Readdressing working fluid selection with a view to designing a variable geometry ejector

Transcription

1 International Journal of Low-Carbon Technologies Advance Access published May 2, 2013 *Corresponding author: Readdressing working fluid selection with a view to designing a variable geometry ejector... Szabolcs Varga *, Pedro S. Lebre and Armando C. Oliveira New Energy Technologies Unit, Institute of Mechanical Engineering, Faculty of Engineering, University of Porto, Rua Dr Roberto Frias, Porto , Portugal... Abstract A number of factors influence the performance of an ejector, e.g. working fluid, geometry and operating conditions. In the present work, six low-environmental-impact working fluids were evaluated for their use in an ejector cooling system running on low-temperature thermal energy. The numerical analysis was based on a model applying the 1D constant-pressure mixing theory. Ejector performance was assessed for the temperatures of the generator, evaporator and condenser in the range of C, 5 158C and C, respectively. The results indicated that owing to its high coefficient of performance and moderate operating pressures throughout the entire ejector cycle, isobutane is a good choice for a refrigerant. The area ratio required for running the ejector in critical mode, under changing operating conditions, varied in a significant range regardless of the selected refrigerant. This clearly indicates the importance of a variable geometry ejector design to strengthen the position of ejector cooling systems among other refrigeration technologies. Keywords: ejector system; working fluids; theoretical analysis; ejector performance Received 2 October 2012; revised 9 January 2013; accepted 2 April INTRODUCTION Air-conditioning applications have been responsible to a large extent for the increase in electricity consumption in most developed and developing countries during the last decade. To reverse this tendency, there is a strong need to develop reliable and costeffective cooling systems, running on low-grade thermal energy such as solar thermal. Among the existing technologies, absorption cooling is the most widely used, representing about two-thirds of the total number of installations [1]. A considerable barrier for a wider spread of absorption systems is their relative complexity because of the simultaneous mass and heat transfer processes in the absorber and desorber [2]. Single-effect absorption chillers typically require a heat source in the range of C and have a cooling cycle coefficient of performance (COP) of 0.6 [1]. A review of solar-driven sorption technologies can be found in [3]. Ejector cooling seems to be an attractive technology because of its structural simplicity, low capital cost and higher flexibility in terms of refrigerant selection. Although the COP of an ejector cycle is relatively low (typically lower than that of sorption systems), they require little maintenance and have a long lifespan. Therefore, with careful design and development, they could successfully compete with other cooling technologies. There are three interconnected factors with a strong influence on ejector performance: operating conditions, ejector geometry and working fluid properties. Over the last two decades, considerable research effort has been reported concerning the effects of these factors. A detailed review of the available literature on ejector applications can be found in [4] and on ejector modelling in [5]. Abdulateef et al. [6] summarised the recent progresses in ejector cooling in the context of systems driven by solar thermal energy. The influence of working conditions on ejector performance is relatively well established. Several experimental (e.g. [7 9]) and theoretical works have been carried out (e.g. [10 12]) to evaluate the effect of generator, evaporator and condenser conditions. Similarly, the effect of geometrical factors was studied both experimentally [9, 13, 14] and numerically [15]. In most works, the area ratio and nozzle exit position were identified as the most relevant variables influencing ejector performance indicators. As mentioned earlier, one of the advantages of ejector cooling is the relatively wide choice for the working fluid. Nevertheless, selecting the appropriate refrigerant is one of the most important steps in the design process [6]. Owing to its environmental benefits and availability, water has been widely used as working fluid since the early days of ejector cooling. However, as it will be shown later, its performance for low-temperature applications is not satisfactory and the ejector geometry using water is the most sensitive to any change in operating conditions. To improve cooling cycle performance, chlorofluorocarbon (CFC) and International Journal of Low-Carbon Technologies 2013, 0, 1 11 # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com doi: /ijlct/ctt026 1 of 11

