International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 7, July 2015)
Solar Refrigeration Absorption System Mahavir Anurup Bhattacharya1, Kashinath Nimba Patil2 1
2
PG scholar, Associate Professor, Department of Mechanical Engineering, K.J. Somaiya College of Engineering, Vidyavihar (E), Mumbai-400 077, Maharashtra, India The charts were plotted by varying the generator and evaporator temperatures for each set of readings. In 2010, many researchers had performed works in this field. B. Kundu et al. had performed a work in which collector efficiencies, COPs and overall efficiencies plotted against collector inlet water temperatures for solar vapour absorption system with different absorber plate profiles. Chan Woo Park and Yong Tae Kang studied the effects of UA ratios and chilled water inlet temperature on the evaporator pressure and COP independently. Jawahar et al. performed thermodynamic studies on a system using pinch technology. Satish Raghuvanshi and Govind Maheshwari, in 2011, studied the variations in COP with corresponding variations in heat exchanger effectiveness and temperatures of generator, absorber, evaporator and heating water inlet to generator. Mariappan et al., in 2012, conducted a thermodynamic analysis and studied the effect of generator temperature on COP, generator energy, refrigerant concentration, circulation ratio, mass flow rate and heat duty. In the same year, V. K. Bajpai had also presented a designing procedure for a solar powered absorption system. In 2013, there were many works, out of which, four were extensively studied. Cuenca et al. studied the effect of ammonia mass fraction on the thermal conductivity. Emin Acikkalp studied the effects of absorber, evaporator, generator and condenser temperatures and some constants against exergy destruction function and COP. The plots for COP were studied in this work. M. Meza et al. studied the influence of absorber mass flux, absorber temperature, generator temperature and generator temperature increments on the absorber heat flux. Also studied the effects of change in generator temperature increments on the generator temperature and COP. Vazhappilly et al. presented a modeling and experimental analysis of the generator. Then in 2014, many studies were conducted in the same domain. A. Acuna et al. studied the influence of generator temperature on COP, collector efficiency and system efficiency for different types of solar collectors. Farshi et al. studied the COP and exergetic efficiencies against the generator temperature with different refrigerant absorber combinations and different ejector dimensions.
Abstract-- Currently significant attention is focused on solar based absorption systems due to increasing demand from the refrigeration industry. Large electric power is used for refrigeration applications. Current attempt is to analysis the solar absorption system. The absorption system performance depends on various parameters are, Generator, Absorber, Condenser and Evaporator Temperatures (Tg, Ta, Tc, Te), Condenser and Evaporator Pressures (Pc, Pe), Effectiveness of Solution Heat Exchanger (ε), Mass Flow Rate of Water Through the Generator, Absorber, Condenser and Evaporator (Mwg, Mwa, Mwc, Mwe) and Heat Flux Through the Generator, Absorber, Condenser and Evaporator (Qg, Qa, Qc, Qe). The maximum COP obtained is 0.75 for FPC, 0.81 for ETC & 0.73 for CPC. It is also observed that the rise in the evaporator temperature reduces the difference in temperature between the load and the evaporator, thus reducing the energy duty and eventually raising the COP. Keywords-- Solar Energy, Absorption Refrigeration, Water Ammonia Systems, Solar Refrigeration.
