Integrating Electrical Heating Fluidized-Bed Heat Storage with Coal-Fired Power Plant for Deep Peak Shaving

Abstract

An electrical heating fluidized-bed thermal energy storage (EH-FB-TES) system is proposed for integration with a coal-fired power plant (CFPP) for deep peak shaving (DPS) due to its high energy storage density and extensive heat exchange performance. The primary objective of this study is to evaluate the thermodynamic performance and economic feasibility of the integrated EH-FB-TES system, specifically focusing on identifying the optimal coupling and heat recovery strategies for enhanced deep peak shaving performance. Since EH-FB-TES uses air flow for fluidization in the heating storage process, its coupling with the CFPP differs from other TES technologies, and the associated thermodynamic performance and cost are thereby analyzed. The results show that, in EH-FB-TES, the heat release efficiency is predominantly constrained by thermal losses. To increase the energy utilization efficiency, a two-stage heat recovery strategy is proposed to release the stored energy in the integration. The first stage is to heat up the feedwater extracted from the deaerator and the second one is to heat up the condensate water. The analyses also show that the selection of reinjection positions for the heated medium from EH-FB-TES greatly influences the system performance. Returning the stored thermal energy to heat up feedwater can effectively increase the output of the unit, while directly generating steam can be beneficial for coal saving. The integrated system achieves a maximum equivalent round-trip efficiency of 32.9% under 20 MW/800 °C conditions. An economic analysis reveals that, compared with other energy storage methods, EH-FB-TES can realize a relatively high energy storage density with a rather low cost. Under the present DPS compensation policy, for a 315 MW subcritical CFPP integrated with a 50 MW EH-FB-TES system, when heat storage is 8 h, heat release is 4 h per day, and the plant operates 100 days per year, the estimated static and dynamic payback periods are 3.06 years and 3.67 years, respectively. The integration of CFPP with EH-FB-TES could be promising for meeting DSP requirements.

1. Introduction

With the demand for carbon neutrality and the depletion of fossil fuels, constructing a power generation system with an increasing portion of renewable energy sources becomes a priority worldwide. Wind and solar power are the main kinds of renewable energies. However, due to their randomness, intermittency and volatility, a high proportion of wind and solar power poses a great challenge to the stability of the power grid. Thus, during the transition period from a high-carbon to a green power grid, in many countries such as China, coal-fired power plants (CFPPs) are still needed. However, instead of being a main power supplier, CFPPs play the role of supporting and regulating power supplies and often operate in a deep peak shaving (DPS) mode. Although, by retrofitting the boiler and steam turbine system, many CFPPs can operate at 30–40% of the rated load, they are desired to have a higher flexibility due to the fast development of renewable energies [1]. Consequently, a CFPP is highly recommended for integration with an energy storage system that can store surplus steam/electric energy and then release it when necessary [2].

Among the energy storage technologies, thermal energy storage (TES) is a promising one that can achieve a rather high energy storage density and power density at a relatively low cost [3]. The energy storage density refers to the amount of energy stored in a unit volume of medium, while the power density represents the rate of energy charging and discharging, reflecting the system’s ability for rapid heat exchange and dynamic response. Certainly, the higher the medium temperature, the higher the energy storage density. Presently, in commercial applications, molten salt-based TES (MS-TES) is the one that has the highest working temperature, up to around 560 °C [4]. This technology was originally used for concentrating solar power (CSP) plants [5] and was recently applied to CFPPs. When coupled with a CFPP, MS-TES can store thermal energy from main steam and reheat steam or flue gases [6,7,8,9], thereby reducing the electricity output, or store surplus electricity to realize DPS operation through a power-to-heat (P2H) approach [10].

So far, several studies have been conducted on the performance of MS-TES-coupled CFPP. Kosman and Rusin [11] proposed using MS-TES for a supercritical CFPP with an additional subcritical turbine. MS-TES was connected with two turbines to store excess electricity/steam for the supercritical cycle, which efficiently operated in nominal conditions. The coupling enabled the unit to supply the power grid below the minimum level for the supercritical cycle and enabled a fast start-up for the subcritical turbine. Miao et al. [12] proposed using MS-TES to store the heat released from reheat steam from a CFPP and found, through simulation, that the coupling could reduce the minimum power load from 30% to 16%, and further to zero by incorporating a P2H approach. The stored thermal energy could increase the output power of the CFPP by an additional 9.65%. Ma et al. [13] conducted a design and performance analysis of the DPS scheme for a CFPP integrated with a MS-TES system. They assessed the thermal efficiency, peak shaving depth, and economic performance of eight coupling schemes and found that the optimal one achieved a maximum peak shaving depth of 90.2%. Xu et al. [10] confirmed that a CFPP integrated with TES was capable of zero-power output, and evaluated the energy efficiency, exergy efficiency, heat rate, and peak shaving capability under five coupling schemes.

