Wind induced heat losses from solar dish-receiver systems
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Parabolic dish-receiver concentrating solar power (CSP) plants are a promising technology for the generation of renewable electricity. However, the high operating temperatures of the cavity receivers mean the performance of these CSP systems is very sensitive to heat losses, in particular by wind. A comprehensive literature review revealed a lack of work undertaken on wind flow around parabolic dish CSP systems and its impact on the heat loss from them. Previous studies investigating the effect of convective heat loss treated the receiver as an isolated entity, decoupled from the dish/reflector structure. Hence, the effect of the dish on the airflow around the receiver had not previously been considered. This gap in the literature indicated a need to understand the effect of the dish in order to develop realistic heat loss models for the design of parabolic dish CSP systems. Hence, the research focused on the interaction between the wind and the dish structure causing local effects of air motion at the cavity inlet and the resulting in convective heat loss. To verify the assertion that the dish would affect the flow of air around the receiver, and hence the heat loss, wind tunnel testing was performed on a scale model of a parabolic dish reflector. This study showed a significant disturbance to the flow field both qualitatively and quantitatively near where the receiver would be located. On this basis, a computational fluid dynamics (CFD) model of the airflow around a scaled dish and receiver was developed. These simulations showed good agreement with the quantitative measurements and qualitative visualization undertaken in the wind tunnel, thus validating the computational approach. Having validated the simulation scheme, a detailed CFD study was undertaken to determine the heat loss from a 20m2 parabolic dish and receiver system, developed by the Australian National University for a range of dish orientations, wind speeds and incidence angles. The CFD simulations confirmed that the dish’s presence had a significant impact on the convective heat loss experienced by the system. With the flow around the dish structure being considered, the heat loss experienced by the receiver was markedly different, and in some cases lower (up to 40%), than when it was assumed the receiver acted in isolation. Furthermore, it was found that for dish tilt angles and wind incidence angles between +/-30° and 0°, the heat loss significantly increased. This can be attributed to the receiver moving into the free stream and being subject to stronger forced flow than experienced in the dish’s wake. In summary, the results delivered quantitative data as to the effect of the dish’s orientation, wind speed and wind incidence angle on heat loss. Using this information, a series of correlations were established to allow designers of parabolic dish CSP systems to incorporate the impact of the dish on the convective heat loss from the receiver. More broadly, the work demonstrated the importance of considering the influence of the dish when determining the heat loss from a parabolic dish receiver to avoid designing overly conservative, and hence costly, CSP systems.