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# The Increased Cost of Cycling Operations at Combined Cycle

Power Plants TP203

November 2012

Prepared By

Steven Lefton

Nikhil Kumar Doug Hilleman

Dwight Agan

Intertek APTECH Sunnyvale, CA USA

Presented at the International Conference on Cycling Plant

Charlotte, North Carolina

November 2012

## The Increased Cost of Cycling Operati ons at Combined Cycle Power Plants

Presented at the International Conference on Cycling Plant

Charlotte, North Carolina, November 2012

Steven Lefton

Nikhil Kumar Doug Hilleman

Dwight Agan

Intertek APTECH Sunnyvale, California

1. Abstract

The increased cycling operations (on/off and load following) at combined cycle power plants in the United States will increase the cycling-related produc tion and maintenance costs at these plants. The increased cycling is primarily the result of additio nal variable generation of renewable wind and solar megawatts. Additional influences include market prici ng for gas fuel and additional emission restrictions on existing coal-fired units. The resulting penetrati on of renewables into the North American grid is increasing the number of on-off and load cycling opera tions, which increase these combined cycle plants’ damage and, thus, maintenance costs. This will in crease the need for spinning reserve megawatts, their costs, and the startup charges for putting combined cycle plants online. The number and the desire for

faster online times for market conditions increases the da mage severity of the gas turbine starts. They are increasing thermal transients with more rapid gas tu rbine acceleration and higher mass gas flows at higher exhaust temperatures that are reaching the heat recovery steam generators (HRSG). These factors impact the gas turbine and the HRSGs, as well as the balan ce of plant, plant water chemistry, and these ultimately reduce the overall plant reliability. The av erage starts on these gas turbines/combined cycle are increasing and the gas turbines/co mbined cycle run times are generally decreasing. Owners and operators, as well as regulators, need to be aware of the increased costs th at occur. While capacity factors decrease, the production costs will also rise significantly due to cycling operations. This is a discussion of the negative impacts of cycling operations on a comb ined cycle power plant. In order to quantify these impacts, a new real-time costing code, Cost Adviso r, which was developed by Intertek APTECH for a Original Equipment Manufacturer (OEM) of gas turbin es to provide improved cycling cost information to owners and operators of combined cycle and cogene ration power plants, in order to provide improved cycling cost information to owners and operators of combined cycle and cogeneration power plants will be presented. An overview of the cost of cycling will also be presented and the lower-bound of the cost of cycling range of a limited number of th ese studies will be discussed and quantified.

2. Introduction

Every time a power plant is turned on and off, the gas turbine, HRSG or boiler, steam lines, steam turbines, and auxiliary components go through unavoi dably large thermal and pressure stresses, which cause damage and incur additional future costs. This damage is made worse by the phenomenon that we call fatigue and creep-fatigue inte raction damage. These cycling -related costs and damages areattributable to current startup, shutdown, and rapid load following, but they have not yet crystallized and occurred in newer combined cycle and fossil plants. Th erefore, we have to estimate and compare them to similar combined cycle plants with longer histories, where we have estimated start/stop and load following costs, the so-called cycling costs. Typical start definitions vary with size, and damage increases associated with these operations as shown in Figure 1.

Figure 1 Definitions of Hot, Warm, and Cold Starts for Combined Cycle Plants.

During the startup and shutdown of combined cycle pow er plants, there are operational constraints that are designed to avoid damage to the combustion turbin e, steam turbine, and to a limited extent, the HRSG components. In the case of the HRSG, the start up and shutdown maximum permissible ramp rates are primarily determined by the gas turbine startup ramp rates that approach 3000 ° F/hour when measured on a minute basis during their occurrence, and these rapid rates significantly affect and reduce the components creep-fatigue life and increase the str esses in the HRSG and gas turbine hot section components. The ultimate result is that these combined cycle units incur significant increases in cost of maintenance and increase in forced outages due to cy cling that would not impact a base loaded unit.

In our work for National Renewable Energy Labo ratory (NREL), Western Electricity Coordinating Council (WECC), and the Department of Energy on the impact of the wind and solar renewable penetration of the western United States grid, we observe a significant increase in starts as well as cycling cost on combined cycle power plants. Figure 2A represents the number of starts (multiplied by unit capacity) for combined cycle units, which start more with increasing renewable penetration. Figure 2B represents the total increase in cyclin g cost of combined cycle units in the western United States grid. These costs are developed based on Intertek APTECH’s lower bound cost submission to NREL and

WECC. 

