Thermodynamic analysis of a High Temperature Pumped Thermal Electricity Storage (HT-PTES) integrated with a parallel organic Rankine cycle (ORC)

Energy Conversion and Management 177 (2018) 150–160

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Thermodynamic analysis of a High Temperature Pumped Thermal Electricity Storage (HT-PTES) integrated with a parallel organic Rankine cycle (ORC) Long Xiang Chena, Peng Hub, Pan Pan Zhaoc, Mei Na Xiea, Feng Xiang Wanga,

T

⁎

a

Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Jinjiang 362200, China Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230027, China c Hefei General Machinery Research Institute, Hefei 230088, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Pumped thermal electricity storage High temperature HT-PTES Parallel ORC Thermodynamic analysis

Pumped thermal electricity storage (PTES) using packed bed is an attractive large-scale energy storage technology. The performance of conventional PTES is limited by the existing technology of compressor, such as low isentropic efficiency and cannot bear high temperature. In this work, a high temperature PTES (HT-PTES) based on an additional electric heater is proposed to enhance the energy storage capacity of PTES. Waste heat, which produced due to the irreversibility of heating, compression and expansion process of both PTES and HT-PTES, is recovered by the organic Rankine cycle (ORC) to generate power. Air and argon (Ar) are investigated as working fluid for PTES and air is selected due to its high thermal performance and economy. Five types of PTES combined with ORC system namely, PTES, HT-PTES, PTES + ORC, HT-PTES + ORC and HT-PTES + parallel ORC are investigated based on transient analysis method. The simulation results show that combined with ORC is an effective approach to improve the round trip efficiency (RTE) of both PTES and HT-PTES. In the five types of combined systems, the HT-PTES + parallel ORC is considered as a more promising large-scale energy storage technology which advantages can be illustrated as follows: (1) it with an acceptable RTE of 47.67%, which is 5.68% higher that of HT-CAES and is only 2.46% lower than the maximum RTE of the five types; (2) it shows an appropriate operating pressure, which are 1.05 MPa for HT-PTES subsystem and 12.20 MPa for ORC subsystem (significantly lower than that of 31.2 MPa for ORC in the HT-PTES + ORC); (3) it presents a considerable energy storage density of 218.69 MJ/m3, which is more than twice that of PTES + ORC (88.14 MJ/m3).

1. Introduction Renewable energy sources (RESs) such as sources of wind energy and solar energy are rapidly emerging as solutions to the challenging climate-change problem which caused by utilization of fossil fuels [1]. With the development of RESs, many challenges have presented due to their unpredictable and cannot be easily controlled [2]. Energy storage systems (ESSs) provide a wide array of technological approaches to solve these challenges due to their potential in load balancing in the electricity grid as well as storing the surplus power during peak production periods for later use at peak demand periods [3]. Energy can be stored in various forms, such as mechanical, chemical, electrostatic, magnetic, biological, and thermal [4]. Despite the large number of available storage technologies, only three of them can be considered as large-scale electricity storage technologies currently (> 100 MWh): pumped hydro storage (PHS), compressed air energy ⁎

storage (CAES) and flow batteries [5]. The PHS has much advantages such as fast response, high round trip efficiency (RTE), low self-discharge rate and long time, hence it is the dominating large-scale ESS technology currently [6]. However, in developed countries, the most suitable sites for PHS are almost being used [7]. Furthermore more, it may have a significant impact on the environment in terms of soil, biodiversity, and water quality [8]. CAES is proposed as an effect energy storage technology due to its high reliability and economic feasibility [9]. However, it suffers from a low RTE and is dependent on fossil fuels results in CO2 emissions [10]. This problem can be partially solved by an adiabatic CAES (A-CAES). In A-CAES, the heat of compression is absorbed and stored by a thermal energy storage (TES) system in charging process, and it is employed to heat the compressed air in discharging process [11]. As a result, the RTE over 70% can be obtained and fossil fuels are ignored. To meet the different energy requirements in terms of electricity, cooling energy,

Corresponding author. E-mail address: [email protected] (F.X. Wang).

https://doi.org/10.1016/j.enconman.2018.09.049 Received 17 July 2018; Received in revised form 10 September 2018; Accepted 15 September 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.

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Nomenclature

f gen hot in out p s turb w ∞

Symbols A cp d D Ex G h hv k L ṁ P r t T U V Ẇ W ρ μ

area, m2 specific heat capacities, kJ/(kg·K) diameter, m diameter, m exergy, kJ mass flow rate per unit cross section, kg/(m3·s) specific enthalpy, kJ/kg volumetric heat transfer coefficient, W/(m3·K) coefficient of thermal conductivity, W/(m·K) length, m mass flow rate, kg/s pressure, kPa radius, m time, s temperature, K heat transfer coefficient, W/(m2·K) volume, m3 work input rate, kW work, kJ density, kg/m3 dynamic viscosity, Pa·s

Acronyms CP CON EH G HP HE HT HHE LP LT LHE M PH RTE TB

Subscripts 0 c com cold con char dischar ES

fluid generation hot fluid inlet outlet particle solid turbine wall ambient conditions

ambient conditions compression process compressor cold fluid consuming charging process discharging process energy storage

compressor condenser electric heater generator high pressure heat exchanger high temperature turbine high temperature heat exchanger low pressure low temperature turbine low temperature heat exchanger motor pre-heater round trip efficiency turbine

