The Photovoltaic Challenge
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Photovoltaics
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 demand1. Traditional use of biomass, mainly in developing countries, accounts for more than 10% of the total energy consumption,1'2 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.
Among the solar energy technologies, the modern use of biomass through energy plantations, and photovoltaic solar energy (PV) have been identified as rapidgrowth technologies with a very large potential.1 A PV system is a modular, fully solid-state electricity generator (see Figure 1). It is therefore reliable and can be used in many different applications, varying from calculators and stand-alone singleor dual-module solar home systems3 (see Figure 2) to grid-connected roof-integrated systems (see cover photo) and central power plants. A major obstacle to large-scale use of PV is the high initial investment cost. Typical electricity generation costs are be-
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module with support
battery
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Figure 1. Main types of PV systems: (a) stand-alone system with battery storage and (b) grid-connected system (MPPT = maximum power point tracker).
tween US $0.25 and $1.00 per kWh for different types of systems and different locations. As a result, the use of PV is limited mainly to applications in which electricity is given a high value or w
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