Caisson energy storage

From Energy-Experts

Energy storage in submerged, open bottomed, anchored caissons

One of the biggest problems with a large penetration of Wind power, (according to Paul – Frederik Bach of this network, who should know - he ran the Western Denmark Power grid for a number of years) is the sudden curtailment of wind farm output over a wide area as storms sweep over. Hence a means of prolonging the output whilst fossil stations on warming ramp up; and load is shed is needed. One possibility is sunken caissons, which could be integrated with the floating support structure for the wind turbine which the Norwegians are already developing. Whilst perfectly feasible, using existing technology, whether or not this is worthwhile is another matter. Thought experiment

Imagine a mug filled with water submerged and inverted in a bowl of water. Drill a small hole in the bottom of the mug, and glue a straw in it Blow on the straw and air will enter the mug forcing water out through the bottom (You will have to hold the mug under the water as it tries to float) Stop blowing and the air will be forced out as water re enters the mug.

Energy storage in sunken caissons

One possibility is to sink the type of caisson used in the Normandy Mulberry Harbours of which - about 115 of these huge structures were built in about 9 months http://www.everything2.org/index.pl?node_id=1306625

They initially were sunk off Folkestone to hide them, and then re floated prior to D Day using compressed air then towed into position and sunk.

In theory similar structures could be constructed and floated into much deeper water, sunk and anchored to the bottom using rock anchors.

A flexible air pipe could be brought to the surface, to say a wind turbine base, housing an air compressor and an air turbine generator set.

By simply pumping compressed air down to the caisson, water would be ejected lowering the internal water surface to the bottom of the caissons.

To recover the stored energy the air valve would be opened and the energy recovered via the air turbine as the water re enters and air is forced out.

Note that because the container is open bottomed, there is no internal or external pressure force - both inside and outside the caisson will be at identical pressures at all times.

However there will be massive floatation forces to be restrained but these present no real engineering difficulty. The flexible pipe would simply be a solid steel pipe which over 100 m can cope with significant bending - the Pluto pipelines, to bring fuel to allied troops after D Day were basically a 3 inch dia steel pipe unreeled from a gigantic 10 m diameter cotton real - 17 were laid in a few days across the Channel after D Day just by being unreeled (These Pluto pipes are the basis for modern submerged cables incidentally). The deeper the water, the more energy can be stored per caisson. Since the compressed air will be at very high pressure a lower cost air turbine can be used (compared to a low pressure one). Note most of the energy will be in the displaced water not the compressed air itself.

My calculation is as follows: (Checked and initial error corrected by Roger Button, thanks)

Assume the top of the caisson is 100 m below the surface.

A 1 m sq surface will have a water pressure of 100 m3 above it = 100 x 1000kg = 10e5 kg = 10e6 newtons.

Force this surface up by 1 m and you perform 10e6 Newton metres of work = 1.0 M Joules = 1.0 MJ

There are 3.6 MJ / kWh, so 1.0 MJ = 10MJ/3.6MJ = 0.277 kWh.

Scale that up by the volume and you get 0.277 x 20 x 20 x 70 = 7777 kWh = 7.7 MWh

Assuming an in out efficiency of say 50%,

Then the energy available as electric is probably say 3.5 MWh = 3.5 MW for 1.0 hours.

However, as Bob Carne has pointed out, since he average output of a 3.5 MW turbine is only about 1.2 MW, the caisson wold give the average output for 3 hours, or if tailed of linearly, 6 hours, which is enough time to start a coal station from warm.

On the face of it, it would work to have one of these per 3 MW floating wind turbine, and this would store sufficient energy to allow the wind turbine output to be prolonged sufficiently for thermal plants to start up, and or loads to be shed. Whether it is an economically worthwhile proposition is another matter. However considering that the Norwegians are planning constructing floating wind turbines for the North Sea, it would seem to be a logical place to put them, ie as integrated into part of the floatation structure.

An alternative to compressed air could be a water pump and turbine, but this would have to be located 100m down which may post maintenance problems - but is pretty normal stuff in the North Sea oil business for example. This would be more efficient than the compressed air system and probably cheaper.

Mulberry Harbours

Phoenix: The primary breakwater was to be provided by concrete caissons. These were to be from 25 to 60 feet high and 174 to 204 feet long. The displacement was up to 6,000 tons.

http://www.hksw.org/despatches_107_1_a.htm

115 of these were built in about 9 months:

http://www.everything2.org/index.pl?node_id=1306625

Pluto – pipeline under the ocean

http://en.wikipedia.org/wiki/Operation_Pluto

After full-scale testing of a 45 nautical mile (83 km) HAIS pipe between Swansea in Wales and Watermouth in North Devon, the first line to France was laid on August 12, 1944, over the 70 nautical miles (130 km) from the Isle of Wight through the English Channel to Cherbourg. A further HAIS pipe and two HAMELs followed. As the fighting moved closer to Germany, 17 other lines (11 HAIS and 6 HAMEL) were laid from Dungeness to Ambleteuse in the Pas-de-Calais.

Note the 3 inch pipe bent around the 10 m dia drum Pluto //http://news.bbc.co.uk/1/shared/spl/hi/picture_gallery/04/uk_d_day_inventions/html/6.stm//

Pluto was Pipeline Under The Ocean, vital arteries that pumped millions of gallons of fuel from Britain to Allied forces in France.

Two lines were built, codenamed Dumbo and Bambi. Pumping stations were disguised as ice cream shops, garages and bungalows.

The sea bed cables were laid by 30ft diameter Conun drums pulled by tugs. This is one of more than 1,000 secret photographs of the pipeline operation newly released and available to be viewed free at The National Archives in Kew.