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Contract No. DE-AC36-08GO28308

Impact of Generator Flexibility 

on Electric System Costs and 

Integration of Renewable 

Energy

D. Palchak and P. Denholm

Technical Report

NREL/TP-6A20-62275

July 2014
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Impact of Generator Flexibility 

on Electric System Costs and 

Integration of Renewable 

Energy 

D. Palchak and P. Denholm

Prepared under Task No. SA12.0384

Technical Report

NREL/TP-6A20-62275

July 2014
NOTICE

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Table of Contents

Tables........................................................................................................................................................... 1

Figures ......................................................................................................................................................... 2

Abstract........................................................................................................................................................ 3

1. Introduction........................................................................................................................................... 3

2. Test System........................................................................................................................................... 4

2.1 Base System Description............................................................................................................... 4

3. Results................................................................................................................................................... 7

3.1 Base Case ......................................................................................................................................... 7

3.2 Increased Flexibility ........................................................................................................................10

3.3 Unconstrained Instantaneous Renewable Energy Penetration ........................................................17

4. Conclusions ........................................................................................................................................ 20

5. References .......................................................................................................................................... 22

Tables

Table 1. System Capacity in the Base Case............................................................................................. 5

Table 2. Total VG Penetration and Reserve Requirement for the Base Case....................................... 6

Table 3. Summary of Main Scenarios ....................................................................................................... 7

Table 4. Total Production Cost Summary for Increased Flexibility From Coal for All VG 

Scenarios............................................................................................................................................. 10

Table 5. Total Revenue and Total Production Costs for all Coal Units in the RE Scenarios 

for the Base and Increased Flexibility Cases .................................................................................. 17

Table 6. Total Production Costs for Base and 100 Max RE Cases, as Well as Both Inc. Coal 

Flex Scenarios .................................................................................................................................... 18
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Figures

Figure 1. Annual energy generation by renewable penetrations in the base case.............................. 7

Figure 2. Total and marginal curtailment rates in the base case........................................................... 8

Figure 3. Generation dispatch of resources for three days in September in three RE cases ............ 9

Figure 4. Changes to the components of the total production costs with increased coal 

plant flexibility..................................................................................................................................... 11

Figure 5. Change in dispatch for the 40% RE scenario from Base to Inc. coal flex case (top) 

and range of operation for coal given unit commitment (red and gray shaded areas) and 

the actual dispatch (lines) for the two cases (bottom). The top graph has some smaller 

generation types omitted for clarity and includes the changes to load which are a result 

of changes to pumped storage operation. Wind, solar, and hydro are also combined for 

clarity... ................................................................................................................................................ 12

Figure 6. (a) Reduced curtailment rate and (b) reduced fuel use in each scenario resulting 

from increased coal plant flexibility ................................................................................................. 13

Figure 7. Additional avoided CO2 emissions due to increased coal plant flexibility......................... 14

Figure 8. Capacity starts for the base case and Inc. coal flex case. Capacity starts are the 

number of starts multiplied by the rated capacity of the unit........................................................ 15

Figure 9. Price duration curves for both base case and Inc. Coal Flex case for 40% RE 

penetration .......................................................................................................................................... 16

Figure 10. Curtailment for all flexibility scenarios................................................................................. 19

Figure 11. Avoided CO2 emissions due to increased flexibility........................................................... 20
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Abstract

Flexibility of traditional generators plays an important role in accommodating the increased 

variability and uncertainty of wind and solar on the electric power system. Increased flexibility 

can be achieved with changes to operational practices or upgrades to existing generation. One 

challenge is in understanding the value of increasing flexibility, and how this value may change 

given higher levels of variable generation. 

This study uses a commercial production cost model to measure the impact of generator 

flexibility on the integration of wind and solar generators. We use a system that is based on two 

balancing areas in the Western United States with a range of wind and solar penetrations 

between 15% and 60%, where instantaneous penetration of wind and solar is limited to 80%. 

We evaluate the impact of reducing the minimum generation level of the coal generation fleet 

from 60% to 40% of nameplate capacity and observe the corresponding decrease in production 

costs. At low wind and solar penetration, this increased flexibility provides very little benefit. 

However, at higher levels of renewable penetration, increased flexibility results in decreased 

curtailments, which reduces fuel consumption and decreases the system production cost. 

We also examine the impact of relaxing the 80% penetration limit, assuming that active power 

controls and other new technologies allow wind and solar to provide system stability services. 

This further decreases production costs, particularly in very high penetration scenarios. In all 

scenarios, emissions of CO2 decrease as flexibility is increased and more variable generation is 

accommodated.