2 S. Varga et al. hydrochlorofluorocarbon (HCFC) refrigerants have also been studied and tested in ejectors over the last few decades (e.g. [7] and [16, 17]); however, these refrigerants are subjected to total phase out in a scheduled time frame because of their negative environmental impact. More recently, a few experimental works have been published with the results of ejector performance using low-environmental-impact refrigerants other than water. Sankarlal and Mani [18, 19] presented their results on ejector cooling systems using ammonia. Relatively moderate COP values were obtained, although the applied generator temperatures were also low (62 728C). Eames et al. [20] reported a better ejector performance (COP ) for an ejector cooling system using R245fa as working fluid. Comparative studies of cooling performance using different refrigerants are limited to theoretical analyses, mostly because of the fact that different working fluids typically require very different ejector geometries. Sun [21] was among the first ones to carry out simulations for various working fluids operating under identical conditions. Eleven working fluids were investigated, seven of which belonged to the CFCs and HCFCs groups, subjected to complete phase out. It was concluded that the hydrofluorocarbon (HFC) refrigerant R152a resulted in the highest cooling cycle COP. Cizungu et al. [22] performed computer simulations in a relatively low generator temperature range (60 808C) with four different working fluids (R123, R134a, R152a and ammonia). It was concluded that the four fluids resulted in similar COPs, however requiring very different geometrical characteristics. Most recently, Roman and Hernandez [23] evaluated ejector performance for HFC (R152a and R134a) and hydrocarbon (R290, R600 and R600a) refrigerants. Although the simulated COP values seem to be somewhat ambitiously high, considering the operating condition range applied, the results clearly indicate that hydrocarbons are good alternatives to HFCs, when precautions are taken taking their flammability into consideration. Dahmani et al. [2] compared the performance of two HFC (R152a and R134a) and two hydrocarbon (R290, R600a) working fluids as a function of generator pressures. It was found that for low generator pressures, R600a provided the highest ejector performance. It was demonstrated by several authors that ejector geometry depends not only on the selected working fluid but also on the operating conditions (e.g. [13] and [15]). This led to the concept of a variable geometry ejector (VGE). Sun [24] was one of the first authors analysing geometrical requirements for use of water as a working fluid. Recently, Dennis and Garzoli [25] presented research results of a VGE using R141b under variable operating conditions. In both cases, the advantages of the concept were clearly demonstrated; however, no technical solutions were given. Potential technical implementations for controlling the area ratio between the constant area section and the primary nozzle throat section can be found in [26] for air, in [27] for carbon dioxide (R744) and in [28, 29] for water as the working fluid. The use of air as a working fluid has a very limited application in refrigeration. R744 has a very low critical temperature and high critical pressure; therefore, its use requires adequate technical features that would contribute to an increase in the initial cost of the cooling system. Using water as a refrigerant in an ejector cooling device is a possible choice; however, low saturation temperatures correspond to very low pressures, which may cause sealing problems, as well as low COPs for moderate generator temperatures. Thus, the objective of the present paper is to numerically assess and compare the performance of an ejector cycle using low-environmental-impact refrigerants for a range of operating conditions that would be applicable in an airconditioning system running on solar thermal energy (generator temperatures between 80 and 1208C). Special attention is given to the geometrical requirements of the ideal ejector under variable operating conditions as well as to the sensitivity of the area ratio for upstream and downstream flow conditions with these working fluids. This is important for the development of future VGEs capable of operating with the best performance over a range of conditions typical of solar-driven systems. 2 EJECTOR REFRIGERATION CYCLE, EJECTOR OPERATION AND PERFORMANCE A basic ejector refrigeration cycle consists of the following major components: a high-pressure generator, a low-pressure evaporator, an ejector, a condenser, a circulation pump and an expansion valve, as shown in Figure 1. The role of the generator is to provide sufficient thermal energy coming from an external source (e.g. solar thermal) for the working fluid to entrain and recompress the secondary stream coming from the evaporator, where the cooling effect takes place. To complete the cycle, the enthalpy of the working fluid at c is released into the environment in the condenser. After condensation, part of the refrigerant is returned to the generator by a pump, increasing its pressure to the generator pressure, and part of it is returned to the evaporator through an expansion valve. Figure 1. A schematic diagram of the ejector cooling cycle with its main components. 2 of 11 International Journal of Low-Carbon Technologies 2013, 0, 1 11

