Thermal Calculation and Experimental Investigation of Electric Heating and Solid Thermal Storage System

energies

Article Thermal Calculation and Experimental Investigation of Electric Heating and Solid Thermal Storage System

Haichuan Zhao 1, Ning Yan 1,*, Zuoxia Xing 1, Lei Chen 1 and Libing Jiang 2

1 School of Electrical Engineering, Shenyang University of Technology, Shenyang 110870, China; [email protected] (H.Z.); [email protected] (Z.X.); [email protected] (L.C.) 2 Shenyang Lanhao New Energy Technology Company, Shenyang 110006, China; [email protected] * Correspondence: [email protected]; Tel.: +86-159-0982-6968

Received: 7 August 2020; Accepted: 2 October 2020; Published: 9 October 2020

Abstract: Electric heating and solid thermal storage systems (EHSTSSs) are widely used in clean district heating and to flexibly adjust combined heat and power (CHP) units. They represent an effective way to utilize renewable energy. Aiming at the thermal design calculation and experimental verification of EHSTSS, the thermal calculation and the heat transfer characteristics of an EHSTSS are investigated in this paper. Firstly, a thermal calculation method for the EHSTSS is proposed. The calculation flow and calculation method for key parameters of the heating system, heat storage system, heat exchange system and fan-circulating system in the EHSTSS are studied. Then, the instantaneous heat transfer characteristics of the thermal storage system (TSS) in the EHSTSS are analyzed, and the heat transfer process of ESS is simulated by the FLUENT 15 software. The uniform temperature distribution in the heat storage and release process of the TSS verifies the good heat transfer characteristics of the EHSTSS. Finally, an EHSTSS test verification platform is built and the historical operation data of the EHSTSS is analyzed. During the heating and release thermal process, the maximum temperature standard deviation of each temperature measurement point is 28.3 ◦C and 59 ◦C, respectively. The correctness of the thermal calculation of the EHSTSS is thus verified.

Keywords: solid sensible heat storage; thermal calculation; ﬂuid-solid coupling; heat transfer characteristics; experimental investigation

1. Introduction In recent years, renewable energy utilization has developed rapidly in China. However, the abandonment of the renewable electricity generation is serious, especially during the wintertime heating period [1]. In the northern parts of China, CHP units are used for central heating in winter. The power output of CHP unit is greatly constrained by the heating load according to the principle of “power determined by heat” [2]. This has become one of the main reasons for the consumption of renewable energy in winter. The decoupling of the heat and power control of the CHP unit can be realized by adding a large amount of electric heating and heat storage unit to the CHP unit, which can effectively improve the adjustment capacity of the CHP unit and the renewable consumption capacity of the grid [3,4]. In addition, the traditional coal-fired boiler heating system for urban and factory thermal energy supply can also be replaced by the EHSTSS. It has become one of the effective ways to control air pollution. In the multi-energy system of combined cooling, heating and power, the thermal storage system can also be used as an energy storage system and a thermal energy supply system [5]. It can improve the diversity of thermal energy supply in a multi-energy system. In various thermal energy storage system, sensible heat storage is relatively popular because of its simple technology application and low cost [6]. More research on sensible heat storage heating systems mainly include the EHSTSS and the electrode boilers. Compared with the electrode boiler, the EHSTSS has advantages

Energies 2020, 13, 5241; doi:10.3390/en13205241 www.mdpi.com/journal/energies Energies 2020, 13, 5241 2 of 20 in heat storage density, safety, area covered and electrothermal response speed. It has become a research hotspot in recent years. The thermal calculation and the heat transfer characteristics analysis of the EHSTSS is a key link in the design and manufacture of the equipment. Similar to the thermal calculation of a traditional coal-fired boiler, the thermal calculation of the EHSTSS is mainly to calculate the key parameters of the equipment when the rated parameters are known [7]. Thermal energy storage (TES) is a technology under investigation since the early 1970s [8]. At present, the research on EHSTSS mainly focuses on the thermal storage medium, heat transfer characteristics of the system and structural optimization of the system. However, less attention has been paid to the thermal calculation of EHSTSS. Zhu et al. [9] report a helical-coil tube heat exchanger. The temperature difference of thermal energy storage is analyzed with and without considering radiative heat transfer. Gasia et al. [10] proposed a heat transfer enhancement technique that adds cheap and commercially available metallic wool. The thermodynamic performance of heat exchanger is analyzed from heat transfer process and enhancement of heat transfer characteristics in the above literature. However, only a single component is analyzed for thermodynamic performance, and the calculation method and process of heat exchanger thermal parameters are not investigated. Mousavi, et al. [11] investigated the melting process of phase change materials in an internal melt ice-on-coil thermal storage system. The effects of different operating parameters such as the inlet temperature and flow rate of the heat transfer fluid are analyzed, but there is no design calculation and heat transfer performance analysis of the heat storage module. Fadl et al. [12] present a latent heat thermal energy storage system. The influence of different heat transfer fluid inlet temperatures and volume flow rates of the system is evaluated by the experimental investigation. However, it lacks the heat exchanger parameter thermal design calculation process, and the optimization parameter curve of the overall system is not very obvious. In the aspect of TSS design, the existing research mainly focuses on the thermal calculation of parts of the TSS, such as separate heat exchangers, heat storage modules, etc. Raczka et al. [13] and Fujii et al. [14] introduced the design and calculation method of flue gas/waste water-heat exchanger and indirect heat exchanger, respectively. Du et al. [15] also reported a design method for heat storage units. However, the abovementioned research lacks any thermal design calculation of heat storage equipment from the perspective of a complete system and further systematic verification of equipment performance. It is necessary for the whole system as the research object to carry out thermal design calculation and systematic verification for EHSTSS. Therefore, a systematic thermal calculation method for the EHSTSS is presented by this work. According to the structural characteristics of the system, thermal calculation of the system mainly calculates the parameters of the heating element, the TSS, the fan-circulating system and heat exchange system. Meanwhile, the fluid-solid coupling characteristics of the heat transfer in the TSS are analyzed. The numerical simulation of temperature distribution is carried out in the condition of heat storage and heat release. An experimental correlation for the EHSTSS is derived in order to verify correctness of the thermal calculation. The two main contributions of this paper are summarized as follows:

