POWER September 2010 - 68

ENERGY STORAGE
PSH and CAES are the best energy storage
candidates for not only integrating renewable
energy facilities but also for optimizing grid
operations and enhancing electricity markets
because they provide:
■ Load and supply. Importantly, bulk storage
systems function as load and generation.
This makes them ideal for meeting
the range of grid ancillary services. Technologies
from the other two categories of
solution (enhanced monitoring and forecasting,
and dispatching gas-fired facilities)
do not have this characteristic.
■ Fast response. Both CAES and PSH can
move from idle position to full load in less
than 10 minutes, sometimes in as little as
3 minutes.
■ Long-duration cycling. Unlike many battery
and flywheel technologies, CAES and PSH
can comfortably charge or discharge for 2-,
6-, or even 12-hour periods, if necessary.
■ Improved emissions profile. PSH has no
emissions profile. CAES, in versions that
can be supplied on a commercial basis today,
does require a minimum input of natural gas,
but its overall emissions profile is significantly
better than cycling/dispatching gas-fired
units or fossil units (Figure 4).
■ A transmission resource. Depending on its
location, bulk storage can optimize transmission
line loadings and defer the need to
build new transmission assets.
System Operations Tool
In simplistic terms, energy storage maximizes
the penetration of actual wind energy megawatt-hours
into the grid rather than replacing
them with megawatt-hours from fossil-fired
power stations. Here's how it works.
Wind facilities operate at between 25%
and 40% annual capacity factors. Importantly,
however, their operating profiles are typically
far worse during peak periods. In Texas,
ERCOT plans for only 8.7% of the total wind
capacity as being " available " during peak
summer days. The 3,000 MW of wind on the
PJM system-according to Mike Kormos,
senior VP operations, PJM Interconnection,
speaking at the ELECTRIC POWER Executive
Roundtable-gets credit for only 13% of
its capacity value during peak periods.
Available capacity at peak is only part of
the story. Wind resources have a tendency to
shift suddenly and even dramatically. Seasonal
wind resource patterns and daily wind
resource patterns must be accommodated by
the grid, but sudden disruptions and changes
also must be accommodated on a time scale
of minutes. (See the Global Monitor story
in this issue on the variability of wind due
to climate phenomena.) Officials at the Midwest
Independent System Operator (MISO)
note that operators manually curtail thousands
of megawatts of wind each day and
that 1,800-MW swings over an hourly period
are common.
For sub-hourly changes to wind resources,
grid operators can request that operating
plants quickly increase or decrease their
output. Because nuclear plants are required
to operate at full capacity 24/7/365, only the
gas and coal power plants already online can
provide this cycling.
4. Low profile. CAES exhibits a much lower emissions profile than gas-fired generation
assets. Source: CAREBS
CAES
2,500
2,000
1,500
1,000
500
Combined cycle
Simple cycle
Coal
Unfortunately, coal-fired plants are illsuited
for this type of cycling. Not only do
emissions increase, often from plants that are
not well-equipped with emissions control devices,
but plant efficiency also suffers at less
than full load. If you stomp on the gas pedal
in your car, it will accelerate quickly, but the
metal parts of the engine will also experience
more rapid degradation and wear. The same
is true of fossil-fired power stations. Cycling
costs are not insignificant, nor are the emissions,
according to at least one recent study
by Bentek Energy, " How Less Became More:
Wind, Power, and Unintended Consequences
in the Colorado Energy Market, " published
in April of this year.
Cycling and dispatching gas-fired plants is
better than cycling coal plants, but it's still not
the best approach. Gas turbine-based power
stations suffer significant reductions in efficiency
when they operate at part load. Combinedcycle
plants, many of which are already cycled
and dispatched heavily, have suffered similar
metallurgical degradation as coal-fired plants.
The fact is, the emissions-free megawatt-hour
from wind is being traded for an emissionsladen
megawatt-hour from fossil fuel.
Now consider a bulk energy storage facility
that is storing renewable, emissions-free
electricity. Instead of a wind farm producing
megawatts at night when no one needs them,
it charges up a bulk storage facility. This
electricity is then discharged to the grid during
the day, when additional load is needed.
Though neither PSH nor CAES will deliver
to the grid all the electricity used to charge
the storage system, the overall economic efficiency
compared to fossil-fired power stations
is much better.
The most advanced commercially available
CAES plant suffers only 7% deterioration
in cycle efficiency between 25% and
100% output. In other words, efficiency
remains fairly constant throughout the load
range. Combined-cycle facilities typically
suffer more than 30% deterioration; a gas turbine
peaking unit is even less efficient. The
CAES cycle efficiency, or heat rate, is at least
30% better than that of the best combinedcycle
facilities operating today and 60% to
70% better than a peaking gas turbine generator.
Combined-cycle units typically require
at least 40 minutes to come up to full load
from a " warm " condition (such as being shut
down overnight) and several hours from a
cold condition. Simple-cycle gas turbines are
more flexible but are limited in the amount
of down-regulating or decremental reserves
they provide to the system operator.
50
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60
70
80
% of maximum net plant output
www.powermag.com
90
100
Avalanche of Support
Last year, DOE Secretary Stephen Chu began
to speak publicly about the benefits of
POWER | September 2010
CO2 emissions (lb/MWh)
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POWER September 2010

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