The Application of Molten Salt Energy Storage to Advance the Transition from Coal to Green Energy Power Systems

energies

Article The Application of Molten Salt Energy Storage to Advance the Transition from Coal to Green Energy Power Systems

Wojciech Kosman * and Andrzej Rusin

Department of Power Engineering and Turbomachinery, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]

Received: 9 April 2020; Accepted: 28 April 2020; Published: 2 May 2020

Abstract: The paper presents technical solutions for a power grid that undergoes the elimination of a significant number of coal-based power generating units. The purpose of the solutions is to adapt the existing machines with sufficient lifespans to the new operating conditions. In particular these include steam turbines. The steam turbines’ cycles may be extended with energy storage systems based on a molten salt. This allows to increase the flexibility of the power generating units while maintaining the largest possible efficiency of the power generation. The solutions presented here allow to connect the steam turbines cycles to renewable energy sources and reduce the overall number of the units that create the fundamental layer of the power grid. The analysis of the solutions involves numerical modeling. The paper describes the assumptions and the results of the modeling for chosen cases of the modernization. The researched considered a number of options that differed in the investment costs and the resulting performance.

Keywords: energy storage; steam turbine; ﬂexibility; renewable sources

1. Introduction According to the current policies in the power generation industry the energy systems should increase the share of the renewables in the generation of the electric power. A transition from a coal-based system to a green energy one is a complex and a very challenging task. It should be conducted with the upmost respect to the local operating conditions of the machines. A key issue is how to introduce the renewables into the power grid while maintaining its stability. There are two principal issues in that matter. The first one is how to build a reliable foundation for the power grid with the lowest possible harmful impact on the natural environment. The presence of the foundation is necessary due to the specific way the renewable energy sources operate, which depends strongly on the environmental conditions. This is especially important for wind and solar energy. For safety reasons power grids must be able to compensate for any drops in the power generated by the renewable energy sources. In the countries where power grids have relied on fossil- fueled plants it is natural and economically justified due to the investment costs paid so far to maintain the foundation in the current form at least until the return of the investment costs is secured [1]. Here the foundation means the power units with large power supercritical steam turbines. Limiting the fossil fueled plants to the foundation does not exclude their optimization with respect to the criterion of the lowest possible harmful impact on the natural environment. Modern power cycles with steam turbines are designed for operation under a wide range of loads. The minimal load is defined at between fifty and forty percent of the full load. However partial load operation means lower efficiency of the electric power generation, which causes high emissions that harm the natural

Energies 2020, 13, 2222; doi:10.3390/en13092222 www.mdpi.com/journal/energies Energies 2020, 13, 2222 2 of 18 environment. Due to the nature of renewable energy sources and the need to compensate for the drops in their power generation the systems must operate with large load amplitudes during short periods (days) and long periods (months, quarters). This translates into frequent partial load operation and frequent shut-downs preceded by start-ups after short stops. There is also a second issue related to the transition from a coal-dominated grid to a green energy system. A significant number of the power cycles does not satisfy the environmental requirements but include machines with the remaining lifetime that still allows further operation. It means that some of the existing machines could be adopted to operate in the transformed power grid. A solution to the two issues defined above is the usage of energy storage systems. The main feature of these systems is that they make the power generating units independent of the current power demand in the grid, at least to some extent. When connected to the foundation of a power system they stabilize the operating conditions. The power generating units may then operate under a full load with the highest possible efficiency while the surplus of energy generated over the current demand is stored. The surplus may later be used when the demand increases. There are a number of different energy storage types. They differ in size and in the complexity of the loading and the unloading processes. Reference [2] is focused on thermal energy storage technologies that provide a way of valorizing solar heat and reducing the energy demand of buildings. The principles of several energy storage methods and calculations of storage capacities are described. Research presented in [3] concerns the capacity value of the energy storage metrics to quantitatively estimate the contribution of energy storage to the generation adequacy. Work [4] presents a methodology for the optimal design and operation of cogeneration system with thermal energy storage. An electrical and thermal system dispatch model based on combined heat and power with thermal energy storage and demand response is proposed in [5]. In [6], an experimental analysis of different methods of determination of the state of charge of thermal storage tank based on established sensor technologies tested using a lab-scale heat exchanger is shown. The efficiency of the energy storage in in compressed air and hydrogen was analyzed in [7]. An experimental study on the temperature distribution and heat losses of a molten salt heat storage tank was presented in [8]. The research described here is based on energy storage in a molten salt. Technology of this type is used in countries with sufficient solar irradiance to store the solar energy [9]. Molten salt energy receivers are commercially available [10]. The systems with the molten salt are usually connected to steam turbine cycles [11]. They may complement fossil-fueled boilers [12] and expand their flexibility [13] or provide the energy source for a cycle with a working fluid other than steam [14]. Since the molten salt may be heated up to over 600 ◦C the stored energy allows one to generate steam even for supercritical steam turbines [15]. Steam generators fed with molten salt were the subject of optimization [16] and so were the steam cycles including the generators [17]. The purpose of this research was the adaptation of the available energy storage technologies to the transition process of a power system mentioned earlier. New technical solutions are suggested here for the integration of the energy storage and the existing steam turbine cycles. An improvement in the power generation efficiency is achieved because the operation of a steam cycle that is a foundation of a power system becomes independent of the current power demand in the grid. The research is conducted on the most efficient method for the connection between a steam cycle and an energy storage installation. The following sections of the paper describe the details of the suggested solutions and the results of their numerical simulations. The simulations involved a model of the off-design operation of the existing machines. They allowed us to determine the efficiency of the power generation for the modernized cycles.

2. Assumptions As mentioned in the Introduction the foundation of a power system is a number of supercritical steam turbine units. In this work the numerical simulations were performed for a 460 MW supercritical Energies 2020, 13, 2222 3 of 18 steam turbine with rather low levels of the live steam pressure and temperature. A unit of this type is one of the smallest supercritical units typically installed in conventional power plants. Technical solutions verified for this unit may easily be applied to larger turbines. The purpose of the investigation was the optimization of the operation under large fluctuations of the load due to the necessity to compensate the power generated by renewable energy sources. The approach towards the optimization involves the combination of the supercritical steam turbine cycle and a subcritical steam turbine cycle. The subcritical turbine chosen here is a modernized 13K225 unit, which remains operational after the removal of a fossil-fueled steam boiler. Both turbines are located in the same power plant. It is assumed that the technical state of the subcritical turbine allows it to operate for a period that guarantees the return of the modernization costs. The evaluation of a remaining life time requires a detailed analysis for each specific machine. The values of the basic parameters that describe both turbines are given in Table1.

Table 1. Comparison of the steam turbines under the investigation.

