Researchers are committed to develop robust and responsive technologies for renewable energy sources to avert from reliance on fossil fuels, which is the main cause of global warming and climate change. Solar energy based renewable energy technologies are valued as an important substitute to bridge gap between energy demand and generation. However, due to varying and inconsistent nature of solar energy during weather fluctuations, seasonal conditions and night times, the complete utilisation of technology is not guaranteed. Therefore, thermal energy storage (TES) system is considered as an imperative technology to be deployed within solar energy systems or heat recovery systems to maximise systems efficiency and to compensate for varying thermal irradiance. TES system can capture and store the excess amount of thermal energy during solar peak hours or recover from systems that would otherwise discard this excess amount of thermal energy. This stored energy is then made available to be utilised during solar off peak hours or night times.
Phase change material (PCM) based TES system is appraised as a viable option due to its excellent adoption to solar and heat recovery systems, higher thermal storage density and wide range of materials availability. However, due to its low thermal conductivity (≅ 0.2 W/mK), the rapid charging and discharging of TES system is a challenge. Therefore, there is a need for efficient and responsive heat exchange mechanism to boost the heat transfer within PCM.
In this study, transient analysis of two-dimensional computational model of vertical shell and tube based TES system is conducted. Commercial grade paraffin (RT44HC) is employed in shell as thermal storage material due to its higher thermal storage density, thermo-physical stability and compatibility with container material. Water is made to flow in tubes as heat transfer fluid. In this numerical study, the parametric investigations are performed to determine the enhancement in charging rate, discharging rate and thermal storage capacity of TES system. The parametric investigations involve geometrical orientations of tubes in shell with and without fins, inlet temperature and volume flow rate of HTF.
It is evident from numerical results that due to increase in effective surface area for heat transfer by vertical fins, the charging and discharging rate of paraffin based TES system can be significantly increased. Due to inclusion of vertical fins, conduction heat transfer is dominant mode of heat transfer in both charging and discharging processes. Furthermore, vertical fins do not restrict natural convection or buoyancy driven flow as compared to horizontal fins. Similarly, the inlet temperature has a noticeable impact on both charging and discharging process. In melting process, the sensible enthalpy is boosted due to rise in inlet temperature and thus the whole system thermal storage capacity is enhanced. Likewise, the effect of volume flow rate of HTF on charging and discharging rate is moderate as compared to inlet temperature of HTF. The numerical results are validated by experimental results.
To conclude, these findings present an understanding into how to increase charging and discharging rate of TES system so as to provide feasible design solutions for widespread domestic and commercial utilisation of TES technology.