The existing studies showed that CFPP coupled with MS-TES can effectively utilize surplus or low-price electricity to achieve DPS. However, MS-TES has a few drawbacks. First, the commonly used salts are noxious, corrosive and rather expensive [14]. Second, most of the salts can only operate within a temperature less than 560 °C, beyond which they decompose, with poison gases released. Third, since salts have to operate in a liquid mode, their working temperature range, normally between 250 and 560 °C, is rather narrow. In addition, the salt has to remain above saturation temperature even in standby mode to prevent coagulation. Therefore, TES with a safer and higher performance is desired.

Researchers found that the solid concrete, rock, refractory bricks, sand, etc., could be stably sustained at high temperatures, even at 1000 °C, and several associated TES technologies were developed [15,16,17,18]. However, the existing technologies used a stationary or moving bed for heating. Due to the low conductivity of the solid media, large-scale application often encountered problems of uneven or inefficient heating and media cracking, and the actual temperature was normally below 400 °C [16], even lower than that of MS-TES. In contrast, solid particles have advantages including an excellent fluidity, a capacity for uniform heating, and a high thermal stress tolerance, which allow particles to be stored stably at very high temperatures exceeding 1000 °C. The concept of using solid particles for high-temperature TES also originated from the CSP field. Various particle technologies have been developed, such as free-falling particle curtains and centrifugal receivers [19]. Among them, the fluidized bed has attracted significant attention due to its ability to enhance heat transfer by fluidizing particles.

The fluidized bed is a proven heat exchanger with a high heat transfer coefficient (HTC) between the gas–solid flow of the heating surfaces. Recently, Ma et al. [18,20] proposed using it to transfer heat from solar energy-heated sand particles to high-temperature steam in a CSP plant. The simulation and experimental studies showed that, at a heating temperature of 800 °C, the HTC increased by 30–50% compared with stationary heating, and the system cost was reduced by 40% [21,22,23]. In our previous studies [24,25,26], a fluidized bed was proposed for use as an electrical heating device for energy storage, and the corresponding TES is called EH-FB-TES hereafter. With conventional SiC-based electric heating elements, sand particles could be effectively heated up to over 800 °C in a fluidized bed [24,25,26]. Since sand can work at a wide temperature range with excellent fluidity and transport properties, EH-FB-TES has the advantages of a low cost, relatively high efficiency and excellent stability in heat storage [26], which thus could have great potential for DPS in a CFPP. Due to its special features, compared with those of MS-TES, dedicated thermodynamic and economic analyses are needed.

In this study, thermodynamic performance analyses on the integration of EH-FB-TES with a 315 MW sub-critical CFPP are conducted by using Ebsilon. The influence of factors including the heating temperature, heat storage power and integration schemes on the system performance are evaluated using a set of indexes. Based on the analyses, the optimal integration scheme is proposed. In addition, an economic analysis is conducted.

2. Thermodynamic Model of CFPP with the Integration of EH-FB-TES

2.1. Thermodynamic Model of the CFPP Without EH-FB-TES

A typical 315 MW sub-critical CFPP used for combined heat and power (CHP) generation in China was selected for this study, and its main design parameters are listed in

Table 1. The operational parameters of the 315 MW sub-critical CFPP.

Parameters	Unit	Value
Main Steam Pressure	MPa	16.7
Reheat Steam Pressure	MPa	3.465
Main Steam Flow Rate	t/h	966.92
Reheat Steam Flow Rate	t/h	804.48
Condensate Flow Rate	t/h	735.32

.

2.2. EH-FB-TES and the System Description

Figure 1 shows the schematic of the EH-FB-TES system, and Figure 2 depicts the associated thermodynamic diagram. The operation of EH-FB-TES consisted of heat storage and heat release stages. In the energy storage stage, cold particles are fed into a fluidized-bed electric heater, where a set of electric heating elements are installed. After the particles are heated up to a preset temperature, they are conveyed by the fluidizing air and then stored in a hot-sand hopper after gas–solid separation. The main parameters for this stage are shown in

Table 2. Main parameters in the particle heat storage stage.