Greek letters π η ε ψ

pressure ratio isentropic efficiency void fraction shape factor

heat engine cycle is employed to convert the stored thermal energy back into electricity. Compared to PHS and CAES, PTES benefits from relatively high energy storage densities, no geographical constraints and a small installation footprint [19]. Thess [20] formulated a simple thermodynamic model based on the Carnot cycle to predicts the efficiency of PTES (defined as Pumped Heat Electricity Storage PHES in literature), which was set as a function of the temperature of the thermal energy storage at maximum output power. Guo et al. [21] proposed a more realistic thermodynamic model for PTES based on the Brayton cycle, at first the finite-rate heat transfer and external heat leakage losses were considered [21], then the internal and external irreversible losses were took into account [22], and more recently a more universal thermodynamic was established which based on the weak dissipative assumption [6]. Brayton cycle is always utilized as thermodynamic cycle for PTES, it uses a single phase gas like air or argon (Ar) as working fluids. Desrues et al. [23] presented a PTES for large-scale electric applications based on Brayton cycle and using argon gas as the working fluid. A RTE of 66.7% can be achieved, while a maximum temperature of 1000 °C should be required, which has further exceeded the working temperature of existing compressor (about 900 K) [24]. The constraint can be partially solved by introducing an additional electric heater. Benato [5] proposed an innovative PTES power system, which also based on Brayton cycle, while air was utilized as working fluid and five types of high storage material properties were investigated. In the PTES an electric heater is employed to enhance the maximum storage temperature, hence this parameter is not affected by the compressor

and thermal energy, the tri-generation CAES (T-CAES) is presented [12]. In normal A-CAES, a throttling process should be required to release the air out of vessels, which causes considerable exergy losses. This problem can be partially addressed by the isobaric A-CAES (IACAES), and many forms of IA-CAES have been studied, such as based on the pumped water [13], phase change process of volatile fluid [14], and hydrostatic pressure of sea water [15]. For conventional CAES and ACAES, suitable geographical conditions (underground hard-rock or salt caverns, porous rock formation and depleted natural gas field) are required for air storage, which limit their application. This constrain can be partially addressed by introducing of artificial vessels. Moreover, the air in the artificial vessels is expected to be cleaner, which should increase the operating lifetime of the turbine [16]. However, an additional artificial vessels bring increment of system cost. Flow batteries are a relatively young technology. Interest in the development of flow batteries for large-scale grid storage is growing, due to their unique features including ease of scalability, high efficiency, flexible operation and long cycle life [17]. However, compared to PHS and CAES, flow batteries provide a limited output power and are still expensive [5]. Pumped thermal electricity storage (PTES) is the last in-developing storage technology suitable for large-scale energy storage applications [18]. The PTES consists a compressor, a turbine, and two man-made thermally isolated tanks: one hot and one cold, and its basic concept is fairly simple. In charging process, a heat pump cycle is adopted, which transforms the renewable energy or off-peak electricity into thermal energy and is stored in hot and cold tanks. In discharging process, a 151

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pressure. However, the proposed PTES with a low RTE of about 10%, which may be caused by two main reasons: one is the exergy losses in electric heater and the other one is the energy losses in the process of removing thermal front out of thermal storage tanks. Parameters that effect on the PTES were studied and optimized results were presented by McTigue et al. [19]. The thermodynamic analysis results demonstrate that the RTE of PTES was governed mainly by the efficiency of the compression and expansion processes, the RTE unlikely to exceed 50% if the compressors and expanders could only achieve efficiencies typical of turbomachinery. Transcritical Rankine cycle is another common used thermodynamic cycle for PTES, which has been investigated by many researchers. Mercangöz et al. [25] proposed a novel type of bulk electricity storage called electrothermal energy storage (ETES), which utilizing transcritical CO2 cycle as thermodynamic cycle. In this case, pumped heat was stored in hot water, while cold energy was also absorbed by water which then melting and transformed into ice. A similar CO2 transcritical cycle was presented by Morandin [26], heat pump was stored in hot water, while the cold energy was stored in salt-water ice. Frate et al. [27] showed a novel PTES, which took advantage of a lowgrade heat source to boost the RTE of system beyond 100%, in this case the low-grade heat is considered as free. Wang et al. [10] considered that the external heat sources are not available everywhere, and cold energy from liquid natural gas (LNG) can be used as the heat sink to enhance the RTE of PTES beyond 100%. However, the PTES utilizing the transcritical Rankine cycle as thermodynamic cycle has energy storage density lower than 22 kWh/ m3 [10]. When the external heat sources are not considered as free, it will be even lower. In this work, the PTES based on the Brayton cycle and packed bed is investigated due to its higher energy storage density (110–170 kWh/m3 [18]). To further improve the energy storage density, an electric heater employed in [5] is also considered. The heat produced by the irreversibility of heating, compression and expansion process should be released out from working fluid by heat exchangers [23], which is an ideal heat source for organic Rankine cycle (ORC) system. The RTE of PTES is improved due to partial of waste heat transformed into electricity by ORC. ORC is considered to be a promising low grade thermal energy recovery technologies due to its simple structure, high efficiency and environment friendly [28]. For standard ORC, the temperature profiles between working fluid and heat transfer medium cannot match mutually causing a large number of exergy lost in the heat-transfer processes [29]. In order to solve this problem, some advanced ORCs are put forward, e.g. the ORC using the zeotropic mixture [30], trilateral cycle [31], dual-loop ORC [32] and transcritical ORC [33]. The efficiency of zeotropic ORC is higher than that of simple ORC due to the better match of the exchange curves, while the uncertainty in the fluid properties and unknown heat coefficient values may cause some issues in their practical realization [34]. Trilateral cycle is the cycle with the best recovery efficiency for sensible heat sources, however it is great limited by the lack of an efficient two phase expander [34]. The dual-loop ORC is more suitable for recover the multi-grades waste heat, such as the heat from exhaust gas and coolant of engine [35]. Transcritical ORC is another method to decrease thermal irreversibilities in the heat-transfer processes. Moreover, the high temperature of the cycle allows to achieve a better thermal efficiency than that of standard ORC, if a recuperator is introduced [36] and to operate with a small turbine [37]. In this work, air and argon as working fluid for PTES were investigated, and promising working fluid was selected based on the comparison results of their thermal efficiencies and operating pressures. A high temperature PTES (HT-PTES) based on an additional electric heater is proposed to improve the energy storage density of PTES. HTPTES combined with ORC is considered to recover the heat which produced due to the irreversibility of heating, compression and expansion process. Two types of ORC is investigated, one is normal ORC

Fig. 1. Schematic diagram of charging process of HT-PTES system.

and the other one is so called parallel ORC. The parallel ORC consists two ORC which aims to recover the heat from heat source at two temperature ranges. Transient analysis method is employed to calculate the performance of five types of PTES combined with ORC system namely, PTES, HT-PTES, PTES + ORC, HT-PTES + ORC and HTPTES + parallel ORC. The comparison of simulation results for five types of PTES combined with ORC are carried out.