A thermal calculation process specifically designed for the EHSTSS is proposed. • Systematic verification of the rationality and correctness of the EHSTSS from three aspects: • case design, simulation analysis and experimental verification. The multi angle and systematic verification results can provide the basis for the optimal design of • energy storage system.

The rest of this paper is organized as follows: Section2 presents the thermal calculation method and process of the electric heating and heating storage system. Section3 analyses the heat transfer characteristics of the TSS. Experimental veriﬁcation is presented in Section4. Section5 concludes the paper. EnergiesEnergies 20202020, ,1313, ,5241 5241 3 3of of 20 20

2. Thermal Calculation Flow and Method of the EHSTSS 2. Thermal Calculation Flow and Method of the EHSTSS 2.1. Thermal Calculation Flow of the EHSTSS 2.1. Thermal Calculation Flow of the EHSTSS The EHSTSS is composed of the TSS, including a thermal storage module and embedded heating element,The the EHSTSS heat exchanger, is composed the of frequency the TSS, including converte ar thermalfan, the storagethermal module insulation and layer, embedded an external heating controller,element, theetc. heat A structural exchanger, diagram the frequency is shown converter in Figure fan, 1. The the thermaloverall dimensions insulation layer, of EHSTSS, an external i.e. length,controller, width etc. and A structuralheight are diagram 1560 mm, is shown720 mm in and Figure 11001. Themm, overall respectively. dimensions In the of EHSTSS, EHSTSS, the i.e., thermallength, widthstorage and unit height uses are a solid 1560 sensible mm, 720 therma mm andl storage 1100 mm, medium respectively. such as In magnesium the EHSTSS, oxide. the thermal The length,storage width unit uses and a height solid sensible of the thermal thermal storage storage unit medium are 240 such mm, as magnesium 115 mm and oxide. 53 mm, The respectively. length, width Theand heating height elements of the thermal using storageNiCr or unit FeCrAl are 240materi mm,als 115are mmembedded and 53 in mm, the respectively.thermal storage The module heating [16].elements In the usingthermal NiCr storage or FeCrAl process materials of EHSTSS, are embeddedthe thermal in is the generated thermal by storage the heating module element [16]. In and the storedthermal in storagethe thermal process storage of EHSTSS, module. the The thermal thermal is generatedis extracted by from the heating the thermal element storage and storedmodule in by the thethermal frequency storage conversion module. fan, The and thermal the thermal is extracted is exchanged from the thermalthrough storage the heat module exchanger by the to frequencymeet the heatconversion load demand, fan, and when the thermal the heat is load exchanged is in demand through [17]. the The heat heat exchanger transfer to process meet the mentioned heat load demand, above, thewhen heat the flow heat of load the EHSTSS is in demand is mainly [17]. composed The heat transfer of heat processgeneration, mentioned thermal above, storage, the heat heat extraction ﬂow of the andEHSTSS heat exchange. is mainly composedTherefore, of taking heat generation,heat flow and thermal system storage, structure heat into extraction account, and the heat EHSTSS exchange. is dividedTherefore, into taking heating heat system, ﬂow and thermal system storage structure syst intoem, account,fan-circulating the EHSTSS system is dividedand heat into exchange heating system.system, thermal storage system, fan-circulating system and heat exchange system.

