Environmental Engineering Reference
In-Depth Information
ways to use heat and cold storage in the context of power systems has been explored.
This technology involves modulation of the energy absorbed by individual consumer
electric heating elements and refrigeration systems for the benefi t of overall system power
balance. An example of this is described in the next section.
3.8.3 Dynamic Demand Control
A scheme is being investigated that uses the already existing stored energy in millions of
consumer appliances and requires the installation of
dynamic demand control
(DDC). These
monitor system frequency and switch the appliance on or off, striking a compromise between
the needs of the appliance and the grid. Initially fridges and freezer applications have been
investigated.
Refrigerators are 'on' in all seasons, throughout the day and night, and are therefore avail-
able to participate in frequency control at all times. The total energy demand on the UK grid
from domestic (excluding industrial and commercial) refrigeration has been estimated as
16.7TW h per year which amounts to an average load of 1.9 GW. The refrigeration load is
dependent on ambient temperature, winter load being approximately two-thirds that in
summer. Daytime load is also slightly higher than that at night. Refrigerators are designed
to handle considerable switching as they typically have a switching cycle of the order of 5
minutes to 1 hour depending on characteristics and contents. Any additional switching caused
by frequency control should not therefore present a problem.
As an example, the power system model of Section 3.7.2 was used with the addition of
DDC refrigerators. For this purpose, a refrigerator with a dynamic demand controller was
modelled. The aggregation of 24.9 million such appliances (one per UK household) with
statistically uncorrelated behaviour and equivalent to 1320 MW of deferrable load was inves-
tigated for its response to fl uctuating wind power with a reserve of 2000 MW. Figure 3.21
shows that the system with the DDC (black trace) considerably reduced the variation in fre-
quency even though the system was operating with substantially less reserve. This is because
the simulated controllers reacted more quickly than the generator governor to changes in
frequency.
The graph also shows that the system frequency with and without DDC fell below the
operational limit of 49.8 Hz. However, the system with DDC provided a considerable breath-
ing time. Frequency did not fall below the operational limit until nearly 2 hours after the
non-DDC system. A team of engineers operating the power network would therefore be given
a wider choice of generation with which to balance the system including slower acting (and
therefore possibly cheaper and more effi cient) options. Also the delay may allow generation
to be scheduled more cost effectively through the electricity market, which may operate a
'gate closure' (see Chapter 7) time of half an hour in advance of any particular generation
slot.
It may be concluded that an aggregation of a large number of dynamically controlled loads
has the potential of providing added frequency stability and smoothing to power networks,
both at times of sudden increase in demand (or loss of generation) and during times of fl uc-
tuating wind or other renewable power. The devices, if incorporated on a real system, could
displace some reserve and result in a signifi cant reduction in governor activity of the remain-
ing generators with assigned headroom. The amount of reserve displaced will depend on the
