Development of an open volumetric air receiver for a rock bed thermal storage system

[摘要] ENGLISH ABSTRACT: Concentrating solar power technology combined with a thermal energy storagesystem offers a sustainable and dispatchable energy technology which can produce electricity on demand. However, the high costs and advanced technologyrequirements associated with these systems need to be addressed in order to make them more cost effective and viable for local implementation. In this study anopen volumetric air receiver was developed as a suitable solution to overcome the mentioned limitations. This technology has an inherently simple design with thebenefit of using air as HTF that is abundantly available and has no environmental impact.The ultimate goal of this work is to use the open volumetric air receiver to charge a rock bed thermal energy storage system. In this project the first steps were taken through developing an absorber concept for this use. The concept was aimed towards a simple and cost effective design, which satisfies local manufacturingcapabilities and material availability. The charging requirements for the rock bed allowed for the use of a metallic absorber, since the maximum charging airtemperature is 600 °C. A stack of stainless steel wire mesh screens with a gradually decreasing porosity through its depth was identified as a cost-effectiveand simple absorber concept. The stack of wire mesh screens was placed inside a modular stainless steel cup to form a scalable absorber module.A numerical model was developed to investigate the heat and radiation transfer inside the porous absorber. The model assumed local thermal non-equilibrium between the screen and the air temperatures and approximated the radiation as a volumetric heat source. The use of low porosity wire mesh screens in the front of the absorber, followed by a finer screen deeper in its volume reduced the thermal losses and allowed better penetration of the incident flux, as well as enhanced the heat transfer through the absorber. The air mass flow rate proved to have a significant influence on the outlet air temperature. A lower flow rate increased the outlet air temperature, but also caused an increase in thermal loss due to thehigher front temperature and lower heat transfer coefficient, and hence lowered the thermal efficiency of the absorber.The absorber prototype and a small scale central receiver test tower was designed,manufactured and installed on an available 1 kWth medium flux concentrator toexperimentally test and validate the concept. Different configurations of mesh screens were tested at different air mass flow rates. The best mesh configurationproduced hot outlet air at 405 °C to 488 °C at thermal efficiencies of 87 % to 58 % in an average incident flux level of 55.3 kW/m2. The results produced by the model were in very good agreement with the experimental results at higher flow rates and adequately predicted its trend. At lower flow rates,however, the model overestimated the thermal performance. Finally this projectproved that the wire mesh screen absorber concept with a gradually decreasing porosity is a suitable solution for the use in an open volumetric air receiver to charge a rock bed thermal energy storage system.

[发布机构] Stellenbosch University