6   Solar

We are estimating how our consumption stacks up against conceivable
sustainable production. In the last three chapters we found car-driving and
plane-flying to be bigger than the plausible on-shore wind-power potential
of the United Kingdom. Could solar power put production back in the

The power of raw sunshine at midday on a cloudless day is 1000W per
square metre. That’s 1000 W per m2 of area oriented towards the sun, not
per m2 of land area. To get the power per m2 of land area in Britain, we
must make several corrections. We need to compensate for the tilt between
the sun and the land, which reduces the intensity of midday sun to about
60% of its value at the equator (figure 6.1). We also lose out because it is
not midday all the time. On a cloud-free day in March or September, the
ratio of the average intensity to the midday intensity is about 32%. Finally,
we lose power because of cloud cover. In a typical UK location the sun
shines during just 34% of daylight hours.

The combined effect of these three factors and the additional compli-
cation of the wobble of the seasons is that the average raw power of sunshine
per square metre of south-facing roof in Britain is roughly 110 W/m2,
and the average raw power of sunshine per square metre of flat ground is
roughly 100 W/m2.

We can turn this raw power into useful power in four ways:

  1. Solar thermal: using the sunshine for direct heating of buildings or
  2. Solar photovoltaic: generating electricity.
  3. Solar biomass: using trees, bacteria, algae, corn, soy beans, or oilseed
    to make energy fuels, chemicals, or building materials.
  4. Food: the same as solar biomass, except we shovel the plants into
    humans or other animals.

(In a later chapter we’ll also visit a couple of other solar power techniques
appropriate for use in deserts.)

Let’s make quick rough estimates of the maximum plausible powers
that each of these routes could deliver. We’ll neglect their economic costs,
and the energy costs of manufacturing and maintaining the power facili-

Solar thermal

The simplest solar power technology is a panel making hot water. Let’s
imagine we cover all south-facing roofs with solar thermal panels – that

Figure 6.1. Sunlight hitting the earth at midday on a spring or autumn day. The density of sunlight per unit land area in Cambridge (latitude 52.) is about 60% of that at the equator.
Figure 6.2. Average solar intensity in London and Edinburgh as a function of time of year. The average intensity, per unit land area, is 100 W/m2.