3 Readdressing working fluid selection The key component of the cycle is the ejector (see Figure 2). The motive ( primary) fluid coming from the generator (g) enters the primary nozzle at high pressure and low velocity. After expansion, it leaves the nozzle exit section at low pressure and typically at supersonic velocity. This draws the low-pressure (secondary) fluid from the evaporator (e) through the suction chamber. A shear layer between the motive and secondary fluids develops, and thus, the secondary fluid gets accelerated. Under normal operating conditions, the secondary fluid starts mixing with the primary flow after it gets chocked. This mixing process after the nozzle exit plane is rather complex because of the interaction between the two fluid streams and the ejector wall. The motive fluid flow can be characterised by a series of oblique/ normal shock waves called the shock train [30, 31]. During this process, the static pressure of the primary stream tends to increase gradually until it levels with the pressure of the secondary fluid. After the mixing process is completed, a final shock occurs somewhere in the constant area section or in the beginning of the diffuser depending on the operating conditions. The resulting flow becomes subsonic; the pressure is then further increased in the diffuser towards the ejector exit. The exit pressure is mostly determined by the condenser conditions (T c ) of the ejector refrigeration cycle. Ejector performance is often measured by the entrainment ratio (l), defined as: l ¼ _m e _m g For a given cooling load, the required evaporator flow rate is approximately constant. The higher the entrainment ratio, the lower the flow rate on the primary nozzle side, and consequently the lower the required generator energy input. The entrainment ratio is related to the COP of the cooling cycle by the following relationship: COP ¼ Q e Q g ¼ l ðh e h 2 Þ ðh g h 3 Þ According to the variables present in Equation (2), ejector performance is affected by both the working fluid properties and the operating conditions. In the following section, a simplified mathematical model is presented that allows the assessment of ejector performance on the basis of these variables. 3 CONSTANT-PRESSURE MIXING EJECTOR MODEL In the present paper, the analysis of the ejector performance is based on the mathematical model published by Huang et al. [10] and limited to ejector cooling systems where the operating fluid is at subcritical state corresponding to a configuration shown in Figure 1. To develop adequate steady-state gas dynamics relationships inside the ejector, the one-dimensional flow ð1þ ð2þ domain was divided into a number of physical and hypothetical sections including (see Figure 2) primary nozzle throat (d t ), primary nozzle exit (d t,ex ), hypothetical throat where the secondary fluid gets choked (y y), mixing plane (m) where the mixing process is completed, transversal shock wave section (s s) and diffuser outlet (c). Several simplifying assumptions were made, which can be summarised as follows: (1) The working fluid obeys ideal gas flow behaviour, except for its thermophysical properties. (2) The refrigerant is at saturation conditions at point e and has 58C superheat at point g (see Figure 2). (3) The pressure at point c is determined from the saturation curve, according to condenser temperature (T c ). (4) Negligible kinetic energy at inlet and outlet sections (g, e and c). (5) Negligible heat transfer to the environment through the ejector wall. (6) Isentropic efficiencies were used to account for irreversibilities in the primary nozzle (h t ), during the mixing process (h m ) and in the diffuser (h d ). (7) Secondary fluid reaches sonic velocity (critical point on ejector operating curve) at section y y. (8) The mixed stream undergoes a single transverse shock at s s, of zero thickness. For a detailed discussion of the complete set of conservation equations (mass, energy and momentum) and thermodynamic state equations, the reader is referred to the study by Huang et al. [10]. Here only some minor modifications are mentioned. In the mathematical model of Huang et al. [10], primary nozzle diameters (d t and d t,ex ) were inputs; however, in this work, they were calculated on the