A review on air heating applications with evacuated tubes: A focus on series and parallel tube configurations

Evacuated tube solar collectors (ETSCs) have gained great interest among researchers in solar energy applications. The reasons behind that are their high thermal efficiency, wide temperature range, and reasonable price. Consequently, ETSCs are increasingly used in air heating applications, including drying and greenhouses, humidification-dehumidification (HDH) desalination, heating/cooling, and many industrial processes. The entirety of the prior literature is concerned with reviewing ETSC modification, thermal performance analysis, economic aspects, and integration with nanofluids and PCM. However, the field still needs to cover the method of connection between tubes besides highlighting thermal applications, especially those related to air. Therefore, this review aims to compare evacuated tube configurations (series and parallel connections) and discuss air heating applications published in the decade from 2012 to 2023. The best configurations for a particular application depend on specific requirements. The results record that series configurations are appropriate for limited space and higher outlet temperatures, but greater flow resistance and the possibility of tube failure have an influence on the overall system. Parallel configurations offer independent operation and lower flow resistance, whereas installations necessitate a larger area and lower output temperatures. Additionally, 53% of the ETSC work is directed towards drying applications. Contrarily, solar cooking and heating/cooling industrial building applications are not adequately covered by ETSC. The presented review would assist engineers and scientists in following the critical updates in ETSC mechanisms and determining a suitable configuration for specific applications at a reasonable cost.