3.7. Living on other countries’ renewables? http://www.ck12.org
Figure 25.9:A 25 kW (peak) concentrator photovoltaic collector produced by Californian company Amonix. Its
225 m^2 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.
Queries
I’m confused! In Chapter Solar, you said that the best photovoltaic panels deliver 20 W/m^2 on average, in a
place with British sunniness. Presumably in the desert the same panels would deliver 40 W/m^2. So how come the
concentrating solar power stations deliver only 15 [U+0080][U+0093] 20 W/m^2? 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 concentrating 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 describe 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 in this greenhouse-
like heat-collector whooshes up the chimney, drawing in cooler air from the perimeter of the heat-collector. Power
is extracted from the air-flow by turbines at the base of the chimney. Solar chimneys are fairly simple to build, but
they don’t deliver a very impressive power per unit area. A pilot plant in Manzanares, Spain operated for seven
years between 1982 and 1989. The chimney had a height of 195m and a diameter of 10 m; the collector had a
diameter of 240m, and its roof had 6000m^2 of glass and 40000m^2 of transparent plastic. It generated 44 MWh per
year, which corresponds to a power per unit area of 0. 1 W/m^2. Theoretically, the bigger the collector and the taller