Investigations of the thermodynamic efficiency limits of a novel subsea energy storage concept

Abstract

In light of the energy transition and the need to reduce emissions, efficient and capable energy storage devices are needed. Different concepts will have their individual pros and cons, an energy storage device placed subsea would provide high energy densities, long lifetime, and high efficiencies given that the unit could be designed so that it takes advantage of the cooling effects of the ocean.To fully evaluate a conceptual subsea energy storage device based on PHS (Pumped Hydro Storage), the theoretical efficiency limits were derived. Although several different subsea PHS concepts have been presented in various papers, a study outlining the basic thermodynamic limits has not been widely published, the intention of this paper is to fill this gap. Different standard thermodynamic cycles such as the Otto and Joule cycles have been analyzed. The best possible efficiency for an adiabatic process is achieved by compressing/expanding the gas in a combined Otto-Joule cycle which for a compression ratio of 50 would give a maximum possible efficiency of 93.2%. If enough time for heat transfer is given during the compression/expansion process an isothermal process would be achievable which would result in an isothermal process with a theoretical efficiency of 100%. It is interesting to note the difference between CAES (Compressed Air Energy Storage) and the subsea-PHS where in the former case the energy that is utilized is the expansion energy of the gas and for the PHS system the energy of the water flowing into the tank is the energy that is converted into electrical power where the compression energy of the gas reduces the available energy.The main criteria for the ideal cycle are low compression energy and high expansion energy. In addition, the heat generated in the compression process is not utilized so a very slow compression process where there are no finite heat transfers and thus very small entropy generation is desired. If a slow process is not possible, the entropy generation could be lowered by splitting the compression process into smaller steps. A case where an adiabatic compression process is split into several steps was investigated and it was found that splitting an adiabatic compression process into two steps with enough time for heat transfer in between would lower the amount of compression energy needed by 15% for an adiabatic case with a compression ratio of 100.The theoretical study provided in this paper will form the basis for detailed parameter studies and subsequent publications.