Dynamic simulation of flexibility measures for coal-fired ... · steam of high-pressure preheaters...

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VGB PowerTech 4 l 2020 Dynamic simulation of flexibility measures for coal-fired power plants

Dynamic simulation of flexibility measures for coal-fired power plants

AuthorsDr.-Ing. Marcel Richter*STEAG Energy Services GmbH Essen, GermanyDr.-Ing. Gerd OeljeklausProf. Dr.-Ing. habil. Klaus GörnerChair of Environmental Process Engineering and Plant Design University of Duisburg-Essen Essen, Germany

* The VGB-FORSCHUNGSSTIFTUNG awarded Dr. Marcel Richter with the VGB Innovation Award 2019 for the dynamic modelling of a coal-fired power plant for evaluation of flexibility measures (award category: future-oriented).

Dynamic simulation of flexibility measures for coal-fired power plantsMarcel Richter, Gerd Oeljeklaus and Klaus Görner

Kurzfassung

Dynamische Simulation von Flexibilitätsmaßnahmen für Kohlekraftwerke

Die Flexibilisierung konventioneller Kraftwerke ist eine der zentralen Herausforderungen bei der Transformation des Energiesystems in Rich-tung eines möglichst hohen Anteils Erneuerba-rer Energien an der Stromerzeugung. In diesem Zusammenhang bietet die dynamische Kraft-werkssimulation ein effizientes Werkzeug, um verschiedene Flexibilisierungsmaßnahmen zu analysieren.In diesem Artikel werden ausgewählte Flexibili-sierungsmaßnahmen für kohlegefeuerte Dampfkraftwerke hinsichtlich ihres Flexibilisie-rungspotenzials und der thermodynamischen Auswirkungen auf den Kraftwerksprozess be-wertet. Zu diesem Zweck wurde unter Nutzung der Modelica-Bibliothek ClaRa ein detailliertes dynamisches Kraftwerksmodell aufgebaut. Die erste betrachtete Flexibilitätsmaßnahme ist der Ein-Mühlenbetrieb, in dem die Mindestlast bis auf 10 % reduziert werden kann. Anschließend wird die mögliche Lastflexibilisierung durch die Nachrüstung eines indirekten Feuerungssys-tems bewertet. Hierbei zeigt sich eine Verdoppe-lung der erreichbaren Laständerungsgeschwin-digkeiten. Eine weitere innovative Maßnahme ist die Integration eines thermischen Energie-speichers in den Kraftwerksprozess. In diesem Zusammenhang wird das Integrationskonzept eines Dampfspeichers vorgestellt. Die Ergebnis-se der dynamischen Simulationen zeigen eine Flexibilisierung hinsichtlich einer zeitweisen Reduzierung der (Mindest-) Last durch Beladen des Ruths-Speichers und die Möglichkeit, sehr schnell eine zusätzliche Leistung von 4,3 % zu aktivieren (z.B. zur Teilnahme am viertelstünd-lichen Intraday-Markt). l

1 Introduction

The International Energy Agency predicts an increasing share of renewable energies in worldwide electricity generation from 24 % in 2016 to 30 % in 2022, mainly driv-en by a capacity growth of wind energy and photovoltaics [1]. In Germany, for in-stance, the market penetration of renewa-ble energies has been supported by the Re-newable Energy Sources Act (EEG), result-ing in an increased share of renewable energies in gross electricity generation from 16.8 % in 2010 to 40.1 % in 2019 [2].Due to the volatile character of the weath-er-dependent power generation from re-newable energies, the requirements for a stable and secure grid operation are rising. In the current energy system, mainly dis-patchable power plants based on nuclear, lignite, hard coal and natural gas compen-sate the fluctuating power generation from renewable energies and thereby ensure the stability of the electrical grid. Considering the expected capacity growth of fluctuat-ing renewable energies while simultane-ously reducing the capacity of convention-al power plants, the remaining dispatcha-ble power plant fleet has to meet ever higher flexibility requirements. The flexi-bility of a power plant comprises mainly the following three dimensions:

