Exergy Analysis of Vapor Compression Cycle

Transcription

1 CHAPTER 4 Exergy Analysis of Vapor Compression Cycle 4.1 Introduction Refrigeration plays a very important role in industrial, domestic and commercial sectors for cooling, heating and food preserving applications. There are innumerable applications of such systems and they are the major consumer of electricity around the world. Energy consumption is directly proportional to the economic development of any nation, however this area is in great interest now because of increase in the cost of conventional fuels and environmental concerns globally. The scientists are looking for new and renewable sources of energy so as to minimize the costs. ue to the increasing energy demand, degradation of environment, global warming and depletion of ozone layer etc, there is urgent need of efficient energy utilization and waste heat recovery for useful applications. The researchers are concentrating on the alternate and environment friendly refrigerants, especially after the Kyoto and the Montreal protocols. However, in a quest to find out alternate and environment friendly refrigerants, the energy efficiency of the equipment having conventional refrigerants is also very important in the present age of competitive business community. The aim of the scientific community all over the world is to switch to new and renewable energy sources besides, efficient utilization of all conventional sources. Air conditioning bears a huge cost because thermal comfort is very essential as far as domestic and industrial sectors are concerned. The big challenge is to use less 102

2 energy for air conditioning applications in order to reduce the associated power consumption so as to make them more efficient and environmental friendly. The quantitative information is required to be obtained that will show the irreversibility of a process in all the components of any plant. For effective use and proper optimization, the detailed understanding of different thermodynamic processes in any conversion system is very important. In order to optimize their design, a thorough thermodynamic analysis is required. The analysis based on first law of thermodynamic is most commonly used in engineering applications, however, it is concerned only with law of conservation of energy and therefore it cannot show how and where irreversibility in the system or a process occurs. On the other hand, the analysis based on second law analysis is well known method being used to analyze all the thermodynamic cycles for better understanding and evaluation of irreversibility associated with any process. Unlike the first law (energy), the analysis based on second law analysis (exergy) determines the magnitude of irreversibility associated in a process qualitatively and thereby, provides an indication to point out the directions in which the engineers should concentrate more in order to improve the performance of this thermodynamic system [1-3]. Thus, the aim of second law based analysis is to determine the exergy losses and to enhance the performance by changing the design parameters and hence, to reduce the cost of the refrigeration cycle [4]. A lot of studies on the performance evaluation and optimization have been carried out experimentally and theoretically are available in the literature [5-10]. Most of the studies carried out so far on the refrigeration systems shows that the performance analysis of refrigeration systems were investigated based on first law of 103

3 thermodynamics. However, this approach is of limited use in view of the fact that the actual energetic losses are difficult to make out because the first law deals with the quantity of energy and not the quality of energy. In order to calculate the actual losses due to irreversibility in the process, exergy analysis based on second law of thermodynamics is the proper tool. Exergy analysis utilizes exergetic efficiency criterion taking into account all the losses appearing in a system, for measuring the actual performance. The exergy analysis is widely accepted as a useful tool in obtaining the improved understanding of the overall performance of any system and its components [11]. Exergy analysis also helps in taking account the important engineering decisions regarding design parameters of a system [12]. Many researchers have carried out exergy studies of different thermal energy conversion systems describing various approach for exergy analysis and its usefulness in a more simple and effective manner [13-23]. Padilla et al [13] carried out the exergy analysis and the impact of direct replacement of R12 with zeotropic mixture R413A. The performance of a domestic vapor compression refrigeration system originally designed to work with R12 was evaluated using a simulated modeling. They concluded that the overall energy and exergy performance of this system working with R413A is better than that of R12. Kumar et al [14] derived a method to carry out the exergetic analysis of a vapor compression refrigeration system using R11 and R12 as refrigerants. The procedure to calculate various losses as well as coefficient of performance and exergetic efficiency of the cycle has been explained by proper example. Arora and Kaushik [15] did a detailed exergy analysis of an actual vapor compression refrigeration cycle. They 104

