Performance evaluation of three latent heat storage designs for cogeneration applications

Abstract

Well-integrated thermal energy storage units can enhance flexibility and profitability for a cogeneration system by enabling its decoupling of electricity and heat production. In the present study, novel latent heat thermal energy storage technologies are numerically investigated on their thermal and economic performance to evaluate their implementation at an existing combined cycle power plant. Three commercially available storage designs are analyzed: one shell-and-tube heat exchanger design based on planar spiral coils, and two types of advanced macro-encapsulated designs with capsules resembling ellipsoid and slab in shape, respectively. For the spiral coil design, three-dimensional flow velocity and temperature fields are simulated with finite volume method to predict the transient storage heat transfer process, including the effect of secondary flow induced by centrifugal forces. For the macro-encapsulated designs, effective heat transfer coefficients between heat transfer fluid (HTF) and phase change material (PCM) are inferred from scaled-down storage prototyping and testing. A one-dimensional two-phase packed bed model was developed based on the apparent heat capacity-based enthalpy method to numerically study the heat transfer in macro-encapsulated PCM. With an operating temperature range of 46–72 °C and a HTF supplying flowrate range of 4.2–8.4 m3/h defined by the cogeneration strategy, thermal power and accumulated storage capacity are calculated and compared for the first three hours of charge and the first hour of discharge for the three designs. The effect from increasing the HTF flowrate to accelerate charging/discharging processes is indicated by the simulation results. Performance comparison among the three designs shows that the slab capsule design exhibits the highest accumulated storage capacity (710 kWh) and state of charge (40%) after three hours of charge, though it has a lower theoretical total storage capacity (1760 kWh) than the spiral coil design (1830 kWh). The ellipsoid capsule design shows a slightly lower accumulated storage capacity (700 kWh) than the slab design for 3-hr charge and an equivalent accumulated storage capacity/depth of discharge (250 kWh/14%) as the latter. Furthermore, the storage power cost of the slab capsule design is the lowest, by 6–12% lower than the spiral coil design and by 2–3% lower than the ellipsoid capsule design. However, with the highest design flowrate of 8.4 m3/h, the low state of charge (below 40%) after three hours and the low depth of discharge (below 14%) after one hour indicate that redesigning the heat transfer boundary conditions and the configurations of the three units are necessary to meet desirable storage performance in cogeneration applications.