– minimum load (in % of installed net ca-pacity)

– load change rates (in %/min) and ability to provide control power (in % of in-stalled net capacity)

– start-up costs (in €) and start-up time (in h)

Along with the growing flexibility require-ments, the importance of dynamic power plant simulation increases as dynamic power plant models offer an efficient tool to calculate and evaluate the transient op-erational behavior. A comprehensive re-view from Alobaid et al. regarding soft-ware, applications and objectives of the dynamic simulation of thermal power plants can be found in [3]. In addition, Hü-bel et al. developed a detailed model of a lignite-fired power plant in Modelica/Dymola focusing on process-inherent en-ergy storages to provide primary control reserves [4] and the optimization of start-up procedures [5] complemented by the calculation of lifetime consumption. Liu et

al. [6] and Yan et al. [7] used a model of a coal-fired power plant in the simulation software GSE to improve ramp rates by the utilization of process-inherent thermal storages, e.g. by regulating the extraction steam of high-pressure preheaters and ad-justing the condensate mass flow. Hentschel et al. [8] applied a dynamic model of a coal-fired power plant in Apros to extend secondary control power output and to investigate necessary modifica-tions in the control system. Gottelt et al. [9] used a dynamic power plant model in Modelica/Dymola to optimize the unit con-trol regarding the impact of sooth blow-ing on the power output whereas Meinke et al. [10] focused on the evaluation of life-time consumption during start-up proce-dures.

2 Dynamic power plant model

2.1 Model descriptionThe dynamic power plant model was built up using the component library ClaRa (Clausius-Rankine) in the simulation envi-ronment Dymola. ClaRa is a free of charge and open source library of power plant components written in the modeling lan-guage Modelica. The library allows de-tailed modeling and simulation of coal-fired power plants as well as of heat recov-ery steam generators, giving deep insight into their dynamic behavior [11].For the buildup of a detailed dynamic pow-er plant model an extensive information base regarding design data and control structures is necessary to set e.g. geometric dimensions and control parameters and thereby to achieve accurate transient simu-lation results. For this reason, the coal-fired power plant Voerde (Unit A) was cho-sen as a reference, as a substantial informa-tion base was available within the scope of the joint research project “Partner steam power plant” [12]. This information base includes all necessary design data and con-trol structures for the model buildup as well as measurement data to ensure a vali-dation of the dynamic simulation model. Although the selected flexibility measures are evaluated for a specific reference power plant in this article, the qualitative results regarding flexibilization potential and thermodynamic effects are transferable to other thermal power plants.

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Dynamic simulation of flexibility measures for coal-fired power plants VGB PowerTech 4 l 2020

F i g u r e 1 shows the process flow diagram of the dynamic simulation model. The wa-ter-steam cycle includes steam turbines, condenser, condensate pump, preheaters, feed water tank, feed water pump and sev-eral valves and pipes. Besides these compo-nents, a detailed dynamic model of the steam generator with a 1-dimensional dis-cretization was built up consisting of four coal mills, four burner levels, nine radia-tive and convective heating surfaces with

collectors and distributors, three injection coolers and one regenerative air preheater. The steam generator is a once-through boiler, switching to forced circulation in load points < 40 %. Hence, also a steam separator, a start-up bottle and a circula-tion pump is modeled, as illustrated in the process flow diagram in F i g u r e 1 .Besides the power plant components, also the control structures (e.g. unit control, feed water control, steam temperature con-

trol and recirculation control) are consid-ered in the dynamic power plant model, based on the real implementation in the underlying reference power plant.

