Analysis of a Multi-Stage finned PCM based storage system for Solar Thermal Power Generation

Abstract

The sensible storage system utilizing a hot and cold tank is the current commercial technology for solar parabolic trough thermal plants. This technology is very expensive, because of its large storage material requirement, two tanks and heat exchanger. It also has high parasitics. The use of phase change material (PCM) offer higher storage capacity per unit mass. The wide operating temperature range (about 100°C) in parabolic trough plants meant that many PCMs with different melting points in series must be used. Investigation of using five PCMs with different melting point resulted into a storage system with storage material inventory higher than that of the two-tank system due to slow discharging rates. In this study, a multistage finned latent heat storage system model was developed and performance analysis was conducted. A model was developed for a four (NaNO3, KNO3/KCl, KNO3 and KOH PCMs) and five (NaNO3, KNO3/KCl, KNO3, KOH and MgCl2/KCl PCMs) stage cascaded storage system. Various charging and discharging mass flow rates were simulated and for each mass flow rate, the length of the storage system that will satisfy the boundary conditions of the plant at a periodically balanced state was determined. Results showed that using A HTF charging and discharging mass flow rate of 0.025kg/s and 0.03kg/s respectively has the highest percentage phase change of 70% meaning better utilization of storage material. Also the four stage cascade was found to have a percentage phase change of 56% which is lower than that of the five stage cascade. Considering a capacity of 875MWhth, which is the capacity suitable for the 6 hours operation of a 50MWe parabolic trough plant, a storage material inventory of about 25, 000 tonnes is required corresponding to a net volumetric specific capacity of 72.8 kWh/m3 which is about 2.5 times that of the existing two tank system. This clearly shows the higher storage density of the multi-stage finned LHS system. The LHS system is passive and thus has very low parasitics compared to the two-tank system.