How do we store low carbon power long term? The answer is in sight

4993115725_411c7d610c_bThis post is by Chris Goodall, author of The Switch, which describes how the world can cost effectively move to a zero carbon economy.

Sometimes we just don’t notice how well things are going in the race to decarbonise the world economy. Solar photovoltaic panels (PV) continue to decline sharply in cost. Batteries are becoming rapidly cheaper and we will have inexpensive electric cars with 200 miles of range within eighteen months. Wind turbines are improving in price and performance, particularly offshore. Energy use is proving easy to manage second by second. Optimism about a prosperous low carbon future for all seven billion people in the world is more justified with each passing month.

Storage is the road block
The rate of progress in these fields is far, far more rapid than anyone imagined would be possible, even a few years ago. The latest indication of the pleasant inevitability of a world of 100 per cent renewables came last month as Chile announced a record low price for electricity bought from solar providers. At 2.2 pence per kilowatt hour, solar PV is providing power at less than a quarter of the projected cost of electricity from the proposed Hinkley Point C nuclear plant in the UK.

Of course, we all know the transition to a low carbon economy will take much longer than it should. The friction caused by vested interests means that solar, wind and other renewable technologies are being held back. But there is now only one genuine road block to a full global transition away from fossil fuel power: the need for long term storage of energy in high latitude countries.

Approximately eight per cent of the world’s population lives north of London. For people in these latitudes, solar and batteries may provide enough power during the summer and wind turbines will assist throughout the winter. But there will be weeks and months when energy supplies are worryingly short. This means that the UK needs to find the means to store many hundreds of terawatt hours of energy.

In The Switch, I look at the various options for creating these deep reservoirs of instantly available power. One set of opportunities clearly stands out as exciting and within reach. We need to use summer surpluses of solar power (or the excess generated by wind during stormy weeks in winter) to convert water to hydrogen and oxygen via electrolysis.

New ‘power to gas’ solution
I argue that hydrogen shouldn’t be the route to long term storage, useful though it is. It is difficult and expensive to store. Countries with large needs for seasonal storage would need to spend many hundreds of billions developing new infrastructure to hold and transport hydrogen. Rather, I think the logical use for hydrogen is to convert it into another energy carrier; in a simple chemical reaction, hydrogen can be combined with carbon dioxide to make methane, the main constituent of conventional natural gas.

Why is methane better than hydrogen? Because most advanced economies have the capacity to store huge amounts of energy in the storage caverns, tanks and pipelines already built to store gas. Germany, for example, can hold about 200 days of use. If we can make low carbon methane indirectly from solar electricity (or ‘power to gas’ as it is known) at times of surplus, we can burn the gas in conventional power stations when renewable electricity is unavailable.

In May of this year, I went to Copenhagen for a day. Sitting alongside one of the city’s wastewater treatment plants is a one megawatt reactor that splits water into hydrogen and oxygen and then combines the stream of hydrogen with the gas coming off the sewage farm. That gas is part methane and part carbon dioxide, and since both gases originate from CO2 captured during photosynthesis, the mixture is almost carbon neutral. The three gases are then bubbled through a vertical column containing ancient microbes called Archaea. These bugs ‘eat’ the hydrogen and CO2, and exude methane as their waste product. That means that what comes out of the top of the column is 100 per cent methane of sufficient purity to pump into the Danish gas grid. What Electrochaea, the German company providing the technology, has done is to generate 100 per cent renewable methane, ready to generate electricity in a gas-fired power station or be used in a domestic boiler to provide heat.

Several other types of microbe are more than willing to provide us with renewable gases and liquid fuels suited to storage in our existing energy infrastructure of tanks and pipelines. These bugs usually need external energy to help them convert spare hydrogen into longer chain molecules. But we can be sure that the continuing sharp decline in solar PV costs around the world means that the energy required is going to get less and less expensive every year.

Industrial strategy could deliver it
I suspect that the beautiful and compact Copenhagen ‘power to gas’ reactor will be seen as the most significant advance towards the low carbon economy since the first solar panels were put on a US satellite in 1958. Three years ago, the UK government turned down Electrochaea’s plan to site its first commercial plant alongside a Midlands sewage farm. The company now wants to build a reactor many times the size of its Copenhagen trial to show that its economics work. A sensible industrial strategy for the UK would find the few million pounds necessary to help Electrochaea prove that it can provide cost effective long term energy storage for high latitude countries. Or, perhaps even better, a far-sighted oil company might see that some small part of the $200 billion spent globally on exploration each year could be diverted to building world class expertise in sustainable energy by sponsoring a British commercial ‘power to gas’ reactor.

chris@carboncommentary.com

Image: bark via Flickr, CC2.0

2 comments

  • This is very interesting. Can you (anyone) compare the cost of this process with that developed by Ecotricity? https://www.ecotricity.co.uk/our-green-energy/green-gas/how-green-gas-works

  • Though I am a long-term advocate of storing energy in the form of hydrogen, there’s no doubt that storing renewably generated electricity as methane, rather than hydrogen, is also a very interesting concept. It certainly can be done, and a 2011 paper gives details of how an Audi-led project was doing it to produce methane to power cars: http://www.greencarcongress.com/2011/05/egas-20110513.html
    One problem however is that, having generated hydrogen by electrolysing water (an energy-intensive process that may well be powered by renewable electricity), the chemistry of the reaction, known as ‘methanation’, whereby the hydrogen is converted into methane (CH4), is such that only half of it ends up as methane: the rest becomes heat and water.
    4H2 + CO2 = CH4 + 2H2O
    In other words, fully half of the energy that went into the electrolysis emerges again as heat where it is not wanted, rather than being stored for later use.
    If the input electricity is virtually free, that is not necessarily a disaster – half a loaf is better than no bread – but the process is clearly not an efficient one. If the methanation is carried out alongside some other process where the heat generated can be profitably used, then this will of course improve the overall economics. But as the methanation is designed for occasions when the available renewable electricity would otherwise be surplus to requirements, and its energy is to be stored for when demand is greater, it is likely to be operated principally at times when electricity from the grid is virtually free. Hence the free ‘methanation’ heat may not be worth all that much.
    There are many methods of chemically (and indeed physically) storing hydrogen. I fully agree that research into these should be a major priority for the UK. It is vital that the current intermittency of solar and wind renewable energy is overcome efficiently and economically, so that the need for fossil fuel and nuclear baseload generators can be finally eliminated.

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