basis of the assumption that the pressure at the primary nozzle exit is approximately equal to the evaporator pressure. Also, the mixing efficiency (h m ) was treated differently by assuming that h m is given as the ratio of the kinetic energy of the mixed fluid and the kinetic energy of the mixed fluid under isentropic conditions at point m as [32]: h m ¼ v 2 m ð3þ v m;is Thus, applying the momentum conservation equation between sections y y and m, the velocity of the mixed fluid stream (v m ) can be obtained by the following equation: pffiffiffiffiffiffi h m v m ¼ðv pr;y þ lv sec;y Þ ð4þ 1 þ l The choice of isentropic efficiency values influences the prediction of the entrainment ratio to a high extent; therefore, they should be carefully selected. In many cases, they are simply chosen by minimising the error between experimental measurements and model results. Varga et al. [33] summarised isentropic efficiency data previously used in the literature and also International Journal of Low-Carbon Technologies 2013, 0, of 11

4 S. Varga et al. Figure 2. A cross-sectional view of a typical ejector. estimated values for different operating conditions in a steam ejector using CFD. On the basis of these results, the following isentropic efficiencies were applied: h t ¼ 0.95, h m ¼ 0.87 and h d ¼ In order to calculate ejector cycle COP, it was necessary to estimate the heat exchanged in the evaporator, condenser and generator (see Figure 1). In general, it was considered that the thermal and frictional losses in the connecting pipes were negligible. Condenser heat was estimated using the following equation: _Q c ¼ð_m g þ _m e Þðh 1 h c Þ In Equation (5), h 1 was determined from saturated liquid state corresponding to the condenser temperature. The expansion valve before the evaporator was assumed to be adiabatic (h 1 ¼ h 2 ); thus, the cooling capacity (or required secondary flow rate for given _Q e ) could be calculated using the following equation: _Q e ¼ _m e ðh e h 2 Þ Finally, the required generator input was estimated as: _Q g ¼ _m g ðh g h 3 Þ In Equation (7), the enthalpy at the generator outlet (h g )was determined from the fluid properties applying 58C superheating, while at the inlet (h 3 ), it was calculated from the pumping power as: h 3 ¼ h 1 þ _ W b _m g In Equation (8), _W b is the power required to increase the fluid pressure from p c to the generator pressure, given by the following equation: _W b ¼ _m gð p g p c Þ r In Equation (9), fluid density was determined according to the condenser temperature. Both the ejector and the cooling cycle ð5þ ð6þ ð7þ ð8þ ð9þ equations were implemented and solved with the EES (F-CHART, USA) software. EES is a computational tool that enables finding the solution of simultaneous non-linear equations and has the advantage of providing internal functions for the thermodynamic properties of many fluids, including the refrigerants investigated in this work. Typical inputs for the analysis were the refrigerant type; generator, evaporator and condenser temperatures (boundary conditions for the model); and cooling capacity. Typical outputs of the simulations were the ejector cycle performance indicators (l, COP) and the geometrical factors that allow the ejector to work under critical operation, including the diameters of the nozzle throat (d t ) and constant area section (d m ), as well as the corresponding area ratio [r A ¼ (d m /d t ) 2 ]. 4 REFRIGERANT SELECTION An ideal working fluid for ejector cooling should have the following thermodynamic properties: - high latent heat values in the evaporator and generator temperature range; - relatively high critical temperature; - not too high a saturation pressure in the generator and not too low in the evaporator; - reasonably low specific volume in vapour state; - should be a dry refrigerant ( positive slope of saturated vapour line). Besides suitable thermophysical properties, the working fluid should not be toxic, highly flammable and harmful to the environment. There are also additional considerations to be taken into account, such as low cost, availability and high compatibility with equipment materials. Consequently, the ideal refrigerant for ejector cooling has not been identified yet. The criteria (COP and entrainment ratio) for refrigerant selection have to be analysed in the context of the system operating conditions. For a solar-driven system, three temperature/ pressure levels are important for the ejector cycle. On the generator side, the temperature level is limited by the solar circuit 4 of 11 International Journal of Low-Carbon Technologies 2013, 0, 1 11

