1.0 INTRODUCTION1.1 Importance of Energy SavingIn the last few decades, the world electricity generation and consumption became a main issue of discussion among researchers, politicians and environmental activist because of increasing in its generation and consumption consequently affected the cost of electric generation paid by the government in term of fuel subsidization and also the use of fuel in high quantity could be contributed to the global warming effect. The world trend of electricity generation is increasingly averagely about 4.45% per year from 1980 to 2010 with the total of generation in 1980 is about 8050 TWh and in 2010 the total generation is 20000 TWh (Fettweis & Zimmermann, 2008). This is expected that the world total electricity generation to be increased at 25030 TWh in 2020. There are three main sectors of electricity consumer worldwide; Industry, Residential and Commercial. In United States for example, electric consumption in residential sector accounted 38% of the total generation in 2009 (Fettweis & Zimmermann, 2008). In European countries, residential sector consume 29% of the total electric generation in 2005 ((EEA), 2009). Meanwhile, in Malaysia residential sector uses about 21% of the total country electricity generation in 2009 (Commision, 2009). Figure 1(a) and (b) show electricity consumption in US and Malaysia for three main consumers in 2009.

(a) (b)Figure 1: Electricity consumption for three main consumer in 2009 (a) United State and (b) MalaysiaGovernment�s policy on energy saving have been implemented in many countries worldwide with the purpose to reduce energy consumption, consequently reduce cost and also greenhouse gas emission. European Commission Energy leads the energy policies and activities in European countries. In United State, the US Department of Energy leads the policies in energy saving. Meanwhile, in Malaysia Energy Commissioner is the responsible agency developing and enforcing energy saving�s policies. In Malaysia, refrigerator and air conditioner use 22% and 14% respectively (Taha, 2003). Many countries worldwide agreed that energy consumption can be reduced by implementing energy efficient products in residential and product sector. Among all other appliances, refrigerator and air conditioner are the products that are closely related to mechanical engineering although several components such as electric motor and control system are outside the scope of mechanical engineering and this overview is intended to be focused on refrigeration system only.1.2 Types of RefrigerantThe working fluid in refrigerator is called refrigerant which 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 early stage 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 refrigerant for refrigeration process, propane (R-290) was marketed in replacing ammonia (R-717) as refrigerant. Propane is considered as neutral chemical, consequently no corrosive action occur and is neither deleterious nor obnoxious and should occasion require, the engineer can work in its vapour without convenience. Carbon dioxide (R-744) has been used in the 1920s by Carrier and Waterfill in field of positive-displacement and centrifugal compression machine for chillers� operation in air conditioning system. Besides that, they were used of ammonia and water (R-718), sulphur dioxide (R-764), carbon tetrachloride (R-10) and dielene (1,2-dichloroethene,R-1130). From these refrigerant, there was only R-1130 can work with centrifugal machine. The rest were unable to work properly due to several finding such as low performance, safety reason and incompatible with metals (Calm, 2008; Radermacher & Kim, 1996)

In 1930, the fluorocarbon refrigerants were introduced by Midgley and Henne (Calm, 2008) Midgley was distinguished by a shift to fluorochemicals which concerned on safety and durability of refrigerant that focuses on removing of neither toxic compound nor flammable property. One year later, the dichlorodifluoromethane (R-12) were introuduced as commercial refrigerant used in refrigerator followed by R-11 in one year later.Since hydrocarbons (HCs) were not suitable to be used as refrigerant fluids in the refrigerator due to their flamability and toxic, the use of HCs was discarded of chlorofluorocarbons (HCFCs) dominated the second generation of refrigerant with application was more on residential and small commercial air conditioners and heat pumps. However, ammonia remains in application for large scale of system. In 1974, Rowland and Molina advanced the hypothesis that anthropogenic emission of certain chlorinated and bromated compounds, particularly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), could accumulate in the stratosphere (the part of the atmosphere located at an altitude of roughly 12km to 50km) 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 (Molina & Rowland, 1974; Purvis, Hunt, & Drake, 2001).The discovery of the ozone deflecting properties of CFCs and HCFCs refrigerants, and their global warming potential (GWP) lead to the Vienna Convention (1985), the Montreal Protocol (1987), London Amendent (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 refrigerant by 1995 and 2030 respectively (http://rivm.nl bibliotheek/rapporten/48150511.html).Development of new refrigerant becomes crucial as the effect of the end of the production fo CFCs and HCFCs refrigerant. Radermaker and Kim reported that the effort to explore for new refrigeration was started since 1960s with two objectives: (1) achieving a low operator temperature with a moderated pressure ratio during single-stage compression and (2) conserving energy when the refrigeration duty consists of cooling a fluid stream through a large temperature range (Radermacher & Kim, 1996). The most preferred refrigerant was HFC134a and expected will be used for long term. However, observation was found that HFC134a and expected will be used for long term. However, observation was found that HFC134a systems tolerate far less contaminants that CFC12 systems did. Therefore, another potential refrigerant was identified to replace CFC12. It was HFC152a.However, fluorochemical is still become the primary focus and the attention was more on the use of hydrofluorocarbons (HFCs) for the longer term. The production of HFCs R-134a is a very attractive as a refrigerant because it has zero ozone depleting potential as well as low direct global warming potential (GWP). For the sake of the EU�s Kyoto Protocol obligations, the European Commission issued a directive in 2006 mandating the phase-out of R-134a in mobile air conditioning systems and its replacement by refrigerants with a GWP no higher than 150. As of 2011, a ban on R-134a systems applies to all new models, and as of 2017, to all cars in European countries because of indirect radioactive effect for GWP is 1300 as tabulated in Table 1.Table 1: ODP and GWP and other properties of selected refrigerantRefrigerant Ozone depleting potential (ODP) *ODP CFC-11=1 Global Warming potential (GWP): indirect and direct radiative effects *GWP CO2=1 ToxityFlammabilityCFC-12 1 6600 Low ZeroHCFC-22 0.5 1300 Low ZeroHFC-134a Zero 1300 Low ZeroHC R290 Zero Zero Low HighR717 Zero Zero High ZeroSource: Purvis et al. 20011.3 Energy Use in Refrigeration SystemIt has discussed in the previous page that with the increasing the numbers of refrigerators worldwide, the energy consumption also increase. The issue of increment in energy consumption has been discussed by researchers, politicians and als

