressure and
removes the heat by condensing at high temperature and pressure. The
most common refrigerant used at the early stages of refrigerator design
was familiar solvent and volatile fluids. Nearly all these early refrigerants were flammable, toxic, or both, and some were also highly
reactive. In developing the refrigerant for refrigeration process, propane (R-290) was marketed in replacing ammonia (R-717) as refrigerant [37]. Propane is considered as a neutral chemical, consequently
no corrosive action occurred and neither deleterious nor obnoxious and
should occasion require, the engineer can work in its vapour with
convenience. Carbon dioxide (R-744) has been used in the 1920s in the
field of positive-displacement and centrifugal compression machines
for chillers’ operating in air conditioning system. Besides that, they
used ammonia and water (R-718), sulphur dioxide (R-764), carbon
tetrachloride (R-10) and dielene (1,2-dichloroethene, R-1130). From
these refrigerants, only R-1130 can work with the centrifugal machine.
The rest did not perform as well through several finding such as low
performance, safety reason and incompatible with metals [37,38].
In 1930, the fluorocarbon refrigerants were introduced. It was a
shift that arose from the concern of safety and durability of refrigerants
that focuses on removing the toxic compound and flammable properties [37]. One year later, the dichlorodifluoromethane (R-12) were
introduced as commercial refrigerant used in refrigerators followed by
R-11 a year later. Since hydrocarbons (HCs) were not suitable to be
Table 1
Ozone depleting potential (ODP), global warming potential (GWP) and other properties
of selected refrigerant [40].
418
Refrigerant
Ozone
depleting
potential
(ODP) *ODP
CFC-11=1
Global warming
potential
(GWP): indirect
and direct
radiative effects
*GWP CO2=1
Toxity
Flammability
CFC-12
HCFC-22
HFC-134a
HC R290
R717
1
0.5
Zero
Zero
Zero
6600
1300
1300
Zero
Zero
Low
Low
Low
Low
High
Zero
Zero
Zero
High
Zero
Renewable and Sustainable Energy Reviews 69 (2017) 415–428
W.H. Azmi et al.
dispersion methods, for example, ultrasonic agitation, homogenizing,
high-shear mixing and magnetic force agitation [58,59].
refrigeration system [43]. In current practices, mineral oil lubricant is
widely used with chlorofluorocarbon (CFC) refrigerants. The systems
with CFC refrigerants have better miscibility with the mineral oil,
helping its re-turn back to the compressor.
However because of the increased awareness of ozone layer
depleting effects of chlorine, people slowly moved on to another
alternative which is the hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) refrigerants. HCFC and HFC refrigerants is not
miscible with mineral oil, this will affect the performance of the
refrigeration system. Synthetic lubricants, Polyalkylene glycol (PAG),
polyol ester (POE), alkyl benzene (AB) are normally used together with
HFC and HCFC refrigerants [44]. PAGs are extensively used in
automotive applications utilizing HCFC-based refrigerants. In HFC
refrigerants like R-134a and HFC-based refrigerants, blend ester based
oils like POE are normally used. Polyolester (POE) oil, another sort of
lubricant oil used in refrigeration systems, is favoured over mineral oil
for use in oil applications because of its solid chemical polarity and
solubility with refrigerant. Likewise, the utilization of suspended
nanoparticles in the lubricants for compressors increases the system's
efficiency and performance, causing no choking in the system [45].
In order to determine which lubricant to be used with certain
refrigerants, there are several properties to be considered like miscibility, solubility and viscosity with refrigerant, and stability, wear and
lubrication [42]. These properties will greatly influence the refrigeration system performance and efficiency.
3.1. Thermo-physical properties evaluation
There are countless applications that can benefit from a superior
comprehension of the thermal conductivity improvement of nanofluids,
including in refrigeration systems. The first experiment to prove
enhancement of thermal fluid of nanofluid was with the use of alumina
as nanoparticle [47]. Al2Cu and Ag2Al nanoparticles were later used in
the experiment, and showed some improvements of 50–150% of its
thermal conductivity when dispersing the nanoparticles in water and
ethylene glycol as base fluid [60]. In another study, results from the
study demonstrated a thermal conductivity increase with an increment
in particle volume fraction with a smaller particle size. Moreover, the
relative increase in thermal conductivity was observed to be more vital
at higher temperatures [61]. At a volume concentration of 0.1%, the
thermal conductivity of the Cu/water mixture can be improved by
23.8% [62]. The larger thermal conductivities of Cu together with the
bigger surface area of Cu nanoparticles are keys to the improvement of
thermal conductivity. However several case studies [63–66]; reported
that there are little increase or no improvement in the thermal
conductivities of nanofluids under such conditions but that is case
dependent, because it could be that the nanoparticles have to be in the
most stable configuration before it can give considerable improvements
in thermal conductivity.
Despite the fact that the research on heat convection in nanofluids
is restricted when contrasted with that in thermal conductivity, the
outcomes and methodologies in the field are various and worth
mentioning [67]. Tavman et al. [68] researched on TiO2, SiO2, and
Al2O3 nanoparticles in water and reported an increase in nanofluid
viscosity with an increment in the nanoparticle concentration; they
likewise demonstrated that established empirical theories, for example,
the Einstein model [69] were not able to anticipate the right viscosity
increment in nanofluids. Later in another study, the viscosity showed a
relationship between the temperature of 10 and 80 °C with the
nanoparticle concentration. For base fluid with CuO volume fraction
of 0.025, they reported an increment in viscosity of roughly 300%, but
it significantly decreases with the rise of temperature [70].