# Generation Unit Cycling

Definitions

Load Cycling

LL1 Lowest Load at Which Design SH/RH Temperatures

can be Maintained

LL2 Current “Advertised” Low Load LL3 Lowest Load at Which the Unit can Remain On-Line (CC-60%) AGC & Load cycling to 60% 1-2 hours

offline 2-4 hours off line 4 hours or more Fossil

Steam

CCFigure 2A shows the number of starts (multiplied by unit capacity) for various renewable penetration scenarios. 1

Figure 2B shows the estimated total cycling cost of combin ed cycle units for increasing penetration of renewables. 2

, 3

This has also been reported by Strategic Power Sy stems in their data on the service hours per start of F-Class gas turbine machines at 33 hours per start in 2007, 38 hours per start in 2008, and increasing to 52 hours per start in 2009. The 2009 data indicates increasing run times per start corresponding with a significant decrease in the gas fuel prices. 4 As a result of this expected increase in cycling of combined cycle power plants, we believe it is necessary to caref ully examine the effect of cycling on the entire combined power plant and analyze how these cycles aff ect the costs due to increased starting and load following. We have observed that an estimated 30% re newable penetration on the grid will result in over a 100% increase in total cycling cost for combined cy cle units alone (see Figure 2B). The bottom line is that many combined cycle plants w ill see a steady diet of increased starts, lower capacity factors, and increasing cycling costs.

3. Effects of the Gas Turbine and Cycling on the Combined Cycle and Heat Recovery Steam

Generator

HRSGs of many new combined cycle units have also been experiencing early cycling-related failures because the combined cycle plant has been designed primarily to attain high thermodynamic performance at base load ratings and not for cycling operati ons until around 2005 when the newer combined cycle plants have been moderately improved for cycling oper ations. However, these plants have yet to prove their abilities in long-term cycling services. While th ese combined cycle plants are designed at a lower

1 The Western Wind and Solar Integration Study Phase 2, D. Lew, G. Brinkman, E. Ibanez, B.-M. Hodge, M. Hummon, A. Florita, and M. Heaney NREL

2 Power Plant Cycling Costs - N. Kumar, P. Besuner, S. Lefton, D. Agan, and D. Hilleman, Intertek APTECH Sunnyvale, California - NREL

3 Analysis of Cycling Costs in Western Wind and Solar Integration Study – GE Consulting

4 The Value of Reliability Data – A look back... A view of the future of an evolving generation mix. Salvatore A. Della Villa, Jr. Strategic Power Systems, Inc.cost to install than coal-fired units, they require more costly overhauls and repairs. Intertek APTECH has discussed these cycling-related issues affecting HRSGs in five Electric Power Research Institute (EPRI) reports and an American Society of Mechanical Engineers (ASME) paper:

1. “Heat Recovery Steam Generator Tube Failure Manual,” Electric Power Research Institute, EPRI Technical Report 1004503, Final Report (November 2002).

2. “Delivering High Reliability Heat Recovery St eam,” Electric Power R esearch Institute, EPRI Technical Report 1004240 (March 2003).

3. “Interim Nondestructive Examination Guidelines for HRSG,” Electric Power Research Institute, EPRI Technical Report.

4. “Evaluation and Correction of Thermal Transi ent and Cycle Chemistry Influenced HRSG Failures,” Electric Power Research Institute, EPRI Technical Report.

5. “Effects of Flexible Operations on Turbines and Generators,” Electric Power Research Institute, EPRI Technical Report 100351, Final Report, Stephen Hesler, PM (December 2004).

6. “Analysis of Cycling Impacts on Combined Cycle Heat Recovery Steam Generators and Evaluating Future Costs of Countermeasures to Reduce Impacts,” ASME Power 2008-60026, July 22-24, Orlando, Florida, USA.

The effects of this additional cycling on the incr eases in wear and tear and associated increases in maintenance costs, equivalent forced outage rat es, heat rates, and production costs are not well understood by utility decision make rs, regional independent system operators, and public utility

commissions. The interactions and tradeoffs of cost s, reliability, and cycling effects are also not well known. In order to properly manage the assets of a po wer plant, a plant operator and asset manager has to obtain reasonably accurate estimates of the full cost s of their typical operations, especially of its operational cycles and start types in order to bid the co rrect costs in an increasingly cycling environment. Only then can you maximize your plant’s value, reduce operational damage, recover your cycling costs with proper cost knowledge in bidding, and re duce both high-impact/low probability and frequent

outages.

