Abstract
Employing phase change materials (PCMs) for latent heat storage (LHS) application has a great potential to improve a solar thermal system performance. Despite this fact, the use of PCM in this area is quite limited due to the poor thermal conductivity of available PCMs. Therefore, heat transfer enhancement is one of the essential strategies that can overcome this obstacle. In this paper and related project, a PCM heat exchanger (HX) is purposely designed with spiral-wired tubes and integrated in an indirect solar assisted heat pump test system. Although the spiral-wired tube has not been applied in a PCM HX, it is expected to enhance significantly the PCM heat transfer and heat storage performance. To verify this and understand the PCM heat storage and releasement processes, a detailed 3D CFD model has been developed for the PCM HX and validated with measurements. The temperature variations and visualizations of the PCM during charging and discharging processes are therefore simulated and presented temporally. Furthermore, the effects of different inlet heat transfer fluid flow rates and temperatures on the PCM melting/solidification time are demonstrated in this study. Some significant simulation results have been obtained which can instruct efficiently the operation of the heat exchanger and its integration with the solar system.
Highlights
Solar thermal systems have been widely applied in domestic hot water production due to their sustainability and stability in operations
The selected phase change material in this study was organic paraffin type A16 which has a melting point temperature 16 °C. This temperature was selected based on the following considerations: (i) The phase change materials (PCMs) heat exchanger (HX) takes a role of heat source to the evaporator of the solar heat pump. (ii) The UK weather is not always sunny and solar irradiance is not high, the PCM can be charged as much as possible from the solar collector, especially during winter time period. (iii) During summer, the ambient temperatures in the UK climate can be above 18 °C
Latent thermal energy storage technologies with PCMs are essential to be applied to solar thermal systems considering the intermittent nature of solar energy resource and requirement of system compactness
Summary
Solar thermal systems have been widely applied in domestic hot water production due to their sustainability and stability in operations. The research outcomes showed that some important design and operating parameters could affect significantly the performance of the PCM energy storage tank These included storage tank structures, PCM types and temperature and flow rate of heat transfer fluid (HTF), etc. Of those applicable design and evaluation methods, CFD modelling can be an efficient simulation tool to predict the melting/solidification behaviour of PCM by numerically solving Navier-Stokes partial differential equations of mass, energy and momentum [18]. With a validated CFD model, some significant and detailed simulation results can be obtained These include dynamic profiles of PCM temperature, melting/ solidification rate, heat transfer rate and energy stored/released, etc. The simulation results are significant to understand the working mechanism of PCM melting/solidification process and effect of operating conditions on the PCM HX performance and optimising the heat exchanger operation
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