4 developed a computational model to calculate the coefficient of performance, exergy destruction, exergy efficiency and the efficiency defects for R502, R404A and R507A for temperature in the range of -50 C to 0 C and condenser temperature range of 40 C to 55 C. They concluded that R507A is a better substitute to R502A than that of R404A. Nikolaidis and Probert [16] studied the behaviour of a two-stage compound compression cycle, with flash intercooling with R22 using the exergy method and gave some useful conclusions. incer [17] asserts that conventional energy analysis, based on the first law of thermodynamics, evaluates energy mainly on its quantity but analysis that are based on second law considers not only the quality of energy but also quantity of energy. In this study, the main objective is to investigate the performance of a simple VCR system based on exergy analysis. The experimental analysis has been done on a 2TR window air conditioning system using R-22 as refrigerant. With the objective to find out the losses at different operating conditions for vapor compression cycle, exergy analysis has been done by varying the quantity of refrigerant charge. The system has been modified for experimental study to find the possible design conditions with the minimum exergy destruction. In the present study, the effects of temperature changes in the condenser and evaporator on the plant s irreversibility rate was determined. It is observed that the greater is the temperature difference between the condenser and the environment or between the evaporator and the cold room, the higher is the irreversibility rate. The analysis is performed by doing energy and exergy balances for the system. The properties of refrigerant at each state point are calculated using Forane software [18] and the results are discussed. 105

5 The present investigation will help us to improve the understanding of vapor compression cycle and will enhance its efficiency. Further study on the design parameters, (external and internal) operating conditions is in process to obtain the optimum performance of the system Basic thermodynamic considerations First law of thermodynamics is used and is based on law of conservation of energy. This law only quantifies the energy at various components and tells nothing about how to reduce the energy consumption in these systems. In addition to this the second law of thermodynamics in its fundamental form is not used in energy utilization analysis and therefore, some of its derived principles are used in energy conservations. In the interest of energy conservation, minimization of irreversible effects is sought in any thermodynamic process. Any process that occurs without any change of initial state. A number of physical effects which cause a loss of energy available for doing work, or cause an increase in energy required to produce desired output are as follows: Temperature difference: Large temperature difference causes greater losses during heat transfer, therefore, temperature difference should be kept as small as practical e.g. in evaporators and condensers. Friction: Friction causes loss of useful energy and therefore should be minimized. For example, regular cleaning of tube walls prevents scale build-up. Thus fluid friction will be less and less energy will be lost in pumping power. 106

6 Rapid expansion: The adiabatic expansion of a high pressure fluid to a low pressure, sometimes called throttling, is a process that wastes some energy available in the high pressure fluid which could have been used to do some work. Mixing: Mixing of fluids will results in a loss of the useful available energy. The term entropy is a physical property of substances related to energy utilization and conservation. Entropy is a measure of the energy that is not available to do work. For a fluid with isentropic nature the amount of work done in compressor is minimum. A constant entropy (i.e. isentropic) process is an ideal reversible process which can never take place. Hence, in a real process where work is required, the entropy increases and efforts are made to minimize this increase Availability Availability is the maximum useful work that can be obtained from a system at a given state. A system is said to be in the dead state when it is in thermodynamic equilibrium with the surroundings. At the dead state, a system is at the temperature and pressure of its surroundings as well as in thermal, chemical and mechanical equilibrium. It has no kinetic or potential energy relative to its surroundings and does not reacts with the surroundings. A system has zero availability at the dead state. The notion that a system must go, the dead state at the end of the process to maximize the work output can be explained as follows: If the system temperature at the final state is greater than or less than the temperature of the surroundings, we can always produce additional work by running a heat engine between these two temperature levels. If the final velocity of the system is 107

7 not zero, we can catch that extra kinetic energy by a turbine and convert it to rotating shaft work and so on. However, no work can be produced from a system that is initially at the dead state. The atmosphere around us contains a tremendous amount of energy. However, the atmosphere is in the dead state and the energy it contains has no work potential therefore, we conclude that a system will deliver the maximum possible work as it undergoes a reversible process from the specified initial state to the state of its environment, the dead state. This represents the maximum work potential of the system at the specified state and it is called availability. It is important to realize that availability does not represent the amount of work that a work producing device will actually deliver upon installation. Rather it represents the upper limit on the amount of work a device can deliver without violating any thermodynamic laws. There will always be a difference, large or small between availability and actual work delivered by a device. This difference represents the scope for improvement. It may be noted that the availability of a system at a specified state depends on the conditions of the environment as well as the properties of the system. Therefore, availability is a property of the system-surroundings combination and not of the system alone. Altering the environment is another way of increasing availability but it is definitely not an easy alternative. Thus it would be desirable to have a property to enable us to determine the useful work potential of a given amount of energy at some specified state. The work potential of the energy contained in a system at a specified state is simply the maximum useful work that can be obtained from the system. The work done during a process depends on the initial state, final state and the process path. In an 108

