Overgeneration from Solar Energy in California - A Field ...The duck chart shape has been noted previously “Proto-duck” chart of California net load with increased penetration

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Overgeneration from Solar Energy in California – A Field Guide to the Duck Chart

Report Summary

P. Denholm, M. O’Connell, G. Brinkman, J. Jorgenson

November 2015 NREL/PR-6A20-65453

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Sponsors

This work was funded by the Solar Energy Technologies Office and the Office of Strategic Programs of the U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) under Contract No. DE-AC36-08GO28308

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The Duck Chart

• The challenge: How will this net load shape impact successful integration of renewable (mainly solar) energy?

• A big concern is overgeneration.

Source: CAISO 2013

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Sources of overgeneration

Too much supply, not enough demand, when considering: • Ramp constraints • Minimum output levels from hydro and

thermal generators o This includes technical, institutional, and

economic constraints o This also includes the need to operate partially

loaded capacity to maintain system reliability

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Overgeneration results in RE curtailment

• Technically easy to do (at least on utility-scale renewable energy generation)

• But reduces economic benefits and increases the cost of meeting renewable portfolio standards or carbon-reduction goals

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The duck chart shape has been noted previously

“Proto-duck” chart of California net load with increased penetration of PV (from Denholm et al. 2008)

Example of an analysis of the impact of concentrating

solar power (CSP) on the duck chart shape (from

Jorgenson et al. 2014)

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Winter SpringMinimum

SummerPeak

Season and Hour

Net

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Base(no PV)2%

4%

6%

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And there has been analysis of its impact on curtailment

Example of an analysis of the impact of changing system

flexibility on demand shape and curtailment from an

analysis from Nelson and Wisland (2015)

Example of an analysis of the impact of high variable generation on net load shape and resulting overgeneration from E3 (2014)

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These previous analyses also suggest mitigation measures

Example of an analysis of how the duck chart shape can be modified to minimize overgeneration (from Lazar 2014)

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Goals of this work

• Provide a general overview of the duck chart and solar curtailment

• Examine how much curtailment will result as a function of solar penetration

• Examine the impact of mitigation options

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Analysis framework

• Use an industry-vetted simulation tool (PLEXOS)

• Use data from publically available sources (2024 TEPPC Common Case and 2014 LTPP)

• Modify for increased photovoltaic penetration

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Scenarios

Start with 11% wind and add increasing amounts of solar PV

Solar Pre-Curtailment

Potential Scenario (%)

Total Solar (PV + CSP) Potential

(GWh)

Total Pre-Curtailment

RPS Potential (%)

Annual Regulation Up Requirement

(GWh)

Annual Flexibility Up Requirement

(GWh)

11 35,331 36.0% 3,499 10,590 15 46,473 39.6% 3,671 11,089 18 56,438 42.7% 3,947 11,651 21 66,155 45.8% 4,282 12,240 24 77,329 49.4% 4,718 12,947 31 98,964 56.3% 5,652 14,361 37 119,682 62.9% 6,607 15,746

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Initial (most conservative) case

Designed to approximate current grid conditions • Wind and solar cannot provide upward reserves. • No net exports of electricity from California are allowed and at least 70%

of California-owned or contracted generation (including Hoover, Palo Verde, and certain renewable generation) from outside of the state must be imported.

• Up to about 1.3% of peak demand (as much as ~900 MW during periods of peak demand) can be shifted via economic demand response programs.

• No new storage is installed beyond what is in service in 2015. • 25% of all generation within certain zones must be met with local thermal

or hydro generation. • Diablo Canyon remains online as a baseload (non-dispatchable) generator.

The plant does not contribute to the 25% local generation requirement. • Instantaneous penetration of variable generation (including PV, wind, and

CSP without thermal energy storage) is limited to 60% of the normal load.

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Net load on March 29

The worst day for potential overgeneration

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Load without VG Net Load (Unconstrained)Total Solar Wind

Load, solar, and wind profiles for California on March 29 in a scenario with 11% annual wind and 11% annual solar

assuming no curtailment

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But not all PV can be used

System constraints do not allow reduction in net load beyond a certain point…

Modeled net load in California on March 29 in a scenario with 11% annual wind and 11% annual solar

in a system with a 60% instantaneous penetration constraint

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Net Load (With Curtailment)

Net Load (Unconstrained)

Thermal, hydro generators not allowed to reduce output below about 12,600 MW

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Resulting variable generation curtailment

