Numerical Modeling of the Thermal Behavior of Subsea Hydro-Pneumatic Energy Storage Accumulators Using Air and CO2

Abstract

This paper numerically models the thermal performance of offshore hydro-pneumatic energy storage (HPES) systems composed of a subsea accumulator pre-charged with a compressed gas. A time-marching numerical approach combining the first law of thermodynamics with heat transfer equations is used to investigate the influence of replacing air within an HPES system with carbon dioxide (CO2). The latter 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 point. The influences of integrating a piston and an inner liner within the accumulator to mitigate issues related to gas dissolution in seawater and corrosion are explored. It is found that the energy storage capacity of subsea HPES accumulators increases substantially when CO2 is used as the compressible fluid in lieu of air, irrespective of the accumulator set up. It is also noted that the length-to-diameter ratio of the accumulator has a considerable influence on the round-trip thermal efficiency for both air- and CO2-based accumulators. Another factor influencing the round-trip thermal efficiency is the presence of the inner liner. Moreover, the CO2-based HPES system yields a lower round-trip thermal efficiency over that of air.