Advanced hybrid thermochemical-compression seasonal solar energy storage and heat pump system (Solar S&HP)

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Solar energy can provide both electricity and heat without greenhouse gas emissions. The amount of solar radiation incident on the roof of a typical UK home still exceeds its heating demand over the year. However, there is only 1% of renewable heat from solar currently exploited in the UK. The paramount reason for that is the seasonal mismatch between heating demand and solar thermal energy availability and the lack of extensive deployment of thermal energy storage in the UK. Secondly, because of relatively weak solar radiation being far away from equator leads to relatively low temperature heat using the existing solar thermal collectors, particularly during periods outside summer. In this case, it is imperative to develop a seasonal solar energy storage that can effectively store abundant but relatively low temperature solar heat in summer and utilise this at the desired temperature for space and hot water heating in winter, so that 100% solar fraction can be used for space and hot water 'zero-carbon' heating. Thermochemical sorption energy storage technology offers higher energy density with minimum loss due to the temperature-independent means of storage, storing energy as chemical potential. However, its desorption temperature (i.e. temperature of the energy charging process) is relatively high, which makes it problematic to recover solar energy in high-latitude regions like the UK when using the most mature and economic solar thermal collector technology (flat-plate or evacuated tube type). Therefore, an advanced hybrid thermochemical sorption and vapour compression processes is proposed in this project, the integration of the electric-driven compressor, using a small amount of electricity input, enables a large amount of low or ultra-low temperature solar heat (<50 degC) to be efficiently used for thermochemical desorption, leading to enhance the efficiency, capability and flexibility of solar energy storage and heat pumping (Solar S&HP). Since such a hybrid system utilises thermal energy and electric energy simultaneously, it is a win-win solution when it couples with a solar hybrid thermal-photovoltaic (T-PV) collector. The solar T/PV collector supplies the hybrid storage system with solar heat and electricity, whilst the timely extraction of solar heat from the hybrid solar T-PV collector also allows the PV cell to operate at a lower temperature to increase its electrical conversion efficiency, leading to substantially improved overall solar energy conversion efficiency. Some other detailed advantages of the proposed system are, (1) the quality (thermal only) and quantity of different energy inputs (both thermal and electrical) can be adjusted to complement each other whilst storing energy so as to cope with highly variable weather conditions whilst maximising solar energy conversion. Even if solar electricity is not available, electricity from the grid in summer can be used, which has a ~15% lower carbon intensity than in winter. (2) The hybrid thermochemical cycle has a lower desorption temperature which reduces sensible heat loss from the solid sorbent and metallic reactor during the energy storage process which further increases the overall energy efficiency of storage system. (3) During thermal discharging in winter: (a) primary energy consumption for heating can be eliminated, and (b) the collective effect of thermal-driven and electric-driven heat pump processes can be used in extremely cold weather conditions. The whole SSTES system can provide heating at near zero carbon intensity, its carbon emission is approximately 92% and 85% lower comparing to gas boiler and electric heat pump technology, as revealed by the preliminary calculation results.