Dealing with excess wind

Wind as a source of energy is clean but not reliable; too much is not quite as bad as too little but it is still a problem. It seems a shame to have to stall the blades and stop generating on windy days, even though wind is free. There are three reasons why you might need to do this: because there is more wind power available than electricity demand, because taking the wind power would mean shutting down something else - like nuclear - which isn't flexible enough to turn on and off, or because there is more wind available than there is grid capacity to take the power (see Why do we pay wind farms to switch off). Apart from the last case we haven't hit these problems yet but other countries like Denmark and Germany have. If only we could store the excess energy to use later - but maybe we can. Lots of engineers have been working on this problem, especially in Germany, and there are pilot plants running for some of the technologies. The ones which store power as gas are particularly interesting because we can store gas over long periods - from summer to winter if necessary.


Energy storage helps in different ways, for which you need different capacities. The smallest storage is useful to smooth out unexpected lulls in the wind. The way the grid works, suppliers agree to provide power in half hour slots just a few hours ahead. The weather is usually predictable over that timescale but if there is an unexpected lull and a wind farm fails to supply what it promised then there will be a headache for the grid and a financial penalty for the farm.

Battery storage
An electric car battery typically stores up to 50 kWh and a turbine might be 3 MW but most of the time will supply only a half of this - so half an hour's worth of energy from the turbine would need the equivalent of 15 car batteries to store (actually rather more because batteries don't like being completely drained). Plans have been approved for a 6 MW lithium ion battery at Leighton Buzzard to demonstrate the technology in the UK [1] but there are already installations in other countries on this sort of scale: in West Virginia, the Laurel Mountain wind farm is 98 MW capacity and has a battery backup capable of supplying 32 MW for 15 minutes. It was commissioned in 2011. There are similar systems in China and Japan [2].

Compressed air
If you have a salt cavern which can be made air tight and is conveniently close to a power source, you have an excellent potential storage facility. You store energy by pumping air into the cavern and you release the energy by allowing the air to escape through turbines. At Huntorf in Germany there is one which can supply 321 MW for 2 hours: the equivalent of 13,000 car batteries [2]. This is not new technology: Huntorf was built in 1978. Here in the UK we have several salt caverns used to store methane gas - which is energy storage but of another sort [3].

Pumped storage
You can store energy by pumping water uphill between two water reservoirs at different heights with a hydroelectric dam between. Dinorwig power station in Wales has capacity to supply 288 MW for 5 hours and it can come on-stream in 16 seconds. This is the biggest in Europe. Unfortunately there are no other suitable sites in the UK [4].

Cryogenic air storage
Back in 2012 I mentioned a novel energy storage system in which energy is used to cool air down until it is liquid. To get the energy back you let it warm up and expand through turbines as with the compressed air system. Highview Power Storage has a 350kW demonstrator running in Slough and has recently won funding for feasibility studies for two full size systems: a 30 MWh system on the Isle of Grain and a 20 MWh system in Canterbury, both in Kent [5].

Hydrogen
Generating hydrogen by electrolysis of water is another old technology. You can turn the hydrogen back into electricity using a conventional gas turbine but that is not necessarily the best option because it is relatively inefficient. You could use it in a fuel cell in a hydrogen fuelled vehicle: there is a plant in Hamburg which makes hydrogen to fuel city buses and cars. It stores the hydrogen in tanks at 700 times atmospheric pressure. It has storage enough for 20 buses [6]. You can also feed hydrogen directly into the gas grid: E.ON has a plant in Frankfurt which is going to be doing that, starting August this year. It will take excess wind power from local wind farms and generate about 360 m3 hydrogen per hour which by my calculations is about 1.3 MW (using the higher heating value, what you should get from a condensing boiler) [7]. The gas grid can only take a small proportion of hydrogen as it has a lower energy density than methane (by volume). To use more of it we would need to adjust all our appliances, putting bigger nozzles on our gas cookers and so on.

Methane
Add CO2 to hydrogen (and apply some serious chemistry) and you can make our old friend methane CH4. This isn't new technology either but it is a lot more complex than simple electrolysis. Also you need to have a source of CO2 and there isn't enough of it in ordinary air to be useful at scale. The Audi e-gas project in Germany gets its CO2 from a local biogas plant - the bugs that generate methane for biogas also generate CO2 which is normally a waste product but here it is captured and fed into the hydrogen to methane converter. It can use 6 MW of power and generates 300 m3/hour natural gas, about 3 MW. The methane generating step generates waste heat and the Audi project will use this back at the biogas plant for disinfection. There are several other similar, though smaller, facilities in Germany [8].

None of these plants are very large, as yet, but they do demonstrate the principle. As with wind turbines, the costs are only that of building and running the plant because the fuel is free. Whether or not they are financially viable depends on the balance of costs against fossil fuel plant, which in turn depends on volatile fossil fuel prices, uncertain carbon taxes and the potential for a carbon emissions cap. In this uncertain world it is a wonder there is any investment at all.

[1] Go ahead for 6MW renewables storage trial facility in UK (Solar Power Portal)
[2] Energy storage case studies (Clean Energy Action Project)
[3] Converted salt caverns will boost UK's gas storage (Atkins, 2011)
[4[ Dinorwig Power Station (First Hydro Company)
[5] Two liquid air projects through to feasibility stage of competition (gasworld)
[6] Hydrogen Station Port City (in German use google translate to read it in English) (www.powertogas.info )
[7] Power to gas unit injects hydrogen into natural gas system for first time (E.ON)
[8] Audi e-gas project (also in German) (www.powertogas.info )
[9] Pilot und Demonstrationsprojekte im Power-to-Gas-Konzept ( Pilot and demonstration projects in power to gas)  (www.powertogas.info )