POWER June 2012 - 46

ENERGY STORAGE
Boosting CSP Production with
Thermal Energy Storage
Combining concentrating solar power (CSP) with thermal energy storage shows
promise for increasing grid flexibility by providing firm system capacity with
a high ramp rate and acceptable part-load operation. When backed by energy
storage capability, CSP can supplement photovoltaics by adding generation
from solar resources during periods of low solar insolation.
By Paul Denholm, PhD and Mark Mehos, National Renewable Energy Laboratory
T
he falling cost of solar photovoltaic (PV)-
generated electricity has led to a rapid
increase in the deployment of PV and
projections that PV could play a significant role
in the future U.S. electric sector. The solar resource
itself is virtually unlimited; however, the
actual contribution of PV electricity is limited
by several factors related to the current grid.
The first is the limited coincidence between
the solar resource and normal electricity demand
patterns. The second is the limited flexibility of
conventional generators to accommodate this
highly variable generation resource. At high
penetration of solar generation, increased grid
flexibility will be needed to fully utilize the variable
and uncertain output from PV generation
and to shift energy production to periods of high
demand or reduced solar output.
Energy storage is one way to increase grid
flexibility, and many storage options are available
or under development. In this article,
however, we consider a technology already
beginning to be used at scale-thermal energy
storage (TES) deployed with concentrating solar
power (CSP).
PV and CSP are both deployable in areas of
high direct normal irradiance such as the U.S.
Southwest. The role of these two technologies
is dependent on their costs and relative value,
including how their value to the grid changes
as a function of what percentage of total generation
they contribute to the grid, and how they
may actually work together to increase overall
usefulness of the solar resource.
Both PV and CSP use solar energy to
generate electricity. A key difference is the
ability of CSP to utilize high-efficiency TES,
which turns CSP into a partially dispatchable
resource. The addition of TES produces
additional value by shifting the delivery of
solar energy to periods of peak demand, providing
firm capacity and ancillary services,
and reducing integration challenges. Given
the dispatchability of CSP enabled by TES, it
is possible that PV and CSP are at least partially
complementary (Figures 1 and 2).
46
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POWER | June 2012
1. Concentrating on CSP technology. Crews are shown installing mirrored parabolic
troughs at the Solana Generating Station in Arizona. The 280-MW concentrating solar power
(CSP) plant is scheduled for completion in 2013. CSP technology uses mirrors to reflect and
concentrate sunlight onto receivers that collect the sun's heat. Courtesy: Dennis Schroeder
2. Storing solar energy. These tanks at the Solana plant will hold molten salts. Those liquid
salt fluids remain very hot for several hours, so the energy stored in them can be recovered
to produce steam that can be expanded in the steam turbine on demand to produce electricity
later in the day. Courtesy: Dennis Schroeder

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POWER June 2012

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