o government leaders. This is because of energy generation is directly related to the use of fuel and also contributes to greenhouse effect as a result of high level of combustion. There are many literatures that focus on energy in refrigerator with the purpose to reduce energy consumption. The effort became popular after oil price shock in the 1970s and in past few decades and the oil shows fluctuation in its price. History of crude oil price is available at (http://www.ioga.com/Special/crudeoil_Hist.htm)With the concern of reduction greenhouse gas emission by reducing CO2 content in atmosphere, the quantity of fuel combustion to produce energy should be reduced (Shekarchian, Moghavvemi, Mahlia, & Mazandarani, 2011). Since refrigerators and air conditioners contributes larger energy use in domestic, these types of home appliances should be more efficient in energy use. In order to achieve energy efficiency in Malaysian refrigeration system, Masjuki (Masjuki, Mahlia, & Choudhury, 2001) reported that by implementing minimum energy efficiency standards as procedure and regulations that prescribe the energy performance of manufactured products, sometimes prohibiting the sales of products that are less efficient than a minimum level. Prediction of CO2 reduction also was estimated with 30336719 kg CO2 produced in 2012 compared with 72357910 kg CO2 in 2003 (Mahlia, Masjuki, Choudhury, & Saidur, 2001). Energy label are informative labels affixed to manufactured products to describe the product�s energy performance. The important of energy labels 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 (Mahlia, Masjuki, & Choudhury, 2002b, 2002c). The implementation of energy label in Malaysia is started since 2005 for domestic refrigerator and open to all manufacturers on voluntary basis. The label is similar to type B which has been investigated by Mahlia (Mahlia, Masjuki, & Choudhury, 2002a). The same author also conducted research for projected electric saving form implementing minimum energy standard for household refrigerators in Malaysia (Mahlia, Masjuki, Saidur, & Amalina, 2004; Mahlia, Masjuki, Saidur, Choudhury, & NoorLeha, 2003).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 largest refrigerator sale worldwide with the household refrigerator reached 15.99 million in 2002. Refrigerator accounted for more than 32% of the total residential electricity use in China with the total consumption of 1620.o TWh a year (Lu, 2006; Tao & Yu, 2011). Chinese has established their energy efficiency standards for household refrigerator since 2003 and replaced by 2008. Replacement or review of the energy efficiency improvement per year is under consideration for all manufacturers.1.4 Effort To Improve Refrigeration SystemSince the domestic refrigerators were introduced in US in early 20th century, the demand of such appliances was increased from year to year. It was recorded that from 1919 to 1924 there was over 100% increases in sales each year and orders in 1924 were 350% higher than a year earlier. Currently, there are about 1000 million units of refrigerator worldwide. There have been many researches and development since a new domestic refrigerator was discussed in 1923 by Anon (Anon, 1923) which was concentrated on insulation method by using balsa wood and quality in sealing method. In the 1950s, urethane foam had been developed and in early of 1960s the rigid urethane foam was produced with fluorinated hydrocarbon expanding agents such as R-11 and R-12. Because of environmental concern under Montreal Protocol for ozone depletion by CFCs, development of nonfluorocarbon of blowing agent was discussed. Vacuum insulation also was seen as an option with commercial potential. However, the production of vacuum insulation was concerned of the cost of production. In 2004, Johnson reported the comparison effect of blowing agent selection on energy consumption and the life cycle climate performance (LCCP) of typical European refrigerator (Johnson, 2004). Two types of blowing agent were analyse which is HFC-245fa and pentane for the same product specification