5 Readdressing working fluid selection Table 1. Summary of previously studied refrigerants for ejector cooling. Working fluid Type Reference Critical temperature (8C) Saturation pressure (abs, kpa) T e ¼ 108C T g ¼ 908C R11 CFC [21] R113 CFC [34] R12 CFC [21] R500 CFC [21] R123 HCFC [21, 22] R141b HCFC [10] R142b HCFC [16, 21] R134a HFC [21, 23] R152a HFC [21, 23] R600 HC [23] R600a HC [23] R290 HC [23] RC318 [21] R717 [22] Water [21] Note: the indicated list is incomplete. performance depending on collector type and solar radiation intensity, varying along time. On the condenser side, temperature/ pressure is determined by ambient temperature values that are also time dependent. Thus, in this study, T g was selected in the range of 80 to 1208C, resulting from the use of moderate-cost evacuated tube solar collectors. On the downstream side of the ejector (condenser), boundary conditions were considered in a wide range of 24 to 408C that may occur using either active or passive condensers. On the evaporator side, typical airconditioning design temperature values were considered, between 5 and 158C. As mentioned in the Introduction section, a number of refrigerants have been investigated over the last two decades. A summary of these refrigerants with their most important characteristics is shown in Table 1. In this work, CFC and HCFC working fluids were excluded from the analysis, since they are subjected to phase out. Also, ammonia requires robust construction and components (heat exchangers and pump) due to high saturation pressures at low temperatures; therefore, it was also not considered here. R600 and R600a are refrigerants of similar properties (isomers). Preliminary simulations indicated very small differences in terms of ejector performance between these two working fluids. Owing to its lower cost, only isobutane (R600a) was selected for further discussion, and R600 was excluded from the analysis. The remaining six refrigerants that were included in the simulations are listed in Table 2, with some relevant properties. It can be seen that water was included in the list, because it is a widely available, low-cost and environmentally friendly fluid. Nevertheless, it also possesses some negative properties such as high specific volume in vapour phase and low saturation pressure at evaporator conditions. Both R134a and R152a are Table 2. Characteristics of the selected working fluids. Working fluid ODP a GWP b Security c Molecular mass (kg/kmol) hydrofluorcarbon refrigerants that require some special considerations regarding their toxicity and global warming potential. In addition, they both have negative slope saturation vapour line (wet fluids). The principal advantage of these HFCs lies in their relatively high latent heat of vaporisation. RC318 is a cyclic, nontoxic, non-flammable organic working fluid having attractive saturation pressures for the operating conditions that could be expected in an ejector cycle running on solar power. The last two refrigerants (R290 and R600a) belong to the hydrocarbons group. R290 has the lowest critical temperature (96.78C) and highest vapour pressure, and thus requires the most robust construction to assure structural integrity. Both R290 and R600a are environmentally friendly refrigerants, the latter having the advantage of being a dry fluid, and therefore, erosive condensation does not occur in the primary stream inside the ejector. They both are flammable, and thus special attention should be paid to avoid leakage. 5 RESULTS AND DISCUSSION Ratio of latent heats at 108C and 908C SVL d Water 0 A Negative R A Negative R134a A Negative RC A Positive R152a A Negative R600a 0 20 A Positive a Ozone depletion potential relative to R11. b Global warming potential relative to CO 2, using an integration time of 100 y. c ASHRAE classification. Toxicity is represented by the capital letter while flammability is represented by the number. Group A1 is the least hazardous and B3 the most hazardous. d Slope of the saturated vapour line on a T-s diagram. For validating the mathematical model, experimental data of Cizungu et al. [22] and Yapici et al. [7] were compared with simulation results obtained by the present model. In the first case, R11, and in the second case, R123 were used as working fluids. The validation results are summarised in Table 3. It can be seen that the model provided good accuracy in estimating the COP for both working fluids. The highest relative error, 8%, was calculated for R11, with an average error of 3.2%. In the case of R123, the average prediction error was as low as 2.5%. Yapici et al. [7] also presented the optimal r A for different generator temperatures based on their experimental measurements. The data were compared with the area ratio calculated by the present model. It was found that the largest difference was 10%, with an average of 8%. In conclusion, the mathematical model developed was considered to be adequate for predicting ejector performance with an acceptable accuracy. International Journal of Low-Carbon Technologies 2013, 0, of 11