4. Heat Recovery Steam Ge nerator Damage and Failures

HRSG tube failures are very costly due to cycling, as space is limited for repairs, and often it is cheaper to replace the section rather than cut your way into a se ction of the HRSG, make repairs, and weld your way out. Common cycling-dominated power plant failur es occur mainly in the HRSG tubing and the HRSG casing. Superheater and reheater sections mainly suff er from thermal quenching transients due to water in headers from improperly sized/designed drains and fro m improperly operated/maintained attemperator spray systems that generally result in tube-t o-header failures, as shown in Figure 3.

Evaporator tube failures occur due to overheating and excessive deposition of iron and other chemical deposits that get formed and transported during cycl ing operations. Economizer tube failures occur from a variety of issues including: thermal fatigue due to quenching hot tubing, economizer steaming, external

corrosion, and flow assisted corrosion, as shown in Fi gure 4. HRSG casing can be subjected to thermal fatigue and the effects of corrosion from air and fu el born corrosion, as well as improper water washing the gas turbine and carry over to the HRSG.Figure 3 Superheater/Reheater Tube Fa ilure/Cracking at Header.

Figure 4

Flow-Assisted Corrosion at HRSG Low Pressure Drum.

4.1. HRSG Header Borehole Cracking and Drum Penetration Cracking The HRSG ramp rates are limited and often constrained by HR SG water chemistry conditions and the rate of heating and cooling of the thicker-wall pressure pa rts. However, when the temperature of metal, steam, or water rates change from 1500 to 3000 ° F/hour due to the gas turbine exhaust, the gas thermal ramp rate is extremely rapid and very significant wh en calculated using the temperatures as measured by their rates of change in less than 1-hour increments in which they actually occur. This rapid heatingcauses changes in temperature of the inner surface of the pressure part. The temperature differences develop through the thickness of the pressure part wall. This through-wall temperature gradient persists until stable steam or water temperat ure conditions are reestablished, a nd then time elapses for the inner surface temperature to stabilize. Further time is necessary to bring the entire wall thickness to a new stable temperature close to that of the inner surf ace. These transients drive the establishment of a temperature gradient through the wall of the com ponent. The hotter inner surface wants to expand relative to the cooler bulk temperature of the header, but the material is restrained from doing so, causing the inner surface to develop a compressive thermal stress while the coolest parts experience a tensile stress. This process results in cracking and more of this type of cracking has been observed on heavily cycled units and is controlled by the transient and the creep-fatigue interaction properties of the

components.

Headers are heated and cooled predominantly by heat transfer at their inner wall surfaces, tube bore hole surfaces, and pipe nozzle penetrations. Steam drum s are heated and cooled predominantly by heat transfer at their inner wall surfaces, as are downcomer/riser pipe/steam outlet pipe nozzle surfaces, and feedwater nozzle penetrations. These high stresses and cracks, especially during shutdown, are well described in many papers. 5

Superheater and reheater attemperators have been doc umented as a routine cause of high temperature tube failures and header cracking from thermal shock damage as these are cycled in r outine load following and on/off cyclic operation.

4.2. HRSG Tube-to-Header Cracking in Economizer, Superheater, and Reheater Due to the rapid gas turbine temperature rise of the g as turbine exhaust into a cold or warm HRSG, there is very uneven heating of large tube banks at very rapid rates noted above, especially when compared to conventional fossil power plant heat-up rates, t ypically at much lower temperatures of 500 to

1500 ° F/hour. The effects of cycling, and especially c ondensate that is not removed from these tube banks from the previous shutdown, make the uneven h eating cause very high tube-to-tube temperature differences leading to differential expansion tube-to-h eader creaking and failures. These effects are very damaging and we have written extensively on the eff ects and the resulting tube-to-header failures, as shown in Figure 3, and as discussed in the ASME pa per, “Analysis of Cycling Impacts on Combined Cycle Heat Recovery Steam Generators and Eval uating Future Costs of Countermeasures to Reduce Impacts.” These failures are in part because of the in ferior creep-fatigue properties, such as those of T91 tubing, generally found in th e high temperature sections of these HRSG units (Figure 5).