Parameter Unit Supercritical Turbine Subcritical Turbine nominal power MW 460 217

life steam temperature ◦C 560 535 life steam pressure MPa 27 12.7 life steam ﬂow rate t/h 1300 668

reheat steam ◦C 580 535 temperature

The cycles of the two turbines are connected through an energy storage system with a molten salt. All the simulations presented here were performed for a so-called solar salt, which consists of 60% sodium nitrate and 40% potassium nitrate. This fluid is commercially available, proven and applied in existing installations. There have been a number of works on the thermal properties of the solar salt. The model built for the purpose of the investigations described here was based on the results given in [18]. The basic design of the storage installation includes two tanks with hot and cold molten salt. During a standard operation the temperature of the salt does not drop below the saturation temperature of the salt. The amount of the stored energy may be increased if the melting and the solidification processes are included in the operation of the installation. However due to the required values of the temperature in the presented application the complexity of the design of the energy storage with the phase change does not justify the additional investment costs. The combined cycle and the energy storage installations are shown in Figure1. The steam turbines in the figure are simplified and shown as high pressure and intermediate and low pressure sections only. The regeneration heat exchangers are omitted. The cycle includes the supercritical turbine (SHP and SIP/SLP) with the steam boiler and the subcritical turbine (HP and IP/LP) without a boiler. The cycles are connected through the molten salt energy storage with hot and cold tanks. The supercritical steam turbine operates as a foundation of a power grid. It compensates the fluctuations of the electric power generated by the renewables. For a single turbine it causes continuous fluctuations of the load. Energies 2020, 13, x FOR PEER REVIEW 3 of 18 of this type is one of the smallest supercritical units typically installed in conventional power plants. Technical solutions verified for this unit may easily be applied to larger turbines. The purpose of the investigation was the optimization of the operation under large fluctuations of the load due to the necessity to compensate the power generated by renewable energy sources. The approach towards the optimization involves the combination of the supercritical steam turbine cycle and a subcritical steam turbine cycle. The subcritical turbine chosen here is a modernized 13K225 unit, which remains operational after the removal of a fossil-fueled steam boiler. Both turbines are located in the same power plant. It is assumed that the technical state of the subcritical turbine allows it to operate for a period that guarantees the return of the modernization costs. The evaluation of a remaining life time requires a detailed analysis for each specific machine. The values of the basic parameters that describe both turbines are given in Table 1.

Table 1. Comparison of the steam turbines under the investigation.

Parameter Unit Supercritical Turbine Subcritical Turbine nominal power MW 460 217 life steam temperature °C 560 535 life steam pressure MPa 27 12.7 life steam flow rate t/h 1300 668 reheat steam temperature °C 580 535

Energies 2020, 13, x FOR PEER REVIEW 4 of 18 Figure 1. TheThe loading loading of of the the energy energy storage storage inst installationallation with an electric heater. In the technical solution shown in Figure 1 the supercritical turbine always operates under the full loadIn the even technical if the share solution of the shown energy in from Figure the1 the renewables supercritical in the turbine power always grid temporarily operates under increases. the fullThis load is the even first if option the share of the of themodernization. energy from theThe renewablesexcess power in thegenerated power gridin the temporarily supercritical increases. turbine Thisis used is the in the first electric option heater of the modernization.to increase the temp The excesserature power of the generated molten salt. in theThe supercritical excess energy turbine is then is usedstored in in the the electric hot tank heater with to the increase molten the salt. temperature Later, when of the the share molten of the salt. energy The excess generation energy from is then the storedrenewables in the decreases, hot tank withthe stored the molten energy salt. is used Later, to whengenerate the the share life of and the the energy reheat generation steam for from the thesubcritical renewables turbine. decreases, The subcritical the stored turbine energy operates is used without to generate a stea them life boiler, and theso its reheat only steamsource for of the subcriticalsteam is the turbine. steam generator The subcritical powered turbine by the operates energy withoutstorage system. a steam boiler, so its only source of the steamThe is themain steam advantage generator of the powered combined by the cycle energy is the storage possibility system. to supply the electric power to the grid Theeven main below advantage the level that of the is defined combined as a cycle mini ismum the possibilityfor the supercritical to supply turbine, the electric while power the turbine to the gridoperates even under below its the nominal level that conditions. is defined asBoth a minimum the supercritical for the supercritical boiler and turbine,the supercritical while the turbine operatesalways operate under its under nominal a full conditions. load independent Both the supercriticalof the power boiler demand. and theFrom supercritical a global perspective turbine always this operateallows one under to decrease a full load the independent total number of theof fossil-f powerueled demand. units Fromthat constitute a global perspective the foundation this allowsof the onepower to decreasegrid. The the steam total generator number of for fossil-fueled the subcritical units turbine that constitute allows thea very foundation fast start-up of the because power grid. the Theboiler steam start-up generator is avoided for the since subcritical there is turbine no boiler. allows If the a verymethods fast start-upfor a rapid because start-up the boilerare applied start-up then is avoidedthe whole since cycle there becomes is no very boiler. flexible. If the methods for a rapid start-up are applied then the whole cycle becomesA similar very flexible.concept is shown in Figure 2. The loading process of the energy storage is different. In the firstA similar option conceptthe excess is shownpower was in Figure used2 to. Theload loading the energy process storage, of the here energy it is the storage excess is steam di fferent. that Inis the the source first option of the the heat excess to increase power was the usedmolten to salt load temperature. the energy storage, here it is the excess steam that is the source of the heat to increase the molten salt temperature.

Figure 2. Energy storage loading with the heat fromfrom the life and the reheat steam.steam.

The supercritical steam turbine is connected to the energyenergy storagestorage systemsystem throughthrough two heatheat exchangers. TheThe ﬁrst first one one (XSHP) (XSHP) is fedis fed with with the lifethe steam life steam and the and second the second one (XSIP) one with (XSIP) the with reheated the steamreheated from steam the from supercritical the supercritical steam boiler. steam Theboiler. fossil-fueled The fossil-fueled boiler boiler operates operates under under a full a load full load and generatesand generates the nominal the nominal amounts amounts of live of andlive reheatedand reheated steam. steam. The turbineThe turbine generates generates power power at a level at a equallevel equal to the to current the current demand demand from from the grid. the grid. The ﬂowThe flow rates rates of the of livethe andlive and the reheatedthe reheated steam steam to the to the turbine correspond to the current demand. The excess steam flows to the steam−salt heat exchangers. These exchangers are located at the by-pass systems of the high-pressure turbine and the intermediate and low pressure turbines. They are the parts of the steam pressure and temperature reduction stations. The excess life steam flows to the high-pressure steam-salt exchanger (XSHP in Figure 2) and then to the pressure and temperature reduction station RS1. The steam parameters are matched to the parameters of the steam at the outlet of the high-pressure turbine. The excess steam mixes with the steam at the outlet of the HP turbine and is returned to the boiler for reheating The excess reheat steam flows to the exchanger marked as XSIP in Figure 2 and then to the station RS2. The temperature of the steam for the supercritical turbine is sufficient to generate the steam at the nominal temperature required for the subcritical turbine later on. The steam generation for the subcritical turbine is the same as in the first modernization option shown in Figure 1. The third option is a modification of the second one. The supercritical cycle now has an additional steam cooler that increases the temperature of the feed water, which flows to the boiler.

Energies 2020, 13, 2222 5 of 18 turbine correspond to the current demand. The excess steam flows to the steam salt heat exchangers. − These exchangers are located at the by-pass systems of the high-pressure turbine and the intermediate and low pressure turbines. They are the parts of the steam pressure and temperature reduction stations. The excess life steam flows to the high-pressure steam-salt exchanger (XSHP in Figure2) and then to the pressure and temperature reduction station RS1. The steam parameters are matched to the parameters of the steam at the outlet of the high-pressure turbine. The excess steam mixes with the steam at the outlet of the HP turbine and is returned to the boiler for reheating The excess reheat steam flows to the exchanger marked as XSIP in Figure2 and then to the station RS2. The temperature of the steam for the supercritical turbine is sufficient to generate the steam at the nominal temperature required for the subcritical turbine later on. The steam generation for the subcritical turbine is the same as in the first modernization option shown in Figure1. Energies 2020, 13, x FOR PEER REVIEW 5 of 18 The third option is a modification of the second one. The supercritical cycle now has an additional steamThis modification cooler that increases is shown the in temperature Figure 3. The of the new feed steam water, cooler which is flows located to the between boiler. Thisthe intermediate modification ispressure shown steam-salt in Figure3 .exchanger The new steamand the cooler station is locatedRS2. Th betweene modification the intermediate is possible due pressure to the steam-salt relatively exchangerhigh temperature and the of station the steam RS2. Theat the modification outlet of th ise possibleintermediate due topressure the relatively steam-salt high exchanger. temperature The of therecovery steam of at the the heat outlet from of the steam intermediate improves pressure the cycle steam-salt efficiency. exchanger. The recovery of the heat from the steam improves the cycle efficiency.

