Concentrating photovoltaics

An alternative to concentrating thermal solar power in deserts is large-
scale concentrating photovoltaic systems. To make these, we plop a high-
quality electricity-producing solar cell at the focus of cheap lenses or mirrors.
Faiman et al. (2007) say that “solar, in its concentrator photovoltaics
variety, can be completely cost-competitive with fossil fuel [in desert states
such as California, Arizona, New Mexico, and Texas] without the need for
any kind of subsidy.”

According to manufacturers Amonix, this form of concentrating solar
power would have an average power per unit land area of 18 W/m2.

Another way to get a feel for required hardware is to personalize. One
of the “25 kW” (peak) collectors shown in figure 25.9 generates on average
about 138 kWh per day; the American lifestyle currently uses 250 kWh per
day per person. So to get the USA off fossil fuels using solar power, we
need roughly two of these 15 m×15 m collectors per person.


I’m confused! In Chapter 6, you said that the best photovoltaic
panels deliver 20 W/m2 on average, in a place with British sun-
niness. Presumably in the desert the same panels would deliver
40 W/m2. So how come the concentrating solar power stations
deliver only 15–20 W/m2? Surely concentrating power should be
even better than plain flat panels?

Good question. The short answer is no. Concentrating solar power does
not achieve a better power per unit land area than flat panels. The concen-
trating contraption has to track the sun, otherwise the sunlight won’t be
focused right; once you start packing land with sun-tracking contraptions,
you have to leave gaps between them; lots of sunlight falls through the
gaps and is lost. The reason that people nevertheless make concentrating
solar power systems is that, today, flat photovoltaic panels are very expensive,
and concentrating systems are cheaper. The concentrating people’s
goal is not to make systems with big power per unit land area. Land area
is cheap (they assume). The goal is to deliver big power per dollar.

But if flat panels have bigger power density, why don’t you des-
cribe covering the Sahara desert with them?

Because I am trying to discuss practical options for large-scale sustainable
power production for Europe and North Africa by 2050. My guess
is that by 2050, mirrors will still be cheaper than photovoltaic panels, so
concentrating solar power is the technology on which we should focus.

What about solar chimneys?

A solar chimney or solar updraft tower uses solar power in a very simple
way. A huge chimney is built at the centre of an area covered by a transparent
roof made of glass or plastic; because hot air rises, hot air created

Figure 25.9. A 25 kW (peak) concentrator photovoltaic collector produced by Californian company Amonix. Its 225 m2 aperture contains 5760 Fresnel lenses with optical concentration ×260, each of which illuminates a 25%-efficient silicon cell. One such collector, in an appropriate desert location, generates 138 kWh per day – enough to cover the energy consumption of half an American. Note the human providing a scale. Photo by David Faiman.