In this paper we thermodynamically assess the performance of an ammonia-water Rankine cycle that uses no boiler, but rather the saturated liquid is flashed by a volumetric expander (e.g., reciprocating, centrifugal, screw or scroll type expander) for power generation. This cycle has no pinch point and thus the exergy of the heat source can be better used by matching the temperature profiles of the hot and the working fluids in the benefit of performance improvement. The second feature comes from the use of the ammonia-water mixture that offers further opportunity to better match the temperature profiles at the source and sink level. This fact brings ∼10% improvement of exergy efficiency with respect to the case when a single substance (e.g., steam) is used as working fluid. The influence of the expander efficiency, ammonia concentration and the coolant flow rate is investigated and reported for a case study. The applications of this cycle can be found in low power/low temperature heat recovery from geothermal sources, ocean thermal energy conversion, solar energy or process waste heat etc where the cycle competes with Kalina, supercritical or multi-pressure steam implementations of the Rankine cycle.

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