1 Introduction

Integration of renewable resources such as wind and solar increases variability and uncertainty in 

the electric power system. Flexibility of the power system may play a vital role in 

accommodating the variation in net demand that occurs at increased renewable penetration. 

Flexibility can come from a number of sources including energy storage, demand-side 

management, or changes to market structures and operational practices, including increased 

cooperation across regional grid operators (Lew et al. 2013; Kirby and Milligan 2008; Corbus et 

al. 2010). Another crucial component of flexibility is the ability of traditional generation 

resources to change their output based on varying load, which is dictated by the parameters of 

minimum up/down times, ramp rates, and minimum generation level, along with start-up costs 

and part load efficiency (NERC 2009). 

Less flexible power systems can be more expensive to operate, as they force more expensive 

units to stay on when less expensive ones could be used to meet load. While flexibility has 

always been a necessary component of power systems given the uncertainty of demand and 

conventional generation outages, the growth in variable generation (VG)

1 increases the need for 

flexible resources (NERC 2009; Adams et al. 2010). The benefits of zero variable-cost VG 

sources include their ability to displace the operating costs and emissions of the conventional 

electric power system (Lew et al. 2013; Corbus et al. 2010). This primarily means avoiding the 

 

1 VG and “wind and solar” are used interchangeably throughout the report. 
4 

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costs of operating fossil-fueled generators and associated emissions of criteria pollutants and 

CO2. If the power system is not sufficiently flexible, the benefits of VG may be reduced. In 

higher levels of VG penetration, the limited flexibility in the system will lead to fossil-generators 

remaining online while cost- and emissions-free wind and solar is not able to be accommodated 

and is therefore curtailed.

Previous analysis has demonstrated the impact of VG deployment on unit cycling (EPRI 2013, 

Troy et al. 2012), and the relationship between system flexibility and curtailment rates has also 

been analyzed previously (Troy et al. 2010; Denholm and Hand 2011; Ummels et al. 2007). 

These studies demonstrate highly non-linear curtailment rates as a function of VG penetration as 

well as a large sensitivity of curtailment to operational flexibility of the conventional generation 

fleet. Other studies explore further mitigation options such as interconnection and storage in 

reducing curtailment (Tuohy and O’Malley 2009; Silva 2010). 

This study evaluates several aspects of generator flexibility2 using a commercial production cost 

model. It uses a subset of the Western Interconnection electric power system to quantify the 

impact of increased generator flexibility on overall production costs. Flexibility of this test 

system has been previously analyzed by Venkataraman et al. (2013), which included a costbenefit analysis of certain retrofits available for combined-cycle and coal generation plants. They 

found that increased flexibility of a few plants decreased the costs of operating the system and 

were commensurate with the costs of installing the necessary upgrades. Here we examine the 

impact of coal generator flexibility over a range of renewable penetrations to analyze the effect

on curtailments, as well as unit commitment and dispatch of various generation types. Our 

analysis also considers other factors that may be affected by altering generator parameters, such 

as unit starts and CO2 emissions. System marginal prices are also examined to quantify the effect 

that flexibility could have on generator revenues. 

2 Test System

2.1 Base System Description

The purpose of this analysis is to explore the general relationships between generator flexibility 

and overall operational cost, including the ability to accommodate variable generation 

resources.

3

 For this analysis, we used a test system derived from a subset of the Western 

Electricity Coordinating Council (WECC) Transmission Expansion Policy Planning Committee 

(TEPPC) model and other publicly available datasets (TEPPC 2011). The system is large enough 

to be realistic but small enough to isolate the impact of changing generator flexibility parameters. 

A number of previous studies use this test system (Hummon et al. 2013a; Jorgenson et al. 2013; 

Hummon et al. 2013b). Transmission is simplified in our representation of this system to focus 

on the relationship of traditional generators to VG resources. We assume that sufficient 

transmission would be available to accommodate new and existing generation resources.

 

2 From this point on, flexibility will refer specifically to individual generator flexibility as defined in (NERC 2009). 3 We used PLEXOS version 6.300 R03 x64 Edition and the Xpress-MP 24.01.04 solver, with the model 

performance relative gap (MIP gap) set to 0.5%. 
5 

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The test system consists of two balancing authorities—Public Service Company of Colorado 

(PSCo) and Western Area Colorado Missouri (WACM). Multiple individual utilities operate 

within this region, which is referred to in this study as the Colorado test system. These vertically 

integrated utilities balance their own operation and interact with their neighbors under 

confidential bilateral agreements. Because the bilateral contracts are not publicly available, this 

analysis assumes an optimal, least-cost dispatch. Hourly load profiles were scaled from 2006 

data to match the projected TEPPC 2020 forecast for annual load. The peak demand is 13.7 GW 

with an annual energy demand of 79.0 TWh. Table 1 shows the installed capacity of the 

generators in the test system.

