通过集成热能存储来提高燃煤电厂的负荷灵活性外文文献

Contents lists available at ScienceDirect Applied Energy journal homepage Improving the load flexibility of coal fired power plants by the integration of a thermal energy storage Marcel Richter Gerd Oeljeklaus Klaus G rner Chair of Environmental Process Engineering and Plant Design University of Duisburg Essen Leimkugelstra e 10 45141 Essen Germany H I G H L I G H T S A detailed dynamic simulation model for a coal fired power plant is developed The integration of a steam accumulator into the water steam cycle is presented Charging the energy storage leads to a minimum load reduction of up to 7 0 Discharging the energy storage leads to an additional net power of up to 4 3 The integrated energy storage enhances the frequency control supply by 2 8 A R T I C L E I N F O Keywords Dynamic simulation Coal fired power plant Power plant flexibility Primary control reserve Thermal energy storage Steam accumulator A B S T R A C T Improving the flexibility of conventional power plants is one key challenge for the transformation of the energy system towards a high share of renewable energies in electricity generation Flexible and dispatchable power plants will contribute to this ongoing transformation process as they compensate the fluctuating electricity generation from renewable energy sources such as wind and photovoltaics In this context dynamic simulation models offer an efficient tool to evaluate flexibility measures and the resulting highly transient power plant operation In this paper the buildup of a dynamic power plant model using the modeling language Modelica in the simulation environment Dymola is presented The detailed dynamic power plant model is validated suc cessfully against measurement data from the underlying coal fired reference power plant The paper then focuses on the integration of a steam accumulator also known as a Ruths storage into the power plant process in order to increase its flexibility The results of the dynamic simulations show that charging the steam accumulator leads to a reduction of the net power up to 7 0 By discharging the Ruths storage an additional net power of 4 3 can be activated very quickly Thus the storage integration leads to an improved load flexibility with regard to a temporary reduction of the minimum load as well as to the possibility of performing a load change at a constant firing rate e g to participate on the quarter hourly intraday markets Furthermore the integrated energy storage enhances the provision of primary control reserve significantly by 2 8 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 driven by a capacity growth of wind energy and photovoltaics 1 In Germany for instance the market penetration of renewable energies has been supported by the Renew able Energy Sources Act EEG resulting in an increased share of re newable energies in gross electricity generation from 16 6 in 2010 to 33 3 in 2017 2 According to the German Federal Government this share is intend to increase further towards a target of 80 in 2050 3 Due to the volatile character of the weather dependent power generation from renewable energies the requirements for a stable and secure grid operation are rising In the current energy system mainly dispatchable power plants based on nuclear lignite hard coal and natural gas compensate the fluctuating power generation from renew able energies and thereby ensure the stability of the electrical grid Considering the expected capacity growth of fluctuating renewable https doi org 10 1016 j apenergy 2018 11 099 Received 5 September 2018 Received in revised form 31 October 2018 Accepted 27 November 2018 Corresponding author E mail address marcel richter uni due de M Richter Applied Energy 236 2019 607 621 Available online 11 December 2018 0306 2619 2018 Elsevier Ltd All rights reserved T energies while simultaneously reducing the capacity of conventional power plants the remaining dispatchable power plant fleet has to meet ever higher flexibility requirements The flexibility of a power plant comprises mainly the following three dimensions as also stated in 4 minimum load in of installed net capacity load change rates in min and ability to provide control power in of installed net capacity start up costs in and start up time in h From the perspective of a power plant operator enhancing the flexibility parameters results in a higher profitability of a specific power plant e g due to reduced losses in minimum load operation or addi tional revenues on intraday and control power markets From the perspective of the energy system flexible power plants reduce the number of units necessary to ensure the stability of the electrical grid Furthermore flexible power plants enable the integration of more wind and solar power by avoiding wasteful curtailment of renewable en ergies leading to a more efficient energy system 5 This is especially the case for power systems characterized by high shares of electricity generation from conventional power plants Along with the growing flexibility requirements the importance of dynamic power plant simulation increases as dynamic power plant models offer an efficient tool to calculate and evaluate the transient operational behavior of existing or planned power plants A compre hensive review from Alobaid et al regarding software applications and objectives of the dynamic simulation of thermal power plants can be found in 6 In addition H bel et al developed a detailed model of a lignite fired power plant in Modelica Dymola focusing on process in herent energy storages to provide primary control reserves 7 and the optimizationofstart upprocedures 8 complementedbythe calculation of lifetime consumption Liu et al 9 and Yan et al 10 