Direct Absorption Process in an Annular Space for Innovative Solar Collector

Abstract

New configuration for direct absorption solar collector has been developed. In the new configuration, none circulated nano-fluid absorbs solar radiation through glass wall. The absorbed heat directly transfers to circulated water flowing inside copper tube submerged into the nano-fluid. Numerical model has been developed for flowing water, nano-fluid annular region and the copper tube which separates the two different fluids. Heat absorption of concentrated solar arrays reflected by parabolic trough is simulated as a heat source in the energy equation solved in nano-fluid region. Three size ratios of copper-to-glass tube diameters have been examined under same solar radiation. The efficiency of one meter of solar receiver unit with glass-to-copper tube size ratio of (1/2) increases significantly with flow rate up to 65%. Since, water flow rate changes increasingly affect the efficiency, heat convection at water side plays very important role. The size ratio of (1/4) shows slightly lower efficiency for all flow rates, while (3/4) size ratio shows very poor efficiency due to insufficient depth of absorption and reduced Reynolds number of water flow. Circulating water restricts the raising of nano-fluid temperature and in turn reduces thermal losses. At the side of nano-fluid, both optimal depth of absorption and characterized length of natural convection seem to be at size ratio of (1/2) or slightly less. Temperature rising per one meter decreases with flow rate despite of performance enhancing.