Pumped Thermal Electricity Storage

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.

The proposed project has significant potential for impact within the academic community and beyond. Low cost, safe and reliable electricity storage will become more and more necessary with the increasing penetration of renewable electricity technologies because it will enable inflexible or intermittent electricity generation to be matched with varying electricity demand. It will also allow more efficient use of existing generation, transmission and distribution hardware since the need to bring on line costly and inefficient standing reserve will be reduced, power plant will be able to operate closer to optimum load, and bottlenecks in the distribution network will be lessened.

The concept of pumped thermal electricity storage (PTES) was devised in the UK and much of its development and manufacture has a high chance of being based here in the future. If so, this would provide excellent wealth creation and employment opportunities as the world market for electricity storage is very substantial indeed. For example, in the UK alone the doubling of the current storage inventory deemed necessary for the further integration of renewable generation plant constitutes an extra 3 GW of installed storage (power) capacity and an extra 30 GWh of (energy) storage.

PTES is a new concept and consequently there remains much research to be done to underpin its development and to optimise future designs and operating strategies. The proposed work is of a fundamental nature and is applicable to a number of other areas, including several other storage technologies (e.g., advanced adiabatic compressed air energy storage and geothermal storage), heat pumps, internal combustion and Stirling engines, and novel forms of combined heat and power. We will also contribute to the field of power networks and markets by examining the benefits that PTES might bring to the electricity supply chain. All of these are areas with vibrant academic and industrial communities.

We will achieve our impact via a number of routes. We intend publication in a range of leading journals (e.g., IEEE, ASME, Applied Energy) and presentation at both national and international conferences (e.g., IEEE, IRES, the UK Heat Transfer Conference). Furthermore, the project is industrially driven with significant participation from the SME developing PTES (i.e., the inventors). We will also receive guidance from representatives of the electricity supply industry. These involvements will provide a clear route to exploitation of the results.

The proposed project constitutes an interdisciplinary collaboration, bringing together expertise in complementary areas. We believe that this will enhance its overall impact. The team members all have strong track records in communicating their research and in organising dissemination events - examples are given in the Academic Beneficiaries section.