Transforming into a society that is driven by sustainable energy rather than fossil fuels is one of the great challenges facing mankind today. While the sun provides almost enough energy to power our global economy for an entire year, it is hard to harvest because it is spread out in low density across the earth’s surface. Since sunlight hits only half of the planet at any one time, it is inevitably an intermittent resource, and difficult to capture and use. But there is hope, thanks to a new technique that turns solar power into storable liquid or gaseous fuels.
Solar power already has a relatively long history, with heating and cooling devices, hot-water boilers on roofs and other solar thermal devices, such as mirrors that concentrate sunlight to generate heat and photovoltaic (PV) panels that convert solar energy into electricity. Since the 1950s, research has mostly focused on developing increasingly efficient (and hence cheaper) PV cells, which are the building blocks of solar panels. Consequently, this technology is now becoming established, already contributing well over 100 gigawatts to total power-production capacity. To compare, about three times this capacity is installed for nuclear energy and wind power around the world, whereas thousands of gigawatts would be needed to meet a substantial part of mankind’s overall electricity needs through PV.
Today, research and development in PV cells mainly focuses on making them cheaper, more efficient and better able to compete in the electricity market. Examples of progress include “dye-sensitized” cells (devices that are based on cheap molecular light absorbers) and new, cheap, inorganic materials such as perovskite. As solar’s share in the world’s energy production increases, so too do concerns about its intermittency. Today’s power grids can only handle a certain amount of fluctuations.
What should we do when we need electricity but there is no sunlight, and how can we store surplus power? These are becoming increasingly pertinent questions. Devices that convert power into gas (combined with ones that convert gas back into power) are among the emerging options. In its 2011 report, the International Energy Agency drew attention to the potential of directly converting solar energy into gaseous or liquid fuels, but more research is still needed.
If we can get the technology right, solar fuels could be promising weapons in the fight against climate change. For example, they could become an effective energy carrier and storer, since they can be converted back into electricity via fuel cells. In addition, fuels generated from sustainable energy could serve the transport sector, which looks likely to keep using combustion engines for at least another decade, and reduce its CO2 emissions.
However, techniques that transform solar energy into fuel are in their infancy, and we aren’t expecting solar fuels to become a significant part of our energy systems over the next couple of decades (IEA, 2014). That’s why we propose a two-pronged approach towards solar fuel research.
First, we need to increase both fundamental and applied research and development, to determine the best way of producing solar fuel out of all the different options. Secondly, we need to focus on actually building and using solar fuel as early as possible, in order to glean insights that can then be fed back into the research-and-development process.
Whereas proof of the feasibility of solar fuels was found as long ago as 1972 (through the use of titanium dioxide and ultraviolet light), it was not until 1998 that a semiconductor solar cell was covered with a catalyst for the direct light-driven production of hydrogen from water, with a 10% yield. Ever since, the number of devices reported has been limited, and either these devices suffer from low stability and low efficiency (less than 10% solar to fuel) or the fabrication requires more expensive, less abundant materials.
A lot of research is currently devoted to systems that are based on devices fabricated with tailor-made molecular components. Based on the blueprints provided by nature, these artificial leaves should be able to reach high efficiencies; yet their practical applicability has still to be proven. So far, the precise organization of molecular components (to prevent energy losses) remains a big challenge in the development of devices that are efficient in solar-to-fuel conversion. For the moment, we still haven’t found solar fuel technologies that are stable, effective and affordable, but given current intensive worldwide research, it may only be a matter of time.
Clearly, the conversion of direct solar energy into fuel is not something that will dominate the energy market, perhaps not for the next few decades. It is entirely possible, however, that in the relatively short term, such technology could be implemented in niche markets – in remote places without access to electricity, for example. This, combined with more research and development, could help solar technology become more mainstream.
One day, solar fuels could play a dominant role, and we’ll see a new generation of electrical cars powered by the sun. For now, one thing is clear: to tackle a problem as massive as climate change, we must explore all of the options available to us to create clean, sustainable energy.
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Authors: Joost Reek is a professor at the Van’t Hoff Institute for Molecular Sciences and a member of the Global Agenda Council on Nanotechnology. Bob van der Zwaan is senior scientist in the Policy Studies department of the Energy research Centre of the Netherlands (ECN) in Amsterdam.
Image: Solar panels are seen at a power plant in Amareleja, southern Portugal, April 24, 2008. REUTERS/Jose Manuel Ribeiro