with considering 10% of form go to land fills instead of current practice with 60%. The author found that with current foam formulations, the use of HFC-245fa as a blowing agent, instead of cyclopentane and n-pentane blend, offers a significant advantage in energy consumption for refrigerator/freezers although the GWP for HFC-245fa is high compared with pentane blend.Besides the insulation of the refrigerator, components efficiency also became the main discussion among researchers. Generally, refrigerator consists of four main component connected in sequence order to form closed system. The components are compressor, condenser, capillary tube and evaporator. Among these four components, compressor is considering as the main part in the system that working to compress refrigerant and produce primary force to circulate the refrigerant throughout the system. The compressor consume electric source and internal design is moderately complex. Figure 2 shows common working principles of reciprocating compressor that typically used in domestic refrigerator. The measure of compressor performance in domestic refrigerator is energy efficiency ratio (EER). 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. Figure 2: Working principle of common reciprocating compressorThe most concentration of research on the compressor domestic refrigerator in the past few decades was in compressor valve dynamics, lubrication, noise emission and vibration and compressor efficiency. Many literatures can be obtained for such field of study in compressor for domestic refrigerator. Shaffer (Shaffer & Lee, 1976) was reported energy consumption in quarter horsepower hermetic refrigerator compressor by analysing several factors of losses. The author was recommended that motor efficiency should be increased, compressor geometry must be improved and reduce suction gas heating. Karll (Karll, 1976) from Danfos, one of the main compressor manufacturer reported that there are three main areas to be tackled:Mechanical losses, electrical losses amd gas circulation losses in order to improve company�s annual production. Riffe (Riffe, 1976) was improved efficiency of reciprocating refrigerator compressor up to 40% with introducing unitary connecting rod wrist pin anf notched piston and cylinder design. Schroeder was introduced design improvement of electric motor for reciprocating refrigerator compressor from 73% to 80% efficiency by using positive temperature coefficient resistor and married to run capacitor (Schroeder, 1976). Nelson and Middleton (Nelson & Middleton, 1980) were developed an energy efficient compressor for refrigerator and freezers by introducing four poles electric motor combined with positive temperature coefficient resistor and also improve compressor�s geometry. Four poles electric motor also implemented by Peruzzi from Italy with the effort to improve EER for a hermetic reciprocating compressor. The works on improvement of efficiency of electric motors, compressor�s geometry and other compressor parts were continued by other researchers in the following years with the improvement methods almost same as previous researchers did (Dreiman, Bunch, Hwang, & Radermacher, 2004; Kawai, Nishihara, Hamada, & Nakaoka, 1982; Ooi & Ng, 2002)It has been mentioned before that the refrigerator consists of four main components: a compressor, a condenser, a capillary tube and an evaporator. Assembled in sequential of its order, compressor will compressed refrigerant in vapour form to high pressure and temperature, then this refrigerant is fed into condenser. In the condenser, high pressure and temperature refrigerant will be cooled by means of free convection heat transfer and then fed into capillary tube. Capillary tube is a metering device which reduces condenser pressure to evaporator pressure. In The meantime, the temperature of refrigerant also decreases and it will change the phase of refrigerant from sub-cooled liquid into mixture. Then the refrigerant is fed into evaporator. Evaporator is heat exchanger devices to absorb available heat in refrigerated space and the heat is then carried by refrigerant into compressor. These processes occurred continuously in all components.Since the condenser and evaporator is both the heat exchanger device, their performance is very significant as a performance indicator in any domestic refrigera

tor. Early work to investigate performance of the condenser and evaporator in domestic refrigerator was conducted by Ding and Chen (Ding & Chen, 1992). The authors reported that the weight of evaporator influencing the energy consumption of refrigerator. Lee et al. (K.-S. Lee, Pak, Lee, & Lee, 1996) were conducted experimental study for the effect of frost formation in a flat plate finned tube evaporator. They were analyzed effect 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.Keratas et al. (Karatas, Dirik, & Derbentli, 1996) were 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 establish correlation were used for the calculation of the heat transfer coefficient for the nonuniform flow. Tri-tube type evaporator was introduced by Lee et al. (JS Lee, Lee, Ham, Oh, & Cho, 2000) to be used in domestic refrigerator. This kind of evaporator was designed to enhance the defrosting efficiency and basic performance of an indirect cooling household refrigerator. This evaporator was able to enhanced energy efficiency by reducing energy consumption about 4% in Korea. This evaporator has been patented under US patent with Application No. 10/368 343 in 2003. However it has been abandoned by patent US7726025 in 2010. Kim and Jang (Cho, Kim, & Jang, 2005) were conducted an experiment of mass and heat transfer coefficient for finned tube evaporator under frost condition. They were tested a single stage and a two-stage evaporator with a variation of operating parameters and fin geometry. The airside heat and mass transfer coefficients were calculated from the measured data and the effects of 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 of the rate of heat transfer is a method to improve energy efficiency and in the field for cooling system in the transportation industry, hydronic heating and cooling systems in buildings, and industrial process heating and cooling systems in petrochemical, textile, pulp, and paper, chemical, food, and other processing plants. In order to improve the rate of heat transfer, Choi and Eastman (Eastman, Choi, Li, Thompson, & Lee, 1996). Introduced suspended metallic nanoparticles in conventional heat transfer fluids. The resulting �nanofluids� exhibits high nanoparticles in conventional heat transfer fluids. The resulting �nanofluids� exhibits high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhanced thermophysical properties and heat transfer performance can be applied in many devices for better performance. Nanofluids is a part of nanotechnology which being used or considered for use in many application targeted to provide cleaner, more efficient energy supplies and uses. The applications of nanotechnology were reported by many researches such as Choi and Eastman (Eastman et al., 1996), Serrano et al. (Serrano, Rus, & Garcia-Martinez, 2009), Saidur et al. (Saidur, Leong, & Mohammad, 2011) in many application as listed below;I. Engine and transmission oil coolingII. Refrigeration (domestic and chillers)III. Boiler exhaust flue gas recoveryIV. Heating and cooling of buildingV. Cooling of electronicsVI. LubricationsVII. Biomedical applicationVIII. Nanofluids in transformer oilSaidur et al. (Saidur, Leong, et al., 2011) 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. (Jiang, Ding, & Peng, 2009) were conducted experimental work to test thermal conductivity characteristic of the carbon nanotubes (CNT) nanorefrigerants and to build a model for predicting the thermal conductivities of CNT nanorefrigerants. Bi et al. (S.-s. Bi, Shi, & Zhang, 2008) were conducted experiment of mineral oil with TiO2 nano

