EXERGETIC ANALYSIS OF A SOLAR ABSORPTION HEAT TRANSFORMER

Transcription

1 EXERGETIC ANALYSIS OF A SOLAR ABSORPTION HEAT TRANSFORMER 1 KHERRIS Sahraoui, 2 ZEBBAR Djallel, AZIZI Aicha, BENADI Noura et AYAD Laila Institut des Sciences et Technologies Centre Universitaire de Tissemsilt Abdelhak Benhamouda, BP. 182, 38000, Tissemsilt, Algérie s : 1 Kherris77@gmail.com, 2 Djallel.zebbar@gmail.com Abstract: In the present study, the first and second laws of thermodynamic have been used to analyze a solar absorption heat transformer (SAHT). A mathematical model of a single stage solar absorption heat transformer using ammonia-water as binary mixtures has been discussed. Furthermore, in this study new thermodynamics properties of ammonia-water mixtures have been used. An analyze of the performances of the SAHT highly affected by the energy efficiency, solar coefficient of performance, and circulation ratio has been carried out in this study. Keywords- exergy; heat transformer; solar ; modelling ; absorption. 2. INTRODUCTION Very large quantities of waste heat at low temperatures are discarded into the atmosphere from power plants and industrial processes. Many attempts have been made in most industrial sectors to recover this useful energy by heat pumps, vapor absorption refrigeration systems and heat transformers. Upgrading this low-level heat can make possible its use in different forms. Among the different possibilities, heat transformers present an attractive solution for upgrading low temperature waste heat to higher temperature useful heat with minimum consumption of external energy [1-3]. The general principles of heat transformation were studied by Altenkirch [1, 4] in 1913, and developed later by Nesselmann [1, 5]. In 1982 Wilkinson [1, 3] proposed different types of single and multi-stage absorption heat transformers. A comparative study of different working fluid combinations with R22 as refrigerant and six absorbents in a single AHT was performed by Fatouh and Srinivasain 1992, [6]. A new type of AHT operating with reverse rectification with water-glycol and ammonia-water mixtures was presented by Le Goff in 1992 [7-8]. In the same year, Rivero and Le Goff described and compared the different performance criteria available for analyzing heat pump and heat transformers experimental work using an ammonia-water heat transformer has been reported by Mostofizadeh [3]. Stephan and Seher [9-10] have discussed the heat transformer cycles for single and doublestage processes. Kripalani et al. [11] have studied the performance analysis of a vapor absorption heat transformer with different working fluid combinations. Exergy analysis permits to know energy quality into heat transformer for the optimization of the operating parameters to yield a better performance in the AHT [12-20]. Zebbar et al. [12] have elaborated a mathematical modeling of an AHT to find out the optimal operating parameters using the so-called structural analysis, for the thermodynamic optimization. Lee and Sherif [13] utilized the second law analysis to know the performance of multi stage water-lithium bromide absorption heat transformers. The results provided theoretical basis for the optimal operation conditions and design of absorption heat transformers. Sozen [14] studied the irreversibilities in a single-stage heat transformer used to increase solar pond s temperature. The results showed that the absorber and the generator need to be thermally improved in order to increase the efficiency of the system.