Parameters	Unit	Value
Temperature of Preheated Air	°C	350
Environment Temperature	°C	20
Heating Temperature	°C	600/800
Heat Storage Power	MW	50
Air/Solid Ratio p	%	50
Heat Dissipation Loss	%	3

. The heating temperature refers to the final particle temperature in the fluidized-bed electric heater. The heat storage power refers to the electricity consumed by the fans and the air–solid mixture heating, which includes particles and fluidizing air. It is assumed that, at the outlet of the fluidized bed, the exhaust gas and the particles have the same temperature, and the gas–solid separator has an efficiency of 99.5%. The heat dissipation loss is set at 3%, which is slightly higher than the 98% efficiency of particle heaters reported by Ma et al. [27]. In the table, the air/solid ratio p is defined by the following formula:

$$p={\displaystyle \frac{M_{\mathrm{a}\mathrm{i}\mathrm{r}}}{M_{\mathrm{p}}}}$$

(1)

in which $M_{\mathrm{a}\mathrm{i}\mathrm{r}}$ and $M_{\mathrm{p}}$ represent the mass flow rates of fluidizing air and particles.

In the heat release stage, the hot particles are sent into another fluidized-bed heat exchanger, where their thermal energy is converted into the enthalpy of steam or water in the high-pressure heater (HPH) and low-pressure heater (LPH) of the CFPP, while the heat of fluidizing air is recycled by the water in the LPH. The main parameters in the heat release stage are shown in

Table 3. Main parameters in the heat release stage.

Parameters	Unit	Value
Terminal Temperature Difference	°C	30
Environment Temperature	°C	20
Storage/Release Duration Ratio		2
Air/Solid Ratio	%	7
Heat Dissipation Loss	%	4.5

. In this stage, the air fluidizing velocity is low, with a small p value, calculated according to the operational parameters, with correlations given in references [28,29,30]. The terminal temperature difference (i.e., the temperature difference between the outlet particles and the outlet water) is adopted as a constant of 30 °C. For the heat release process, a combined heat dissipation loss of 4.5% is adopted, accounting for the cumulative effects of silo insulation dissipation (1–5% [31]) and the high efficiency of the fluidized-bed heat exchanger (around 99% [32]).

2.3. Integration Schemes of EH-FB-TES to CFPP with Two-Stage Heat Recovery

The coupling of the EH-FB-TES system with CFPP is distinct for the heat storage and release stages. To achieve DPS under stable combustion, in the heat storage stage, it is assumed that the fluidized-bed electric heater and auxiliary fans consume power directly from the CFPP, lowering the net on-grid power. Simultaneously, the enthalpy of the exhaust fluidizing air is used to preheat the condensate water, acting as a part of the low-pressure heater (LPH) to enhance the overall efficiency.

In contrast, the heat release process is thermally coupled with the steam–water cycle of the CFPP. The stored thermal energy is used to heat up the circulating water in the regenerative unit, partially substituting the high-pressure heaters (HPHs) and LPHs. For this selected CFPP, the regenerative unit consisted of three HPHs (HPH #1 to #3, with decreasing pressure), one sliding-pressure-operated deaerator, and four LPHs (LPH #4 to #7, with decreasing pressure). To achieve efficient energy utilization, a two-stage thermal energy recovery strategy is proposed. As shown in Figure 3, in the first stage, high-temperature particles and exhausted gases are used to heat up the feedwater extracted after the deaerator (the lowest-temperature point in the HPH). The heated medium can be reinjected from the outlet of HPH#1 to HPH#3 and then superheated in the boiler, or can be directly mixed with the main steam flow, partially replacing the HPH. In the second stage, the particles and exhaust air at lower temperatures are used to heat up the condensate water. The heated water could be reinjected into the outlets of any LPH. The reinjection temperature decreases progressively from LPH#4 to LPH#7, while the permissible flow rates of extracted feedwater increase accordingly.

2.4. Performance Evaluation Indexes

In order to evaluate the performance of the coupling of EH-FB-TES and CFPP, the following key parameters are defined and used.

The heat storage efficiency of the EH-FB-TES itself without integration, ${\eta }_{\mathrm{p}\mathrm{s}}$, is defined as

$${\eta }_{\mathrm{p}\mathrm{s}}={\displaystyle \frac{Q_{\mathrm{s}}}{P_{\mathrm{s}}}}$$

(2)

in which $Q_{\mathrm{s}}$ represents the thermal energy stored by the particles, and $P_{\mathrm{s}}$ represents the electrical power consumed in the heat storage stage.

The heat release efficiency of the EH-FB-TES itself without integration, ${\eta }_{\mathrm{p}\mathrm{r}}$, is defined as

$${\eta }_{\mathrm{p}\mathrm{r}}={\displaystyle \frac{Q_{\mathrm{r}}}{Q_{\mathrm{s}}+P_{\mathrm{r}}}}$$

(3)

in which $Q_{\mathrm{r}}$ represents the thermal energy utilized, and $P_{\mathrm{r}}$ represents the electrical power consumed in the heat release stage.

The sensitivity of the operational parameters in heat storage and release mode is defined as

$${\beta }_{\mathrm{y}}={\displaystyle \frac{∆y}{∆x}}$$

(4)

in which $∆y$ represents the change value of ${\eta }_{\mathrm{p}\mathrm{s}}$ or ${\eta }_{\mathrm{p}\mathrm{r}}$, and $∆x$ represents the change value of the main operational parameters including ambient temperature, heat loss, etc.

The heat rate of the unit $b_{\mathrm{h}}$ is defined as

$$b_{\mathrm{h}}={\displaystyle \frac{M_{\mathrm{m}}\left(H_{\mathrm{m}}-H_{\mathrm{f}\mathrm{w}}\right)+M_{\mathrm{r}\mathrm{e}}(H_{\mathrm{r}\mathrm{e}\mathrm{h}}-H_{\mathrm{r}\mathrm{e}\mathrm{l}})}{P_{\mathrm{e}}\times 3600}}$$

(5)

in which $P_{\mathrm{e}}$ represents the power output of the unit; $M_{\mathrm{m}}$ and $M_{\mathrm{r}\mathrm{e}}$ respectively represent the main and reheated steam flow rates; $H_{\mathrm{m}}$, $H_{\mathrm{f}\mathrm{w}}$, $H_{\mathrm{r}\mathrm{e}\mathrm{h}}$, and $H_{\mathrm{r}\mathrm{e}\mathrm{l}}$ respectively represent the enthalpy of the main steam, feedwater, high-temperature reheated steam and low-temperature reheated steam. The heat rate refers to the amount of heat consumed for 1 kW·h electricity for the steam turbine.

The coal consumption of the unit $b_{\mathrm{c}}$ is defined as

$$b_{\mathrm{c}}={\displaystyle \frac{M_{\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}}}{P_{\mathrm{e}}\times 3600}}$$

(6)

in which $M_{\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}}$ represents the mass of standard coal consumed by the unit per time, and $b_{\mathrm{c}}$ refers to the amount of coal consumed for generating 1 kW·h of electricity for the boiler.

The steam rate of the unit $b_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{a}\mathrm{m}}$ is defined as

$$b_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{a}\mathrm{m}}={\displaystyle \frac{M_{\mathrm{m}}}{P_{\mathrm{e}}\times 3600}}$$

(7)

b_steam refers to the amount of main steam consumed for generating 1 kW·h of electricity.

The thermal efficiency of the unit with the integration of the heat storage part, ${\eta }_{\mathrm{s}}$, is defined as

$${\eta }_{\mathrm{s}}={\displaystyle \frac{P_{\mathrm{e}}+Q_{\mathrm{s}}+Q_{\mathrm{a}}}{Q_0}}$$

(8)

in which $Q_{\mathrm{a}}$ represents the energy recycled from the exhaust gas of the heat storage stage, and $Q_0$ represents the thermal energy absorbed by the main and reheated steams from the boiler.

The thermal efficiency of the unit with the integration of the heat release part, ${\eta }_{\mathrm{r}}$, is defined as

$${\eta }_{\mathrm{r}}={\displaystyle \frac{P_{\mathrm{e}}}{Q_0+Q_{\mathrm{s}}}}$$

(9)

The equivalent round-trip efficiency of the unit with the integration of a cycle, ${\eta }_{\mathrm{e}}$, is defined as

$${\eta }_{\mathrm{e}}={\displaystyle \frac{P_{\mathrm{e}\mathrm{r}}}{P_{\mathrm{e}\mathrm{s}}}}\times {\displaystyle \frac{{\tau }_{\mathrm{r}}}{{\tau }_{\mathrm{s}}}}$$

(10)

in which $P_{\mathrm{e}\mathrm{r}}$ and $P_{\mathrm{e}\mathrm{s}}$ respectively represent the power generated in the heat storage and release stage, and ${\tau }_{\mathrm{s}}$ and ${\tau }_{\mathrm{r}}$ respectively represent the time duration of the heat storage and release stage.

The static investment payback period $T_{\mathrm{p}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}}$ [33] is defined as

$$\sum _{t=1}^{T_{\mathrm{p}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}}}{(CI-CO)}_{\mathrm{t}}=0$$

(11)

in which CI represents the cash inflow, which includes the revenue from selling electricity and peak shaving compensation. CO represents the cash outflow, which includes the investment cost of equipment, operation and maintenance costs, equipment depreciation costs, financial expenses, taxes, etc., and ${(CI-CO)}_{\mathrm{t}}$ represents the net cash flow in Year t.

