Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 213

generation plant, based on integrated gasification combined cycle (IGCC) tech-
nology, similar off-peak storage can be envisaged. IGCC plant are likely to be
relatively inflexible, so energy storage will also enable such plant to maintain
operation during off-peak periods.
5.5.2 Demand-side participation
Often neglected by utilities, management of load behaviour can have a major impact
on the operation and planning of power systems. Measures can be categorised as
either reducing energy consumption or rescheduling to a later time. The former
include techniques such as energy-efficient lighting and household appliances,
building energy management systems and architectural design to reduce heating and
ventilation requirements. Additionally, for large industrial and commercial custo-
mers, maximum demand charges have encouraged the growth of on-site, embedded
generation. In the United States, consumers can also sell saved electricity to the
power system during periods of peak demand and/or shortages (Sweet, 2006).
Consequently, the peak system demand is reduced, delaying or even avoiding the
construction of new generation plant and expansion of the transmission network.
Of greater interest here is the ability to time shift the electrical demand from
periods of high demand, when generation capacity is stretched, to periods of low
demand, when spare capacity is available. Utilities attempt to encourage this
through 'time of day' tariffs, with rates often considerably higher during peak
periods. Interruptible load tariffs may also be offered to large consumers, whereby a
price discount is given in exchange for a commitment to reduce demand when
requested by the utility. Triggered by under-frequency sensitive relays, fast standing
reserve can thus be provided during emergency conditions. Such schemes reduce
demand variability and are thus of great benefit to the utility. In New Zealand, for
example, interruptible load has become the primary source of operating reserve.
However, although there is likely to be a diurnal variation in wind output, neither of
the above measures directly assists wind integration.
The nature of low grade water and space heating, air-conditioning equipment,
heating of swimming pools or even refrigeration systems, is that they have inherent
storage capabilities, i.e. electrical power can be disconnected without any initial,
obvious effects. Similar storage potential also exists for pneumatic compressed air
supply, production line inventory, municipal water pumping systems, water desa-
lination and purification, aluminium smelting, etc. Typically, such loads require no
advance warning of curtailment, and provide an instantaneous and full response.
For some processes time may be required for valves to operate and/or shut down
procedures to be completed. Consequently, the heating/refrigeration load can be
charged during periods of low demand and switched out during peaks. Alter-
natively, the system can be activated during periods of high wind generation and
switched out during lulls, reducing the impact of high wind energy penetration.
However, a common characteristic of load storage is that it is limited in duration,
e.g. a consumer may accept (or not even notice) a curtailment in hot water/heating
load for 30-60 minutes, but a sustained outage period of discomfort could not be
accepted (particularly during cold, winter evenings when electrical demand is at