Tuesday, October 02, 2007

Redox batteries and Wind Power


I am publishing, with permission, a paper sent to me by Shay McGowan on Wind Generation and the use of battery backup to eliminate power fluctuations. I had intended to quote you parts of this paper, but I liked what I read so much, that I have decided to put the complete essay on the site. Thanks for the paper Shay.

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

According to the European Wind Energy Association most parts of the UK and Ireland have the largest wind energy resource in Europe.

At present, there is an installed capacity of 745 MW in Ireland of wind energy. The island has a theoretical 125.7 GW or 344.9 TWh/yr accessible resource available according to ESB International and Sustainable Energy Ireland (SEI) respectively. With winter peak electricity demand of 5000 MW estimated by ESB National Grid for the period 2007-2010, it can be seen that a substantial wind energy resource is there to be exploited.

Ireland could presently generate 25% of its electricity from the wind with no increase in electricity prices to the consumer. If this was done, there would be wind turbines scattered across only 1/2 of one percent of the country, assuming no offshore development. (1)

Recent binding commitments by the EU to provide 20% of the unions energy needs from renewable sources calls for a rapid deployment of renewable energy technologies.
This 20% target is conservative and well short of what is required if the carbon reduction levels required to mitigate the worse effects of climate change as recently indicated by the I.P.C.C. (Intergovernmental Panel on Climate Change) are to be achieved.

In order to substitute fossil fuel generated electricity to a significant degree a number of different renewable technologies will be required with wind energy most likely contributing the majority of this renewable electricity.

However, due to the intermittent or variable nature of wind the output from a wind turbine can range from above its maximum rated level down to zero. At present, due to the small number of commercial scale wind farms connected to the grid this variability is not an issue and at present has no effect on grid stability. The national grid requires a base load of generated electricity to accommodate the normal every day demands but also requires a fast responsive generating capacity to facilitate peaks in demand (typically Combined Cycle Gas Turbine Generators) otherwise the grid would get unstable with brownouts or blackouts occurring.

In countries such as Denmark and Germany which have a large amount of wind energy penetration most of the variability is ironed out by been interconnected to the UCTE grid, i.e. the continental European power system supplying approximately 450 million people. This allows the importation across borders of electricity when peaks in demand dictate.

Island nations such as Ireland and the U.K. have some interconnectivity with each other and Europe but not with the same capacities and flexibility.

Presently grid stability from wind generation is maintained by:

• More accurate wind forecasting
• Wind management and curtailment
• Interconnectors
• Storage

(1) Staudt.L -Centre for Renewable Energy, Dundalk Institute of Technology

These options shall be briefly explored with a focus on vanadium-based redox flow batteries as a storage solution. Following the recently published 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’2 commissioned by Sustainable Energy Ireland, it would appear that this storage technology could offer the most viable solution to wind energy variability and allow wind generated electricity to be ‘dispatched’ in a similar fashion as conventional fossil fuel generated electricity.

Current methods for accommodating wind variability:

More accurate wind forecasting
Presently in order to account for wind intermittency (where significant interconnectivity is unavailable) meteorological forecasts are used to assess trends and eventually to produce day-ahead forecasts and then hourly production estimates. The predicted hourly loads are compared to the hourly production estimates with the shortfall accounting for the amount of reserve electricity generation required from conventional plant.

The accuracy of meteorological forecasts could be marginally increased but with fast moving weather systems tracking across the Atlantic, affecting Ireland and Northwestern regions of the UK, errors in the region of 30% in predicted generation output can occur.

Wind management and curtailment
Sometimes the only option is to curtail the amount of wind generated electricity entering the grid so as to avoid instability. This leads a greater dependence on fossil fuel plant to make up the reserve capacity and negates the environmental and fuel saving benefits of wind energy. Such uncertainties about generating capacity would have an adverse affect on wind farm developments as the return on the investment would be reduced and difficult to predict.

Ireland at present is interconnected to Europe via Northern Ireland and Scotland with a capacity of 450MW. In addition they is a proposal for a submarine interconnector of 500MW below the Irish sea to Wales.
Concern over security of energy supplies is for ever increasing with the continuing growth of international terrorism, conflict in the middle east and political unrest in Nigeria but to name a few. A more confident and wealthy Russia has proved to be far from a reliable gas supplier and has appeared to have used its natural gas monopoly as a political weapon, most notably by turning off gas supplies to the Ukraine in January 2006.

For this reason It could be argued, that from a security of supply point of view that increasing dependence on imported electricity from Europe should be minimised if possible, as total electricity energy consumption by the EU-25 comprised 37% of oil and 24% gas. Germany for example gets 40% of its natural gas from Russia and with the continuing growth of gas powered generating stations and gas fuelled space heating this dependence will only increase further.
Furthermore, traditional centralised energy production feeding into pan national grids is a highly inefficient way to generate electricity with only approximately 30% of the primary energy reaching the distant end user. These losses occur due inefficient generation, non-recovered waste heat and transmission and distribution losses. Reliance on interconnectivity across borders would increase this inefficiency further with increased transmission distances.

Also at issue is the length the time it takes to provide interconnectivity as according to Wolfgang Kerner, policy officer at the European Commission, on talking about interconnection between European nations stated that - The construction of new powerlines and inteconnectors is an arduous process, It takes 10 to 15 years to construct a line....

(2 http://www.sei.ie/index.asp?locID=1030&docID=-1 - Sustainable Energy Ireland)

Storage Systems
The most common storage systems available or in current development are:
• Battery Storage
• Compressed Air Energy Storage
• Hydrogen Fuel Cells
• Super and Ultra Capacitors
• Flywheels
• Pumped Storage
• Flow Batteries

Battery Storage:
Conventional battery storage consists of lead acid batteries in various forms. They are relatively inexpensive when compared to other storage technologies but have performance limitations, short life spans and contain highly toxic substances which are difficult to treat and dispose of. Other battery technologies such as Nickel cadmium, Nickel Hydride, Lithium ion, Lithium polymer and Sodium sulphur are under development for large scale systems but are more expensive than conventional lead acid batteries

Compressed Air Storage:
Compressed air storage relies on the availability of suitable geological formations which can store the compressed air. Such formations are rare which limits the suitability of this system and its wide scale use.

Hydrogen Fuel Cells:
Fuel cells harness the chemical energy of hydrogen to generate electricity without combustion or pollution. They are currently an emerging technology at the prototype stage.
Super and Ultra Capacitors:
Super and Ultra Capacitors store energy electrochemically. They are suitable for high value short burst energy. Their application for large scale storage systems is still under development.

Flywheel Storage:
A flywheel is a mass rotating about a shaft. They store energy mechanically in the form of kinetic energy, have a high turn around efficiency and a relatively long life. They have been used for UPS systems for server farms but are very expensive to construct due to the high precision engineering required and at present have maximum power outputs of around 100kw for 1 hour. They occupy a niche market at present.

Pumped Storage:
Pumped hydroelectric storage plants are characterized by long construction times and high capital expenditure. Similarly to Compressed Air Storage systems, the locations available for pumped storage are limited or have already been exploited.

(I will publish the next section in a day or so. It is taking me a bit of time to re-format the text to suit the blog)

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