The increasing use of renewable energy technologies for electricity generation, many of which have an unpredictably intermittent nature, will inevitably lead to a greater need for grid-scale electrical energy storage schemes. The UK government's target (as part of the EU Renewable Energy Directive) is for 20% of energy to come from renewable sources by 2020. This will require a much a higher proportion of electricity to be generated from uncontrollable sources such as wind, and one of the associated challenges will be providing sufficient electricity storage capacity to deal with the resulting variability in supply. Currently there is about 30 GWh of electricity storage capacity in the UK, with a maximum power output of around 3 GW. Nearly all of this is in the form of Pumped Hydro Storage (PHS), which is expensive and its scope for extension is limited by geographical constraints. Estimates vary, but the expert view is that our storage inventory will need to at least double over the next decade or so in order to efficiently accommodate the expanding fraction of wind and other renewable generation technologies. There is thus strong motivation to develop new, efficient and cost-effective electricity storage methods. This project is aimed at investigating a novel storage technology known as Pumped Thermal Electricity Storage (PTES). PTES uses a high temperature-ratio heat pump to convert electrical energy into thermal energy which is then stored in two large reservoirs - one hot and one cold. The reservoirs contain gravel, or a similar high heat capacity material, and are able to store the energy much more compactly than PHS. When required, the thermal energy can be converted back into electrical energy by effectively running the heat pump backwards as a heat engine. The projected round-trip efficiency is approximately 75%, which is a little lower than PHS, but PTES has a number of potential benefits, including low capital cost and no geographical constraints. Compared to chemical energy storage methods (batteries and flow batteries) it also has the advantage of not requiring any hazardous or scarce substances. The success of PTES will hinge upon minimising the effect of various thermodynamic irreversibilities (for example, heat transfer across substantial temperature differences and losses associated with compression and expansion of the working fluid) whilst simultaneously keeping capital costs low. Accordingly, the proposed work focuses on investigating fundamental thermodynamic, fluid flow and heat transfer processes using a combination of experimental, theoretical and computational methods. An important aim of the work is also to develop and validate an overall system model and to use this to optimise the design and operation strategy, and to examine the benefits that PTES might bring to the electricity supply chain.

Decarbonising the energy system in many countries (including both China and the UK) is likely to involve the large-scale deployment of renewable electricity generators with intermittent output and the electrification of energy services such as heat and transport that have very low load factors. These changes in electricity supply and demand will lead to a great need for energy storage. Our work for the Carbon Trust has estimated that the effective deployment of energy storage in the UK could reduce the cost of a low-carbon electricity system by £15 billion in 2030. The deployment and operation of storage is a complex task, since it can provide many different services, including energy arbitrage (buying off-peak and selling at higher peak prices), energy reserves, resolving transmission and distribution constraints and improving system reliability. The weight placed on each of these could affect the pattern of investment, and sophisticated planning tools are required to ensure that optimal decisions can be taken. We will improve these tools so that we can accurately represent a variety of storage systems. Much of the existing work on storage assumes that it might be used simply to postpone the "like for like" replacement of network assets that would otherwise be overloaded, but we will consider more radical options, using storage to actively manage the distribution network as part of the broader smart grid. We can use our models to calculate the economic value of energy storage when it provides a range of services to network companies and system users. We will measure the option value of having a storage system that can be deployed in much less time than it takes to get consent to build a transmission line, adding flexibility in how we respond to uncertainty over the future evolution of the energy system. It is important that such decisions are made on the basis of appropriate models, capable of quantifying the wide range of services that storage can provide, taking account of the way that electricity generation and demand varies over the course of the day and the year, and measuring the impact of transmission and distribution network effects. It is important to recognise that many countries (including the UK) have liberalised their electricity industries, and storage will only be pursued if companies believe that a viable business case exists. Work has started (via the Low Carbon Networks Fund) to test particular business models for demonstration projects, but this needs to be generalised. We will provide a quantified assessment of whether there is a business case for energy storage at present, and of what needs to be done to create one. This will involve a detailed study of the contracts that would be written around electricity storage, drawing lessons from existing arrangements for other kinds of storage, such as for gas or agricultural commodities. We will study how the rules of the electricity market could affect the choices of storage and generation technology. We will ask what policies are needed to ensure that storage can be economically viable when sensibly deployed and operated. We will identify technology policies that can help move energy storage from prototypes to large scale deployment. International transmission may complement energy storage within a country, and we will assess the potentially conflicting incentives if neighbouring countries adopt different strategies for dealing with intermittency. Too many debates around energy policy today are based on assertions without sufficient evidence. The models that we will develop, and the analysis that we will perform, will provide numerical estimates of the effectiveness of a range of policies, allowing regulators and other policy-makers to choose options that will lead to decarbonisation in the most effective way.

