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NREL
July 5, 2011
National Renewable Energy Laboratory Innovation for Our Energy Future

• As penetration of variable generation (solar, wind) increase, it is increasingly important to consider the interaction between these resources and the entire grid system
• Dispatchable energy (e.g. CSP w/storage) has a higher value than non-dispatchable energy
.
– At low penetration of solar and wind this difference is small
– At higher penetration (15% on an energy basis) this difference may increase by as much as 4 cents/kWh
• Overall penetration of solar energy can be increased by the use of CSP with storage which provides grid flexibility
– Allows for higher levels of PV penetration by providing the ramping rate and range needed to accommodate the variable output of PV systems
Innovation for Our Energy Future

Increase in Energy Value Due to Dispatchability of
Systems with Thermal Energy Storage
Dispatchable solar energy sources:
1.
Maintain high energy value
– Always displaces the highest cost energy sources
2.
Maintain high capacity value even at high solar penetration.
3.
Lower curtailment than solar systems w/o storage
4.
Lower integration/reserve costs
The actual difference in value is largely a function of penetration and overall grid system flexibility
Innovation for Our Energy Future

• Detailed grid simulations of the Western Interconnect
– Simulates the hourly dispatch of the power plant fleet
– Ensures reliability by ensuring availability of operating reserves
– Validates basic transmission operability using DC power flow
– Enforces power plant constraints including ramp limits, operating limits
– Calculates fuel burn and associated cost and emission
– Assumed frictionless markets (best case scenario for PV)
• Two scenarios
– 15% PV and 15% wind
– 10% PV, 5% CSP and 15% wind
• Did not capture full range of integration costs due to uncertainty about reserve requirements of PV, short term variability and forecast errors – assumed perfect knowledge of solar resource
Innovation for Our Energy Future

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0
1 5 9 13 17 21 1 5 9 13 17 21 1 5 9 13 17 21 1 5 9 13 17 21
PV
CSP
Wind
Hydro
PHS/CAES
Other
Biomass
Coal
Nuclear
Geothermal
Gas
Difference in gas burn
Storage enables a relative fuel savings benefit over PV of about 0.5 cents/kWh at
$4.50/mmBTU gas
Example WECC-wide dispatch during a 4-day period in spring
Dispatch of CSP results in less high cost gas and more low cost fuels
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Hour
Innovation for Our Energy Future
PV
Wind
Hydro
PHS/CAES
Other
Biomass
Coal
Nuclear
Geothermal
CSP
Gas

Normal peak at ~4-5 pm
92000
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12 14 16
Hour (Ending)
18
Normal Load
2%
5%
10%
15%
20 22
At 10% PV, peak is shifted to
8-9 pm. PV provides no further peak capacity benefits
At this point PV cannot reduce the need for generation capacity
CSP capacity value remains close to ~100% by shifting energy production to evening
(and morning during spring/winter months)
•Capacity value adder depends on market conditions - typical values of $40-$70/kW/year
•Depending on CSP system design and market conditions, adds a CSP value of 0.7-2.0 cents/kWh
Innovation for Our Energy Future

60000
50000
40000
Normal Load
1% PV
5%
10%
15%
20%
30000
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Ramp rate of conventional generator requirements increases
0
0
Ramp range of conventional generator requirements increases
36
Hour
48 60 72
Curtailment results from two main constraints – ramping requirements and minimum generation constraints. Curtailment results when existing plants to not have the flexibility to ramp
Innovation for Our Energy Future

Curtailment Due to Minimum Generation Constraints
• Marginal curtailment rate of PV moving from
10% to 15% of generation was 5%
• At SunShot goals (~6 cents/kWh) this increases effective PV cost by about 0.3 cents/kWh due to underused capacity
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PV
CSP
Wind
Hydro
PHS/CAES
Gas
Other
Biomass
Coal
Nuclear
Geothermal
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0
1 5 9 13 17 21 1 5 9 13 17 21 1 5 9 13 17 21 1 5 9 13 17 21
Extensive coal and nuclear cycling unlikely to occur in current system
• PV curtailment would be reduced if grid flexibility were increased
PV
CSP
Wind
Hydro
PHS/CAES
Gas
Other
Biomass
Coal
Nuclear
Geothermal
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1 5 9 13 17 21 1 5 9 13 17 21 1 5 9 13 17 21 1 5 9 13 17 21
• CSP/TES provides an option to replace
“baseload” capacity with more flexible generation
Innovation for Our Energy Future

60%
50%
40%
30%
20%
Estimates marginal curtailment as a function of
PV penetration (without additional grid flexibility)
10%
0%
0% 5% 10% 15% 20% 25%
Fraction of System Energy from PV
30% 35%
Without storage or load shifting, marginal LCOE of
PV increases rapidly
“Multiplier” to base LCOE
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0% 5%
9
35%
6
8
7
15
14
13
12
11
10
10% 15% 20% 25%
Fraction of System Energy from PV
30%
Innovation for Our Energy Future

• Variability and uncertainty of solar resource requires changes in operation, typically some re-dispatch of system resources to maintain reliability
12000
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Normal Load
10% PV
15%
20%
Very large ramping of conventional generators is required. This potentially means more use of fast responding but lower efficiency generators
4000
0
0
-4000
-8000
-12000
12 24
Hour
Innovation for Our Energy Future

