Analysis and Design of Novel Absorption Power Cycle Plants

Abstract

Absorption Power Cycles (APCs) provide an interesting field within power cycles. The multicomponent mixture with variable temperature across boiling is employed as a working fluid. This has a potential for decreasing exergy loss associated with heat transfer during phase change processes (boiling and condensation). Absorption process has also an effect of lowering exhaust pressure of a turbine. The APCs hold a potential for heat recovery applications at very low temperatures, where constant temperature of boiling and condensation largely limits performance and economic effectiveness of Organic Rankine cycles (ORCs). Theoretical calculations show superiority of APC over extensive range of considered ORC working fluid. The advantage of APC further increases when air cooled condenser needs to be used instead of wet cooling tower. With the same boundary conditions for all cycles the APC provides higher utilization efficiency and power output at source temperatures below approximately 120 °C, for temperatures as low as 60 °C the net power output can be surpassed even more than three times. The proposed APC employs aqueous solution of salts considered generally for absorption cooling (Lithium Bromide, Lithium Chloride, Calcium Chloride) as a working fluid. Unlike ammonia used in mixture with water in Kalina APC or often ORC working fluids, used salts are non-toxic, environmentally friendly and pure water in expander simplifies its design. After summary of theoretical research from thermodynamics point of view are discussed principles, aspects and issues for design of single components of the cycle. Results of sizing are presented on two examples with 100 °C heat source. First one is 20 kWe unit using hot air as a heat source and air cooled condenser, second one is 500 kWe unit with heat source being pressurized water and using wet cooling tower heat rejection. Results show possibility of building relatively efficient system for even small power output with turbine isentropic efficiency nearly 80 % for the 20 kWe unit, but relatively large heat exchangers.