6 S. Varga et al. Table 3. Comparison of simulated and experimental COPs (experimental results of Cizungu et al. [22] and Yapici et al. [7]). Cizungu et al. [22], R11, T e ¼ 8.88C, T c ¼ 27.78C T g, 8C COP[22] COP relative error, % Yapici et al. [7], R123, T e ¼ 108C, T c ¼ 33.18C T g, 8C COP[7] COP relative error, % The entrainment ratio as a function of generator temperature (for constant T c and T e ) is shown in Figure 3, for the six selected working fluids presented in Section 4. A horizontal line is also depicted for an entrainment ratio of 0.2, as the minimal acceptable performance considered by the authors. As expected, the entrainment ratio increased with generator temperature, independently of the working fluid used. However, it is obvious from Figure 3 that water performed considerably worse than any of the other refrigerants. For example, for low generator temperatures (T g ¼ 808C), the entrainment ratio for R600a was 5 times larger than that for water. An entrainment ratio l of 0.2 was obtained for T g as high as 1108C, while for all the other refrigerants, the ejector performed better in the entire range of generator temperatures considered in this work. It is also clear from Figure 3 that up to 888C, all working fluids resulted in similar entrainment ratios (approximately 0.32 within 7% for T g ¼ 888C). Beyond this temperature, the refrigerants having relatively low T cr (e.g. R290) started to perform worse than those having a higher one (e.g. R152a). Based on the results for the entrainment ratio, R600a seems to be a good choice for the working fluid in the generator temperature range presented. A slightly different behaviour can be observed in Figure 4, where the variation of COP with T g is presented for the selected refrigerants. As before, water seems to perform significantly poorer than the other five refrigerants. It required a generator temperature of 1158C for the cycle to operate at a COP of 0.2, which is 308C higher than in the case of the other fluids. All the other refrigerants showed a significantly better performance; however, the dispersion of the COP at constant T g increased considerably because of the difference in their latent heats. At a generator temperature of 808C, the difference between the COP obtained for RC318 (0.15) and for R152a (0.21) was as high as 30%. To work at the same performance, RC318 required a generator temperature of 88C higher than R152a. Again, considering the lower limit for an acceptable ejector cooling performance to be COP ¼ 0.2, working fluids R152a, R134a, R290 and R600a reached this limit for a T g of 858C. Having an ejector cycle operating at relatively low generator temperatures is of great importance when the system is running on solar thermal energy. Collector efficiency decreases with average collector temperature, which negatively influences the overall system performance. The dependence of ejector performance on evaporator temperature is well known, that is, both l and COP increase with T e. The relative order of the different working fluids remained practically the same over the range of the evaporator temperatures considered (5 158C) while keeping the generator (908C) and condenser (358C) conditions constant as shown in Figure 5. R152a had the highest COP, while water had the lowest. For water, the minimum evaporator temperature required to provide COP ¼ 0.2 was 138C, while all the other fluids resulted in a better cooling performance below 108C (typical design value in air conditioning). The influence of condenser temperature on cooling cycle COP for constant T g and T e is shown in Figure 6. An increase in T c rapidly decreased the ejector performance. This decrease was most evident in the case of water. For instance, changing the condenser temperature from 308C to368c decreased COP nearly four times (from 0.27 to 0.07). For the other fluids, this dependence was somewhat less strong, but still very significant, varying in the range of 53% (RC318) to 61% (R290). It is also clear from the figure that although at very low T c ( 248C) an ejector using water as working fluid performed comparably with the other refrigerants, for any condenser temperature over 328C, the COP dropped below 0.2. In hot climates, a T c over 328C can often be expected in cooling systems using a passive condenser. The other refrigerants provided acceptable performance up to 358C (RC318) and 398C (R152a). Among all the working fluids, R152a operated at the highest COP over the range of condenser temperatures considered, followed by R134a. In the previously presented analysis, ejector dimensions were determined under optimal (design) operation, that is, when the secondary fluid reaches sonic velocity in the mixing section (double choking). The higher the sensitivity of the optimal dimensions on operating conditions, the worse its performance under varying upstream and downstream