Review and performance evaluation of roughened solar air heaters

Solar air heater is an eco-friendly, economical and simple device which is used to harness solar energy for space heating, process heating and agricultural applications. The thermal performance of solar air heater can be improved by the application of artificial roughness on the underside of absorber surface. The heat transfer and friction characteristics of artificially roughened solar air heaters with different roughness geometries have been reviewed in this article. The article presents the authoritative account of the current progress on topic, discusses the previous developments, and throws light on the future directions. An attempt has been made to compare the performance of solar air heater having different types of roughness geometries based on correlations proposed in the literature. Thermo-hydraulic performance parameter (η), thermal efficiency (ηth), thermal efficiency improvement factor (TEIF), effective efficiency (ηeff), and exergetic efficiency (ηex) are evaluated to gauge the performance of different roughness geometries.