just like problems that engineers have already solved. We simply need to
figure out how to match ever-changing supply and demand in a grid with
no fossil fuels. I’m not saying that the wind-slew problem is already solved
– just that it is a problem of the same size as other problems that have been
solved.

OK, before we start looking for solutions, we need to quantify wind’s
other problem: long-term lulls. At the start of February 2007, Ireland had
a country-wide lull that lasted five days. This was not an unusual event, as
you can see in figure 26.2. Lulls lasting two or three days happen several
times a year.

There are two ways to get through lulls. Either we can store up energy
somewhere before the lull, or we need to have a way of reducing demand
during the entire lull. (Or a mix of the two.) If we have 33 GW of wind
turbines delivering an average power of 10 GW then the amount of energy
we must either store up in advance or do without during a five-day lull is

10 GW × (5 × 24 h) = 1200 GWh.

(The gigawatt-hour (GWh) is the cuddly energy unit for nations. Britain’s
electricity consumption is roughly 1000 GWh per day.)

To personalize this quantity, an energy store of 1200 GWh for the nation
is equivalent to an energy store of 20 kWh per person. Such an energy store
would allow the nation to go without 10 GW of electricity for 5 days; or
equivalently, every individual to go without 4 kWh per day of electricity
for 5 days.

## Coping with lulls and slews

We need to solve two problems – lulls (long periods with small renewable
production), and slews (short-term changes in either supply or demand).
We’ve quantified these problems, assuming that Britain had roughly 33 GW
of wind power. To cope with lulls, we must effectively store up roughly
1200 GWh of energy (20 kWh per person). The slew rate we must cope
with is 6.5 GW per hour (or 0.1 kW per hour per person).

There are two solutions, both of which could scale up to solve these
problems. The first solution is a centralized solution, and the second is
decentralized. The first solution stores up energy, then copes with fluctuations
by turning on and off a source powered from the energy store. The
second solution works by turning on and off a piece of demand.

The first solution is pumped storage. The second uses the batteries of
the electric vehicles that we discussed in Chapter 20. Before I describe
these solutions, let’s discuss a few other ideas for coping with slew.