Simulation of a hot mirror parabolic trough solar collector receiver

Kaluba, Victor Siuluta
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Parabolic trough solar collectors (PTSC) are currently the most mature solar collecting technology, applied commercially for electricity generation. High input temperature in power plants are desired for improved efficiency and help reduce electricity costs. However, at high temperatures, heat losses through radiation increase significantly. Thermal radiation transfer is the dominant heat loss mechanism in PTSC receivers operating at high temperatures. In existing systems, the radiation losses are reduced by using a selective absorber coating placed on the absorber pipe. It has optical properties that suppress infrared radiation (IR) emissions. The material absorbs well in the solar wavelength range (0.3 to 2.5 um ) but emits poorly in the infrared (IR) wavelength range. However, the material degrades at temperatures beyond 400oC resulting in high IR emission. This study developed a theoretical framework to characterize a different approach, alternative to the selective coating in reducing heat losses. This is the use of a hot mirror coating in a PTSC receiver. The coating is placed on the inner surface of the glass cover to reflect infrared radiation emanating from the absorber pipe back for reabsorption. The hot mirror is transparent to solar radiation and reflective in the wavelengths above 2.5 um. The formulations developed described the thermal interactions in a hot mirror coated PTSC receiver. To describe the heat loss reduction mechanisms, the study modelled theoretically the long range thermal radiation interactions inside the receiver unit. The model used discretization of the active surfaces to account for all the dominant radiation interactions. Different simulations scenarios were done to predict receiver performance. The performance of various candidates for hot mirror coating (ITO, Gold and Silver) were investigated. The effects of variation of some hot mirror optical parameters on overall plant efficiency was also investigated. The simulation was validated using other simulation works and experimental data. The results showed a close match with a discrepancy of 0.7%. High HTF temperatures were attained using hot mirrors. ITO gave the highest HTF temperature. The hot mirror thermal stability is not compromised since the glass cover stays cooler than the absorber pipe (< 400oC). The glass cover temperatures were far less than the absorber pipe temperatures. Even with the higher temperature of ITO (500oC), the glass cover never reached above 400oC. Solar transmissivity for the hot mirror materials is as important as the need for high HTF out temperatures.
A thesis submitted to the Faculty of Science in fulfilment of the requirement for the degree of Doctor of Philosophy, University of the Witwatersrand, Johannesburg, 2017.
Kaluba, Victor Siuluta (2018) Simulation of a hot mirror parabolic trough solar collector receiver, University of the Witwatersrand, Johannesburg,