Seasonal and Short-term Energy Storage Through the Integration of Solar PV/T with Thermochemical Sorption Technology for Domestic Applications

Abstract

To maximize the utilisation of solar energy and improve the solar fraction in domestic households, this PhD study explored the novel integration of solar Photovoltaic/Thermal (PV/T) collector and thermochemical sorption energy short-term & seasonal storage system for the high latitude regions, using the weather condition in the England city of Newcastle upon Tyne as a case study. Solar Photovoltaic/Thermal (PV/T) hybrid collector was used to convert solar energy into both electrical and thermal energy simultaneously, and this represents in principle one of the most efficient ways to utilize solar energy. Thermochemical sorption energy storage, as one type of thermal energy storage, exhibits its compelling advantages of much higher storage density and capacity than sensible and latent heat storages as well as feasibility for long-duration energy storage. The thermal energy output from the PV/T collector was used to provide domestic hot water and stored in the thermochemical sorption storage system for shifting seasonal load to supply the domestic hot water and space heating demands in winter. One of the key challenges in the development of such an integrated system are the operational condition match (running temperature and system capacity, etc.) between these two units and the supply and demand balance for heating within a typical household. This study looked at the sorption material and solar conversion technology selection, thermodynamic cycle development, system configuration optimization, parametric analysis and numerical modelling and simulation. The model of the Photovoltaic cell electrical power generation was developed by using the modelling tool of Matlab Simulink and subsequently coupled with a Computational Fluid Dynamics model for the simulation of simultaneous electrical and thermal energy production of the water-cooled Photovoltaic/Thermal collectors. The combined model was validated with commercial modules and used for performance evaluation and comparison of two types of PV/T collectors, with and without air-gap. The dynamic performance of a water-jacket-type heat exchanger reactor used in the thermochemical sorption unit was numerically and parametrically investigated and integrated with the PV/T collector model for the whole system simulation. One of the key point to implement the integrated system is that the mass flow rate of the heat transfer fluid (i.e. the water loop) that extracted heat from the PV/T collectors was adjusted to deliver heat at desired temperatures, especially for thermochemical sorption energy storage due to the variation of solar irradiation during the day and the mono-variant equilibrium characteristics of the thermochemical sorption cycle. It was found that the Photovoltaic/Thermal collector with air gap could produce 28 ~133 litre of hot water per day per m2 collector for the output temperature range of 40~100 °C; whereas, the Photovoltaic/Thermal collector without air gap was not competent for the purpose studied in this work especially in the cold regions. The application case studies suggested that an installation of 26 m2 air-gap Photovoltaic/Thermal collectors integrated with the thermochemical sorption energy storage (TSES) system using 6.26 m3 of adsorbent volume with SrCl2-NH3 working pair can fully satisfy the domestic hot water (DHW) demand of an ordinary single household in the city of Newcastle upon Tyne all year round with 100% solar source, and cover at least half of the annual electricity consumption. To contribute high solar fraction for annual heating demands, including the DHW and space heating (SPH), in a typical household in high latitude regions, the compressor-assisted thermochemical sorption energy storage (CATSES) system was introduced and investigated. The PV/T supplies electrical energy for the compressor to assist lower desorption temperatures for the TSES resulting in a considerable improvement on utilising a low-grade heat from the PV/T. The optimisation was conducted to explore the optimal temperature of the heat transfer fluid (HTF) used to carry thermal energy from the PV/T to store in the TSES. The system performance of using a variety of reactive salts such as CaCl2, NaI, BaBr2 and SrCl2 were studied and analysed for the application in terms of, e.g., winter temperature output, storage volume, minimum compression ratio (CR) of the compressor, minimum PV/T installation area to meet the 100% solar fraction for heating demands and electricity required in the proposed system. The results suggested that CaCl2 and SrCl2 were the two promising reactive salts to be employed in the integrated system to provide 100% solar fraction for annual heating demands in a typical household in high latitude regions with sufficiently high winter temperature output for domestic hot water and space heating demands. The minimum CR that work with CaCl2 and SrCl2 were 9.44 and 11.35 respectively when 30 m2 of PV/T was installed (averaged installable rooftop area for a household in the UK) with sufficient amount of electricity supplied by the PV/T. If the compressor has the ability to work at the CR of 16, the PV/T installation area can be reduced to 22.31 and 25.82 m2 when CaCl2 and SrCl2 were used respectively with the additional amount of electricity imported from the grid. When the adsorbent density of 450 kg/m3 was applied, storage material volume of CaCl2 and SrCl2 were 28.52 and 21.91m3 respectively to be able to supply 100% thermal energy in wintertime in a typical household in Newcastle upon Tyne, UK. The future works may focus on analysing the CASTES with denser composite adsorbents to obtain a higher energy density. Moreover, other types of heat exchanger reactor, such as the fin-tube one, may be used to achieve better heat transfer performances. The operational control of the CATSES may be studied to explore the way to physically store and release thermal energy to balance the real-time thermal energy supply (from the PV/T) and demand (from a household) to obtain the maximum efficiency.