particles mixtures as lubricant in domestic refrigerator. The refrigerant was R-134a with the common lubricant was 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 were conducted experiment work for performance of a domestic refrigerator using TiO2-R600a nanorerigerant as working fluid with 0.1 g/L and 0.5 g/L concentration of TiO2-R600a nanorefrigerant. The performance of refrigerator shows 9.6% less energy used with 0.5 g/L. TiO2-R600a nanorefrigerant (S. Bi, Guo, Liu, & Wu, 2011). Table 2 shows summary of previous works from other researcher regarding to application of nanoparticles in refrigeration system.Table 2: Summary of previous work regarding to application of nanoparticles in refriegration systemAuthors Nanoparticles Size of nanoparticles Refrigerant type % of concentration Performance(Park & Jung, 2007)Carbon nanotubes TiO2 20nm OD and 1µm length R123 and R134a 1% by volumeEnhance heat transfer coefficient up to 36.6%(Trisaksri & Wongwises, 2009)TiO2 Average 21nm R141b 0.01, 0.03 and 0.05% by Volume Nucleate pool boiling heat transfer detiorated with increasing particle concentration(Peng, Ding, Jiang, Hu, & Gao, 2009)CuO - R113 0-0.5% by weight Maximum enhancement of heat transfer coefficient is 29.7% is obtained.(Peng et al., 2010)Diamond 10nm R113 0-5% by weight The nucleate pool boiling heat transfer coefficirnt ofR113/oilmixture with nanoparticles is increase by 63.4%(Kedzierski, 2011)Al203 10nm R134a 0.5, 1 and 2% by weight The average heat flux improvement for heat fluxes less than 40kW/m2 was approximately 105%, 49%, and 155% for the 0.5%, the 1% and the 2% mass fractions, respectively.(Sabareesh, Gobinath, Sajith, Das, & Sobhan, 2012)TiO2 20nm R12 0.005-0.015 by volume Increase COP of 1.43 instead of 1.22

2.0 Nanofluid.Nanofluids are colloidal suspensions of nanosized (1-100 nanometers) solid particles in a base liquid. It is proven that nanofluids usually have properties of enhanced thermal conductivity if we compare with its base fluids (Pawel Keblinski, Eastman, & Cahill, 2005). In refrigeration system, the effort to improve the efficiency of the system by introduces nanoparticles in refrigerant (nanorefrigerant) and in lubricant oil (nanolubricant). Nanofluids show great potential to enhance the thermodynamic and mechanical performance of refrigeraton 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 refrigeration system, we can have either only nanorefrigerant or nanorefrigerant in the system, or we can have them both in the system. The nanorefrigerant and nanolubricant, it both thus help in energy saving, but in different way to another. 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 seperate system. Nanorefrigerant helps more on improving heat transfer, while nanolubricants work more by lubricate the moving piston by having more better tribology characteristic. By adding nanoparticles on both refrigerant and lubricant oil can improve greatly on energy saving.As in previous years, a numerous effort has been employed to research in tribulogy to reduce friction, enhance lubrication, and reduce wear of interacting surfaces that are in relative motion which can be applied in many fields such refrigeration, compressor amd many more. To those in the maintenance department, one sh

