# 8 Hydroelectricity

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](http://www.origins-photography.co.uk).

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

For this hydroelectric forecast, Iâ€™ll divide Britain into two: the lower, dryer bits, which Iâ€™ll call â€śthe lowlands;â€ť and the higher, wetter bits, which Iâ€™ll call â€śthe highlands.â€ť Iâ€™ll choose Bedford and Kinlochewe as my representatives of these two regions.

Letâ€™s do the lowlands first. 1 To estimate the gravitational power of lowland 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. Thatâ€™s the power per unit area of land on which rain falls.

Erratum. Kinlochewe is shown incorrectly. The correct location is about 60km further north.

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 â€śrun-of-the-riverâ€ť hydro and other small-scale hydroelectric schemes, but such lowland facilities can never deliver more than 1 kWh per day per person.

Figure 8.2. Altitudes of land in Britain. The rectangles show how much land area there is at each height.

Letâ€™s turn to the highlands. Kinlochewe is a rainier spot: it gets 2278 mm per year, four times more than Bedford. The height drops there are bigger too â€“ large areas of land are above 300 m. So overall a twelve-fold increase in power per square metre is plausible for mountainous regions. The raw power per unit area is roughly 0.24 W/m2. 2 If the highlands generously share their hydro-power with the rest of the UK (at 1300 m2 area per person), we find an upper limit of about 7 kWh per day per person. As in the lowlands, this is the upper limit on raw power if evaporation were outlawed and every drop were perfectly exploited.

Figure 8.3. Hydroelectricity.

Figure 8.4. A 60 kW waterwheel.

What should we estimate is the plausible practical limit? Letâ€™s guess 20% of this â€“ 1.4 kWh per day, and round it up a little to allow for production in the lowlands: 1.5 kWh per day.

The actual power from hydroelectricity in the UK today is 0.2 kWh/d per person, 3 so this 1.5 kWh/d per person would require a seven-fold increase in hydroelectric power.