When I think about the future of renewable energy, I picture the inner workings of a leaf – any leaf. A green plant is a remarkable solar-energy collector, effortlessly pulling sunlight, water, and carbon dioxide from the environment, and converting it into stored chemical energy. And the total amount of energy processed by photosynthesis is enormous. The Sun bathes the Earth with 173,000 terawatts of solar energy annually. On land alone, plants convert that energy into more than 100 billion metric tonnes of biomass. Our global energy use is just 18 terawatts per year, in contrast. As solar energy proponents are fond of saying: ‘The Sun provides in an hour enough energy to supply the world for a year.’
Humans already have a long tradition of exploiting sunlight trapped by plants. That is where coal, petroleum and natural gas came from: they are the fossil remains of ancient biomass, accumulated over many millions of years. The problem is that burning fossil fuels releases millions of years’ worth of carbon dioxide back into the atmosphere all at once. What we really want to do is replicate the process now, creating new fuel as quickly as we consume it, with the whole process driven by sunshine. Then we could bring solar energy to places it has never gone before. We could provide an unlimited supply of liquid fuels for aircraft and heavy-duty vehicles such as tractors and trucks, not to mention feedstocks for the plastics, paints and pharmaceutical industries – all with no net carbon emissions.
The obvious first thought is: why not just let the plants do the work? They have already mastered the necessary molecular technology. We’ve tried that with biofuels derived from corn, soya, or algae – but if we grow crops like corn for fuel, we’re robbing the Peter of food production to pay the Paul of carbon-neutral energy. We could install algal bioreactors in places where crops can’t be grown – but then the amounts of water, fertiliser, and energy consumed in processing the fuel are formidable.
We therefore need to tap the sun’s energy in a novel, synthetic way. And that way actually needs to improve on nature, audacious though that sounds, because the solar energy figures I just mentioned are not quite the cause for optimism they seem. Natural photosynthesis is only one per cent efficient. The biomass that became fossil fuels was based on sunlight falling unhindered on every square centimetre of exposed ground, every second of every day, for as long as there have been green plants. To make a meaningful, environmentally sound contribution to the energy supply, we have to create an industrial process that can make a serious dent in the 36 billion tonnes of CO2 emitted annually by human consumption of fossil fuels, year in and year out. In other words, we need to do what plants do, but even better.
Although that sounds daunting, the more we know about natural photosynthesis, the more we can see that, since it has been cobbled together piecemeal by evolution, rational design ought to be capable of improving the yield. The essence of the natural process is to split water to yield hydrogen and to use the hydrogen to remove the oxygen from CO2 to make hydrocarbons. What nature accomplishes – and what we want to do – is to remove some CO2 from the atmosphere to create biomass. If our human nanotechnology can mimic that process, we will use up CO2 as quickly as we produce it. It is almost too elegant that the key ingredient for addressing climate change could be the substance that is causing the problem in the first place.
The eventual goal is to obtain the CO2 for fuel production from the atmosphere itself, but there the CO2 concentration – even at its swollen level of 400 parts per million – is impractically low by current industrial process standards. At present, waste gases from industrial sources such as coal- and gas-burning power stations, steelworks and cement factories constitute the best source of CO2 for fuel generation. They also neatly encapsulate the appeal of liquid solar fuels, as we could transform smokestack fumes from polluting industries into the raw material for a new kind of green energy.
Fortunately, engineers are not heading into entirely uncharted territory. Chemical reduction of CO2 to make hydrocarbon fuel is already a tried and tested process. Based on a German invention of 1925, it uses cobalt or iron catalysts plus energy to make a range of hydrocarbons for fuel, lubricants, or feedstock. The process has been embraced where economic circumstances render the extra energy cost acceptable. During the Second World War, Germany, with no access to oil, used this technology to create fuel. South Africa today derives about 25 per cent of its fuel by similar means.
The German process doesn’t achieve the desired environmental goals – it actually increases CO2 emissions – but it has inspired a promising step toward true artificial photosynthesis in the hands of George Olah, a Hungarian-American chemist, now 88 years old. Olah’s approach uses hydrogen produced via renewable electricity in a catalytic process to ‘reduce’ CO2 to hydrocarbon or alcohol fuels.
