such as glass production.

Other demand-management and storage ideas

There are a few other demand-management and energy-storage options,
which we’ll survey now.

The idea of modifying the rate of production of stuff to match the
power of a renewable source is not new. Many aluminium production
plants are located close to hydroelectric power stations; the more it rains,
the more aluminium is produced. Wherever power is used to create stuff
that is storable, there’s potential for switching that power-demand on and
off in a smart way. For example, reverse-osmosis systems (which make
pure water from sea-water – see p92) are major power consumers in many
countries (though not Britain). Another storable product is heat. If, as sug-
gested in Chapter 21, we electrify buildings’ heating and cooling systems,
especially water-heating and air-heating, then there’s potential for lots of
easily-turn-off-and-onable power demand to be attached to the grid. Well-
insulated buildings hold their heat for many hours, so there’s flexibility
in the timing of their heating. Moreover, we could include large thermal
reservoirs in buildings, and use heat-pumps to pump heat into or out of
those reservoirs at times of electricity abundance; then use a second set of
heat pumps to deliver heat or cold from the reservoirs to the places where
heating or cooling are wanted.

Controlling electricity demand automatically would be easy. The simp-
lest way to do this is to have devices such as fridges and freezers listen
to the frequency of the mains. When there is a shortage of power on the
grid, the frequency drops below its standard value of 50 Hz; when there is
a power excess, the frequency rises above 50 Hz. (It’s just like a dynamo
on a bicycle: when you switch the lights on, you have to pedal harder
to supply the extra power; if you don’t then the bike goes a bit slower.)
Fridges can be modified to nudge their internal thermostats up and down
just a little in response to the mains frequency, in such a way that, without
ever jeopardizing the temperature of your butter, they tend to take power
at times that help the grid.

Can demand-management provide a significant chunk of virtual storage?
How big a sink of power are the nation’s fridges? On average, a
typical fridge-freezer draws about 18 W; let’s guess that the number of
fridges is about 30million. So the ability to switch off all the nation’s
fridges for a few minutes would be equivalent to 0.54 GW of automatic ad-
justable power. This is quite a lot of electrical power – more than 1% of the
national total – and it is similar in size to the sudden increases in demand
produced when the people, united in an act of religious observance (such
as watching EastEnders), simultaneously switch on their kettles. Such “TV
pick-ups” typically produce increases of demand of 0.6–0.8 GW. Auto-
matically switching off every fridge would nearly cover these daily blips