ould understand the important of precision lubrication. It makes sense that good lubrication is a good investment because machines run better when they are appropriately greased up. The topic of adding nanoparticles in lubricants or we can call it as nanolubricants is one of the most active areas in tribulogy improvement in research today. Among the most significant current discussions in nanotechnology is cost reduction and energy saving. The term �real cost� was the correct way to explain this. Real Cost is measured by the expenses connected with the inability to have the proper lubrication, in the correct spot, 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 if we improve the tribulogical performance (Mang & Dresel, 2007). Statistically, the refrigerator is the second-biggest client of power (13.7%), directly after air conditioner system (14.1%). (Dept. of Energy) With most apparatuses you spare vitality by utilizing them less, however you can't exceptionally well do that with your air conditioner, especially when one sitting in car considering current climate. Current automotives air-cond aren't only somewhat more efficient, they're amazingly more efficiently than previous model because great deal of effort was used to improve the design and the performance of these system, but there are still less effort to use the nano-particles in them. Nanofluids or nanolubricants have the following attributes when contrasted with ordinary normal fluid suspensions. (Saidur, Kazi, Hossain, Rahman, & Mohammed, 2011)

Higher heat transfer between the particles and liquid because of high surface area of the particles.

Better dispersion stability. Reduces particle clogging. Reduces pumping force when contrasted with base liquid to acquire compar

able warmth exchange.Lubricants with nano particles are a basicly new class of liquids which comprise of a base lubricants with nano-sized particles (1�100 nm) suspended inside them which are metal or metal oxide focussing towards increment conduction and convection coefficients, taking into consideration more heat rejected out of the coolant.(Phillbot Keblinski, Phillpot, Choi, & Eastman, 2002). There are still many thing have to be investigated before we can fully use nanoparticles in fluid (Pawel Keblinski, Prasher, & Eapen, 2008). The definitive theory on nanolubricants still does not exists based on these reasons (Das, Choi, Yu, & Pradeep, 2007):

The thermal properties is excessively different from solid�solid composites or standard solid�liquid suspensions.

Th thermal transport in nanofluids, other than being shockingly efficient contrasted with standard solid�liquid suspensions, relies on upon nontraditional variables, for example, molecule size, shape, and surface treatment.

Th comprehension of the material science behind nanofluids obliges a multidisciplinary approach.

Tribological performance According to the theory of thermodynamics, coefficients of performance (COP) for vapor compression refrigeration cycles was affected by the desired cooling output over the power input by the compressor. Is is known that if we reduce the work required by compressor to run the system, we can reduce the COP of the system. One of the way was to improve the performance of tribology to achieve that goal. Previously, the lubricant industry is dealing with numerous work of research activity in the field of nanotechnologies with the aim to improve the performance of tribological of lubrications to reduce the friction, wear, and improve lubrication(Liu, Li, Lu, & Fan, 2005). In this subchapter, we only focus on the improvement of lubrication performance. Friction and wear are important as sources of energy and material in mechanical equipment. Lubricant is a main concern to increase mechanical durability and energy efficiency. Lubricants in refrigeration system grease up inner parts, remove heat generated during compression, clean the framework, go about as a fluid seal and decrease works required by compressor(Hundy, Trott, & Welch, 2008). The existance of nano particles in lubricants impacts the conduct of every part of refrigeration units. It is, then again, general

ly conceded that the condenser is the slightest delicate segment to the presence of the lubricant, and the writing is therefore rare on that subject, while the compressor, the evaporator and pipes are the subject of a bigger number of productions, some of which are examined in the following areas.

3.1 Impacts on components performance3.1.1 CompressorA noteworthy impact of 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 increase rotational rate of the blade and expanded stream rate of the bloqing vapor (Yanagisawa, Shimizu, & Fukuta, 1991). This means that the faster the movement os compressor, the higher the percentage that foam formation occur. Actually, there are less work that focus on this area involving lubricant with nanoparticles. One of the study 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, lesser (Majano, Ng, Lakiss, & Mintova, 2011).There are also several studies in the literature reporting of increasing performance of compressor that use nanolubricants as its lubricating oil. It is proven that carbon nano-particles can be used and enhanced the lubrication on the contact surfaces having friction. Carbon nano-oil increase the anti-wear properties at the thrust slide-bearing 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 (Ahamed, Saidur, & Masjuki, 2011). A review on exergy analysis of vapour compression system, show the reduction in the energy losses when nanofluids are used as nanolubricants instead of baselubricants due to better thermal dissipation, lower wearing and improved lubrication properties, thus resulting the exergy losses in the compressor to be lessen (Ahamed et al., 2011). It was stated that the addition of Copper Oxides (CuO) and carbon nano-tubes helps to enhance heat transfer. However, not all show positive result because (Fedele, Colla, Scattolini, Bellomare, & Bobbo, 2014), the study show 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 comercial oil.3.1.2 Impacts on Capillaries and pipes tubeThe common problem of oil in circuit funnels is connected again to that of oil come back to the compressor. In the event that oil stays in the refrigeration circuit, the system performance is reduced and the compressor durability will be affected. In the study carried out by (Azmi, Sharma, Sarma, Mamat, & Najafi, 2014), it can be conclude that friction factor increments with both density and absolute viscosity of the nano liquid. It can be interpreted that the two nano liquids, SiO2 and TiO2, having comparable estimations of viscosity can have distinctive estimations of friction factor. The increase of friction factor when using nanolubricants will probably have good effect on flow characteristic, thus improve the oil return to the compressor.3.2 Tribological improvementThe topic of tribological improvement in enclosures is one of the most active areas in nanotechnologies research today. Author found that 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 (Kalita, Malshe, Jiang, & Shih, 2010). 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 (Sayuti, Sarhan, & Salem, 2014) they experimenting SiO2 nanolubricants on apparatus wear and surface harshness utilizing fuzzy logic and response analysis to figure out which prepare parameters are measurably significant. In this experiment, is was found out that minimun tool wear when using lubricants with 0.5%wt nanoparticle conce

