Researchers at the University of Malta have kicked off research to assess how the significant amounts of CO2 available from carbon capture and storage (CCS) facilities may support offshore energy storage systems co-located within the same footprint of wind farms.
The Investigating the thermal performance of Subsea Energy Storage Accumulators with a 2-Phase Fluid Transition (SEA2F) project will see the application of a novel approach that compresses the gas within an accumulator located underwater to leverage the excellent heat sink and source characteristics provided by open seawater.
The proposed technology is based on a Hydro-Pneumatic Energy Storage (HPES) principle, and builds on standard off-the-shelf components, making it more reliable, the university writes.
Hydraulic pumps and turbines are used for energy conversion, enabling a higher power density than gas compressors and expanders while also offering the possibility to be deployed subsea.
During periods of an oversupply of wind power, seawater is injected into the accumulator, compressing the gas through the formation of a liquid piston and storing energy. In periods of low wind and high energy demand, the pressurized water is allowed to flow to a turbine and generate electricity.
“Through advanced computational modelling of the thermal behaviour operating in subsea conditions, it is predicted that replacing air with CO2 as the compressible fluid has the potential of increasing the storage density by at least 50% in lower pressure applications,” said Tonio Sant , professor at the University of Malta and project coordinator.
“This is achievable at sea depth ranges where both fixed and floating turbines are being deployed, thus permitting co-location with offshore wind farms.”
According to the University of Malta, a wide range of emerging storage technologies are present but are not necessarily suitable for deployment in the tough offshore environment on a scale that caters to the demand of offshore wind farms.
Co-locating energy storage systems offshore within the same footprint of the wind farm avoids the need for space on land to accommodate additional infrastructure.
The subsea environment is said to allow the CO2 to undergo phase change, from gas to liquid during compression, to increase the stored energy substantially while achieving quasi-isothermal conditions.
Seawater enables high thermal round-trip efficiencies to be attained without the need for thermal energy storage systems that are presently being integrated within recent compressed gas energy storage technologies. The novel concept simplifies the storage solution significantly, without loss of efficiency, making it more adaptable for offshore use.
In 2017, the team at the University of Malta deployed an air-based HPES first prototype in the Grand Harbour of Valletta. The experimental campaign demonstrated that high thermal efficiencies exceeding 95% are achievable using the novel approach.
The research team is now working with the university spin-off company FLASC B.V. to evaluate market opportunities for CO2-based offshore applications demanding long-term energy storage.