Figure 3. Energy storage loading with the heat from the excess steam with an additional steam cooler.

The unmodifiedunmodified cycle of the supercritical steam turbine already includes a steam cooler for the steam flowing flowing from from a a steam steam extraction extraction in in a aturbine turbine to toa feed a feed water water heater. heater. However, However, during during an off- an odesignff-design operation operation period period the the feed feed water water temperat temperatureure remains remains much much lower lower than than under under full loadload operation and therefore thethe additionaddition ofof aa newnew steamsteam coolercooler isis thermodynamicallythermodynamically satisfied.satisfied.

3. Numerical Models of the Turbines The analysisanalysis of of the the cycles cycles modernized modernized according accord toing the suggestionsto the suggestions given above given involved above numericalinvolved simulations.numerical simulations. A quasi one-dimensional A quasi one-dimensional model was preparedmodel was for prepared the simulations for the simulations at diﬀerent levels at different of the loadlevels [19 of]. the The load models [19]. were The developed models were on the develo basisped of measurement on the basis dataof measurement from real turbines. data from The datareal allowedturbines. us The to data determine allowed the us performance to determine maps the performance for the machines maps that for makethe machines up the steam that make cycle. up the steamApart cycle. from the performance maps the numerical model includes the mass and the energy equations for eachApart machine from the or deviceperformance that is maps a part the of thenumerical steam cycle. model In includes particular the the mass model and includes the energy the equationequations of for the each turbine machine capacity or device that relates that is thea part pressure of the steam distribution cycle. In along particular the steam the path,model the includes steam temperaturethe equation andof the the turbine ﬂow rate. capacity For a groupthat relates of turbine the pressure stages this distribution relation may along be describedthe steam as:path, the steam temperature and the flow rate. For av group of turbine stages this relation may be described as: t 2 2 s m p pout T = in2 − 2 in (1) m 2pin −p2out Tin mn p p Tin,n = in,n2 out,n2 (1) mn pin,n−−pout,n Tin,n

Two functions heavily influence the operation of the supercritical steam turbine. The first one is related to the life steam pressure. For a stand-alone turbine the pressure depends on the mass flow rate of the life steam generated in the boiler:

pSHP,in = fmb (2) In the modernization options 2 and 3 the flow of the steam generated in the boiler is different than the flow to the turbines. Due to this reason the life steam pressure is related to the flow of the steam to the turbine:

pSHP,in = fmt,in (3) The Equations (2) and (3) include the same function f but a different flow. If not stated otherwise the proceeding simulation results were obtained for Equation (2). The second important function is related to the temperature of the reheated steam. This value also depends on the load. For a low load the supercritical boiler generates the reheated steam at a

Energies 2020, 13, 2222 6 of 18

Two functions heavily influence the operation of the supercritical steam turbine. The first one is related to the life steam pressure. For a stand-alone turbine the pressure depends on the mass flow rate of the life steam generated in the boiler:

pSHP,in= f(mb) (2)

In the modernization options 2 and 3 the flow of the steam generated in the boiler is different than the flow to the turbines. Due to this reason the life steam pressure is related to the flow of the steam to the turbine:

pSHP,in= f(mt,in) (3) Energies 2020, 13, x FOR PEER REVIEW 6 of 18 The Equations (2) and (3) include the same function f but a different flow. If not stated otherwise thetemperature proceeding that simulation is lower resultsthan the were design obtained value for du Equatione to the (2).small mass flow rate of the flue gas. Therefore:The second important function is related to the temperature of the reheated steam. This value also depends on the load. For a low load the supercritical boiler generates the reheated steam at T = fm (4) a temperature that is lower than the designSIP,in value dueb to the small mass flow rate of the flue gas. Therefore:The reheated steam temperature depends solely on the steam flow in the boiler, so for the modernization options analyzed here this isT alwaysSIP,in= fconstant,(mb) since the flow of the steam in the boiler(4) is always equal to the nominal (design) value. The reheated steam temperature depends solely on the steam flow in the boiler, so for the The assessment of the modernization bases on the efficiency of the power generation. A current modernization options analyzed here this is always constant, since the flow of the steam in the boiler is efficiency may be calculated as a ratio of the electric power to the heat delivered to the steam cycle in always equal to the nominal (design) value. the boiler: The assessment of the modernization bases on the efficiency of the power generation. A current Nel efficiency may be calculated as a ratio of the electricη = power to the heat delivered to the steam cycle(5) in the boiler: Qd N Figure 4 presents the power generationη efficien= el cies for the supercritical and the subcritical(5) Q turbines working separately before the modernizationd and each with its own boiler. The efficiencies are markedFigure4 aspresents “spr” theand power “sub” generation respectively e ffi inciencies the Figure for the 4. supercriticalThey are shown and the in subcriticalthe relation turbines to the workingload. separately before the modernization and each with its own boiler. The efficiencies are marked as “spr” and “sub” respectively in the Figure4. They are shown in the relation to the load.

Figure 4. Current efficiencies eﬃciencies of the power generationgeneration for the analyzed turbines.

The graphsgraphs fromfrom thethe Figure Figure4 are4 are extrapolated extrapolated in in the the range range of theof the low low load. load. The The minimal minimal load load for thefor the supercritical supercritical turbine turbine is 40% is 40% and and for thefor subcriticalthe subcritical one one 55%. 55%. Under Under the the lower lower loads loads the the boilers boilers are notare not able able to sustain to sustain the requiredthe required temperature temperature of the of steam.the steam. The The extrapolation extrapolation was was prepared prepared under under the assumptionthe assumption that thethat boilers the boilers uphold uphold the temperature the temperat requiredure required by Equation by Equation (3). This (3). is doneThis inis orderdone toin prepareorder to aprepare reference a reference basis for thebasis comparison for the comparison of the modernization of the modernization options. options. The current eefficiencyﬃciency of the power generation is not a proper deﬁnitiondefinition for systems with an energy storage. Since Equation (5) involves the cu currentrrent values it does not includeinclude any informationinformation about the heat delivered earlier to the energy storage. Therefore, the options of the modernization suggested here are compared through the power generation efficiency defined as:

Eel,spr +Eel,sub η = (6) Ed The first term in the numerator of the fraction in the above equation is the energy produced by the supercritical turbine during the loading of the energy storage and the second term is the energy produced by the subcritical turbine during an unloading process that completely drains the stored energy. The efficiency is calculated for a chosen period of the operation. In the analysis this period was defined in the following way. The energy storage is loaded up to the level that allows the subcritical turbine to operate for 24 h without any other energy source. Thus, the period for which the efficiency is calculated is the period of the energy loading plus the 24 h of the energy unloading. The energy delivered to the system that appears in the denominator of the fraction in Equation (6) is the energy delivered to the steam cycle in the supercritical boiler during the loading process. The