Temperature variations of the nanofluid viscosity obtained in the study
seem consistent with the modified Andrade equation, reported by Kole
and Dey [70].
3. Nanoparticle dispersion
As nanotechnology emerge as the smallest size of particle that could
be commercialized, the idea of using nanoparticles in the form of
suspended particles as thermal fluid was investigated by Masuda et al.
[46] and Choi [47], who named the thermal fluid as ‘Nanofluid’. The
aim of understanding nanofluids is to improve heat transfer ability of
thermal fluids with the highest possible thermal properties using the
smallest possible concentrations with uniform dispersion and stable
suspension of nanoparticles in host fluids. The resulting “nanofluids”
exhibit high thermal conductivities compared to those currently using
heat transfer fluids, and they represent the best hope for enhanced
thermo-physical properties and heat transfer performance that can be
applied in many devices for better performance. As reviewed by Azmi
et al. [48], nanofluids provide enhancement in thermo-physical properties such as thermal conductivity and viscosity compared to traditional
base fluids such as water and ethylene glycol. The previous study on
heat transfer enhancement showed improvement in heat transfer
coefficient compared to the base fluid [49–51]. Nanofluids is a part
of nanotechnology which are being used or considered for use in many
applications targeted to provide cleaner, more efficient energy supplies
and uses. The applications of nanotechnology were reported by many
researches [52–54] in many applications such as engine and transmission oil cooling, refrigeration (domestic and chillers), boiler exhaust
flue gas recovery, heating and cooling of building, cooling of electronics, lubrications, biomedical application and nanofluids in transformer oil.
There are two methods to produce nanofluid, which are the one step
method and two step method [55]. In the one-step preparation
methods, the condensed vapour phase nanophase powders, are condensed into a fluid having low vapour pressure and at the same time
dissolved in liquid. The nanoparticles are created by applying a
physical vapour decomposition technique or liquid chemical techniques [56]. In the two step method, the nanomaterials are integrated as
dry powders by thermal decomposition and photochemical routines,
displaced from organometallics, metal vapour synthesization and
electrochemical synthesization techniques, salt reduction of transition
metal methods, and ligand reduction [57]. After the process, the
nanosized powder is put into the oil to shape the nanoparticle/oil
blend. At that point, this blend is dispersed by utilizing variable types of
3.2. Heat transfer augmentation with nanoparticles
There have been several studies in literature reporting on possible
heat transfer improvements when adding nanoparticles in base fluid.
In recent years analyses have shown that nanofluids have a tendency to
have generously higher thermal conductivities than the base liquids
[71]. Among the significant advantages of nanofluids are higher surface
area, higher stability of the colloidal suspension, lower pumping force
needed to accomplish the identical heat transfer, reduced particle
clogging contrasted with ordinary colloids, flexible control of the
thermodynamic properties and transport properties by changing the
particle concentration, size, and shape resulting in higher heat transfer
capabilities [72].
In spite of the fact that an improved thermal conductivity in
nanofluids is a better element for application in heat transfer devices,
it is not always a sufficient condition. Actually, nanofluids should also
be investigated for execution under convective modes. This section will
specifically discuss improvement of heat convection using nanofluids.
The first known experiment of nanofluids heat convection was by Pak
and Cho [73]. The results showed that convective heat transfer
coefficient of the dispersed fluids with submicron metallic oxide
particles Al2O3 at a volume concentration of 3% was 12% lower than
that of base when analyzed under the state of consistent average
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W.H. Azmi et al.
velocity. Consequently, a better choice of particles having higher
thermal conductivity and bigger size is recommended so as to use
dispersed liquids as a working medium to improve heat transfer
performance. However, Xuan and Li [74] demonstrated an increment
as much as 40% in the heat transfer coefficient of the nanofluid, at
equivalent constant speed. They used pure copper with particle size of
100 nm in their experiments. The difference of their result was
probably because the heat transfer coefficient must depend on the
volume concentration as well as size of nanoparticle and type of
nanomaterial.
Pak and Cho [73] and other researchers studied the heat transfer
coefficient of nanofluids. Yang et al. [75] discovered that the heat
transfer coefficient depends on volume concentration, material, temperature, and type base fluid. Duangthongsuk and Wongwises [76] and
Kakaç and Pramuanjaroenkij [77] critically reviewed the heat convection, and are focussing on the forced convective heat transfer in the
numerical and experimental aspects of nanofluids. Although all the
above work was on forced heat convection, there are also researches of
natural convection of nanofluids. The first research about natural
convection was done in 2003 by Putra et al. [78]. Al2O3 nanoparticles
with 131.2 nm and 87.3 nm CuO particles were tested in water based
fluid. They observed that as the nanoparticle volume fraction increases,
the natural convective heat transfer in nanofluids is lower than pure
water. This condition was recorded to be higher for CuO nanofluid than
Al2O3 water nanofluid. In another experiment, Duangthongsuk and
Wongwises [79] studied the heat transfer coefficient and friction factor
of the TiO2-water nanofluids streaming in a flat horizontal double tube
counter-flow heat exchanger under turbulent flow conditions, tentatively. The heat transfer coefficient of nanofluids was roughly 26% more
prominent than that of pure base fluid. The outcomes additionally
demonstrated that the heat transfer coefficient of the nanofluids at a
volume concentration of 2.0% was around 14% lower than that of base
liquids for given conditions.
