Thursday, October 04, 2007

Redox Batteries and Wind Power part 2


Wind Energy Intermittency and Grid Stability
-Are Flow Batteries the Answer?

Here is the continuation of Shay McGowan's paper:

Flow Batteries:
Flow batteries date back to the 19th century. They are best described as: ‘…..a form of battery in which electrolyte containing one or more dissolved electro-active species flows through a power cell / reactor in which chemical energy is converted to electricity. (3)

Vanadium-based redox (VRB) flow batteries operate by having two separate electrolyte solutions with a different ‘redox’ potential. This solution is stored in tanks outside the battery. When there is a demand for electricity the electrolyte solutions are fed into two halves of the reaction chamber. They are kept separated by a membrane which allows one electrolyte to ionise the other by exchanging electrons. The membrane allows this exchange to happen while preventing the two solutions to physically mix.

Until recently VRB’s were considered prohibitively expensive in common with other storage technologies. However, the technology has progressed beyond the development stage with a number of proven installations. It is on verge of wide spread commercialisation. Costs have considerably reduced and will further with economies of scale and ongoing refinements of the technology.

‘The additional layer of equipment adds perhaps 15% to the cost of a stand-alone wind farm, but more than makes up for the additional cost by increasing the value of the electricity generated.’ (4)

Advantages of VRB’s:
• Scalability – unlimited capacity - easily expanded by increasing the number of electrolyte storage tanks.
• Low toxicity – does not use heavy metals
• Raw material readily available and cost effective.
• Long life – 10-15 years plus - replacing the membranes in the cell stacks can extended this further. The electrolyte solution is reusable and will retain a residual value close to its original.
• Low maintenance – High tolerance to charge/discharge (up to 10,000 cycles) – only two moving parts
• Instantaneous response – can discharge energy in milliseconds.
• No harmful emissions.
• Quite operation – suitable for urban sites if space permits.

Disadvantages of VRB’s :
• Large area required (generally not a problem with wind farm remote locations)
• Greater system complexity compared to conventional batteries i.e. control units, sensors, pumps etc. required.

Commercial Scale
Operational VRB Energy Storage Systems to date:

Kings Island, Tasmania, Austrailia

This VRB System provides a 200kW x 4 hour storage to complement the existing 5 wind turbines (ranging from 250 to 850 kW) and diesel generators. The use of the VRB has allowed for considerable reduction in the use of the diesel generators and provides a dependable deliverable electricity supply to the island.

PacifiCorp, Moab, Utah, U.S.A.
This installation was the first large-scale commercial VRB-ESS in North America. The system consists of a 250kW x 8 hour (2 MWh) storage unit. It is being used as a load levelling device to supply peak power to a remote location in southeast Utah allowing for the deferral of the need for a new electrical sub-station.

Other applications of the technology include a 1.5MW UPS system for a semiconductor fabrication plant in Japan and a 275 kW output balancer for a wind power project in the Tomari Wind Hills of Hokkaido, Japan.

Sorne Hill Wind Farm, Buncrana, Donegal, Ireland
The 32MW Sorne Hill wind farm will have the largest installation of VRB Storage System worldwide and is the subject of a comprehensive feasibility study ‘VRB ESS Energy Storage & the development of dispatchable wind turbine output – Feasibility study for the implementation of an energy storage facility at Sorne Hill, Buncrana, Co. Donegal’ commissioned by Sustainable Energy Ireland, the statutory authority charged with promoting and assisting the development of sustainable energy and Tapbury Management Ltd., a private company formed to provide administrative and management services to Sorne Wind Energy Ltd.

Findings of the Sorne Hill feasibility study:
The Sorne Hill feasibility study is extremely important as it validates economic viability of the VRB Storage systems for wind farms such as this. It will facilitate the further rollout of up 3000 MW of contracted and proposed wind generation in Ireland, removing uncertainties about intermittency and income streams.

SEI estimates that up to 700MW of storage will be required in the years to come. Final contracts worth US$9.4 million are about to signed with a planning permission application for the VRB storage building about to be processed for the Sorne Hill site.

The report comes to the conclusion that a 2MW of power VRB 6 hour (12MWh) Storage System is required which will have the ability to dispatch 3MW for 10 minutes ever hour to accommodate fluctuations in the wind energy generation.

Wind generated electricity is bought at a discounted rate of €57 per MW because of the variability of supply.

The Sorne Hill Wind Farm will now receive €85 for each MW delivered to the grid, the same price as conventional electricity supply such as coal and gas. This will allow the projected rate of return (known as the IRR) to increase from the normal 9-11% to approximately 17.5% according to the study.

The maturation of VRB technology could release the full potential of wind generated electricity worldwide and make such targets as Ireland’s 33% of electricity from renewables by 2020 a very realistic goal. The ability of renewables with VRB Storage Systems to dispatch electricity as efficiently or more efficiently than conventional plant, (commonly combine cycle gas turbines which can take up to 15 minutes to dispatch (5) , removes one of the major obstacles to their wide scale deployment and potential significant contribution to world energy.

The essay could be broadened in its scope to detail the other discussed storage technologies to a further degree as they all have merits and potential for the future. Furthermore, zinc bromine and polysulphide bromide flow batteries offer many of the advantage of VRB’s and are worthy of further discussion.

Another development that’s worth noting is the research by Skyllas-Kazacos (of the University of News South Wales who developed the VRB technology in the 1980’s) into vanadium bromide revox batteries. This chemical would be twice as soluble as vanadium sulphate allowing battery plant to half in size opening up their use for smaller applications from cars to domestic applications.

(5) Mechanical Engineering Magazine – September 2005 –

Thanks again to Shay for his permission to publish in full his excellent essay. I have not included the extensive references and appendix as I am a bit lazy and the formatting would take a good bit of work. If anyone wants the information included, you can e-mail me and I will ask Shay if I can send you the material.


No comments: