POWER June 2012 - 49

ENERGY STORAGE
it cannot be directly compared with other
electricity storage options, which can charge
from any source. However, TES provides
some potential advantages for bulk energy
storage, including round trip efficiency in
excess of 95%.
As part of our assessment, we used a reduced
form dispatch model designed to
examine the general relationship between
grid flexibility, variable solar and wind gen5.
CSP: the dispatchable renewable
energy. Simulated system dispatch is
shown from April 7 to 10 with 15% contribution
from PV and 10% from dispatchable CSP.
This chart illustrates the importance of dispatchability
at high solar penetration. The figure
shows two CSP profiles. The blue line at
the bottom of the chart is the non-dispatched
CSP without thermal storage, which aligns
well with PV generation. The red line denotes
the thermal storage used to shift energy to
the end of the day. Source: NREL
Curtailed PV
Wind
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Dispatched CSP
Conventionals
Non-dispatched CSP
Load
Dispatched CSP
Usable PV
eration, and curtailment. We calculated the
hourly electrical output of a CSP plant with 8
hours of storage.
Figure 5 illustrates the importance of dispatchability
at high solar penetration over a
four-day period. The figure shows two CSP
profiles. The " non-dispatched CSP " line
(in blue) is the output of CSP alone, without
thermal storage; it aligns with PV production
when the sun shines, as you would
expect. Without storage, the result would be
significant CSP curtailment because the sum
of CSP and PV generation exceeds the grid
energy requirement at that time. The orange
line is the actual dispatched CSP but with the
effect of TES included, showing its response
to the net demand pattern after wind and PV
generation are considered. It shows how a
large fraction of CSP energy is sent to energy
storage to be shifted toward the end of the
day, thus allowing the system to absorb more
of the PV generation in the middle of the day.
In the first day, this ability to shift energy
eliminates curtailment of PV generation.
On the other days, the wind and PV resources
exceed the " usable " demand for energy
in the early part of the day, resulting in
curtailed energy even while the CSP plant is
storing 100% of thermal energy. However,
overall curtailment is greatly reduced.
The addition of CSP/TES can increase
0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96
Hour
the overall penetration of solar by moving
energy delivery to the grid from periods of
low net demand in the middle of the day to
morning or evening.
Figure 6 also demonstrates the importance
of dispatchability to reduce curtailment and
6. Achieving a good balance. This chart depicts the curtailment of solar, assuming an
equal mix (on an energy basis) of PV and CSP. This demonstrates how the addition of CSP and
TES can increase the overall penetration of solar by moving energy from periods of low net
demand in the middle of the day to morning or evening. Source: NREL
PV only marginal
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PV + CSP marginal
PV only average
PV + CSP average
increase the overall penetration of solar via the
ability to shift solar energy over time. However,
the analysis to this point assumes that CSP and
PV are complementary only in their ability to
serve different parts of the demand pattern. We
have not yet considered the additional benefits
of CSP to provide system flexibility by replacing
baseload generators and generators online
to provide operating reserves.
Adding a highly flexible generator such as
CSP/TES can potentially reduce the possible
generation constraints on the system. In the
near term, this means that fewer conventional
generators will be needed to operate at part
load during periods of high solar output. In
the longer term, the ability of CSP/TES to
provide firm system capacity could replace
retiring baseload generators.
CSP plants with TES add system flexibility
because of their fast ramp rate and large
operating range relative to large baseload
generators. Many CSP plants, both existing
and proposed, are essentially small steam
(Rankine cycle) plants whose " fuel " is concentrated
thermal energy. Few of these plants
are deployed, so it is not possible to determine
their performance with absolute certainty.
However, historical performance of
the SEGS VI power plant located in Kramer
Junction, Calif., and small gas-fired steam
plants provides some indication of CSP flexibility.
These plants operate at well over a
50% capacity range with only about a 5% increase
in heat rate at 50% load. This provides
a strong indication that CSP plants should be
able to provide high flexibility.
Implementing a flexible grid, as described
above, with solar thermal and PV plants re7.
CSP boosts PV penetration. Simulated
system dispatch from April 10 to 13 is
shown with 25% contribution from PV and
10% from dispatchable CSP, where CSP reduces
the minimum generation constraint. By
shifting energy over time and increasing grid
flexibility, CSP enables greater overall solar
penetration and, in particular, greater penetration
of PV. Source: NREL
Curtailed solar
Wind
June 2012 | POWER
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Energy from solar (%)
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Dispatched CSP
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Load
Dispatched CSP
Usable PV
Generation (MW)
Solar curtailment rate (%)
Generation (MW)

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POWER June 2012

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