Table 1. System Capacity in the Base Case

System Capacity (MW)

Coal 6,178

Gas Combined Cycle 3,724

Gas Turbine/Gas Steam 4,045

Hydro 773

Pumped Storage 560

Wind 3,347 (10.7 TWh)

Solar PV 878 (1.8 TWh)

Othera 513

Total 15,793

Generator parameters are based largely on the database used in the Western Wind and Solar 

Integration Phase 2 Study (2013). Key assumptions regarding generator flexibility in our base 

case include:

• Coal units have a minimum stable level at 60% of rated capacity with typical minimum 

run times of 168 hours (seven days). 

• Combined-cycle units have a minimum stable level of 55% with typical minimum run 

times of eight hours. 

• Combustion turbines have unit-specific minimum stable levels in a range of 35%-45% 

with typical minimum run times of two hours. 

• Instantaneous penetration of wind and solar is limited to 80%. 

Ten VG scenarios are considered for this study, with details provided in Table 2. The scenarios 

include total potential (before curtailment) penetrations ranging from 16% to 55% in roughly 5% 

increments on an energy basis. The ratio of wind to solar is held close to a 5.5:1 ratio, 

respectively, in all scenarios.4

 As noted previously, instantaneous VG penetration is limited to 

80% of the total load. This means that at least 20% of the generation in each hour must come 

 

4 In the lowest penetration scenario, this corresponds to about 2.3% from PV and 13.4% from wind. For comparison, 

Colorado received about 11% of its electricity from wind generation in 2012. (EIA 2013)
6 

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from traditional thermal or hydro generators. Any VG above this threshold is either curtailed or 

can be held for operating reserves.5 This 80% limit is somewhat arbitrary, but reflects concerns 

about stability limits and other technical constraints regarding systems with large amounts of 

non-synchronous generators (Ela et al. 2014; Ruttledge et al. 2012). As a result, we use this 80% 

limit to explore the impacts of placing limits on instantaneous penetration of VG. We also 

examine the impact of relaxing this constraint in section 3.3. 

Three spinning reserve products are being held in our model: contingency, regulation, and a 

load-following service. The spinning contingency reserve requirement is based on the single 

largest unit in the system and is not changed in all scenarios. The regulation and load following 

reserve requirement is based on the combination of the wind and solar penetration in a given 

interval. The technique to calculate the reserve is outlined in Lew et al. (2013) and Ibanez et al. 

(2012), which uses uncertainty in the forecasts of wind and solar in determining the reserve 

requirement. Table 2 shows the annual wind and solar penetration as a percentage of total 

generation, as well as the total wind and solar generation and the total annual reserve 

requirement. It includes the potential generation, as well as the actual (post- curtailment 

generation) discussed in more detail in the results section. 

Table 2. Total VG Penetration and Reserve Requirement for the Base Case

VG Generation Potential/Actual

(GWh)

Annual Reserve Requirement 

VG Penetration (GW-h)

(Potential/Actual)

(%) PV Wind Contingency Flex Regulation

16 / 16 1,834 / 1,834 12,539 / 12,539 3,548 502 1,050

21 / 21 2,556 / 2,556 16,394 / 16,393 3,548 600 1,134

27 / 27 3,168 / 3,163 21,266 / 21,254 3,548 769 1,281

32 / 32 3,755 / 3,723 25,200 / 25,102 3,548 855 1,364

36 / 35 4,276 / 4,202 28,080 / 27,836 3,548 918 1,422

40 / 39 4,805 / 4,595 31,776 / 31,042 3,548 1,096 1,626

45 / 44 5,379 / 4,976 35,664 / 34,325 3,548 1,183 1,702

50 / 47 5,991 / 5,303 39,666 / 37,053 3,548 1,304 1,835

55 / 50 6,549 / 5,370 43,603 / 39,127 3,548 1,456 2,091

60 / 52 7,165 / 5,679 47,330 / 41,193 3,548 1,511 2,140

We examine two opportunities to increase system flexibility, described in Table 3. The first is 

modifying assumptions regarding the minimum stable operating level of coal plants. Because 

this system derives a large fraction of its energy from coal, this study focuses on the impact of 

coal generator flexibility. Future analysis will consider additional generator mixes and the impact 

of various other generator flexibility parameters. The second flexibility option considered is 

increasing the maximum instantaneous penetration of VG. We also consider a scenario where the 

two flexibility options are combined. All flexibility scenarios are run for all ten VG penetration

levels to show how the value of flexibility changes with increasing penetrations of wind and 

solar. 

