Solar Fuels
There are currently two solar fuels technologies in use for which performance and economic
assessments are possible. These are biomass-derived fuels and hydrogen produced by
electrolysis. In the latter case, electricity is derived principally from fossil fuel sources, which
ultimately derive from biomass as well.
The state of biomass-derived fuel production is reviewed in the first part of this assessment,
while the second portion describes the state of the research effort to produce hydrogen fuel using
electrolysis.
BIOMASS-DERIVED FUELS
Biomass is the oldest form of solar energy to be used by humanity. Until about 1800, biomass
was the predominant fuel used for heating, metallurgy, lighting, and transportation (as animal
fodder), as well as supplying materials needs in the form of timber and fiber and, of course, as
food for human sustenance. The energy use of biomass is primarily from the 14.9-Gha terrestrial
surface of the Earth, of which only 56% is productive terrain with forests, savannas, prairies, and
arable land.
The current biomass primary energy (bioenergy) supply is about 11% of the total energy demand
of 13.3 TW (IEA 2004), or 1.4 TW. Human food intake for metabolic needs is around half that at
about 0.65 TW. The industrial country use pattern is for heat and electricity (very often in
combined heat and power [CHP]) using mainly solid biomass, along with a growing use of liquid
fuels such as ethanol and biodiesel produced from crops (Chum and Overend 2003). The capture
of methane from environmental technologies, such as anaerobic digestion, is also widely
practiced^ (Schulz and Eder 2001).
The developing country pattern is more artisanal and inefficient; biomass is often used as
fuelwood in cook stoves and to provide energy in small-scale industries (tea-drying, brick kilns,
charcoal manufacture, etc.) The major development areas for biomass-to-fuels are concerned
with the production of liquid fuels such as ethanol; electricity and co-produced heat; gaseous
fuels such as methane, fuel gas, and syngas; and in the future, hydrogen. The United States has a
major program to develop processes that co-produce energy and higher value products, such as
biorefineries (DOE 2003).
The large scale of the biomass and bioenergy supply chain is set in a framework of constraints
with respect to the environment, food and fiber supply, and sustainability. Most of the biomass
used in industrial countries is part of a cascade of use, reuse, and recycling of biomass materials
with post-consumer residues being very important, along with the use of residues generated on
farms, in the forest, and in the industries that process biomass.
However, crops for energy are of growing importance as sugar cane (Goldemberg et al. 2004),
cereals (Bullion 2004), and oilseeds (Martini and Schell 1998) become significant inputs to the
expanding liquid biofuels markets. These tend to be the most expensive biomass resources,