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A cautionary flow cell tale -- Storing energy I

A flow cell battery, a device that its latest marketers claim would be useful for storing energy from wind turbines, is currently being tested at the University of Alaska Fairbanks (UAF) Energy Center. The experimental Vanadium Redox Battery, supplied by VRB Power Systems of Vancouver, Canada, is rated at 3.3 kW and uses a pair of tanks filled with liquid electrolytes to store energy. The two solutions are pumped through the battery with positive and negative charges. The liquid storage medium differentiates the system from traditional lead-acid batteries, according to UAF's Jack Schmid, who adds that the device is roughly the size of three refrigerators. "All the energy is stored in the electrolyte," says Schmid. "In a conventional lead-acid battery, a constant chemical change is going on, and when the plates degrade, the battery degrades." The new system has no such solid surfaces so cannot suffer from the same problems.

Rigorous testing is now required. The university installed the device on campus in August and testing is expected to continue for several months. But, at least in theory, the battery will offer a long lifespan. "It's simple by design but it's got a lot of mechanical parts," Schmid says. "Operating it over a long period will allow us to get the feel of it. You have pumps and joints and you're moving fluid from one point to another. It has lots of potential benefits but you also have potential for problems." Initial testing will be limited to the lab but the battery will eventually be brought outdoors. "In the future, we'll expose it to cold weather and see what happens," he says. "Since the fluid is mostly sulphuric acid, it should be able to withstand extreme temperatures. Right now lead-acid batteries are good for storing energy, but this flow design might improve on that."

The flow cell battery has been around as a concept for 30 years or so, but has yet to make the grade commercially. Its Canadian sponsor, however, has faith in its potential. The battery was pioneered in Australia by Maria Skyllas-Kazacos of the University of New South Wales, the licence for which was sold to Melbourne listed company Pinnacle VRB in 1998. That company's international patents for the battery were acquired by VRB Power System of Canada in 2004 in a share swap deal, along with the rights to patents licensed by Pinnacle to Sumitomo Electric Industries (SEI) in Japan. VRB Power also acquired patents to another flow battery, Regenesys, from European utility RWE AG. RWE terminated work on Regenesys undertaken by British National Power on its takeover of the British utility in 2002; the economics of Regenesys had been subject to debate (Windpower Monthly, October 2000). SEI, too, apparently had similar reservations. "The bean counters said the technology was not commercial," says Rob Blackwell of VRB Pinnacle, which has only retained the Australian patents. He adds that there is a VRB battery at Tomamae Wind Farm Villa in Hokkaido, Japan, which came online in January 2005.

Although storage has the potential to enable power systems to operate more efficiently -- absorbing power at periods of low demand and releasing it at periods of high demand -- commercial viability can be elusive. Electricity from storage has a generation cost and will only be economic if the differential between the energy prices paid to the storage operator at times of high demand, and paid by the storage operator at times of low demand, is adequate. As the installed costs of most utility-scale storage systems are high (at least EUR 1000/kW) and load factors are low (because the system must spend at least half of the time charging), it is difficult to make ends meet. Several studies have suggested that around EUR 750/kW is a target figure for storage costs, but much depends on the economics of particular electricity systems. Dedicated storage to level the output of wind plant adds to their cost and its possible economic benefits are even more uncertain than storage for an entire power system. Installed costs and load factors of storage are similar to those of wind energy, which means that generation costs are roughly similar. If the rating of a dedicated store is half the rating of the wind farm with which it is associated, electricity prices from the facility will go up by about 50%. That corresponds to at least EUR 20/MWh, which is double the extra value and more of "firm power" above variable power (Windpower Monthly, February 2006).

In isolated island systems, however, where variable renewable energy supplies a high proportion of electricity requirements, the economic prospects for storage for this niche application are better. A 200 kW redox battery with a storage capacity of 800 kWh was installed by Pinnacle at Hydro Tasmania's Huxley Hill wind farm on King Island, which consists of two 850 kW Vestas turbines and three 250 kW Nordex turbines. The island is not connected to the mainland grid, but relies on its wind-diesel power system. VRB Power Systems has claimed that the pay back period is less than four years. Pinnacle's Blackwell is not so sure. "The provability of those claims is still subject to discussion," he says.

Strenuous marketing efforts by VRB Power Systems, which is publicly traded in Canada and interested in seeing its share price rise, have raised some recent interest in testing the company's product beyond North America. The Danish national laboratory at Risø, together with national transmission company, has bought a 15 kW unit with a storage capacity of 120 kWh, capable of delivering 15 kW an hour over an eight hour period. Delivery of the unit is delayed but expected in April, when it will be included in a miniature test-bench power system, SYSlab, where the load is provided by students and the power by Risø's wind turbines and other renewable energy units. The attraction of a vanadium battery, says Risø's Jens Carsten Hansen, is that its capacity can be incrementally increased in size, unlike a traditional battery. Risø is intending to evaluate if a VRB battery can improve the economics of a power system with a large penetration of wind energy, like that of Denmark.

Prior to the share swap deal with VRB Power Systems, Pinnacle had also licensed its technology to a company called Vanteck VRB, publicly listed in Canada, for the African continent. A 250 kW VRB system was subsequently commissioned by Vanteck in conjunction with South African utility ESCOM. Blackwell says a "friendly divorce" took place between Pinnacle and VRB Power in 2005. Meantime, VRB Power Systems claims to have made its largest sale to date -- of a 12 MWh battery system to be linked to a planned 6.9 MW extension of Ireland's 32 MW Sorne Hill wind farm in Donegal, should the extension get building consent. According to VRB, the purchaser of the battery is the project's operator, Tapbury Management Ltd, which is funding a study into the potential benefits, if any, of coupling the batteries to wind farms in Ireland. Tapbury is paying $6.3 million for the unit, claims VRB Power systems. The owner of Sorne Hill, wind investment specialist HgCapital, is definitely not involved in the project, says the company's Tom Murley.

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