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 pre heaters and adjusting the condensate mass flow Hentschel et al 11 applied a dynamic model of a coal fired power plant in Apros to extend secondary control power output and to investigate necessary mod ifications in the control system Gottelt et al 12 used a dynamic power plant model in Modelica Dymola to optimize the unit control regarding the impact of sooth blowing on the power output whereas Meinke et al 13 focused on the evaluation of lifetime consumption during start up procedures The previous studies have a detailed dynamic model of a specific reference power plant and a focus on the optimization of an existing power plant process in common The novelty of the research presented in this article which is also based on a detailed dynamic model of a specific power plant lays in the integration of an additional thermal energy storage to further improve load flexibility Scope and purpose of this work is to evaluate the short term dynamic power plant operation while charging and discharging the integrated heat storage Regarding the heat storage integration into thermal power plants most research and applications are in the area of combined cycle gas turbine CCGT plants as well as concentrated solar power CSP plants Wang et al 14 for example focused on the flexibilization of a CCGT plant using a model in Aspen Plus introducing the integration concept of a latent heat storage Angerer et al 15 presented the integration of a sensible heat storage for the thermal decoupling of gas turbine and heat recovery steam generator during startups and shutdowns of CCGT plants A widely used application are sensible heat storages based on molten salts at CSP plants such as at the Andasol solar power plant Aldeire Spain 16 and the Gemasolar solar power plant Fuentes de Nomenclature Abbreviations CCGTcombined cycle gas turbine CHPcombined heat and power CSPconcentrated solar power Ecoeconomizer EEGGerman Renewable Energy Sources Act FWTfeed water tank HP Thigh pressure turbine HPPhigh pressure preheater ICinjection cooler IP Tintermediate pressure turbine LP Tlow pressure turbine LPPlow pressure preheater NRWNorth Rhine Westphalia PRCprimary control reserve RHreheater SHsuperheater SRCsecondary control reserve TESthermal energy storage TRCtertiary control reserve Greek symbols water level difference efficiency density kg m3 Mathematical symbols henthalpy kJ kg i jcount variables mmass kg ppressure bar Ppower W Qheat W ttime s vspecific volume m3 kg Vvolume m3 Subscripts 1begin 2end condcondensation elelectrical evapevaporation ininlet liqliquid outoutlet ththermal vapvapour M Richter et al Applied Energy 236 2019 607 621 608 Andalusia Spain 17 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 dis trict heating systems e g at the Simmering power plant Vienna Austria the Lausward power plant D sseldorf Germany and at the GKM power plant Mannheim Germany 18 With both technologies molten salt storages and non pressurized water storages high storage capacities 1000 MWhthand high storage times in the range of several hours can be realized By contrast steam accumulators provide limited storage capacities but high power outputs and very good dynamic properties 18 For this reason the integration of a steam accumulator into a coal fired power plant is considered within the scope of this paper as a Ruths storage integration promises further optimization of the short term dynamic behavior especially regarding fast reaction times of the net power when charging or discharging the storage vessel Currently Ruths storages are mainly installed to buffer imbalances between steam generation and steam demand e g in textile industry metal manu facturing and tobacco processing 19 In the context of power gen eration Ruths storage systems are mainly installed to provide saturated steam which is directly flowing to a steam turbine like in the solar tower plant PS 10 20 or the steam storage power plant Berlin Char lottenburg with two separate storage turbines of 20 MWeleach 21 no longer in operation A deeper analysis of applications of thermal en ergy storages in the context of power generation and heat supply can be found in 22 Contrary to the direct use of the discharged saturated steam in an additional storage turbine or a downstream consumer this paper pre sents the integration of a Ruths storage into the high pressure pre heating line of a large scale coal fired power plant This innovative integration concept promises fast load changes of the steam turbine without waiting for the dynamic response of the steam generator when changing the firing rate The storage integration is evaluated by de tailed dynamic power plant simulations during highly transient char ging and discharging processes which so far has not been addressed in literature in such a level of detail and temporal resolution Therefore the paper is structured as follows Section 2 describes the buildup and the validation of the dynamic power plant model In Section 3 the investigated integration concept of a Ruths storage into the water steam cycle is introduced including operational limitations and the design of the steam accumulator for the case of the underlying reference power plant process Section 4 presents the results of the dynamic power plant simulation and the identified flexibilization po tential concerning the possibility of adjusting the net power at a con stant firing rate and the improvement of primary control reserve by the storage integration Section 5 provides a short conclusion and an out look on further research The main findings of this work which is based on a specific power plant process are transferable to other steam power plants and thereby are expected to provide a detailed reference