We all know that the main contender to provide electricity in a renewable manner - wind turbines - tends to have a highly fluctuating output on the short term basis - seconds, minutes, hours, days, weeks and in some cases, on a monthly basis. On a yearly basis, it tends to be very predictable - something like +/- 3 to 6 %, typically. So the main trick is to match the instantaneous supply with the varying (but more or less predictable) demand. One way to do this is to expand the size of the grid where wind turbines provide energy to/where energy is taken from. Another is to use short term (day, hour, weekly) storage; pumped hydropower is probably one of the most energy efficient and is the best practiced large scale example of this.

This works well if the combination of hills and water (salt water can also be used) is available. But there are lots of places where there is water but no hills - for example, Florida, southern Saskatchewan, Long Island and Cape Cod. So, along comes this idea (see http://www.kema.com/press_releases):

KEMA Collaborates on Large-Scale Offshore Energy Storage System

BURLINGTON, Mass., Sep 13, 2007 (News Release)

KEMA, in partnership with civil engineering firm Bureau Lievense and technology illustrators Rudolph and Robert Das, has developed an "Energy Island" concept to store power generated from an offshore wind farm. The innovative concept design is the initial result of an on-going feasibility study being conducted for Dutch energy companies.

The Energy Island designed by KEMA, Lievense and the Das brothers incorporates a new concept in pumped hydro storage - an inverse offshore pump accumulation station (IOPAC). On the Energy Island when there is a surplus of wind energy, the excess energy is used to pump sea water out of the interior 'subsurface-lake' into the surrounding sea. When there is a shortage of wind power, sea water is allowed to flow back into the interior 'lake' through commercially available generators to produce energy. The IOPAC is unique from conventional pumped hydro storage systems in that it would be stationed on an artificial island off the Dutch coast in the North Sea and comprised of a ring of dikes surrounding a 50 meter deep reservoir. The island itself would be built from materials dredged to deepen the interior reservoir.

KEMA analysis estimates that the proposed Energy Island storage system would have a maximum generation capacity of 1,500 MW, depending on the water level. It also would have an annual storage capacity of more than 20 GWh - enough energy to offset 500 to 840 kilotons of CO2 emissions. In the next phase of the feasibility study underway KEMA is further analyzing the costs and benefits of additional regulating reserve, download wind power, CO2 reduction, and environmental impact.

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Anyway, the 1.5 GW capacity is a really big example - about the size of the US portion of the Average Niagara Falls power plant output. However, smaller example could also be used for areas without the huge proposed wind power systems for the North Sea/Europe. And a variation on this could be tried in the US midwest "flatlands", where the combination of an island in the middle of/net to large rivers could be used. Or, an above ground/below ground reservoir could be employed. After all, what is needed is a an average head (potential drop) of 164 feet.

nb41