 

5 Wind generation is already used for provision of operating reserves by some utilities (Bird et al. 2014). 
7 

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Table 3. Summary of Main Scenarios

Scenario Description

Base Base system including 60% minimum stable level of coal plants and 80% 

maximum instantaneous wind and solar penetration

Inc. Coal Flex

Decrease minimum stable level/ maximum capacity of coal units to 

40% from 60% and 80% maximum instantaneous wind and solar 

penetration

100 Max VG Base system including 60% minimum stable level of coal plants and 

100% maximum instantaneous wind and solar penetration

100 Max VG + Inc. Coal Flex

Decrease minimum stable level/ maximum capacity of coal units to 

40% from 60% and 100% maximum instantaneous wind and solar 

penetration 

3 Results

3.1 Base Case

We begin with an examination of the operation of the test system in its base case configuration in 

all renewable scenarios. 

Figure 1 shows the annual generation mix of the VG scenarios for the base case. At low 

penetration, wind and solar tend to displace natural gas (primarily combined-cycle) generators. 

As penetration increases, coal begins to be displaced, particularly in the off-peak periods. Figure 

1 also shows increased levels of curtailment at the higher VG penetrations (the x-axis represents 

the penetration after curtailment). 

Figure 1. Annual energy generation by renewable penetrations in the base case

Curtailment is an important economic challenge to large-scale deployment of renewables. Figure 

2 summarizes the total and marginal curtailment rates in the base case. Total curtailment is 

defined as the total amount of renewable energy curtailed divided by the total potential 

generation in that scenario. Marginal curtailment represents the incremental curtailment from one 

scenario to the next divided by the incremental potential generation between one scenario and the 

next. Overall, curtailment in these simulations is quite low until penetration of about 32%, and 

then rises steadily. The relatively low curtailment rates are due in part to the assumption that 
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sufficient transmission capacity exists or is constructed to accommodate 100% of all wind and 

solar generation. While we effectively ignore transmission constraints due to our objective of 

understanding the impact of generator flexibility, transmission constraints are a major source of 

curtailment in the existing grid. Additional discussion of current curtailment rates is provided by 

Bird et al. (2014), who find that while a large fraction of historic curtailments in the U.S. appear 

to be driven by transmission constraints, other operational practices lead to curtailment at 

penetration levels below those simulated here. Many of these practices, such as manual dispatch, 

are evolving, and curtailment rates in the U.S. are generally dropping even while penetrations are 

increasing. 

Figure 2. Total and marginal curtailment rates in the base case

An additional source of relatively low curtailment rates is our assumption that curtailed wind and 

solar can be used to provide operating reserves, enabling de-commitment of thermal units during 

extended hours of high solar and wind production. The model assumes that each MWh of 

curtailed energy can provide 1 MW-h of reserves (1 MW of reserves service for one hour). If 

wind and solar were not allowed to provide reserves, more thermal units would have to stay 

committed, and would likely result in a greater number of committed units during certain 

periods. In this study, more than 50% of the curtailed capacity is typically providing reserves

(Figure 2 shows curtailment that represents both lost energy and curtailment that provides at least 

some value in the form of reserve provision). Had those reserves been provided by partially 

loaded generation, it is possible that the curtailment rates would have been even higher than that 

simulated here. While VG providing reserves is not as valuable to the system as delivering 

energy,6 it still provides significant system value, as discussed by Hummon et al. (2013a). For 

this reason, this analysis considers all VG that is not providing energy to be “curtailed” even 

though there are periods where reserves are more valuable than energy. Bird et al. (2014) 

 

6 For example, the average price of energy in the lowest penetration case is $33.0/MWh, while the average price of 

reserves is $12.7/MW-h, with regulation, (the most valuable reserve), averaging $16.6/MW-h. In the 39% VG case 

the average price of energy is $24.4/MWh, while the average price of reserves is $11.0/MW-h with regulation 

averaging $17.8/MW-h.
9 

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discusses current curtailment practices in more detail, including the use of curtailed capacity for 

reserve provision. 