Wen and Ding [80] studied TiO2 water based nanofluids. They
found that the decreasing effect of the natural convection of nanofluids
affected the gradients of concentration of the nanoparticle, to the
particle–surface and particle–particle communication, and to the
modification of the properties of the dispersion. An experiment was
carried out by analyzing their heat transfer performance for singlephase natural convection in bottomheated enclosures, assuming that
nanofluids behave like single phase fluids [81]. The results also showed
that the heat transfer improvement is maximum at an ideal molecular
concentration and the maximum enhancement of heat transfer increases as the temperature increases. Table 2 shows a summary of
previous works from researchers regarding the enhancement in heat
transfer of nanofluids.
Table 2
Summary of previous work regarding the heat transfer enhancement of nanofluids.
3.3. Nanofluids in heat exchanger of refrigeration system
In the refrigeration system, the system operates by taking advantage of the fact that high compressed vaporized fluids at a certain
temperature tend to get colder when they are allowed to expand. The
heat transfer between two or more fluids at different temperatures was
done by heat exchanger equipments [82]. In order to reject and absorb
heat, the system uses two heat exchangers in order to achieve ideal
vapour-compression refrigeration cycle. The optimization nanofluid
performance in the heat exchanger will make the system run more
efficiently thus, more energy will be saved. The nanofluids have a
potential to enhance the heat transfer coefficient in the heat exchanger
of refrigeration systems.
An extensive review has been done for the characteristic of heat
transfer in straight tubes. Unfortunately there is still debate about
anomalous heat transfer enhancements that has been achieved [83,84].
Statistically the majority of the past studies demonstrated low heat
transfer improvements; 11% of the specimen indicated decreasing heat
transfer coefficients and 3% showed no enhancements by any means
Author
Base
fluid
Nanoparticle
size (nm)
Finding
Kumar
et al.
[114]
Mineral oil
50
Keblinski
et al.
[71]
−
1–100
Ahamed
et al.
[72]
−
−
Choi [47]
−
1–100
Chopkar
et al.
[60]
Ethlyene
Glycol
18
Mintsa
et al.
[61]
Water
−
Das et al.
[67]
Water
−
Tavman
et al.
[68]
Water
−
Kole and
Dey [70]
Gear oil
40
Pak and
Cho
[73]
Water
13–27
Xuan and
Li [74]
Water
100
Yang et al.
[75]
Synthetic
oil
20–40
Improvement of heat transfer
when using Al2O3/MO as
lubricants in refrigerants
(R600a), also the power
consumption decreases by
11.3%.
Nanofluids have a tendency to
have generously higher warm
conductivity than the base
liquids
Nanofluids have higher surface
area, higher stability of the
colloidal suspension, lower
pumping force needed to
accomplish the identical heat
transfer, reduced particle
clogging contrasted with
ordinary colloids, flexible
control of the thermodynamics
properties and transport
properties by changing the
particles concentration, size,
and shape resulting in higher
heat transfer capability
Al2O3 as nanoparticles enhance
of thermal fluid of nanofluid
Al2Cu and Ag2Al nanoparticles
show some improvement of 50–
150% of its thermal conductivity
when dispersing the
nanoparticles in water and
ethylene glycol as base fluid
Thermal conductivity increase
with an increment in particle
volume fraction and with a
smaller in particle size
Increase in nanofluid viscosity
with an increment in the
nanoparticle concentration
TiO2, SiO2, and Al2O3
nanoparticles in water reported
an increase in nanofluid
viscosity with an increment in
the nanoparticle concentration
For base fluid with CuO volume
fraction of 0.025, the increment
in viscosity of roughly 300%, but
decreases with the rise of
temperature
Convective heat transfer
coefficient of the dispersed
fluids with submicron metallic
oxide particles Al2O3 at a
volume concentration of 3% was
12% littler than that of base
when analyzed under the state
of consistent average velocity
increment of as much as 40% in
the heat transfer coefficient of
the nanofluid, at the same
steady average speed
Heat transfer coefficient was
depend on nanoparticle
concentration, material,
temperature, and type base fluid
[85]. However, in another study, an arrangement of exact solutions
have been acquired for hydrodynamically and thermally fully developed laminar nanofluid flows in channels and tubes, which is subjected
to constant heat flux. From the arrangements, it has been inferred that
the anomalous heat transfer rate, surpassing the rate anticipated from
the increment in thermal conductivity, is possibly in certain cases such
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boiling and pool boiling. The use of nanolubricant can also benefit to
the compressor performance by giving better tribological properties of
friction coefficient and wear of the compressor. Thus resulting to low
pumping power requirement and better wear characteristic consequently more life cycle of the moving components. However, the
increase in concentration of nanoparticles will affect significantly to
the viscosity of the nanofluid. Therefore, a stable solution of nanorefrigerant and nanolubricant mixture need to be identified in order to
have better performance in vapour compression of refrigeration system
for specific application. Redhwan et al. [94] provided a comprehensive
review on the latest development of nanorefrigerant for various types of
refrigerant bases and their performance. They found that the massive
potential of nanorefrigerant to enhance heat transfer capability will
drive the use of nanorefrigerant in various applications.
as titania-water nanofluids in a channel or tube and alumina-water
nanofluids in a tube [86].
There are limited studies on heat transfer characteristics for
complex geometries such as heat exchangers, helically coiled tubes,
microchannels and enhanced tubes. Huminic and Huminic [87]
reviewed the importance of distributed articles on the improvement
of the convection heat transfer in heat exchangers by utilizing nanofluids on two points and focuses on the use of nanofluids in different
sorts of heat exchangers. An experiment was carried out by Tiwari et al.
