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9a Read the document on energy saving aimed at industrial plant and facility managers.
Complete the text using the words in Exercise 8e.
Dynamic demand control systems can be fitted to electrical appliances that operate on duty
cycles, i.e. appliances that start up, run for a time, shut down again, and then remain on standby
for a while before repeating the same cycle. Heating and refrigeration units are common
examples of power-hungry equipment that operate on this start-run-stop-wait basis.
Dynamic systems exploit the fact that duty cycle appliances do not require(1) continuous
power. The purpose of the systems is to help smooth power demand for the benefit of electric
utilities. To achieve this, they delay the start-up of the appliances they control during periods of
(2) . However, only minor adjustments are made to timing as, generally, the
appliances concerned can only be held on standby for short periods as they need to run on a
(3) basis. But this still benefits electric utilities as it helps to avoid
problematic, momentary (4) on the demand curve. Dynamic controls work by
detecting slight (5) in the frequency of the mains AC supply. Although this
varies only within a very narrow (6) , small drops in frequency indicate
that power station turbines are working close to full capacity. The dynamic control system can
therefore hold the appliance on standby for a short time until mains frequency increases again.

b Read the text again and answer the following questions.
1 What is meant by duty cycle?
2 What problem is dynamic control designed to avoid?
3 What sensor input do dynamic controls rely on to allow them to function?
4 What does the sensor input in (3) indicate with regard to power stations?
5 To what extent do dynamic systems modify duty cycles?

10a Read the extract from the talk and, in pairs, discuss the following questions.
... electrical charge is extremely difficult to store in large amounts, you can’t just charge up huge
batteries. So we use an innovative technique to store up power potential during off-peak periods.
1 Why do you think this is such a major issue for power stations?
2 Can you suggest what innovative technique is used to solve the problem?

b 8.7 Helen goes on to describe the solution to the problem. Read the talk and make notes
on pumped storage.

Helen: The concept of pumped storage is relatively simple. During off-peak periods when main
power stations have got spare capacity, some of the extra power they produce is used by pumped
storage stations to pump water from a low-level reservoir up to a high-level reservoir, where the
water's stored. It’s then held there until there’s a peak in demand at some point the next day. At

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