Energies 2020, 13, 2222 7 of 18 about the heat delivered earlier to the energy storage. Therefore, the options of the modernization suggested here are compared through the power generation eﬃciency deﬁned as:

Eel,spr + Eel,sub η = (6) Ed

The first term in the numerator of the fraction in the above equation is the energy produced by the supercritical turbine during the loading of the energy storage and the second term is the energy produced by the subcritical turbine during an unloading process that completely drains the stored energy. The efficiency is calculated for a chosen period of the operation. In the analysis this period was defined in the following way. The energy storage is loaded up to the level that allows the subcritical turbine to operate for 24 h without any other energy source. Thus, the period for which the efficiency is calculated is the period of the energy loading plus the 24 h of the energy unloading. The energy delivered to the system that appears in the denominator of the fraction in Equation (6) is the energy delivered to the steam cycle in the supercritical boiler during the loading process. The equation does not include the energy produced in the supercritical turbine during the unloading process because it requires additional energy delivered to the steam cycle. The current values of the power and the heat delivered to the steam cycle may be inserted into Equation (6), which then becomes:

Nel,spr tload+Nel,sub 24h η = (7) Qd tload

The numerical simulations include the decrease of the salt temperature in the hot tank due to the imperfect insulation. The hot tank is loaded with the molten salt at the temperature that is 5 degrees Kelvin higher than the temperature of the salt later used to generate the steam for the subcritical turbine.

4. Methodology The numerical model described in the preceding section allows one to simulate the performance of the investigated power plant in the chosen cases of the modernization. The simulations were conducted for a rather wide range of loads from 35% to 100% for the subcritical steam turbine and 35% to 90% for the supercritical steam turbine. Since the design power outputs are different for both turbines the load percentages translate into different values in megawatts. A number of combinations of the loads were analyzed. During a typical operation the power demand undergoes continuous fluctuations and the turbine output must match the demand. In order to compare different options of the modernization one may chose two approaches. The first one is to assume a particular curve of the demand over time and conduct the simulations for that curve matching it with the turbine load. However, the shape of the demand curve strongly depends on the location of the power plant and local conditions of the operation (cooling possibilities, ambient conditions etc.). For that reason a second approach was applied. It was assumed that the energy storage loading and unloading is conducted under a constant power supply to the grid. The power supply may be different for the loading and the unloading but remains constant during both processes. Each run of the simulation model started with the given values of the power delivered to the grid from the supercritical and the subcritical turbine. The subcritical turbine always matches its power output to the power demand from the grid. The supercritical turbine matches the power demand in the options 2 and 3 of the modernization but in the option 1 it operates always at 100% delivering the excess power to the electric heater that increases the temperature of the molten salt. The simulation model then matched the turbines performance to the selected levels of the power demand. The model calculated the required flow rates and the parameters of steam in the characteristic locations of the power plant (inlets and outlets of the turbine sections and heat exchangers, etc.). Based on the parameters in the steam cycles the flow rates of the molten salt were calculated and then the Energies 2020, 13, 2222 8 of 18 time required for the energy loading and the unloading processes. Finally the efficiency of the electric energy generation could be determined. The numerical simulations were conducted for a constant value of the energy delivered to the steam cycle in the supercritical boiler. This constant value is equal to the value for the full load operation of the supercritical steam cycle. The algorithm that drove the calculations and assured the convergence of the numerical model for the options 2 and 3 had to match the two main flows. The flow of the life steam at the inlet to the supercritical turbine had to match the given power demand from the supercritical turbine and the flow of the steam generated in the supercritical boiler had to match the constant value of the energy delivered to the steam cycle in the boiler. The modeling results are presented in the next section of the paper in the form of the graphs that present the efficiency and the loading time for different combinations of the power demand from the analyzed turbines.

5. Modernization Eﬀects

5.1.Energies Option 2020, 113, x FOR PEER REVIEW 8 of 18

The addition of the energy storage system to the steam cycles generally lowers the power generation eefficiency.fficiency. ThisThis decrease decrease is is shown shown in in Figure Figure5 for 5 for the the first first option option of the of modernization.the modernization. The graphThe graph in the in figure the figure presents presents the values the values calculated calculated from Equationfrom Equation (6) for (6) diff forerent different levels of levels the electric of the powerelectric supply power/ demand.supply/demand. The power The supplied power supplied to the grid to fromthe grid the from supercritical the supercritical turbine N turbineel,spr refers Nel,spr to therefers loading to the process.loading process. This is a This relative is a valuerelative in va Figurelue in5 andFigure its reference5 and its reference value is the value nominal is the (design)nominal power(design) output power of output the supercritical of the supercritical turbine Nturbineel,n,spr .N Accordingel,n,spr. According to the to assumptions the assumptions for the for option the option 1 the supercritical1 the supercritical turbine turbine operates operates under under the full the load full independent load independent of the current of the current powerdemand power demand and the excessand the power excess is power used foris used the loading for the ofloading the energy of the storage. energy storage.

Figure 5. Power generation efficiency—option eﬃciency—option 1.

The power power demand demand for for the the subcritical subcritical turbine turbine Nel,sub Nel,sub is givenis given for the for unloading the unloading process. process. The Thesubcritical subcritical turbine turbine power power is always is always equal to equal the power to the supplied power supplied to the grid. to theIts value grid. in Its Figure value 5 inis Figurerelative5 andis relative its reference and its value reference is the nominal value is the(design) nominal power (design) output power of the subcritical output of theturbine subcritical Nel,n,sub. turbineThe N graphel,n,sub .shows that the efficiency depends mostly on the level of the power demand during the loadingThe graph process. shows This that is the due effi tociency the differen dependsce mostly of the onnominal the level power of the between power demandthe two duringturbines the − loadingthe nominal process. power This of is the due supercritical to the difference turbine of the is nominalover two power times betweenlarger than the twothe turbines—thepower of the nominalsubcritical power turbine. ofthe The supercritical values in Figure turbine 5 should is over be two compared times larger to the than values the powerin Figure of the4. The subcritical current turbine.efficiencies The of values the supercritical in Figure5 shouldand subcritical be compared turbines to the for values a full load in Figure operation4. The are current 48.4% e andfficiencies 42.0%, ofrespectively. the supercritical and subcritical turbines for a full load operation are 48.4% and 42.0%, respectively. An important piecepiece ofof informationinformation in in the the evaluation evaluation of of the the combined combined cycle cycle is is the the time time required required to loadto load the the energy energy storage storage system. system. Figure Figure6 presents6 presents the the dependency dependency between between the the time time of of the the loading loading process and the power demands during the two processes: the loading and the unloading. According to the assumptions described in the preceding sections this time is always determined for the amount of energy that allows the subcritical turbine to operate for 24 h during the given load. Of course the amount of the stored energy is different for different loads. The graph is divided into two sections with different ranges on the vertical axes due to the large density of the lines. The low power demand during the loading process results in more excess power supplied to the electric heater to increase the temperature of the molten salt and to store the energy. The time required to load the storage is then shorter. If the power demand from the supercritical turbine is under 50 percent of the nominal turbine power then the loading period is around two days.