The term ‘reduce’ has a special meaning in chemistry, and is central both to the chemistry of life and to the quest for renewable solar fuels. Look around the countryside on a nice, sunny day and you can see the central chemical principle of life on Earth. The dense mass of greenery and the blue sky represents the twin poles of life: oxidation and reduction, or redox. Air in the sky contains oxygen that liberates energy when it combines with organic compounds; oxidation is the process that creates fire, and also that powers your metabolism. The mass of green, on the other hand, is matter in a chemically reduced state, which is the opposite of what happens in respiration and combustion. In the presence of oxygen, reduced compounds can be thought of as having stored energy. Just as oxygen is the element of oxidation, hydrogen is the element of reduction.
These two elements have been linked in a close dance ever since Earth was formed, but to complicate matters there is a third partner: carbon. Carbon can exist in an oxidised state (that’s carbon dioxide – CO2) or in a reduced state with hydrogen atoms attached, as in biomass and fuel. All living things consist of reduced carbon, great long chains and helixes and complicated clumps of carbon and hydrogen with other key elements attached in strategic places. Redox reactions – the molecular dance between carbon, hydrogen and oxygen – underlie three great mysteries: the origin of life, how to mitigate global warming, and how to tap the Sun’s energy without plants.
The laboratory for Olah’s CO2-reducing process is located in Iceland because of its abundant renewable electricity, generated from that country’s natural thermal springs. Since 2011, the George Olah Renewable Methanol Plant, operated near Reykjavik by Carbon Recycling International, has been using electricity from a thermal power station to split water into water and hydrogen. A nearby cement works provides a source of waste CO2. The hydrogen produced by the plant reduces the CO2 to methanol. The methanol (sold by Carbon Recycling International as Vulcanol) can be used as fuel for vehicles, either straight or mixed with petrol. In July 2015, Carbon Recycling linked with the UK division of the engineering firm Engie Fabricom to develop large, standardised CO2-to-methanol plants. Although Iceland’s energy situation is unique, George Olah notes that many parts of the world have access to other forms of cheap renewable electricity (hydropower or solar-thermal power, for instance) that could drive the plants.
The Olah process is far from artificial photosynthesis, however. Turning sunlight directly into useful liquid fuels requires understanding the detailed electro-chemistry of what goes on in green plants, and then learning how to beat nature at its own game. The details of the photosynthesis process are immensely complicated: the water-splitting system in plants, called photosystem II, has two almost identical halves, each of which has 19 protein subunits that use 35 chlorophyll molecules. But at the most basic level, scientists understand quite well how plants use sunlight to generate electricity.
Photosynthesis ultimately depends on the photoelectric effect, explained by Albert Einstein in 1905, in which photons of light interact with electrons, knocking them free of their atoms. It is the process behind silicon solar panels. Normally, when sunlight knocks an electron out of any substance, the electron jumps straight back in. What the natural photosystems do is to prevent the electrons recombining by smuggling them down a chemical pathway from which the electron cannot return. A combination of minerals – magnesium in chlorophyll, manganese and calcium in the water-splitting photocentre – and a surrounding protein matrix constrain the electrons so they have no choice but to be shuffled away.
if our technical catalytic systems fall short of nature’s, why not just work with natural organisms?
The task for artificial photosynthesis researchers is to find an equivalent for the natural pass-the-electron-parcel chains. A lot of the research has centred on photosystem II, built around an unusually structured group of manganese, oxygen and calcium atoms (Mn4O5Ca) known as a cubane, which is embedded in proteins. The bonds between the atoms of cubane are distorted by the protein matrix; the resulting strain is what enables its catalytic (reaction-inducing) activity. This manganese, oxygen and calcium reaction centre is perhaps the chemical crux of life on Earth. But it turns out that slavishly copying it might not be the best way to create an artificial photosynthesis system of our own.
Researchers have tried many, many alternatives to the cubane-based catalyst in green plants, with only limited success. That slow progress has inspired a whole other approach: if our technical catalytic systems fall short of nature’s, why not just work with natural organisms that already have their own alternatives to green-plant photosynthesis? We’ve seen the drawbacks of using off-nature’s shelf biomass from corn, soya, or algae, but could there be a useful halfway point between natural photosynthesis and a full-blown artificial version? It turn out there is.