temperatures/pressures, and thus the higher the benefit of developing a VGE. For example, if an ejector with a fixed geometry is designed to operate at T g ¼ 908C, any increase in generator temperature would increase the primary flow rate and the under expansion of the flow at the nozzle exit section, consequently contributing to a decrease in the entrainment ratio. In contrast, any decrease in T g would result in insufficient momentum transfer between the two streams and thus in a significant decrease in ejector performance [7]. Besides operating conditions, ejector geometry also depends on system cooling capacity. To make the results more general and independent of cooling capacity, ejectors can be characterised by the area ratio (r A ). Figure 7 represents the influence of generator temperature on r A for the six working fluids. It can be seen that r A was considerably larger for water than for the other refrigerants; for an easier interpretation, the results for water are presented on a secondary axis. It was found 6 of 11 International Journal of Low-Carbon Technologies 2013, 0, 1 11

7 Readdressing working fluid selection Figure 3. Entrainment ratio as a function of generator temperature for the six selected working fluids and for constant T e ¼ 108C and T c ¼ 358C. Figure 4. COP as a function of generator temperature for the six selected working fluids and with T e ¼ 108CandT c ¼ 358C. Figure 5. COP as a function of evaporator temperature for constant T g ¼ 908C and T c ¼ 358C. International Journal of Low-Carbon Technologies 2013, 0, of 11

8 S. Varga et al. Figure 6. COP as a function of condenser temperature for constant T e ¼ 108CandT g ¼ 908C. Figure 7. The influence of the generator temperature on r A with T e ¼ 108CandT c ¼ 358C. that the area ratio increased with the generator temperature, as shown in Figure 7. The sensitivity of optimal r A on T g was most pronounced for water, corresponding to an increase of more than six times over the temperature range of C, or an average of 11.9%(8C) 21. The lowest sensitivity was observed for R290, resulting in a variation of 1.9%(8C) 21, which is still considerable when a few degrees of variation are allowed in the generator. All the other fluids resulted in sensitivities between those for R290 and water. R152a had the highest COP (see Figure 4) and showed a variation of 31.9% between generator temperatures of 848C and 968C. Therefore, it can be concluded that although the extent of the dependence of the area ratio on generator temperature was different for the different fluids, in all cases using a variable area ratio ejector would clearly be advantageous when variations in T g occur during system operation. The influence of evaporator temperature on r A is shown in Figure 8. The area ratio increased almost linearly with T e in the range of 5 158C. This variation was much smaller than that in the case of T g, 1%(8C) 21, except for water. Such as before, an ejector using water required very different area ratios for a small change in T e. The sensitivity was as high as 4.8% (8C) 21. In the simulations, ejector outlet pressure was calculated from condenser temperature, according to saturation conditions, and thus condenser temperature directly influenced the area ratio. The results of r A for varying T c in the range of C are shown in Figure 9. The temperature range was selected sufficiently wide, so that it would cover most situations occurring in air-conditioning systems using either a passive or an active condenser. The data presented in the figure were determined according to the critical operating point on the ejector 8 of 11 International Journal of Low-Carbon Technologies 2013, 0, 1 11

9 Readdressing working fluid selection Figure 8. The influence of the evaporator temperature on r A with T g ¼ 908CandT c ¼ 358C. Figure 9. The influence of the condenser conditions on area ratio with T g ¼ 908CandT e ¼ 108C. operating curve for constant generator and evaporator temperatures, that is P c ¼ P cr. Please note that in this case the critical point is not a thermodynamic state: it is defined as the highest back pressure for which the entrainment ratio is independent of p c. For more details on the critical point on the ejector operating curve, the reader is referred to the study by Varga et al. [35]. It can be seen from Figure 9 that the area ratio required for the ejector to operate in critical mode decreased with T c in a nonlinear manner. The sensitivity of r A on the downstream conditions was again highest for water. The results indicated that a one-degree increase in the condenser decreased the corresponding area ratio of 5.3% on average. The ejector using R290 was the least sensitive to condenser temperature. The estimated sensitivity was 23.2%(8C) 21, a value considerably higher than the sensitivity calculated for the generator