5. Effect of Cycling on GT Operating Costs

During every startup and shutdown and in load cyclin g to minimum load of the gas turbine, the hot-gaspath components sustain significant thermal cycling. These thermal cycles frequently shorten the lives of these parts and also reduce the inspection and overhaul intervals required for the engine as compared with an engine running continuous duty (base load). This shortening of the life of the parts and overhaul intervals increase operating costs in two ways. First, more parts have to be replaced during the overhauls and inspections, so-called parts fall out, which increase the costs of the overhaul. Often at the hot gas path inspection, parts are found to be damaged and are not repairable. For example, when hot section parts crack (thermal barrier coating crack) due to cycling, the cracks often proceeds into the base metal (see Figure 6). Figure 7 is an example that shows th e effects of gas turbine bl ade coating cracking that

5 “ Optimization of F-Class Combined Cycl e Unit Starts and Shutdowns Based on HRSG High Pressure Superheater Header Tubehole Cracking Avoidance Constraints”, M. Pe arson of J. Michael Pearso n and Associates Co Ltd., J. Grover and S. Paterson of Aptech Engineering Serv ices, R. Anderson of Competitive Power Resources, and B. Dooley of Structural Integrity Associates.proceeds into the base metal. The part is not repairab le and must be replaced at a cost not expected or generally included in most long-term service agreemen ts. We have seen these very significant costs for the replacement of 10 to nearly 100% (depending on the number of cycles, the thermal barrier coatings, and the gas turbine manufacturer) of the blades and van es in the first three rows of the gas turbine hot

section and the combustor. In addition, there are the compressor thrusting an d compressor-to-case rubs that have increased the number of total gas turbine catastrophic failures that has been shown to be startrelated. Secondly, because the costs of these replacem ent parts and total overhauls are disbursed earlier in time, there is an additional cost due to the time value of money.

Reference Intertek APTECH analysis of Yukio, Takahashi’s “Follo w Up Investigation on Creep-fatigue Evaluation of Various Alloys ”

Figure 5 Creep-fatigue Interaction Curve.

Figure 6 Gas Turbine First Stage Blade Thermal Barrier Coating Cracking Extending into Base Platform.Figure 7 Cross-section View of Pressure Side Airfoil Cracking of Surface Coating Extending into Vane Base Metal of a Gas

Turbine Blade.

6. Steam Turbine and Generator Effects of Cycling

Figure 8 shows severe turbine nozzle block fatigue cracking and case cracking from a heavily cycled steam turbine. In order to predict damage in a steam turbine, gas turbin e, or HRSG, known damage mechanisms must be correlated to temperatures and pr essures in real-time and alarmed for the operators. The correlation of high pressure rotor bore surface temper atures and their ramp rates to thermal stresses are shown in Figure 9. Lacking a detailed stress anal ysis model, we have applied a statistical model to thermal transient data stress output from the manufact urer’s on-board turbine thermal stress analyzer.

Our stress model had a 0.97 correlation coefficient and, t hus, a nearly perfect fit to actual plant data that shows increased stresses with increased megawatt ramp rates. Intertek APTECH then combined the temperature-to-stress statistical model with its other cost models developed during a cost of cycling project. This allowed us to estimate the effects of changing thermal ramp rates on damage and, ultimately, on costs. More details are reported in th e EPRI report “The Effects of Flexible Operations on

Turbines and Generators.”Figure 8 Actual Nozzle Block Cracking.

Figure 9 Large Turbine Rotor Stress Shown as a Function of Megawatt Ramp Rate.7. Effects of increased cycling on the Generator

Increased generator failures and insurance claims have been noted and generators which were previously not a concern are now reported to be the number three in insurance damage claims. 6 With the increased starts, we have seen an increase in the generator lead failure, hydrogen leakage, seal oil in the generator stator, and the need to rewedge and rewind generators in cycling duty. Lube and seal oil ingress into generators cause wedges and windings to become loose as stator iron gets very hot with oil pluggage, causing reduced localized cooling of the stator core . The hotter core iron then expands causing areas of the core iron to loosen its tight compaction. Piece s of the loose stator core iron have been dislodged, eroded, and in some areas fatigued-off, causing these small pieces to act like magnetic termites and short windings. All these mechanisms cause high-impact/low -probability events to become more common in cycling units with long outages and high repair costs.

