may be true, but efficiency should not be confused with delivered power.

39Typical solar panels have an efficiency of about 10%; expensive ones per-
form at 20%
. See figure 6.18. Sources: Turkenburg (2000), Sunpower www.
, Sanyo, Suntech.

A device with efficiency greater than 30% would be quite remarkable. This
is a quote from Hopfield and Gollub (1978), who were writing about panels
without concentrating mirrors or lenses. The theoretical limit for a standard
“single-junction” solar panel without concentrators, the Shockley–Queisser
limit, says that at most 31% of the energy in sunlight can be converted to
electricity (Shockley and Queisser, 1961). (The main reason for this limit
is that a standard solar material has a property called its band-gap, which
defines a particular energy of photon that that material converts most ef-
ficiently. Sunlight contains photons with many energies; photons with en-
ergy below the band-gap are not used at all; photons with energy greater
than the band-gap may be captured, but all their energy in excess of the
band-gap is lost.) Concentrators (lenses or mirrors) can both reduce the
cost (per watt) of photovoltaic systems, and increase their efficiency. The
Shockley–Queisser limit for solar panels with concentrators is 41% efficiency.
The only way to beat the Shockley–Queisser limit is to make fancy photo-
voltaic devices that split the light into different wavelengths, processing each
wavelength-range with its own personalized band-gap. These are called
multiple-junction photovoltaics. Recently multiple-junction photovoltaics
with optical concentrators have been reported to be about 40% efficient.
[2tl7t6], In July 2007, the University of Delaware
reported 42.8% efficiency with 20-times concentration [6hobq2], [2lsx6t]. In
August 2008, NREL reported 40.8% efficiency with 326-times concentration
[62ccou]. Strangely, both these results were called world efficiency records.
What multiple-junction devices are available on the market? Uni-solar sell a
thin-film triple-junction 58 W(peak) panel with an area of 1 m2. That implies
an efficiency, in full sunlight, of only 5.8%.

40Figure 6.5: Solar PV data. Data and photograph kindly provided by Jonathan
Kimmitt. See figure 6.19.
A similar system is made by Arontis

Figure 6.17. Part of Shockley and Queisser’s explanation for the 31% limit of the efficiency of simple photovoltaics. Left: the spectrum of midday sunlight. The vertical axis shows the power density in W/m2 per eV of spectral interval. The visible part of the spectrum is indicated by the coloured section. Right: the energy captured by a photovoltaic device with a single band-gap at 1.1 eV is shown by the tomato-shaded area. Photons with energy less than the band-gap are lost. Some of the energy of photons above the band-gap is lost; for example half of the energy of every 2.2 eV photon is lost. Further losses are incurred because of inevitable radiation from recombining charges in the photovoltaic material.
Figure 6.18. Efficiencies of solar photovoltaic modules available for sale today. In the text I assume that roof-top photovoltaics are 20% efficient, and that country-covering photovoltaics would be 10% efficient. In a location where the average power density of incoming sunlight is 100 W/m2, 20%-efficient panels deliver 20 W/m2.