The UK electricity system faces challenges of unprecedented proportions. It is expected that 35 to 40% of the UK electricity demand will be met by renewable generation by 2020, an order of magnitude increase from the present levels. In the context of the targets proposed by the UK Climate Change Committee it is expected that the electricity sector would be almost entirely decarbonised by 2030 with significantly increased levels of electricity production and demand driven by the incorporation of heat and transport sectors into the electricity system. The key concerns are associated with system integration costs driven by radical changes on both the supply and the demand side of the UK low-carbon system. Our analysis to date suggests that a low-carbon electricity future would lead to a massive reduction in the utilisation of conventional electricity generation, transmission and distribution assets. The large-scale deployment of energy storage could mitigate this reduction in utilisation, producing significant savings. In this context, the proposed research aims at (i) developing novel approaches for evaluating the economic and environmental benefits of a range of energy storage technologies that could enhance efficiency of system operation and increase asset utilization; and (ii) innovation around 4 storage technologies; Na-ion, redox flow batteries (RFB), supercapacitors, and thermal energy storage (TES). These have been selected because of their relevance to grid-scale storage applications, their potential for transformative research, our strong and world-leading research track record on these topics and UK opportunities for exploitation of the innovations arising. At the heart of our proposal is a whole systems approach, recognising the need for electrical network experts to work with experts in control, converters and storage, to develop optimum solutions and options for a range of future energy scenarios. This is essential if we are to properly take into account constraints imposed by the network on the storage technologies, and in return limitations imposed by the storage technologies on the network. Our work places emphasis on future energy scenarios relevant to the UK, but the tools, methods and technologies we develop will have wide application. Our work will provide strategic insights and direction to a wide range of stakeholders regarding the development and integration of energy storage technologies in future low carbon electricity grids, and is inspired by both (i) limitations in current grid regulation, market operation, grid investment and control practices that prevent the role of energy storage being understood and its economic and environmental value quantified, and (ii) existing barriers to the development and deployment of cost effective energy storage solutions for grid application. Key outputs from this programme will be; a roadmap for the development of grid scale storage suited to application in the UK; an analysis of policy options that would appropriately support the deployment of storage in the UK; a blueprint for the control of storage in UK distribution networks; patents and high impact papers relating to breakthrough innovations in energy storage technologies; new tools and techniques to analyse the integration of storage into low carbon electrical networks; and a cohort of researchers and PhD students with the correct skills and experience needed to support the future research, development and deployment in this area.

The ability to store and release energy on demand is essential to an energy future that is based on clean, non-polluting and sustainable renewable energy. This includes both electrical and thermal energy and a large number of technologies are being developed to fulfil this need. Energy storage will become a major industry in our century and will employ hundreds of thousands of people globally. Energy storage will be everywhere - in large scale batteries connected to electrical networks, in homes to store energy generated from solar panels and in cars, replacing petrol engines. In order to meet this challenge and to ensure that UK plays an important role in this industry we will form a Centre of Doctoral Training in to train researchers at the highest level to help form and influence the direction of Energy Storage technologies. Our students will receive training in all aspects of energy but concentrating on the core technologies of electrochemical storage (batteries and supercapacitors), mechanical storage, thermal storage and superconducting magnetic energy storage. They will have the opportunity to interact with industrialists and gain experience in running a grid connected Lithium-ion battery. They will also undertake a major three-year research project allowing them to specialise in the topic of their choice.