2. System configuration and operating principle Fig. 1 shows the schematic diagram of high temperature PTES (HTPTES) in charging process, which is almost the same with conventional PTES. The corresponding T-s diagram of both HT-PTES and PTES are shown in Fig. 2. In conventional PTES, the working fluid from low pressure tank (LP tank) is regulated to a designed temperature (450 K) by heat exchanger 1 (HE1). Then it flows into compressor 1 (CP1) and is compressed to a target temperature (900 K) by using surplus or intermittent electricity. The heat of high pressure and high temperature working fluid is absorbed by the thermal storage material when it goes through high pressure tank (HP tank). A heat exchanger (HE2) is required to cool down the working fluid to near ambient temperature (T0). After that, it expands in the turbine (TB1) to near ambient pressure and its temperature decreases rapidly which is much lower than the ambient temperature. Finally, the cold working fluid is heated to a medium temperature (slightly higher than 450 K) and the cold energy from working fluid is stored in the LP tank. Differ from conventional PTES, the working fluid from CP1 is firstly heated by the electric heater (EH) to a higher temperature (1400 K), then it flows into HP tank.

Fig. 2. T-s diagram of both PTES and HT-PTES systems in charging process. 152

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can be achieved as follows,

In discharging process, the system consists two main subsystems, HT-PTES system and ORC system. As seen in Fig. 3, the HT-PTES system (with a solid line in the figure) has the same schematic diagram with conventional PTES, while the working fluid works in different operation conditionals (seen in Fig. 4, single quotation marks refer to the state of PTES). Firstly, the low pressure and cold working fluid from LP tank is compressed by the compressor 2 (CP2) to a designed pressure. Then the warm and high pressure working fluid flows into HP tank directly and absorbs heat from thermal storage material. After that, the high temperature (1400 K for HT-PTES and 900 K for PTES) and high pressure working fluid expands in the turbine (TB2) to generate power. The working fluid released from TB2 are regulated to a medium temperature by high temperature heat exchanger (HHE) and low temperature heat exchanger (LHE). Finally, the working fluid entering the LP tank will release heat and obtain cold energy from thermal storage material. The heat from HHE and LHE is higher than 450 K, which seems to be an ideal heat source for ORC to transform heat into electricity. For normal ORC, the condensed working fluid is firstly compressed to a target pressure by the pump. Then, it through preheater (PH1) to receive heat from outlet turbine (HT). After that, the working fluid absorbs heat from outlet of TB2 in HHE, and evaporates to vapor phase. The high temperature and high pressure working fluid flows into HT and expands to generate electricity. The outlet of HT is firstly cooled in PH1 and then condensed in condenser (CON). From Fig. 4, it also can be found that the outlet temperature of TB2 (state 4b) in HT-PTES is much higher than that in PTES (state 4b′). For a normal ORC, to transform more thermal energy from the high temperature heat source into power, means high expansion ratio for expander is required, which will raise up the system operating pressure and increase system costs. This problem can be partially solved by a novel parallel ORC. In the parallel ORC, the thermal energy from heat source is recovered at different temperature range, such as a heat source at temperature range 450–750 K can be divided into two temperature ranges 450–600 K (from LHE) and 600–750 K (from HHE). As seen in Fig. 3, the parallel ORC consists two normal ORCs, a single pump is utilized to raise up the working fluid pressure, while two expanders (HT and LT) working in different operation temperature. Several operating configurations of PTES combined with ORC systems are evaluated in this work, their schematic diagrams can be decomposed from Figs. 1 and 3 for charging and discharging process, respectively. Their processes and characteristics are summarized in Tables 1 and 2 for charging and discharging process, respectively.

Pc, out = πc Pc, in

(1)

where Pc,in and πc are inlet pressure and pressure ratio of the compressor, respectively. The power consumed by compressor can be expressed as,

̇ = ṁ c, in (hc, out −hc, in ) Wcom

(2)

where ṁ c, in is mass flow rate of inlet fluid for compressor, hc,out and hc,in is the specific enthalpy of outlet and inlet of working fluid, respectively. The isentropic efficiency of compressor ηcom is defined as:

ηcom = (hc, out , is−hc, in )/(hc, out −hc, in )

(3)

where hc,out,is is the isentropic specific enthalpy of compressor outlet. Pump which is a component of ORC, has the same function with the compressor, while the working fluid is operating in liquid phase. Hence, its outlet pressure, power consuming and isentropic efficiency also can be calculated by Eqs. (1)–(3), respectively. 3.2. Turbine Turbine is employed to extract energy from working fluid and converts it into electrical power when combined with a generator. The outlet pressure of turbine can be described as follow,

Pturb, out = Pturb, in/ πturb

(4)

where Pturb,in is inlet pressure of the turbine, πturb is pressure ratio of turbine. The power generation of turbine can be expressed as follow,

̇ = ṁ turb, in (hturb, in−hturb, out ) Wturb

(5)

where ṁ turb, in is the inlet mass flow rate for turbine, and hturb,out and hturb,in is the specific enthalpy of outlet and inlet of turbine, respectively. The isentropic efficiency of turbine ηturb is defined as:

ηturb = (hturb, out −hturb, in)/(hturb, out , is−hturb, in)

(6)

where hturb,out,is is the isentropic specific enthalpy of turbine outlet. 3.3. Heat exchanger Heat exchangers are applied to regulate the working fluid to a designed temperature (such as HE1, HE2 and CON) or recover heat from

