Micro-scale Co-generation Near-isothermal-Adiabatic Compressed Air Energy Storage

Decarbonisation of the UK's energy system will require substantial action at a regional and local level. Therefore, the UK's energy system is growing rapidly to a more decentralised model by 2050 with a great level of small-scale electricity and heat generation at the distribution level, where wind and solar renewable energies will play a large role. However, the intermittent nature of these renewable sources presents a great challenge in energy generation and load balance maintenance to ensure stability and reliability of the power network. This highlights the need for electricity storage technologies as they provide flexibility to store excess electricity for times when it is in demand. The majority of recent installations deploy fast response electricity storage systems (e.g. batteries) with short-duration electricity storage (minutes-days) and short-discharge duration of up to 4 hours. However, technologies with long-duration electricity storage (days-weeks) and medium-duration discharge of over 4 hours, with negligible capacity and efficiency degradation are required to ensure power supply security in all weather conditions (e.g. wind or solar energies are not available for several days).

There are several possible technologies for long-duration energy storage, e.g., pumped-hydro storage, liquid air energy storage and compressed air energy storage (CAES). Among them, adiabatic CAES systems (ACAES) has the lowest installed energy capital costs (2-50$/kWh) for a wide range of storage applications from micro scale (few kW) to large scale (few MW). In conventional ACAES systems, the electricity is used to compress air in compressors, generating high levels of heat during the process. The heat of the compressed air is removed at the outlets of the compressors and stored in a thermal energy storage (TES) unit, while the cool compressed air is stored in a cavern at depths of hundreds of metres. To discharge the energy on demand, the cool compressed air heats up in the TES before expansion in turbines to generate electricity.

Despite its promising features for decarbonising the electricity power system, there are major challenges which hinder further development of ACAES systems, including (1) limitations on the underground geology, (2) low roundtrip efficiency and (3) thermal and structural challenges on the TES unit because of high-temperature air at the outlets of the compressors. This proposal aims to address these major challenges through development of an affordable micro-scale co-generation near-isothermal and adiabatic CAES system with overground air storage vessels (micro-Ni-ACAES). The system utilizes near-isothermal and high-efficiency compressor/expander devices, TES and heat exchanger units based on an innovative composite phase change material and air storage vessels. The project will perform a fundamental experimental and modelling analyses to gain deep insight into the flow and thermal fields in the near-isothermal compressors/expanders as well as charging and discharging kinetics of the TES unit. Both isochoric and isobaric storage processes will be analysed. These fundamental studies will lead to efficient designs of the micro-Ni-ACAES system components and further support the development of a thermodynamics-based design tool. The design tool will be used to identify the system's optimum operating condition and control strategy for steady-state and dynamic operations of the system. Additionally, the project will include a techno-economic and environmental impact assessment in order to evaluate the economic viability of the system, as well as CO2 abatement and fossil fuel savings over the system's lifetime. The proposed high efficiency co-generation micro-Ni-ACAES systems are believed to be the future of the CAES technology, eventually culminating in decentralised microgrid power network in application to district energy network or commercial sectors (e.g. business parks).