Integrating Compressed Air Energy Storage (CAES) in Floating Offshore Wind Turbines

Abstract

The design of an offshore energy storage system carries unknowns which need to be studied at an early stage of the project to avoid unnecessary costs of failures. These risks have led to an increasing dependence on more sophisticated mathematical models. This paper refers specifically to energy storage in the offshore wind farming industry and has the objective of proposing an adiabatic compressed air energy (A-CAES) system which would be integrated on a semi-submersible offshore wind turbine (OWT) platform. Calculations in respect to the sizing of the main sub-components of the system are included and estimates for the overall round trip efficiency are presented. Preliminary calculations to size the various parts of the energy storage system (ESS) have been carried out based on the energy availability of an offshore 8 MW wind turbine with real wind data from the North Sea. The load data to determine the lowest 12-hour demand period was taken from the Nordpool database. The calculations of the proposed conceptual design are based on an operational scenario in which the 24-hour period of a particular day is split in a 12-hour charging and 12-hour discharging cycle. For charging, a 5-bank, 2-stage compressor train is used to pressurize a number of steel cylindrical vessels with compressed air. This is followed by a process in which the compressed air is discharged across 12 hours using a 2-bank, 2-stage expander turbine. The multiple compression banks enable a modular power delivery to the air storage vessels, with the number of compressors utilized varying subject to wind availability. The two stages allowed for the air to be cooled in between the stages using heat exchangers, transferring the heat of compression to a pressurized sea water circuit. The hot water would be stored in thermally insulated vessels at 350°C to heat the inlet expanding air in the discharge period. A 70 and 100 Bar charging scenarios, both with a cushion pressure (CP) in the air storage vessel (ASV) of 10 Bar at the end of the discharge cycle have been considered. Standard performance criteria are calculated such as compression and expansion ratios, inlet and outlet temperatures for the respective expansion and compression air streams and flow rates within the heat exchangers to come up with an indicative sizing proposal for the respective turbo machinery and storage vessels making up the system. Round trip efficiencies are also calculated. The study determined that a CAES system consisting of 9 compressed air storage vessels operating with a peak pressure of 100 Bar should meet the storage requirements. It is also estimated that the entire CAES system would require around 1082 m2 of deck area on the platform to accommodate the pressure vessels, the compressor and expander trains, the heat exchanger and the hot water storage vessel.

Summary of research on operation control of electrochemical energy storage power plants for offshore wind power

“…Peter P. Vella has presented A scheme to integrate an adiabatic Compressed air Energy storage (A-CAES) system onto a semi-submersible offshore wind turbine (OWT) platform. The scheme includes the calculation of the size of the main sub-components of the system and the estimation of the overall round-trip efficiency [33].Chen Y presents a conceptual solution aimed at harnessing wind energy within offshore wind farms and converting it into heat. This system involves the storage of heat in a thermochemical storage unit, its distribution to heating users when needed, and its release upon request.…”

Research on a power smoothing control strategy for energy storage hydraulic wind turbines

“…The HESS state space model can be obtained by joining Equations ( 13), ( 15), ( 16), (18), and (19). When the hydraulic pump motor is operating under motor conditions, the state space model is expressed as…”

“…8 At the same time, the efficiency of hydraulic accumulator is about 90%, and the hydraulic energy storage system (HESS) can realize seamless connection with the HWT, further improve the performance of the HWT and realize the smooth short-term power of the HWT in. 18 The efficiency comparison between hydraulic accumulator and other energy storage methods is shown in Table 1.…”