Light years ahead [report]

Aldo Steinfeld, Michael Epstein
Consider an area of less than 5 per cent of the Sahara desert, roughly 500km by 500km: a minute fraction of the Earth's surface. Even with solar collectors of a limited (20 per cent) efficiency, sunlight falling on this area would be sufficient to supply the current yearly energy needs (1.2 x 10 14 kWh) of the world's entire population. In practice, of course, it doesn't, because solar radiation reaching the Earth has some serious drawbacks: it is very dilute (only 1kW m -2 ), intermittent
more » ... , intermittent (available only during daytime and under clear sky conditions); and unequally distributed (mostly falling near the equator). But we can overcome these drawbacks by converting solar energy into solar fuels that can be stored over long periods of time, and transported over long distances, from the sunny regions of the Earth to the industrialised and populated centres, where much of the energy is needed. Solar fuels can be burnt to generate heat, further processed into electrical or mechanical work, or used directly to generate electricity in fuel cells and batteries, to meet customer's energy demands whenever and wherever needed. There are three pathways for making solar fuels from solar energy: • solar electrochemical path: solar-made electricity, from photovoltaic or solar thermal systems, followed by an electrolytic process; • solar photochemical path: direct use of the photon energy; and • solar thermochemical path: solar-made heat followed by a thermochemical process. Combinations of these three pathways are possible, but the thermochemical path offers some intriguing thermodynamic advantages. It is this approach that is of most interest to a consortium of scientists, including ourselves, at the Paul Scherrer Institute (PSI)/Swiss Federal Institute of Technology (ETH), Switzerland and the Weizmann Institute of Science (WIS) in Israel. Crucial to the solar thermochemical path are parabolic-shaped reflectors that concentrate the solar radiation by up to 5000 times. Solar receivers absorb this concentrated solar radiation and deliver it, in the form of h igh-temperature heat, to a chemical reactor wherein an endothermic reaction results in the production of solar fuels. These solar fuels ultimately store solar energy within their chemical bonds. Regardless of the fuel, the higher the temperature of the reaction process, the higher the efficiency of energy conversion. Higher temperatures, however, also lead to greater losses by re-radiation from the solar receiver. For a given solar concentration, there is an optimum temperature for maximum efficiency. For a solar concentration of 5000, the optimum temperature of a solar receiver is 1500K -giving a maximum theoretical efficiency of 75 per cent (the solar energy that could in principle be converted into the chemical energy of fuels). High efficiencies translate into smaller solar collection areas for the same chemical energy output and, consequently, lower costs. An example of a thermochemical process for producing fuels using solar power involves thermally reducing a metal oxide at elevated temperatures using concentrated solar energy. The product metal can be used to generate power directly in metal-air batteries or fuel cells, or it can
doi:10.3929/ethz-a-004274060 fatcat:vquj23ehpvdllp6whlmphtbnva