8 availability analysis, the initial state is specified and thus it is not a variable. The work output is maximized when the process between the two specified states is executed in a reversible manner. Therefore all the irreversibilities are disregarded in determining the work potential. Finally the system must be in the dead state at the end of the process to maximize the work output Reversible work The availability analysis is useful for determining the useful work potential of the energy of a system at a specified state. It serves as a valuable tool in determining the quality of energy and comparing the work potential of different energy sources. The availability analysis alone however, is not of great value for studying energy devices operating between two fixed states. This is because in an Availability analysis, the final state is always assumed to be the dead state, which is hardly ever the case for actual thermodynamic systems. The adiabatic efficiencies are of limited use because the exit state of the model (isentropic process) is not the same as the actual exit state. There are two new quantities that are related to the exact initial and final states of the actual process. These two quantities are reversible work and irreversible work. They serve as valuable tools in the optimization studies of the components in complex thermodynamic systems. Reversible work (W rev ) is defined as the maximum amount of work that can be obtained as a system undergoes a process between the specified initial and final states. This is the useful work output (or input) obtained when the process between the initial and final states is executed in a totally reversible manner. That is, any heat 109

9 transfer between the system and the surroundings must take place reversibly, and no irreversibilities should be present within the system during the process. When the final state is the dead state, the reversible work equals availability Irreversibility For processes, that require work, reversible work represents the minimum amount of work necessary to carry out that process. Any difference between the reversible work W rev and the useful work W u is due to the irreversibilities present during the process and this difference is called irreversibility I, it is expressed as lost work potential i.e. I = W rev W u For a totally reversible process, the actual and reversible work terms are identical and thus irreversibility is zero. This is expected since totally reversible processes generate no entropy which is a measure of irreversibilities occurring during a process. For all actual (irreversible processes, irreversibility is a positive quality since the work term is positive and W rev > W u for work producing devices ad the work term is negative since W rev < W u for work consuming devices. Irreversibility can be used as the lost opportunity to do work. It represents the entropy that could have been converted to work. The smaller the irreversibility associated with a process, the greater the work that will be produced (or the smaller the work that will be consumed). To improve the performance of thermodynamic systems, the primary sources of irreversibility associated with each component in the system should be located and effort should be made to minimize them. 110

10 4.2 Energy conservation aspects in VCR systems Refrigeration and air conditioning system mainly consists of four major components, i.e. compressor, condenser, evaporator and throttling device along with piping network, suitable refrigerant and heat transfer liquids. It is imperative that system operating criteria should be analyzed completely to assure improvements in components, protective devices, advanced and complex thermodynamic cycle designs and proper working fluid selection. However, the working of these components has to be understood deeply in order to understand the working of the overall system and to look for the options of energy conservation and savings. A summary of cyclic effects that result in reduced energy consumption per unit of refrigeration capacity are described as follows: Energy conservation in evaporator The function of the evaporator is to cool the space by transferring heat into refrigerant in terms of its latent heat of vaporization. Evaporators may be of the types as, water-cooling, air-cooling shell and tube heat exchangers. It has been observed that when evaporator temperature changes from +5 to -20 C, the refrigeration tons drop to nearly one third its value and power/ton increases from 0.81 to 1.67 which is more than double. A high value of evaporator temperature will increase cooling output and decrease the input power and hence, will have a more pronounced effect on improving COP of the system. Performance of an actual practical machine also shows this effect very clearly. Thus, it is advantageous to operate machine at as high as evaporator temperature and as low as condensing temperature as possible. The 111

11 correct choice for designing an evaporator and their sizing is the most important factor for an efficient and economic refrigerating system. For energy conservation, promotion of increased heat transfer and maintaining of the highest reasonable evaporating temperature is required. It should be noted that under no circumstances these two factors are opposing. That is, one way of increasing evaporator heat transfer without decreasing evaporating temperature. Effectively this means that for a given heat transfer (refrigeration) capacity, the evaporating temperature is higher, and hence less power is used. The energy conservation and saving options in the use of evaporators are as follows: 1. Counter flow arrangement increases heat transfer over parallel flow, since mean effective temperature difference is greater. 2. A flooded evaporator surface is more effective than a dry expansion surface, which is only partially wetted. 3. Refrigerant sprays may increase surface wetting. 4. Maintenance of clean surfaces reduces the heat transfer coefficient. 5. Increased surface area, through use of fins/more tubing, increases heat transfer. 6. Having higher fluid velocities increase the heat transfer coefficient, in both the refrigerant and air or water side. In liquid chillers this is achieved by increasing the number of passes. 112

