Application Research On Second Heat Transfer Type Solar Water Heater

At present, as the world economy has developed rapidly, conventional energy crisis has been more and more serious. It is an important task that new energy especially solar energy should be exploited and utilized. All-glass solar vacuum tubular collector and flat-plate solar collector which are most widely used, have disadvantage of low thermal efficiency, lacking of energy storage equipment, and low cleanness of water. The concept about a new type solar collector which combines heat absorption with energy storage as well as thermal extraction, is proposed in this paper. The new type solar collector which uses second heat transfer form, makes use of phase transition material to absorb solar energy in the day and produce hot water at night. By this means, not only the problem of energy storage can be solved, but also the generation and usage of hot water can be available at the same time which makes the water more clean.Heat transfer performance of second heat transfer type solar water heater and solar collector with energy storage are studied in the paper through theoretical and experimental research. First of all, working principle of vacuum tube solar collector and principle of solar thermal storage are analyzed theoretically. Secondly, relevant heat transfer parameters of solar collector are tested through experiments, which includes temperature variation of heat transfer medium in vacuum tube,law of phase transition and temperature distribution of energy storage material,temperature variation of outlet water, as well as the impact of water flow on heat transfer performance of copper pipe.The experimental results show that,first,when water or glycol solution is used as heat transfer medium,and coating vacuum tube uses as collector,average daily efficiency of the second heat transfer type solar water heater can be achieved to 54%,however,its average heat loss coefficient is only 0.63W/(m2·K).The absorption energy of heat transfer medium which can be transfered to water in storage tank through heat conduction and thermal convection.It will guarantee that the heat can be transmitted to storage tank when the hot water be used during the night.Secondly,the average daily efficiency of the storage type solar water heating system is 52.03%,in addition, the average heat loss coefficient of direct system is 0.58W/(m2·K),and the average heat loss coefficient of indirect system is 0.58W/(m2·K).The storage type solar water heating system is forced circulation system,system circulating water transfer energy to phase change materials through spiral coil heat exchanger during the process of energy storage. In the curse of heating removal,phase change materials surrounded by copper pipe transfer energy to the water which flows through copper pipe.After the release of phase change materials,when temperature belows the phase transition point,it will appears that phase change materials condensed in brass appearance surface,and then phase change materials which more distant from the coil are still in a liquid form.The condensed materials on outer surface hinder liquid energy storage materials from transferring energy to the water within the copper pipe,Therefore,it will cause the stored energy can not be extracted.Second heat transfer type solar water heater has advantage of good resistance to frost and easy for energy storage as well as producing hot water with high cleanness. By comparison to conventional solar collectors, there also exist some shortcomings such as low thermal efficiency and high cost.In order to enhance the practicality of the system, much improved experiments should be carried out in the future.