20 - Concentrating and multijunction photovoltaics

Summary

Focus

Sunlight has two fundamental characteristics that make its use as a cost-effective large-scale source of electricity challenging: its low power density and its broad spectrum. The first characteristic means that sunlight has to be collected over large areas to gather significant amounts of power, and the second characteristic means that conventional solar cells are inherently limited in converting this power to electricity. The “concentrator photovoltaics” (CPV) approach using multijunction solar cells addresses these two challenges head-on.

Synopsis

Sunlight shines with a power of about 1 kW m⁻², on average, onto the Earth's surface. Although this might feel considerable to a beachgoer on a hot sunny day, for electrical power generation, this power density is actually inconveniently small. For perspective, consider that a typical electrical power plant generates 1 GW, enough to supply the needs of a rather small city and about 0.1% of the total electricity generation capacity in the USA. The very best conventional solar cells are of efficiency about 20%, so to make a 1-GW solar photovoltaic power plant would require at least 5 km² of solar cells. To gather sunlight over such large areas and convert it to electricity economically is a fundamental challenge of photovoltaics. One approach, discussed in other chapters, is to reduce the cost of the solar cells. In contrast, this chapter describes an alternative approach that reduces the amount of cells needed: CPV.