Investigating thermal performance characteristics of subsea compressed gas energy storage systems with gas-liquid phase change

Abstract

One of the main sustainable development goals for the United Nations is the reduction of greenhouse gases from fossil fuel combustion. Reduction of fossil fuels is possible by shifting the electricity energy market towards renewable energy sources (RES). Although RES technologies such as wind and solar power are now mature enough to effectively combat climate change, the intermittency in supply presents severe engineering challenges when large RES farms are interconnected to electricity grids. Such challenges of RES could be mitigated through the integration of energy storage systems (ESS). Due to the ocean covering the vast majority of Earth’s surface while a considerable amount of Earth’s population lives within the vicinity of the sea it is logical to invest in offshore ESSs. The study presented throughout this dissertation deals with a novel concept of a subsea hydro-pneumatic energy storage (HPES) system utilising carbon dioxide (CO2) as the compressible fluid. Carbon dioxide was studied since it is able to experience a phase change (gas-liquid-gas) during the storage cycle in typical subsea temperatures when limiting the peak operating pressure below the critical value. The influence of integrating a piston and an inner liner within the accumulator, to mitigate issues related to gas dissolution in sea water and corrosion were explored. A preliminary thermal analysis of the novel concept was carried out through in-house developed codes using Python® V3.8. The proposed accumulator was initially studied when utilising air as the compressible fluid, thus providing an initial understanding of the thermal behaviour of the accumulator in the presence of the integrated piston and inner liner. Air was then substituted with CO2 to study its behaviour within the proposed accumulator. Conclusions for this study were drawn for the separate accumulators and through a comparison between the results of the two gases. Based on the results obtained, it has firstly been established that the diameter-to-length ratio influences the accumulator thermal behaviour irrespective of the fluid utilised for compression. The introduction of the piston in the accumulator only resulted in a marginally influence on the round-trip thermal efficiency of the system. On the other hand, the introduction of the inner liner was found to have a considerable influence on the thermal efficiency, especially when CO2 was used. This study concluded that a CO2-based accumulator manages to store more energy per unit accumulator volume and per unit mass of steel than one working with air.