I. INTRODUCTION For over two centuries, many studies and researches have been conducted so as to bring an improvement in artificial cooling methods. This has resulted in the development of the refrigeration and air conditioning systems that we are using today. Many scientists and researchers have, in different ways, studied the systems and have made their own contribution in the literature. Out of this prolific literature, we have studied only those papers which are more recent and whose subject is related to this study. All these papers show the effects of some parameter/s on the COP of the system. A brief chronological outline of the same is given below: Hans-Martin Hellmann, in 2002, studied the effect of the condenser temperature on the carnot COP for systems working between different temperature levels. Jose´ Ferna´ndez Seara and Jaime Sieres, then, in 2006, presented the effects of refrigerant concentration on refrigerating effect and COP. They also studied the effect of blow-down fraction, evaporator pressure and reflux ratio on the COP. A. Ramesh kumar and M. Udaya kumar, in 2008, studied the corresponding change in COP and heat duty for a change in the pressure ratio and condenser temperature for a GAXAC cycle. 159
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 7, July 2015) The entropies, exergies, pressures and temperatures at each point are also evaluated the EES. This data was helpful for the system designing. Kim et al. studied the effects of generator temperature on circulation ratio, exergetic efficiency and the COP. This was done by both the first and second law analysis. They also assessed pinch point characteristics of heat exchangers and condensers and plotted the corresponding charts. López-Villada et al. performed overall thermodynamic modeling and analysis. Plotted chilled water temperature characteristics against 1st law efficiency and power for different cycles. Shaarawi et al. performed a comparative analysis between onstant pressure and constant temperature system. Plotted the effects of change in generator temperature on the energy capacities of all the components. Urueta et al. performed exergy and energy analysis and studied the influence of generator temperature on exergy efficiency and COP under varying evaporation temperatures and for different components. Zeyu Li et al. solar CPC collector efficiency over a year's insolation data is plotted at different conditions against the colletor inlet temperature. The evaporation and condensation temperatures are varied and the efficiencies and COPs are plotted. It is apparent that much work has been conducted in this field as pertains to maximizing the COP by optimizing various parameters. In the above studies, the COP and refrigerating system efficiencies are plotted against many significant factors such as the pressures, component temperatures, heat exchanger efficiencies, mass fractions, water mass flow rates and heat transfer in the components. However, the following gaps were discovered: In every paper, the effects of only a part of the above parameters on the system COP are studied. The combined effect of all these parameters on the COP in any single paper is missing since the objective of each paper was to study only those particular effects. The effects of the heat transfer in all the four components i.e. the generator, the absorber, the evaporator and the condenser, on the COP of the system, is not studied in any of the above papers. The effects of the water mass flow rates in the generator, evaporator, absorber and condenser on the COP have not been thoroughly studied. In the present work, a study of some of the above mentioned effects have been attempted. The variations in the COP with respect to all the above mentioned system parameters and conditions are studied. Moreover, the variation of the COP with the heat exchanger effectiveness is also studied. But the most important highlight of this study is that the heat exchange values at the four components are plotted against the COP.
Analysis is carried with the help of the data reported elsewhere [A. Acuna et. Al., 2014, Satish Raghuvanshi and Govind Maheshwar & Sieres and Seara, 2006]. II. ANALYSIS AND DISCUSSION Effect of generator temperature variation on the collector efficiencies: The efficiency of the solar collectors increases rapidly in the low generator temperature zone. Then it reaches a high efficiency value and starts stabilizing at that value. Initially, at low temperatures, the refrigerant starts vaporizing. As the temperature keeps rising, the vaporizing rate also increases and leads to a corresponding rise in the efficiency. But, on the other hand, the temperature differential between the generator and the ambient also keeps on rising. This leads to heat loss from the collector as well as from the generator to the ambient. Moreover, once the maximum temperature required to vaporize the whole refrigerant is reached, any further increase in temperature would not be useful for running the system. The ETC has the maximum efficiency value, followed by the FPCI, the FPC and the CPC, in that order. Given the structure and design of the CPC, its efficiency should be highest. But it is not so, because, in its collector efficiency equation, the beam radiation component is also present. Since the maximum achievable temperature at the generator has a limit, the efficiency is less. Despite this, the CPC is favored more than the other collectors because of its ability to achieve elevated temperatures. The collector efficiency values decrease with an increase in the temperature difference between the collector outlet and the generator inlet. This can be seen clearly from the graphs. Effect of generator temperature on the Coefficient of Performance The COP values of the refrigeration system rise with a corresponding rise in the generator temperature. The generator temperature has a direct effect on the COP since it features in the equation of the COP. Moreover, as the generator temperature rises, more vapour is released at the generator, leading to more ammonia being evaporated at the evaporator. This raises the potency of the refrigerating effect, leading to a greater value of the COP. The maximum value of achievable COP is the highest for the CPC, followed by the ETC, the FPCI and the FPC, in that order. The COP also rises with a decrease in the temperature difference between the collector outlet and the generator inlet. 160
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 7, July 2015) Effect of generator temperature on the net COP of the solar vapour absorption system The net COP is the product of the performances of both the solar system as well as the refrigeration system, i.e. it is the total performance parameter of the solar absorption refrigeration system.