Energies 2020, 13, 2222 9 of 18 process and the power demands during the two processes: the loading and the unloading. According to the assumptions described in the preceding sections this time is always determined for the amount of energy that allows the subcritical turbine to operate for 24 h during the given load. Of course the amount of the stored energy is different for different loads. The graph is divided into two sections with different ranges on the vertical axes due to the large density of the lines. The low power demand during the loading process results in more excess power supplied to the electric heater to increase the temperature of the molten salt and to store the energy. The time required to load the storage is then shorter. If the power demand from the supercritical turbine is underEnergies 502020 percent, 13, x FOR of PEER the nominal REVIEW turbine power then the loading period is around two days. 9 of 18

Figure 6.6. Loading time of the energy storage for the 24 h of the subcritical turbine operation—option 1—full1—full graphgraph ((aa)) andand itsits enlargedenlarged sectionsection ((bb).).

The valuesvalues ofof thethe molten molten salt salt ﬂow flow rate rate and and the the total total required required amount amount of theof the molten molten salt salt in the in hotthe tankhot tank are shownare shown in Figure in Figure7. Both 7. Both values values are shown are shown in relation in relation to the to subcritical the subcritical turbine turbine load. load.

Figure 7.7. Molten saltsalt ﬂowflow raterate andand totaltotal amount amount for for the the 24 24 h h of of subcritical subcritical turbine turbine operation—option operation—option 1. 1. The design of the tanks, the electric heater and the steam-salt heat exchangers must match the largestThe values design shown of the in tanks, the Figure the electric7. The designheater isan ad separate the steam-salt subject heat not discussedexchangers in must this paper. match the largestIt shouldvalues beshown noted in that the dueFigure to the7. The diﬀ designerence inis a the separate nominal subject power not outputs discussed between in this the paper. turbines the unloadingIt should be will noted probably that due be conducted to the difference under in the the higher nominal load power then the outputs power between demand the during turbines the loadingthe unloading process. will probably be conducted under the higher load then the power demand during the loading process.

Energies 2020, 13, 2222 10 of 18

5.2. Option 2 In the modernization options 2 and 3 the power output of the supercritical turbine during the energy storage loading always matches the current demand from the power grid. However, the amount of the heat supplied to the steam cycle in the boiler is always equal to its nominal value. The excess steam flows to the steam—salt exchangers and increases the temperature of the salt to store the energy. The power generation efficiency for the energy storage loading and the unloading are shown in Energies 2020, 13, x FOR PEER REVIEW 10 of 18 Figure8 for option 2. Compared to option 1 this technical solution has lower e fficiency, especially for theoption lower 1 the power temperature demands. of the the change molten is salt the is result due ofto dithefferent electric type power, of the energywhile in conversion. option 2 it Inis optiondue to 1the the heat temperature carried in ofthe the hot molten steam. salt is due to the electric power, while in option 2 it is due to the heat carried in the hot steam.

Figure 8. Power generation efficiency—option eﬃciency—option 2.

The loading timetime ofof thethe energyenergy storagestorage required required for for the the 24 24 h h operation operation of of the the subcritical subcritical turbine turbine is longeris longer than than in in option option 1. 1. The The length length of of the the loading loading period period is is shown shown in in Figure Figure9 .9. The The graph graph is is again again divided into two sections with different different ranges on the vertical axes axes due due to to the the density density of of the the lines. lines. The didifferencefference between options 1 and 2 is around 10 h with option 1 providing the shorter period. period. The total amount of the molten salt required to guarantee the 24 h operation of the subcritical turbine and the flowflow rate of the salt is the same as in optionoption 1.1.

Figure 9. Loading time of the energy storage for the 24 h of the subcritical turbine operation—option 2—full graph (a) and itsits enlargedenlarged sectionsection ((bb).).

The results described so far for option 2 are from the simulations where the life steam pressure at the inlet to the supercritical turbine depends on the steam flow in the boiler according to Equation (2). The results are different if the pressure is adjusted to the load according to Equation (3). The values of the pressure for both Equations (2) and (3) are shown in Figure 10a. The line for Equation (2) is not horizontal in the figure because the constant value in the calculations was the amount of energy delivered to the steam cycle in the boiler and not the steam flow.

Energies 2020, 13, 2222 11 of 18

The results described so far for option 2 are from the simulations where the life steam pressure at the inlet to the supercritical turbine depends on the steam flow in the boiler according to Equation (2). The results are different if the pressure is adjusted to the load according to Equation (3). The values of the pressure for both Equations (2) and (3) are shown in Figure 10a. The line for Equation (2) is not horizontal in the figure because the constant value in the calculations was the amount of energy

EnergiesdeliveredEnergies 20202020 to,, 1313 the,, xx FORFOR steam PEERPEER cycle REVIEWREVIEW in the boiler and not the steam ﬂow. 1111 ofof 1818

FigureFigure 10. 10. LifeLife steamsteam pressurepressure ( (aa)) and and the the flow ﬂowflow of of the the molten molten salt salt during during the loading of the energy energy

storagestorage ( (bb)) forfor thethe optionoptionoption 2,2,2, steamsteamsteam pressurepressurepressure depends dependsdepends on onon the thethe ﬂow flowflow in inin the thethe boiler boilerboiler p pSHP,inpSHP,inSHP,in === f(mf(mbb)) or or at at the the turbineturbine inlet inlet p pSHP,inSHP,in === f(mf(mf(mtt).).t ).

AccordingAccording to to Figure Figure 10b1010bb the the cycle cycle with with the the lower lower pressure pressure of of the the life life steam steam delivers delivers more more molten molten saltsalt to to the the hothot tanktank whilewhilewhile the thethe amount amountamount of ofof energy energyenergy delivered delivereddelivered to toto the thethe steam steamsteam cycle cyclecycle in inin the thethe supercritical supercriticalsupercritical boiler boilerboiler is istheis thethe same. same.same. The TheThe larger largerlarger flow flowflow of ofof the thethe molten moltenmolten salt saltsalt results resuresultslts in inina aa shorter shortershorter period periodperiod ofofof thethethe loadingloadingloading ofof the the energy energy storage.storage. FigureFigure 1111 comparescompares the the chosen chosen cases cases of of the the loading loading process process for for different didifferentfferent levels levels of of the the power power demanddemand during during the the loading, loading, different didifferentfferent load load during during the the unloading unloading and and two two types types of of the the function function that that determinesdetermines the the life life steam steam pressure pressure (Equations(Equations (1)(1) oror (2)).(2)).

FigureFigure 11. 11. LoadingLoading time time of of the the energy energy storage storage for for 24 24 h h of of subcritical subcritical turbine turbine operation—option operation—option 2, 2, life life steamsteam pressure pressure adjusted adjusted to to the the steam steam flow ﬂowflow in in the the boiler boiler or or at at the the turbineturbine inlet—full inlet—full graph graph ( (aa))) and and its its enlargedenlarged section section ( (bb)).)