3. Thermodynamic analyses The following assumptions are made to simplify the analysis of PTES, HT-PTES and ORC systems. (1) The composition of air in PTES and HT-PTES systems consist of 75.57% N2, 23.16% O2 and 1.2691% Ar (mass fraction). (2) The isentropic efficiency is set constant for compressors, pump and expanders. (3) The heat and pressure loss in the pipes connecting all the components are negligible. (4) All the kinetic and potential effects are ignored. (5) The electric efficiency of the compressor motor and of the expander generator is equal to one. (6) The thermal energy from electric heater is absorbed completely by working fluid. 3.1. Compressor and pump The compressors (CP1 and CP2) are utilized to increase the pressure of working fluid in PTES and HT-PTES, in this situation the working fluid is operating in the gas phase. The outlet pressure of compressor

Fig. 3. Schematic diagram of HT-PTES combined with parallel ORC system in discharging process. 153

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(1−ε ) ρs cp, s

∂Ts ∂ 2T = ks 2s + h v (Ts−Tf ) ∂t ∂x

(9)

In Eqs. (8) and (9) cp,f and cp,s are the fluid and solid specific heat capacities, uf is the superficial velocity of the fluid moving through the bed, Ac is the surface area of the side of storage tank, V is the volume of storage tank, Uw is the heat transfer coefficient of the wall of storage tank. The void fraction is denoted as ε, which is evaluated as function of the ratio particle diameter dp to packed bed diameter D [38]:

ε = 0.375 + 0.17

dp 2 + 0.39 ⎛ ⎞ D ⎝D⎠

dp

⎜

⎟

(10)

The parameter hv is the volumetric heat transfer coefficient, the correlation by Coutier and Farber [39] is used to calculate hv:

G h v = 700 ⎜⎛ ⎟⎞ d ⎝ p⎠

Table 1 Process and characteristics of PTES and HT-PTES in charging process. Process

Characteristics

PTES HT-PTES

1a-2a-3a-5a-6a-7a-1a 1a-2a-3a-4a-5a-6a-7a-1a

Without electric heater With electric heater

ks = 5.85 +

Cp, s = 1117 + 0.14T −411exp(−0.006T )

1.75ρf (1−ε ) 2 150μ (1−ε )2 ΔP = 2 2 uf + uf L ψdp ε3 ψ dp ε 3

3.5. Energy balance equation for each components The detailed energy balance equation for each components in charging and discharging process are shown in Tables 3 and 4, respectively. 3.6. System evaluation Round trip efficiency (RTE) is defined as the ratio between total power consuming in the discharging process and the total power generation in discharging process in a cycle period. In charging process, the power is mainly consumed by the PTES/HT-PTES, while in discharging process both PTES/HT-PTES and ORC systems can generate power. As the heating energy is not considered in the reference, RTE can be calculated as follow:

For fluid phase:

+ ερf cp, f uf

∂Tf ∂x

= h v (Ts−Tf )−

Ac Uw (Tf −T∞) V

(14)

where L is the length of packed bed, μ is the dynamic viscosity of the fluid and ψ is the shape factor to correct for the filling material not being spherical.

A quasi-one-dimensional two-phase transient model is formulated, which considers the losses through the walls and temperature-dependent thermophysical properties of the fluid and solid phases. To simplify the analysis, some assumptions are made in this work: (1) temperature gradient in radial direction is ignored; (2) no internal heat generation; (3) uniform temperature particles; (4) no mass transfer between solid and fluid phases; (5) radiation heat transfer is neglected; (6) heat conduction in the fluid phase is neglected, due to it is small compared to the convective heat transfer. The model consists a set of two energy balance equations, the first one for fluid (subscript f), while the second one for the solid filler material (subscript s):

∂t

(13)

Pressure losses in the packed bed is accounted for using the Ergun equation, which providing the void fraction is in the range 0.33 < ε < 0.55 [41]. The Ergun equation states:

3.4. Packed bed

∂Tf

(12)

(7)

where ṁ and h are the mass flow rate and specific enthalpy, respectively. The subscript hot, cold and HE are denoted hot fluid, cold fluid and heat exchanger, respectively.

ερf cp, f

15360exp(−0.002T ) T + 516

where T (°C) is the temperature of α-Alumina. Munro also provides the corresponding interpolation formula of specific heat capacity for αAlumina:

higher grade heat source (such as LHE, HHE, PH1 and PH2). For the heat exchangers the energy balance equation is:

ṁ hot , HE (hhot , in−hhot , out ) = ṁ cold, HE (hcold, out −hcold, in )

(11)

where G is the mass flux flowing through the packed bed thermal storage system. The α-Alumina is employed as thermal storage material in this work, hence the thermal conductivity ks is computed by the Munro’s corresponding interpolation formula [40]:

Fig. 4. T-s diagram of both PTES and HT-PTES systems in discharging process.

System

0.76

(8)

For solid phase:

Table 2 Process and characteristics of PTES, HT-PTES and ORC in charging process. System

Process

Characteristics

PTES HT-PTES for ORC HT-PTES for parallel ORC ORC Parallel ORC

1b-2b-3b-4b-6b-1b 1b-2b-3b-4b-6b-1b 1b-2b-3b-4b-5b-6b-1b 1c-2c-3c-4c-5c-6c-7c-1c 1c-2c-3c-4c-5c-6c-7c-13c-1c 1c-2c-8c-9c-10c-11c-12c-13c-1c

Without LHE HHE is employed as the heat source for ORC and LHE is ignored Both LHE and HHE are employed as heat source for ORC With preheater The parallel ORC have the same expansion ratio while operating at different temperature range

154

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Table 3 Energy balance equation for each components in charging process. Components