2.2 Model validationIn order to validate the dynamic power plant model, the simulation results are compared to measurement data. The load profile chosen for the validation is charac-terized by negative load changes from 80 % part load down to minimum load in the night hours and load changes vice versa from minimum load up to nearly full load in the morning hours. The minimum load of 25 % is determined by the firing system in two-mill operation (four-mill operation in full-load). A reduction of the minimum load could lead to the occurrence of unsta-ble combustion. Thus, further reducing the firing rate is not possible, while further re-ducing the load within the water-steam cycle is, as considered in Section 3.1 (one-mill operation) and Section 3.3 (integra-tion of a thermal energy storage).The trajectory of the power output, shown top left in F i g u r e 2 , is mainly influenced by the unit control. The target power out-put is the only time-dependent input vari-able in the dynamic power plant model. The results of the dynamic simulation model show a high correlation to the meas-ured values in stationary load points and also during transient load changes. Some dynamic fluctuations in the power output

SH-5 RH 2IC 2 IC 3

SH 4IC 1

RH 1

SH 1-3

HP-T IP-T

Eco

Allocator

Classifier

Coal mills HPP 7 HPP 6

Feed-waterpump LPP 3 LPP 2 LPP 1

FWT Condensatepump

LP-T G

Con

dens

er

Evap

orat

or

Circ

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art-u

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Fig. 1. Process flow diagram of the dynamic power plant model.

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Target power outputMeasurement power outputSimulation power output

Measurement live steamSimulation live steamMeasurement inlet economizerSimulation inlet economizer

Measurement live steamSimulation live steamMeasurement outlet evaporatorSimulation outlet evaporatorMeasurement inlet economizerSimulation inlet economizer

Measurement outlet circulation pumpSimulation outlet circulation pumpMeasurement injection coolingSimulation injection cooling

Fig. 2. Validation of the dynamic power plant model: Comparison of measured and simulated parameters.

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are underestimated by the dynamic power plant model as switching processes (e.g. the start-up of two coal mills between 5:00 and 6:00) are modelled in a simplified manner and boundary conditions (e.g. the heating value of the hard coal) are con-stant over the considered period.The time curves of the simulated mass flows at inlet of economizer and live steam (top right in F i g u r e 2 ) also show a high accuracy in comparison to the measured values. In load points < 40 % the steam generator switches from once-through to forced circulation operation. In this period between 1:00 and 5:00 the feed water is not completely evaporated at inlet of the steam separators. The separated liquid wa-ter then flows into the start-up bottle and is recirculated to inlet of the economizer by a circulation pump. Thus, this period is char-acterized by a constant mass flow at inlet of economizer while further reducing the live steam mass flow. The time curves of the re-circulation mass flow also show a good ac-cordance, regarding the start and end point of the recirculation mode and also the recirculated mass flow rate (bot-tom right in F i g u r e 2 ). The accurate modelling of live steam temperature con-trol is proven by comparing the accumu-lated cooling water mass flow of IC1 and IC2.Furthermore, the validation of water-steam temperatures (bottom left in F i g -u r e 2 ) proves the accuracy of the detailed dynamic steam generator model. The time curves of the temperatures at inlet of econ-omizer, outlet of evaporator and live steam show a high correlation. Due to the switch of the steam generator from once-through to forced circulation mode, the economizer inlet temperature rises between 1:00 and 5:00 in measurement data and simulation results.In summary, the comparison of simulated and measured values shows a good accord-ance in stationary load points and during transient load changes. The detailed dy-namic power plant model reaches a simula-tion speed of 20x real time. Consequently, the model allows reasonable investigations about flexibility measures, as presented in the next Section.

3 Flexibility measures

3.1 One-mill operationThe minimum load of coal-fired power plants is around 40 % in once-through op-eration (benson point) and around 25 % if forced circulation mode is possible for con-tinuous operation. A further minimum load reduction in two-mill operation is typically limited by the combustion system, in particular the control range of the coal mills and the dust load of the primary air mass flow. However, a minimum load re-duction below 25 % is possible by switch-

