How do you generate power, when the wind is not blowing, at offshore wind farms? Use Compressed Air.

Compressed air energy storage for offshore turbines

This article was first published in WindMax, the quarterly technical data publication of Windpower Monthly. Click here to find out more. The first large-scale commercial plant was built in Huntorf in Germany and has now been in successful operation for many years.

 From article, (In its simplest form, a CAES [Compressed Air Energy] system for an offshore wind turbine would use a multistage compressor driven from the electrical output of the turbine generator, and a separate expander-driven generator, to feed back the stored energy when there is sufficient demand.

It would be impractical on a single turbine to burn fossil fuel to enhance energy recovery in the expander – and would defeat the objective of a totally green system.

The expansion of air in unfired turbo expander systems to recover energy has well-known history in air-separation plants, factory air systems and aero engine testbeds, so the technology required is reasonably well established, as it is for the compressor, which would likely be of a standard packaged centrifugal type for which there are a wealth of references.

In a single-stage expansion from 10 bar down to atmospheric pressure the expander exhaust temperature would reach cryogenic levels.

Energy would be recovered and power would be available from the expander to drive a generator, but without a fossil fuel combustion system the power available would be limited. The power can be substantially increased however, and very low air temperatures avoided, by a variety of methods.

In a multi-stage expander, ambient temperature sea water could be used to provide interstage heating of the expanding air and could also preheat the first stage of air as the temperature inside the storage vessel falls due to the declining pressure in the reservoir. This would make use of free heat in the ocean as the energy source for what is in effect a heat pump.

Compressing air from atmospheric pressure to 10 bar requires an intercooled multi-stage compressor. The heated water from the compressor intercoolers can be stored in an insulated vessel, and also used, in the energy recovery phase, to preheat the expander inlet air stream, or for interstage heating. 

The cooling water could be retained in a closed system in which the water is repeatedly cycled to and from hot and cold water storage tanks.

To adapt a buoyancy column for CAES would require a machinery room between the column itself and the tower to house the necessary compressor, expander and ancillaries. 

Strengthening of the end closures of the column would also be necessary. Elimination of the need for a standby oil- or gas-fired turbine generator, plus the opportunity to offer stored energy at a premium price during peak demand would offset the capital costs involved.

The system is entirely flexible and the calculated powers are based on an arbitrary choice of a 10-bar pressure.

If more power is required the storage pressure can be increased, or the column diameter increased to give more storage volume; if less power is acceptable the storage pressure could be reduced.

The system described above offers a potential solution to the problem of the intermittent and unpredictable nature of wind.

Not only does a large buoyant turbine support offer a CAES reservoir of exactly the right size, but the reservoir is located exactly where it is required.)

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