Energy balance equation

LP tank

QLP, char = − ∫0 char (ṁ 7a h7a−ṁ 1a h1a)−Ac Uw (Tf −T∞) dt

HE1

QHE1, release = ∫0 char ṁ 1a (h1a−h2a) dt

CP1

t WCP1 = ∫0 char ṁ 2a (h3a−h2a) dt t Qelectricity = ∫0 char ṁ 3a (h4a−h3a) dt

t

t

EH

t

HP tank

QHP, char = ∫0 char (ṁ 4a h4a−ṁ 5a h5a)−Ac Uw (Tf −T∞) dt

HE2

QHE 2, release = ∫0 char ṁ 5a (h5a−h6a) dt

t

TB1

WTB1 =

RTE =

∫0tchar

ṁ 6a (h6a−h7a) dt

Wgen, PTES + Wgen, ORC Wcon, PTES

(15)

where the Wgen,PTES and Wgen,ORC are the total power generation of PTES/HT-PTES and ORC systems, respectively. Wcon,PTES is the total power consuming of PTES/HT-PTES. The energy storage density (ρES) is defined as the ratio between total power generation in discharging process and total volume of HP tank and LP tank (Vtotal), it can be computed as:

ρES =

Fig. 5. Comparison between numerical predictions and experimental data [42] of packed bed.

properties which is calculated by the REFPROP 9.1 [43]. Fig. 5 and Table 5 give the comparison results between our numerical predictions and experimental data of packed bed from [42] at different instants of time. As seen in Table 5, although the maximum error between experimental and predicted data is 6.78%, the error between most of experimental and predicted data are less than 1%. Hence, the comparison results demonstrate that temperature and position of the thermal front can be well predicted by the numerical model.

Wgen, PTES + Wgen, ORC (16)

Vtotal

4. Results and discussion In this work, an electric heater is added after the compressor in charging process for conventional PTES aims to enhance the inlet temperature of high pressure tank and improve the energy storage density of the PTES system. Additionally, the ORC system is employed to recover the heat which produced due to the irreversibility of heating, turbines and compressors. Several operating configurations of PTES combined with ORC are evaluated in this section.

4.2. Thermodynamic analysis In this section, air and argon (Ar) are investigated as the working fluid for PTES. R123 is proposed as the working fluid for ORC, due to its high efficiency and acceptable environmental properties (ozone depletion potential ODP 0.02 and global warming potential GWP 77 [28]). Several operating configurations of PTES combined with ORC are evaluated and compared in this section. The thermodynamics properties of all the working fluids are calculated by the REFPROP 9.1 developed by the National Institute of Standards and Technology of the United States (NIST) [43].

4.1. Thermodynamic model validation The calculation model of the compressor, turbine, pump and heat exchanger are validated in our previous work [14]. Hence, only the numerical model of packed bed is validated in this work. To validate the model, the numerical predictions are compared with experimental results by Hänchen et al. [42]. In our numerical model, the diameter and length of packed bed considered in [42] is adopted, while the gas properties calculated from ideal gas equation is replaced by the real gas

4.2.1. Working fluid selection for PTES Ar is proposed as a promising working fluid for PTES [19], due to its high specific heat ratio, which implies that a designed outlet temperature of compressor can be achieved with a lower pressure ratio. Air

Table 4 Energy balance equation for each components in discharging process. Components

Energy balance equation

LP tank

QLP, dischar = ∫0 dischar (ṁ 6b h6b−ṁ 1b h1b)−Ac Uw (Tf −T∞) dt

LHE

t QLP, dischar = ∫0 dischar (ṁ 5b h5b−ṁ 6b h6b) dt t QLP, dischar = ∫0 dischar (ṁ 4b h4b−ṁ 5b h5b) dt tdischar WTB2 = ∫0 ṁ 3b (h3b−h4b) dt

HHE TB2 HP tank

t

t

= ∫0 dischar (ṁ 10c h10c−ṁ 9c h9c ) dt t

= ∫0 dischar (ṁ 5c h5c−ṁ 4c h4c ) dt

t

QHP, dischar = ∫0 dischar (ṁ 3b h3b−ṁ 2b h2b) + Ac Uw (Tf −T∞) dt

∫0tdischar

CP2

WCP 2 =

CON

QCON , release = ∫0 dischar ṁ 13c (h13c−h1c ) dt

Pump

Wpump = ∫0 dischar ṁ 1c (h2c−h1c ) dt

PH1

QPH 1, dischar = ∫0 dischar (ṁ 4c h4c−ṁ 3c h3c ) dt = ∫0 dischar (ṁ 6c h6c−ṁ 7c h7c ) dt

HT

t WHT = ∫0 dischar ṁ 5c (h5c−h6c ) dt t QLP, dischar = ∫0 dischar (ṁ 9c h9c−ṁ 8c h8c ) dt tdischar WLT = ∫0 ṁ 10c (h10c−h11c ) dt

PH2 LT

ṁ 1b (h2b−h1b) dt t

t

t

155

t

t

= ∫0 dischar (ṁ 11c h11c−ṁ 12c h12c ) dt

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Table 5 Validation results for the packed bed. Column height (m)

0.09 0.33 0.51 0.72 0.91 1.08

t = 1200 s

t = 3000 s

t = 4800 s

Texp (K)

Tcal (K)

Error (%)

Texp (K)

Tcal (K)

Error (%)

Texp (K)

Tcal (K)

Error (%)

635.0 350.0 302.0 298.0 295.0 294.0

641.2 348 300.5 293.6 293.1 293

0.98% −0.57% −0.50% −1.48% −0.64% −0.34%

795.0 600.0 420.0 325.0 301.0 294.0

794 602.3 430.1 327.9 300.6 294.55

−0.13% 0.38% 2.40% 0.89% −0.13% 0.19%

810.0 750.0 590.0 445.0 355.0 325.0

814 750.9 630.0 466.2 365.2 318.35

0.49% 0.12% 6.78% 4.76% 2.87% −2.05%

is also proposed as the working fluid for PTES [5] because it can be obtained from anywhere which reduces the plant purchasing cost. In this section, the performance of conventional PTES with Ar and air as working fluid are evaluated. The basic conditions and assumptions of the simulated of PTES with air and Ar as working fluid are shown in Table 6. Additionally, in our computing process, a control system has been implemented in order to stop the charging process and discharging process. For the charging process, a minimum admissible temperature increasing (ΔTcharge) of 30 K for outlet temperature of HP tank (THP,out,charge) should firstly be achieved, which aims to guarantee the HP tank can be fully used. When the first limitation has satisfied, charge time (tcharge) is considered. Hence the control conditions of charging process are expressed as:

