# 8   Hydroelectricity

To make hydroelectric power, you need altitude, and you need rainfall.
Let’s estimate the total energy of all the rain as it runs down to sea-level.

For this hydroelectric forecast, I’ll divide Britain into two: the lower,
dryer bits, which I’ll call “the lowlands;” and the higher, wetter bits, which
I’ll call “the highlands.” I’ll choose Bedford and Kinlochewe as my repre-
sentatives of these two regions.

Let’s do the lowlands first. To estimate the gravitational power of low-
land rain, we multiply the rainfall in Bedford (584 mm per year) by the
density of water (1000 kg/m3), the strength of gravity (10 m/s2) and the
typical lowland altitude above the sea (say 100 m). The power per unit
area works out to 0.02 W/m2. That’s the power per unit area of land on
which rain falls.

When we multiply this by the area per person (2700 m2, if the lowlands
are equally shared between all 60 million Brits), we find an average raw
power of about 1 kWh per day per person. This is the absolute upper
limit for lowland hydroelectric power, if every river were dammed and
every drop perfectly exploited. Realistically, we will only ever dam rivers
with substantial height drops, with catchment areas much smaller than the
whole country. Much of the water evaporates before it gets anywhere near
a turbine, and no hydroelectric system exploits the full potential energy of
the water. We thus arrive at a firm conclusion about lowland water power.
People may enjoy making “run-of-the-river” hydro and other small-scale
hydroelectric schemes, but such lowland facilities can never deliver more
than 1 kWh per day per person.

Figure 8.1. Nant-y-Moch dam, part of a 55 MW hydroelectric scheme in Wales. Photo by Dave Newbould, www.origins-photography.co.uk.
Figure 8.2. Altitudes of land in Britain. The rectangles show how much land area there is at each height.