Evaluation Method for Low-Grade Thermal Energy Conversion Systems Including Finite Heat Exchanger Thermal Performance

Low-grade thermal energy conversion (LTEC), including ocean thermal energy conversion (OTEC) and waste heat recovery, holds substantial potential for sustainable power generation. Because LTEC systems operate under extremely small temperature differences, conventional thermal efficiency of cycle alone cannot adequately characterize the maximum power extractable from finite-flow-rate heat sources. Although previous studies have analyzed heat exchanger irreversibilities and theoretical power limits, a compact analytical framework that simultaneously couples finite heat-capacity flow rates with the thermal performances of both high- and low-temperature heat exchangers remains limited. In this study, a theoretical framework for a single-stage endoreversible cycle is developed, expressing heat exchanger performance in terms of the number of transfer units (NTU). Closed-form analytical equations are derived for the maximum gross power (Wm,NTU), optimal heat-source temperature changes, and maximum power efficiency (ηm). Comparative evaluation against the classical model by Ikegami and Bejan demonstrates that finite heat exchanger thermal performance not only reduces the attainable maximum power, but also shifts the optimum outlet temperature changes of both warm and cold streams. Furthermore, the performances of the high- and low-temperature heat exchangers are strongly coupled; increasing the NTU on one side yields diminishing returns in power output if the other side remains limited, eventually reaching a fixed upper limit. Conversely, the thermal efficiency of cycle at maximum power (ηth,m) remains independent of NTU and is determined by the high- and low-temperature heat source inlet temperatures. Comparison with an ammonia Rankine cycle model under representative OTEC conditions confirms close agreement in predicted maximum power and optimal temperature shifts. This analytical framework provides a simple, robust benchmark for evaluating and optimizing LTEC and OTEC systems while accounting for heat source flow limits and heat exchanger performance.