The net COP shows a steep initial rise after which there is a linear rise. The value of the net COP is the highest for the ETC. Even the CPC can reach the same efficiency, provided that the generator temperature be raised. The FPC and FPCI show lower total COP values.
Figure 1 Solar collector efficiency plotted against the generator temperature for the four collector types.
Figure 2 Collector efficiencies plotted against the generator temperatures
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Figure 3 Effects of the generator temperature variation on the COP of the FPC, FPCI, ETC and CPC.
Figure 4 Effects of generator temperature change on the net COP of the solar absorption system for the different collectors.
Effect of condenser pressure on the COP The COP shows a decline with an increase in the condenser pressure (high side pressure). With an increase in the condenser pressure, the saturation temperature rises along with a fall in the vapour pressure. Both these effects are accompanied with an increased heat demand in the generator, thus resulting in lowering of the COP (Figure 5).
This particular graph has been plotted at different values of the condenser pressure. A rise in the condenser pressure is accompanied by a drop in the COP range. With an increase in the condenser pressure, the saturation temperature rises along with a fall in the vapour pressure. Both these effects are accompanied with an increased heat demand in the generator which leads to a lowering in the COP (Figure 6).
Effect of change in evaporator (low side) pressure on the COP The COP vs evaporator temperature graph of the flooded evaporator shows a linear rise in the COP with respect to the evaporator temperature. Rise in the evaporator temperature reduces the difference in temperature between the load and the evaporator, thus reducing the energy duty and eventually raising the COP.
Influence of heat exchanger effectiveness on the COP and absorber heat exchange In this graph, both the aforementioned effects are shown simultaneously i.e. direct rise in the COP as a result of increasing effectiveness as well as the drop in absorber heat exchange with a rise in the effectiveness. The latter also leads to a rise in the COP (Figure 7). 162
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Figure 5 Graph of COP plotted against the condenser pressure
Figure 6 Graph of COP plotted against the evaporator pressure
Figure 7 Plot of COP, Qa against the heat exchanger effectiveness
Effect of condenser cooling water flow rate on the COP Water enters the condenser and condenses the saturated ammonia vapours. It can be seen from the graph that there is an almost linear rise in the COP with a rise in the mass flow rate of water at the condenser. The assumption here, again, is that the heat exchange remains same, but at an increased mass flow rate. This requires reduction in the enthalpy (or temperature) difference. Increasing the mass flow rate of water for the heat exchange by keeping these parameters constant facilitates more condensation along with more reduction in temperature (subcooling). This leads to a better heat exchange at the evaporator leading to a rise in the COP (Figure 8).
This raises the generator load which subsequently lowers the COP (Figure 9). Effect of generator heating water flow rate on the COP The graph shows that there is a decreasing trend of the COP for an increase in the flow rate of the heating water to the generator. There are two reasons for this. First, rise in the flow rate without changing the other design conditions can raise the entropy. Second, increasing the flow rate can increase the heat supply, although, the temperature of the generator remains the same. The excess heat is thus lost (Figure 10). Effect of evaporator load water flow rate on the COP It can be seen from the following graphs that there is a linear rise in the COP with a rise in the mass flow rate of water at the evaporator. The assumption here is that the heat exchange remains same, but at an increased mass flow rate. This requires reduction in the enthalpy (or temperature) difference. This leads to COP rise as the evaporator load is reduced (Figure 11).
