We need to store surplus electricity as gas
(The comments underneath this article are particularly interesting. I recommend reading. Chris.) Whatever renewable energy advocates say, the intermittent nature of solar, wind and marine energy production represents a difficult problem. Although we can adjust electricity demand to match supply to a far greater extent than we do today, the huge expected growth in UK offshore wind power is going to give the electricity grid major problems. When the gales blow, we’ll be dumping power but running short of electricity on cold, still days.
That’s why the recent contract win by ITM Power, the Sheffield hydrogen electrolysis company, is so interesting. ITM is supplying some of its units to a 360 kW hydrogen production plant in Germany that take surplus electricity, convert it into hydrogen and feed the gas into the national gas grid. (Please read the comments for discussion of some of the problems this might cause). In a second stage, hydrogen can be converted to methane, the main constituent of natural gas, and injected into the grid or into storage caverns. The Germans have been quicker to recognise than other countries that the gas network can store far more energy than any other media. Forget batteries, compressed air storage or pumped water: the national gas grid has a capacity several orders of magnitude greater. And the network and its storage sites already exist. No need to spend billions on new facilities. Complete reliability.
The problem
We can see the issue already. When the wind is strong the UK grid sometimes cannot cope with the electricity produced. Wind farms are paid to disconnect. We mustn't exaggerate the current problem: the amounts are small and paying generators to shut down has long been a feature of all electricity grids. But as the capacity of working wind farms rises from 7 gigawatts now (providing – at peak – about 20- 25% of UK summer night demand) to thirty gigawatts and beyond we know the problem is going to get more and more severe. Electricity is wasted, increasing the long run cost.
And, of course, the reverse situation is also a problem. Cold December weather is often correlated with low wind speeds. Those thirty gigawatts of turbines might be only producing 1 or 2 gigawatts of power at times when electricity is really needed. Fossil fuel power stations will have to work instead. Most of the time these plants will stand ideal, and creating the right incentives to build them is proving one of DECC's many challenging problems.
Most analysis of renewable energy deployment suggest that the UK and other countries need to invest heavily in energy storage and/or massive increases in the capacity to ship electricity around Europe. At the moment, we have very little storage of any form. The two large ‘pumped hydro’ plants provide a few gigawatts for a few hours. In Germany, the total amount of non-fossil energy that can be quickly converted into electric power is about one twenty fifth of one percent of annual electricity demand.
Some expansion of pumped hydro is possible; I’m told Japanese companies are pumping water up sea cliffs ready to be released when power demand rises. A few more large reservoirs are possible in the UK. But getting to the energy equivalent of more than a day’s supply of electricity is almost impossible to envisage. We could use a 100% electric car fleet to provide power but one German study suggested that this would provide, in total, only about a third of a day’s power. Other battery sources would be astronomically expensive.
Unfortunately, those periods of calm in mid-winter can last weeks or more in the UK, and longer elsewhere. The main potential sources of energy storage are insufficient.
The answer
This is why we need to consider the possible role of the gas grid. In the UK, total gas demand is very approximately 3 times total electricity use. (I’m using rounded figures only here).
| Total demand | Power source |
| Electricity | 400 TWh |
| Gas | 1100 TWh |
| Of which, used for electricity | c. 300 TWh |
Most countries, but not the UK, have maintained substantial gas storage. Gas is bought when cheap, usually in the summer, and put into depleted hydrocarbon reservoirs and other storage reservoirs for use in winter and to meet unexpected needs. German has storage capacity of about 200 TWh. A gas power station is about 60% efficient, meaning that German gas storage could provide the energy to meet about 100 days of continuous UK electricity demand.
In the UK, the malfunctioning energy markets have held back investment in storage and we can only store about 18 days continuous gas use. But sites have been found, and planning permission often granted, to multiply this fourfold.[2] This is enough to overcome all the problems of intermittent renewables.
This, of course, is similar to what the government already intends. New gas-fired capacity will be given payments just for being ready to fire up when the wind stops blowing. The real innovation that the Germans are beginning to explore is to use the gas grid both as a back-up to wind in calm condition AND as storage for energy when the wind is too strong.
This is why the ITM Power contract is so intriguing. Its units will be employed to turn surplus electricity into hydrogen through simple electrolysis, the splitting of water into its components, hydrogen and oxygen. The intention is then to put the hydrogen into the gas grid, mixing it with the methane already there. (I didn’t know this, but it seems that 1 or 2 percent concentrations are safe). This means we’ve potentially got energy storage from surplus wind in the gas grid. At times when wind is over-abundant, and usually this means wholesale electricity is cheap, the wind farm output can be diverted to electrolysis in a process that is about 80% efficient. (This means that 100 kilowatt hours of electricity can be converted into hydrogen that when combusted produces 80 kilowatt hours of heat). We've got some storage, and energy that would otherwise have been dumped or sold for less than nothing.
But, you might say, adding 1 or 2 percent hydrogen into the gas grid doesn’t provide enough storage for more than a few days. The logical next step is even more interesting, and just beginning to be explored in the Germany and Austria. Hydrogen can easily be converted to methane using a well-understood process. Find a source of CO2 (not scarce) and hydrogen be turned into conventional natural gas. Except that, in effect, it is 'renewable' because it is sourced from water and CO2.
2H2 + CO2 = CH4+O2
Very roughly, this methanation process is also 80% efficient . That is, 100 units of chemical energy in the hydrogen turn into 80 units of chemical energy in methane. Conceivably the lost heat could be reused, possibly in a simple Organic Rankine Cycle (ORC) plant to produce electricity. More about all this here.
The storage process is complete. When the wind is blowing, the surplus electricity gets converted into hydrogen and then methane. The total efficiency is about 64% (80% times 80%).This isn’t great, but the wind farms’ power might otherwise be wasted. And it is not much worse than other conceivable large scale energy storage mechanism.
If the UK wants thirty gigawatts of wind (equal to total UK demand on a summer night), we have to find a way to enable electricity to gas conversion to happen at a very large scale. It seems to me that there is no alternative if we want to use renewables, decarbonise the power supply and keep the lights on as well. Electricity-to-gas hugely increases the capacity of the electricity grid to cope with intermittent renewables and provides ‘zero-carbon’ gas to power stations in times of low wind. Perhaps critically, it also helps stabilise the price of gas and reduces the UK’s increasing dependency on imports. We can engineer the market so that gas-fired power stations can work most of the time on ‘zero-carbon’ methane, reducing the overall cost of renewable power.
Electrolysis and methanation are relatively cheap. I can’t see a good reason not to go down this route. Am I missing something?
http://www.oxfordenergy.org/wpcms/wp-content/uploads/2011/08/NG-54.pdf
UK storage estimates
http://www.uni-kassel.de/upress/online/frei/978-3-89958-798-2.volltext.frei.pdf