As energy demand grows it becomes increasingly important to be free from fossil fuels. As it is now, more energy is produced from renewable sources, but the challenge is to meet the transport sector’s specific requirements for storability. Maybe the solution is to transform solar energy directly into fuel, without detouring past oil and biofuel and by mimicking a plant’s own energy storage in an integrated system?

The need for energy continues to increase. At present, about one-sixth of the global energy consumption comes from renewable energy sources: solar, wind, hydro, wave, biomass and geothermal energy.

But the percentage is growing.

The IEA estimates in its World Energy Outlook for renewable energy in 2035 that we will be able to generate three-to-four times as much electricity as today. At the same time, about a quarter of the world’s total energy consumption comes from transport, so the sector is extensive.

Within the transport sector, the picture is very different, where renewable sources account for only three percent of the energy used.

The slow transition and massive oil dependence is due to liquid hydrocarbons, which have a number of good qualities as an energy carrier and makes them more difficult to replace: the energy is stably stored in chemical form, with high energy density, and is easy to release in a relatively efficient manner in internal combustion engines. The infrastructure is also well developed.

What are the alternatives?

How can today’s fuels be replaced? Biofuels are an option, but they come with a built-in conflict. They are made from commodity crops – often corn or sugarcane – that might otherwise be used for food. In the future, the hope is to cultivate microalgae, microorganisms or fungi (such as the so-called myco-diesel) without interfering with food production – or to ferment cellulosic ethanol from waste products from agriculture and forestry. The most common form of bio-fuel is bio-ethanol from the fermentation of carbohydrates, and biodiesel from organic oils.

In addition, there are electric vehicles and fuel cells. The dilemma here is scalability. Nearly half of today’s shipments are made by air, heavy trucks or by ship and it would be difficult to electrify them. Replacing the world’s vehicle fleet is also no small task; of over a billion vehicles, approximately one in fifteen is suited for alternative fuels. And this takes into account electric hybrids and pure electric cars, which make up less than one percent of the fleet. (At present, electric cars pose significantly greater environmental impact in the production part of their life cycle). Finding a fossil-free fuel to use in existing internal combustion engines would of course facilitate the conversion significantly.

A solar-based fuel would be ideal; an hour of sun on the earth’s surface receives the equivalent energy of the annual consumption of mankind. Photovoltaic cells directly on a vehicle are not near enough to be effective at this stage. Excluding nuclear power, solar energy is the way to go. It has been stored in chemical bonds – using fossil biomass to become oil, coal and gas, or through crops – and in both cases through the process of photosynthesis.

Natural and artificial photosynthesis

In the natural photosynthesis process, water combined with carbon dioxide turns to sugar and oxygen, using chlorophyll molecules. Of the captured light energy, there is a small percentage that is converted to biomass and which can be harvested, a large part of the energy consumed by the plant itself in so-called dark reactions. The binding energy, the light reaction, is very efficient, with an efficiency of 30-40 percent – compared with, at best, 20 percent of today’s solar cells.

The driving force in the process is charge separation: the ability to keep the charges spaced, instead of recombining them and emitting heat. When plants absorb light energy, a supply of electrons is released. In Sweden, there is extensive research on artificial photosynthesis.

The electrons that arise are charged from a catalyst, a mix of manganese ions. The manganese fills its electrons from water molecules that come to the surface. The electrons that are transported away reduce carbon dioxide from the air and turn them into carbohydrates.

Chlorophyll cannot operate without plants – but of course it is an attractive idea to seek a biomimetic solution: to copy nature and try to create artificial photosynthesis, where fuel can be produced in a photochemical system. Process ingredients are almost inexhaustible – sun and water – and by using carbon dioxide as a resource instead of as a pollutant.

In Sweden, extensive research is being conducted in the field. Different research groups gather in the Consortium for Artificial Photosynthesis, with the bulk of its operations at the Ångström Laboratory in Uppsala.

Objectives: robust, integrated and fast

Research on artificial photosynthesis has two parallel tracks. One is to try to modify cyanobacteria. They give off hydrogen gas naturally under certain conditions, and there is hope that by genetic means to enhance these properties.

The second is to produce molecules that may mimic the photosynthetic function. One can of course use the power from solar cells to split water, but the energy transformation is not yet cost efficient. The dream is instead to have an artificial system: an integrated, super light-sensitive molecule that oxidizes water with a catalyst and produces hydrogen from protons.

Hydrogen can be used as a fuel – or it can be used to construct other fuels, synthetic hydrocarbons by Fischer-Tropsch, or other chemical processes. The British company Cella Energy claims to be able to encapsulate hydrogen microbeads, so that they can be used with the same infrastructure as liquid fuel.

Chlorophyll molecules are broken down gradually by light, and plants need to regenerate and heal themselves continuously. An important goal of research in artificial photosynthesis is to find synthetic materials that are stable and do not degrade too fast.

Several artificial leafs that capture sunlight and split water and produce hydrogen gas have already been developed. But expensive construction materials like ruthenium and palladium have shown insufficient response rates and have prevented a commercial breakthrough. In terms of cost, they cannot yet compete with fossil fuels.

The Consortium for Artificial Photosynthesis tries to combine the ruthenium with manganese as a catalyst, and there are other experiments where the catalyst material is made of titanium oxide or cobalt oxide. The latter is promising because it is both stable and abundant.

Recently, the Canadian company FireWater Fuel reported successes with an amorphous nano-material based on iron oxide as a catalyst.

A mirage solution

Artificial photosynthesis is a complex composition of difficult sub-problems. Although there is much work before we can add them together, we have already begun to master some of the individual steps.

The transition to renewable energy is a big challenge, especially when it comes to transportation. Being in a self-sustaining process of converting sunlight into an easy-fuel is a beckoning solution – but one that is already established and proven in the form of plants’ own success formula. Studying nature’s methods and developing them further might open an energy efficient shortcut for future clean fuel solutions.

The article was published in May 2013