TheThe eefficiencyffiefficiencyciency of ofof the thethe power powerpower generation generationgeneration for the forfor whole ththee combinedwholewhole combinedcombined cycle also cyclecycle increases. alsoalso Theincreases.increases. comparison TheThe comparisonofcomparison the efficiency ofof thethe values efficiencyefficiency is presented valuesvalues isis in presentedpresented Figure 12 .inin The FigureFigure figure 12.12. includes TheThe figurefigure the includesincludes results of thethe the resultsresults simulations ofof thethe simulationssimulations forfor thethe lowlow levelslevels ofof thethe loads.loads. TheThe didifferencesfferences betweenbetween thethe twotwo typestypes ofof thethe adjustmentadjustment ofof thethe lifelife steamsteam pressurepressure areare smallersmaller forfor thethe operatiooperationn thatthat isis closeclose toto thethe fullfull loadload becausebecause thethe steamsteam thatthat flowsflows toto thethe turbineturbine isis almostalmost equalequal toto thethe totatotall flowflow ofof thethe steamsteam generatedgenerated inin thethe boilerboiler andand bothboth pressurepressure adjustmentadjustment EquationsEquations (1)(1) andand (2)(2) givegive similarsimilar results.results. TheThe adjustmentadjustment ofof thethe lifelife steamsteam pressurepressure anandd itsits lowerlower valuesvalues forfor thethe part-loadpart-load operationoperation generallygenerally leadlead toto thethe improvementimprovement ofof thethe powerpower genegenerationration efficiency.efficiency. InIn thethe combinedcombined cyclecycle withwith thethe energyenergy storagestorage systemsystem presentedpresented herehere thisthis imimprovementprovement isis visiblevisible forfor thethe wholewhole cycle.cycle.

Energies 2020, 13, 2222 12 of 18 for the low levels of the loads. The differences between the two types of the adjustment of the life steam pressure are smaller for the operation that is close to the full load because the steam that flows to the turbine is almost equal to the total flow of the steam generated in the boiler and both pressure adjustment Equations (1) and (2) give similar results. The adjustment of the life steam pressure and its lower values for the part-load operation generally lead to the improvement of the power generation efficiency. In the combined cycle with the energy Energies 2020, 13, x FOR PEER REVIEW 12 of 18 storageEnergies 2020 system, 13, x presentedFOR PEER REVIEW here this improvement is visible for the whole cycle. 12 of 18

Figure 12. Power generation efficiency—option 2, life steam pressure adjusted to the steam flow in Figurethe boiler 12. or Power at the generationgeneration turbine inlet. eefficiency—optionffi ciency—option 2, 2, life life steam steam pressure pressure adjusted adjusted to theto the steam steam flow flow in the in boilerthe boiler or at or the at turbinethe turbine inlet. inlet. A larger improvement of the power generation efficiency may be achieved through the decrease of theA life larger and improvement the reheat steam of the temperaturepower generation for the eefffi ficiencysubcriticalciency may turbine. be achieved Of course, through this the leads decrease to a ofdecrease the life of andand the thecurrentthe reheatreheat efficiency steamsteam temperatureoftemperature the subcritical forfor thetheturbine subcriticalsubcritical but it improves turbine.turbine. theOf course, efficiency this of leads the whole to a decreasecombined of system. the current Lower eefficiencyffi ciencytemperature of the subcriticalof the steam turbine generated but it for improves the subcritical the eefficiencyfficiency turbine of requires the whole a combinedlower temperature system. Lower Lowerof the moltentemperature temperature salt in of ofthe the hot steam tank. generated This allows for one the to subcritical increase the turbine flow rate requires of the a lowermolten temperature salt in the steam-salt of the molten exchangers salt in the while hothot ke tank.tank.eping This constant allows theone amountto increase of energythe flow flow delivered rate of the to moltenthe supercritical saltsalt inin thethe cycle. steam-saltsteam-salt The flow exchangers exchangers rate of whilethe while salt keeping keis epingcompared constant constant in theFigure the amount amount 13 for of two energyof energy different delivered delivered values to the toof supercriticalthe temperaturesupercritical cycle. ofcycle. the The steamThe flow flow ratefor ratethe of thesubcritical of the salt salt is compared turbine.is compared in Figure in Figure 13 for 13 twofor two diff erentdifferent values values of the of temperaturethe temperature of the of steamthe steam for thefor subcriticalthe subcritical turbine. turbine.

Figure 13. Flow rate of the salt during the loading process for didifferentfferent temperature of the steam for theFigure subcritical 13. Flow turbine—optionturbine—option rate of the salt 2.2.during the loading process for different temperature of the steam for the subcritical turbine—option 2. A larger flowflow rate results in aa shortershorter loadingloading period and this leads to a better eefficiencyfficiency of the powerA generationlargergeneration flow of rateof the the results whole whole cyclein acycle shorter with with the loading energy the energy period storage. storage. and The this values The leads ofvalues to the a ebetterffi ofciency the efficiency efficiency are compared of theare powercompared generation in Figure of 14a. the The whole improvement cycle with of thethe effienergyciency storage. in the referenceThe values to theof thenominal efficiency values are of comparedthe steam temperaturein Figure 14a. for The the improvement subcritical turbine of the is effi aroundciency 0.5 in thepercentage reference points. to the nominal values of the steamThe shortened temperature time for of the the subcritical loading process turbine is is shown around in 0.5 Figure percentage 14b. The points. hot tank is filled faster whenThe the shortened subcritical time steam of thetemperature loading process is lower. is shownThe comparison in Figure 14b.to the The results hot tank with is the filled nominal faster whentemperature the subcritical value shows steam that temperature the loading is processlower. The is 2–3 comparison h shorter, todepending the results on with the load.the nominal This is temperatureespecially important value shows for the that low the loads loading because process the 2–3 is 2–3h are h 10%shorter, of the depending total loading on the time. load. This is especially important for the low loads because the 2–3 h are 10% of the total loading time.

Energies 2020, 13, 2222 13 of 18 in Figure 14a. The improvement of the eﬃciency in the reference to the nominal values of the steam temperature for the subcritical turbine is around 0.5 percentage points. The shortened time of the loading process is shown in Figure 14b. The hot tank is ﬁlled faster when the subcritical steam temperature is lower. The comparison to the results with the nominal temperature value shows that the loading process is 2–3 h shorter, depending on the load. This is especiallyEnergies 2020 important, 13, x FOR PEER for theREVIEW low loads because the 2–3 h are 10% of the total loading time. 13 of 18

Figure 14. Power generation efficiency eﬃciency ( a) and the loading time ( b) for different diﬀerent life and reheat steam temperature for thethe subcriticalsubcritical turbine—optionturbine—option 2.2.

The results in in Figures Figures 13 13 and and 14 14 were were obtained obtained in inthe the simulations simulations under under the theassumption assumption that that the thevalues values of the of thelife lifeand and the the reheat reheat steam steam temperatur temperaturee are are equal. equal. The The values values of of 535 535 and and 515 515 °C◦C are respectively the nominal value and the minimal value that may be appliedapplied withoutwithout the increased erosion process in the low-pressure partpart ofof thethe subcriticalsubcritical turbine.turbine.

5.3. Option 3 The steam temperaturetemperature atat thethe outletoutlet ofof thethe intermediateintermediate steam-salt steam-salt exchanger exchanger (XSIP (XSIP in in Figure Figures2 or 2 Figureor 3) is3 )relatively is relatively high. high. Its Itsvalue value is iswell well above above the the saturation saturation temperature. temperature. For For this this reason optionoption 33 was investigated. In this option the heat in the steam at the outlet of the exchanger is used to increase the temperature of the feed water that flowsflows to the supercritical boilerboiler (Figure(Figure3 3).). AA newnew steamsteam coolercooler is installedinstalled downstream of the boiler.boiler. The results ofof thethe simulationsimulation includingincluding thethe powerpower generationgeneration eefficiencyfficiency and the time of the loading process for thethe energy storage for this option are gathered in Figure 1515.. The addition of the new steam cooler increases the power generation efficiency and decreases the loading period. However, the results of the simulations prove that the profit in terms of the efficiency and time is small. The investment costs of the new steam cooler would probably not be justified in an economical analysis. It should be emphasized here that the variants of option 2 described earlier did not include the new steam cooler but involved the adjustments of the supercritical steam pressure or the subcritical steam temperature. These adjustments do not require additional investment cost but the change of the parameters of the working fluid in the already existing machines.