An example of VG providing reserves is provided in Figure 3. It shows the impact of wind and 

solar on the system dispatch for the period of September 18-21 for three different penetrations of 

wind and solar. Of note in the highest penetration is the significant curtailment, and also the use 

of curtailed energy for provision of reserves. A large portion of the total VG curtailed as energy 

actually holds reserve provision. For example, in the 39% case, while about 5% of the VG 

energy is curtailed, 80% of this energy (or about 4% of the total curtailment) is effectively used 

to provide reserves (where 1 MWh of energy provides 1 MW-h of reserve capacity). In the 

highest VG case, where about 12.5 % of VG is curtailed, about 75% of this curtailed energy is 

used to provide reserves, and actually provides about 80% of the system reserve requirements. 

Further analysis is needed of the ability of VG to provide reserves considering resource 

uncertainty and operational implications of curtailed VG providing a large fraction of system 

reserve requirements. 

Figure 3. Generation dispatch of resources for three days in September in three RE cases 

Figure 3 illustrates multiple challenges of high VG penetration, as well as possible opportunities 

for increased generator flexibility to accommodate RE. Specifically noticeable in the 56% case 

are two separate instances where inflexibility is causing higher production costs: 1) during high 

load the VG contribution is being capped at 80% (midday September 19), but 2) during 
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succeeding hours when load decreases, the coal units online are not turning down by the same 

margin as the load, meaning they are operating at or near their minimum generation. The 

following sections address both of these opportunities for increased system flexibility. 

3.2 Increased Flexibility

We begin our examination of flexibility by looking at the scenarios where we decrease the 

minimum generation point of coal from 60% to 40% of the maximum capacity. The expectation 

is that reducing the minimum generation of the coal units will increase flexibility in the system, 

and therefore, reduce curtailments and potentially improve the unit commitment and dispatch of 

the system. Table 4 summarizes the total production cost for the base case and increased coal 

flexibility case.

Table 4. Total Production Cost Summary for Increased Flexibility 

From Coal for All VG Scenarios 

VG

Penetration7

Base Inc. coal flex Savings from increased 

flexibility

(%) $M (%)

16 1423.6 1423.6 0.0 0%

21 1306.4 1305.8 0.6 0%

27 1169.8 1167.8 2.0 0%

32 1072.5 1066.4 6.1 1%

36 1006.4 997.6 8.8 1%

40 943.0 927.0 16.0 2%

44 874.3 856.1 18.1 2%

48 817.0 796.0 20.9 3%

50 776.0 751.9 24.1 3%

52 725.9 701.8 24.1 3%

Table 4 demonstrates that at low penetration, the flexibility of the coal units, which are 

traditionally used as baseload plants, has little impact on system savings. At low penetration of 

VG these plants are rarely cycled, indicating that there is no savings associated with allowing a 

lower minimum generation point. As the penetration of wind and solar increases, the system 

requires more coal cycling, and there is greater opportunity for savings associated with improved 

coal flexibility.

There are several sources of the savings associated with increased flexibility. Figure 4

demonstrates the source of the savings in each scenario, measured by the difference between the 

base case and increased flexibility case. Variable operations and maintenance (VO&M) refers to 

the costs associated with operation of a generator that is not fuel, while start and shutdown costs 

are the VO&M attributed specifically to starting or shutting down a unit. Regulation cost 

represents the additional cost to the system associated with providing operating reserves.

 

7 These penetration numbers represent the wind and solar penetration in the base case, as do the rest of the tables in 

the report. The Inc. coal flex case has slightly higher penetrations due to its ability to accommodate more VG.
11 

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Figure 4. Changes to the components of the total production costs 

with increased coal plant flexibility

The dominant source of savings associated with increased flexibility is a reduction in fuel costs. 

This reduction is itself associated with two factors—reduced curtailment and more efficient 

commitment and dispatch. Both of these factors are illustrated in Figure 5, which represents the 

40% VG case. The ability to decrease coal generator output accommodates more wind and solar 

generation. This can be observed during the overnight hours on September 23, 24, and 26, where 

coal is being displaced by the zero-cost VG and hydro resources. In addition, there are instances 

when increased coal flexibility allows coal to displace higher cost CC generation, such as during 

the middle of the day, September 23 and in the nighttime hours of September 25. Figure 5 

(bottom) illustrates this more directly. The shaded regions show the operating ranges allowed for 

the coal based on the committed units’ maximum and minimum generation levels. The solid 

lines show the actual dispatch. The most obvious change is the ability of the coal to decrease to a 

lower combined generation which causes VG and hydro to increase generation output (reduce 

curtailment). The other change is the range increase due to the maximum generation (grey area 

above) which is a result of more committed units. This leads to coal displacing gas because the 

combination of units have a higher ramp capacity and can maintain a higher output during 

multiple hours in the timeframe.
12 

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Figure 5. Change in dispatch for the 40% RE scenario from Base to Inc. coal flex case (top) and 

range of operation for coal given unit commitment (red and gray shaded areas) and the actual 

dispatch (lines) for the two cases (bottom). The top graph has some smaller generation types 

omitted for clarity and includes the changes to load which are a result of changes to pumped 

storage operation. Wind, solar, and hydro are also combined for clarity.
13 

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The annual reduction in curtailment and fuel use associated with increased flexibility are 

illustrated in Figure 6. 

