Beyond Constant-Temperature Reservoirs: A Stirling Cycle with Constant Heat-Generation Rate

TL;DR

This paper develops a theoretical model for heat engines powered by constant heat-generation sources, like radioisotope systems, revealing universal power-efficiency relationships and analyzing a Stirling cycle with unique temperature control protocols.

Contribution

It introduces a novel framework for finite-time heat engines with constant heat-generation sources and analyzes a Stirling cycle under these conditions, including decay effects.

Findings

Universal power-efficiency relationship independent of cycle configuration

Oscillatory temperature behavior in hot source dynamics

Performance decline due to radioactive decay over time

Abstract

Conventional heat-engine models typically assume two heat reservoirs at fixed temperatures. In contrast, radioisotope power systems introduce a fundamentally different paradigm in which the hot sources supply heat at a constant generation rate rather than maintaining a constant temperature. We develop a theoretical framework for finite-time heat engines operating between constant heat-generation-rate hot sources and constant-temperature cold reservoirs. A universal proportion between average output power and efficiency is established, independent of the specific cycle configuration or working substance. As a representative case, we analyze a finite-time Stirling cycle employing a tailored control protocol that maintains the working substance at constant temperatures during the quasi-isothermal processes. An intrinsic oscillatory behavior emerges in the temperature dynamics of the hot source, reflecting the interplay between heat accumulation and release. We further quantify the long-term decline in engine performance resulting from radioactive decay and demonstrate its impact over the system's operational lifespan. This work establishes a new theoretical prototype for heat engines and shall provide guidings for the analysis and design of radioisotope power systems.