Let’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 – 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. If the highlands generously
share their hydro-power with the rest of the UK (at 1300 m2 area per per-
son), 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.

What should we estimate is the plausible practical limit? Let’s guess
20% of this – 1.4 kWh per day, and round it up a little to allow for produc-
tion in the lowlands: 1.5 kWh per day.

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

55Rainfall statistics are from the BBC weather centre.

56The raw power per unit area [of Highland rain] is roughly 0.24 W/m2. We
can check this estimate against the actual power density of the Loch Sloy
hydro-electric scheme, completed in 1950 (Ross, 2008). The catchment area
of Loch Sloy is about 83 km2; the rainfall there is about 2900 mm per year
(a bit higher than the 2278 mm/y of Kinlochewe); and the electricity output
in 2006 was 142 GWh per year, which corresponds to a power density of
0.2W per m2 of catchment area. Loch Sloy’s surface area is about 1.5 km2,
so the hydroelectric facility itself has aE per unit lake area of 11 W/m2. So
the hillsides, aqueducts, and tunnels bringing water to Loch Sloy act like a
55-fold power concentrator.

The actual power from hydroelectricity in the UK today is 0.2 kWh per day
per person
. Source: MacLeay et al. (2007). In 2006, large-scale hydro pro-
duced 3515 GWh (from plant with a capacity of 1.37 GW); small-scale hydro,
212 GWh (0.01 kWh/d/p) (from a capacity of 153 MW).
In 1943, when the growth of hydroelectricity was in full swing, the North
of Scotland Hydroelectricity Board’s engineers estimated that the Highlands
of Scotland could produce 6.3 TWh per year in 102 facilities – that would
correspond to 0.3 kWh/d per person in the UK (Ross, 2008).
Glendoe, the first new large-scale hydroelectric project in the UK since 1957,
will add capacity of 100 MW and is expected to deliver 180 GWh per year.
Glendoe’s catchment area is 75 km2, so its power density works out to 0.27 W
per m2 of catchment area. Glendoe has been billed as “big enough to power
Glasgow.” But if we share its 180 GWh per year across the population of
Glasgow (616 000 people), we get only 0.8 kWh/d per person. That is just
5% of the average electricity consumption of 17 kWh/d per person. The 20-
fold exaggeration is achieved by focusing on Glendoe’s peak output rather
than its average, which is 5 times smaller; and by discussing “homes” rather
than the total electrical power of Glasgow (see p329).

Figure 8.3. Hydroelectricity.
Figure 8.4. A 60 kW waterwheel.