Maximum efficiency of absorption refrigerators at arbitrary cooling power

TL;DR

This paper derives the maximum efficiency at arbitrary cooling power for absorption refrigerators composed of Carnot-type heat engines and refrigerators, revealing how efficiency increases as power decreases, with bounds established for low-dissipation models.

Contribution

It provides a theoretical framework for the maximum efficiency of absorption refrigerators at any given power, incorporating the interplay between internal engine and refrigerator constraints.

Findings

Efficiency sharply increases when cooling power slightly decreases from maximum.

Derived bounds on maximum efficiency for low-dissipation absorption refrigerators.

Efficiency increase is nonlinear unless the power ratio diverges.

Abstract

We consider absorption refrigerators consisting of simultaneously operating Carnot-type heat engine and refrigerator. Their maximum efficiency at given power (MEGP) is given by the product of MEGPs for the internal engine and refrigerator. The only subtlety of the derivation lies in the fact that the maximum cooling power of the absorption refrigerator is not limited just by the maximum power of the internal refrigerator but, due to the first law, also by that of the internal engine. As a specific example, we consider the simultaneous absorption refrigerators composed of low-dissipation (LD) heat engines and refrigerators, for which the expressions for MEGPs are known. The derived expression for maximum efficiency implies bounds on the MEGP of LD absorption refrigerators. It also implies that a slight decrease in power of the absorption refrigerator from its maximum value results in a large nonlinear increase in efficiency observed in heat engines whenever the ratio of maximum powers of the internal engine and the refrigerator does not diverge. Otherwise, the increase in efficiency is linear as observed in LD refrigerators. Thus, in all practical situations, the efficiency of LD absorption refrigerators significantly increases when their cooling power is slightly decreased from its maximum.