SEB Bulletin January 2006 - Future Perspectives
Solar Biological Energy - How to split water using energy from sunlight
James Barber FRS and Christine Foyer
James Barber, Division of Molecular Biosciences, South Kensington Campus, Imperial College London, SW7 2AZ, UK
Christine Foyer, Crop Performance and Improvement Division, Rothamsted Research, Harpenden, Herts, AL5 2JQ
As a signatory to Kyoto and the recent Montreal agreements, the UK recognises the threat posed by climate change. There is a wide-spread acknowledgement by UK scientists of the urgent need for groundbreaking research to discover new, safe and non-polluting systems of energy production to fill the energy generation “gap” and reduce the present reliance on fossil fuels, whose use is considered by many to be the major driver of the present climate change crisis. The mean global energy consumption rate in the year 2000 was 13 TW and this will rise towards 20 TW within this decade. At present about 11% of global fuel demand comes from biomass (combustion and fermentation) while 85% is derived from fossil fuels. We consider that plants provide key answers concerning carbon-free safe energy generation, as well as systems for capturing and storing atmospheric carbon dioxide.
The world is already witnessing the beginnings of extreme weather phenomena associated with climate change. Picturing the future effects of global warming is not easy but it is estimated that climate change could raise the global temperature by about three degrees centigrade over the next hundred years. Indeed, seven out of the ten warmest years on record were in the 1990's. In 2005 the UK made climate change a key priority of its presidency of both the G8 group of nations and the EU. However, it is not a simple task to implement means of reducing the pollution associated with current energy generation that underpins climate change. Key questions concern how to make energy sustainable and secure zero carbon emissions.
We consider that given recent advances in our understanding of the way that plants split water using the solar energy-driven metallo-protein complexes of photosystem (PS)II, the time is right to re-consider this paradigm of safe and efficient energy generation as a basis for future sustainable energy sourcing. It is widely acknowledged that we need to take a holistic view of carbon management that includes new systems of energy generation, and this incorporates the design of energy-generating systems that mimic the light-driven water-splitting systems activities of plants and algae. We are now poised to exploit the new knowledge that we have gained concerning PS II structure and function.
In terms of solar energy conversion the early photosynthetic processes, including the water splitting reaction of plants are highly efficient. In contrast, the later stages of energy conversion into complex carbohydrates and other molecules that underpin the production of biomass are rather less efficient. The energy contained within the biomass produced annually is only about 10 times greater than that required for present energy demand. This relatively small yield is because the conversion of solar energy into organic material on a global scale is limited to 0.2%.
Conversion efficiencies greater than 10% are required if solar energy is to be utilised on a massive scale. Moreover, the interception area required for harvesting sunlight has to be minimised. Commercially available photovoltaic cells can convert solar energy to electricity at 10% and above and currently global photovoltaic installations provide about 3 GW. Unfortunately the cost of silicone-based photovoltaic cells is high, being about £2 per watt. However, the working lifetime of such systems is currently 25 years or more. In order to produce hydrogen as a fuel source, solar-generated photovoltaic electricity would have to be used to split water by electrolysis, which is an inefficient process requiring expensive platinum electrodes.
We consider that potentially significant alternative technologies for the future will mimic photosynthesis. They will use photo-chemically active catalysts that can capture solar energy to split water directly into oxygen and hydrogen. Work in the Barber laboratory, has recently revealed precise molecular details of the PSII catalytic site where water splitting occurs in photosynthesis. The site is composed of four manganese atoms (Mn) and a calcium ion (Ca2+) surrounded by a highly conserved protein matrix*. This new information provides a framework on which to elucidate the unique chemistry which underpins Nature's solution to the energy problem.
Our developing knowledge and appreciation of how plants use metal ions bound to proteins to harness solar energy and split water provides mankind with a new avenue to explore for generating hydrogen as a fuel. Hydrogen gas is considered to be the perfect carbon-free fuel as it produces only heat and water when it burns. Creating a world hydrogen economy will require enormous technological investment and advances, but within this agenda plants and plant-based systems are viewed as crucial clean sources for the production of hydrogen and energy to meet future needs.
Our developing knowledge and appreciation of how plants use metal ions bound to proteins to harness solar energy and split water provides mankind with a new avenue to explore for generating hydrogen as a fuel.
The energy provided by solar radiation is over 100,000 TW. Hence, more solar energy strikes the surface of the earth in one hour than all the global fossil energy consumption in an entire year. Early in the development of life on earth, some living organisms developed molecular mechanisms that took advantage of this vast energy resource and captured it using the process that we now call photosynthesis. The evolution of photosynthetic competence allowed life on our planet to develop and prosper on an enormous scale.
The majority of photosynthetic organisms (plants, algae and cyanobacteria) use the visible region of the solar spectrum to split water into molecular oxygen and 'hydrogen' (in the form of reducing equivalents). The oxygen released into the atmosphere is used to generate energy through respiration by aerobic organisms (plants, animals and microbes) and for the combustion of fuels (biomass and fossil). The 'hydrogen' released from water is combined with carbon dioxide to form the organic molecules of life which constitute the global biomass and also responsible for the laying down of fossil fuels millions of years ago.
We are now poised to exploit the recent knowledge that we have gained concerning photosystem (PS) II structure and function as a new system of energy generation.
* For further specialist reading concerning advances in our understanding of these processes we recommend Volume 2 of the Series on Photoconversion of Solar Energy titled “Molecular to Global photosynthes, edited by Mary Archer and Jim Barber [ISBN 1-86094-256-3] and Volume 22 of the Advances in Photosynthesis and Photorespiration Series titled: “Photosystem II: The light-driven water: Plastoquinone oxidoreductase”, edited by Tom Wydrzynski and Kimiyuki Satoh [ISBN-10 1-4020-4249-3(HB)].