ing to a one-mill operation. Such one-mill operation has been successfully demon-strated in the past years at the power plants Bexbach, Heilbronn (Unit 7) and Mannhe-im (Unit 8) [13] [14]. Another typical limi-tation of the minimum load is the mini-mum flow rate through the heating sur-faces of the steam generator and the steam turbine stages, which is around 8 to 10 % of the nominal mass flow rate [15]. Also the minimum flue gas temperature of around 280 to 300 °C at the inlet of the DeNOx plant has to be considered [16].In order to identify the achievable mini-mum load reduction and the thermody-namic effects on the power plant process, a reduced minimum load in one-mill opera-tion is simulated with the dynamic pow-er plant model. F i g u r e 3 shows the time curves of the net power with the mini-mum load point in two-mill operation (blue) and in one-mill operation (orange). By applying a one-mill operation, a mini-mum load reduction from 25 % to 10 % is possible.Ta b l e 1 summarizes the main process pa-rameters in full load and in minimum load with two-mill operation as well as with one-mill operation:The live steam temperature decreases from 530 °C in two-mill operation to 490 °C in the reduced minimum load point in one-mill operation. Similarly, the temperature of the hot reheated steam decreases from 505 °C to 460 °C. The 15 K higher econo-

mizer inlet temperature in one-mill opera-tion at 10 % minimum load can be ex-plained by a higher circulating mass flow rate to ensure the minimum evaporator mass flow rate of 260 kg/s. The live steam mass flow rate of 72 kg/s is still slightly above the typical minimum mass flow rate of 8 to o10 % of the steam turbines and heating surfaces.The flue gas temperature at the outlet of the economizer or at the inlet of the DeNOx plant decreases from 318 °C in the mini-mum load point with two-mill operation to 300 °C in the optimized minimum load point in one-mill operation, still above the typical limit of around 280 to 300 °C.

3.2 Indirect firing systemIn coal-fired power plants typically a direct firing system is installed, in which the mix-ture of primary air and coal dust at the out-let of the coal mills flows directly to the burner levels. Depending on design, coal properties and wear condition, the inertia of the grinding process in the coal mills can be approximated with a transfer function (PTn element) with transition times of sev-eral minutes. Thus, the inertia of the grind-ing process has a direct impact on the achievable load change rates and thereby on the flexibility of the entire power plant process.An improvement in load change rates and power plant flexibility can be achieved by installing or retrofitting an indirect firing

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Target power outputPower output (two-mill operation)Power output (one-mill operation)

two-mill operation

one-mill operation

Fig. 3. Power output with minimum load in two-mill operation (blue) and one-mill operation (orange).

Tab. 1. Process parameters in different load points.

Parameter Full load in four-mill operation (100 %)

Minimum load in two-mill operation

(25 %)

Minimum load in one-mill operation

(10 %)

Live steam mass flow 600 kg/s (100 %) 150 kg/s (25 %) 72 kg/s (12 %)

Live steam temperature 530 °C 530 °C 490 °C

Hot reheat temperature 530 °C 505 °C 460 °C

Evaporator mass flow rate 564 kg/s 260 kg/s 260 kg/s

Circulation mass flow rate --- 120 kg/s 210 kg/s

Temperature inlet economiser 250 °C 234 °C 259 °C

Temperature outlet evaporator 395 °C 300 °C 300 °C

Flue gas temperature inlet DeNOx 375 °C 318 °C 300 °C

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system (F i g u r e 4 ). In the indirect firing system, primary air and coal dust are sepa-rated by mechanical or electrical separa-tors at the outlet of the coal mills and the coal dust is temporarily stored in a dust silo. By extracting the coal dust from the silo by a rotary valve, the inertia of the decoupled milling process can be ig-nored, resulting in higher load change rates. In addition, according to [17], also the control accuracy and the coal dust quality is improved with an indirect firing system.F i g u r e 5 shows the step response of the net power output when changing the target power output. In contrast to the nominal power plant operation, the maximum per-missible rate of change (gradient limita-tion in unit control) is deactivated during this dynamic simulation study.The comparison of the two time curves shows a significantly faster response in the case of the indirect firing system. The step response of the indirect firing system can be approximated by a PT2 element with a time constant of 40 seconds. The direct fir-ing system shows a higher inertia, which can be approximated by a PT3 element with a time constant of 60 seconds. In the case of the indirect firing system some dynamic fluctuations in the power output occur, which can be explained in particular by subordinate control loops that are not set optimally (e.g. feed water control).During nominal power plant operation, where the improved dynamic behaviour is considered within the control system, the power plant process with an indirect firing system shows approximately a doubling of the achievable load change rate.

