Comparison of system performance in a hybrid solar receiver combustor operating with MILD and conventional combustion. Part I: Solar-only and combustion-only employing conventional combustion

Abstract

The paper reports a detailed heat transfer study of a hybrid solar receiver combustor operating in either the solar-only or combustion-only modes. It aims to investigate the differences between performance in these two modes of operation and to reduce the uncertainties of an analytical model of the device developed previously for the techno-economic assessments. The influence of the dominant geometrical parameters, burner configuration and flux distribution of the incident solar radiation on the thermal efficiency, heat losses, heat transfer mechanisms and heat flux distribution within the cavity was investigated with a three-dimensional computational fluid dynamics (CFD) model of the device. For the solar-only mode of operation, the model employs a Monte-Carlo ray tracing approach to calculate the heat flux distribution incident on the pipes containing the heat transfer fluid (HTF) and the overall thermal efficiency. For the combustion-only mode, two different burner designs were considered as representative cases for non-premixed and partially premixed flames, one with and the other without swirl. It was found that the device can achieve a similar thermal performance in each mode of operation given the use of either a cavity of sufficient size (length-to-diameter cavity ratio, L/D>5) or an appropriate arrangement of the HTF pipes within the cavity. In addition, that the thermal efficiency of the device was found to increase with the length-to-diameter cavity ratio and/or solar peak flux at the focal plane. A comparison of the heat flux distribution on the HTF pipes in the two modes of operation showed that the solar-only and the combustion-only mode, employing swirl-stabilised flames, can provide similar heat flux distributions in comparison with non-swirl flames, suggesting that the materials selection would be controlled by the combustion mode. The investigation also highlights that the analytical model developed previously can be effectively employed to evaluate the thermal efficiency of the device although its use should be limited to long cavities (L/D>5). Recommendations are also made on how to further refine the analytical model for the solar-only mode of operation.