[88] to find the hydraulic and thermal performance of different
nanofluids in a plate heat exchanger. The CeO2/water nanofluid seems
to enhance the heat transfer performance of a plate heat exchanger
when an ideal molecule concentration was achieved [88]. However in
another study, Taws et al. [89] demonstrated that there is no significant
increase in heat transfer for the nanofluid with 2.0% volume concentration at the same Reynolds number. However, a reduction in the
Nusselt number was reported for volume concentrations of 4.65%,
where the CuO/water nanofluids in a chevron-sort modern plate heat
exchanger was tested [89].
The utilization of nanofluids in heat pipes has indicated impressive
improvement in reduction of thermal resistance and improvement in
performance, although the previous studies demonstrated particle
accumulation and deposition in smaller scale channel heat sinks [90].
There are also reports that nanofluids enhance the performance of
evaporators, one of the heat exchangers in refrigeration systems. From
the recent experiment, researchers observed that the boiling heat
transfer coefficient and the enhancement of critical heat flux using
nanofluids resulting from a thin porous nanoparticle deposition layer
on the evaporator surface, serves to enhance the wettability and
capillarity of the boiling surface [91–93].
4.1. Development of nanorefrigerant
In a study done by Pak and Jung [95], the author investigated the
impact of carbon nanotubes (CNTs) on nucleate boiling heat transfer.
Test outcomes demonstrated that CNTs enhance nucleate boiling heat
transfer coefficients for these refrigerants. Nucleate pool boiling heat
transfer of a refrigerant-based-nanofluid was examined at distinctive
concentrations and pressures of nanorefrigerants dispersed with
nanoparticles [96]. The outcomes show that the nucleate pool boiling
heat transfer decrease with increase of nanorefrigerant concentration,
particularly at high heat fluxes. Saidur et al. [54] reported that there is
no comprehensive literature on the nanoparticles as additives with
conventional refrigerants and oils used in refrigeration system.
Previously, Jiang et al. [97] conducted experimental work to test
thermal conductivity characteristics of the carbon nanotubes (CNT)
nanorefrigerants and to build a model for predicting the thermal
conductivities of CNT nanorefrigerants. Bi et al. [98] conducted an
experiment of mineral oil with TiO2 nanoparticle mixtures as lubricant
in domestic refrigerators. The refrigerant was R-134a with the common
lubricant, Polyol-ester (POE). The refrigerator performance with the
nanoparticles was investigated using energy consumption test facilities
with 26.1% less energy consumption. The same authors also conducted
experiment work for performance of a domestic refrigerator using
TiO2-R600a nanorefrigerant as working fluid with 0.1 g/L and 0.5 g/L
concentration of TiO2-R600a nanorefrigerant. The performance of the
refrigerator shows 9.6% less energy used with 0.5 g/L TiO2-R600a
nanorefrigerant [99]. Table 3 shows a summary of previous works from
other researchers regarding to application of nanoparticles in the
refrigeration system.
4. Potential of nanorefrigerant and nanolubricant
In refrigeration systems, the effort to improve the efficiency of the
system by introducing nanoparticles in refrigerant (nanorefrigerant)
and in lubricant oil (nanolubricant). Nanorefrigerant and nanolubricant are special types of nanofluids. Nanofluids show great potential to
enhance the thermodynamic and mechanical performance of refrigeration systems. Adding nanoparticles to the base liquids can altogether
increase their transport properties and the efficiency of the system,
regardless of the possibility that the impact on pressure must be
deliberately evaluated. In addition, nanolubricants can enhance their
tribological properties (lubricity, against wear properties, high pressure
condition) with clear advantages for the compressors. In the refrigeration system, the nanorefrigerant act as the heat absorber or cooling
agent while nanolubricant just lubricate the compressor so they work in
a separate system. Nanorefrigerants help on improving heat transfer;
while nanolubricants work more by lubricating the moving piston by
having better tribology characteristics. By adding nanoparticles on both
refrigerant and lubricant oil, it can greatly improve on energy saving of
the refrigeration system.
Nowadays, there are extensive of equipment and application can
take benefit from the usages of nanofluids, including in the vapour
compression system performances. Nanorefrigerant and nanolubricant
are a great alternative to enhance the performance of refrigeration
system due to better heat transfer characteristic, better tribological
performance, and the properties enhancement of refrigerant-lubricant
mixture. The suspended nanoparticles enhance the thermal conductivity of the base lubricant and refrigerant-lubricant mixture, and also the
pool boiling heat transfer in the system. The increase in thermal
conductivity property can benefit to the heat exchanger of the system.
In condenser, more heat will be removed from the system due to better
convective heat transfer. This will result with higher heat transfer
coefficient however with minimum penalty in pumping power [50]. The
nanofluid also can probably enhance the evaporator performance. This
is due to better heat transfer coefficient and critical heat flux in flow
4.2. Development of nanolubricant
As in previous years, numerous efforts have been employed to
research on tribology to reduce friction, enhance lubrication, and
reduce wear of interacting surfaces that are in relative motion which
can be applied in many fields such as refrigeration, compressor and
many more. To those in the maintenance department, one should
understand the importance of precision lubrication. It makes sense that
good lubrication is a good investment because machines operate better
when it is greased up appropriately. Among the most significant
current discussions in nanotechnology is cost reduction and energy
saving. The term ‘real cost’ is the correct way to explain this. Real cost
is measured by the expenses connected with the inability to have
proper lubrication, in the correct spot and at the ideal time. Scientific
studies show that 0.4% of gross domestic product could be saved in
terms of energy in western industrialized countries by the improvement of tribological performance [100]. Statistically, the refrigerator is
the second-biggest client of power, directly after air conditioner
systems. The modern automotive air-conditioning system is more
efficient because of the great effort to improve the design and the
performance of the system, but there is still little effort to use
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Table 3
Summary of previous work regarding applications of nanoparticle in refrigeration system.
