Combined-cycle gas turbine power plant integration with cascaded latent heat thermal storage for fast dynamic responses

Abstract The combined-cycle gas turbine (CCGT) power plants are often required to provide the essential fast grid balance service between the load demand and power supply with the increase of the intermittent power generation from renewable energy sources. It is extremely challenging to ensure CCGT power plants operating flexibly and also maintaining its efficiency at the same time. This paper presents the feasibility study of a CCGT power plant combined with the cascaded latent heat storage (CLHS) for plant flexible operation. A 420 MW CCGT power plant and a CLHS dynamic models are developed in Aspen Plus based on a novel modelling approach. The plant start-up processes are studied, and large amount of thermal energy can be accumulated by CLHS during the start-up. For load-following operation, extensive dynamic simulation study is conducted and the simulation results show that the extracted exhaust gas can be used for thermal energy storage charging, and the stored heat can be discharged to produce high temperature and high pressure steam fed to the steam turbine. Besides, the stored heat can also be used to maintain the heat recovery steam generator (HRSG) under warm condition to reduce plant restart-up time. The simulation results demonstrate that the integration of CLHS with CCGT power plant is feasible during the start-up, load-following and standstill operations.


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The combined-cycle gas turbine (CCGT) power plants are often required to provide the essential fast 10 grid balance service between the load demand and power supply with the increase of the intermittent 11 power generation from renewable energy sources. It is extremely challenging to ensure CCGT power 12 plants operating flexibly and also maintaining its efficiency at the same time. This paper presents the 13 feasibility study of a CCGT power plant combined with the cascaded latent heat storage (CLHS) for 14 plant flexible operation. A 420 MW CCGT power plant and a CLHS dynamic models are developed 15 in Aspen Plus based on a novel modelling approach. The plant start-up processes are studied, and 16 large amount of thermal energy can be accumulated by CLHS during the start-up. For load-following 17 operation, extensive dynamic simulation study is conducted and the simulation results show that the 18 extracted exhaust gas can be used for thermal energy storage charging, and the stored heat can be 19 discharged to produce high temperature and high pressure steam fed to the steam turbine. Besides, the 20 stored heat can also be used to maintain the heat recovery steam generator (HRSG) under warm 21 condition to reduce plant restart-up time. The simulation results demonstrate that the integration of 22 CLHS with CCGT power plant is feasible during the start-up, load-following and standstill 23 operations. 24 25 Keywords: combined-cycle gas turbine; cascaded latent heat storage; flexible operation; dynamic 26 modelling; Aspen Plus 27 28 Highlights: 29 · Dynamic modelling of combined-cycle gas turbine power plant with thermal storage. 30 · Cascaded latent heat storage integration strategies to plant operation processes. 31 · Complete system dynamic simulations of the plant with cascaded latent heat storage. 32 · Quantified analysis of stored and released thermal energy for different strategies. 33 34

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Combined-cycle power generation technology has been developed and served as an effective means 36 for base load supply worldwide since the 1960s due to its inherent advantages in high efficiency and 37 operational flexibility [1]. Although the technology in design and operation of combined-cycle gas 38 turbine (CCGT) plants is now widely available, CCGT plants face new technical challenges nowadays 39 in terms of efficient flexible operation to support the integration of intermittent renewable energy. 40 Over the past 10 years, the capacity of intermittent renewable energy has increased dramatically, 41 which has a significant impact on maintaining the balancing of the power generation and demand. 42 This forces CCGT power plants into a role change: from base load supply to fast response operating 43 services. This has led to a series of potential issues, such as low plant operation energy efficiency, low 44 load factors, and potentially shortened plant life time. To address those issues, this paper investigates 45 a new potential solution -to integrate the plant with thermal storage to create an energy buffer for fast 46 energy dispatch to support plant flexible operation. 47 48 In recent years, the study on flexible plant operation has started being given important consideration 49 and several studies the start-up process of CCGT power plants are reported [2,3]. Those paper 50 focused on optimizing the start-up process, but the dynamic performance of CCGT power plants 51 operating flexibly under different load conditions have not been extensively studied. With the increase 52 of renewable generation, the impact of passive operation of power plants during load changes has 53 received more attention. The flexible operation of CCGT power plants could enhance the stability of 54 the grid dynamics and maximise short-term high profits, but it will lead to a significant reduction in 55 the This paper is organised as follows: Section 2 brief describes the CCGT power plant and its operating 98 conditions; Section 3 presents the mathematical models of the gas turbine, HRSG, steam turbine, and 99 CLHS; Section 4 offers results and discussion of the proposed integration strategies; finally, in 100 Section 5 conclusions in relation to this overall study are drawn, with clearly outlined suggestions for 101 future exploitation. 102 103

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A CCGT power plant generally consists of the gas turbine, HRSG and steam turbines, as shown in 105 Figure 1. Air is compressed via a compressor and is mixed with natural gas (NG) in the combustion 106 chamber for combustion, then hot combustion gas expands in the gas turbine, which forms a Brayton 107 cycle; the heat from the gas turbine exhaust is used to generate steam for steam turbine, that is, the 108 heat passes through the HRSG to heat the water flow, which formulates a Rankine cycle. In this way, 109 the CCGT power plant can achieve a much higher thermal efficiency than a single cycle gas turbine 110 power plant, because the waste heat from the gas turbine exhaust is recovered via the HRSG which is 111 then used by the steam turbines for electricity generation. in Table 1  The temperature of the compressor outlet stream is given by: 153 where, out T is the outlet temperature, and in T is the inlet temperature. The air composition used in 155 modelling are given in Table 2. 156 The natural gas composition used in the modelling is given in Table 3. It consists of methane, ethane,  161 propane, nitrogen, carbon dioxide, and other gases and the methane and ethane account make up more 162 than 99% of the total volume [27]. Therefore, only two reactions are considered in the combustion 163 process: 164  For the turbine, it was modelled as an isentropic process, and its output power is calculated by Eq. (6) 170 [22]: 171 The isentropic efficiency of the turbine is defined as [28]: 177 where, t n t n is the ratio of rotating speed to its designed value, and t m t m is the ratio of mass flow rate to 179 its designed value. 180 181 The temperature of the turbine outlet stream is given by: 182 (9) 183

HRSG section modelling 185
The HRSG is modelled as a group of heat exchangers in this study. The exhaust gas from the gas 186 turbine enters the HRSG, where the waste heat is recovered to produce steam at different pressure 187 levels (HP, IP, and LP). The heat exchanger dynamic model was developed based on energy and mass 188 balance equations. 189 190 The energy conservation equation is given by [29,30]: and mass balance gives [3]: 193 The heat flux can be calculated by Eq. (12): 196 (12) 197 198 In order to capture the dynamics of the heat exchanger, the heat exchanger is discretized into several 199 zones, as shown in Figure In the model simulation, the thermodynamic properties (e.g. heat capacity and density) of the exhaust 211 gas and water/steam are updated at every time-step based on the current temperature and pressure 212 using Aspen Plus's thermodynamic database. In the CLHS system, thermal energy is transferred to the storage media during charging, and is 226 released in later discharging step. There are mainly three types of thermal energy storage: sensible 227 heat storage, latent heat storage, and chemical heat storage [7]. The latent heat storage will be used for 228 this study because its energy density is much higher than sensible heat storage [32, 33] and the cost is 229 lower than chemical heat storage. Besides, heat transfer irreversibility of a latent heat storage system 230 can be significantly reduced using cascaded phase change materials [7].  Figure 5 (b). The 241 entire CLHS system consists of 5600 sets of such concentric tubes in parallel. 242 243 The consideration for such an arrangement is that heat is required to be quickly absorbed or released 244 during the charging or discharging processes. At rated state, the temperature of gas turbine exhaust gas is 846 K, therefore the material PCM1 is 254 chose whose melting temperature is 773 K. In this way, the outlet temperature of PCM1 will not 255 exceed 773 K for the charging process. This guarantees the maximum temperature of PCM2 will be 256 less than 773 K. Moreover, the PCMs have to operate around the melting point to ensure safety and 257 without poisonous gas generated. For these reasons, the materials listed in Table 4  from the inner tube to the PCM and the heat diffusion in the PCM, the heat transfer is by means of 266 heat conduction. The heat loss through the outer tube of the CLHS system is assumed negligible. 267 Figure 6 shows a portion of a three-dimensional heat conduction grid. 268 269 In a cylindrical-coordinate system, the three-dimensional heat conduction equation for the point P in 273 the Figure 6 is given by [37]: where, subscript P denotes the point P shown in Figure 6. 276 277 Due to the cylinder is symmetrical, the unique temperature in θ direction is assumed. Therefore, the 278 heat conduction equation in the cylinder is given by [38]: The discretization equation is obtained by integrating the differential equations in the control volume 282 over the time interval from t to t t + D . The discretized equation is shown as follows [37]: The ΔV is the volume of the control volume, which is given by: 290 There are three methods available for solving the discretised partial differential equation that depends 293 on the value of the weighting factor ( f ). In particular, 0 f = leads to the explicit scheme, 0.5 f = 294 to the Crank-Nicolson scheme, and 1 f = to the fully implicit scheme. The explicit scheme is used to 295 discretize the differential equation in this study, as follows: 296

a T a T a T a T a T a a a a a T
The discretization equation is given by: 311

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This section presents the integration strategies of CCGT power plant with CLHS during the start-up, 325 load-following operation, and standby, respectively. In particular, the start-up procedure is studied, 326 and the idea of energy storage during plant start-up is proposed. The paper examines how the 327 integration of CLHS impact on the performance of the plant regarding to the output power and CLHS 328 charging or discharging processes. The plant output power can be regulated through variation of 329 CLHS charging and discharging processes. only a small part of the exhaust gas passes through the HRSG at the start-up, and most of the exhaust 339 gas is directly discharged into the atmosphere, resulting in energy loss. As described in [42], 340 approximately 75% of the exhaust gas (513 kg/s in this study) from the gas turbine is discharged into 341 the atmosphere for 25 minutes during the plant start-up. However, this waste energy is potential to be 342 captured by the CLHS, as shown in Figure 7. The 75% of exhaust gas may first pass through the 343 CLHS before discharging into atmosphere, and the other 25% of exhaust gas flows into HRSG, during 344 the plant start-up process. A filter is needed to remove the corrosive gases of the exhaust gas, as 345 shown in Figure 7, and the gas pressure of CLHS outlet is assumed to be the same as the atmosphere. 346 In this way, waste heat in the exhaust gas can be captured by the PCM layers in the CLHS. 347 For PCM layers filled at the same height in the CLHS system, it can be assumed that they have the 350 same temperature distribution due to their parallel structure [33]. Then the study of the entire CLHS 351 system can be simplified as a study of one set of concentric tubes ( Figure 5 (a)). In order to establish a 352 reasonable initial temperature distribution of the PCM layers such that a phase change process occurs 353 in the simulation, a temperature below the phase change point of each PCM is used to start up the 354 CLHS, as listed in Table 5; when the local temperature reaches the phase transition point, the 355 temperature distribution of each PCM at that time is its initial temperature distribution, as shown in 356 Figure 8. The figure presents the temperature distribution of the shaded area in the Figure 5 (a). For 357 each PCM layer, the phase change temperature is reached first in the lower left corner as expected. 358 The axial temperature distribution coincides with the exhaust gas in the inner tube, while the radial 359 temperature distribution also follows the heat conduction from the inside to the outside of the PCM. 360 361  After 1500 seconds of simulated charging process, waste heat in the exhaust gas is further diffused 365 and stored in the PCMs. The lowest local temperature of each PCM layer reaches the phase transition 366 point, and the temperature in the region where the local temperature is higher than the phase change 367 point continues to increase after undergoing the phase change process. The updated temperature 368 distribution of different PCM layers are shown in Figure 9. The plotted temperature is the right side of 369 the concentric tubes (see Figure 5 (a)) and the gas flows from bottom to top, therefore, the heat 370 diffuses from left side to right side, and from bottom side to top side as well.

