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).

Multiplying

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.

## Solar photovoltaic

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.

Figure 6.3. Solar power generated by a 3 m2 hot-water panel (green), and supplementary heat required (blue) to make hot water in the test house of Viridian Solar. (The photograph shows a house with the same model of panel on its roof.) The average solar power from 3 m2 was 3.8 kWh/d. The experiment simulated the hot-water consumption of an average European household – 100 litres of hot (60°C) water per day. The 1.5–2 kWh/d gap between the total heat generated (black line, top) and the hot water used (red line) is caused by heat-loss. The magenta line shows the electrical power required to run the solar system. The average power per unit area of these solar panels is 53 W/m2.
Figure 6.4. Solar thermal: a 10 m2 array of thermal panels can deliver (on average) about 13 kWh per day of thermal energy.
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