8. Balance-of-Plant Effects of Cycling

In addition, there is significant damage in the bala nce of plant that is caused by 1) cycling-related chemical upsets in feedwater and boiler water, 2) equipment corrosion during offline and startup periods, 3) excessive deposits, buildup, and under-deposit corro sion in the boiler, e.g., corrosion transport from air-cooled condensers to the HRSG, and 4) other tran sient behavior causing damage during starts, load cycling, and shutdowns.

We observe more switchgear, transformer, and large mo tor failures due to cycling operations. The impact of cycling on NO

x control system and ammonia injection grids causes ammonium bisulfate to participate out on the HRSG tubing, causing more starts a nd offline cleaning to remove the deposits.

Piping systems valves and check valves are particular ly susceptible to cracking and failure when multiple unit 2:1, 3:1 combined cycle plants with multiple HRSG s are started up, and cooler steam is blended into an operating hot piping system. Piping systems need to be subjected to transient stress analysis and brought up to much higher standards to include these transient effects. All of these component failures

cause increased costs.

9. Top-Down Method for Estimating Cycling Costs

Intertek APTECH has found that accurate estimates of total unit damage costs can be derived using a regression analysis of historical unit damage with historical costs and equivalent forced outages, along with component-specific data that indicate the br eakdown of cycling costs among various cycle types (e.g., hot, warm, and cold starts, load follows). In tertek APTECH’s damage model is intended to provide information on the cycling-related damage for the enti re unit. It is founded on physical models and uses plant temperature and other real-time thermocouple data(we call this “signature data”) to provide crossvalidation and calibration relative to the megawatt changes, but requires only hourly megawatt operational data to estimate damage. Relying solely on hourly megawatt unit load data is an inherent advantage due to the fact that these types of data are more readily available. In addition, hourly megawatt data provide an accurate history of past unit opera tions. Our damage model accounts for creep damage, fatigue damage, erosion, corrosion, and all other t ypes of damage that are known to occur in most combined cycle power plants.

Intertek APTECH’s damage model validation pro cess has included the assessment of key components with finite element analysis and creep/fatigue analys is methods to determine remaining useful component

6 Reference Combined Cycle Journal.life, augmented by information from thousands of la boratory failure and stress analyses of power plant components. These life analyses of key high cycling co st components are statistically calibrated to the failure history of these components. Then all damage is calibrated to actual plant costs and these costs must back predict or back cast historical costs to be validated. Traditional, uncalibrated engineering fatigue and creep analyses, however ambitious and expe nsive, are rarely useful and are often misleading in predicting cycling costs.

Real-time, minute-by-minute thermocouple data (or “c ycling signature data”) provide information about the damage mechanisms and accumulation rates that may occur as a result of cycling operations. The signature data are used to calibrate, revise, and/or confirm the results of the general damage and cost

models described above. Th is is done by using the signature data as input to analytical models, as well as to qualitative “expert opinion reviews” for the ev aluation of cycling-related damage during load transients. This helps to calibrate and confirm the susceptibility of a given unit to cycling-related damage, as reflected in capital and maintenance spending and cost per cycle calculated using the top-down and bottom-up regression techniques.

Critical components for which detailed plant signature data are analyzed include the steam drums, water wall/evaporator tubing, superheater and reheater tubing and headers, economizer inlet, and startup system components, as well as turbine/generator-related com ponents, such as valves, cases, generator windings, and steam chests. The maximum temperature ramp rate and the overall range of temperature change experienced by a component during the transient are key indicators of cycling-related creep and fatigue damage. These parameters are used to quantify the severity of each unit’s load, startup, and shutdown transients. The signature data are also used in evaluating and troubleshooting each unit’s cycling operations, leading to recommendations for temperature ramp rate limits, e.g., superheater ramp rates, economizer ramp rates in startup and during shut down/cooling, and other operational improvements aimed at minimizing damage, maximizing the asset’s lif e and reliability, and reducing maintenance costs.

Intertek APTECH uses a statistical regression model to develop best estimates of the total combined cycle equipment damage costs due to typical cycling, which include incremental equivalent forced outage rates, capital, and maintenance costs. These costs are first estimated for an equi valent hot start used to describe each cycle type, and then estimated for the typical cy cles experienced by the specific units studied based on how they are operated. Figure 10 shows the results of this type of regressi on analysis typically run with and without equivalent forced outage costs to sep arate the forced outage effects. This, and our other correlations of starts to plant outages, confirm our assertions that cycling causes increased forced outages.Figure 10 Best Estimate Forced Outage, Maintenance, Operation, and Capital.