THP, out , charge > T0 + ΔTcharge

and

tcharge ≥ 4h

discharging process, results in a maximum of RTE exists. When the dTLP,TB2 are set as 80 K and 60 K, the PTES with air and Ar get the maximum RTE of 0.4788 and 0.4232, respectively. The results indicate that the PTES with air is more efficient than the PTES with Ar. In this case, the maximum operating pressure of the PTES with air is about 10 bar, which is higher than that of PTES with Ar (4.7 bar), while in practice, the pressure of 10 bar is acceptable for storage tanks. Hence, in this work, air is proposed as the working fluid for PTES due to its high thermal performance and it with an acceptable operating pressure. 4.2.2. Comparison of PTES and HT-PTES The energy storage density is an essential parameter to assess the performance of PTES, with a higher energy storage density means the size of the system reducing. The energy storage density of PTES is severely depended on the maximum storage temperature, which is characterized by the outlet temperature of compressor. An advanced compressor is considered by REW Power in the EU project “ADELE” [24], while it can only withstand temperature about 600 °C (nearly 900 K). Hence, it’s difficult to improve the energy storage density of conventional PTES with existing technology of compressor. The above problem can be partially solved by using an electric heater [5,44]. In the HT-PTES, the air from the compressor CP1 is heated to a higher temperature, then the thermal energy of air is absorbed by the HP tank. To obtain a maximum RTE, the heat exchanger is required to regulate the outlet temperature of turbine TB2. As shown in Fig. 8, with the variation of dTLP,TB2, that the RTE of HT-PTES has the same trend with that of conventional PTES (shown in Fig. 7). When dTLP,TB2 is set as 320 K, the curve is found to plateau at an RTE value of 41.99%. The comparison simulation results of both PTES and HT-PTES based on the optimum dTLP,TB2 are shown in Table 7. The results show that the

(17)

For the discharging process, either the discharge time (tdischarge) reaches the designed value (4 h) or the minimum outlet temperature of HP tank (THP,out,discharge) achieved, the discharging process will be finished. Hence the control conditions of discharging process are descripted as:

THP, out , discharge > Tmax −ΔTdischarge

or

tdischarge ≤ 4h

(18)

where Tmax (900 K) is the maximum designed temperature of HP tank, ΔTdischarge (30 K) is the maximum admissible temperature reduction which aims to ensure the outlet temperature of HP tank approximate to the designed value. Additionally, to make sure that the minimum inlet temperature of LP tank in discharging process higher than 450 K, the designed value of LP tank inlet temperature is ΔTdischarge higher than 450 K. Fig. 6 shows the charge energy and discharge energy of each successive cycles for both air and Ar as working fluid in PTES. The charge energy in the first cycle is much higher than that in the residual cycles. It is because that the initial temperature of HP tank in the first cycle is identical with ambient temperature, while for other cycles, partial thermal energy remains in HP tank from the previous cycle, which results in more energy is required in the first cycle. For both air and Ar as working fluid, the periodic steady states are reached after about 10 cycles. As seen in Fig. 7, the RTE of PTES is major affected by the temperature difference (dTLP,TB2) between the outlet temperature of turbine TB2 (T4b) and the inlet temperature of LP tank (T6b) in discharging process. The RTE of PTES with both air and Ar raise up at first then go down later. It is because, for a constant inlet temperature of LP tank, a lower dTLP,TB2 implies a higher temperature reduction in TB2, hence a higher expansion ratio of TB2 is required. The expansion ratio of TB2 is depended on the compression ratio of CP2. Both power generation of TB2 and power consuming of CP2 increase with the dTLP,TB2 decreases. With a higher dTLP,TB, the increment rate of power generation of TB2 is faster than the increment rate of power consuming of CP2. However, with the decreasing of dTLP,TB2, the increment rate of power generation of TB2 decreases while the increment rate of power consuming of CP2 increases. Hence there is a maximum of net power generation in

Table 6 Parameters setting for conventional PTES system.

156

Term

Unit

Value

Ambient pressure Ambient temperature Idle time after charging process Idle time after discharging process Inlet temperature of CP1 Inlet temperature of HP tank in charging process Inlet temperature of LP tank in discharging process Volume of the HP tank Volume of the LP tank Ratio of length to diameter of LP and HP tanks Pressure of the LP tank Isentropic efficiency of compressors Isentropic efficiency of turbines Density of Al2O3 Shape factor of the Al2O3 Diameter of Al2O3 particle Mass flow rate of air Mass flow rate Ar Dynamic viscosity of air Dynamic viscosity of Ar

kPa K h h K K K m3 m3 – kPa – – kg/m3 – m kg/s kg/s kg/(m·s) kg/(m·s)

101.32 293.15 2 2 450 900 480 200 250 2 101.32 0.85 0.88 3990 [5] 1 0.02 15 30 3.0 × 10−5 2.2 × 10−5

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Table 7 Comparison of simulation results of PTES and HT-PTES systems.

Power consuming of CP1 Power consuming of CP2 Power consuming of EH Power generation of TB1 Power generation of TB2 Energy released from HHE Energy released from LHE Charge time Discharge time Expansion ratio of TB2 Optimum dTLP,TB2 ρES RTE

PTES

HT-PTES

105,785 MJ 38,479 MJ 0 MJ 26,525 MJ 76,438 MJ 8631 MJ 0 MJ 4.00 h 3.87 h 8.2 80 K 84.35 MJ/m3 47.88%

107,509 MJ 48,260 MJ 127,122 MJ 27,182 MJ 135,368 MJ 35,279 MJ 0 MJ 4.00 h 3.63 h 14.6 320 K 193.57 MJ/m3 41.99%

consuming in compressor CP2. The results also show, that the RTE of HT-PTES is 5.89% lower than that of PTES, while the energy storage density of HT-PTES is 193.57 MJ/m3 is more than twice that of PTES (84.35 MJ/m3). Table 7 also shows that 8631 MJ of heat energy is wasted in HHE for PTES, which is more than 10% of available total energy absorbed from HP tank in discharging process, while this value is more than 20% for HT-PTES. The waste heat from HHE shows a high great due to their high temperature (higher than 450 K), which is valuable to be recovered.

