only 2%.

Let’s quantify the fluctuations in country-wide wind power. The two

issues are short-term changes, and long-term lulls. Let’s find the fastest

short-term change in a month of Irish wind data. On 11th February 2007,

the Irish wind power fell steadily from 415 MW at midnight to 79 MW at

4am. That’s a slew rate of 84 MW per hour for a country-wide fleet of

capacity 745 MW. (By slew rate I mean the rate at which the delivered

power fell or rose – the slope of the graph on 11th February.) OK: if we

scale British wind power up to a capacity of 33 GW (so that it delivers

10 GW on average), we can expect to have occasional slew rates of

84 MW/h × | 33 000 MW | = 3700 MW/h, |

745 MW |

assuming Britain is like Ireland. So we need to be able to either power

*up* replacements for wind at a rate of 3.7 GW per hour – that’s 4 nuclear

power stations going from no power to full power every hour, say – *or* we

need to be able to suddenly turn *down* our *demand* at a rate of 3.7 GW per

hour.

Could these windy demands be met? In answering this question we’ll

need to talk more about “gigawatts.” Gigawatts are big country-sized units

of power. They are to a country what a kilowatt-hour-per-day is to a person:

a nice convenient unit. The UK’s average electricity consumption is

about 40 GW. We can relate this national number to personal consumption:

1 kWh per day per person is equivalent to 2.5 GW nationally. So if

every person uses 16 kWh per day of electricity, then national consumption

is 40 GW.

Is a national slew-rate of 4 GW per hour completely outside human

experience? No. Every morning, as figure 26.3 shows, British demand

climbs by about 13 GW between 6.30am and 8.30am. That’s a slew rate of

*6.5 GW per hour*. So our power engineers already cope, every day, with slew

rates bigger than 4GW per hour on the national grid. An extra occasional

slew of 4 GW per hour induced by sudden wind variations is no reasonable

cause for ditching the idea of country-sized wind farms. It’s a problem