3.3 Integration of a thermal energy storage

Applications of thermal energy storagesIn accordance with [19] the integration of a thermal energy storage (TES) is one pos-sible measure to enhance the power plant flexibility. Depending on points of integra-tion, storage technology and storage capac-ity a TES can have various effects on the flexibility parameters. In general, a TES integration leads to a (partially) decou-pling of firing rate and net power. This de-gree of freedom can e.g. be used to adjust the net power at a constant firing rate. Fur-thermore, an integrated TES can enhance the provision of control power and improve start-up procedures.Regarding the storage integration at ther-mal power plants, a widely used applica-tion are sensible heat storages based on molten salts at concentrated solar power (CSP) plants such as at the Andasol solar power plant (Aldeire, Spain) [20] and the Gemasolar solar power plant (Fuentes de Andalusia, Spain) [21]. In the context of combined heat and power (CHP) plants, thermal energy storages with water under ambient pressure conditions are more and more used in combination with district heating systems, e.g. the Lausward power plant (Düsseldorf, Germany) and at the GKM power plant (Mannheim, Germany) [22]. With both technologies, molten salt storages and non-pressurized water stor-ages, high storage capacities > 1,000 MWhth and long storage times in the range of sev-eral hours can be realized.By contrast, steam accumulators provide limited storage capacities, but high power outputs and very good dynamic properties [22]. Due to their promising short-term dy-

namic behavior, the integration of a steam accumulator into the power plant process is considered in this article. Currently, Ruths storages are mainly installed to buff-er imbalances between steam generation and steam demand, e.g. in textile industry, metal manufacturing and tobacco process-ing [23]. In the context of power genera-tion, Ruths storage systems are mainly in-stalled to provide saturated steam which is directly flowing to a steam turbine, like in the solar tower plant PS 10 [24].

Integration and operation conceptRuths storages are characterized by a slid-ing pressure operation and the release of saturated steam when discharged. F i g -u r e 6 shows the integration concept of such a Ruths storage into the reference pro-cess.During discharge-mode saturated steam flows out of the storage vessel and replaces the extraction steam of the first high-pres-sure preheater (HPP 6) at a pressure of around 20 bar in power plant full load. Re-placing this extraction steam leads to an ad-ditional steam mass flow in the IP- and LP-turbine and thereby to an increased net power. Hence, the plant-specific overload capacity of this turbine sections has to be verified, especially during discharge-mode in full load operation. The maximum per-missible discharge mass flow is limited by the extraction steam to HPP 6, which is 50 kg/s in full load and 10 kg/s in minimum load in the case of the reference process.Charging of the Ruths storage is realized with steam from the cold reheat at around 40 bar and 310 °C in full load. Thus, the pressure in the storage vessel can also rise up to 40 bar. As the pressures within the water-steam cycle decrease in part load, an additional integration point in the live steam line – with a fixed pressure of 80 bar in load points < 40 % – is necessary. This enables charging in minimum load up to 40 bar and subsequent discharging in full load. Charging the Ruths storage with steam from the cold reheat or live steam line leads to a reduced mass flow within the reheating surfaces and the affected tur-bine sections. Thus, a minimum mass flow rate has to be considered to ensure a steady flow and cooling of the reheating surfaces and to avoid ventilation in the steam tur-bine. For the dynamic simulation studies, a charging mass flow of 50 kg/s is assumed, in accordance with the maximum dis-charge mass flow to HPP 6 in full load. Dur-ing charging-mode the mass flow rate in the IP- and LP-turbine is thereby reduced to 93 % (charging in full load, cold reheat) and to 18 % (charging in minimum load, live steam), which is still sufficient high compared to the typical value for the mini-mum mass flow mentioned in Section 3.1.