on the integration of a steam accumulator into a steam power plant to improve short term dynamic behavior 2 Dynamic power plant simulation 2 1 Simulation software The dynamic power plant model presented in this paper was built up using the component library ClaRa Clausius Rankine in the si mulation environment Dymola ClaRa is a free of charge and open source library of power plant components written in the modeling language Modelica The library allows detailed modeling and simula tion of coal fired power plants as well as of heat recovery steam gen erators giving deep insight into their dynamic behavior 23 The ClaRa library is structured component wise including models for pumps fans turbines heat exchangers furnaces coal mills valves pipes as well as storage tanks and flue gas cleaning units The library provides these component models at different levels of detail sup porting the user in creating system models tailored to their specific needs The advantage of this concept is that the physical precision of a complex power plant model can be adapted to cope with the given si mulation task without an unnecessary excess of computing time The component models are validated against literature or measurement data of existing power plants Moreover the ClaRa library uses robust and fast media data for water CO2and gaseous flue gas and air mix tures from a free of charge version of the TILMedia library In addition special heat transfer correlations and radiation models are available for the flue gas path inside of a detailed dynamic steam generator model 2 2 Dynamic power plant model For the buildup of a detailed dynamic power plant model an ex tensive 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 simulation results For this reason the coal fired power plant Voerde Unit A was chosen as a reference in this paper as a substantial information base was available within the scope of the joint research project Partner steam power plant 24 This information base includes all necessary design data and control structures for the model buildup as well as measurement data to ensure a validation of the dynamic simulation model Table 1 summarizes the key parameters of the power plant Voerde and the dynamic power plant model respectively 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 unstable combustion Thus further reducing the firing rate is not possible while further reducing the load within the water steam cycle is as considered in Section 3 focusing on the in tegration of a thermal energy storage Fig 1 shows the process flow diagram of the dynamic simulation model The water steam cycle includes steam turbines condenser condensate pump preheaters feed water tank feed water pump and several valves and pipes Besides these components a detailed dynamic model of the steam generator with a 1 dimensional discretization in flow direction was built up consisting of four coal mills four burner levels nine radiative and convective heating surfaces with collectors Table 1 Key data of the dynamic power plant model Key datagross electric power730MWel net electric power695MWel minimum load 25 steam generatortypeonce through forced circulation 40 single reheating modified sliding pressure live steam mass flow600kg s live steam parameters187bar 530 C reheated steam parameters36bar 530 C M Richter et al Applied Energy 236 2019 607 621 609 and distributors three injection coolers and one regenerative air pre heater 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 circulation pump is modeled as illustrated in the process flow diagram 2 3 Modelling approach The power plant component models in the ClaRa library are based on fundamental physical equations As an example the modelling ap proach of a water storage vessel with a non ideal interaction between the vapor and liquid phase is presented This is especially the case in condensers preheaters feed water tanks and start up bottles as wells as in the steam accumulator integrated into the power plant process in Section 3 The dynamic mass balances 1 and 2 consider evaporation and condensation mass flow rates volume changes and mass flows through inlet and outlet ports The dynamic energy balances 3 and 4 con sider inflowing and outflowing enthalpy flows as well as energy transport between the phases due to heat and mass transfer Heat transfer occurs in the case of a temperature difference between vapor and liquid phase thermodynamic disequilibrium Furthermore heat exchange with the steel mass of the surrounding wall is considered as illustrated in Fig 2 t Vmmmm V t d d d d i N i j N j liq liq 1 in liq 1 out liq condevap liq liq inout 1 t Vmmmm V t d d d d i N i j N j vap vap 1 in vap 1 out vap condevap vap vap inout 2 Fig 1 Process flow diagram of the dynamic power plant model h tm mhmhmhmhp V t V p t hV t V t QQ d d 1 d d d d d d d d i N ii j N jj liq liq 1 in liq in 1 out liq out condevap liq liq liq liq liqliq liq liq liq phaseswall liq inout 3 h tm mhmhmhmhp V t V p t hV t V t QQ d d 1 d d d d d d d d i N ii j N jj vap vap 1 in vap in 1 out vap out cond evap vap vap vap vap vapvap vap vap vap phaseswall vap inout 4 Fig 2 Modelling approach of non ideal phase interaction between two phases 25 M Richter et al Applied Energy 236 2019 607 621 610 2 4 Power plant control system Besides the dynamic behavior of the power plant process e g due to mass and energy storage also the set points and control variables calculated in the control system have a major impact on the transient power plant operation Hence in addition to the power plant compo nents the following control structures are considered in the presented dynamic power plant model based on the real implementation in the underlying reference power plant unit control feed water co