There is a group of primitive bacteria – the acetogens – that can reduce oxides of carbon without photosynthesis. These microbes perform the special trick of being able to live off the very gases we are concerned with: oxides of carbon (carbon monoxide and carbon dioxide), along with hydrogen. They can generate alcohols from these raw materials and, even better, can do so using a variety of ratios of hydrogen and carbon monoxide. This flexibility makes them well-suited for industrial use, because just such mixtures of gases are produced as the polluting waste products of electricity generation, as well as steel and cement manufacture.
LanzaTech, a US energy company devoted to producing liquid fuels from industrial waste gases, is one of the leading proponents of acetogens. These ancient bacteria are found naturally today around hydrothermal vents in the deep ocean, where they live on the hot gases that well up from the ocean floor. LanzaTech is focusing on one specific bacterium, Clostridium autoethanogenum, to generate ethanol from waste gases, mostly carbon monoxide and dioxide from steel mills.
Jennifer Holmgren, LanzaTech’s CEO, recognises that having a clever idea is not enough if you are trying to shift the enormous fossil-fuel industry. ‘Scaling up is the most important thing for any new technology,’ she says. ‘If it doesn’t scale, it doesn’t matter.’ To that end, the company has created a demonstration plant at the Baosteel mill in Shanghai, China, and last year they signed an agreement with the world’s biggest steel-makers, ArcelorMittal, to build a €87 million fuel-generating plant at their Ghent steelworks in Belgium. LanzaTech has also signed a deal to supply Virgin Atlantic with bio-aviation fuel.
This last venture touches on one of Holmgren’s key concerns, bringing carbon reductions to the parts of the energy economy that green electricity cannot easily reach. ‘If we go to electric vehicles on a large scale, how do we balance the system?’ she asks. ‘The system requires production of fuels – ground and aviation – and chemical coproducts. If the ground fuels portion goes off to electric, let’s say 30 per cent of ground transport, what happens to the economics of aviation fuel and chemicals production?’
LanzaTech’s approach is an important step toward true ‘artificial photosynthesis’, since it yields biofuels without relying on the usual green plants, but it is still only a beginning. More far-reaching are the experiments now underway to develop hybrid fuel-production systems – ones that still exploit energy-harvesting mechanisms found in nature, but that add synthetic components to make them serve our needs more effectively.
This work has been greatly aided by the remarkable discovery that some bacteria can live directly off a diet of electricity. Peidong Yang, a Chinese-born professor of chemistry at the University of California, Berkeley, has exploited this appetite for electrons by matching the bacteria with microscopic semiconductors that act as tiny solar cells. The bacteria grab electrons from the semiconductors and use them to reduce CO2. It’s a brilliant synthesis: semiconductors are the most efficient light harvesters, and biological systems are the best scavengers of CO2.
Methane is not a liquid fuel, but it can readily be converted to one. It can also be used directly as natural gas to run power plants
Yang’s team is currently studying three different systems. In one, the researchers built a forest of silicon and titanium dioxide nano-wires as the light harvester, and then cultured the bacterium Sporomusa ovata to grow over the wires and feed on the electricity. In another system, the researchers precipitated light-harvesting cadmium sulphide nanoparticles onto Moorella thermoacetica; the particles enable the previously non-photosynthetic bacteria to turn light, water and CO2 into acetic acid, which can readily be transformed into fuels such as butanol, or synthesised into plastics and pharmaceuticals. It is ‘artificial photosynthesis’ in a truly profound way, bringing the photosynthetic ability to an organism that never had it for billions of years.
The third method is the most conventional, but it also looks like the most likely one to scale up. Combining an electrochemical cell (driven by electricity, sunlight, or a combination of the two) with the bacterium (Methanosarcina barkeri) produces methane with an impressive 10 per cent solar-to-fuel conversion rate. Methane is not a liquid fuel, but it can readily be converted to one. It can also be used directly as natural gas to run power plants. This approach could solve one of renewable energy’s most pressing problems. Electricity cannot be easily stored, and both sun and wind are powerful but intermittent energy sources. Solar-generated methane can be stored to provide electricity generation when the sun doesn’t shine and the wind doesn’t blow.
