Energy Vol. 21, No. I, pp. 21-27, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-5442/96 $15.00+0.00
O. M. IBRAHIM~'$ and S. A. KLEIN§
tDepartment of Mechanical Engineering and Applied Mechanics, University of Rhode Island, Wales Hall, Kingston, RI 02881-0805 and §Solar EnergyLaboratory, University of Wisconsin-Madison, Madison, WI 53706, U.S.A. (Received 28 March 1995) A b s t r a c t - - W e present a thermodynamic analysis of the Maloney and Robertson and the Kalina absorption power cycles. The maximum power for specified external conditions is identified and used as a reference to evaluate the performance of these two absorption power cycles. The evaluationfocuses on power cycles operating with low temperature heat sources such as geothermal heat, solar energy or waste heat.
Many of the recent proposals for alternative power cycles employ a non-azeotropic binary mixture in a Rankine or absorption-type power cycle. The motivation for using mixtures is that heat transfer can occur at variable temperature while at a constant pressure. Thevariable temperature heat-transfer processes reduce the temperature mismatch between the hot and cold streams and the cycle, thereby reducing the availability destruction in the heat exchangers. Maloney and Robertson I studied the performance of an absorption cycle. Their conclusion showed that the absorption power cycle has no thermodynamic advantage over the Rankine cycle. More recently, Kalina2 proposed an absorption power cycle using ammonia-water. In apparent contradiction to Maloney and Robertson's conclusion, Kalina shows that his cycle has a thermal efficiency which is 30--60% higher than comparable steam power cycles. Kalina 3 and Kalina and Leibowitz4-7 explained the basic advantages of what has become known as the Kalina cycle technology. E1-Sayed and Tribus 8 compared theRankine and Kalina cycles theoretically when both cycles are used as a bottoming cycle with the same thermal boundary conditions. They conducted a first and second law thermodynamic analysis and concluded that the Kalina cycle can have 10-30% higher thermal efficiency than an equivalent Rankine cycle. A recent publication by Stecco and Desideri 9 presents the results of an analytical study showingboth thermodynamic and practical advantages for the Kalina cycle compared to a Rankine cycle using the exhaust of a gas turbine as energy source. Marston m developed a computer model of the cycle analyzed by EI-Sayed and Tribus. 8 The results of this model show good agreement with published results of EI-Sayed and Tribus. s This paper provides a detailed evaluation of the Kalina and Maloney andRobertson absorption power cycles and a comparison of their performance with the maximum power (MP) cycle. A methodology for providing an engineering evaluation of absorption power cycles is described. The evaluation focuses on power cycles operating with low temperature heat sources such as geothermal heat, solar energy or waste heat. A case study is considered in which the heat source for the powercycle is a fluid stream with an inlet temperature of 455 K and a thermal-capacitance rate of 10 kW/K. The sink temperature is 286 K. Heat transfer to and from the power cycle occurs through heat exchangers which are described with traditional heat-exchanger relations. In case study, the ratio of the hot-side to cold-side heat-exchanger conductances is 1.0, i.e. UA. = UAL in all cycles. Similarresults are obtained for other heat-exchanger conductance ratios. The turbines and pumps are modeled as reversible adiabatic processes.
STo whom all correspondence should be addressed. 21
O.M. Ibrahim and S. A. Klein MAXIMUM POWER (MP) CYCLEMODEL
The best cycle that will result in the upper limit of the maximum power for specified external conditions has been studied in Refs. 11-13....