Solar Energy Perspectives - IEA

Chapter 9: Solar fuels
back to water…. you notice a cyclic process. No material is consumed. No material is
discharged. The only energy that enters into the process is sunlight. The energy available in
the hydrogen used to produce electricity or power is solar energy in disguise.”
However, research suggests the process is made easier, and the required temperature
lowered (but still above 1 200°C) if some natural gas is also used in the process, again not
as an energy source but as a reactant to reducing the zinc oxide and providing syngas.
Solar chemical reactor prototypes have been tested and developed further at the Paul
Scherrer Institute in Switzerland and Weizmann Institute in Israel. Other “redox” cycles
have been tested, and some require lower temperature levels, such as the tin dioxide
carbon reduction (900°C). Then again, liquid fuels could be manufactured with carbon
atoms.
Solar-enhanced biofuels
As most liquid fuels require carbon atoms, the more climate-friendly option would be
gasification or pyrolysis of biomass in concentrating solar towers, using CO 2 captured
from the atmosphere by the plants. This would reduce the land and water requirement of
current or future (advanced) biofuels, as concentrating solar in sunny countries is more
land-efficient than burning part of the biomass in providing high-temperature process
heat. Indeed, one company in Colorado has already tested the technology on a small
scale: Sundrop Fuels, a spin-off of the US National Renewable Energy Laboratory.
Cellulosic biomass of any kind was almost instantaneously gasified at temperatures above
1 100°C on a solar tower (Figure 9.4). At this temperature level, no volatile hydrocarbon
tar was produced 2 (Perkins and Weimer, 2009). The resulting syngas could then be turned
into any kind of liquid fuels through Fischer-Tropsch or other similar processes, with
properties quite close to those of petroleum products As natural gas is currently cheaper,
the company is using it as the energy source for its first large-scale biofuel plant.
Concentrating solar heat or electricity remain options for the future deployment of
advanced biofuels.
It is probably too early to tell if this particular technology, which has been demonstrated
on a small scale, can be scaled up with sufficient efficiency. Sceptics note that advanced
biofuels using any type of cellulosic material are not yet fully mature technologies, and
believe that solar heat would add to the cost and complications. Others insist that the
advantage of high-temperature solar heat with no combustion residues will, to the
contrary, help overcome the current difficulties in developing advanced biofuel
technologies.
There are other options. For example, solar distillation of ethanol reduces the consumption
of fossil fuel in the manufacturing of standard 1 st generation biofuels and has likely reached
competitiveness with oil prices by now, e.g. in Thailand (Vorayos et al., 2006). Similarly, the
torrefaction of raw biomass – a mild form of pyrolysis at temperatures ranging between 200-
320°C – to make it a more energy-dense, cleaner, hydrophobic and stable solid fuel,
2. Tar production during gasification driven by burning biomass (or natural gas) at temperatures below 1 000°C is one of the main
difficulties in manufacturing advanced biofuels. It causes fouling of downstream catalytic surfaces and clogging of processing
equipment.
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© OECD/IEA, 2011