Effect of absorber cooling water flow rate on the COP It is observed from the graph that the value of the COP decreases with an increase in the absorber cooling water mass flow rate. As the mass flow rate of cooling water is raised, the temperature and hence, the enthalpy of the rich solution exiting the absorber reduces.
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Figure 8 COP against the condenser cooling water flow rate
Figure 9 COP against the absorber cooling water flow rate
Figure 10 COP against the generator heating water flow rate
Figure 11 COP against the evaporator load water flow rate
doesn’t yield any useful output, thus reducing the COP as a result of wastage of useful energy. In the case of the condenser, the COP value remains almost constant initially, starts taking a decline at a particular value and then falls at a constant heat value. This is because of more heat exchange in the condenser. If the temperature of the solvent becomes too low, it will hinder with the vaporizing of the refrigerant at the evaporator. This leads to the fall in the COP at a particular value of the exchanged heat (Figure 12).
Effect of variation in the heat transfers in generator, absorber, condenser and evaporator on the COP From the figure, it is quite visible that the COP rises and then begins to fall steeply when the evaporator and absorber heats reach a certain value. The same is the case for the generator as well, the only difference being that the rise is comparatively steep and the fall is slow. Increasing the heat exchange in these components reduces the loads in both the generator and the evaporator. Although, after some particular value, the excess heat exchange goes waste as it
Figure 12 Plots of variation in the heat transfers in generator, absorber, condenser and evaporator on the COP
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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 7, July 2015) [13] Kajano Joseph, Lucas Doug, Muyuka Glorius, (2013), Design and analysis of an absorption refrigeration system. A Major Qualifying Project Report Submitted to the Faculty of the Worcester Polytechnic Institute in partial fulfillment of the requirements for the Degree of Bachelor of Science in Mechanical Engineering. [14] Kalogirou S. A., (2004), Solar thermal collectors and applications. Progress in Energy and Combustion Science, 30, 231–295. [15] K Karthik, (2014). Design, fabrication and analysis of solar vapour absorption refrigeration system. International Journal of Emerging Technology and Advanced Engineering, 4(9). [16] L. Garousi Farshi, C.A. Infante Ferreira, S.M.S. Mahmoudi, M.A. Rosen, (2014), First and second law analysis of ammonia/salt absorption refrigeration systems. International Journal of Refrigeration, 40, 111-121. [17] Misra et al., (2006), Thermoeconomic evaluation and optimization of an aqua-ammonia vapour-absorption refrigeration system. International Journal of Refrigeration 29, 47-59. [18] Satish Raghuvanshi & Govind Maheshwari (2011), Analysis of Ammonia –Water (NH3-H2O) Vapor Absorption Refrigeration System based on First Law of Thermodynamics. International Journal of Scientific & Engineering Research, 2(8,) 1. [19] Shaarawi et. Al., (2014), Comparative analysis between constant pressure and constant temperature absorption processes for an intermittent solar refrigerator. International Journal of Refrigeration, 41, (2014), 103-112. [20] Sözen A, Özalp M., (2003), Performance improvement of absorption refrigeration system using triple-pressure-level. Appl. Therm. Eng., 23, 1577–1593. [21] Staicovici M D., (1995), Polybranched regenerative GAX cooling cycles. Int J Refrig, 18(5), 18–29. [22] Tozer RM, James RW, (1997), Fundamental thermodynamics of ideal absorption cycles. International Journal of Refrigeration, 20(2), 120–35. [23] V.K.Bajpai, (2012), Design of solar powered vapour absorption system. Proceedings of the World Congress on Engineering, 2012, 3, London, U.K. [24] Zohar Jelinek and A. Levy, (2012), The influence of Diffusion Absorption Cycle Configuration on Performance. Applied Thermal Engineering, 7, 2219.
