Lithium bromide high-temperature absorption heat pump: Coefficient of performance and exergetic efficiency

Abstract

A theoretical study on the employment of a lithium bromide absorption heat pump in Spain, used as machine type I and aimed to produce heat at 120°C via waste heat sources at 60°C, is given in the paper. Real performance conditions are stated for each component of the machine, namely the absorber, the heat recoverer, the generator, the condenser, the solution pump, the expansion valve and the evaporator. By means of thermodynamic diagrams (p, t, x) and (h, x), the required data are obtained for calculation of the heat recovered in the evaporator Qe, and the heat delivered to the absorber Qa and to the condenser Qc, as well as the heat supplied to the generator Qg. In addition, the heat delivered by the hot solution to the cold solution in the heat recovered Qr, and the work Wp done by the solution pump are calculated. The probable COP is calculated, and values are obtained close to 1.4. The working temperature in the generator is determined; it ranges from 178 to 200°C. The heat produced by the lithium bromide absorption heat pump is 22% cheaper than the heat obtained from a cogeneration system comprising natural gas internal combustion engine and a high temperature heat pump with mechanical compression. Compared with a high temperature heat pump with mechanical compression, the heat produced by the absorption heat pump is 31% cheaper. From (h, x) and (s, x) diagrams, exergy losses for each component can be determined, and, from these results, an exergetic efficiency of 75% is obtained, which provides the quality index of absorption cycle.