Development and validation of direct contact gas liquid heat exchange for energy storage
Item statusRestricted Access
Embargo end date03/07/2021
McKinley, Daniel Howard
Decarbonisation of the electrical grid, necessitated by international targets to limit further global warming, will require a steadily increasing penetration of non-dispatchable intermittent renewable electricity generation sources. Energy storage has the potential to substantially increase the grid’s ability to accept greater quantities of renewables while maintaining stability. Pumped-Heat Energy Storage (PHES) is a form of electrical energy storage targeted to provide storage on the order of days or weeks, as opposed to short durations of storage currently available through battery technologies. PHES systems could be utilised at substations across the country to help the grid endure diurnal load fluctuations and periods of low wind and solar resource. This system is based upon the Joule-Brayton cycle, which operates in the reverse direction to store exergy and the forward direction to generate electricity. An inert gas is the cycle working fluid, and a liquid is used to transfer heat to and from thermal exergy stores. Exergy is stored as temperature differences from ambient in balanced hot and cold stores. PHES development at the University of Edinburgh has iteratively explored different system architectures, and focused on increasing confidence in components within these architectures where there is uncertainty with regard to performance. Early work concentrated on gas-liquid mixing within the cylinder of a compressor/expander machine, while current work has eliminated such mixing and instead proposes the use of large scale, direct-contact heat exchangers. Such exchangers suffer from significant uncertainty for this application owing to the lack of existing experimental correlations with which to predict their behaviour at the proposed operating pressure and temperature. As a result, gas liquid surface interactions and heat-transfer between gas and liquid streams are largely unknown, hindering system development. Two experimental campaigns were conducted to verify components in both the early and current system iterations. The first demonstrated a novel in-cylinder gasliquid mixing device and quantified device behaviour against the no-mix condition. The second campaign demonstrated operation of a scaled pilot packed-column direct contact heat exchanger, where gas and liquid comingled to exchange heat. Existing experimental correlations for high pressure packed column flooding were verified against experimental results, and the overall heat exchange coefficient was calculated. Results were used to validate a finite volume heat transfer model based upon previous correlations. Successful gas-liquid heat exchange in the temperature and pressure range of interest was demonstrated, advancing PHES development and informing future iterations of the system.