Author
Nanoparticle
Nanoparticle size
(nm)
Refrigerant
type
Particle
concentration
Performance
Park and Jung [95]
Carbon nanotubes
TiO2
TiO2
20 nm OD and 1 µm
length
21
R123 and R134a
1% by volume
Enhance heat transfer coefficient up to 36.6%
R141b
CNT
15–80
R113
0.01, 0.03% and 0.05%
by volume
0.2–1.2% by volume
Bi et al. [98]
TiO2
50
R134a
0.06–0.1% by weight
Bi et al. [99]
TiO2
50
R600a
0.06–0.1% by weight
Nucleate pool boiling heat transfer detiorated with
increasing particle concentration
The thermal conductivities of CNT nanorefrigerants
increase altogether with the increment of the CNT volume
fraction.
The performance was superior to the HFC134a and POE oil
refrigeration system, with 26.1% less energy utilization
utilized with 0.1% mass fraction TiO2 nanoparticles
contrasted with the HFC134a and POE oil system.
Al2Cu and Ag2Al nanoparticles show some improvement of
50–150% of its thermal conductivity when dispersing the
nanoparticles in water and ethylene glycol as base fluid.
Trisaksri and
Wongwises [96]
Jiang et al. [97]
equipments. Lubricants are a main concern to increase mechanical
durability and energy efficiency. Lubricants in refrigeration systems
grease up inner parts, remove heat generated during compression,
clean the framework, go about as a fluid seal and decrease the work
required by the compressor [104]. The existence of nanoparticles in
lubricants impacts the contact of every part of the refrigeration unit. It
is, then again, generally conceded that the condenser is the slightest
delicate segment to the presence of the lubricant, and writing is
therefore rare on that subject. The compressor, the evaporator and
pipes are the subject of a bigger number of productions, some of which
are examined in the following areas.
nanoparticles in them. Nanolubricants have the following attributes
when contrasted with ordinary normal fluid suspensions [72].
i. Higher heat transfer between the particles and liquid because of
high surface area of the particles.
ii. Better dispersion stability.
iii. Reduces particle clogging.
iv. Reduces pumping force when contrasted with base liquid to acquire
comparable warmth exchange.
Lubricants with nanoparticles are basically a new class of liquids
which comprise a base lubricant with nanosized particles (1–100 nm)
suspended inside which are metal or metal oxide and focussing towards
the increment of conduction and convection coefficients, taking into
consideration more heat rejected out of the coolant [101]. There are
still many thing that have to be investigated before consumers can fully
use nanoparticles in the lubricants [102]. The definitive theory on
nanolubricant still does not exist based on these reasons [67]:
5.1. Impacts on compressor performance
A noteworthy impact of the decrease in the performance of
compressor is the foaming phenomenon. Foam is formed by mechanical action. The movement of the piston to the oil draws air into the
sump. This happens despite any design and assembly issues. The
foaming gets to be rough with increased rotational rate of the blade and
expanded stream rate of the blowing vapour [105]. This means that the
faster the movements of the compressor, the higher the percentage that
foam formation occurs. Actually, there is less work that focuses on this
area involving lubricants with nanoparticles. One of the studies stated
that lubricants that have LTL-type zeolite crystals as nanoparticles
slows down the oxidation rate of the oils and also resulted in a lower
rate of production of solid polymeric residues, potentially causing the
effect of foam forming to be lesser [106].
There are also several studies in the literature reporting the
increasing performance of compressors that use nanolubricants as its
lubricating oil. It is proven that carbon nanoparticles can be used and
enhance the lubrication on the contact surfaces by reducing friction.
Carbon nano-oil increases the anti-wear properties at the thrust slidebearing of scroll compressors. This will decrease frictional loss in the
thrust slide-bearing that occupies a large part of total mechanical loss
in scroll compressors [72]. A review by Ahamed et al. [72] on energy
analysis of vapour compression systems showed the reduction in the
energy losses when nanofluids are used as nanolubricants instead of
base lubricants as it had better thermal dissipation, lower wearing and
improved lubrication properties. Thus resulting in the energy losses in
the compressor to be less [72]. It was stated that the addition of Copper
Oxides (CuO) and carbon nano-tubes help to enhance heat transfer.
However, not all showed positive results because the study showed
no noticeable improvement on performance of system even when using
titanium oxide (TiO2) or single wall carbon nano-horns (SWCNH) as
nanoparticles in lubricant, instead of using commercial oil [107].
i. The thermal properties are excessively different from solid–solid
composites or standard solid–liquid suspensions.
ii. The thermal transport in nanofluids, other than being shockingly
efficient contrasted with standard solid–liquid suspensions, relies
on nontraditional variables, for example, molecule size, shape, and
surface treatment.
iii. The comprehension of the material science behind nanofluids
obliges a multidisciplinary approach.
The topic of adding nanoparticles in lubricant or nanolubricant is
one of the most active areas in tribology improvement in research
today. The development of nanolubricant can be found in tribological
evaluations of the next section.
