Energy storage system to top up peak electricity supply

Energy storage system to top up peak electricity supply
Storing excess energy could help base supply power plants meet spikes in demand (Image: 0x6612390)
Storing excess energy could help base supply power plants meet spikes in demand (Image: 0x6612390)
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Storing excess energy could help base supply power plants meet spikes in demand (Image: 0x6612390)
Storing excess energy could help base supply power plants meet spikes in demand (Image: 0x6612390)

The amount of power drawn from the electricity grid can vary greatly at different times of the day. It usually peaks in the early evening for a couple of hours after the mass exodus from school and work, while short-lived spikes are also common after major televised sporting events, during commercial breaks and in the morning hours. This can cause headaches for energy companies as they struggle to match supply with demand. But researchers have now found a way to manage these short-lived draws on the electricity grid far that could halve the fuel needed.

Matching the highs and lows in demand with a steady supply is a major challenge. Energy companies typically top up a 'base' supply of energy with electricity from power plants that are just switched on to cope with the peaks. However, the gas-fired generators often used to feed these peaks are notoriously inefficient, expensive to run and sit idle for long periods of time. In short, the system wastes both energy and resources.

Storing excess energy

University of Leeds Professor of Engineering, Yulong Ding, and colleagues are proposing a more environmentally friendly system that would also be cheaper to run. Crucially, the system would store excess energy made by a plant supplying the 'base' demand and use this to supply the 'peaks' in demand - as and when they happen.

"This integrated system is truly novel," said Professor Ding, who led the research. "Because we are storing the excess energy for later, there is less need to ramp up the output of gas-fired plants whenever a peak in demand is expected, generating electricity that may simply not be used."

The key idea is to use excess electricity to run a unit producing liquid nitrogen and oxygen - or 'cryogen'. At times of peak demand, the nitrogen would be boiled – using heat from the environment and waste heat from the power plant. The hot nitrogen gas would then be used to drive a turbine or engine, generating 'top up' electricity.

Meanwhile, the oxygen would be fed to the combustor to mix with the natural gas before it is burned. Burning natural gas in pure oxygen, rather than air, makes the combustion process more efficient and produces less nitrogen oxide. Instead, this 'oxy-fuel' combustion method produces a concentrated stream of carbon dioxide that can be removed easily in solid form as dry ice.

Fuel savings

Using such an integrated system, the amount of fuel needed to cater for peak demand could be cut by as much as 50 percent. Greenhouse gas emissions would be lower too, thanks to the greatly reduced nitrogen oxide emissions and the capture of carbon dioxide gas in solid form for storage."This is a much better way of dealing with these peaks in demand for electricity. Greenhouse gas emissions would also be cut considerably because the carbon dioxide generated in the gas-fired turbine would be captured in solid form."

"On paper, the efficiency savings are considerable. We now need to test the system in practice," Professor Ding said.

Full details of the system will be published in the International Journal of Energy Research.

Why not use this system to store excess energy from renewables that are operating during times of low demand?

This is already being done by using wind farms to compress air into deep underground storage. During peak times, the compressed air is released, and interestingly the best way to use it is for the front end of a natural gas turbine generator. As these don\'t then have to compress their own air, and as the intake air is cooler, then run *very* efficiently.

If this system was 50 eff it would be amazing. Vs a slightly more expensive battery system that would be 80-90% eff.
OK up to the dry ice part. Taking hot exhaust CO2 to dry ice is going to use a lot of power and to what end. Do you store the dry ice forever or what? I am thinking large batteries may be more efficient and less complicated with the possible use of ultra capacitors to smooth out the charge/discharge cycle.
This is one of the most intelligent methods I have ever heard of for storing energy! The delta between storage and usage is the smallest with the least amount of non-renewable components necessary to store the energy! Now if we can only get this in to production!
Corey Johnson
Check out Isentropic (http://www.isentropic.co.uk/). Their system is similar, though the energy storage is in the form of a porous bed (gravel, stone) in a tank through which air (or any other gas) is compressed and expanded.
It is essentially a heat pump driven by excess electricity to create a temperature difference between two tanks. When the electricity is to be retrieved, it becomes a heat engine producing work from the previously created temperature difference.
I have written a detailed post on the Isentropic technology myself as well which explains some of the issues and benefits that I see with the system - http://yeroc.us/weblog/post/index/62/Pumped-Heat-Electricity-Storage.