POWER June 2012 - 50

ENERGY STORAGE
8. The power of grid flexibility. The curtailment of solar is shown, assuming an
equal mix (on an energy basis) of PV and CSP. The chart illustrates the potential overall effect
of grid flexibility introduced by CSP and the corresponding opportunities for increased
use of PV. Source: NREL
PV only marginal
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quires CSP plants that are more flexible in
operation than conventional fossil-fueled
plants. Because it is not possible to determine
the exact mix of generators that would
be replaced in high renewables scenarios, we
consider a range of possible changes in the
minimum generation constraints resulting
from CSP deployment.
CSP flexibility is defined as the fraction
of the CSP-rated capacity that is assumed to
reduce the system's potential generation constraint.
For example, deployment of a CSP
plant with TES that can operate over 75% of
its capacity range could replace a baseload
plant that normally operates over 50% of
its range. In this scenario, each unit of CSP
could reduce the minimum generation constraint
by 25% of the plant's capacity.
This very simplistic assumption illustrates
how the dispatchability of a CSP plant could
allow for a lower minimum generation limit
and allow for greater use of wind and PV. As
a result, as CSP is added, the grid can actually
accommodate more PV than in a system
without CSP.
This is illustrated conceptually in Figure
7, which also shows a four-day period. CSP
still provides 10% of the system's annual energy,
but now we assume that the use of CSP
allows for a decreased minimum generation
point, and the decrease is equal to 25% of the
installed CSP capacity. In this case about 21
GW of CSP reduces the minimum generation
point from about 18 GW to 13 GW. This gen50
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Energy
from solar (%)
eration " headroom " allows for greater use of
PV, and enough PV has been added to meet
25% of demand (up from 15% in Figure 5).
As a result, the total solar contribution is now
35% of demand. By shifting energy over time
and increasing grid flexibility, CSP/TES enables
greater overall solar penetration and
greater penetration of PV.
The potential overall impact of the flexibility
introduced by CSP/TES and the corresponding
opportunities for increased use of PV
are shown in Figure 8, which builds on Figure
6 by adding the energy supplied by CSP/TES.
The figure assumes that each MW of capacity
of CSP energy reduces the minimum generation
constraint by 25% of its capacity, and an
equal mix of PV and CSP/TES on an energy
basis. In this case, the addition of CSP/TES
allows PV to provide 25% of the system's energy
with very low levels of curtailment.
The relationship between the reduction in
minimum generation constraint and potential
increase in PV penetration is illustrated in
Figure 9, which shows how much more PV
could be incorporated at a constant marginal
curtailment rate of 20% when CSP is added.
In this scenario, the x-axis represents the
fraction of annual system energy provided
by CSP. Increased penetration of CSP results
in a linear decrease in minimum generation
constraints. The figure illustrates two CSP
flexibility cases. In one scenario, each unit of
CSP reduces the minimum generation constraint
by 20% of its capacity; in the other,
www.powermag.com
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PV only average
PV + CSP (with flex benefits) marginal
PV + CSP (no flex benefits) marginal
PV + CSP (with flex benefits) average
PV + CSP (no flex benefits) average
9. A dynamic duo. The figure shows
the relationship between reducing generation
constraints through the addition of
CSP/TES and the potential increase in PV
penetration. CSP flexibility is defined as the
fraction of the CSP rated capacity that is
assumed to reduce the system's minimum
generation constraint. Source: NREL
40% CSP flexibility
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20% CSP flexibility
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Energy from CSP (%)
the CSP flexibility is assumed to be 40%.
These amounts are not meant to be definitive
but represent a possible impact of CSP in reducing
minimum generation constraints.
Further Quantifying the Benefits
of CSP Deployment
This analysis is a preliminary assessment of
the potential benefits of CSP in providing
grid flexibility using reduced form simulations
with limited geographical scope and
many simplifying assumptions. Gaining a
more thorough understanding of how CSP
can enable greater PV and wind penetration
will require detailed production simulations
using security-constrained unit commitment
and economic dispatch models currently
used by utilities and system operators.
Future and ongoing studies at NREL and
elsewhere will evaluate the benefits of TES
in more detail. To perform these simulations,
production cost models will need to include
the ability of CSP to optimally dispatch the
solar energy resource. These simulations will
consider the operation of the entire power plant
fleet, including individual generator characteristics
and constraints, and the operation of the
transmission system. These simulations will
provide a better estimate of the benefits of grid
flexibility enables by CSP deploying TES. ■
-Paul Denholm, PhD (paul.denholm@
nrel.gov), a senior analyst, and
Mark Mehos (mark.mehos@nrel.gov), a
principal program manager, work at the
National Renewable Energy Laboratory.
This article is based on an abridged version
of the report titled " Enabling Greater
Penetration of Solar Power via the Use of
CSP with Thermal Energy Storage. " The
full report (Technical Report NREL/TP6A20-52978)
is available electronically at
no cost at www.osti.gov/bridge.
POWER | June 2012
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Solar curtailment rate (%)
Increase in energy from PV at
constant curtailment rate (%)

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POWER June 2012

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