Design procedure for energy efficient latent heat storages with heat transfer fluid channels and finite inlet power

Over the past years, the greenhouse gas emission reduction policy has been pushing the growth of renewables in our current energy market, which for heat and cold production has been hampered by an imbalanced thermal energy supply and demand. Embedding compact and efficient thermal energy storages enables this problem to be overcome. Recent studies have been focusing on latent heat storages, because of their superior storage capacity. However, their storage capacity is substantially reduced due to the insuperable need for heat transfer enhancement techniques and heat carrier fluid channels. Heat transfer enhancement techniques for latent heat storages are well known nowadays, such as high conductive fin structures and encapsulation techniques. Both aim at reducing the characteristic length of the PCM (Phase Change Material), i.e. the PCM thickness, to improve the charging characteristics. Most of the numerical studies focus on charging the storage with a constant wall temperature boundary condition. In this research, numerical simulations of latent heat storages are performed by incorporating a heat transfer fluid. The fluid delivers and extracts heat, which is limited by the fluids temperature change and heat capacity rate. Charging and discharging power of the storage are therefore limited. In order to reduce the computational cost, an averaged convection-diffusion equation is solved for the heat transfer of the fluid flow. The flow is therefore solved only in the streamwise direction, whereas heat transfer over the channel boundaries is incorporated by lateral diffusion. By adjusting the design of the storage, such as the width of the heat transfer fluid channels, the amount of channels, etc., the storage performance can be improved. Improvements are both with respect to storage capacity, charging power and residual fluid heat, which results in a higher energy efficiency. The proposed design procedure suggests designing the storages with many narrow parallel heat transfer fluid channels surrounded by associated thin layers of PCM. This procedure holds irrespective of the presence of highly conductive fin structures in the PCM. This leads to the conclusion that optimal performance will mainly be determined by production limits and pumping power.status: publishe