About 5% of the potential VG is curtailed on this day

Used and curtailed VG in California on March 29 in a scenario with 11% annual wind and 11% annual solar

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VG Potential

VG With Curtailment

Curtailment

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Most curtailment due to 60% limit

PV could potentially provide up to nearly 70% of the total demand in California at noon on this day

Instantaneous penetration of VG on March 29 with and without curtailment

in a scenario with 11% annual wind and 11% annual solar

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% E

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VG WithCurtailment

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Existing storage is important

Larger amount of PV would be curtailed without the existing pumped hydro storage in California

Increase in VG use resulting from schedulable pumped storage in a scenario with 11% annual wind and 11% annual solar

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VG With Pumping

VG Without Pumping

Pumping Load (Avoided Curtailment)

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Little curtailment during the summer During peak demand days, there is sufficient demand to absorb all the potential VG

Load, solar, and wind profiles for California on July 27 in a scenario with 11% annual wind and 11% annual solar

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Load without VG Net Load Total Solar Wind

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Lower summer VG penetration

VG contribution is much smaller on the day with the greatest demand

Instantaneous penetration of VG in California on July 27 in a scenario with 11% annual wind and 11% annual solar

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Adding more PV makes the problem more challenging

Additional PV increases the size of the duck “belly” and requires even greater flexibility

Load in California and VG profiles on March 29 in a scenario with 11% and 15% annual solar

assuming no curtailment

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Load without VGSolar Potential 15%Net Load (Unconstrained - 15% Solar)Solar Potential 11%Net Load (Unconstrained - 11% Solar)

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Without greater flexibility, the net load cannot be reduced

Without reducing the minimum generation level, adding PV does not change the shape of the duck

Net load on March 29 in a scenario with 11% and 15% annual solar

considering operational constraints

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Load without VG

Net Load with Curtailment (11% Solar)

Net Load with Curtailment (15% Solar)

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And PV curtailment increases

Most (65%) of additional PV is curtailed on this day (5% over the entire year)

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VG with Curtailment (15% Solar)VG with Curtailment (11% Solar)Curtailment (15% Solar)Curtailment (11% Solar)

Usable and curtailed VG on March 29 in a scenario with 11% and 15% potential annual solar

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Impact of VG penetration in the base case Marginal curtailment = curtailment of all incremental VG moving from one penetration level to the next Total curtailment = curtailment rate of all PV installed on the system at a certain penetration level

Marginal and average curtailment due to overgeneration under increasing penetration of PV in California

with a 60% instantaneous penetration limit

35% 37% 39% 41% 43% 45% 47%

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10% 12% 14% 16% 18% 20% 22% 24%

Annual RPS Penetration

Annu

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G C

urta

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Annual Solar Energy Penetration

MarginalCurtailment

Total Curtailment

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Impact of VG curtailment on LCOE Curtailed energy means less can be sold and incremental costs of additional PV rise dramatically

Marginal and average PV LCOE (based on SunShot goals) due to overgeneration under increasing penetration of PV in California

with a 60% instantaneous penetration limit

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10% 12% 14% 16% 18% 20% 22% 24%

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Marginal Cost

Average Cost

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Enabling greater solar penetration How can we accommodate or change the duck chart shape to increase VG penetration and reduce curtailment?

• Fatten the Duck – All approaches that increase the flexibility of the grid and allow greater instantaneous penetration of VG resources—includes changing operational practices to allow more frequent cycling, unit starts and stops, and minimizing the amount of thermal units held at part load.

• Flatten the Duck – Shrink the belly shape by shifting supply/demand patterns to allow solar energy to meet parts of the load that would not normally be provided in the middle of the day. Typically involves demand response or storage.

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Option 1: Fatten the duck Make a bigger belly

Accommodate increased penetration of PV by reducing system minimum generation requirements

and fattening the duck

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Load without VG NetLoad (Actual)Net Load (Unconstrained) Min Generation-Base CaseMin Gen More Flexible

Decrease system minimum generation

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Option 2: Flatten the duck Or “streamline” the duck…by shifting demand

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Ideal Shifted Load Load without VGNet Load (Unconstrained) Net Load (Actual)Min Generation-Base Case

Accommodate increased penetration of PV by flattening the duck

(increasing mid-day demand)

Net shift in load

Added load

Reduced load

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Example – New storage flattens the duck New storage can provide shift load. Here is the additional load produced by the California storage mandate

Impact of flattening the duck on March 29 with 1,285 MW of added storage

in a scenario with 15% annual solar

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Load Before New Storage Curtailment Before New Storage

Load with New Storage Curtailment with New Storage

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But storage can also help fatten the duck • New storage can provide a highly flexible source of

operating reserves, and allow for greater penetration of VG. Storage can reduce the need to keep partially loaded generators operating.