The dynamic investment payback period $T_{\mathrm{p}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}}^{\prime }$ [33] is defined as

$$\sum _{t=1}^{T_{\mathrm{p}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}}^{\prime }}{\displaystyle \frac{{(CI-CO)}_{\mathrm{t}}}{{(1+i)}^{\mathrm{t}}}}=0$$

(12)

in which i represents the industry standard discount rate.

3. Results and Discussion

3.1. Validation of Thermodynamic Model

Before the analyses of the integrated system, a validation of the thermodynamic model built using Ebsilon 16 for the original CFPP is conducted. The output power and main operational parameters of the unit under the turbine heat acceptance (THA) condition are calculated and compared with the designed ones, as shown in

Table 4. Design values and simulation results of CFPP under THA case.

Parameters	Design Value	Simulation Value	Relative Error/%
Output Power (MW)	315
Main Steam Temperature (°C)	538	538	0
Reheat Steam Temperature (°C)	538	538	0
Main Steam Pressure (MPa)	16.7	16.7	0
Reheat Steam Pressure (MPa)	3.465	3.442	−0.66
Main Steam Flow Rate (t/h)	966.92	959.9	−0.73
Reheat Steam Flow Rate (t/h)	804.48	812.67	1.02
Condensate Water Flow Rate (t/h)	735.32	729.94	−0.73
Steam Rate (kg/kWh)	3.070	3.047	−0.75
Heat Rate (kJ/kWh)	7949.3	7917.8	−0.40

. The relative errors are small and acceptable.

The operational performance of the CFPP under off-design conditions was also simulated, with the unit operating in sliding-pressure mode.

Table 5. Design values and simulation results of CFPP under various partial loads.

Parameters		Main Steam Pressure (MPa)	Main Steam Flow Rate (kg/s)	Reheat Steam Pressure (MPa)	Reheat Steam Flow Rate (kg/s)
75%THA	Design data	14.3	696.43	2.56	591.95
	Simulation result	14.3	702.61	2.59	608.21
	Relative error (%)	0	0.89	−1.16	−2.67
50%THA	Design data	9.86	466.68	1.74	405.20
	Simulation result	9.86	469.45	1.75	415.32
	Relative error (%)	0	−0.59	−0.57	2.50
40%THA	Design data	8.08	383.07	1.43	335.24
	Simulation result	8.08	382.39	1.42	341.29
	Relative error (%)	0	−0.18	−0.70	1.80
30%THA	Design data	6.30	297.61	1.12	262.73
	Simulation result	6.30	293.46	1.09	264.59
	Relative error (%)	0	−1.39	−2.68	0.71

presents the key thermodynamic parameters for the main steam and reheat steam. The relative errors between the simulation results and the design values are minimal, which is acceptable, thereby validating the reliability of the CFPP model. However, regarding the integrated CFPP and EH-FB-TES system, a direct validation against experimental data was not feasible due to the lack of industrial reference systems and available measurement data.

3.2. Influence of Main Operational Parameters of EH-FB-TEC on Efficiency

Figure 4 shows the influences of the main operational parameters on ${\eta }_{\mathrm{p}\mathrm{s}}$ and ${\eta }_{\mathrm{p}\mathrm{r}}$, which are studied respectively at heating temperatures of 600 °C and 800 °C. The dominant factors affecting ${\eta }_{\mathrm{p}\mathrm{s}}$ are the preheated air temperature, heating temperature and air/solid ratio. At a higher heating temperature, the impacts of the preheated air temperature, heating temperature, heat loss and ambient temperature on ${\eta }_{\mathrm{p}\mathrm{s}}$ are weaker, while the influence of the air/solid ratio is stronger.

In the heat release stage, the most dominant parameter affecting ${\eta }_{\mathrm{p}\mathrm{r}}$ is the heat loss, followed by the final temperature difference, which directly governs the heat transfer capacity. As the heating temperature increases, the effect of heat loss on ${\eta }_{\mathrm{p}\mathrm{r}}$ increases, while the influences of other parameters become weaker. For the engineering design, it is critical to enhance the insulation properties of the storage vessel, minimize thermal storage duration, and maximize the heat transfer surface area of the exchanger. The fluidized-bed heat exchanger is a bubble bed with a relatively low air velocity and small air/solid ratio, whose impact on ${\eta }_{\mathrm{p}\mathrm{r}}$ is obviously weaker than that in the heat storage stage.

