The most promising routes to eventual full-scale commercial solar energy conversion directly into fuels were identified at a recent international meeting in Regensburg, sponsored by the European Science Foundation (ESF). An interdisplinary task force was established at this meeting to make the case for substantial investments in these technologies to EU and national government decision makers.
The fundamental issue is that total annual global energy consumption is set to at least double from its current level of 14 TW by 2050, while fossil fuels will start to run out. The use of fossil fuels also produces unacceptable levels of carbon dioxide, causing global warming and has disastrous effects in many areas, such as food production.
Apart from solar energy, the shortfall can only be made up by renewable sources such as wind, along with the other non-fossil, non-renewable fuel source of energy, nuclear. But these will be unable to satisfy the predicted increased energy needs and certainly will not be able to replace fossil fuels entirely, even for electricity production alone. Another problem is that they will not readily yield stored fuels. Without an unexpected breakthrough in electricity storage, there will be a continued need for fuels for around 70 percent of total global energy requirements, particularly in transportation, manufacturing, and domestic heating. Electricity only accounts for 30 percent of global energy consumption at present.
However solar energy is plentiful since enough reaches the earth's surface every hour to meet the world's annual energy needs. The problem lies in harnessing it. Nature has perfected, in photosynthesis, a highly efficient and flexible means of doing this across a wide variety of scales, from isolated bacterial colonies to large forests.
Substantial progress has been made recently, particularly in Europe, in understanding and mimicking these natural processes, sufficient for scientists to be confident that they could use them to produce fuels on a commercial scale. The focus of research should therefore be on drawing inspiration from biological systems for the creation of both natural and artificial solar energy conversion systems that allow in the long run for a stable and sustainable energy supply. There should also be an aim to reduce the human ecological footprint and thereby increase the global ecological capacity using technology that is environmentally clean, for instance by conversion of carbon dioxide back into fuels in a cyclic process.
The ESF task force is recommending that three parallel avenues of solar energy research for generating clean fuel cycles should be pursued in Europe:
- Extending and adapting current photovoltaic technology to generate clean fuels directly from solar radiation.
- Constructing artificial chemical and biomimetic devices mimicking photosynthesis to collect, direct, and apply solar radiation, for example to split water, convert atmospheric carbon dioxide and thus produce various forms of environmentally clean fuels.
- Tuning natural systems to produce fuels such as hydrogen and methanol directly rather than carbohydrates that are converted into fuels in an indirect and inefficient process.
These three research themes will overlap, and all will exploit fundamental research elucidating the precise molecular mechanism involved in the splitting of water into hydrogen and oxygen in photosynthesis by both plants and oxygenic bacteria. This process, which evolved 2.5 billion years ago, created the conditions for animal life by converting atmospheric carbon dioxide into oxygen and carbohydrates, and also produced all the fossil fuels, which humans are turning back into carbon dioxide at an increasing rate, threatening catastrophic environmental effects. The same process now holds our salvation again.
Although the principal products of photosynthesis in plants and bacteria are carbohydrates, some hydrogen is produced in certain algae and bacteria, providing a basis for genetic modification to increase yields, and for the creation of suitable artificial systems. Furthermore, photosynthesis is capable of generating other chemicals currently made industrially, such as nitrates amino acids, and other compounds of high value for chemical industry. The European research programme will therefore seek to develop systems for converting solar energy directly into such chemicals with much greater efficiency, offering the prospect not just of producing unlimited energy, but also fixing atmospheric carbon dioxide to bring concentrations back down to pre-industrial levels as part of the overall thrust for clean renewable energy.
There are considerable challenges, with the first being to mimick the functioning of natural photosynthetic systems, particularly photosystem II, the enzyme complex in the leaves of plants that splits water into hydrogen and water via a catalyst comprising four manganese atoms along with some calcium. Significant progress has been made recently on this front. Participants at the ESF's brainstorming conference, describe the solar fuels project as the quest for building the "artificial leaf." There is growing conviction in Europe and elsewhere that, by 2050, a large proportion of our fuels will come from such "artificial leaves," and that there is no time to lose starting the crucial enabling research, in order to gain technology leadership in this important future key technology.