Unlike natural photosynthesis, all of these artificial systems at present require concentrated CO2 to work. ‘Ideally we’d be working with 400 ppm [parts per million] CO2 in the atmosphere, but no one knows how to do that yet, no one,’ Yang says. There is an upside, though. The current approaches can be readily coupled with carbon-capture technology to pull CO2 from smokestack emissions and convert it into fuel. This is the essential element of a closed carbon cycle that mimics nature, consuming the carbon created by human industry rather than dumping it into the environment. But that cycle still ultimately depends on the presence of the polluting industries.
Then again, we do now know how to use CO2 drawn directly from the air, on a laboratory scale at least. In January this year, George Olah’s group at the University of Southern California reported dramatic new work. Olah’s colleague G K Surya Prakash along with the PhD student Jotheeswari Kothandaraman have developed a combined process that uses a polyamine (a class of organic molecules essential both to life and to many industrial chemical processes) to capture carbon dioxide from the atmosphere, in conjunction with a ruthenium-based catalyst to reduce the CO2 to methanol. Ruthenium catalysts have been employed before to reduce carbon dioxide, but making the process work at atmospheric levels of CO2, in a unified process with the carbon-capturing reaction, is a notable advance. In tests, up to 79 per cent of the CO2 captured from the air was converted into methanol.
The Olah group have been pursuing their vision of a ‘methanol economy’ for many years and, with their experience from the Carbon Recycling plant in Iceland, they are well-placed to figure out how to make it work in a commercially viable way. Doing so will involve juggling a bewildering array of processes and market variables, though. Large-scale capture of atmospheric CO2 would require prodigious quantities of polyamine, which raises issues of environmental safety. Ruthenium is a rare-earth metal that has seen considerable volatility in supply and cost. Its current price is around $42 per ounce, but a decade ago the price was more than $850.
These challenges should not deter us. We have grown used to accepting that we have to follow wherever the market leads us, which is how fossil fuels have remained so entrenched in the global economy for so long. But today there are bigger concerns than short-term market efficiency. We must have a reliable, secure, long-term, carbon-neutral fuel supply. That is the cornerstone of our future energy needs, and the other arrangements will have to be fitted around it.
‘Carbon is precious. We must learn to recycle it. There should be no waste. There is no waste in nature’
Back in 2008, the photosynthesis expert James Barber of Imperial College London advocated an Apollo-style programme, comparable in scale and urgency to the 1960s Moon race, to develop solar fuels. It’s taken a while, but their call is finally being heeded. Once the least known of renewable energy technologies, solar liquid fuels now have powerful advocates. In particular, Bill Gates recently organised the Breakthrough Energy Coalition, a group of 28 investors aiming to boost world spending on carbon-free energy development to $20 billion a year. He also catalysed Mission Innovation, a 20-major-nation governmental initiative launched at the Paris climate change conference in December 2015.
Gates has some powerful advantages. He understands both the technological and financial challenges, and has plenty of financial resources himself, having pledged $2 billion to the project. His thinking is outlined in a paper, Energy Innovation: Why we need it and how to get it. The US government is also getting on board with the new approach. In April 2015, the Department of Energy’s Joint Center for Artificial Photosynthesis (JCAP) announced renewed funding of $75 million and a change of direction, away from hydrogen production and toward the kind of solar-generated liquid fuels I’ve been describing. With researchers, foundations, major world governments, and large investors all pulling in the same direction, success is looking far more probable that it did just a couple years ago – although, as Gates points out, such major technological shifts have typically taken decades in the past. At the same time, programs like JCAP are puny compared to the total magnitude of the R&D effort needed.
The costs are high, but the potential payoff is even higher. Holmgren at LanzaTech lays out a compelling vision: ‘Carbon is precious. This means we must learn to recycle it. If you can extend its life by reusing it in a fuel, you will keep that equivalent amount of fossil fuel in the ground. There should be no waste. There is no waste in nature.’
Redox reactions – the dance between carbon, hydrogen, and oxygen – produced the cornucopia of life on Earth. Right now, we are merely running down those reactions, unwinding millions of years of biochemistry that is locked away in the planet’s fossil fuels, and systematically polluting the atmosphere in the process. We need to understand the redox reactions, so we can master the biomechanical machinery of photosynthesis and start building up with it. Success could transform the world economy, and the global environment. It is a challenge we cannot afford to pass up.