12 4.2.2 Energy conservation in compressor The compressor pumps refrigerant round the circuit as well as produces the required substantial increase in the pressure of the refrigerant i.e. its function is to raise the gas refrigerant pressure and temperature and they are of various types such as reciprocating, centrifugal, positive displacement, screw and fully or partially hermetically sealed. The desired operating conditions appropriate for energy conservation in the use of compressor are given as below: 1. Compressor capacity decreases with higher suction pressure and lower discharge pressure. 2. Compressor power requirement decreases with decreasing compression ratio and increases with greater clearance volume. 3. Speed control results in excellent reduction of power with capacity decrease. 4. Cylinder unloading provides good power reduction with capacity decrease. 5. Screw compressors have excellent full load (and part load with slide valve control) power requirements. 6. An economizer arrangement with screw compressors reduces energy requirements considerably. Using multiple compressors in a system can also save energy, since at partial load some of the compressors can be operated and at full load (generally the most efficient operating condition) while others are shut down. Of course this also results in the extra benefit of stand-by capacity in case of failure of a compressor. Energy conservation methods applied to centrifugal chillers have become an especially important 113

13 consideration because of the vast quantity of power used. These energy saving methods can be summarized as: 1. Multistage compression with the economizer cycle decreases power requirements over single-stage compression. 2. Variable inlet guide vane capacity control will result in good reduction of power with capacity reduction. 3. Increasing the number of water passes in the heat exchangers (evaporator and condenser) will reduce compressor power requirement. However, increased pumping cost must be balanced against this. 4. emand of limiting devices that limit the current drawn by the motor will reduce utility company demand charges. 5. Non condensed hot gas discharge temperature is high which can be used to recover heat energy Energy utilization in condensers Condenser s function is to reject the heat of condensation and change the state of the refrigerant from gas to a liquid. Condensers may be of the types such as, watercooled/air-cooled shell and tube and water-cooled evaporative condenser. In many applications of refrigeration, temperatures lower than necessary are specified. A low value of the condensing temperature can reduce the hot gas discharge temperature which is too high. esign and operation of condenser can contribute for the energy conservation in many ways as given below: 114

14 1. Maintain clean heat transfer surfaces, through proper water treatment and maintenance. 2. Compare the required power for air cooled condenser with that of either evaporative or water cooled condenser and then select the condenser design. 3. Select condensers with a large heat transfer surface area Sub-cooling of liquid before the expansion valve It is possible to sub-cool the condensed liquid after it comes out of the condenser. This sub-cooling can be provided by using the surplus cooling of the refrigerant gas which might otherwise be lost. When evaporator coils are defrosted by water sprays, the water of melted ice can be used for sub-cooling. The defrost water is also heated in the process. For short term increase of capacity, sub-cooling by liquid nitrogen can be used. In general, it can be said that sub-cooling improves the compressor efficiency, provided the heat given up during the sub-cooling leaves the system and does not re-enter. Following are some of the points relating to energy utilization and conservation as affected by flow control devices: 1. The thermostatic expansion valve (TEV) provides efficient use of the evaporator surface over a wide range of loads. 2. If there is a significant pressure drop in either the evaporative or the refrigerant distributor, an external equalizer type TEV must be used. 3. The internal equalizer valve will result in inefficient use of evaporator surface. If the system is subject to hunting a cross charge or other anti- hunting type TEV must be used. 115

15 4. The capillary tube is a less-energy-efficient flow control device than the TEV. However, for small capacity units, capillary tube is used. 5. The constant pressure expansion valve is a very inefficient flow control device if there is significant load variation Energy conservation in connecting pipe network Pipes are used to circulate refrigerant and cooling liquid through various components of the system. The major considerations in refrigerant piping that affect energy use are summarized as follows: 1. Suction and hot gas lines should be sized for reasonably low recommended pressure drops, since compressor power increases with pressure difference. 2. Pressure drop in the liquid line should be low enough to prevent flashing. 3. If a liquid-suction heat exchanger is used for sub-cooling, it will also improve compressor energy efficiency. 4. Excess oil in the system will coat heat transfer surfaces of the evaporator and condenser thereby reducing their performance. It is desirable to use recommended oil products for improved performance 4.3 Experimental setup and procedure The experimental setup used in the present study is a 2TR window air conditioner which was further modified to incorporate the compound gauges for temperature and pressure measurements at suction and discharge side. The main loop of the system under investigation is similar to that of a common VCR system and composed of four basic components only i.e. compressor, an evaporator, condenser 116