Fig. 6. Evolution of the charge energy and discharge energy with cycles for both air and Ar as working fluid.

4.2.3. HT-PTES and PTES combined with ORC As described in Section 4.2.1, heat exchanger is required to regulate the outlet temperature of TB2 to a lower temperature due to the irreversibility of compression and expansion process. The energy released from heat exchanger, which with temperature higher than 450 K, seen to be an ideal heat source for ORC to recover and change it into electricity. Hence, the low RTE of HT-PTES can be partially addressed by an additional ORC system. In this section, three configurations of HT-PTES and PTES combined with ORC are investigated. R123 is selected as working fluid for ORC due to its better thermal efficiency and environmental performance (low ODP and GWP) [28]. The parameters setting for ORC system are listed in Table 8. Fig. 9 shows the trends of RTE for PTES + ORC with the variation of dTLP,TB2. It can be seen when the dTLP,TB2 is at the value of 100 K the RTE reaches a maximum value of 50.13%, which is 2.25% higher than that without ORC. As presented in Fig. 10, the RTE of HT-PTES + ORC has the same trend with that of PTES + ORC. The optimum dTLP,TB2 is 340 K and a corresponding maximum RTE of 47.10% can be obtained. Compared with the HT-PTES, the HT-PTES + ORC shows a 5.11% increasing in RTE. It improves more significant than that of PTES + ORC due to the two main reasons: one is the ratio of wasted heat to available heat absorbed from HP tank for HT-PTES + ORC (35873 MJ/ (35873 MJ + 135616 MJ) = 20.92%) is higher than that of PTES + ORC (10735 MJ/(10735 MJ + 72937 MJ) = 12.83%); the other one is the heat recover ratio of HT-PTES + ORC (10967 MJ/ 35873 MJ = 30.57%) is higher than that of PTES + ORC (2165 MJ/ 10735 MJ = 20.17%).

Fig. 7. Effect of dTLP,TB2 on RTE of PTES with air and Ar as working fluid.

Fig. 8. Effect of dTLP,TB2 on RTE of HT-PTES.

Table 8 Parameters setting for ORC system.

power consuming of CP1 and TB1 are almost the same with both PTES and HT-PTES. However, large amount of power is required for electric heater in HT-PTES, which aims to reheat the air from compressor CP1 to a higher temperature (1400 K). Higher expansion ratio of TB2 is needed to make full use of high temperature thermal energy in HTPTES, results in more power generation in turbine TB2 and more power 157

Term

Unit

Value

Condensation temperature Pinch point temperature difference Pressure loss of heat exchanger Isentropic efficiency of pump Isentropic efficiency of HT and LT

K K – – –

313.15 30 2% 0.70 0.80

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tank is departed into two range (T5b–T4b and T6b–T5b) as heat sources for ORC1 and ORC2. The thermodynamic data for R123 in HT-PTES with ORC and parallel ORC systems are shown in Table 10. The variation of RTE for HT-PTES + parallel ORC with the increasing of dTLP,TB2 is also shown in Fig. 10, which has the same trend with that of HT-PTES + ORC. Compared with HT-PTES + ORC, the RTE of HT-PTES + parallel ORC is slightly less than that of HTPTES + ORC at low dTLP,TB2, while it increases rapidly with the dTLP,TB2 increases and finally higher than that of HT-PTES + ORC. The phenomenon can be explained as follows. The T-s diagram of air and R123 in heat exchangers (LHE and HHE) for HT-PTES + ORC and HTPTES + parallel ORC are shown in Fig. 11. It can be seen that the average heat transfer temperature difference of heat exchanger in the HT-PTES + parallel ORC is higher than that of HT-PTES + ORC, hence more exergy destructed in the heat exchanger results in less power generation for HT-PTES + parallel ORC. However, as seen in Fig. 12, the pump pressure increases rapidly for the HT-PTES + ORC while it raises up gently for HT-PTES + parallel ORC with the dTLP,TB2 increases, which causes high power consuming of pump in HTPTES + ORC. The exergy destruction in the heat exchangers is the major contributor on the effect of RTE at low dTLP,TB2, while power consuming of pump play a significant role on the effect of RTE at high dTLP,TB2. Hence the RTE of HT-PTES + parallel ORC is slightly less than that of HT-PTES + ORC at low dTLP,TB2, while it increases rapidly with the dTLP,TB2 increasing and finally higher than that of HT-PTES + ORC. When the dTLP,TB2 is set as 380 K, the maximum RTE of 47.67% can be achieved for HT-PTES + parallel ORC, which is 0.57% and 5.68% higher than that of HT-PTES + ORC and HT-PTES systems, respectively. From Table 9, we can also find that the maximum operating pressure of ORC in HT-PTES + parallel ORC is 12.20 MPa, which is much lower than that of HT-PTES + ORC (31.4 MPa). Additionally, the maximum operating pressure for subsystem HT-PTES in HTPTES + parallel ORC is 0.26 MPa less than that of HT-PTES + ORC. It is because that a higher outlet temperature of TB2, a lower expansion ratio is required, results in the maximum pressure of HT-PTES subsystem decreases. The comparison results show that the HT-PTES + parallel ORC is more efficient and more stable than HT-PTES + ORC. The HTPTES + parallel ORC with an acceptable RTE, an appropriate operating pressure and a considerable energy storage density, seems to be a more promising large-scale energy storage system.

Fig. 9. Effect of dTLP,TB2 on RTE of PTES combined with ORC.

