Geoscience Reference
In-Depth Information
scheduled maintenance (North American Electric Reli-
ability Corporation (NERC), 2009), but modern wind
turbines have a down time of only 0 to 2 percent over
land and 0 to 5 percent over the ocean (Dong Energy et
al., 2006, p. 133). Similarly, commercial solar projects
have downtimes of
Although concern exists about stability of the cur-
rent grid, it generally works, increasing the desire of
grid operators to maintain the status quo. In Denmark,
wind energy supplies about 20 percent of the total elec-
tric power, suggesting that a large penetration of WWS
power may also work. However, variable renewables
have not been used to date to power larger percentages
of a grid, and thus concerns persist. Such concerns,
though, have not yet been based on real experience or
scientific data.
There are at least seven ways to design and operate
a WWS energy system so that it will reliably satisfy
demand:
1 percent on average, although
some have zero downtime during a year and others up
to 10 percent (Banke, 2010).
Adifference, though, exists between outages of
cen-
tralized power plants
(coal, nuclear, natural gas) and
outages of
distributed power plants
(wind, solar,
wave). When individual solar panels or wind turbines
are down, only a small fraction of electrical production
is affected. When a centralized plant is down, a large
fraction of the grid is affected. When more than one
large, centralized plant is offline at the same time, an
entire grid can be affected.
∼
1.
Interconnect
geographically
dispersed
variable
energy sources (e.g., wind, solar, wave).
2.
Combine WWS resources as one commodity and
use a nonvariable energy source, such as hydroelec-
tric power, to fill in remaining gaps between energy
demand and supply.
13.8. Reliably Matching Demand
with Variable Wind, Water,
and Sunlight Resources
Wind, wave, and solar power produce output that
varies in time according to the weather. Thus, they
are referred toas
variable WWS resources
.Another
term commonly used to describe variability is
intermit-
tency
.However, all energy resources are intermittent
due to scheduled and unscheduled maintenance (Sec-
tion 13.7), whereas variable resources are those whose
energy outputs vary with the weather in addition to
being affected by maintenance.
One concern with the use of variable WWS resources
is whether such resources can provide reliable electric
power. Any electricity system must respond to changes
in demand (also referred to as
load
) over periods of sec-
onds, minutes, hours, seasons, and years, and accom-
modate unanticipated changes in the availability of gen-
eration. It is not possible to control the weather; thus,
asudden change in demand often cannot be met by a
variable resource.
The concern about matching demand with supply,
though, applies to all energy sources, not just the vari-
able ones. For example, because coal and nuclear pro-
vide constant (
baseload
)supplies during the day, they
do not match power demand, which varies continu-
ously. As a result, gap-filling resources, such as nat-
ural gas and hydroelectric, are used to meet demand
peaks. Some WWS technologies, including geother-
mal and tidal power, can also serve as baseload power
sources.
3.
Use demand-response management to shift times
of demand to better match the availability of WWS
power.
4.
Store electric power at the site of generation for
later use.
5.
Oversize WWS peak generation capacity to min-
imize the times when available WWS power is
less than demand, and provide power to produce
hydrogen for transportation and heating when WWS
power exceeds demand.
6.
Store electric power in electric vehicle batteries.
7.
Integrate weather forecasts into system operation.
13.8.1. Interconnecting Geographically
Dispersed Generators
Interconnecting geographically dispersed wind, wave,
or solar farms to a common transmission grid smoothes
out electricity supply significantly (Kahn, 1979). Sim-
ilarly, the combined energy from colocated wind and
wave farms reduces variability of wind and wave power
individually (Stoutenburg et al., 2010). For wind alone,
interconnection over regions as small as a few hundred
kilometers apart can eliminate hours of zero power,
accumulated over all wind farms. When nineteen geo-
graphically dispersed wind sites in the Midwest, over
aregion850
850 km, were hypothetically intercon-
nected, about 33 percent of yearly averaged wind power
was calculated to be available at the same reliabil-
ity as a coal-fired power plant (Archer and Jacobson,
×
Search WWH ::
Custom Search