16 and an expansion device. The vapors from low temperature evaporator are sucked into the compressor and compressed to increase the pressure and temperature. The compression is assumed to be polytropic and the vapors are condensed in the air cooled finned condenser in which the fan is driven by an electric motor which is also driving the evaporator fan at other end. The speed of the condenser and evaporator fan is kept constant. The compressor is hermetic type reciprocating compressor with 2100 watts nominal input power at 220V (50Hz). The evaporator was designed for a cooling capacity of 24K BTU. The refrigerant is charged in four steps and the performance is evaluated at each stage. The unit also comprises of other devices such as filters and compressor protection device. The refrigerant volume flow rate was measured using a rotameter which is specifically calibrated for R-22. The temperature and pressure were measured by using compound gauges. The photographic view of the experimental set up and line diagram is shown in Fig.-4.1(a) & 4.1(b) and the specifications of the VCR system are given in Table

17 Fig.-4.1a: Experimental setup of 2 TR Window Air Conditioner based on VCR Fig.-4.1b: Typical diagram of a vapor compression cycle. 118

18 Table 4.1: Specification of Experimental Set-up Type Capacity Condenser Evaporator Expansion evice Compressor Window Air Conditioner 2 TR (24000 BTU) Finned Coils, Air Cooled Finned Coils Capillary Tube Hermetically Sealed, Reciprocating 4.4 Exergy Analysis Exergy analysis has two advantages over the conventional heat balance method for design and performance analysis of energy related systems. It provides a more accurate measurement of the actual inefficiencies in the system and the true location of these in efficiencies. In refrigeration cycle, with the heat balance analysis, it is not possible to find out the true losses. Exergy analysis is based on the assumption that there is an infinite equilibrium environment that ultimately surrounds all systems that are to be analyzed. The exergy or available energy of a system is the maximum work that could be derived if the system were allowed to come to equilibrium with the environment. It is a consequence of the second law of thermodynamics that the combined exergy of all systems can only decrease or remain unchanged. Unlike energy, exergy is not conserved, once it is lost, it is lost forever. In other words, exergy (quality) is degradable, while energy (quantity) is conserved. Exergy can be exchanged between systems, but if there are thermodynamic irreversibility s such as, friction or heat transfer with finite temperature differences, some of the potential for the production of work is destroyed. In all real processes, therefore, the total exergy of the system decreases [19-23]. 119

19 For a specified system boundary a clear distinction can be made between exergy destruction and exergy loss. Exergy loss is exergy that is passed on to some other system often the environment and which cannot be considered useful in the context of the purpose of the system. The term exergy destruction is used when the potential for the production of work is destroyed within the system boundary. The exergy of a system is a co-property of the system and the environment. In exergy analysis of compressors the environment consists of the local surroundings of the compressor. These local surroundings are modeled as being in equilibrium and infinite. Given sufficient information, the exergy of all the systems can be determined at any time. On the basis of first law, the performance of refrigeration cycle is based on the coefficient of performance, which is defined as the ratio of net refrigerating effect (cooling/heating load) obtained per unit of power consumed. It is expressed as: Q W e COP = (4.1) c Exergy balance for a control volume can be expressed as [2] EX [ in o ] out ( me ) ( me ) + Q(1 T / T) ( Q(1 T T ) = x x ( ± W in out 0 (4.2) / For the present system shown in Fig.4.1b, the component wise exergy balance equation can be written as below: a) Compressor ( EX ) comp = Ex 1 + Wc Ex2 = mr ( To ( s2 s1)) (4.3) b) Condenser ( EX ) cond = Ex2 Ex3 = mr ( h2 To s2) mr ( h3 To s3) (4.4) 120

20 c) Expansion evice ( EX ) exp = Ex3 Ex4 d) Evaporator = mr ( h3 Tos3 ) mr ( h4 To s4) = m T ( s 2 s )) (4.5) r ( o 1 ( 1 To Tr ) E 1 ( EX ) evap = Ex4 + Qe / x ( 1 T / T ) m ( h T ) = mr ( h4 Tos4 ) + Qe o r r 1 os1 (4.6) The total exergy destruction in the system is the sum of exergy destruction in different components of the system and is given by e) Total exergy destruction ( EX ) = ( EX ) + ( EX ) + ( EX ) + exp ( EX ) (4.7) total comp cond evap f) Exergy Efficiency Q e η exergy = ( 1 To / T (4.8) r Wc 4.5 Results and iscussions In order to have a comparative study of vapour compression cycle by varying the quantity of refrigerant charged, the real time data was measured and the calculations were made using Forane software [18]. The temperature, pressure and mass flow rate of the refrigerant was measured using compound gauges and rotameter respectively at different state points. The basic properties such as entropy and enthalpy of the refrigerant at different state points were calculated using Forane software [18]. The 121

