41The Solarpark in Muhlhausen, Bavaria. On average this 25-hectare farm is
expected to deliver 0.7 MW (17 000 kWh per day).
New York’s Stillwell Avenue subway station has integrated amorphous sili-
con thin-film photovoltaics in its roof canopy, delivering 4 W/m2 (Fies et al.,
The Nellis solar power plant in Nevada was completed in December, 2007,
on 140 acres, and is expected to generate 30 GWh per year. That’s 6 W/m2
Serpa Solar Power Plant, Portugal (PV), “the world’s most powerful so-
lar power plant,” [39z5m5] [2uk8q8] has sun-tracking panels occupying 60
hectares, i.e., 600 000 m2 or 0.6 km2, expected to generate 20 GWh per year,
i.e., 2.3 MW on average. That’s a power per unit area of 3.8 W/m2.

41The solar power capacity required to deliver 50 kWh/d per person in the UK
is more than 100 times all the photovoltaics in the whole world.
To deliver
50 kWh/d per person in the UK would require 125 GW average power, which
requires 1250 GW of capacity. At the end of 2007, world installed photo-
voltaics amounted to 10 GW peak; the build rate is roughly 2 GW per year.

... paving 5% of this country with solar panels seems beyond the bounds of
. My main reason for feeling such a panelling of the country
would be implausible is that Brits like using their countryside for farming
and recreation rather than solar-panel husbandry. Another concern might be
price. This isn’t a book about economics, but here are a few figures. Going
by the price-tag of the Bavarian solar farm, to deliver 50 kWh/d per person
would cost €91 000 per person; if that power station lasted 20 years without
further expenditure, the wholesale cost of the electricity would be €0.25 per
kWh. Further reading: David Carlson, BP solar [2ahecp].

43People in Britain throw away about 300 g of food per day. Source: Ventour

Figure 6.10. In the USA, Miscanthus grown without nitrogen fertilizer yields
about 24 t/ha/y of dry matter. In Britain, yields of 12–16 t/ha/y are re-
ported. Dry Miscanthus has a net calorific value of 17 MJ/kg, so the British
yield corresponds to a power density of 0.75 W/m2. Sources: Heaton et al.
(2004) and [6kqq77]. The estimated yield is obtained only after three years
of undisturbed growing.

The most efficient plants are about 2% efficient; but the delivered power per
unit area is about 0.5 W/m2
. At low light intensities, the best British plants are
2.4% efficient in well-fertilized fields (Monteith, 1977) but at higher light in-
tensities, their conversion efficiency drops. According to Turkenburg (2000)
and Schiermeier et al. (2008), the conversion efficiency of solar to biomass
energy is less than 1%.
Here are a few sources to back up my estimate of 0.5 W/m2 for vegetable
power in the UK. The Royal Commission on Environmental Pollution’s esti-
mate of the potential delivered power density from energy crops in Britain is
0.2 W/m2 (Royal Commission on Environmental Pollution, 2004). On page
43 of the Royal Society’s biofuels document (Royal Society working group
on biofuels, 2008), Miscanthus tops the list, delivering about 0.8 W/m2 of
chemical power.

Figure 6.19. A combined-heat-and-power photovoltaic unit from Heliodynamics. A reflector area of 32 m2 (a bit larger than the side of a double-decker bus) delivers up to 10 kW of heat and 1.5 kW of electrical power. In a sun-belt country, one of these one-ton devices could deliver about 60 kWh/d of heat and 9 kWh/d of electricity. These powers correspond to average fluxes of 80 W/m2 of heat and 12 W/m2 of electricity (that’s per square metre of device surface); these fluxes are similar to the fluxes delivered by standard solar heating panels and solar photovoltaic panels, but Heliodynamics’s concentrating design delivers power at a lower cost, because most of the material is simple flat glass. For comparison, the total power consumption of the average European person is 125 kWh/d.