Figure 15. Power generation efficiency (a) and the loading time for 24 h of operation of the subcritical turbine (b)—options 2 and 3.

The addition of the new steam cooler increases the power generation efficiency and decreases the loading period. However, the results of the simulations prove that the profit in terms of the

Energies 2020, 13, x FOR PEER REVIEW 13 of 18

Figure 14. Power generation efficiency (a) and the loading time (b) for different life and reheat steam temperature for the subcritical turbine—option 2.

The results in Figures 13 and 14 were obtained in the simulations under the assumption that the values of the life and the reheat steam temperature are equal. The values of 535 and 515 °C are respectively the nominal value and the minimal value that may be applied without the increased erosion process in the low-pressure part of the subcritical turbine.

5.3. Option 3 The steam temperature at the outlet of the intermediate steam-salt exchanger (XSIP in Figures 2 or 3) is relatively high. Its value is well above the saturation temperature. For this reason option 3 was investigated. In this option the heat in the steam at the outlet of the exchanger is used to increase the temperature of the feed water that flows to the supercritical boiler (Figure 3). A new steam cooler is installed downstream of the boiler. The results of the simulation including the power generation efficiency and the time of the loading process for the energy storage for this option are gathered in EnergiesFigure 202015. , 13, 2222 14 of 18

Energies 2020, 13, x FOR PEER REVIEW 14 of 18 efficiency and time is small. The investment costs of the new steam cooler would probably not be justified in an economical analysis. It should be emphasized here that the variants of option 2 described earlier did not include the new steam cooler but involved the adjustments of the supercritical steam pressure or the subcritical steam temperature. These adjustments do not require additional investment cost but the change of the parametersFigure 15. Power of the generation working efficiencyfluid efficiency in the (a) alreadyand the loading existing time machines. for 24 h of operation of the subcritical turbine (b)—options 2 and 3.3. 5.4. Direct integration with renewables 5.4. Direct Integration with Renewables AllThe the addition technical of thesolutions new steam presented cooler here increases are me antthe for power the foundation generation of efficiency a power system.and decreases Again itthe should loadingAll the be technicalrecalledperiod. However,that solutions the main presentedthe taskresults of here theof arethefoundation meantsimulations for is the the prove foundation compensation that the of aprofit powerof the in fluctuations system. terms Againof the of theit should power be generated recalled thatin the the renewable main task energy of the foundation sources. According is the compensation to the assumptions of the fluctuations given in the of Introductionthe power generated the source in of the the renewable energy for energy the foundation sources. Accordingis fossil fuel. to the assumptions given in the IntroductionThe structure the source of the of presented the energy power for the plant foundation with a storage is fossil system fuel. allows one to use renewable energyThe sources structure (RES) of thein the presented foundation. power This plant may with be done a storage through system the connection allows one of to the use renewables renewable notenergy directly sources to the (RES) power in the grid foundation. but rather This to the may energy be done storage through system the connectionin the cycles of presented the renewables here. Thisnot directly idea is depicted to the power in Figure grid 16. but rather to the energy storage system in the cycles presented here. ThisThe idea thermodynamic is depicted in Figure cycle 16 from. Figure 16 is a modified version of option 1 from Figure 1. The differenceThe thermodynamic is in the renewable cycle source from that Figure is connect 16 ised a modifiedto the energy version storage of optioninstallation. 1 from The Figure electric1. Thepower diff fromerence wind, is in thesun renewable or another source is used that to isincrease connected the temperature to the energy of storage the molten installation. salt in Thethe electric powerheater. fromThe electric wind, sun power or another from the is usedRES is to stored increase only the iftemperature its level exceeds of the the molten current salt demand in the electric in the grid.heater. This The solution electric poweris similar from to the the RES solutions is stored in onlythe countries if its level exceedswith sufficient the current solar demand irradiance in the or grid. the Thisstrength solution of the is similarwind, where to the solutionsthe renewables in the countriesfeed the withenergy su ffistorage.cient solar In the irradiance countries or thewith strength worse conditionsof the wind, for where the theextraction renewables of the feed energy the energy from storage.the renewables In the countries the presence with worseof the conditions supercritical for turbinethe extraction guarantees of the the energy continuity from theof the renewables power supply the presence to the grid. of the supercritical turbine guarantees the continuity of the power supply to the grid.

Figure 16. 16. CycleCycle with with a storage a storage system system for energy for from energy the fromrenewable the renewablesources and sources the supercritical and the turbine.supercritical turbine.

The results ofof thethe simulationssimulations for for the the cycle cycle integrated integrated with with the the renewables renewables are are shown shown in in Figures Figures 17 17and and 18 .18. They They were were conducted conducted without without deﬁning defining the the particular particular typetype ofof thethe renewable energy source. source. Figure 17 presents the efficiency of the power generation for different shares of the power from the renewables. In order to compare this case with the previous cases the simulations were conducted for constant levels of the power from the renewables. The efficiency is defined as:

Nel,spr tload1 + Nel,RES⁄Nel,n,spr+Nel,sub 24h η = (8) Qd tload In the above equation the share of the power from the renewables is relative to the nominal power of the supercritical turbine. The values of the efficiency from Figure 17 are high because the power from the renewables is treated here as an effect that does not require an input in the form of a

Energies 2020, 13, 2222 15 of 18

Figure 17 presents the efficiency of the power generation for different shares of the power from the renewables. In order to compare this case with the previous cases the simulations were conducted for constant levels of the power from the renewables. The efficiency is defined as:

Energies 2020, 13, x FOR PEER REVIEW 15 of 18 Nel,spr tload 1+ Nel,RES/Nel,n,spr +Nel,sub 24h η = (8) delivered energy. The graph in Figure 17 was obtainedQd tload for a simulation with the full load operation of the subcritical turbine, so Nel,sub/Nel,n,sub is 100%. In the above equation the share of the power from the renewables is relative to the nominal power Energies 2020, 13, x FOR PEER REVIEW 15 of 18 of the supercritical turbine. The values of the eﬃciency from Figure 17 are high because the power deliveredfrom the renewables energy. The is treatedgraph in here Figure as an 17 e ﬀwasect thatobtained does notfor requirea simulation an input with in the the full form load of a operation delivered energy. The graph in Figure 17 was obtained for a simulation with the full load operation of the of the subcritical turbine, so Nel,sub/Nel,n,sub is 100%. subcritical turbine, so Nel,sub/Nel,n,sub is 100%.

Figure 17. Power generation efficiency for different share of the power from the renewables - modernization according to the option 1.

The integration of the steam turbines and the renewables significantly shortens the loading time of theFigure energy 17. 17. storage. PowerPower Ageneration comparison generation efficiency of eloadingﬃ ciencyfor different times for is dishare shownﬀerent of inthe share Figure power of 18. thefrom powerthe renewables from the - modernizationrenewables—modernization according to the according option 1. to the option 1.

The integration of the steam turbines and the renewables significantly shortens the loading time of the energy storage. A comparison of loading times is shown in Figure 18.

