Exergy analysis of electrically- and thermally-driven engines to drive heat pumps: An exhaustive comparative study

Abstract

The choice of driving a heat pump with an electrically-or a thermally-driven engine is a vexing question complicated by the carbon footprint and environmental impact of using electricity versus natural gas (or waste heat) as the main driver for the respective engines. Useful work generated by these two distinct engines is the focal point of this paper, addressing a key question: which engine presents a better choice for a given heat pumping application within the constraints of energy and environmental stewardship? We examine this question comprehensively through the methodology of energy, exergy, and availability analysis, explaining clearly, why the output of work from these two distinct engines is inherently vastly different. Thermodynamic consistency is guaranteed by satisfaction of the First and Second Laws applied to closed systems and their subsystems. The general conclusion is that thermally-driven engines are not industrious converters of heat to mechanical work, for heat pumps.ing coal. Absorption (a subset of sorption) is receiving increased attention as an enabling technology to utilize fuel more effectively, reduce pollution, and to serve multiple thermal loads simultaneously. For example, integration of absorption with micro gas turbines uses a wide range of fuels to simultaneously produce electricity, cooling, and heating (so called combined cooling, heating, 20 and power, or CCHP systems) [3, 4]. Other applications include fuel-driven Absorbtion Heat Pump (AHP) water heating [5,6,7], compression absorption heat pump for large temperature (90 o C) lifts [8, 9], solar-assisted absorption heat pumps [10,11,12, 13,14], district heating [15, 16, 13,17], and thermal energy storage [18, 19]. Many examples of industrial applications of absorption 25 heat pumps can be found in drying [20, 21, 22], distillation [23, 24, 25, 26], and evaporation [27,28]. The predominant working pairs in absorption technology are LiBr-H 2 O, NH 3 -H 2 O, and to a limited extent, NaOH-H 2 O with few exceptions [20].