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By Neil Bowdler Science reporter, BBC News
In the prototype, sunlight heats a ceria cylinder which breaks down water or
carbon dioxide In the prototype, sunlight heats a ceria cylinder which breaks
down water or carbon dioxide
A prototype solar device has been unveiled which mimics plant life, turning the
Sun's energy into fuel.
The machine uses the Sun's rays and a metal oxide called ceria to break down
carbon dioxide or water into fuels which can be stored and transported.
Conventional photovoltaic panels must use the electricity they generate in
situ, and cannot deliver power at night.
Details are published in the journal Science.
The prototype, which was devised by researchers in the US and Switzerland, uses
a quartz window and cavity to concentrate sunlight into a cylinder lined with
cerium oxide, also known as ceria.
Ceria has a natural propensity to exhale oxygen as it heats up and inhale it as
it cools down.
If as in the prototype, carbon dioxide and/or water are pumped into the vessel,
the ceria will rapidly strip the oxygen from them as it cools, creating
hydrogen and/or carbon monoxide.
Hydrogen produced could be used to fuel hydrogen fuel cells in cars, for
example, while a combination of hydrogen and carbon monoxide can be used to
create "syngas" for fuel.
It is this harnessing of ceria's properties in the solar reactor which
represents the major breakthrough, say the inventors of the device. They also
say the metal is readily available, being the most abundant of the "rare-earth"
metals.
Methane can be produced using the same machine, they say.
Refinements needed
The prototype is grossly inefficient, the fuel created harnessing only between
0.7% and 0.8% of the solar energy taken into the vessel.
Most of the energy is lost through heat loss through the reactor's wall or
through the re-radiation of sunlight back through the device's aperture.
But the researchers are confident that efficiency rates of up to 19% can be
achieved through better insulation and smaller apertures. Such efficiency
rates, they say, could make for a viable commercial device.
"The chemistry of the material is really well suited to this process," says
Professor Sossina Haile of the California Institute of Technology (Caltech).
"This is the first demonstration of doing the full shebang, running it under
(light) photons in a reactor."
She says the reactor could be used to create transportation fuels or be adopted
in large-scale energy plants, where solar-sourced power could be available
throughout the day and night.
However, she admits the fate of this and other devices in development is tied
to whether states adopt a low-carbon policy.
"It's very much tied to policy. If we had a carbon policy, something like this
would move forward a lot more quickly," she told the BBC.
It has been suggested that the device mimics plants, which also use carbon
dioxide, water and sunlight to create energy as part of the process of
photosynthesis. But Professor Haile thinks the analogy is over-simplistic.
"Yes, the reactor takes in sunlight, we take in carbon dioxide and water and we
produce a chemical compound, so in the most generic sense there are these
similarities, but I think that's pretty much where the analogy ends."
The PS10 solar tower plant near Seville, Spain. Mirrors concentrate the sun's
power on to a central tower, driving a steam turbine The PS10 solar tower plant
near Seville, Spain. Mirrors concentrate the sun's power on to a central tower,
driving a steam turbine
Daniel Davies, chief technology officer at the British photovoltaic company
Solar Century, said the research was "very exciting".
"I guess the question is where you locate it - would you put your solar
collector on a roof or would it be better off as a big industrial concern in
the Sahara and then shipping the liquid fuel?" he said.
Solar technology is moving forward apace but the overriding challenges remain
ones of efficiency, economy and storage.
New-generation "solar tower" plants have been built in Spain and the United
States which use an array of mirrors to concentrate sunlight onto tower-mounted
receivers which drive steam turbines.
A new Spanish project will use molten salts to store heat from the Sun for up
to 15 hours, so that the plant could potentially operate through the night.