ntration and enhanced surface roughness. In another study, Author examined nano-lubricants with vegetable oils as base fluid which are more preferable and studies are to be done to assess the performance of vegetable oils when nanoparticles are added to them (Koshy, Rajendrakumar, & Thottackkad, 2015). The surface geography and surface roughness investigations done by AFM and FESEM uncover 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 level are included to the lubricants with suitable surfactants. This was one of the effort made by them to introduce more environmental solution that lave great lubrication performances. Recently, Author gave a comprehensive review on nanolubricant with boron nitride as nanoparticles (Wan, Jin, Sun, & Ding, 2015). The result show that the nanolubricant with a little measure of boron nitride nanoparticles could display good tribological performance optimal concentration of nanoparticles was found to be around 0.1wt.%.

Heat transfer enhancement4.1 Using nanoparticles to improve heat transfer

When the heat transfer improve in certain refrigeration system, there are significant improvement in the performance of the system. In recent study (R. R. Kumar, Sridhar, & Narasimha, 2013) there are improvement of heat transfer when using Al2O3 as nanoparticles in refrigerants, also the power consumption decreases, this is because the improvement of heat transfer reduce the effort that needed to run the working fluid by the compressor, thus the system work more efficiently. There have been several studies in the literature reporting on possible heat transfer improvement when adding nanoparticles in base fluid. In recent years analyses have shown that nanofluids have a tendency to have generously higher warm conductivity than the base liquids (Pawel Keblinski et al., 2005). 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 thermodynamics properties and transport properties by changing the particles concentration,size, and shape resulting in higher heat transfer capability (Saidur, Kazi, et al., 2011).4.1.1 Thermal conductivity enhancement

Thre is countless applications that can benefit from a superior comprehension of the thermal conductivity improvement of nanofluids, including in refrigeration system. The first experiment to prove enhancement of thermal fluid of nanofluid was (Chol, 1995) where the author use alumina as nanoparticles. Al2Cu and Ag2Al nanoparticles was later use in the experiment, and it show some improvement of 50% to 150% of its thermal conductivity when dispersing the nanoparticles in water and ethylene glycol as base fluid (Chopkar, Kumar, Bhandari, Das, & Manna, 2007). In another study, Results from the study demonstrate the thermal conductivity increase with an increment in particle volume fraction and with a smaller in particle size. Moreover, the relative increase in thermal conductivity was observed to be more vital at higher temperatures (Mintsa, Roy, Nguyen, & Doucet, 2009).

However in several cases of study (Buongiorno et al., 2009; Putnam, Cahill, Braun, Ge, & Shimmin, 2006; Shalkevich et al., 2009; Turanov & Tolmachev, 2009); reported that there are little increase or no improvement in thermal conductivities of nanofluids. However that is case dependent, because maybe the nanoparticles have to be in the most stable configurration before it can give considerable improvement in thermal conductivity.4.1.2 ViscosityDespite the fact that the research on heat convection in nanofluids is restricted contrasted with that in thermal conductivity, the outcomes and methodologies in the field are very various and worth mentioning (Das et al., 2007). Tavman et al. (2010) researched TiO2, SiO2, and Al2O3 nanoparticles in water furthermore reported an increase in nanofluid viscosity with an increment in the nanoparticle concentration; they likewise demonstrated that established oempirical theories, for example, the Einstein model (Einstein, 1906) were not able to anticipate the right viscosity increment in nanofluids. Later in another study, the viscosity