5. Tribological performance
According to the theory of thermodynamics, coefficients of performance (COP) for vapour compression refrigeration cycles was affected
by the desired cooling output over the power input by the compressor.
It is known that by the reducing the work required by the compressor
to run the system, the COP of the system can be improved. One of the
ways is to improve the performance of tribology to achieve that goal.
Previously, the lubricant industry is dealing with numerous works of
research activity in the field of nanotechnologies with the aim to
improve the performance of tribology of lubrications to reduce the
friction, wear, and improve lubrication [103]. Therefore, this section
focuses on the improvement of lubrication performance. Friction and
wear are important as sources of energy and material in mechanical
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5.2. Impacts on capillaries and pipelines
The common problem of oil in circuit funnels is that it is connected
again to that oil that comes back to the compressor. In the event that
the oil stays in the refrigeration circuit, the system performance is
reduced and the compressor durability will be affected. A study done by
Motta et al. [108] studied on the effects of refrigerant/lubricant mix on
critical flow in adiabatic capillary tubes. It was presumed that the effect
on critical flow depends unequivocally on some key refrigerant/oil
properties: viscosity, vapour pressure, and density. In a horizontal tube
with internally enhanced heat transfer surfaces, the two-phase pressure
drop for POE lubricant oil at low and medium mass flux did not
increase, but increased at higher mass flux [109]. In one study, it can
be concluded that the friction factor increments with both density and
absolute viscosity of the nanofluid [50]. It can be interpreted that the
two nanofluids, SiO2 and TiO2, having comparable estimations of
viscosity can have distinctive estimations of friction factor.
5.3. Tribological improvement
The topic of tribological improvement in enclosures is one of the
most active areas in nanotechnology research today. The researcher
found that nanolubricants adequately decreases sliding frictional losses
by a nonstop supply of active lubricant additives and by developing a
steady, low friction tribofilm at the sliding interface of the workpiece
surface [110]. Nanolubricants containing inorganic MOS2 nanoparticles researched by deliberate tribological testing under a simulated
machining cooperation between abrasive crystals and a workpiece in a
surface grinding procedure. Later, in the study they experimented with
SiO2 nanolubricants on apparatus wear and surface harshness utilizing
fuzzy logic and response analysis to figure out significant parameters
[111]. In the experiment, it was found that minimum tool wear was
when using lubricants with 0.5 wt% of concentration and enhanced
surface roughness.
In another study, Koshy et al. [112] examined nanolubricants with
vegetable oils. The surface geography and surface roughness investigations done by AFM and FESEM uncovered that the roughness of the
friction surface of the pin is diminished and the surface gets smoother
at the point when nanoparticles at ideal concentration levels are
included to the lubricants with suitable surfactants as shown in
Fig. 5. They observed that the lubricated surfaces using coconut oil
(without nanoparticles) to be less rough after sliding with the roughness value of 104 nm (initial surface roughness of the pin surface
before the test is 143 nm). However, the surface roughness of the pin
surface after sliding with coconut oil added with MoS2 nanoparticles
was found less with further reduction to 68 nm. This was one of the
efforts made by them to introduce more environmental solutions that
have greater lubrication performances. Recently, the author gave a
comprehensive review on nanolubricants with boron nitride as nanoparticles [113]. The result showed that the nanolubricant with a little
measure of boron nitride nanoparticles displayed good tribological
performance with optimal concentration of nanoparticles found to be
around 0.1 wt%. Table 4 shows a summary of previous works for the
application of nanoparticles in tribological improvement.
6. Refrigeration system performance
When the heat transfer improves in certain refrigeration systems,
there are significant improvements in the performance of the system.
In recent studies, there are improvements of heat transfer when using
Al2O3 as nanoparticles in refrigerants, also the power consumption
decreases. This is because the improvement of heat transfer reduces the
effort needed to run the working fluid by the compressor, thus the
system works more efficiently [114]. The COP of cooling in the
refrigeration system is the ratio of the heat removed from the cold
reservoir to input work. The best performance for refrigeration system
Fig. 5. The AFM images before and after the presence of coconut oil with nanoparticles
[112].
is described as process that uses the lowest amount of inputs to create
the greatest amount of outputs. The higher the COP of a certain system,
the higher energy can be saved, and the more efficiently the system
runs. Fig. 6 shows the energy that can be saved compared to expected
100000 million watts hours in 2030 [115]. It was observed that using
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W.H. Azmi et al.
Table 4
Summary of previous works for the application of nanoparticles in tribological improvement.
Author
Type of
nanoparticle
Nanoparticle size
(nm)
Concentration (wt%)
Performance
Kalita et al.
[110]
MOS2
100
0.2
Sayuti et al.
[111]
Koshy et al.
[112]
MOS2
100
0.2
MOS2
90
0.25, 0.5, 0.75 and 1
Wan et al.
[113].
boron nitride
30
0.1, 0.5 and 1.0
Nanolubricants adequately decrease sliding frictional losses by a nonstop supply
of active lubricant additives and by developing a steady, low friction tribofilm at
the sliding interface of the workpiece surface.
Minimun tool wear when using lubricants with 0.2 wt% of concentration and
enhanced surface roughness.
The roughness of the friction surface of the pin is diminished and the surface
gets smoother at the point when nanoparticles at ideal concentration level are
included to the lubricants with suitable surfactants.
