Performance improvement in solar water heating systems-A review

Solar energy is free, environmentally clean, and therefore it is accepted as one of the most promising alternative energy sources. The effective use of solar energy is hindered by the intermittent nature of its availability, limiting its use and effectiveness in domestic and industrial applications especially in water heating. Conversion of solar energy into thermal energy is the easiest and most used method. The efficiency of solar thermal conversion is around 70% but solar electrical direct conversion system has an efficiency of only 17%. Solar water heating systems are mostly suited for its ease of operation and simple maintenance. Many research papers revealed that the improvement on thermal efficiency of solar water heating systems resulted in techniques to improve the convection heat transfer. Solar water heating systems are classified into two broad categories as passive and active systems. Passive techniques have been used to improve the convective heat transfer. The techniques like insertion of twisted tapes and its geometry, etc., play a vital role to improve the performance of solar water heating systems. This review paper summarizes the previous works on solar water heating systems with various heat transfer enhancement techniques include collector design, collector tilt angle, coating of pipes, fluid flow rate, thermal insulation, integrated collector storage, thermal energy storage, use of phase change materials, and insertion of twisted tapes. This paper also discussed the methods to optimize and simulate the solar water heating systems to understand flow and thermal behavior in solar collectors that would lead to the improvement of the thermal performance of solar collectors. (C) 2014 Elsevier Ltd. All rights reserved.