Figure 5 shows ${\eta }_{\mathrm{s}}$ at different heating temperatures and storage powers. Due to energy conversion from electricity to thermal energy and then to electricity, the ${\eta }_{\mathrm{s}}$ of the integrated system is always lower than that of the original system. The results also show that a higher thermal storage power or a lower heating temperature will result in an even lower ${\eta }_{\mathrm{s}}$. This is mainly because more particles are heated up at a higher temperature and storage power, and a larger particle flow rate needs more fluidizing air, resulting in a higher heat loss. The results are consistent with the sensitivity analyses shown in Figure 4, i.e., ${\eta }_{\mathrm{s}}$ is sensitive to the air/solid ratio. They also confirm that the heat recovery of the exhaust gas is necessary for EH-FB-TES.

3.3. Influence of Coupling Scheme on Thermodynamic Performance

Figure 6 compares the impacts of the reinjection positions of the heated media in an LPH on the performance of the integrated CFPP, evaluated using the heat rate $b_{\mathrm{h}}$ and integrated system efficiency ${\eta }_{\mathrm{s}}$. The results show that, after integration, $b_{\mathrm{h}}$ becomes smaller, indicating an enhancement in energy efficiency. Specifically, a higher heating temperature reinjection results in a lower $b_{\mathrm{h}}$. Position #4 at 800 °C achieves the minimal heat rate and is regarded as the optimal reinjection.

Table 6. Unit performance under different reinjection positions.

Parameters	Original System	#0	#1	#2	#3
Reinjection Position	/	Inlet of HPT	Outlet of HPH #1	Outlet of HPH #2	Outlet of HPH #3
Reinjection Temperature (°C)	/	538	277	246	\
Reinjection Flow Rate (t/h)	/	72.48	416.13	624.47	\
Heat Rate (kJ/kWh)	7917.8	7177.5	7405.8	7422.04	\
Power Generation (MW)	315.0	324	343.5	342.0	\

presents the reinjection temperatures, flow rates, heat of the unit, and power generation capacity for different potential reinjection positions in HPH under a storage energy of 50 MW. Due to the low temperature and excessive flow rate, Position #3 (referred to Figure 3) is not considered for reinjection. As the reinjection temperature decreases, the heat rate increases but is still lower than that of the original CFPP. The power generation output of CFPP for Position #1 reinjection is the highest, followed by Position #2, and the lowest is Position #0. Position #1 reinjection achieves a lower unit heat rate and a higher power generation output compared to Position #2 reinjection, indicating that Position #1 reinjection is better. This is consistent with the fact that Position #4 has a higher reinjection water temperature and a better performance than Positions #5 to #7 in the LPH. The evaluation of reinjection at Position #0 versus Position #1 indicates that the reinjection temperature at Position #0 is higher, and the unit can utilize higher-grade thermal energy for direct power generation, resulting in a lower heat rate but a reduced overall output power. Therefore, Positions #0 and #1 have their own advantages, and users can select them based on their needs.

Figure 7 shows the influence of the reinjection temperature on the performance of the integrated CFPP. The dashed curve corresponds to the unit’s power output, while the solid one represents the standard coal consumption of the unit. As the reinjection temperature increases, both the coal consumption and output power decrease. With the same energy input from EH-FB-TES to CFPP, the lower coal consumption causes a lower power generation. The discontinuity phenomena in both curves are due to the phase transition at the saturation temperature of 351 °C and pressure of 16.7 MPa. The reinjection flows on the left side of the curves are hot water, while those on the right side are superheated steam. For hot water, EH-FB-TES partially substitutes HPHs, thereby reducing the heat rate and coal consumption. Meanwhile, steam reinjection allows EH-FB-TES to behave not only as an HPH but also as the steam drum and superheater. The heated media can be directly integrated into the main steam. This approach is similar to using an additional waste heat boiler to directly generate electricity. Based on the distinct characteristics, Position #0 is named the Coal-Saving Position and Position #1 is the Power Generation Position. Under the condition of a storage–release duration ratio of 2, total thermal losses during the heat storage and release process of 7.5%, and full-load unit operation, the coupled system performance is analyzed at different thermal storage powers and heating temperatures.

As shown in Figure 8, the thermal loss during the heat storage–release cycle leads to a reduction in the overall system efficiency. The system efficiency marginally increases with the heating temperature, while it significantly declines with increasing storage power. Coal-Saving Position coupling achieves a superior thermal efficiency.

Figure 9 further shows that Coal-Saving Position coupling provides a limited power enhancement against Power Generation Position coupling, and the increased heating temperature reduces the output power. Consistent with Figure 8, Coal-Saving Position coupling leads to a reduction in the feedwater flow rate of the boiler for main steam generation. Consequently, coal consumption, and thereby the total power generation, decreases. Similarly, the higher storage temperature enhances the steam generation capacity of the heat storage module, further saving coal.