21 coefficient of performance (COP), refrigerating effect, compressor work, exergy destruction and exergy efficiency are calculated using simple Excel sheet. A 2TR window air conditioner equipped with different pressure, temperature and flow measuring devices has been studied experimentally using energy and exergy analysis. The unit is charged with refrigerant R-22 in four steps i.e. 25%, 50%, 75% and 100% respectively and the system performance is analyzed in each case. The reference temperature is measured to be 25 o C. The coefficient of performance (COP), cooling load, exergy destruction, exergy efficiency has been evaluated against running time and evaporator temperature and the discussion of results is given below: It is observed from Tables ( ) that the evaporator temperature is least in case when the system is 100% charged but this increases pressure ratio of the compressor and hence compressor work also increases as a result the coefficient of performance goes down as can be seen from Fig Figure-4.2 also shows the comparison of coefficient of performance against time in all the cases and it is observed that the COP fluctuates with time having number of peaks after certain intervals except when the system is 75% charged. In the case when the system is 75% charged the COP has the highest peak in the middle of running time followed by small peaks on either sides. However, when the system is 25% charged, the COP first increases along the highest peak in the first few minutes and then decreases gradually with small ups and downs before finally reaching steady state condition in an hour s time. Figure 4.2 show that when the system is 50% charged, the COP initially drops and finally achieves a consistent value. After an hour of running the system, the steady state coefficient of 122

22 performance was observed in all the case and hence, the data after a certain period was not plotted as can be seen in Fig Coefficient of Performance Coefficient of Performance (25% charged) Coefficient of Performance (50%charged) Coefficient of Performance (75% charged) Coefficient of Performance (100% charged) Running Time/minutes Fig.-4.2: Variation of COP with respect to running time Table 4.2a: Parameters of the VCR system when the unit is 25% charged Time P disch. / kg/cm² P suc. / kg/cm² m/ kg/s Cond. In/ c -15 cm of Hg Evap. Inlet / c

23 Table 4.2b: Exergetic performance results of the VCR system when the unit is 25% charged RE/ kj W comp./ kj COP EX evap. EX comp. EX cond. EX exp. EX total Exergy Efficiency % Exergy Efficiency Table 4.3a: Parameters of the VCR system when the unit is 50% charged Time P disch. / kg/cm² P suc. / kg/cm² m/ kg/s Cond. In/ c Evap. Inlet / c

24 Table 4.3b: Exergetic performance results of the VCR system when the unit is 50% charged EX Exergy W comp. / EX EX EX EX Efficiency % Exergy RE/ kj kj COP evap. comp. cond. exp. total Efficiency Table 4.4a: Parameters of the VCR system when the unit is 75% charged Time P disch. / kg/cm² P suc. / kg/cm² m/ kg/s Cond. In/ c Evap. Inlet / c

25 Table 4.4b: Exergetic performance results of the VCR system when the unit is 75% charged EX Exergy RE/ W comp./ EX EX EX EX Efficiency % Exergy kj kj COP evap. comp. cond. exp. total Efficiency Table 4.5a: Parameters of the VCR system when the unit is 100% charged Time P disch. / kg/cm² P suc. / kg/cm² m/ kg/s Cond. In/ c Evap. Inlet/ c

26 Table 4.5b: Exergetic performance results of the VCR system when the unit is 100% charged RE /kj W comp. /k J COP EX evap. EX comp. EX cond. EX exp. EX total Exergy Efficiency % Exergy Efficiency Figures ( ), shows the exergy efficiency variation with respect to evaporator temperature. It is observed that the highest exergy efficiency is found when the system is 100% charged. This is due to the fact that, as observed the refrigerant temperature (evaporator) is minimum when the system is 100% charged and therefore the term Q 1 increases. The highest exergy efficiency is found to be 45.9% at an evaporator temperature of 7 o C when the system is 100% charged while the least exergy efficiency was found to be 3.5% at an evaporator temperature of 24.5 o C and 25% system charging. The reason for reduced exergy efficiency when the refrigerant is only 25% charged is because the refrigerant temperature is almost same as the reference temperature and hence exergy content is very small in the later case as compared to the former case. Thus the results are consistent with the hypothetical 127

27 observations. However, as far as energy analysis is concerned. This particular result is reverse as can be seen by Carnot cycle efficiency. TL COPC = ( T T ) (5.9) L H Exergy Efficiency /% Evaporator Temperature / o C Fig.-4.3: Variation of exergy efficiency with respect to evaporator temperature when 25% of the total refrigerant is charged 128

