Heat transfer enhancement for U-pipe evacuated tube solar absorber by high-emissivity coating on metal fin

In the solar thermal field, the U-pipe evacuated tube collector has been widely used to provide hot water for domestic usage and space heating, or integrated with concentrating reflector to deliver thermal power in higher temperature range. Enhancing heat transfer process inside the U-pipe evacuated tube has significant effect on improving the overall thermal efficiency of the collector. However, the current heat transfer enhancement methods require either structure modification or material property change, and are therefore difficult to be promoted for commercial use. Herein, we proposed an easy and feasible method of depositing a high-emissivity coating on a metal fin to enhance the interior heat transfer. The effect of applying this method on the U-pipe evacuated tube with and without compound parabolic concentrator integration was investigated by numerical simulation. We found that this method exerts elevating improvement effect with an increasing operating temperature of the collector. The proposed method yields a maximum improvement of 2.7% for a single U-pipe evacuated tube operating up to 150 ?degrees C. Higher optimization effect was achieved in the case of compound parabolic concentrator integration. By employing this method, we found an 8.4% performance improvement when the tube is combined with a concentrator with concentration ratio of 6.0 at an operation temperature of 200 ?degrees C. At the background that the structure of the U-pipe has been mature and commercialized for a long time, the proposed method with easy-feasibility and low cost is expected to have potential for implementation.