• We examine this scenario by increasing the maximum VG penetration level to 80% and eliminating the local generation requirement.

• This assumes that distributed storage, including behind-the-meter storage, is controllable and able to respond to grid system requirements – storage acts as part of a “smart grid.”

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Impact of fattening the duck The minimum generation level has dropped to about 10,000 MW, increasing the amount of load served by PV

Net load on March 29 in a scenario with 15% annual solar increasing the maximum penetration of VG to 60% to 80% and

eliminating the local generation constraint

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60% Max. Penetration


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This can substantially reduce curtailment By reducing minimum generation constraints, the curtailments on this day have been reduced by about 50%

VG curtailment on March 29 in a scenario with 15% annual solar increasing the maximum penetration of VG to 60% to 80% and

removing the local generation requirement

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Greater penetration of VG A fatter and flatter duck increases the penetration of VG at the same level of curtailment

Marginal and average annual curtailment due to overgeneration under increasing penetration of PV in California after adding mandated storage, removing local generation constraint, and increasing maximum instantaneous VG penetration to 80%

35% 37% 39% 41% 43% 45% 47% 49% 51% 53% 55%

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80% Limit (Avg.)

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Engaging responsive demand? What if demand response provides BOTH shiftable load and more flexible operation?

Impact of additional demand response and increasing maximum penetration to 90% on system net load on March 29 and July 27

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Net Load w/o DRNet Load w/DRLoad ReductionAdded Load

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Net Load w/o DRNet Load w/DRLoad ReductionAdded Load

Limited load shifting on the duck curve day, but flexible operation may allow greater penetration

Benefits on high-demand days include capacity reduction and less use of

peaking generation

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Even greater penetration of VG PV penetration of 25% with less than 20% marginal and 5% total curtailment

Marginal and average curtailment due to overgeneration under increasing penetration of PV in California

after additional demand response and increasing maximum penetration to 90%

35% 37% 39% 41% 43% 45% 47% 49% 51% 53% 55%

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90% Limit (Marg.)

90% Limit (Avg.)

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Additional opportunities to fatten and flatten the duck

This analysis does not consider many additional options to increase VG penetration • Electricity storage • CSP with thermal storage • Regional interchange • Additional generator flexibility • Alternative renewable sources

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Conclusions

• This analysis indicates that near-term technology options are capable of helping mitigate challenges demonstrated by the duck chart, and increase PV penetrations exceeding 25% and total renewable penetration exceeding 50% with modest curtailment.

• Changes under way or proposed elsewhere—such as shorter scheduling intervals, increased interaction across regions, and creating new market incentives for generator dispatch—will reduce the minimum generation challenge and enable greater utilization of VG.

• In the longer term, grid operators will need non-traditional resources to supply reserves and grid stability services. In turn, this will require system operators to have visibility and control of distributed PV, storage, and load, and will likely require new market mechanisms to incentivize these resources to participate in providing grid services.

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References

• Brinkman, G., J. Jorgenson, A. Ehlen, J. Caldwell (2015). “The California 2030 Low Carbon Grid Study,” NREL TP- 6A20-64884

• CASIO (2013). What the Duck Curve Tells us about Managing a Green Grid. https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf

• Denholm, P., M. O'Connell, G. Brinkman, J. Jorgenson (2015). Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart. NREL/TP-6A20-65023

• Energy and Environmental Economics, Inc. (2014). Investigating a Higher Renewables Portfolio Standard in California. San Francisco, CA: January. Accessed May 2, 2014: https://ethree.com/documents/E3_Final_RPS_Report_2014_01_06_with_appendices.pdf

• Lazar, J. (2014). Teaching the Duck to Fly. Regulatory Assistance Project. • Nelson, J. and L. Wisland (2015). Achieving 50 Percent Renewable Electricity in

California. Union of Concerned Scientists. Online at www.ucsusa.org/California50RPSanalysis.

https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf

https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf

https://ethree.com/documents/E3_Final_RPS_Report_2014_01_06_with_appendices.pdf

https://ethree.com/documents/E3_Final_RPS_Report_2014_01_06_with_appendices.pdf

http://www.ucsusa.org/California50RPSanalysis

Contact: Paul Denholm [email protected] Full report at http://www.nrel.gov/docs/fy16osti/65023.pdf