28 25 Exergy Efficiency/% Evaporator Temperature / o C Fig.-4.4: Variation of exergy efficiency with respect to evaporator temperature when 50% of the total refrigerant is charged Exergy Efficiency /% Evaporator temperature / o C Fig.-4.5: Variation of exergy efficiency with respect to evaporator temperature when 75% of the total refrigerant is charged 129

29 48 Exergy Efficiency /% Evaporator Temperature / o C Fig.-4.6: Variation of exergy efficiency with respect to evaporator temperature when 100% refrigerant is charged From Tables ( ) above, it is observed that the exergy destruction in the compressor is found to be the highest as compared to other components i.e. evaporator, condenser and expansion devices. This is attributed to the reciprocating compressor being used in the system and the losses are huge due to friction between the cylinder and piston rings. Although, lubricant is used to minimize these losses but still frictional losses are more prominent in reciprocating compressors. Another component of loss is due to the wire drawing effect at entry and exit through the valve plates which changes the entropy of the system and increases the irreversibility of the system. The exergy destruction in each case with respect to the percentage of refrigerant charged is given in the order as below: 130

30 For 25% refrigerant charged ( EX ) > ( EX ) > ( EX ) > exp ( EX ) comp evap cond For 50% refrigerant charged ( EX ) > ( EX ) exp > ( EX ) > ( EX ) comp evap cond For 75% refrigerant charged ( EX ) > ( EX ) > ( EX ) > exp ( EX ) comp cond evap For 100% refrigerant charged ( EX ) > ( EX ) exp > ( EX ) > ( EX ) comp evap cond The exergy destruction in the evaporator is found to be the least when the system is 75% charged. This is because of the higher evaporator temperature observed during the test and this reduces the term Q 1 in Eq. (4.6) The variation of total exergy destruction with respect to the evaporator temperature for four different cases viz. 25%, 50%, 75%, and 100% charging is shown in Figs. ( ) respectively. It is observed that the total exergy destruction is comparable when the system is 75% and 100% charged and it is least when the system is 25% charged. This is because the evaporator temperature is higher due to insufficient amount of charging and is comparable to the reference temperature. 131

31 Total Exergy estruction /kw Evaporator Temperature/ o C Fig.-4.7: Variation of total exergy destruction with respect to evaporator temperature when 25% of the total refrigerant is charged Total Exergy estruction /kw Evaporator Temperature / o C Fig.-4.8: Variation of total exergy destruction with respect to evaporator temperature when 50% of the total refrigerant is charged 132

32 Total Exergy estruction /kw Evaporator Temperature/ o C Fig.-4.9: Variation of total exergy destruction with respect to evaporator temperature when 75% of the total refrigerant is charged Total Exergy estruction /kw Evaporator Temperature / o C Fig.-4.10: Variation of total exergy destruction with respect to evaporator temperature when 100% of the total refrigerant is charged Tables ( ), also shows that the average coefficient of performance is highest when the system is 50% charged. This is because the refrigerating effect is 133

33 higher in this case and the compressor consumers less work. It is also observed from the tables that the exergy destruction is almost comparable when the system is 75% and 100% charged while it is least when the system is 25% charged, this is due to the fact that when the system is 25% charged, the evaporator temperature is higher and therefore the term Q 1 is significantly low. 5.6 Conclusions Exergy analysis is a technique to present the process and this further aid in reducing the thermodynamic losses occurring in the process. This is an important tool in explaining the various energy flows in a process and in the final run helps to reduce losses occurring in the system. In this experimental study, a window air conditioning system based on vapor compression cycle is modified for experimental analysis. The system comprises of four components i.e. compressor, a capillary tube (expansion device), a condenser and an evaporator and is having a cooling capacity of 24K BTU. Based on the experiment testing following conclusions are drawn: 1. Although the quantity of refrigerant charged do affect the exergy losses but the maximum losses in all the cases are in the compressor. This is attributed to the frictional losses and losses due to wire drawing effect during suction and delivery of the refrigerant. This will augment the study of tribology to exactly study the friction characteristics and also the design aspects needs to be improved to reduce the wire drawing effect to have efficient compressor. 134

34 2. It is observed that the total exergy destruction is comparable when the system is 75% and 100% charged and it is least when the system is 25% charged because the evaporator temperature is very close to the reference temperature. 3. The average coefficient of performance is highest when the system is 50% charged and this is because of higher refrigerating effect and reduced compressor work. 4. The exergy efficiency of the system varies from 3.5% to 45.9% which is mainly due to the variation of evaporator temperature. 5. The average values of the system exergy efficiency are more when the system is 100% charged. These values show that the overall exergy performance is better when the system is fully charged but the compressor work is the highest in this case and the COP is also less as compared to other situations. When the actual requirements are less the system should be operated with variable refrigerant flow so as to achieve optimum balance between the exergy efficiency and energy saving. 135