Figure 18. LoadingLoading time time of of the the energy storage for 24 h of operation of the subcritical turbine with differentdifferent shareshare ofof thethe power power from from the the renewables—option renewables - option 1—full 1 - full graph graph (a) ( anda) and its enlargedits enlarged section section (b). (b). The integration of the steam turbines and the renewables significantly shortens the loading time of theItFigure energyshould 18. storage. beLoading emphasized Atime comparison of thatthe energy a constant of loading storage powe timesfor r24 supply ish shownof operation from in the Figure of renewabl the 18 subcritical. es during turbine the with loading was assumeddifferentIt should share bein order emphasized of the to power compare thatfrom a the constant renewablesoptions power and - option supplyvariants 1 - from full of graphthe the thermal renewables (a) and cyclesits enlarged during analyzed thesection loading in the wasstudy. assumed(b ).In practice in order the renewable to compare sources, the options especial andly variantsthe wind, of exhibit the thermal significant cycles fluctuations analyzed in of the a currentstudy. Inpower. practice the renewable sources, especially the wind, exhibit significant fluctuations of a currentIt should power. be emphasized that a constant power supply from the renewables during the loading 5.5.was Comparison assumed in order to compare the options and variants of the thermal cycles analyzed in the study. In practice the renewable sources, especially the wind, exhibit significant fluctuations of a The results of the simulations for different cases are compared in Figure 19. The figure presents current power. the power generation efficiency for the energy storage loading with the power supply from the supercritical turbine of 35%. The following options are compared: 5.5. Comparison • O1, O2 and O3—options 1, 2 and 3, respectively, The results of the simulations for different cases are compared in Figure 19. The figure presents the power generation efficiency for the energy storage loading with the power supply from the supercritical turbine of 35%. The following options are compared: • O1, O2 and O3—options 1, 2 and 3, respectively,

Energies 2020, 13, 2222 16 of 18

5.5. Comparison The results of the simulations for different cases are compared in Figure 19. The figure presents the power generation efficiency for the energy storage loading with the power supply from the supercritical turbine of 35%. The following options are compared: Energies 2020, 13, x FOR PEER REVIEW 16 of 18 O1, O2 and O3—options 1, 2 and 3, respectively, • • SHP,in t,in O2, pSHP,in =O2,f(m pt,in)—option = f(m 2,) - supercritical option 2, supercritical steam pressure steam adjusted pressure according adjusted to Equationaccording (3), to • Equation (3), O2, THP,sub,TIP,sub = 515 ◦C, • • o O1, RES—option O2, THP,sub 1 integrated, TIP,sub = 515 with C, RES, the power supplied from the RES is 5% of the nominal • • supercritical O1, steam RES turbine - option power. 1 integrated with RES, the power supplied from the RES is 5% of the nominal supercritical steam turbine power.

Figure 19. Power generation efficiency eﬃciency for the analyzed cases.

The comparison of the options show showss that the option 1 always provides the largest efficiency efficiency of the power generation. The The conversion conversion of of the the electric electric power into the heat stored in the molten salt is more efficient efficient than than the the conversion conversion of of the the heat heat in in th thee hot hot steam. steam. The The efficiency efficiency may may be improved be improved if the if thesteam steam parameters parameters are areadjusted. adjusted. This This applies applies to tothe the steam steam parameters parameters in in the the supercritical supercritical and and the subcritical turbine. Option 3 that is a variant of option 2 with an additional steam cooler provides an efficiency efficiency gain but it is smaller than the gain from the adjustment of the steam parameters. Of course the adjustment should also also be be made made for for option option 3. 3. However However the the cost cost of ofthe the additional additional steam steam cooler cooler may may not notjustify justify the theupgrade. upgrade. It should It should be noted be noted that that the the adjustment adjustment of ofthe the steam steam parameters parameters is is done done without without any investment cost.

6. Conclusions The application of the energy storage system as a buffer buﬀer between a supercritical and a subcritical turbine allows one to create a unit thatthat generatesgenerates power over a veryvery widewide rangerange ofof loads.loads. A unit of this type may be an important part of a power sy systemstem with a large number of the renewable energy sources. The wide range of the loads allows one to decrease the total number of similar units that are required in the system foundation. It also makes the unit less dependent on the ambient conditions.conditions. The presented presented solutions solutions are are designed designed for for power power grids grids that thatundergo undergo a transformation a transformation from fossil from fossilfuels fuelsto green to green energy. energy. The Thesolution solution involves involves the the machines machines from from existing existing systems systems that that are are still operational. Their harmful impact on the natural en environmentvironment is reduced due to the integration with newer technologies. Among the optionsoptions presentedpresented herehere the the option option that that allows allows to to use use the the renewable renewable energy energy sources sources in thein the power power plant plant that that is is a foundationa foundation of of a a power power grid grid showed showed thethe bestbest eefficiency.ﬃciency. However,However, this was obtained in the simulations under the assumption th thatat the energy supplied from the RES is is constant, constant, which is hardly true. The integration of the RES with a core power plant should be carefully analyzed for a particular location and local conditions regarding the availability of the renewables. The difference between the options presented here is the type of the energy conversion during the loading process. Option 1 with the electric energy supplied to the heater that increases the temperature of the molten salt performed better in the simulations in terms of the power generation efficiency. Option 2 with the heat in the steam converted to the heat in the molten salt had lower

Energies 2020, 13, 2222 17 of 18 which is hardly true. The integration of the RES with a core power plant should be carefully analyzed for a particular location and local conditions regarding the availability of the renewables. The difference between the options presented here is the type of the energy conversion during the loading process. Option 1 with the electric energy supplied to the heater that increases the temperature of the molten salt performed better in the simulations in terms of the power generation efficiency. Option 2 with the heat in the steam converted to the heat in the molten salt had lower efficiency even with the adjusted parameters of the steam. Option 3 is just a minor improvement when compared to option 2. In all the options the supercritical steam boiler always operates with the highest possible efficiency. This translates to the lowest possible emissions per a unit of the produced electric power. The energy storage system with the molten salt that was analyzed here is very often applied to solar power plants. The salt is pushed through vertical solar tower in a number of tubes exposed to the sun. The flow must be divided into a considerable large number of pipes due to the exposure to the sunlight. The salt heater in the cycles described here is free from this restriction.

Author Contributions: Conceptualization, W.K. and A.R.; methodology, W.K. and A.R.; software, W.K.; validation, A.R.; investigation, W.K. and A.R.; data curation, W.K.; writing—original draft preparation, W.K.; writing—review and editing, A.R.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The research was supported by research funds of the Silesian University of Technology, Gliwice, Poland. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

HP high pressure subcritical steam turbine IP intermediate pressure subcritical steam turbine LP low pressure subcritical steam turbine m mass ﬂow rate [kg/s] Nel electric power [MW] p pressure [MPa] Qd heat delivered to a cycle [MW] SHP high pressure supercritical steam turbine SIP intermediate pressure supercritical steam turbine SLP low pressure supercritical steam turbine T temperature [◦C] XHP high pressure heat exchanger, subcritical steam section XIP intermediate pressure heat exchanger, subcritical steam section XSHP high pressure heat exchanger, supercritical steam section XSIP intermediate pressure heat exchanger, supercritical steam section η power generation eﬃciency [%] Subscripts b boiler in inlet out outlet n nominal, design RES renewable energy source spr supercritical sub subcritical t turbine Energies 2020, 13, 2222 18 of 18

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