Endoreversible terrestrial solar energy conversion

Abstract

The analysis of reversible radiation conversion is insightful in process optimization and provides the upper limit to performance of any conceptual device. However, in practice, irreversibilities are unavoidable and play an important role in performance optimization. The finite absorption of a radiative source flux, and the simultaneous emission of radiation, is an inherently irreversible process. Likewise, heat rejection from the conversion device is unavoidable and dependent on local environmental conditions and resources. The endoreversible model treats these irreversibilities as inherent but external to the conversion process. In this paper, the effect of these irreversibilities on performance is investigated for a model with irreversible radiative absorption combined with irreversible conductive or convective heat rejection. Previous models either do not include the magnitude of entropy rejection required by the second law or do not otherwise accurately represent terrestrial solar conversion. Analysis of the model provides a guide for optimal operating conditions. For relatively poor heat rejection, as can occur in arid climates, analysis of the model reveals regions of operating parameters that will result in zero or low theoretical work output, and/or high operating temperatures and increased risk of failure. This is of particular concern for photovoltaic systems with low maximum temperature limits. Analytical expressions for maximum ideal work are provided, given the specific radiative source flux and heat rejection conditions. A fair evaluation of system performance is thereby obtained by comparing actual work production to the ideal, given the specific external operating conditions or restrictions.