CLHS integration strategy during load-following operation 382
In addition to avoiding the energy loss of the exhaust gas during the start-up process, the real-time 383 output power of the CCGT power plant can be regulated within a certain range by the CLHS charging  384 and discharging processes. The response speed of CCGT power plant is mainly limited by the 385 water-steam cycle, therefore, this section focuses on the utilization strategies of thermal storage in 386 water-steam cycle. During off-peak time, part of the high-temperature exhaust gas is extracted from 387 the gas turbine as a heat source for CLHS charging (same as the layout shown in Figure 7). As the 388 result, the power generated by the steam turbines will be reduced, but the gas turbine section is still 389 operating under the rated load condition. The minimum steam turbine power is 66 MW when 363 kg/s 390 exhaust gas by pass to the CLHS for thermal storage. On the contrary, during peak time, part of the 391 feed water from the deaerator flows into the CLHS, undergoing the reverse process of charging, it 392 evaporates into high temperature steam, and then leaves CLHS as superheated steam, as shown in 393 Figure 10. The maximum steam turbine output power increases to 143 MW. In order to produce dry 394 steam for steam turbine, a separator is needed to separate water droplets from steam. Finally, the 395 stored thermal energy is released from the CLHS to the feed water, thereby increasing the power 396 output of the steam turbines. The simulated discharging process is as follows. At beginning, the power plant operates at the 401 nominal load condition, and the total output power is 420 MW, in which 285 MW is from the gas 402 turbine and 135 MW is from the steam turbines. Figure 11 shows the designed load demand dynamics. 403 At the 300th second, the load demand was reduced from 420 MW to 408 MW. After 1800 seconds, 404 the load demand returned to 420 MW. At the 2800th second, the load demand increased again from 405 420 MW to 428 MW and lasted 1200 seconds. During this period, the gas turbine has been operating 406 under rated conditions with an output power of 285 MW. As a result, the real-time power output of 407 the power plant is determined by the steam turbines. It should be pointed out that the initial 408 temperature distribution of the CLHS layers used for the load-following operation simulation is the 409 same as the initial temperature distribution (Figure 8)  To meeting the load demand reduction from 420 MW to 408 MW, correspondingly the steam turbine 416 output power was reduced from 135 MW to 123 MW, 60 kg/s of exhaust gas was extracted from the 417 gas turbine outlet and sent to the CLHS. This is under charging conditions, so the extracted gas also 418 flows from the bottom of the CLHS to its top, which is the direction along the PCM melting point in 419 decreasing order. Figure 12 shows the temperature distribution of different PCM layers at the end of 420 charging in the load-following operation (time = 2160s). Compared to the temperature distribution of 421 different PCM layers in the start-up operation (Figure 9), the radial temperature difference of each 422 PCM layer is significantly reduced. This is because the charging time in the load-following operation 423 is longer than that in the start-up operation. Thus, the thermal diffusion in the PCM is more fully. To meet the load demand increase from 420 MW to 428 MW, correspondingly the steam turbine 441 output power was increased from 135 MW to 143 MW, 10 kg/s of superheated steam produced by 442 CLHS was sent to IPTB. This is under discharging conditions, so the extracted feed water flows from 443 the top of the CLHS to its bottom, which is the direction along the PCM melting point in ascending 444 order. Figure 14 shows the temperature distribution of different PCM layers at the end of discharging 445 in the load-following operation (time = 4000s). Compared to the temperature distribution of different 446 PCM layers at the end of charging in the load-following operation (Figure 12), the radial temperature 447 is slowly reduced from the right end to the left end at the same height of each PCM layer. This proves 448 that an amount of heat has been transferred from the PCM layers to the feed water. It can be seen from the simulation results that since the latent heat energy density is much higher than 454 the sensible heat, although the temperature change is small, the amount of stored or released is large. 455 The CLHS system with different melting temperatures can make the temperature difference between 456 the working fluid and PCM large enough to ensure all PCMs phase changes. So that the CLHS system 457 makes heat transfer more efficient for both charging and discharging processes. 458 459 4.2.4 Load-following dynamics 460 Figure 15 shows the real-time output power of the steam turbines during load-following operation. 461 The steam turbines can correctly respond the load dynamics. Whenever the load changes, the steam 462 turbines can respond to them within 6 mins. The response time meets the Secondary Frequency 463 Response requirements of generating units specified in the GB Grid Code [43]. Figure 16 further 464 reveals the amount of heat stored and released over charging and discharging during load-following 465 operation. According to the calculation, a total of 54 GJ heat is stored in the CLHS system in the 1860 466 seconds and a total of 27.5 GJ heat is released to the feed water in the 1200 seconds. It can be seen 467 that each PCM layer stores a relatively equal amount of heat during charging, but that are very 468 different during discharging. The discharged heat from PCM4 is very small (0.1714 GJ), therefore it 469 is not visible from the figure. This is because heat transfer is mainly determined by the heat sink 470 (PCMs for charging and water for discharging) in both processes. During charging the local initial 471 temperature of each PCM layer is close to its own phase change temperature and phase change occurs 472 gradually throughout the PCM layers, so heat is stored primarily through latent heat of phase change 473 and the thermodynamic reversibility of the process is relatively greater. However, during discharging 474 the evaporation temperature of water does not change much, which causes its phase change to occur 475 in only a few layers and the thermodynamic reversibility of the process is relatively smaller. This 476 explanation can also be verified by the results shown in Figure 17. As can be seen, during charging 477 the temperature of the exhaust gas entering and exiting each PCM layer crosses its phase change 478 temperature (Figure 17 (a)), but during discharging only the temperature of the water entering and 479 exiting the PCM layer 2 and 3 crosses its phase change temperature (Figure 17 (b)). Therefore, based 480 on the different thermal properties of PCMs and water, it can be expected that there is an optimal 481 thickness for each phase change layer to maximize the charge and discharge performance. According to the initial temperature of the material, the start-up procedure of the CCGT power plant 494 can be divided into: hot, warm and cold start depending on the initial temperature of the material, with 495 standstill for up to 8 hours, 48 hours and 120 hours, respectively [1]. The start-up speed is limited by 496 the thermal stress of the steam turbine and HRSG. The longer the standstill time, the longer the 497 start-up time is required if there is no heat preservation measure adopted. Therefore, keeping the 498 HRSG warm is crucial vital for the CCGT power plant to restart faster. In fact, the stored thermal 499 energy can also be used to keep HRSG warm during plant standstill period. As shown in Figure 18, 500 during the off-load period, ambient air is fed into the CLHS to produce hot air, which is then sent to 501 the HRSG to compensate for the heat loss of the HRSG, thereby keeping the HRSG in a hot or warm 502 state ready for faster start-up. The potential approach is to keep the HRSG warm through the CLHS 503 instead of maintaining the natural circulation, so the gas turbine and steam turbines can be shut down. 504 This approach does not change the inherent structure of the HRSG and the working fluid, there should 505 be no major technical barrier in the implementation process. In addition, the air flow rate fed into the 506 CLHS is determined by the current temperature drop in the CLHS, and this process can be controlled 507 by a feedback loop. 508