Fig. 10. Effect of dTLP,TB2 on RTE of HT-PTES combined with ORC.

However, from Table 9, it can be found that a high expansion ratio of 203 is required for turbine HT in the HT-PTES + ORC system, results in an extremely high operating pressure of 31.4 MPa. The problem can be partially solved by a novel parallel ORC system, which consists of ORC1 (1c-2c-3c-4c-5c-6c-7c-13c-1c) and ORC2 (1c-2c-8c-9c-10c-11c12c-13c-1c). The ORC1 and ORC2 using the same pump to enhance the pressure of R123, resulting in an identical expansion ratio for LT and HT, while the major difference between two ORC are the temperature range of heat sources. The heat from outlet of turbine TB2 to inlet of LP

5. Conclusion In this work, an electric heater is employed to raise the maximum storage temperature of conventional pumped thermal electrical storage system, which aims to reduce the ratio of compressor and enhance the energy storage density of the system. The proposed system is denoted as

Table 9 Comparison of simulation results of HT-PTES and PTES combined with ORC systems.

Expansion ratio of TB2 Maximum pressure of PTES Optimum dTLP,TB2 Expansion ratio of HT in ORC Maximum pressure of ORC Mass flow rate of R123 Power generation of TB2 Energy released from HHE Energy recovered from HHE Energy released from LHE Energy recovered from LHE ρES RTE

PTES + ORC

HT-PTES + ORC

HT-PTES + parallel ORC

7.0 1.05 MPa 100 K 37 5.72 MPa 5.21 kg/s 72934 MJ 10967 MJ 2165 MJ 0 MJ 0 MJ 88.14 MJ/m3 50.13%

12.9 1.31 MPa 340 K 203 31.4 MPa 14.31 kg/s 135616 MJ 35873 MJ 10735 MJ 0 MJ 0 MJ 216.71 MJ/m3 47.10%

10.2 1.05 MPa 380 K 79 12.20 MPa 20.50 kg/s 129657 MJ 20929 MJ 7930 MJ 20078 MJ 5698 MJ 218.69 MJ/m3 47.67%

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Table 10 Thermodynamic data for R123 in HT-PTES with ORC and parallel ORC systems. ORC

1c 2c 3c 4c 5c 6c 7c 8c 9c 10c 11c 12c 13c

Parallel ORC

T (K)

P (MPa)

h (kJ/kg)

ṁ (kg/s)

T (K)

P (MPa)

h (kJ/kg)

ṁ (kg/s)

313.15 332.77 332.77 460.00 770.97 575.70 352.77 – – – – – 352.77

0.154 31.358 31.358 30.73 30.116 0.161 0.158 – – – – – 0.158

240.59 271.18 271.18 404.43 761.12 622.56 434.74 – – – – – 434.74

14.31 14.31 14.31 14.31 14.31 14.31 14.31 – – – – – 14.31

313.15 321.12 321.12 423.32 630.10 478.25 341.12 321.12 553.28 810.19 657.02 341.12 341.12

0.154 12.203 12.203 11.959 11.720 0.161 0.158 12.203 11.959 11.720 0.161 0.158 0.158

240.59 252.56 252.56 362.36 623.45 535.81 426.00 252.56 526.36 828.96 699.80 426.00 426.00

20.5 20.5 10.8 10.8 10.8 10.8 10.8 9.7 9.7 9.7 9.7 9.7 20.5

carried out, and comparative researches of five systems are conducted. The main concluding remarks have been given as follows: (1) The RTE of PTES with air as working fluid is 5.56% higher than that of PTES with Ar as working fluid. Additionally, the air can be obtained from anywhere. Hence, with an acceptable maximum operation pressure (lower than 1.5 MPa), air is selected as the working fluid of PTES and HT-PTES. (2) Compared with PTES, the RTE of HT-PTES is 5.89% less than that of PTES due to the high exergy loss in electric heater. However, the energy storage density of HT-PTES is more twice that of PTES. (3) ORC is employed to recover the heat produced due to the irreversibility of system operating process. The simulation results show that RTE of both PTES and HT-PTES combined with ORC can be improved, while RTE of HT-PTES increased more significant than that of PTES due to an additional irreversibility of heating process should be considered. (4) The maximum RTE of HT-PTES + ORC achieved with an extremely high operating pressure (31.4 MPa) in ORC subsystem, which may limit its application and enhance its manufacturing cost. A novel parallel ORC is proposed to reduce the operation pressure of ORC system. The simulation results show that the maximum operation pressure of HT-PTES + parallel ORC is merely 12.2 MPa. Additionally, the RTE of HT-PTES + parallel ORC is 47.67%, which is slightly higher than that of HT-PTES + ORC (47.10%). (5) The HT-PTES + parallel ORC with an acceptable RTE (only 2.46% lower than that of PTES + ORC), an appropriate operating pressure (1.05 MPa for HT-PTES subsystem and 12.20 MPa for ORC subsystem) and a considerable energy storage density (218.69 MJ/m3, more twice that of PTES + ORC), seems to be the most promising large-scale energy storage system in the five types of storage system.

Fig. 11. T-s diagram of air and R123 in heat exchangers LHE and HHE for HTPTES + ORC and HT-PTES + parallel ORC.

Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant No.: 51706221 and 51576187), Natural Science Foundation of Fujian (Grant No.: 2017J05143) and Scientific Equipment Research Project of Chinese Academy of Sciences (Grant No.: YZ201611).

Fig. 12. Effect of dTLP,TB2 on power of pump and maximum operating pressure for ORC and parallel ORC.

HT-PTES. Air and argon (Ar) are investigated as the working fluid of PTES. The heat generated due to the irreversibility of heating, compression and expansion process is recovered by ORC system. A novel parallel ORC is proposed to recover the heat with high temperature. R123 is selected as working fluid for ORC due to its high thermal efficiency and environment properties. Thermodynamic analyses of five types of PTES combined with ORC namely PTES, HT-PTES, PTES + ORC, HT-PTES + ORC and HT-PTES + parallel ORC are

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