POWER June 2012 - 47

ENERGY STORAGE
The dispatchability of CSP with TES
can enable higher overall penetration of
the grid by solar energy by providing solar-generated
electricity during periods of
cloudy weather or at night, when PV-generated
power is unavailable. Such systems
also have the potential to improve grid
flexibility, thereby enabling greater penetration
of PV energy (and other variable
generation sources such as wind) than if
PV were deployed without CSP.
Challenges of Solar Deployment
at High Penetration
The benefits and challenges of high PV penetration
(feeding high percentages of solar
3. Syncing up with demand. This chart, showing a simulated dispatch in California for
a summer day with PV penetration from 0% to 10%, illustrates how PV has the ability to reduce
the need for peaking capacity due to its coincidence with demand patterns. Each set of curves
represents a 24-hour day. Source: National Renewable Energy Laboratory (NREL)
PV
Wind
Gas turbine
Geothermal
60,000
50,000
40,000
30,000
20,000
10,000
Pumped storage
Hydro
Combined cycle
Imports
Coal
Nuclear
generation to the grid) have been described
in a number of analyses. At low penetration,
PV could displace the highest cost generation
sources and may also provide reliable capacity
to the system. Figure 3 shows a simulated
system dispatch for a single summer day in
California with PV penetration levels from
0% to 10% (on an annual basis). This figure
is from a previous analysis that used a production
cost model simulating the western
U.S. This particular scenario illustrates how
PV reduces the need for peaking capacity
due to its coincidence with demand patterns.
As penetration increases, the value of PV
capacity drops. This can be observed in Figure
3, where the peak net load (normal load
minus PV) stays the same between the 6%
and 10% penetration curves. Beyond this
point, PV no longer adds significant amounts
of firm capacity to the system.
Several additional challenges for the economic
deployment of solar PV occur as penetration
increases. These are illustrated in
Figure 4, which shows the results of the same
simulation, except on a spring day. During
this day, the lower demand could result in
PV displacing lower-cost base-load energy,
should CAISO dispatch PV first. At 10% PV
penetration in this simulation, PV completely
eliminates net imports, and California could
export energy to neighboring states, if the
cost of the exported power were attractive.
Several factors limit the ability of conBase
(no PV)
2%
6%
PV penetration (summer day)
4. The impact of diminished demand. Simulated dispatch in California for a spring
day is shown with PV penetration from 0% to 10%. During this day, the lower demand results
in PV displacing lower-cost baseload generation, if PV is dispatched first. Source: NREL
PV
Wind
Gas turbine
Geothermal
35,000
30,000
25,000
20,000
15,000
10,000
5,000
Base (no PV)
June 2012 | POWER
2%
6%
PV penetration (spring day)
www.powermag.com
10%
Pumped storage
Hydro
Combined cycle
Imports
Coal
Nuclear
10%
ventional generators to reduce output to
accommodate the variability of renewable
generation. One is the rate at which generators
can change output, particularly in the
evening, when they must increase output rapidly
in a high-PV scenario.
Another limitation is the overall ramp
range, or generator turndown ratio. This represents
the ability of power plants to reduce
output, which is typically limited on large
coal and nuclear units. Accommodating all
of the solar generation, as shown in Figure 4,
requires nuclear generators to vary their output,
which is not current practice in the U.S.
nuclear industry. Most large thermal power
plants cannot be shut down for short periods
of time (a few hours or less), although brief
shutdowns would be required to accommodate
all the energy generated during the period
of peak solar output. Additionally, many plant
operators have limited experience with cycling
large coal plants, and extensive cycling could
significantly increase the cost of maintenance.
(See " Mitigating the Effects of Flexible Operation
on Coal-Fired Plants " and " Make Your
Plant Ready for Cycling Operations " in the
August 2011 issue of POWER or the magazine's
archives at www.powermag.com.)
The ability to " de-commit " or shut down
power plants may also be limited by the need
47
Generation (MW)
Generation (MW)

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POWER June 2012

Table of Contents for the Digital Edition of POWER June 2012