Environmental Engineering Reference
In-Depth Information
stantial, there might be periods when the available power from renewables exceeds demand,
or cannot be accommodated by the transmission or distribution system. However, even before
this stage is reached, energy from variable sources will have to be shed because the power
system would need to keep a minimum level of thermal plant generation in order to maintain
adequate operating reserve.
Discarding energy from variable sources poses no particular operational diffi culties. Output
from wind turbines can be controlled through blade pitch variation, from photovoltaics
through inverter control and from hydro, wave and tidal schemes through similar
control techniques. However, this discarding or
curtailment
of energy results in an economic
penalty on variable sources, which becomes increasingly important at high penetrations. This
penalty is diffi cult to assess as it depends heavily on the fl exibility of the base load units, i.e.
the extent to which they could be operated in a stable regime at low power and upon how
rapidly their output could be increased if required. For the level of penetration expected over
the next decade or so, the penalties due to discarded energy are unlikely to be of major sig-
nifi cance. It should also be noted that curtailment may also be required due to distribution or
transmission system constraints. Reference [4] states that 5 out of 6 studies show that at a
penetration of 20%, curtailed energy ranges from 0 to 7%. Most studies show that, with sen-
sible design, curtailment due to local network capacity limitations would be rarely
required.
3.5.7 Overall Penalties Due to Increasing Penetration
In previous sections, operational penalties due to increases in variable source penetration
were reviewed. A number of studies have been carried out to provide estimates of these
penalties as the penetration of renewables increases. The majority of these studies relate to
wind power, as this is the variable source with the largest installed world capacity to date.
Table 3.1 gives some indicative fi gures based on EU-funded studies and on Danish and UK
thresholds linked to operational experience of wind farms. The additional costs are tabulated
in terms of the level of penetration.
Table 3.1
Implications of increasing wind energy supply on the UK network
Wind power
penetration
Measures required
Cost penalties
Up to 5%
None
Negligible
5-10%
Occasional instances when some energy from the wind is
discarded and more part loading of the thermal plant
required
0.1
-
0.2 p/kW h
10-20%
As above, plus more use of pumped storage or hydro to
balance wind power
0 . 2
-
0.8 p/kW h
20-50%
May be necessary to build more storage, or peaking plant, or
retain old coal plant, depending on relative costs (extra
storage will benefi t the system as a whole)
> 0.8 p/kW h