Design dataThe Ruths storage integrated into the pow-er plant process is designed for a charge

Allocator Allocator

Classifier Classifier

Primary air Primary air

Coal mill Coal millSecondary air Secondary air

Separator

Dustsilo

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Fig. 4. Direct (left) and indirect (right) firing system [18].

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Target power output .Power output direct firing systemPT3 (T=60s)Power output indirect firing systemPT2 (T=40s)

Fig. 5. Step response of the power plant with direct (left) and indirect (right) firing system.

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and discharge time of 30 minutes. Thus, the evaluated storage concept can be clas-sified more as a power storage than as a capacity storage. With the chosen storage time of 30 minutes and the mass flow dur-ing charging and discharging of 50 kg/s, the utilizable water mass can be calculated to 90,000 kg. Taking into account an upper pressure limit of 40 bar (pressure of cold reheat in full load), a lower pressure limit of 20 bar (pressure of extraction steam to HPP 6 in full load) and a maximum liquid water level of 90 %, a storage volume of 1,331 m3 is determined. Ta b e l e 2 gives an overview of the operational parameters and the design data of the steam accumula-tor integrated into the water-steam cycle of the reference power plant.Besides the storage vessel, also connection pipes between the integration points in the water-steam cycle and the Ruths storage have to be installed, each equipped with a control valve. These valves at inlet and out-let of the storage vessel need to be con-trolled appropriately by the power plant control system. In particular, the unit con-trol has to be modified and a subordinated control loop for the adjustment of the net power during charging/discharging has to be implemented.

Dynamic simulationsF i g u r e 7 shows the results of the dynam-ic power plant model during a discharge process at a constant fire rate of 100 %. The additional net power of 4.3 % is realized by opening the outlet valve of the Ruths stor-

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Fig. 6. Process flow diagram of the dynamic power plant model with integrated Ruths storage.

Tab. 2. Design data of the integrated Ruths storage concept.

Operational parameter Discharging in full load

Discharging in minimum load

Charging in full load

(cold reheat)

Charging in minimum load (live steam)

Steam mass flowHeat flow

kg/sMWth

50136

1027

50136

50136

Design steam accumulator

Pressure rangeStorage volumeStorage capacityUsable steam mass

barm3

MWhth

kg

20 - 401,331

6890,000

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Mass flow outletRuths storage

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Fig. 7. Process variables during a discharge process of the Ruths storage at a constant firing rate of 100 %.

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age within 60 seconds, leading to an outlet mass flow of saturated steam which tempo-rarily replaces the extraction steam to HPP 6. The load of 104.3 % can be kept al-most constant for about 23 minutes. Dur-ing this time period the pressure in the Ruths storage decreases due to the out-flowing steam and energy. Thus, the con-trol valve position increases and reaches full opening after 23 minutes. Subsequent-ly, the mass flow of saturated steam at out-let of the Ruths storage as well as the ad-ditional net power decrease until the pres-sure in the Ruths storage is equivalent to the pressure of the extraction to HPP 6 af-ter about 60 minutes.Charging the Ruths storage with steam from the cold reheat at a constant firing rate of 100 % leads to a decrease in net power of about 5 percentage points to 95 % (see F i g -u r e 8 , left). The pressure in the steam ac-cumulator increases. After 23 minutes the pressure difference between Ruths storage and cold reheat is again so small so that the control valve at inlet of the storage tank is fully open and the inlet mass flow as well as the impact on the net power is decreasing.Taking into account reproducible and con-stant initial values after each storage cycle, the round trip efficiency (power to power) of the storage system can be calculated to be 83 %:

