Daniel Nocera, the rock-star of artificial photosynthesis, and his colleagues at Harvard published a paper in June that shows a viable route to long-term energy storage.

The Science article demonstrates how surplus power from solar PV can be converted into liquid fuels. The electricity is used to split hydrogen which is then fed to an engineered bacterium (Ralstonia Eutropha) alongside carbon dioxide.

The bug 'eats' the gases and exudes useful, and highly storable, alcohols such as isobutanol. The conversion efficiency of the energy in electricity to valuable fuels is about 40% in his laboratory - representing an overal process efficiency from sunlight to biofuel of about 10%, about ten times greater than the most efficient plants.

Albeit only at the early experimental level, we now have the potential for an all-natural green refinery for renewable liquid fuels.

Of critical importance, Nocera's team shows that the bacterium will generate alcohols in the presence of oxygen (which is not the case with most methanogens and acetogens, two other classes of microbes being investigated as the potential workforces in green refineries). Equally vitally, the work seems to indicate that Ralstonia Eutropha is happy to consume CO 2 at the very low concentrations found in ambient air. No necessary requirement for expensive carbon capture.

Liquids such as isobutanol are energy dense - perhaps holding 25 times as much power per litre as a battery - and are safe to store and easy to ship. We can use the existing infrastructure of pipelines, tankers and storage tanks. Burn isobutanol in your existing petrol car and you will get motion (although you might have to add 15% conventional gasoline as well). It can also be stored and eventually combusted in a turbine to make electricity at a later date if necessary.

Price of liquid fossil fuels versus solar isobutanol

Will generating liquid fuels in green refineries will eventually become cost-competitive with fossil energy sources? Yes. But, based on the numbers in the Nocera paper, it looks at first sight as though this might take some time. First, the wholesale caost of gasoline / petrol.

Barrel of oil: $49 (August 10th 2016)

Number of litres per barrel: 159

Therefore, cost of crude per litre: $0.295

Approximate cost of processing crude to get to petrol / gasoline: $0.080

Therefore, wholesale cost of petrol / gasoline: About 37.5 US¢ per litre.

What might it cost to get isobutanol using artificial photosynthesis today?

Amount of energy in a litre of isobutanol: About 8 kilowatt hours

Efficiency of conversion of solar electricity into isobutanol in Nocera work: About 40%

Therefore, required solar-produced electricity to generate one litre of isobutanol: 20 kilowatt hours.

The cost of electricity purchased from solar farms in best locations in 2016: 4¢ a kilowatt hour. This is the approximate price paid by the electricity utility for PV in recent auctions in the Middle East for all the yearly output of a solar farm. However, this number has been as low as just under 3¢ a kilowatt hour in some 2016 auctions.

Cost of making a litre of isobutanol from solar PV in the best locations: About 80¢ a litre.

These two tables show that solar gasoline is apparently over twice as expensive as the fossil equivalent: 80¢ versus 37.5¢ per litre for the average cost of production. (And this is after making the wholly unfair assumption that the green refinery costs nothing to operate).

Solar costs will continue to fall sharply around the world, but parity with $49 oil for generating liquid fuels is probably the best part of a decade away. However the numbers in the boxes do suggest that the world will never see oil prices sustained above $100 again because at that level using electricity to make fuels may be already cheaper today than that level.

Editor's note: much will also depend on tax policy: if the 'green' fuel is taxed at a lower rate than petrol / gasoline, or not all all, it would be cheaper even at current prices.

The free energy revolution is coming

At this point we need to ask the question: 'Why would anybody want to convert valuable electricity into less valuable oil? In the UK, wholesale electricity today is usually worth about £40 a megawatt hour while gasoline / petrol sells for the equivalent of £25 a megawatt hour before taxes at today's oil prices.

The answer is that gasoline can be stored easily and electricity cannot be. So when we have too much electricity, the world needs to convert it into fuel gases and liquids. Otherwise it is wasted. So, even before the further fall in solar PV costs in years to come, Nocera's technology has a possible use. It will help us cope with otherwise problematic surpluses of power.

Take Sunday 7th August, for example. Strong winds and reasonable sun caused near-havoc in the electricity market, in the UK and also around northern Europe. The average price of power over the course of the day bought and sold by the UK National Grid as it balanced supply and demand was just over £1 per megawatt hour.

This isn't a typo; electricity was essentially worthless last Sunday. During the early afternoon, the price fell to less than negative £60. What precisely does this mean? In those four hours National Grid was offering £60 a megawatt hour to anybody who would either cut their production of electricity or add to their demand. (I think this may have been the day of lowest average very short term electricity prices ever seen in the UK - please correct me if I am wrong).

If your business had taken negative £60/MWh electricity on Sunday and used it to make isobutanol in a Nocera refinery you would have made a turn of almost £100 per megawatt hour. That is why we will see artificial photosynthesis soon.

That Sunday was atypical. But it will become increasingly common to see very low prices as offshore wind grows (and even solar continues to edge up as companies put unsubsidised panels on their warehouse and factory roofs).

On these occasions, the value of converting power to liquids, or indeed power to gas, as Electrochaea is showing so impressively at its commercial trial in Copenhagen using methanogens to make natural gas, is clear-cut. It will stabilise the electricity market as well.

This is where large-scale government investment is needed!

And when solar and wind are in short supply, the liquid fuels made originally from solar PV via artificial photosynthesis can be productively combusted in turbines to make the electricity needed to 'keep the lights on'.

To make a complete transition to renewable energy sources the world needs energy storage on a truly massive scale. High latitude countries, such as the UK, will need to have perhaps one third of their annual energy demand available in storage buffers. Power to liquids and power to gas are the way forward, providing the responsiveness and flexibility that new nuclear power stations unfortunately lack.

There are about ten early stage technologies around the world for turning seasonal surpluses of power into gas or liquid form. Daniel Nocera's route is therefore one of many. The sensible industrial strategy for the UK government would to use the country's skills in bioengineering to build commercial operations using several of these approaches.

Government R+D money is vital now. It is no accident that much of Nocera's team's groundbreaking work is funded by US government agencies, including the navy and air force.

Chris Goodall is an expert on energy, environment and climate change, and a frequent contributor to The Ecologist. He blogs at Carbon Commentary where this article was originally published.



Chris's new book: 'The Switch' is written and researched by Chris Goodall and published by Profile Books. Several of the other power to gas and power to liquids approaches are covered in the final chapter.

Special offer for readers of The Ecologist: Enter the discount code SWITCHECO at the checkout on Profile Books' web site to buy the book at £7.99 including postage and packing, a price lower than currently available at the main online booksellers.

Author's note: Thank you to Phil Levermore, Managing Director of the not-for-profit utility Ebico, for his help on prices in the UK balancing market.