Heat Transfer and Thermodynamic Analysis of Heat Pipe-Assisted Latent Heat Thermal Energy Storage Systems for Concentrating Solar Power Applications

Thermal energy storage (TES) is the key advantage of concentrating solar power (CSP) systems. Among various types of TES, latent heat thermal energy storage (LHTES) benefits from large energy density and virtually isothermal operation that can potentially reduce the cost of TES systems significantly. However, most of the phase change materials (PCMs) used in LHTES systems have a prohibitively low thermal conductivity. This work presents a novel approach to substantially improve the heat transfer rates of a LHTES system by incorporating heat pipes (HPs) and/or thermosyphons (TSs) to circumvent the large thermal resistances of the PCMs. The basic design of a HP-assisted LHTES module is presented and a thermal network modeling approach is developed for system level analysis of the thermal response of the module. The model is further extended to predict the thermal performance of a large-scale LHTES system. An exergy analysis is also presented to investigate the second-law efficiency of the LHTES systems and guidelines are provided for the design of LHTES systems to achieve maximum second-law performance accounting for the practical constraints imposed on the operation of solar LHTES systems. The performance of HPs and TSs as the heart of the proposed technology is also studied in detail to ensure that the HPs/TSs are capable of providing the assigned heat transfer load. Numerical analysis of LHTESs is necessary for fully understanding the complex physical phenomena involved, including the solid-liquid and liquid-vapor phase changes and hydrodynamics of the HPs/TSs. To this end, a full numerical simulation of a HP-assisted LHTES for dish-Stirling applications is presented. Conjugate heat transfer effects in a HP-PCM system are analyzed and the effects of HP spacing on the heat transfer response are investigated. The benefits offered by the integration of LHTES with CSP systems, such as damping the temporal variations of solar radiation and shifting the power generation to times of higher demand, are proved