Figure 6. (a) Reduced curtailment rate and (b) reduced fuel use in 

each scenario resulting from increased coal plant flexibility

Because increased generator flexibility results in reduced fossil fuel use, there will be an 

additional benefit of reduced CO2 emissions. While we assume no cost or carbon constraints in 

this simulation, CO2 reduction benefits can be calculated based on the fuel consumption values 

from Figure 6.

8 Figure 7 (a) provides the avoided total CO2 emissions that result from the 

increased generator flexibility. This represents as much as a 2.5% reduction in total emissions in 

the highest renewable case. This data is also expressed per unit of renewable generation (b). The 

 

8 The emissions calculations only consider coal and gas, which are the primary fuels utilized in our test system. The 

assumed CO2 intensity is 206.2 lb/MMBTU for coal and 117.0 lb/MMBTU for natural gas. (EIA 2013; EIA 1994). 
14 

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slight decrease in emissions benefits at the highest penetrations in (b) is a result of the decreased 

marginal benefit of VG, as well as the inability of VG to provide more than 80% of the energy in 

this scenario. Therefore, the baseload coal is not changing much beyond 50% even though more 

VG is added. 

Figure 7. Additional avoided CO2 emissions due to increased coal plant flexibility

While reduced fuel use is the dominant source of savings, an important secondary benefit is 

reduced plants starts. Figure 8 provides the total system start data for coal, combined-cycle and 

combustion turbines for each scenario. The data follows previous analysis by Lew et al. (2013) 

that indicates that the addition of wind and solar tends to decrease combined-cycle starts 

(because they are less often on the margin) while increasing coal unit starts. Overall, additional 

coal flexibility results in a substantial reduction in starts of all types. 
15 

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Figure 8. Capacity starts for the base case and Inc. coal flex case. Capacity starts are the number 

of starts multiplied by the rated capacity of the unit.

Finally, there is a small contribution from decreased operations and maintenance (O&M) costs. 

Of note is the increase in costs associated with providing regulation reserves. We model the nonsteady-state costs associated with thermal generators, equivalent to a “bid” price for regulation in 

restructured markets. These costs are assumed to vary from $2/MW to $10/MW for thermal 

generators, with a more detailed discussion provided by Hummon et al. (2013a). Because we 

allow curtailed wind and solar to provide this service at no additional cost, when we increase 

coal flexibility and decrease curtailment, this slightly increases the system-wide cost of 

providing regulation. This small increase in cost is more than made up for by decrease in cost 

associated with generation from thermal generators. 

There are two important implications of decreased production costs from the perspective of 

system wide benefits or benefits to individual plant operators. From the perspective of the 

system, (or a vertically integrated utility) the savings associated with increased coal flexibility 

must be compared to the cost of changing operational practices, plant retrofits, or potentially 

increased O&M requirements. The annual system-wide operational savings is about $24M in the 

highest renewable penetration case, or about $3,900/MW of coal capacity in the system. As 

noted previously, the mechanisms to change the minimum generation of coal generators could 

include both changes to operational practices as well as physical modifications. Further 

explanation of retrofits to individual generators is provided in Venkataraman et al. (2013), which 

found that benefits to the system are comparable to the costs of implementation. Cochran et al. 

(2013) addresses some of these physical modifications and also summarizes many of the 

implications and challenges of the operational changes that would be required to run coal plants 

at lower output. 

From the perspective of a participant in a restructured market, flexibility can also have a positive 

impact. Overall, the introduction of zero marginal cost renewables will displace fossil 

generation, reducing their capacity factor and revenue. There is also a strong interaction between 

VG and prices which can be influenced by generator flexibility. Figure 9 shows the price 
16 

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duration curves for the 40% VG penetration scenario in the base case and the increased coal 

flexibility case. Prices are determined by the marginal generators on the system at a given time 

interval. Gas combined cycle units on the margin are represented by prices from about $25-

$60/MWh. Coal is generally less expensive at about $18-$24/MWh. As VG penetration increases

and gas is displaced, the number of hours with coal on the margin increases. However, there are

also an increased numbers of hours where the inflexibility in the system produces hours of zero 

marginal prices. This is due to a higher number of hours with curtailment, which causes the 

locational marginal prices to be zero because there are free resources not being utilized.

