Experimental Study And Numerical Simulation Of The Double Helix Coil Phase Change Thermal Storage

Solar energy is among renewable energy that has the most abundant quantity,but it is distributed and intermittent and these heavily inhibit its utilization.Fortunately,thermal energy storage technologies can be used to remove the instability and volatility of renewable energy and they are key factors for achieving reliable and effective utilization of solar energy.Thermal energy storage can not only realize controlled storage and release of low and medium grade energy such as solar energy,industrial waste heat,waste heat from movable heat source,but can also achieve stabile output for heat and power conversion system by initiative adjustment.Therefore,effective and stabile thermal energy storage technologies are a focus of research in utilization of renewable energy.And the material modification and heat accumulator structure optimization are the technological difficulties that should be comprehensively considered in thermal energy storage technologies studies.Based on the energy flow characteristics of solar energy and the intended temperature range of thermal energy storage,the erythritol which has a high thermal energy density and a good stability was chosen to be the heat storage material,and a double spiral coil pipe phase change accumulator using erythritol as the heat storage material was designed.Firstly,the effects of the tube diameter,the curvature radius and the flow rate on the pressure drop and convective heat transfer of the accumulator are analyzed and compared by calculation and simulation.The results showed that as the tube diameter of the spiral coil pipe increased,the heat transfer coefficient increased while the pressure drop decreased;as the curvature radius of the spiral coil pipe decreased,the pressure drop increased;and the flow rate had a much greater effect on the pressure drop and heat transfer coefficient.Secondly,a low and medium temperature solar phase change thermal energy storage experiment rig which had a visual window was established,the thermal energy storage characteristic of erythritol in the double spiral coil pipe phase change accumulator was experimented,the temperature variation and the melting process of the PCM in the double spiral coil pipe phase change accumulator was achieved and the effect of the flow rate on thermal energy storage characteristic was analyzed.The experiment results indicated that the melting of the PCM in the accumulator took 450 min,the PCM showed a very uniform phase change in the melting process,and the total melting ratio was 95%;the temperature of the testing point at the top of the phase change accumulator and close to the pipe wall increased fast,the temperature of the phase change material(PCM)near the bottom of the tank increased slower;changing the inlet flow rate of the heat conduction oil had little effect on the inlet and outlet temperature of the accumulator and almost had no effect on the heat storage rate;changing the inlet temperature of the thermal conduction oil could notably accelerate the melting rate of the PCM.Then,the temperature variation and the melting process of the PCM in the double spiral coil pipe phase change accumulator and the effect of the flow rate was simulated.Finally,the costs and benefits of a solar water heating system with/without phase change thermal energy storage were compared.