The Photovoltaic Challenge
Wim C. Sinke, Guest Editor
The term "solar energy" refers to a wide variety of techniques for using the energy available as sunlight.
Well-known examples are active and passive thermal solar energy and photovoltaic solar energy but, strictly
speaking, hydropower, wind energy, and biomass are also forms of solar energy. Today, only hydropower is used in
significant quantities, covering approximately 6% of the world's energy demandl. Traditional use of biomass, mainly
in developing countries, accounts for more than 10% of the total energy consumption1,2 but is sometimes left out of
statistics because it falls outside the category of organized and commercial use.
The global potential for solar energy is huge, since the amount of energy that reaches the earth's surface every
year exceeds the total energy consumption by roughly a factor of 10,000. There are, however, various barriers to the
large-scale use of solar energy technologies. Most technologies have in common that the power density of the
generator is low; in other words, one needs large areas to generate significant amounts of energy. This is especially
true for biomass, with typical conversion efficiencies (solar energy to chemical energy) of 1% or less. Further, many
solar energy technologies have proved technically feasible, but have yet to be proved economically feasible. Last,
but not least, the large-scale use of solar energy requires substantial modification of our global energy supply
system, which is based largely on fossil fuels.
Fortunately, these disadvantages are offset by the intrinsic advantages of solar energy. Overall emissions of CO2
and other gases (for instance, those associated with the construction of solar hardware) are generally small because
no fuel is used during operation. In addition, solar energy, if used correctly, is a renewable source of energy which
can be used indefinitely. Finally, it can be made available in some form to people in almost all areas of the world.