Cold energy storage: fundamentals and applications

This thesis concerns cold energy storage (CES) technology. Such a technology produces cold energy by consuming electricity in a refrigerator and stores cold energy in an eutectic phase change material (PCM) in a temperature range of (TPCM is the PCM storage temperature and Ta is the environmental temperature), resulting in a cold exergy efficiency less than 100%. The stored cold energy can be either directly extracted by a cold discharge process or utilized through a Rankine cycle at peak hours for electricity generation. The aim of the research is to study fundamental aspects and address the scientific and technological challenges associated with the CES technology. Methods for storing high grade, high energy density and temperature-adaptive cold energy are to be developed. Another objective is to develop innovative solutions for enhancing charge/ discharge processes. Particular attention is paid to the use of a prototype CO2 based CES system to investigate the feasibility of CES technology for small scale systems. In this work, a criteria for PCM selection for high grade and high energy density cold storage is established. For enhancing charging/ discharging rate of a PCM device, metal foams are embedded in PCM to form a PCM composite. Parametric study on the CES system is done based on a CO2 Rankine cycle for achieving an optimal cold storage efficiency. Investigations have been carried out on the performance of a small scale CES system. These include CES with an open and a close Rankine cycle and a piston based engine for cold to power conversion in the cycle. A method for improving grade of stored cold energy is using eutectic salt-water solutions for forming a binary/ ternary cold storage system, by which the eutectic temperature is lowered. PCMs with lower freezing temperature and smaller molecular weight are selected as components in the binary/ ternary system. However, due to the potential issue of compatibility of PCM molecular structures, it is critical to select PCMs which have comparable melting temperatures and compatible molecular structures. PCM composite is formed by embedding metal foams in PCM solutions. Cold discharging rate, defined as the power transfer of cold energy per unit time during the discharge process, is greatly affected by thermal diffusivity and thermal conductivity of the PCM composite. Combined effect of cold radiation and convection is to be considered for assessing the value of cold discharging rate, which becomes more significant for large PCM capsules under low PCM temperatures and low Reynolds number (Re). Cold utilisation in a CES system using Rankine cycles is theoretically studied. Storage efficiency of the CES system is a round trip efficiency of electricity, which is defined as ratio of output electricity to the input electricity. A storage efficiency as high as 43.9% has been shown to be possible for the CES system. However, the storage efficiency is generally between 30%~40% in consideration of the actual efficiencies of cryogen pump, regenerator, engine and refrigerator. Piston based engines with a new valve scheme is experimentally investigated. Compared with small engine, large engine system has apparently larger capacity for power generation, but the engine efficiency is reduced due to the block of the exhaust gas in the chamber. In the presented case study, the efficiency of the large engine is 38.5% while the storage efficiency of the CES system is approximately 22.0%. In the point view of net electricity output for peak-shifting, CES is a feasible technology that need to be further developed. In brief, the work of the CES research are summarized as follows: • Improvement of cold charging/ discharging rate by embedding open-cell metal foams in PCM; • Assessment of cold discharging rate by considering the combined effect of cold radiation and convection; • Optimization of cold storage efficiency by developing computer program based on sub-critical CO2 properties; • Cold to power conversion by using a piston based engine coupled with a new valve scheme.