Low temperature High temperature fluid fluid Temperature Heating element measuring hole Heat Exchanger Heat storage Thermal channel storage module Hot air 53mm

Water supply 115mm Outlet Thermal s torage u nit Heat rel ea si ng hole

Return water 40mm 1100mm

Inlet of heat 720mm 1440mm exchanger 1560mm Circulation fan Cool air Heating resistance wire

FigureFigure 1. 1. StructureStructure diagram diagram of of the the EHSTSS. EHSTSS.

TheThe four four subsystems subsystems of of the the EHSTSS EHSTSS are are interrel interrelated.ated. The The thermal thermal calculation calculation of of the the EHSTSS EHSTSS shouldshould clarify clarify thethe parameterparameter relationship relationship between between the the subsystems. subsystems. The parametricThe parametric relationship relationship among amongthe subsystems the subsystems of the EHSTSSof the EHSTSS is shown is shown in Figure in 2Figure. 2. Thermal calculation of EHSTSS usually start with thermal storage system. Before that, the key parameters of the EHSTSS need to be determined, mainly including thermal storage capacity, heating power, initial and ﬁnal temperature of thermal storage module, temperature of supply and return water, heating time, heating voltage, etc. Thermal calculation of the thermal storage system mainly determines the number of the thermal storage units and their arrangement. When the arrangement of thermal storage units is determined, the number of thermal storage channels and structure parameters of the TSS can be obtained. The number of thermal storage channels and structure parameters of the TSS are the vital input parameters of the heating system. The length and surface load of the heating element are calculated according to heating voltage, heating power, number of the thermal storage channel and other parameters. The parameter of the heat exchange system is aﬀected by the maximum heat load, the temperature of upper and lower air ducts determined by the thermal storage system. The parameter

Energies 2020, 13, 5241 4 of 20

Flow( S af ) Supply and return water temperature(T 3 , T 4 ) Heat exchange Circulating air Motor power( P ) Upper and lower air duct system Flow system m resistance(Δ P ) temperatures(T 1 , T 2 )

Maximum heat load( P ) Heat storage power(P h ) mhl Flow resistance(Δ P ) Heating time(t 1 ) Thermal storage system Initial and final tempera- Heat storage brick hole number(N ) Energies 2020, 13, 5241 4 of 20 ture of regenerator(T , T 0 )

Rated heating power(P h ) Heating wire length(L) of fan-circulating system is mainly affected by the airHeating flow system and the flow resistance which is determined byEnergies heat 2020 exchanger, 13, 5241 system and theHeating thermal voltage(U storage) system. Heating wire surface load(W) 4 of 20

S Figure 2. Thermal calculation relation of energyFlow( storage af ) system Supply and return water temperature(T 3 , T 4 ) Heat exchange Circulating air Motor power( P ) Thermal calculation of EHSTSSUpper and usually lower air duct start withsystem thermal storageFlow system.system Before that, the keym temperatures(T , T ) resistance(Δ P ) parameters of the EHSTSS need to be determined,1 2 mainly including thermal storage capacity, heating power, initial and final temperature of thermal storage module, temperature of supply and return Heat storage power(P ) Maximum heat load( P mhl ) water, heating time,h heating voltage, etc. Flow resistance(Δ P ) ThermalHeating time( calculationt 1 ) ofThermal the thermal storage system mainly determines the number of the thermal storage system storageInitial and units final tempera- and their arrangement.Heat When storage the brick arra holengement number(N ) of thermal storage units is determined, ture of regenerator(T , T ) the number of thermal0 storage channels and structure parameters of the TSS can be obtained. The number of thermal storage channels and structure parameters of the TSS are the vital input Rated heating power(P h ) Heating wire length(L) parameters of the heating system. The length andHeating surface system load of the heating element are calculated according to heating voltage, Heatingheating voltage( power,U) number of the thermalHeating wirestorage surface load(channelW) and other parameters. The parameter of the heat exchange system is affected by the maximum heat load, the

temperature of upper and lower air ducts determined by the thermal storage system. The parameter Figure 2. Thermal calculation relation of energy storage system of fan-circulating systemFigure is 2. Thermalmainly calculationaffected by relation the air of energyflow and storage the system. flow resistance which is determined by heat exchanger system and the thermal storage system. TheThermal thermal calculation calculation of EHSTSS process forusually the four start subsystem with thermal of the storage EHSTSS system. discussed Before above that, isthe shown key The thermal calculation process for the four subsystem of the EHSTSS discussed above is shown inparameters Figure3. Theof the calculation EHSTSS need methods to be ofdetermined, key parameters mainly in including each subsystem thermal are storage investigated capacity, below. heating in Figure 3. The calculation methods of key parameters in each subsystem are investigated below. power, initial and final temperature of thermal storage module, temperature of supply and return