3.4. Economic Analysis

3.4.1. Cost Estimation

To evaluate the economic viability of the integrated CFPP, the cost estimation of the retrofit and construction investment was conducted. The heating temperature, storage power, and capacity of the EH-FB-TES are assumed to be 800 °C, 50 MW, and 400 MWh, respectively. It should be noted that all capital costs and financial parameters listed below are standardized to the price level of the base year 2024.

Table 7. Cost of the particle storage tanks (CNY/kWh_th).

Component	Material Cost	Construction Cost	Total Cost
High-temperature tank body	6.3	12	18.3
Low-temperature tank body	1.5	1	11.5
Insulation material	4	2	6
Foundation and supporting structure	8	8	16
Total cost	19.8	32	51.8

shows the costs of the tank bodies, materials, foundation and supporting structure of the solid particle storage tanks. The calculation method of the wall thickness refers to the model proposed by Zhao et al. [34], and the mass of the material is estimated using the tank diameter and wall thickness. Considering the allowable stress variation in materials with temperature and material cost, poured refractory material and 20# steel are selected for high-temperature and low-temperature storage tanks, with unit costs of 6000 and 5500 CNY/t, respectively. Assuming that the ratio of material processing fees to raw material costs is 1:1, the construction cost, insulation material cost, and the cost of the foundation and support structure are calculated through the estimation method used for the molten-based salt storage tank [35].

The main cost of the fluidized-bed electric heater comes from the material of the built-in heating elements and its electric control box cost and the material and construction costs of the heater body.

Table 8. Cost of the fluidized-bed electric heater (CNY/kWh_th).

Component	Material Cost	Construction Cost	Total Cost
Fluidized-bed electric heater body	50	50	100
Built-in heating elements	150	100	250
Insulation material	20	20	40
Foundation and supporting structure	50	50	100
Total cost	270	220	490

presents the cost based on the quotations from equipment manufacturers.

There are two types of heat exchangers in EH-FB-TES. The air preheater and the heat exchanger are used to recover the residual heat of the exhaust gas. Their cost estimations refer to the method used for the tubular heat exchanger [36], and the material adopted is 20# steel. The particle–water heat exchanger for the heat release process adopts the fluidized-bed heat exchanger designed by Gomez-Garcia et al. [32], and its shell structure is made of poured refractory material. The costs include manufacturing and construction, etc. It is assumed that the total cost of each heat exchanger is four times the material cost.

Table 9. Detailed cost list of all the components.

Component	Total Cost (Million CNY)	Unit Cost
Particle storage tanks	20.72	51.8	CNY/kWhth
Fluidized-bed electric heater	24.5	490	CNY/kWe
Particle-water heat exchangers	6.5	16.25	CNY/kWhth
Air preheater	0.8	2	CNY/kWhth
Fans	0.45	1.13	CNY/kWhth
Cable	0.4	1	CNY/kWhth
Particle flow rate controller	6	120	CNY/kWe
Auxiliary equipment	6	120	CNY/kWe
Particles	0.15	3	CNY/kWe
Electrical instrument	3	60	CNY/kWe
Pipes, valves and other fittings	2	40	CNY/kWe
Total cost of the system	70.52	833	CNY/kWe
		72.18	CNY/kWhth
Initial investment cost	77.57
Operation and maintenance cost		2.2 million	CNY/Year

lists the cost of the components of the integrated system, including the auxiliary cost, management expenses and land cost incurred during construction. It is assumed that the initial investment cost is 1.1 times the total manufacturing cost. The results show that the initial investment cost of the entire system is slightly less than that of the reported MS-TES system [37].

Meanwhile, it is assumed that the annual operation and maintenance cost of the system is average throughout the life cycle, including labor costs, material costs, insurance costs, maintenance costs, and others. For labor costs, it is assumed that the annual labor cost is 250,000 CNY, the withdrawal rate of welfare and other funds is 60%, and the average annual growth rate of wages is 6%. For material costs, it is assumed that the maintenance cost per unit of the heat storage system during the system’s life cycle is 10 CNY/kWh_th, and the maintenance cost of the heating system is 20 CNY/kW_e. The insurance rate is 0.25% of the value of fixed assets. Maintenance costs account for 0.5% of the value of fixed assets in the 2nd year, 1% from the 3rd to the 16th year, and 1.5% after the 17th year. Other charges are assumed to be at a unit price of 10 CNY/kW_e. The calculated average annual operation and maintenance cost is 2.2 million CNY.