9

 We do 

not consider the impact of the production tax credit where wind generators bid negative costs. 

Including this effect would increase the benefits of increased coal flexibility on plant revenue. 

The impact of increased flexibility of coal units is to increase the number of hours of non-zero 

energy prices by reducing the hours with curtailment.

Figure 9. Price duration curves for both base case and 

Inc. Coal Flex case for 40% RE penetration 

As a result, the increased generator flexibility results in higher revenues, despite reduction in 

output and the associated decrease in total generation costs. This leads to a somewhat counterintuitive result that in some circumstances reducing output can actually increase revenues for 

fossil generators. The costs and revenues for coal generators are shown in Table 5. 

 

9 As noted previously, a large portion of the curtailed VG resource is held as reserve provision, especially at the 

lower wind and solar penetrations. When all of the energy curtailed can be used as reserves, the energy price does 

not go to zero. This leads to two mechanisms for the increased flexibility of coal to reduce the number of hours that 

there is a zero marginal energy price: (1) reducing the curtailed energy to zero, or (2) allowing VG to hold more 

reserve provision through changes in thermal dispatch. 
17 

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Table 5. Total Revenue and Total Production Costs for all Coal Units in the RE Scenarios for the 

Base and Increased Flexibility Cases

VG 

Penetration

Total Generation 

Cost Revenue Net Revenue Change in Net 

Revenue Due to 

Increased 

Flexibility Base Inc. Coal 

Flex Base Inc. Coal Flex Base Inc. Coal Flex

(%) $M

16 858 857 1459 1454 601 597 -4

21 845 844 1355 1358 510 514 4

27 821 818 1236 1239 415 421 6

32 786 782 1110 1130 324 349 25

36 760 753 1026 1048 266 296 29

40 721 713 932 960 211 248 37

44 676 664 835 866 159 202 43

48 640 627 750 788 110 161 51

50 608 596 700 746 92 151 59

52 579 567 645 680 66 113 47

3.3 Unconstrained Instantaneous Renewable Energy Penetration

The benefits of increased coal plant flexibility appear to largely saturate at the highest renewable 

penetration levels. This is partially due to the instantaneous penetration limit imposed in the base 

case. In the highest renewable scenarios, an increasing fraction of the curtailed energy is due to 

this limit, and increased generator flexibility does little to accommodate further renewable 

penetration. We imposed this limit because the impacts of large scale deployment of nonsynchronous generators on inertia and primary frequency control have yet to be studied in great 

detail. However, relaxing our constraint can provide additional insights into the benefits of 

increased penetration potentially enabled via provision of synthetic inertia, and active power 

control (Ela et al. 2014; Ruttledge et al. 2012). 

We consider a case where VG can provide up to 100% of instantaneous demand.10 All other 

system constraints from the base case are enforced. Two cases were simulated: one with the base 

coal plant flexibility, and one with the increased coal flexibility, which effectively combines two 

system flexibility cases. 

Table 6 summarizes the savings in production cost for all flexibility scenarios examined in this 

report. The first two values are the base case total production cost and savings from the increased 

coal flexibility case, repeated from Table 4. This is followed by the savings from the 100 Max 

RE case and also the case with 100% allowable renewable penetration and increased coal 

flexibility. 

 

10 To achieve 100% penetration of wind and solar would actually require greater than what is needed to meet 100% 

of demand because of the operating reserve requirements, which would need to be derived from curtailed wind and 

solar. 
18 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Table 6. Total Production Costs for Base and 100 Max RE Cases, 

as Well as Both Inc. Coal Flex Scenarios

VG 

Penetration Base Case 

Savings in Each Flexibility Scenario

Inc. Coal Flex 100 Max RE 100 Max RE + 

Inc. Coal Flex

(%) ($M)

16 1423.6 0.0 -0.1 -0.1

21 1306.4 0.6 -0.2 0.5

27 1169.8 2.0 0.0 2.1

32 1072.5 6.1 0.1 5.9

36 1006.4 8.8 0.3 9.3

40 943.0 16.0 0.8 17.2

44 874.3 18.2 1.7 20.9

48 817.0 21.0 1.3 28.1

50 776.0 24.1 7.2 37.1

52 725.9 24.1 10.3 42.4

Savings from the 100% penetration cases do not appear significantly different than the base case

until much higher penetration, simply from the fact that renewables do not exceed 80% 

instantaneous penetration until annual penetration levels reach about 40%. Also of note is the 

small negative value associated with the low renewable cases. Because the model finds a nearoptimal solution, but not necessarily the optimal solution, there can be small differences between 

runs with nearly identical parameters.