The round trip efficiency of 83 % is a high value for energy storage systems. This high value can be explained by the moderate pressure difference between cold reheat and extraction steam to the first high-pres-sure preheater, leading to relatively small exergy losses.Charging the Ruths storage with live steam in minimum load leads to a temporarily re-duction of the electrical minimum load from 25 % to 18 %. As the live steam has a fixed pressure of 80 bar in load points < 40 %, the pressure difference to the

steam accumulator is sufficiently high to enable a constant reduction of the net pow-er over the entire period of 30 minutes (see F i g u r e 8 , right).If the Ruths storage is charged with live steam in minimum load and discharged to HPP 6 in full load, the round trip efficiency decreases to 61 % (due to the higher exergy of live steam), which is still a reasonable value for energy storage systems.

Based on the identified potentials on the net power, summarized in Ta b l e 3 , an exemplary load profile was simulated with the dynamic power plant model (see F i g -u r e 9 ). Between 0:15 and 0:30 (1) the Ruths storage is discharged, leading to an additional power output of 4 % and a de-creasing pressure in the storage vessel. During minimum load operation the stor-age is charged with live steam twice from 2:15 to 2:30 (2) and from 4:00 to 4:15 (4),

resulting in an increasing pressure in the storage vessel. Thus, the electrical mini-mum load can be reduced temporarily to 18 % for these two quarters of an hour. Be-tween 3:00 and 3:45 (3) the steam accu-mulator is discharged, taking into consid-eration the limited potential regarding outflowing steam mass flow and impact on the net power in minimum load (see Ta -b l e 3 ). At the time the power plant reach-es full load (5), a discharge process of the Ruths storage is performed, demonstrating the ability of charging the storage in mini-mum load (low spot market prices) and discharging the storage in full load (high spot market prices). In addition, two dis-charge and two charge periods of 15 min-utes each are assumed during the follow-ing full load operation (6), illustrating the potential of arbitrage trading on the quar-ter-hourly intraday markets.

4 Conclusion and Outlook

The load flexibility of coal-fired power plants becomes increasingly important due

Time in min

Net

pow

er /

firin

g ra

te in

%

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

Net

pow

er /

firin

g ra

te in

%

Time in min

-5 0 5 10 15 20 25 30 35 40 45 50 55 60



100

99

98

97

96

95

94

28

26

24

22

20

18

16

Fig. 8. Net power output during a charge process of the Ruths storage at a constant firing rate of 100 % (left, cold reheat) and 27.5 % (right, live steam).

Tab. 3. Operational parameters of the integrated Ruths storage concept.

Operational parameter Discharging in full load

Discharging in minimum load

Charging in full load

(cold reheat)

Charging in minimum load

(live steam)

Steam mass flowHeat flowΔ net powerRound trip efficiency

kg/sMWth

% %

50136

+4.3 %

1027

+0.9 %

50136

-5.0 %83 %

50136

-7.0 %61 %

Time in h

Net

pow

er in

%

100

80

60

40

20

00:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00

Set point net power (reference process)Net power (reference process)Net power (with integrated ruths storage)

1

2 3 4

5

6

Fig. 9. Net power of the reference process and the power plant with integrated Ruths storage during a typical load profile for coal-fired power plants.

