Understanding thermal energy storage

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    Lindsay Wright, Policy Engagement Manager at the UK Energy Research Centre (UKERC), outlines the potential for thermal energy storage in the UK…

    Thermal energy storage uses different technologies to collect heat for future use, whether that’s hours, days, or even months later. This can happen at building, district, town or regional scale, depending on the technology used. Recent research for the UK Energy Research Centre (UKERC), led by Philip Eames at the University of Loughborough, has helped us to better understand how we use heat in the UK, which technologies are available for thermal energy storage and where they might be deployed and distributed, and the potential for thermal energy storage to balance peak grid load.

    Heat networks currently supply less than 2% of the UK’s space heating, compared to approximately 16% in Germany. Just under half (45-47%) of UK energy consumption is for heating purposes, and of the total national heat demand, space and water heating account for 63% and 14% respectively. Domestic heating makes up 57% of total heat use, but around 80% of the UK’s heat is currently generated by fossil fuels. If we are to achieve our greenhouse gas emission reduction targets, then it’s vital we develop low emission heating approaches.

    Until the UK’s building stock is transformed to be more thermally efficient, or replaced with energy efficient new build, the greatest use of heat in the UK will continue to be for space heating. To assess the feasibility and potential of a range of thermal storage approaches, the UKERC research team undertook 2 case studies using data for a house in Derby, and for the Pimlico district heating scheme in London.

    In the Derby case study, daily winter heat requirements and daily peak heat requirements were determined for a large family house and scaled, based on the predicted performance if the house was compliant with the Building Regulations of 1980, 1990 and 2010. Thermal stores were sized to meet the maximum space heat load for a 3 hour period to allow heat pump operation at periods of low electrical grid load. For a water-based sensible heat store, the storage volumes needed were found to range from 2.6m3 for the house constructed to 1980s Building Regulations, to 0.56m3 for construction to 2010 Building Regulations. A ‘theoretical’ phase change material (PCM)-based store was found to reduce these volumes by two thirds.

    Given that PCM storage is likely to become a viable technology in the next few years, the researchers found that combining it with an electric air-source heat pump as part of a retrofit could be technically possible. If an appropriate demand side management strategy was also in place, demand could be reduced, and supply challenges for electricity utilities minimised.

    In the second case study, the researchers analysed the Pimlico District Heating Undertaking which includes a 2500m3 thermal store built in the 1950s, providing a balancing function to match variable supply and demand as well as an emergency buffer to ensure seamless supply in the event of planned or unexpected maintenance. The thermal store allows better control and plant efficiency; without it, the system would need to vary in operation to meet the changing demand, and so run inefficiently.

    The team investigated the potential additional national electrical generation and peak grid load resulting from the deployment of different numbers of air source heat pumps with different performance characteristics, and calculated the potential storage in GWh of heat and electric equivalent that could be achieved with distributed thermal storage. They found that 2 million air source heat pumps with a winter COP of 2, each meeting a 12kW thermal load, would require an extra 12GW of electrical generation (compared to a current winter peak load of just under 60GW). If each dwelling equipped with a heat pump system had 3 hours of thermal storage, then the equivalent electrical storage would be 36GWh. This would enable improved capacity factors of generation plant to be realised and reduce the amount of additional power generation capacity needed to meet this additional load.

    The expansion of heat networks in the UK is possible in areas of high heat demand, although installation costs are high at present. If the electricity supply is decarbonised, CHP will no longer be the lowest carbon option and large MW-scale heat pumps may prove preferential.

    The wide-scale adoption of air source heat pumps for space heating will also require significant investments due to the seasonal variation and magnitude of peak winter loads. Strengthening of the low voltage electrical network and significant additional generation capacity will be needed in addition to major building refurbishment to reduce heat loads.

    Distributed thermal energy storage can provide a significant diurnal load shifting capability. However, without the development of effective latent or thermochemical heat storage systems, the storage volumes required will be large and difficult to integrate into existing domestic dwellings.

    This article is based on the findings of the 2014 UKERC Research Report: Eames, P., Loveday, D., Haines, V. and Romanos, P. (2014) The Future Role of Thermal Energy Storage in the UK Energy System: An Assessment of the Technical Feasibility and Factors Influencing Adoption

    Lindsay Wright

    Policy Engagement Manager

    UK Energy Research Centre (UKERC)

    www.ukerc.ac.uk

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