Page 255 - Fluid Power Engineering

222 Chapter Eleven

explain the place of wind energy in a grid and how variability in wind
resource is managed in a grid.
Generation capacity on a grid can be classified into three types:
Base-load generators, spinning reserves, and nonspinning reserves.
Base-load generators are large thermal or nuclear power plants that
run at fixed capacity 7 days a week, 24 hours a day. These plants
run at capacity factors of 90-plus%, because they are most efficient at
high-capacity factors. These plants supply energy to cover the “base
load.” Base load is the minimum amount of hourly energy consump-
tion across the entire grid on any day. Since the base load amount of
demand will exist on the grid at all times, base-load generators meet
this demand. Among renewable energy sources, geothermal can sup-
ply to base load, because it can be designed to produce energy at a
constant rate 24 × 7. Spinning reserves is the second type of genera-
tion capacity comprising of natural gas or diesel-powered generators.
These generators, as the name suggest, are spinning or running at low
capacity all the time and can react very quickly to increase in demand.
These generators meet the variable portion of demand during the day.
Nonspinning reserves are generators that have a reaction of time of
10 min to 1 h. As the name suggests, these reserves are not running
and need time to warm up and start producing energy. The goal of
these reserves is to fill in for known changes to supply and demand,
for example, scheduled maintenance of a base-load generator. In a
highly reliable grid, the capacity of the three types of generators is
planned to meet the variability in demand.
Wind and solar plants supply energy to offset the production of
spinning reserves. The amount of wind and solar energy is becom-
ing larger and is likely to be a significant fraction of total electricity
production in a grid. With wind penetration of up to 20% of system
total peak demand, additional spinning reserves will be required. It
has been found that this results in additional cost of up to 10% of the
1
wholesale value of wind energy, which is a modest increase in cost.
Wind resource varies second-to-second, hour-by-hour, and has
diurnal and seasonal patterns. As wind energy is aggregated from
multiple turbines in a wind farm, the short-term variations in wind
resources is attenuated as a percentage of the overall power output.
When wind energy from multiple wind farms is aggregated, minute-
to-hour variation in energy is attenuated and an hour-to-day variation
in wind energy production and delivery to the grid is observed. Grid
system operators manage such variations using spinning reserves.
Consider another case when wind energy penetration into a grid
is high and there is insufficient load on the grid. This may occur during
high wind periods. Because wind energy is considered an intermittent
resource, wind plants are asked to curtail energy production, after all
the spinning reserves have attained the minimum energy output level.
Note the base-load generators typically do not curtail production.