Figure 10 shows the unit cost and energy density of EH-FB-TES compared with a few other types of energy storage systems [35,38,39]. Among them, the heat storage density of EH-FB-TES is 250–350 kWh/m³, and the equivalent energy storage density is estimated as 100–140 kWh/m³ based on the assumption of a thermal–electric conversion efficiency of 40%. Figure 10 illustrates the comparative advantages of EH-FB-TES. While electrochemical batteries suffer from high costs and their mechanical storage (e.g., pumped hydro) is limited by a low energy density, EH-FB-TES strikes a balance. It achieves the lowest cost (<100 CNY/kWh) among all compared technologies and maintains a high energy storage density. Notably, compared to the mature MS-TES, EH-FB-TES offers a comparable energy density but with a distinct cost advantage, confirming its economic feasibility for large-scale applications.

3.4.2. Economic Benefit of DPS Operation

The integrated CFPP can be operated in DPS mode, in which it stores the redundant power of the CFPP during the day when there is sufficient wind and solar power and releases the energy back to the unit during the peak electricity consumption period at night. The unit operates at 50%THA, 40%THA and 30%THA, respectively, in the heat storage stage, with an energy storage power of 50 MW and a heat storage duration of 8 h. In the heat release stage, the unit operates in power generation mode at 100%THA, with a heat release duration of 4 h.

Based on the DPS compensation policies for thermal power plants in different regions of China [40] for the actual market conditions in 2024, the typical compensation prices under different peak shaving depths are classified and shown in

Table 10. The typical quotations for DPS compensation under different peak shaving depths.

Peak Shaving Depth (%THA)	Compensation (CNY/kWh)	Peak Shaving Depth (%THA)	Compensation (CNY/kWh)
(45, 50]	0.1	(30, 35]	0.5
(40, 45]	0.3	(25, 30]	0.6
(35, 40]	0.4	(0, 25]	0.7

. The on-grid electricity price of CFPPs is assumed to remain at 0.34 CNY/kWh, while the coal price fluctuates, significantly affected by supply and demand. It is also assumed that the auxiliary electricity consumption of the power plant is 5%, i.e., the power supply to the grid accounts for 95% of the power generation. Figure 11 shows the operating profits of the system before and after coupling EH-FB-TES under different coal prices within one cycle.

According to the cost estimation in Section 3.4.1, the static and dynamic payback periods $T_{\mathrm{p}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}}$ and $T_{\mathrm{p}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}}^{\prime }$ are 9.58 years and 15.46 years, respectively, when the system stores heat at 50%THA, 4.31 years and 5.03 years at 40%THA, and 3.06 years and 3.67 years at 30%THA.

4. Conclusions

This study conducts thermodynamic and economic analyses of the integration of an electrical heating fluidized-bed thermal energy storage (EH-FB-TES) system with a coal-fired power plant (CFPP) for deep peak shaving (DPS). EH-FB-TES employs sand particles electrically heated to 600 °C or 800 °C at a storage power surpassing conventional thermal storage temperature limits. The thermodynamic analyses demonstrate that the preheated air temperature, heating temperature, and air/solid ratio are the dominant factors affecting the heat storage efficiency, with heat loss being the most influential parameter for the heat release efficiency. The thermal efficiency increases with particle temperatures but declines with increasing storage power levels.

To increase the energy utilization efficiency, a two-stage heat recovery strategy is proposed. Namely, the high-grade thermal energy from particles and exhaust gases is used to heat up feedwater extracted after the deaerator, partially replacing the high-pressure heater (HPH), or is directly evaporated and supplied to the high-pressure turbine (HPT), while the intermediate temperature energy is used to heat up the condensate water, partially substituting the low-pressure heater (LPH). Coal-Saving Position coupling uses thermal energy stored in particles to generate steam and apply it into the HPT, which achieves a lower coal consumption but results in lower power generation. Meanwhile, Power Generation Position coupling heats up feedwater, which can generate more power but consumes more coal. The system achieves a maximum equivalent round-trip efficiency of 32.9% under 20 MW/800 °C conditions when using the Power Generation Position. Compared with other energy storage methods, EH-FB-TES has a relatively high energy storage density, while having a low energy storage cost. When the system for DPS stores heat at 30%THA while integrated with an FB-TES of 50 MW and a heat storage duration of 8 h, the static and dynamic payback periods are 3.06 years and 3.67 years.

However, the heat loss induced by the introduced fluidized air remains a limitation of the EH-FB-TES system; the stable and efficient circulation of particles requires further optimization, and the associated research should be carried out in the future.