POWER June 2012 - 48
ENERGY STORAGE
to provide operating reserves from partially
loaded power plants. As the amount of PV on
the system increases, the need for operating
reserves also increases due to the uncertainty
of the solar resource, as well as its variability
over multiple time scales.
Previous analysis has demonstrated the
economic limits of PV penetration due to
generator turn-down limits and supply/demand
coincidence. Because of these factors,
at high penetration of solar energy, increasing
amounts of solar generation may need to be
curtailed. Generator constraints would likely
prevent the use of all PV generation potentially
available in Figure 4's 10% scenario.
Nuclear plant operators would be unlikely to
reduce output for such a short period. Furthermore,
PV generation could be offsetting
other low- or zero-carbon sources such as
wind and geothermal generation.
Although the percentage of solar energy
on the U.S. grid is currently far too small to
result in significant impacts, the curtailment
of wind energy is an increasing concern.
Though a majority of wind curtailments in
the U.S. are due to transmission limitations,
curtailments due to excess generation during
times of low net load (as happened last year
in the Pacific Northwest with Bonneville
Thermal energy storage provides some
potential advantages for bulk energy
storage, including round trip efficiency in
excess of 95%.
Power Administration) are a significant factor
that will increase if grid flexibility is not
enhanced.
One measure of a flexible grid is the ability
of the aggregated set of generators to rapidly
change output at a high rate and over a
large range. Flexibility depends on many factors,
including these:
■ Generator mix. Hydro and gas-fired generators
are generally more flexible than
coal or nuclear ones.
■ Grid size. Larger grids are typically more
flexible because they share a larger mix
of generators and can share operating reserves
and a potentially more spatially diverse
set of renewable resources.
■ Use of forecasting in unit commitment.
Accurate forecasting of the wind and solar
generation units reduces the need for operating
reserves.
■ Market structure. Some grids allow more
rapid exchange of energy and can more
efficiently balance supply from variable
generators and demand.
■ Other sources of grid flexibility. Some locations
have access to demand response,
which can provide an alternative to partially
loaded thermal generators for provision
of operating reserves. Other locations may
have storage assets such as pumped hydro.
Increasing Solar Deployment
Using CSP
An alternative to storing solar-generated
electricity is storing solar thermal energy via
CSP/TES. Because TES can only store energy
from thermal generators such as CSP,
circle 25 on reader service card
48
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POWER | June 2012
http://www.powermag.com
POWER June 2012
Table of Contents for the Digital Edition of POWER June 2012
Contents
POWER June 2012 - Cover1
POWER June 2012 - Cover2
POWER June 2012 - Contents
POWER June 2012 - 2
POWER June 2012 - 3
POWER June 2012 - 4
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POWER June 2012 - Cover3
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