2 Fartaj [15] compared the energy, exergy and entropy balance methods for analysing double stage absorption heat transformer cycle which is in fact a modification of a two stage heat transformer cycle. The results obtained show the influence of irreversibilities of individual components on deterioration of the effectiveness and the coefficient of performance of the system. In this study a new thermodynamics properties of ammonia-water mixtures have used [21]. The proposed correlations cover equilibrium conditions of phases at high pressures and temperatures. The performances of absorption heat transformer are defined by the energy efficiency, the solar coefficient of performance and circulation ratio have been analyzed and compared. 3. MATHEMATICAL MODEL A schematic representation of SAHT is shown in Fig. 1. It consists from the following elements: generator, evaporator, condenser, absorber, solution heat exchanger (SHE), refrigerant heat exchanger (RHE), two pumps and throttling valve. The generator and evaporator receive waste heat at the same medium temperature. The absorber delivers useful heat at a higher delivery temperature, where as part of the heat flowing into the process is rejected at ambient temperature from the condenser. The discussion of the mathematical modelling is based on the laws of the mass and energy balances for each SAHT element and the properties of ammonia-water mixtures at various points of the system. 3. RESULTS AND DISCUSSIONS Fig. 1 Schematic representation of solar absorption heat transformer For the simulation of the solar ammonia-water absorption heat transformer, the software "SARM" (Simulation of Absorption Refrigeration Machine) is used [22-25] in this study. The performances of the system have been evaluated by varying the operating parameters Variation of the with different heat source temperatures The comparison of the results obtained for the coefficient of performance according to the absorber and generator temperatures, with those of Ismail I.M., [20] for three temperature values of generator and evaporator on (60, 70 and 80 C) is carried out (Figs. 2: a, b and c). The operating conditions chosen are:

3 The condenser temperature Tc = 25 C; The generator an evaporator temperatures Tg =Te= (60, 70, 80) C; The efficiency of the two exchangers = 0.8. The comparison shows a good agreement of the coefficient of performance obtained in this study with those of Ismail I.M., [20], for three values of temperatures. The average error is less than 2.82% ,48 0,48 0, ,48 =60 C «a» =80 C =70 C Circulation retio f c «b» Tc=25 C Tc=30 C Tc=35 C Tc=40 C Tg=Te=70 C ,28 0,26 0,28 0, «c» Fig. 2 Comparison of with different heat source temperatures Ta (Absorber) C Fig. 3 Variation of fc=f(ta, Tc) with Tg=Te=70 C 3.2. Effect of the absorber and condenser temperatures to the circulation ratio This analysis is performed when the variation of the absorber and condenser temperatures for the evaporator and generator temperatures set at 70 C. The efficiency of the two exchangers is assumed equal to 75 %. From Fig. 3 it can be seen that the increase of the absorber temperature leads to an increase in the circulation ration. The value of this last is higher in the lower range of the condenser temperatures. This increase reflects the growth of low pressure Effect of the absorber, generator and condenser temperatures to the exergy efficiency The exergy efficiency is plotted in Fig. 4 as a function of generator, condenser and heat delivery temperatures.

4 It is clear that the exergy efficiency decrease with an increase in generator, condenser and heat delivery temperatures. 4. CONCLUSION Fig. 4 Variation of exergy efficiency with different heat source temperatures The conclusions of this study are summarized as follows: The performance of the heat transformer strongly depends on the properties of the refrigerant-absorbent solutions; A mathematical modeling of solar ammonia-water AHT was carried out; The of the SAHT system was analyzed and compared with other results; The variation of of the SAHT system with different waste heat source temperatures against the temperature boost (absorber) for the given condenser, generator and evaporator temperatures show that the increase in temperature boost causes a decrease in and s; The variation of delivery temperature has very little influence over the of the system but the exergy efficiency varies significantly. Finally, it is necessary to note the good agreement of the calculated of the of the AHT with Ismail I.M. NOMENCLATURE REFERENCES SUBSCRIPTS T temperature, [ C, K] a Absorber coefficient of performance g Generator heat rate [kj/ s, kw] e Evaporator x solution concentration c condenser NH 3 Ammonia s Solution H 2 O Water r Refrigerant fc circulation ratio RHE refrigerant heat exchanger AHbsorption heat transformer SHE solution heat exchanger SAHT Solar absorption heat transformer TST thermal storage tank η exergy efficiency exchanger efficiency [1] K.P. Tyagi, Aqua-ammonia heat transformers, Heat Recovery Systems and CHP, 7 (5), pp , [2] HEROLD, K.E., RADERMACHER,R., KLEIN, S., Absorption Chillers and Heat Pumps : CRC Press, 1996.