The nanolubricant with a little measure of boron nitride nanoparticles could
display good tribological performance with optimal concentration of
nanoparticles was found to be around 0.1 wt%.
et al. [119] investigates the relation between thermal performance and
the increase of COP performance for certain refrigeration systems. The
outcomes demonstrated that thermal conductivity, dynamic viscosity,
and density of Al2O3/R-134a nanorefrigerant increased by 28.58%,
13.68%, and 11%, respectively contrasted with the base refrigerant (R134a) for the same temperature. Additionally, Al2O3/R-134a nanorefrigerant demonstrates the maximum COP of 15%, 3.2%, and 2.6% for
thermal conductivity, density, and specific heat, individually contrasted
with R-134a base refrigerant. The replacement of R134a with R152a
gives a green and clean environment, with zero ozone depleting
potential (ODP) and less GWP [120]. The performance of refrigeration
was significantly enhanced with 21% less energy utilization with the
replacement of 0.5% volume concentration of ZnO-R152a refrigerant.
Both the suction pressure and discharge pressure were brought down
by 10.5%. In addition, the evaporator temperature was lessened by 6%
with the utilization of the nanorefrigerant. Table 6 shows a summary of
previous works for the enhancement in refrigeration system performance.
0.1% volume concentration of TiO2 nanofluids is the most energy
saving.
6.1. Energy saving using nanorefrigerant
As already mentioned earlier, dispersing nanoparticles greatly
enhances heat transfer, and better system stability is achieved and this
helps in energy saving. To prove that, Shengshan and Lin [116] has
conducted an experiment to investigate the refrigeration performance
with R134a/TiO2 nano-refrigerants with different concentrations of
TiO2 particles with no change of the first refrigeration system. The test
results demonstrate that the R134a/TiO2 nano-refrigerant works
typically and securely in the cooler with lower power utilization and
quicker refrigeration speeds with an ideal TiO2 nano-particle concentration of 10 mg/L which decreases the energy used by 7.43%.
Solubility and dissolvability of the mineral oil with the hydrofluorocarbon (HFC) refrigerant is improved when TiO2 nanoparticles is
utilized as added substances [117]. In a different study, the scientist
concentrates on five distinctive nanorefrigerants with Al2O3 nanoparticles and their pure liquids: R12, R134a, R430a, R436a, and R600a.
The outcomes show that COP is improved by adding nanoparticles to
the unadulterated refrigerant and the highest increment of COP was
obtained when utilizing the R600a/Al2O3 mixture with 43.93% [118].
The research study by Bi et al. [99] also carried out with TiO2R600a nano-refrigerants to test the refrigerator performance using
energy consumption test and freeze capacity test. The outcomes
demonstrated that TiO2-R600a nano-refrigerants work ordinarily and
securely in the fridge. The fridge execution was superior to the original
R600a system, with 9.6% less energy utilized with 0.5 g/L TiO2-R600a
nano-refrigerant. Table 5 shows the energy saving when using a
nanorefrigerant in the experiment by Bi et al. [99]. Later, Mahbubul
6.2. Energy saving using nanolubricant
Lubricant used in the refriegeration system has an impact to
diminish the exergy losses in the compressor [121]. There are a lot
of experiments to test the effectiveness of the nanofluid to achieve
better performance in refrigeration systems by dispersing nanoparticles into the refrigeration lubricant. According to Wang et al. [122], the
performance of residential air conditioners, the energy efficiency ratio,
EER increased 6% by replacing the Polyol-Easter oil VG32 lubricant
with nanolubricant. They used NiFe2O4 nanoparticles into naphthene
based oil B32, and using R134a, R407C, R410a and R425a as
refrigerant. In another study, Sabareesh et al. [123] using nanolubricants to study the impact of dispersing a low concentration of TiO2
nanoparticles in the mineral oil based lubricant. They investigated the
effect on its viscosity and lubrication qualities, and the performance of
refrigeration systems utilizing R12 (Dichlorodifluoromethane) as the
working liquid. Sharif et al. [124] investigated the thermal conductivity
and viscosity of Al2O3 nanoparticles suspended in polyalkylene glycol
(PAG) lubricant for various volume concentrations and different
working temperature. They were recommended to use the Al2O3/PAG
nanolubricants with volume concentration of less than 0.3% for
application in automotive air conditioning systems.
Table 5
Energy saving when using nanorefrigerant [99].
Fig. 6. The potential of energy saving when using nanoparticles by the year 2030 [115].
424
Concentration (g/L)
Energy consumption (kWh)
Energy saving
0
0.1
0.5
0.9567
0.8999
0.8649
−
5.4
9.6
Renewable and Sustainable Energy Reviews 69 (2017) 415–428
W.H. Azmi et al.
An improvement in the COP of the refrigeration system resulted
from an ideal volume fraction, with low concentrations of nanoparticles
suspended in the mineral oil, and the compressor work reduced by
about 11%, which ultimately resulted in the COP improvement of about
17% [123]. Lou et al. [125] investigateD the effectiveness of nanolubricants toward enhancing performances in domestic refrigerators. The
study also showed power utilization of the domestic refrigerators
reduced by 4.55% when utilizing graphite nanolubricants with a mass
division of 0.1%. In another experiment done by Subramani [126], the
freezing capacity is higher, and the power utilization reduces by 25%
when a blend of mineral oil and alumina nanoparticles used instead of
POE oil. Furthermore, the replacement of nanolubricant in the system
enhances the evaporator performance by an enhancement factor of
1.53. In addition, Kumar et al. [114] in one study found out that
nanolubricant helps in obtain better subcooling at the condenser, thus
it helps in improving the COP of the system. The experiment also
recorded an enhancement of power consumption up to 11% when the
nanolubricant was used instead of original lubricant.
