Cost evaluation of two concepts for the integration of hydro-pneumatic energy storage in floating wind turbines

Abstract

As costs of offshore wind are declining, it is sensible to evaluate options for integrating largescale energy storage to address grid management issues resulting from higher penetration of renewables. Integrating storage within the offshore turbine structure itself will reduce space requirements onshore and offer opportunities for cost reductions associated with longer and heftier power transmission cables. This is even more important for floating offshore wind turbines (FOWTs), which are being targeted for far offshore sites. Energy storage technologies may be classified into four main groups: mechanical (e.g. pumped-hydro, flywheels, compressed air), thermal (e.g. sensible/latent heat storage), electrochemical (batteries, capacitors, fuels cells) and chemical (e.g. power-to-gas, synthetic fuels). For an overview of technologies, refer to [2-4]. Pumped-hydro storage (PHS) is still the storage option with the largest capacity globally due to low cost, with efficiencies now reaching 80%. This is followed by compressed air energy storage (CAES), though this generally operates at lower round trip efficiencies (< 70 %) mainly due to the thermal losses incurred when air at atmospheric pressure is compressed to high storage pressures. Both PHS and CAES have a long service life (50-100,000 cycles), but they are geographically restricted by a need for high altitude terrain or underground caverns.