10. Bottom-Up Method for Estimating Cycling Costs

The “top-down” method described above provides a measure of total cost of cycling based on historical spending of the whole plant without regard for sp ecific components. The “bottom-up” method seeks the relative distribution of those costs as a means of apportioning the total costs and providing a sound methodology for evaluating potential countermeasures. By identifying the major costs of damage to components that currently contribute to the wear-and-tear elements of cycling costs, the calculated cost of cycling can also be adjusted in the future. Impr ovements identified reflect improvements made through identification of problem components with high costs and forced outage rates. Root cause analysis, hardware fixes, and changes in operating practic es, component replacements, and upgrades can be implemented. The cycling costs estimated using the bottom-up method also serve as a “sanity check” on the unit-wide costs derived in the top-down method. When we also perform the bottom-up analysis, it is generally within 5% to 10% of the costs derived from the top-down method, which provides validation of our top-down methodology. Figure 11 gives an example of the distribution of cycling damage costs among the major plant systems of a combined cycle unit. Best Estimate of Smoothed Plant X Units 2, 3, 4, Gas Turbine Portion of the Combined Cycle Plant including Forced Outage, Maintenance, Operation and Capital Costs

$0K $5,000K $10,000K $15,000K $20,000K $25,000K $30,000K $35,000K $40,000K

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013

YearAnnual Outage, Maintenance, Operationand Capital Costs (in 2009 currency) Smoothed Annual Candidate Costs in Year 2009 Dollars Best Fit of Annual Costs (Results in $XX per Equivalent Hot Start and a COV=5%)

A djusted for major overhauls and assumes 474 EHS per year for cycling the three gas turbines in the futureFigure 11 Total Operations and Maintenance Costs for Combined Cycle.

11. Results of the Cycling Costs

The overall range of cycling costs compared with co mmonly assumed costs is show n in Figure 12, which includes all cycle types of hot, warm, and cold star ts for the three unit types of small drum, large supercritical, and gas turbines. The unit’s specific analysis results depend heavily on the regression analysis of the costs versus cycles and the unit signatu re data during cyclic opera tions, including the range of all load changes. The increased incremental costs that are attributed to cycling are broken down into the following categories:

1. Increases in maintenance and overhaul capital expenditures

2. Forced outage effects, including forced out age time, replacement energy, and capacity

3. Cost of increased unit heat rate, both long-term efficiency, and while at low/variable loads

4. Cost of startup fuel, auxiliary power, chemicals, and extra startup manpower

5. Long-term generation capacity cost increases due to unit life shortening

Figure 11 gives an example of the breakdown of main tenance and overhaul cycling costs for a combined cycle power plant. Cost ranges of the lower-bound of studies that we have performed are significant. We published the lower-bound range of cost for combin ed cycle plants in the publication for NREL. 7

7 Impact of Wind and Solar on Emissions and Wear and T ear of Fossil-Fueled Generato rs, D. Lew, G. Brinkman, N. Kumar, P. Besuner, D. Agan, and S. Lefton.Figure 12 Typical Cycling Costs.

12. COSTCOM Methods Properly Predict Future Damage

Intertek APTECH’s methodology, first developed in 1989 for fossil power plants, correlates plant operations with past damage and then calibrates real component costs to these damages. We make the effort to verify that the cost models accurately “pre dict” past costs so that they can quantify and predict future costs more accurately. When possible, we estimate cycling cost using at least two different methods with independent investigators (e.g ., top-down regression methods vs. bottom-up cost accounting and in plant studies). The major combined cycle plant components are broken down by costs in Figure 11 and separate damage models are develope d from analysis of plant signature data collected from the plant data acquisition systems. Cost calib rations are executed in all of these major component

and cost areas.