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to the transformation of our energy system towards a high share of renewable energies with their weather-dependent and fluctuat-ing electricity generation. Thereby, dynam-ic simulation models are gaining in rele-vance as they offer a powerful helping tool to evaluate different flexibility measures. This article presents the successful buildup of a physical based dynamic simulation model for a coal-fired steam power plant with the open source library ClaRa in the modeling language Modelica and the simu-lation environment Dymola. The dynamic simulation model includes a detailed wa-ter-steam cycle and steam generator model together with the most relevant control structures. The validation of the dynamic simulation model shows a good accord-ance to measurement data of the underly-ing reference power plant with a high sim-ulation speed of about 20x real time.The dynamic power plant model was then used to evaluate selected flexibility meas-ures. Ta b l e 4 summarizes the main re-sults concerning the achievable flexibiliza-tion potential. By applying a one-mill op-eration, the minimum load can be reduced from 40 %/25 % to 10 %. Installing an indi-rect firing system can enhance the flexibil-ity of the power plant process by means of doubling the achievable load change rate. Furthermore, the integration of a steam ac-cumulator into the water-steam cycle en-hances the load flexibility and the short-term dynamics of the power plant. The in-tegrated Ruths storage can be used to temporarily reduce the minimum load from 25 % to 18 % (charging in minimum load) and to supply additional net power of 4 % in power plant full load (discharging in full load). Also a highly dynamic arbitrage trading on the quarter-hourly intraday markets as well as the supply of control power is possible with the integrated en-ergy storage.All in all, the dynamic simulation results show the feasibility of dynamic power plant simulations to evaluate different flex-ibility measures and thereby to provide de-tailed information for the highly dynamic power plant operation in electrical grids with a high share of fluctuating renewable energies.Further research will be done on the poten-tial of the selected flexibility measures with

a dynamic power plant model based on the study of the “Reference Power Plant North Rhine-Westphalia” [25] with live a steam temperature of 600 ° C and a net efficiency of around 46 %, representing the current state-of-the-art for modern coal-fired steam power plants. Also the flexibilization of Combined Cycle Power Plants (e.g. by storage integration) will be part of further research.

Acknowledgments

The results presented in this article have been achieved within the scope of the re-search projects “Partner steam power plant for the regenerative power generation” and “FLEXI-TES – Power Plant Flexibility by Thermal Energy Storage” funded by the German Federal Ministry for Economic Af-fairs and Energy (project ref. no. 03ET7017G/03ET7055G). The authors would also like to thank STEAG GmbH for the acquisition of design and process data of the reference power plant.

Nomenclature

AbbreviationsCHP combined heat and powerCSP concentrated solar powerEco economizerEEG German Renewable Energy Sources ActFWT feed water tankHP-T high-pressure turbineHPP high-pressure preheaterIC injection coolerIP-T intermediate-pressure turbineLP-T low-pressure turbineLPP low-pressure preheaterRH reheaterSH superheaterTES thermal energy storage

SymbolsΔ differenceη efficiency, %P power, W

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[4] Hübel M, Prause J, Gierow C, Meinke S, Hassel E. Simulation of ancillary services in thermal power plants in energy systems with high impact of renewable energy, In: Pro-ceedings of the ASME 2017 Power and En-ergy Conversion Conference, June 26-30, 2017, Charlotte, USA.

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Tab. 4. Comparison of flexibility parameters affected by the TES integration.

flexibility parameter

initial state potential (flexibility measure)

minimum load 40.0 % (once-through operation)25.0 % (forced circulation mode)

10.0 % (one-mill operation)18.0 % (integration of TES)

maximum load 100.0 % 104.3 % (integration of TES)

load change rate ±1.3 %/min ±2.5 %/min (indirect firing system)

capability of load change at constant firing rate (e.g. for short-term intraday trading)

---

+4.3 % / 15.4 MWhel (discharge of TES)

-5.0 % / 18.6 MWhel (charge of TES, cold reheat)

-7.0 % / 25.1 MWhel (charge of TES, live steam)

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[16] Lens, H.; Nolte, M.: Absenkung der Genera-tormindestlast von Steinkohlekraftwerken durch regelungstechnische und verfahren-stechnische Maßnahmen, VGB PowerTech, 4 ,41-46, 2015.

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The electricity sector

at a crossroads

The role of renewables energy

in Europe

Power market, technologies and acceptance

Dynamic process simulation as an engineering tool

European Generation Mix

Flexibility and Storage

1/2

2012




The electricity sector

at a crossroads

The role of

renewables energy

in Europe

Power market,

technologies and

acceptance

Dynamic process

simulation as an

engineering tool

European

Generation Mix

Flexibility and

Storage

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2012

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