would be about 10 m2 of panels per person – and let’s assume these are
50%-efficient at turning the sunlight’s 110 W/m2 into hot water (figure 6.3).
50% × 10 m2 × 110 W/m2
we find solar heating could deliver
13 kWh per day per person.
I colour this production box white in figure 6.4 to indicate that it describes
production of low-grade energy – hot water is not as valuable as the highgrade
electrical energy that wind turbines produce. Heat can’t be exported
to the electricity grid. If you don’t need it, then it’s wasted. We should bear
in mind that much of this captured heat would not be in the right place.
In cities, where many people live, residential accommodation has less roof
area per person than the national average. Furthermore, this power would
be delivered non-uniformly through the year.
Photovoltaic (PV) panels convert sunlight into electricity. Typical solar
panels have an efficiency of about 10%; expensive ones perform at 20%.
(Fundamental physical laws limit the efficiency of photovoltaic systems to
at best 60% with perfect concentrating mirrors or lenses, and 45% without
concentration. A mass-produced device with efficiency greater than 30%
would be quite remarkable.) The average power delivered by south-facing
20%-efficient photovoltaic panels in Britain would be
20%× 110 W/m2 = 22 W/m2.
Figure 6.5 shows data to back up this number. Let’s give every person
10 m2 of expensive (20%-efficient) solar panels and cover all south-facing
roofs. These will deliver
5 kWh per day per person.