5 [3] Sava PORNEALA, Emilia MURINEANU, performance thermodynamic criteria of the absorption heat transformers, TERMOTEHNICA 1 2/2007, UNIVERSITY DUNAREA DE JOS, GALATI, [4] ALTENKIRCH, Reversible absorption maschine,z.ges. Kalteind. 19 (21), [5] NESELMANN, K., Wiss veroff Siemens- Werken, 12, pp , [6] FATOUH, M., SRINIVASA MURTHY, S., Comparison of R22 absorbent pairs for vapour absorption Heat Transformers based on P T x h data : Heat recovery Systems & CHP, 13 (1), pp.37-48,1993. [7] LABIDI, J., LASALLE, A., LE GOFF, P., A New Heat Transformer for Upgrading Industrial Waste heat :Proceedings of the International Symposium on Efficiency, Costs, Optimization and Simulation of Energy Systems (ECOS 92), Zaragoza, Spain, pp , [8] LE GOFF, P., LABIDI, J., RANGER, P., JEDAY, M.R., and MATSUDA, H., The Concept of Reverse- Rectification Exergy Analysis and Applications: Proceedings of the International Conference Analysis of Thermal and Energy Systems ATHENS, pp , June [9] STEPHAN, K., SEHER, D., Heat transfoermer cycle:i) One and two-stage process ; J. Heat Recovery Systems, 4, pp , [10] STEPHAN, K., SEHER, D., Heat transfoermer cycle:ii)thermodynamic analysis and optimization of single stage absorption heat transformer ; J. Heat Rec. Systems 4, pp , [11] KRIPALANI, V.M., SRINIVASA MURTHY, S., and KRISHNAMURTY, V.M., Performance analysis of a vapour absorption heat transformer with different working fluid combinations; J. Heat Recovery Systems 4, pp , [12] Djallel Zebbar, Sahraoui Kherris, Souhila Zebbar et Kouider Mostefa, Thermodynamic optimization of an absorption heat transformer, International Institute of Refrigeration, pp. 88, France, [13] S.F. Lee, S.A. Sherif, Second law analysis of multi-stage lithium bromideewater absorption heat transformers. ASHRAE Transactions Atlanta, U.S. 106 (2000). [14] A. Sozen, Effect of irreversibilities on performance of an absorption heat transformer use to increase solar pond s temperature. Renewable Energy 29 (4) (2004) pp [15] S.A. Fartaj, Comparison of energy, exergy, and entropy balance methods for analyzing double-stage absorption heat transformer cycles. International Journal of Energy Research 28 (14) (2004) pp [16] Xiaoyong Qin, Lingen Chen, Fengrui Sun, Chih Wu, An absorption heat-transformer and its optimal performance, Applied Energy,78, pp , [17] Rivera W, Romero RJ. Evaluation of a heat transformer powered by a solar pond. Solar Energy Mater Solar Cells, 63, pp. 413:x:x 422, [18] Rivera W. Experimental evaluation of a single-stage heat transformer used to increase solar pond s temperature. Solar Energy, 69 (5), pp , [19] Rivera W, Cardoso MJ, Romero RJ. Single-stage and advanced AHT operating with lithium bromide mixtures used to increase solar pond s temperature. Solar Energy Mater Solar Cells, 70, pp , [20] Ismail IM, Upgrading of heat through AHT. Int J Refrig, 18(7), pp , [21] Kherris S., Makhlouf M., Zebbar Dj., Sebbane O., Contribution study of the thermodynamics properties of the ammonia-water mixtures, Thermal Sciences, Vol. 17, N 3, pp , [22] Kherris S., Makhlouf M. et ASNOUN, " SARM", Simulation of Absorption Refrigeration Machine, Revue des énergies renouvelables, 11 (4), pp , [23] S. Kherris, Simulation des cycles de machines frigorifiques à absorption, mémoire de magister, Université Ibn Khaldoun, Tiaret, [24] S. Kherris, Contribution à l optimisation des installations frigorifiques à absorption solaire, thèse de doctorat, Université Djillali Liabes, Sidi Bel Abbes, 2011.