Table 6
Summary of previous works for the enhancement in refrigeration system performance.
Author
Refrigerant
Based
Fluid
Nanoparticle
size (nm)
Evaluation
Wang et al.
[122]
R134a,
R407C,
R410a and
R425a
R12
NiFe2O4/
Mineral
oil B32
32
Energy efficiency
ratio, EER
increased by 6%
TiO2/
Mineral
Oil
30 to 40
R600a
Graphite/
Mineral
oil
50
Compressor work
reduced by 11%,
which ultimately
resulted in the
COP
improvement of
17%.
Power utilization
of the domestic
refrigerator
reduced to 4.55%
when utilizing
graphite
nanolubricant
with a mass
division of 0.1%.
Lower power
utilization and
quicker
refrigeration
speed with an
ideal TiO2 nanoparticle
concentration of
10 mg/L which
decreases the
energy used by
7.43%.
Al2Cu and Ag2Al
nanoparticles
showed thermal
conductivity
enhancement of
50–150% when
dispersing the
nanoparticles in
water and
ethylene glycol as
base fluid.
The maximum
COP of 15%,
3.2%, and 2.6%
for thermal
conductivity,
density, and
specific heat,
respectively and
individually
contrasted with R134a base
refrigerant.
21% less energy
utilization when
0.5 vol% ZnOR152a refrigerant.
Both the suction
pressure and
discharge
pressure were
brought down by
10.5% when
nanorefrigerant
was utilized. The
evaporator
temperature was
lessened by 6%
with the
utilization of
nanorefrigerant.
Sabareesh
et al.
[123]
Lou et al.
[125]
Shengshan
and Lin
[116]
R134a
TiO2/
Mineral
oil
50
Bi et al. [99]
R600a
TiO2/
Mineral
oil
50
Mahbubul
et al.
[119]
R134a
Al2O3/R134a
–
Kumar and
Elansezhian [120]
R152a
ZnO/PAG
40
7. Pressure drop characteristic
The characteristics of refrigerant suspension may change when
nanoparticles are dispersed in refrigerant, this includes the pressure
drop characteristics. Liquid solid phase pressure drop attributes and
liquid solid and vapour phase (phase change) pressure drop qualities of
nanofluids were observed by researchers. Before selecting any refrigerant, the effect of pressure need to be studied carefully to ensure the
nanorefrigerant stability for a longer period of time. Li and
Kleinstreuer [127] studied the pressure drop of solid and liquid phase
of fluid charateristics by simulation. According to them, there are two
properties that affect the pressure drop which is density and viscosity.
The addition of nanoparticles into the base working fluid has made the
nanofluid increase in density and viscosity, so this will be compensated
with additional pressure drop. Further research have been done to
confirm that viscosity of nanofluids is higher than basefluids with
addition of nanofluids where Al2O3 nanofluids and ethylene glycol
based ZnO nanofluids were tested [120,128].
Namburu et al. [129] in their numerical study also agreed that the
previous two causes of pressure drop properties is proportional with
nanoparticle volume fraction. In another study, it was reported that
with the same Reynolds number, single-phase pressure drop increased
when nanoparticle concentration increases compared to pure fluids
[130]. Later they found that pressure drop increases significantly for
≥1 vol% concentrations of nanoparticles [131]. In this study, an effort
has been made to examine the viscosity of nanorefrigerants for TiO2
nanoparticles with R123 refrigerant. Results show viscosity increments
with the increment of the molecule volume fraction and pressure drop
increments with the increment of volume fraction and vapour quality
[132].
Later, based on the analysis by Mahbubul et al. [133], it was found
that both heat transfer and pressure drop properties increased with the
increase of nanoparticle volume concentrations. So, the improvement
of a refrigeration system's performance must take account the correct
optimum concentration of nanoparticles of nanorefrigeration to obtain
better pressure drop and heat transfer characteristic.
8. Conclusions
A great number of studies on the blend of nanofluids (nanorefrigerant and nanolubricant) in the writing are being summarized in this
study. Because of their improved heat transfer attributes and improvement in COP and energy saving, it is safe to assume that nanorefrigerants and nanolubricants will be utilized as a part of numerous
modern refrigeration system and gadgets sooner rather than later. The
accompanying comments can be extracted from the present study:
425
Renewable and Sustainable Energy Reviews 69 (2017) 415–428
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i. Nanolubricants have good tribological performance thus can reduce
the friction, wear, and improve lubrication. This can help the longer
service of some mechanical parts in the refrigeration system,
especially the compressor.
ii. The use of nanoparticles as additives can enhance heat transfer
through the increase of thermal conductivities, and also the
convective mode of heat transfer. However, the effect of viscosity
needs to be observed because a certain increase of viscosity can
affect performance of the refrigeration system.
iii. Nanofluids help in performance enhancement of heat exchanger
(Condenser and evaporator).
iv. Nanorefrigerants and nanolubricants enhance the refrigeration
system performance. This can help in increasing the efficiency of
the system thus improving the energy saving feature.
v. An optimal concentration of nanoparticles dispersed in base fluid
need to be investigated in order to obtain the best performance
enhancement. This is because of the effect of pressure drop caused
by the higher viscosity of nanofluids.
Acknowledgements
The financial support by Universiti Malaysia Pahang (UMP) under
RDU151411 (RAGS/1/2015/TK0/UMP/03/2) and also Automotive
Engineering Centre (AEC) under RDU1603110 are gratefully acknowledged.
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