Reciprocating thermocompressor performance with non-ideal component characteristics

Abstract

Using heat to compress a gas can have many technological uses including power generation and heat-activated cooling. A model is presented here for a reciprocating, displacer-based thermocompressor. The model is based upon expressions derived from mass and energy conservation for each component of the system through each part of the cycle. It also incorporates features not found in previous works, such as specified regenerator effectiveness and component dead volumes. In order to form a thermodynamic cycle, work is generated from an ideal isothermal expansion of the compressed gas. The model shows that Carnot efficiency is achieved from the ideal work-generating cycle for a regenerator with unity effectiveness. However, for non-ideal conditions, thermal efficiency is highly sensitive to the regenerator performance. For example, assuming zero dead volume in the system, for a temperature ratio of 2 and a pressure ratio of 1.4, a regenerator effectiveness of 0.9 corresponds to a thermal efficiency of only 44 per cent of the Carnot efficiency. Dead volume also adversely impacts performance, with cold space dead volume being the most critical.