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