35 References 1. Aphornratna, S and Eames, I.W., Thermodynamic analysis of absorption refrigeration cycles using the second law of thermodynamics method, International Journal of Refrigeration,Vol.18(4), (1995), pp Izquierdo, M., Vega, M.., Lecuona, A., Rodriguez, P., Compressors driven by thermal solar energy: Entropy generated, exergy destroyed and exergetic efficiency, Solar Energy, Vol. 72(4), (2001), pp Kaushik, S.C., Solar refrigeration and air conditioning, ivya-jyoti parkashan, Gio-environ academia press, Jodhpur, (1989). 4. Kotas T.J., The exergy method of thermal plant analysis. London: Butterworth, (1985). 5. Halimic, E., Ross,., Agnew, B., Anderson, A., Potts I., A comparison of the operating performance of alternative refrigerants, Applied Thermal Engineering Vol.23,(2003), pp Spatz, M. W and Motta, S.F.Y., An evaluation of options for replacing HCFC-22 in medium temperature refrigeration systems, International Journal of Refrigeration, Vol.27, (2004), pp Xuan, Y. and Chen, G., Experimental study on HFC-161 mixture as an alternative refrigerant to R502, International Journal of Refrigeration, Vol.28, (2005), pp Fatouh, M and El Kafafy, M., Assessment of propane/commercial butane mixtures as possible alternatives to R134a in domestic refrigerators, Energy Conversion and Management, Vol. 47, (2006), pp

36 9. Han, X.H., Wang, Q., Zhu, Z.W., Chen, G.M., Cycle performance study on R32/R125/R161 as an alternative refrigerant to R407C, Applied Thermal Engineering, Vol.27,pp Mani, K and Selladurai, V., Experimental analysis of a new refrigerant mixture as drop in replacement for CFC12 and HFC134a, International Journal of Thermal Sciences, Vol.47, pp Bejan, A., Tsatsaronis, G., Moran, M., Thermal esign and Optimization. Wiley, New York, USA 12. Rosen, M.A., incer, I., Kanoglu, M., Role of exergy in increasing efficiency and sustainability and reducing environmental impact. Energy Policy, Vol.36, pp Padill, M., Revellin, R., Bonjour. J., Exergy analysis of R413Aas replacement of R12 in a domestic refrigeration system, Energy Conversion and Management vol. 51, (2010), pp Kumar. S., Prevost, M., Bugarel, R., Exergy analysis of a compression refrigeration system, Heat Recovery Systems & CHP vol. 9(2), (1989), pp Arora. A and Kaushik. S.C., Theoretical analysis of vapor compression refrigeration system with R502, R404A and R507A, International Journal of Refrigeration vol. 31,(2008), pp Nikolaidi, C and Probert,., Exergy-method analysis of a two stage vapor compression refrigeration plants performance, Applied Energy vol. 60,(1998), pp

37 17. incer, I.,Refrigeration Systems and Applications. Wiley, UK,(2003), pp Forane ver 4.0 : Refrigeration Simulator 19. Kaushik, S.C., Reddy, V.S., Tyagi, S.K., Energy and Exergy Analysis of Thermal Power Plants : A Review. Renewable and Sustainable Energy Reviews Vol.15, (2011) pp Tyagi, S.K., Park, S.R., Tyagi, V.V.,Anand, S., Second law based performance evaluation and parametric study of a sea water source cascade heat pump, International Journal of Exergy, Vol. 7(3), (2010), pp Tyagi, S.K., Kim, M.S., Park, S.R., Anand, S., Second law based performance of a modified VAC hybrid heat pump system using NH 3 -H 2 O as the working fluid, Indian Journal of Pure & Applied Physics, Vol.48, (2010), pp Tyagi, S.K., Wang, W., Kaushik, S.C., Singhal, M.K., Park, S.R., Exergy Analysis and Parametric Study of Concentrating Type Solar Collectors, International Journal of Thermal Science, Vol. 46, (2007), pp Kaushik, S.C., Singhal, M.K., Tyagi, S.K., Solar Collector Technologies for Power Generation and Space Air Conditioning Applications : A State of the Art, Internal Report, Centre for Energy Studies, Indian Institute of Technology, elhi, India, (2001). 138