The Ammonia-Water Mixture Rankine Cycle: Considerations on its Applicability as Bottoming Cycle

The use of a mixture of ammonia and water as working fluid in power cycles has been proposed for various cycle configurations such as the Kalina and the regenerative Rankine cycles. The purpose of this study is to investigate the performance of a non-regenerative single-pressure Rankine cycle using an ammonia-water mixture, with the ammonia mass fraction ranging from 0 to 1, as working fluid, for a number of applications where the Rankine cycle is the bottomer in a combined cycle with a topping gas turbine. The applications include condensing operation, and three cogeneration cases: namely hot water production for district heating and for the textile industry, and steam generation. An increase in power output from the bottoming cycle is found when an ammonia-water mixture is used instead of water for hot water production for district heating and for the textile industry. Pure water provides a higher power output in the case where steam is generated for cogeneration, while in the condensing operation pure water and pure ammonia give better results than any mixture. An exergetic analysis of the plant was performed in order to explain the different behaviour of the cycle. Performance of a simple Rankine cycle can be improved by using an ammonia-water mixture in all the applications where the heat sink temperature rise is greater than or equal to 25 K. Ammonia-related safety risks are much lower when a mixture is used as opposed to pure ammonia cycles, because improvements in the cogeneration cases studied are possible using a mixture with an ammonia mass fraction lower than 0.16