This work presents the numerical investigation of transient behaviour and heat storage characteristics of 10 MJ of sensible heat storage (SHS) bed designed for discharging the heat in the temperature range of 523-623 K for solar power plant applications. Thermal model of heat storage bed in cylindrical configuration has been developed considering the heat transfer enhancement technique in the storage bed by incorporating the axial fins on the discharging tube surfaces. Materials choosen for the present analysis are high thermal conductivity, cast iron and cast steel bed and low thermal conductivity, concrete bed. Numbers of discharging tubes with axial fins have been optimized based on the discharging time.

 

Energy saving and development of efficient energy storage systems have been the main objectives especially where the energy source is intermittent like solar energy. Integration of thermal energy storage (TES) system can facilitate a continuous generation of power from solar thermal power plants. There are mainly three types of heat storage systems, sensible heat storage (SHS), latent heat storage (LHS) and thermo-chemical storage (TCS). In SHS, heat is stored by raising the temperature without causing the phase change of storage medium (liquid or solid). Liquid media (mostly molten salts) is the proven technology for SHS but major problems with liquid media system are bulky storage tanks for hot and cold fluids and expensive heat exchangers. Although one tank can be eliminated by using thermocline system but freezing of molten salts at high temperature is still a key problem and requirement of auxiliary heating units during freezing time leads to higher operating cost. Also most of the molten salts become unstable beyond 550 °C. An alternative to liquid media storage is LHS that involves phase transition (i.e., solid to liquid and vice versa) of storage material for storing/releasing heat. LHS technology is in development stage and presently, no commercially large scale storage heat applications are available. Fernandez et al. and Khare et al. have shown that concrete, cast iron and cast steel can be the suitable materials for high temperature SHS applications.  The TES system using solid state sensible heat storage material (SHSM) is usually implemented by embedding a tube register heat exchanger in SHSM to transfer thermal energy to or from the heat transfer fluid (HTF), such as water, steam, molten salt, air and synthetic oil. The advantages of concrete systems include low cost of storage media, ease of handling of the material and low degradation of heat transfer between the heat exchanger and concrete. The main requisite properties of SHSM are high heat capacity, density and thermal conductivity. Tamme et al. explored the possibility of employing ceramic and concrete as SHSM for high temperature heat storage application. Laing et al. investigated the performance of SHS system employing ceramic and concrete for the maximum heat storage temperature of 663 K and 350 kW of storage capacity. The concrete was selected due to its low exergy loss, low cost and easy handling although ceramic has 20% higher storage capacity and 35% more thermal conductivity. Laing et al. presented the design and test results of a SHS system employing concrete as SHSM and thermic oil as HTF in the temperature range of 573-673 K. For storage capacity of 400 kWh, the reported charging and discharging time were 6 hr. According to John et al. concrete has better resistance to thermal cyclic loading during charging and discharging and could retain its mechanical properties over such cycles.  The main difficulty in using concrete as SHSM is larger in size and slower in heat transfer rate. However, heat transfer characteristics in low conductivity solid storage media such as concrete, magnesia, etc. can be improved by incorporating heat transfer enhancement techniques and size of storage bed can be minimised by selecting high heat capacity materials with high temperature swing. Nandi et al. reported that concrete and castable ceramic were low cost (25-30 $ kWh) and durable SHS systems