showed a relationship between temperature of 10 °C 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 decreases significantly with the rise of temperature (Kole & Dey, 2011). Temperature variation of the nanofluid viscosity obtain in the study seem consisten with the modified Andrade equation, reported by Chen et al.4.1.3 Heat ConvectionIn spite of the fact that an improved thermal conductivity in nanofluids is an better element for application in heat transfer devices, it is not always sufficient condition. Actually, nanofluids also should be investigated for execution under convective modes. In this section, we will specificly discuss improvement of heat convection in nanofluids. The first known experiment of heat convection was by (Pak & Cho, 1998). On the other hand, the results showed that convective heat transfer coefficient of the dispersed fluids with submicron metallic oxide particles ?Al?_(2 ) O_3 at a volume concentration of 3% was 12% littler than that of base when analyzed under the state of consistent average velocity. Consequently, 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 execution. However in Xuan and Li (2003) study show that demonstrated an increment of as much as 40 % in the heat transfer coefficient of the nanofluid, at the same steady average speed. The principle diffrence between their experiment was in the decision of nanoparticles, which on account of Xuan and Li were 100­nm copper particles. The difference of their result was probably because the heat transfer coefficient must depend on the molecule volume fraction as well as on the molecule size and material. Again in (2007), Pak and Cho and others reseached instead of using Nusselt numbers, deal with dimensional heat transfer coeffficient that includes the thermal conductivity of the fluid. In (2005), Yang et al. discover that heat transfer coefficient was depend on nanoparticle concentration, material, temperature, and type base fluid. Daungthongsuk et al. and Kakac et al. critically review the heat convection in (Daungthongsuk & Wongwises, 2007; Kakac & Pramuanjaroenkij, 2009), there are focussing on the forced convective heat transfer in numerical and experimental of nanofluids. Although all above work was on forced heat convection, there are also research of natural convection of nanofluids. The first research about natural convection was done in 2003 (Putra, Roetzel, & Das, 2003). The 131.2­nm ?Al?_(2 ) O_3 nanoparticles and the other with 87.3­nm CuO particles was tested in water based fluid, they observed that as the nanoparticle volume fraction increases, the natural convective heat transfer in nanoflids is lower than pure water, and this situation was recorder higher in the CuO nanofluid than for ?Al?_(2 ) O_3 water nanofluid. Wen and Ding (2005) study TiO2�water based nanofluid where they found out that the decreasing effect of natural convection of nanofluids affected by 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. In 2011, an experiment was carried out by analyzing their heat transfer performance for single­ phase natural convection in bottom­heated enclosures, assuming that nanofluids behave like single phase fluids (Corcione, 2011). The results also show that the heat transfer improvement is maximum at an ideal molecule concentration and the maximum enhancement of heat transfer increases as the temperature increases.5.0 Nanofluids performance enhancement in heat exchanger.5.1 Introduction In refrigeration system, the system operates by taking advantages of the fact that high compressed fluids at a certain temperature will tend to get colder when they are allowed to expand. The transfer of heat between two or more fluids at different temperatures was done by heat exchanger equipment (Sundén & Wu, 2015). To reject heat and absorbs heat, the system uses two heat exchanger in order achieve ideal vapor-compression refrigeration cycle. The optimization of performance of nanofluids in heat exchanger will make the system run more efficient and more energy will the be saved. Early prediction predicticted that nanofluids have potential to enhance heat transfer coeffient and critical heat flux in pool boiling and

flow boiling.5.1 Performance of nanofluids in tubes and Heat exchangersAn extensive review have been reviewed about the characteristic of heat transfer in straight tubes, unfortunately there is still debates about anomalous heat transfer enhancement has been achieved (Dalkilic et al., 2012; Hussein, Sharma, Bakar, & Kadirgama, 2014). Statistically that the majority of the past studies demonstrated low heat transfer improvement; 11% of the specimen indicated decreasing of the heat transfer coefficient and 3% showed no enhancement by any means (Sergis & Hardalupas, 2011). 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 consistent 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 such cases as titania�water nanofluids in a channel, alumina�water nanofluids in a tube furthermore titania�water nanofluids in a tube (C. Yang, Li, Sano, Mochizuki, & Nakayama, 2013). In complex geometries such heat exchangers, helically coiled tubes, microchannels, and enhanced tubes, the topic on the heat transfer characteristic is still few of them. In 2012, review the important distributed articles on the improvement of the convection heat transfer in heat exchangers utilizing nanofluids on two points, focuses on use of nanofluids in different sorts of heat exchangers (Huminic & Huminic, 2012). In the year of 2013, an experiment was carried out to find the hydraulic and thermal performance of different nanofluids in a plate heat exchanger. The CeO2/water nanofluid seems to enhances the heat transfer performance of plate heat exchanger when an ideal molecule concentrations was achieved (Tiwari, Ghosh, & Sarkar, 2013). However in another study, demonstrated that there is no signifiant Nusselt number increase for the nanofluid with 2.0% molecule volume fraction taking into account the same Reynolds number, although for the nanofluid with 4.65% volume fraction, a reduction in the Nusselt number was reported, where the CuO/water nanoflids in a chevron-sort modern plate heat exchanger was tested (Taws, Nguyen, Galanis, & Gherasim, 2012). There are also reports that nanofluids enhance the performance of evaporators, one of the heat exchanger in refrigeration system. From the recent experiment, researchers reported that the boiling heat transfer coefficient and the enhancement of critical heat flux by nanofluids resulting from a thin porous nanoparticle deposition layer on the evaporator surface, which serves to enhance the wettability and capillarity of the boiling surface (Kim, 2009; Wen, Corr, Hu, & Lin, 2011; White, Shih, & Pipe, 2011). 6.0 Enhancement in refrigeration system performance.6.1 IntroductionThe coefficient of performance or COP of cooling in refrigeration system, is the ratio of the heat remove from the cold reservoir to input work. The best performance for refrigeration system is describes 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. 6.2 Energy performance improvement of refrigeration system using nanolubricantThere are lot of experiment to test the effectiveness of the nanofluid to achieve better performance in refrigeration system by dispersing nanoparticles into the refrigerant, lubricant or both of it. According to an investigation by (Wang, Wu, & Wu, 2010) where they use NiFe2O4 nanoparticles into naphthene based oil B32, and using R134a, R407C, R410a and R425a as refrigerant, the performance of residential air conditioners, energy efficiency ratio, EER increased 6% by replacing the Polyol-Easter oil VG 32 lubricant with the nanolubricant. In another study using nanolubricant, where the study explores the impact of dispersing a low concentration of TiO2 nanoparticles in the mineral oil based lubricant, on its viscosity and lubrication qualities, and in addition on the performance of refrigeration system utilizing R12 (Dichlorodifluoromethane) as the working liquid (Sabareesh et al., 2012). An improvement in the COP of the refrigeration system has been results from an ideal volume fraction , with low concentration of nanopar