3

 This is apparent in the lowest penetrations of the 100% 

Max RE case. While allowing 100% instantaneous penetration does not affect any hours in these 

lowest RE scenarios, the presence of that constraint in the objective function causes a different 

local solution to be found. 

The 100% RE penetration cases have a lower savings than the Inc. coal flex cases in all scenarios

at all VG penetrations. However in the highest penetration scenarios the combination scenario 

has savings that exceed the sum of the two individual cases in isolation. This is because of the 

increased number of hours that coal is at or near minimum generation when higher levels of RE 

are allowed. This combined effect is observed in both curtailment reduction and CO2 emissions 

reductions. 

Figure 10 shows the curtailment rates for all four scenarios, demonstrating a significant reduction 

at the higher levels of curtailment. It shows both the reduced curtailment as a function of 

flexibility (shift downward), and also the corresponding increase in renewable penetration (shift 

rightward). In the highest penetration scenario, the same amount of installed wind and solar 

renewable capacity produces an annual energy penetration of 52.4% in the base case, and 55.4% 

in the combined flexibility case due to reduced curtailment. 
19 

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Figure 10. Curtailment for all flexibility scenarios

This analysis demonstrates that increased generator flexibility can be a very effective option to 

reduce curtailment. Below 45% penetration, the combined flexibility cases reduced curtailments 

by greater than 50%. This reduction (on a percentage basis) falls as the supply/demand mismatch 

of load and renewable generation increases at higher penetration, because there is simply not 

enough load at the correct time (particularly during the spring) to absorb the times when the 

largest amounts of renewable energy are available. At these higher penetrations additional 

flexibility options, such as demand response or storage, may be needed for further curtailment 

reduction.11

The improved dispatch efficiency and lower curtailment in the combined flexibility cases results 

in a significant CO2 emissions reduction potential, illustrated in Figure 11. Overall, up to a 7% 

reduction in CO2 emissions is created, which again indicates the potential importance of grid 

flexibility in high renewable, low carbon grid scenarios. 

 

11 The availability of wind is often highest during nighttime hours, when load is at its lowest over the course of a day 

(GE Energy 2010). Another possible alternative to enabling technologies is a more balanced mix of wind and solar, 

which are often complimentary in their production profiles. 
20 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

 

Figure 11. Avoided CO2 emissions due to increased flexibility 

4 Conclusions

This analysis finds that increasing the flexibility of the electric power system will lead to 

decreased operation costs, especially as VG becomes a larger part of the generation fleet. This 

study looked specifically at changing the minimum generation levels of coal plants. At low 

penetration of renewables, there is little savings from increased flexibility; however, at increased 

penetration, additional flexibility allows increased use of VG and decreased curtailments. 

The study also examined the impact of instantaneous penetration limits of VG. Penetration limits 

can exist due to concerns of system stability and limited ability of some wind and solar 

technologies to provide primary frequency response and inertia. Cases comparing 80% and 

100% instantaneous penetration limits find reduced curtailment in the 100% case, implying 

further examination of the benefits of wind and solar providing active power control and 

synthetic inertia are warranted. 

Finally, the study examined the possible revenue implications for coal plants in a market setting. 

While introduction of zero-cost generation will always decrease revenue in general, changing 

flexibility can impact the decreased revenues. We found that in some cases increased flexibility, 

while decreasing actual generation from the coal fleet, can actually increase revenue. This is 

largely due to reducing curtailment and the number of hours of zero marginal prices. 

While this analysis is focused on a coal-dominated generation mix, we would expect similar 

trends in systems with greater contribution from gas generation. This is because much of the 

avoided curtailment is during short periods with small levels of curtailment. Units with shorter 

minimum run times, such as gas generators, would likely remain committed during many of 

these periods and still be able to accommodate VG more effectively with lower minimum 

generation levels. Further analysis is needed on the actual impact of other generation mixes, as 

well as altering other generator parameters, such as ramp rate or minimum up and down times. 

Insight could also be gained by changing the parameters of a subset of generators, rather than 
21 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

altering the whole fleet as we did in this case study. Also, it is important to understand how these 

changes in either operational practice or enabling upgrades compare to the economic benefit of 

establishing them. This is especially important when comparing different methods for adding 

flexibility to the electric power system.
22 

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