• We have not yet analyzed the increased need for frequency regulation or forecast uncertainty for either PV or CSP
– One previous PV study estimated costs of re-dispatch at 0.4-0.7 cents/kWh, but used limited data sets and is not reproducible
– Estimates from wind integration studies are in the range of 0.2-0.4 cents/kWh
• Storage enables operation at part load and ability to hold back energy during periods of high uncertainty or large reserve requirements
Innovation for Our Energy Future

With gas prices in the range of $4.50-$9.00 mmBTU, the estimated value of CSP with storage is an additional 1.6-4.0 cents/kWh relative to PV due to:
• Energy shifting value: ~0.5-1.0 cents/kWh
• Capacity Value ~0.7-2.0 cents/kWh
• Reduced curtailment: Depends on PV cost. At 6 cents/kWh, corresponds to ~0.3 cents/kWh
• Reserve/integration costs 0.1-0.7 cents/kWh
Innovation for Our Energy Future

• The ability of a the grid to accommodate PV is inherently limited by the increased variability and uncertainty of net load
• As PV penetration increases other generators will need:
• Short start-up times
• Large ramp rates
• Large turn-down ratios
• Good part load efficiency
CSP with storage can provide these requirements
Historical performance of U.S. small gas steam plants which are a good proxy for
CSP – typical operating range of 78% with only a 7% heat rate penalty at 50% load.
Innovation for Our Energy Future

45
Dispatch in a “conventional” system
Relying on thermal generators and ignoring flexibility benefits of CSP limits amount of demand that can be met with variable generation
40
35
30
25
20
15
Curtailed PV
Dispatched CSP
Usable PV
Wind
Conventionals
Load
Non-Dispatched CSP
Dispatched CSP
Additional PV will largely be curtailed due to minimum generation constraints
10
5
0
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96
Hour
CSP energy is shifted to morning and evening, increasing the contribution of solar technologies, but not providing a direct benefit to PV or wind.
Total RE contribution is 35% on an energy basis (solar provides 23%). About 5% is curtailed.
Innovation for Our Energy Future

Dispatch in a “CSP-flexible” system
45
40
Adding the flexibility of CSP enables a greater fraction of the load to be served by variable generation
35
30
25
20
15
Curtailed PV
Dispatched CSP
Usable PV
Wind
Conventionals
Load
Non-Dispatched CSP
Dispatched CSP
10
5
Minimum generation constraint reduced
0
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96
Hour
CSP energy is still shifted, but also used to provide quick-start reserve capacity during periods of high PV output.
CSP provides additional ramping capacity in the evening and morning.
Total RE contribution is increased to 46% (solar contribution at 29%) with no increase in curtailment.
Innovation for Our Energy Future

• As penetration of variable generation (solar, wind) increase, it is increasingly important to consider the interaction between these resources and the entire grid system
• Dispatchable energy (e.g. CSP w/storage) has a higher value than non-dispatchable energy
.
– At low penetration of solar and wind this difference is small
– At higher penetration (15% on an energy basis) this difference may increase by as much as 4 cents/kWh
• Overall penetration of solar energy can be increased by the use of CSP with storage which provides grid flexibility
– Allows for higher levels of PV penetration by providing the ramping rate and range needed to accommodate the variable output of PV systems
Innovation for Our Energy Future

References (Note that several of the results in this presentation have not yet been published).
Madaeni, S., R. Sioshansi, and P. Denholm, "How Thermal Energy Storage Enhances the Economic Viability of Concentrating Solar Power" accepted in Proceedings of the IEEE.
Madaeni, S. H., Sioshansi, R., Denholm, P. (2011) “Capacity Value of Concentrating Solar Power Plants”
NREL Report No. TP-6A20-51253.
Brinkman, G.L., P. Denholm, E. Drury, R. Margolis, and M. Mowers. (2011) “Toward a Solar-Powered Grid -
Operational Impacts of Solar Electricity Generation” IEEE Power and Energy 9, 24-32 .
Denholm, P., and M. Hand. (2011) “Grid Flexibility and Storage Required to Achieve Very High Penetration of
Variable Renewable Electricity” Energy Policy 39 1817-1830 .
Sioshansi, R. and P. Denholm. (2010) “The Value of Concentrating Solar Power and Thermal Energy
Storage.” IEEE Transactions on Sustainable Energy . 1 (3) 173-183.
Denholm, P., E. Ela, B. Kirby, and M. Milligan. (2010) “The Role of Energy Storage with Renewable Electricity
Generation” NREL/TP-6A2-47187.
Denholm, P., R. M. Margolis and J. Milford. (2009) “Quantifying Avoided Fuel Use and Emissions from
Photovoltaic Generation in the Western United States” Environmental Science and Technology . 43, 226-232.
Denholm, P., and R. M. Margolis. (2007) “Evaluating the Limits of Solar Photovoltaics (PV) in Electric Power
Systems Utilizing Energy Storage and Other Enabling Technologies” Energy Policy . 35, 4424-4433.
Denholm, P., and R. M. Margolis. (2007) “Evaluating the Limits of Solar Photovoltaics (PV) in Traditional
Electric Power Systems” Energy Policy . 35, 2852-2861.
Innovation for Our Energy Future