ticles suspended in the mineral oil, and the compressor work reduced by about 11%, which ultimately resulted in the COP improvement of about 17%. In (2015), Lou et al. investigates the effectiveness of nanolubricant towards enhancing performances in domestic refrigerator. The study also show power utilization of the domestic refrigerator reduce about 4.55% when utilizing graphite nanolubricant with a mass division of 0.1%.6.3 Energy performance improvement of refrigeration system using nanorefrigerantAs already mentioned above, by dispersing nanoparticles int greatly enhance heat transfer, and achieve better system stability and this helps in energy saving. To prove that, in (2007) Shengshan & Lin has conducted experiment to investigates the refrigeration performance with R134a/TiO2 nano-refrigerants with different concentration 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 speed with an ideal TiO2 nano-particle concentration of 10 mg/L which decreases the energy used by 7.43%. The research study by Bi, Shengshan, et al. also being carried out, TiO2-R600a nano-refrigerants were utilized to test refrigerator performance by using energy consumption test and freeze capacity test (S. Bi et al., 2011). The outcomes demonstrate that TiO2-R600a nano-refrigerants work ordinarily and securely in the fridge. The fridge execution was superior than original R600a system, with 9.6% less energy utilized with 0.5 g/L TiO2-R600a nano-refrigerant. Later, Mahbubul, I. M., et al. investigates the relation between thermal performance and the increase of COP performance for certain refrigeration system (Mahbubul, Saadah, Saidur, Khairul, & Kamyar, 2015). The outcomes demonstrate that thermal conductivity, dynamic viscosity, and density of Al2O3/R-134a nanorefrigerant increase around 28.58%, 13.68%, and 11%, individually contrasted with the base refrigerant (R-134a) for the same temperature. Additionally, Al2O3/R-134a nanorefrigerant demonstrates the maximun 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 in this way gives a green and clean environment, with zero ozone depleting potential (ODP) and less GWP (D. S. Kumar & Elansezhian, 2014). Kumar et al. The performance of refrigeration was significantly enhanced with 21% less energy utilization when 0.5%v ZnO-R152a 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.7.0 Pressure drop characteristic of nanorefrigerantThe characteristic of refrigerant suspension may change when nanoparticles dispersed in it, this including the pressure drop characteristic. Liquid solid phase pressure drop attributes and liquid solid and vapor phase (phase change) pressure drop qualities of nanofluids are researched by many scientists. Before selecting any refrigerant, the effect of pressure need to be study carefully to ensure the nanorefrigerant stability for a longer period of time. Li and Kleinstreuer study the pressure drop of solid and liquid phase of fluid charateristic by simulation (Li & Kleinstreuer, 2008). According to them, they are two properties that affect the pressure drop which is density and viscosity. The addition of nanoparticles into base working fluid has made the nanofluid became higher density and higher viscosity, so this will increase the chances of pressure drop happen. Further research have been done to confirm this, viscosity of nanofluids is higher than basefluid, when Al2O3 nanofluids and ethylene glycol based ZnO nanofluids were tested (D. S. Kumar & Elansezhian, 2014; Yu, France, Choi, & Routbort, 2007). Namburu, Praveen K., et al in their numerical study also agree that above two causes of pressure drop properties proportional with nanoparticles volume fraction (Namburu, Das, Tanguturi, & Vajjha, 2009). In another study, reported that the same Reynolds number, single-phase pressure drop increase when nanoparticle concentration increases compared to pure fluids (Jaeseon Lee & Mudawar, 2007). Later in (2011) they found out that pressure drop increases significantly for =1 volume % concentrations of nanoparticles. Later,based on Mahbubul et al. analysis it was found that both heat transfer and pressure drop porperties increase with the increase of nanoparticle volume concentration. So, the improvemen

t performance of a refrigeration system must take account the correct optimum (Mahbubul, Saidur, & Amalina, 2013). concentration of nanoparticles of nanorefrigeration to obtain good pressure drop and heat transfer characteristic.