Fun Bags  

CNN has a report on a novel form of compressed air energy storage - using giant plastic bags in relatively deep (600 metres) of water - being researched in the UK.

The inventor envisages these being used in conjunction with offshore wind, wave and tidal energy as a way of dealing with the intermittency problem "on site" (or thereabouts).

The article quips that this is one case of plastic bags in the ocean serving a good cause, for a change.

While these bags have been dubbed "wind bags" and "energy bags", neither are particularly catchy or positive monikers - I think dubbing them "fun bags" would be much more attention grabbing.

More at the BBC and Digital Journal.

Since it became a viable energy resource around 20 years ago, wind power has emerged as a leading renewable technology. At the end of 2006 the worldwide capacity of all wind turbines was close to 75 gigawatts, which represents around one percent of all electricity use in the world. Three quarters of that wind power usage is currently based in Europe. The Danes lead the way with nearly 20 percent of their electricity created by wind. They are followed by Germany, which generates around half that amount and Spain around seven percent.

But although it's clean, plentiful and relatively cheap, there is an inherent problem with wind power. It's not always there when you need it, leaving more conventional, more polluting energy resources to take up the slack.

The wind's variability has been one of the sticking points for wind power growth in the UK energy market. Given the UK's famously inclement weather and its island status -- which offers unrivalled offshore facility - you might think that wind power capacity was being filled at a rate of knots. But in reality, wind power currently contributes about one percent of the UK's energy needs, with around 2000 on and offshore turbines up and running.

Whilst wind is no silver bullet to the energy problem, it can make an important contribution to the equation. This is especially true in the UK, which possesses 40 percent of Europe's total wind energy. The prospects for wind power could be greatly enhanced if cost-effective storage could be implemented. Some, like Minnesota based Xcel Energy, are putting their faith in new battery technology. But a UK professor, Seamus Garvey thinks he might have found another solution -- storing energy in flexible containers on the ocean floor.

Professor Garvey's idea of using Compressed Air Energy Storage (CAES) isn't a new one, but his methods are. Traditionally, CAES stows energy in a vast underground reservoir. During peak energy hours, air is released powering a turbine, which in turn produces electricity. There are currently only two CAES sites in the world -- in Huntorf, Germany and in McIntosh, Alabama. ...

The prospects for his energy storage idea with tidal power are perhaps even better. "With tidal power you can use a hydraulic ram. This can take a large flow of water at a low pressure. Out of that it can then give you a small flow of water at a high pressure."

Naturally, storing vast amounts of air requires vast amounts of storage. Professor Garvey envisages a cone-like structure stretching 50 meters wide at the top to around 80 meters across at the base. The bags are made of a combination of plastics. "A polyester reinforcement at the core with probably a polythene layer around that," Garvey said.

At a depth of around 600 meters, Professor Garvey calculates that the bags would be able to store 25 megajoules of energy for every meter cubed. The deep water is essential. "Only in deep water, where the pressure is greatest, are the bags a good economic proposition," Garvey explained.

Although there is an additional cost in fixing reinforcement cables and ballast, Garvey believes the future economic prospects for his invention are good. He plans to put the storage bags through smaller scale land-based tests, with four-meter-diameter bags, to prove that his calculations are right.

The centrifugal force required to compress the air is too great for small wind turbines to cope with, so much larger turbines will have to be installed for the project to realize its goals. Currently, wind turbines are situated in relatively shallow water -- around 40 meters. So how will the project work if the bags need to be at a depth of 600 meters? Well, a series of pipes will link turbine and bag and Professor Garvey believes the distances, in Europe at least, wouldn't have to be too long.

Research into floating turbines is underway and, as Professor Garvey points out there are steep ocean shelves off the west coast of France and Portugal and around the entire periphery of the Mediterranean. "You could put wind turbines on these shelves and within a few hundred meters, or kilometers you could be in 600-meter-deep water," he said.

Professor Garvey, who has secured a three year grant from German energy provider E.ON, is confident that with the right funding the UK can achieve its stated aim of providing 20 percent of its energy from renewables by 2020. "We're probably the richest country per head in the world in terms of renewable energy," he said. "But we're way behind Denmark, Germany and the United States."

Something that might work quite well with the "fun bags" concept is the idea of floating wind turbines - Technology Review has a look at advances in this area, noting "Advances in floating platforms could take wind farms far from coasts, reducing costs and skirting controversy" - Wind Power That Floats.

Offshore wind-farm developers would love to build in deep water more than 32 kilometers from shore, where stronger and steadier winds prevail and complaints about marred scenery are less likely. But building foundations to support wind turbines in water deeper than 20 meters is prohibitively expensive. Now, technology developers are stepping up work in floating turbines to make such farms feasible.

Several companies are on their way to demonstrating systems by borrowing heavily from oil and gas offshore platform technology. In December, the Dutch floating-turbine developer Blue H Technologies launched a test platform off Italy's southern coast; last month, the company announced its plans to install an additional test turbine off the coast of Massachusetts, and possibly begin constructing a full wind farm off the Italian coast, next year. Close behind is SWAY, based in Bergen, Norway, which raised $29 million last fall and plans to field a prototype of its floating wind turbine in 2010.

If these efforts succeed, they could open up a resource of immense scale. For example, according to a 2006 analysis by the U.S. Department of Energy, General Electric, and the Massachusetts Technology Collaborative, offshore wind resources on the Atlantic and Pacific coasts exceed the current electricity generation of the entire U.S. power industry.

The success of the floating turbine could hold the key to exploiting that resource. Wind farms such as those installed in Denmark, Germany, and other European waters and proposed for Nantucket Sound, in Massachusetts, suffer from a limited supply of marine construction equipment such as pile drivers and cranes. Emerging Energy Research, a consultancy based in Cambridge, MA, said last week that the global market for offshore wind energy could reach 40,000 megawatts by 2020--enough to power more than 30 million U.S. homes, and more than twice the scale of last year's wind installations worldwide--but only with greatly expanded marine construction capacity. Building even 2,000 megawatts of offshore wind over the next five years will require a significant increase in the marine supply chain, according to Keith Hays, the consultancy's research director.

Floating turbines can be assembled onshore and towed into position, making an end run around the offshore construction bottleneck. The platform that Blue H towed out of Brindisi Harbor in Puglia, Italy, this winter is called a tension-leg platform, a conventional offshore oil and gas platform design that floats below the surface, held rigidly in place by chains running to steel or concrete anchors on the seabed. Installed on top is an 80-kilowatt wind turbine fitted out with sensors to record the wave and wind forces experienced 10 kilometers offshore. Much bigger floating versions--2.5-megawatt and 3.5-megawatt turbines of the scale used in today's offshore wind farms--are under construction by Blue H and could be installed as soon as this fall.