Environmental Engineering Reference
In-Depth Information
In the UK system the transmission system operator (TSO) currently carries a balancing
reserve of 2.5GW [4]. Within limits, the larger the available headroom of the frequency
response plant the greater the reliability of the power system; however, the presence of these
additional part-loaded generating sets on the system signifi cantly increases the system fuel
costs. Any part-loaded machine has a poor effi ciency due to
standing losses
: fi xed losses
incurred by a generator when running unloaded.
The risk of demand being unmet is measured by a statistical quantity known as the
loss -
of - load probability
(LOLP). In the UK pre-privatized system a LOLP of 9% was considered
acceptable, i.e. expected loss of supply incidents for each customer in nine winters per
century. The LOLP gives no information on how much load will be shed or for how long. It
is understood that reliabilities of the same order are expected in present privatized systems
of developed countries although fi gures are diffi cult to obtain. For such a reliability to be
maintained in addition to the short term reserves discussed earlier, a
system
or
plant margin
must be maintained. This is the difference between
installed capacity
, including imports and
exports, and maximum annual
peak demand
. This margin is necessary because at any time
there are bound to be plants that have broken down or are out for maintenance. For reliable
operation, a power system must have a plant margin that is substantially greater than the
operating margin. Taking as an example a system with a 70GW winter peak demand, to
guarantee that the demand will be met to a typical reliability expected in an industrialized
nation about 84 GW of conventional capacity (20% above peak demand) should be available
on the network.
In privatized or
deregulated
power systems all these activities are managed through the
complex fi nancial instruments that defi ne the contributions and obligations of several inde-
pendent generation providers all feeding into the same network. For example, in the UK and
in other similarly privatized networks, system operators pay considerable premiums to power
generators who provide response and reserves.
3.4.7 Capacity Factor and Capacity Credit
[4]
When renewables displace signifi cant amounts of conventional generation plant, an extra
conventional plant capacity margin is required to maintain system supply reliability. It is
important to recognize that an additional plant margin will only be required at times of low
electricity demand and high input from variable sources. At other times an additional margin
will not be required as there will be suffi cient conventional plant available to meet demand
irrespective of the contribution from variable sources.
The capacity factor of a generator, as mentioned in Section 2.4.3, is usually defi ned as the
ratio of its yearly energy output to the output it would have produced if it operated continu-
ously at its nameplate rating. Due to unavailability caused by maintenance schedules, break-
downs, etc., no plant achieves a capacity factor of unity. Base load thermal generators
have a capacity factor of 85-90% when new, declining over the years until they are
decommissioned. Wind turbines achieve capacity factors of 20-40% depending on the windi-
ness of the site. In the UK the average capacity factor of onshore wind farms is ~30% with
offshore schemes achieving higher values. Often, uninformed commentators quote the fi gure
of 30% when they claim that wind turbines require back-up for 70% of the time. In fact wind