III. CONCLUSIONS 1. The different kinds of vapour absorption systems are studied and the best suited one is selected. 2. The properties and parameters that hold significance for the functioning of the system are studied and an appropriate selection of the same is made. 3. The performance of the system is studied under varying values of parameters such as the effectiveness, heat flux, mass flow rate, temperatures and pressures. REFERENCES [1]
Acikkalp, (2014), Modified thermo-ecological optimization for refrigeration systems and an application for irreversible four temperature-level absorption refrigerator. International Journal of Energy and Environmental Engineering, 4, 20. [2] Acuna, F. Lara, N. Velazquez, J. Cerezo, (2014), Optimum generator temperature to couple different diffusion absorption solar cooling systems, International journal of refrigeration, 45, 128-135. [3] ASHRAE Handbook Fundamentals, 1997 (IP). (1997). American Society of Heating. [4] Cho, K., Kim, J., (2000), Thermal and absorbing performance in a vertical absorber. International Journal of Air-Conditioning and Refrigeration, 8 (2), 51–59. [5] Christy V Vazhappilly, Trijo Tharayil, A. P. Nagarajan, (2013), Modeling and experimental analysis of generator in vapour absorption refrigeration system. International Journal of Engineering Research and Applications, 3(5), 63-67. [6] Chua HT, Han Q, Ng KC, Gordon JM, (1996), Thermodynamic modeling and experimental evidence for the optimization and maximum-efficiency operation of absorption chillers. ECOS 1996, Stockholm, Sweden. [7] O. Adegoke, (1987), Evaluation of a Refrigerant/Absorbent Combination for Vapour Absorption Refrigeration Systems Utilising Solar Heat. A thesis submitted for the fulfillment of the requirement for the Degree of Doctor- of Philosophy in Mechanical Engineering of the University of London. [8] Hans-Martin Hellmann, (2002), Carnot-COP for sorption heat pumps working between four temperature levels. International Journal of Refrigeration, 25, 66–74. [9] Herold K E, He X, Erickson D C, Rane M V., (1991), The branched GAX absorption heat pump cycle. in: Proceedings of the absorption heat pump conference, Tokyo, Japanese Society of Refrigeration, Tokyo, Japan, 127–32. [10] H. T. Chua, H. K. Toh and K. C. Ng, (2002), Thermodynamic modeling of an ammonia water absorption chiller, International Journal of Refrigeration, 25(7), 896-906. [11] J. Ferna´ndez-Seara, M. Va´zquez, (2001), Study and control of the optimal generation temperature in NH3–H2O absorption refrigeration systems. Appl. Therm. Eng., 21 (3), 343–357. [12] Jose´ Ferna´ndez-Seara, Jaime Sieres, (2006). Ammonia–water absorption refrigeration systems with flooded evaporators. Applied Thermal Engineering, 26, 2236–2246.
Nomenclature Tg Ta Tc Te Pc Pe ε Qg Qa
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Generator Temperature Absorber Temperature Condenser Temperature Evaporator Temperature Condenser Pressure Evaporator Pressure Effectiveness of Solution Heat Exchanger Heat Exchange in the Generator Heat Exchange in the Absorber
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 7, July 2015) Qc Qe Mwg Mwa Mwc Mwe
Heat Exchange in the Condenser Heat Exchange in the Evaporator Mass Flow Rate of Hetaing Water Generator Mass Flow Rate of Cooling Water Absorber Mass Flow Rate of Condensing Water Condenser Mass Flow Rate of Load Water Evaporator
Mr
Mass Flow Rate of Refrigerant Through Evaporator COP Coefficient of Performance ηFPC Plate Efficiency of Flat Plate Collector ηFPCI Plate Efficiency of Improved Flat Plate Collector ηETC Plate Efficiency of Evacuated Tube Collector ηCPC Plate Efficiency of Compound Parabolic Collector Gt Total Global Irradiation Gb Global Beam Radiation
Through Through Through Through
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