The open volumetric air receiver (OVAR)-based central solar thermal systems provide air at a temperature > 1000 K. Such a receiver is comprised of porous absorbers, which are exposed to a high heat-flux > 800 Suns (1 Sun = 1 kW/m2). A reliable assessment of heat transfer in an OVAR is necessary to operate such a receiver under transient conditions. Based on a literature review, the need for developing a comprehensive, unsteady, heat transfer model is realized. In this paper, a seven-equations based, one-dimensional, zonal model is deduced. This includes heat transfer in porous absorber, primary-air, return-air, receiver casing, and their detailed interaction. The zonal model is validated with an inhouse experiment showing its predictive capability, for unsteady and steady conditions, within the reported uncertainty of ±7%. The validated model is used for investigating the effect of operating conditions and absorber geometry on the thermal performance of an absorber. Some of the salient observations are (a) the maximum absorber porosity of 70–90% may be preferred for non-volumetric and volumetric-heating conditions, (b) the minimum air-return ratio should be 0.7, and (c) the smallest gap to absorber-length ratio of 0.2 should suffice. Finally, suggestions are provided for extending the model.
Electricity and gas-based heat treatment of metal is an energy-intensive process. To mitigate the use of such high-grade energy the concept of an open volumetric air receiver-based solar convective furnace (SCF) system is developed for the heat treatment of metal. This system includes an in-situ waste heat recovery mechanism. This paper presents a Joule heating-based, controlled, experimental assessment of a laboratory-scale, retrofitted, SCF system for generating benchmark data. The reported measurements illustrate the heat transfer for (a) the charging and discharging process of thermal energy storage and (b) the two-stage heat treatment of metal with an in-situ heat recovery process. The overall system efficiency, including heat recovery, heat storage, and heat transfer, is found to be 24%. Thus, the SCF system can serve as a viable alternative to an electrical energy-based heat treatment furnace.
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