temperature (1.9%(8C) 21 ). Considering the remaining four refrigerants, it was found that the sensitivities were very close to each other: 23.4%(8C) 21 for R134a; 23.5%(8C) 21 for R152a and R600a; and 23.7% for RC318. An air-conditioning system using a passive condenser is affected by climatic conditions and is therefore likely to be subjected to a variable T c. Thus, it can be concluded that ejector cooling cycle performance would clearly benefit from a variable geometry device, in order to bring the operation of the ejector to near-critical mode. 6 CONCLUSIONS A mathematical model was developed to evaluate the performance of an ejector cooling cycle for a range of operating conditions adequate for an air-conditioning system using solar thermal energy as heat source. The model was based on the constant-pressure mixing theory and suitable to assess ejector performance for different working fluids, using the physical property libraries of the EES software. In this work, six low-environmental-impact refrigerants were studied (water, International Journal of Low-Carbon Technologies 2013, 0, of 11

10 S. Varga et al. R152a, R134a, R290, R600a and RC318), because of their high potential for the ejector cooling. It was found that the entrainment ratio and COP increased with generator and evaporator temperatures and decreased with condenser temperature. Despite being cheap and widely available, water performed considerably worse than any of the other refrigerants in the entire range of operating conditions considered. Additionally, water has very low saturation pressures in the evaporator (1 to 2 kpa) and the condenser (3 to 6 kpa), which may cause technical problems to vacuum tight connections. All the other refrigerants showed similar entrainment ratios for relatively low generator temperatures (,888C). In terms of COP, R152a performed the best, for the applied range of operating conditions, except for generator temperatures near T cr (113.38C). Using R290 and R134a resulted in relatively high COPs for low generator temperatures (,958C); however, application of these working fluids (including R152a) may require robust constructions and special equipments, because of the high vapour pressures in the generator ( 30 bar) and the condenser. RC318 and R600a have moderate vapour pressures in the entire range of operating conditions; however, RC318 also resulted in the lowest COP among the working fluids other than water. It was found that an ejector using R600a can operate with good performance for a wide range of operating conditions under moderate pressures and thus can be an attractive choice for an air conditioner running on solar thermal energy, as long as special attention is paid to the fluid flammability. The sensitivity of ejector dimensions to upstream and downstream temperatures/pressures was analysed by calculating the area ratio required to allow the ejector to operate in critical mode (choked secondary stream). The results indicated that r A increased with T g and T e and decreased with T c for all six refrigerants. A water-working ejector was the most sensitive to variable operating conditions. For instance, changing the generator temperature by 18C resulted in a variation in r A as high as 11.9% (on average). This also indicates that a steam ejector is likely to fail in systems subjected to variable operating conditions. R290 resulted in the lowest sensitivity of the area ratio to operating conditions. The results showed that a 18C change in the evaporator, generator and condenser temperatures resulted in a change in r A of 1, 1.9 and 23.2%, respectively. This sensitivity can be considered still high, especially for the variations in generator and condenser temperatures, which clearly indicates the urge to develop a VGE that can operate at high performance under such circumstances. The cycle performance results, expressed by COP and the entrainment ratio, do not differ much for the different refrigerants analysed, under the range of operating conditions considered (except for water). The most important conclusion of this work is that regardless the choice of environmentally friendly refrigerant, it is fundamental to implement VGEs for solar-driven systems. This would decisively contribute to a wide dissemination of cost-effective ejector cooling systems running on solar thermal energy. Based on these conclusions, future work by the authors will concentrate on the development of VGEs. ACKNOWLEDGEMENTS The present work was developed within the framework of the Investigation into an Improved Ejector for Variable Operating Conditions research project.the authors acknowledge the financial support of Fundação para a Ciência e a Tecnologia (FCT) through contract PTDC/EME-MFE/113007/2009 and also thank all the partners involved in the project. 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