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This paper describes the dynamic modelling and simulation study for CLHS integration into a 420 513 MW CCGT power plant for flexible plant operation. A modelling method is introduced to achieve 514 whole system dynamic simulation in Aspen Plus by an external FORTRAN code. The integration 515 strategies during start-up, load-following and standstill operations are proposed and studied. 516 517 The dynamic simulation results shown that the strategies for CLHS integration with CCGT power 518 plant is technically feasible. In the plant start-up processes, the gas turbine exhaust gas could pass 519 through CLHS before discharged into atmosphere, and then the waste heat can be captured by CLHS. 520 During the load-following operation, the output power of the CCGT power plant can be reduced by 521 extracting exhaust gas from the gas turbine, the extracted exhaust gas is used to charge the CLHS; and 522 the stored heat can be discharged to produce high temperature and high pressure steam for the steam 523 turbine to increase the output power. Meanwhile the gas turbine section is still running at the rated 524 load condition. Besides, the stored heat can also be used to maintain the HRSG under warm condition 525 to reduce restart-up time after a standstill. 526 527 To further improve the CLHS performance under various operating models, efforts could be directed 528 to its optimising design, such as optimising the layout of phase change materials according their 529 thermodynamic properties, and the air flow rate used to keep the HRSG warm during a standstill. 530 531