12.1. Description of Real Time Operations The developed code COSTCOM and our combined cycle Cost Advisor developed in 2012 for an OEM’s combined cycle power plants, is a real-time softwa re system that allows power plant operators and managers to see the comprehensive costs of their cycling operation within 1-minute of the current operation as shown in Figures 13 and 14). It is a PC -based program which gathers data from the existing plant information system for through-the-plant network. The system includes three modules: 1) an engine that calculates the results based on the plant inputs of some 15-20 variables in real time, 2) a viewer that displays the real time and historical results, and 3) an administrator which is used for changing the display settings, such as the alarm level or localization setti ngs for language, local currency, units of measure, temperature/ramp rate limits in green ( no rmal), yellow (caution), and red (do not exceed). 8

8 Cost Advisor and COSTCOM User Manual. 

## “Quantify True Unit Cost Per Cycle” 

# Estimated Cycling Costs Often Wrong

Small Drum Large Supercritical GT Simple Cycle GT Comb. Cycle 

### Unit Type

$5,000

$10,000

$100

$200 

### Typical Industry Value

(without consideration of true costs)

$3,000 - $100,000

$15,000 - $500,000

$300 - $80,000 $15,000 - $150,000 

### Potential Range

of Total CostFigure 13 Combined Cycle Cost Advisor Main Screen.

Figure 14 Cost Advisor Cost Screen Page 2 Showin g a Start and its Cost in Real Time.Based on the key damage process indicators (DPI) for equipment health showing temperatures, pressures, flow rates, stress levels, and power outputs from the critical plant components (e.g., gas turbines, HRSGs, and steam turbine generators), it classifies the current cycling transient type that is occurring and the damage and other costs being incurred during that tran sient. The relationship between the measured key variables and the damage and costs are determined by c odification of an extensive cost of cycling study of the plant using the methodologies described above. Once that relationship is determined, certain parameters in the computational “engine” are customi zed for that unit so it can accurately determine the damage and cost of current operation from the key DPI being monitored in real time. The basic principle is that given the current plant design, past cycling dama ge at past cycling rates of change have given past historical costs and the damage models must be calibrated via multivariable regression analyses to these past costs in order to predict future damage and costs.

COSTCOM also shows the level of overall plant damage costs per day being incurred during a cycling (or any other) operation. From this, the operator can dete rmine if the unit can be “pushed” to get online and produce megawatts faster (when the costs being incurred are low and the dispatch “rewards” are high) or if the unit’s ramp rates should be reduced because damage levels ar e too high relative to marketplace rewards. For example, the main screen first provides alerts to the operator with colored bar displays to indicate high temperatures and high rate of change of DPI, such as megawatt, stresses, temperatures, and ramp rates in normal (green), normal (yellow cautio n), and alarming (red do not exceed) levels for the overall unit damage being incurred. Secondly, it shows the responsible key measured variables (temperatures, ramp rates, flows, and stress levels) fo r the high damage and real time costs. It can also give advance warning of damage and allow an operator to avoid a damaging transients and it encourages the operators to stay in the green box of normal temp eratures and ramp rates. In studies of combined cycle units, we have had operators reduce the cycling damage as measured by signature data as much as 30-50% during starts and trips/shutdowns by revised procedures and minor changes to plant hardware. This will have the effect on costs and damage rates in the future which will also be reduced by the 30-50% reduction in damage per cycle.

13. Conclusion

Combined cycle plants will bear an increased burden of cycling operation and costs due to the increase in wind and solar integration into the grid. Thus, combin ed cycle plants not only n eed to know these costs, but also control their cycling costs as well as recove r them in the electricity markets. These costs dominate the typical plants variable cost of operatio ns and maintenance that should be broken down into cycling operations and maintenance and base loaded operations and maintenance. As part of the developments we have performe d and published in “Impacts of Re newable Generation on Fossil Fuel Unit Cycling: Costs and Emissions” 9 , we have presented (Figure 15) the lower-bound cost from our analysis of a large number of combined cycle units’ cy cling costs. We believe most plants built before 2005 far exceed these lower-bound costs, and plants built after 2005 are still in the honeymoon stage with low accumulated damage and may still exhibit si milar lower-bound and beyond the lower-bound with significantly elevated cycling costs in the future.

9 G. Brinkman, D. Lew, National Renewable Energy La boratory, N. Kumar, S. Lefton, Intertek APTECH,

Venkataraman, G. Jordan, GE Energy Schenectady.Figure 15

Lower-bound Hot, Warm, and Cold Starts, Maintenanc e and Capital Cost per Megawatt Capacity (Including Outliers).601 West California Avenue Sunnyvale, CA 94086

408.745.7000 Fax 408.734.0445

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