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Correlations for natural convection heat loss from solar cavity receivers are widely based on isothermal surface temperature assumptions, which do not occur in practice due to the local heat balance varying with position. An open question thus exists regarding the suitability of such correlations for non-isothermal conditions. This paper addresses this issue by presenting a new Nusselt correlation developed from an experimental investigation of natural convection heat loss from a non-isothermal scale-model cylindrical cavity receiver. Cavities that are considered in this work have length-to-diameter ratios of 1 and 2, are operated at peak temperatures ranging from 355 °C to 650 °C, and exhibit temperature differences along the cavity wall between 40 °C and 342 °C. Stagnation and convection zones, as well as view factor profiles, are observed to contribute to the wall temperature distribution as the cavity is inclined downwards. An energy balance undertaken for steady state provides insight into the effects of non-uniform surface temperature distribution and inclination-dependent surface areas on radiative and convective losses. Natural convection heat loss results from this work are compared with widely-used correlations from the literature that assume isothermal wall conditions, and systematic discrepancies are observed. The proposed Nusselt correlation which accounts for the temperature non-uniformity, cavity inclination and geometric aspect ratio is evaluated against experimental data from this and other studies. It is found to produce excellent predictions of Nusselt numbers for cylindrical cavity receivers in the Grashof number range of 2.6 × 105 to 1.4 × 107.

This chapter provides an overview of the fundamental principles of CSP systems. It begins with the optical processes and the ultimate limits on the extent to which solar radiation can be concentrated. Practical factors that reduce achievable concentration levels further are discussed. Mechanisms of thermal energy loss from receivers are covered. Available power cycles for electricity generation are reviewed. The second law of thermodynamics is introduced to lead into a consideration of optimization of overall system efficiency via variation of operating temperature and receiver aperture size. Performance modelling of complete systems is introduced and finally the analysis of levelized cost of energy is covered, as a metric for comparing systems, and as a tool to thermo-economic optimization in design.