water, heatingHeat time, extraction heating voltage, etc. Heat exchange Heat storage Heating Thermal calculation of the thermal storage system mainly determines the number of the thermal

storage units Heatand transfer their system arrangement. inlet and outlet air System When supply and the return arra ngement of thermal storage units is determined, Power rating(P h ) Three-phase voltage(U1) temperature(T 1 , T 2 ) water temperature(T 3 ,T 4 ) the number of thermal storage channels and structMaximumure heat parameters load of the TSS can be obtained. The of water supply(P ) Total heat storage(Q) Single-phase voltage(U) Heat transfer 2 coefficient(k) water temperature Heat storage Current Single-phase number of thermal storage channels and structure parametersdifference of themargin( TSSm) are the vital inputpower Heat exchange system Heat exchange system Heat transfer area of Maximum load Single-phase Single-phase Design total heat storage(Q 1) parametersinlet air flow( ofS iaf ) the heatingoutlet airsystem. flow(S af ) Theheat length exchanger ( Aand) corresponds surface to water load flow of the heating elementresistance( areR) calculatedcurrent(I) Heat storage of Heating element Design water a single brick number(N) according to heating voltage, heating power, numberflow of rate( vthe) thermal storage channel and other Low temperature duct Number of bricks(n) Single heating wire voltage Δ High temperature duct Number of heat resistance( P f3 )+Heat exchanger Pipe diameter Single-phase + resistance( Δ P )+back transfer tubes( N 1 ) Single-phase parameters.flow resistance(Δ P )+channel The flow parameter off1 the heat exchange system is affected by theStructural maximum design heat load, the f4 ΔP Single pipe flow current current resistance(Δ ) flow resistance( f2 ) Pf5 cross-sectional area(S1) Rearrangement of three Single heating Single heating temperature of upper and lowerSum air ductsAir cross determined section by the thermaldimensional storage bricks system.wire resistanceThe parameterwire power of heat exchanger(S) Cumulative total Resistivity(ρ) of fan-circulating systemresistance( is Δ P mainly ) affectedHeat exchanger by the air flow andReal the heat storage flow resistanceDiameter( whichD) Diameter( isD ) outlet air flow capacity(Q ) Fan flow(S ) 2 af Actual air flow rate in the Heating wire determined by heat exchanger system and the thermal storage system. length(L) Surface load(W) Motor calculation heat exchange tube(ω) N QQ− power( P ) 21< 0.05? m Q The thermal calculation processReserve for the four subsystem of the EHSTSS1 discussed above is shown coefficient Y in Figure 3. The calculationActual methodsmotor of key parameters in each subsystemHeat storage structure are arrangement investigated below. power( P ) a Heat extraction Heat exchange Heat storage Heating FigureFigure 3.3. ThermalThermal calculation flow ﬂow diagram of of solid solid thermal thermal storage storage system. system.

Heat transfer system inlet and outlet air System supply and return 2.2. Thermal Calculation of Key Parameters of EHSTSS Power rating(P h ) Three-phase voltage(U1) 2.2. Thermal Calculationtemperature( ofT 1 , KeyT 2 ) Parameterswater temperature( of EHSTSST 3 ,T 4 ) Maximum heat load of water supply(P ) Total heat storage(Q) Single-phase voltage(U) ThermalThermal calculationcalculation is mainly aimed aimed Heatat at transfercalculating calculating the the2 key key parameters parameters in the in the four four subsystems subsystems of coefficient(k) water temperature Heat storage Current Single-phase difference margin(m) power Heat exchange system Heat exchange system Heat transfer area of Maximum load Single-phase Single-phase ofthe the EHSTSS. EHSTSS. The The following following is a is detailed a detailed investigat investigationion to the to thecalculationDesign calculation total heat storage( of Q key 1) of keyparameters parameters in the in inlet air flow(S iaf ) outlet air flow(S af ) heat exchanger (A) corresponds to water flow resistance(R) current(I) Heat storage of Heating element Design water a single brick thesubsystem. subsystem. number(N) flow rate(v) Low temperature duct Number of bricks(n) Single heating wire voltage Δ High temperature duct Number of heat resistance( P f3 )+Heat exchanger Pipe diameter Single-phase + resistance( Δ P )+back transfer tubes( N 1 ) Single-phase flow resistance(Δ P )+channel flow f1 Structural design f4 ΔP Single pipe flow current current 2.2.1.resistance( ThermalΔ ) Calculationflow resistance( of Key f2 ) Parameters in Heating System Pf5 cross-sectional area(S1) Rearrangement of three Single heating Single heating Sum Air cross section dimensional bricks wire resistance wire power of heat exchanger(S) Cumulative total Resistivity(ρ) The heating power,resistance( the lengthΔ P ) of heatingHeat exchanger wire and the surface loadReal heat of storage the heating elementDiameter(D) areDiameter( theD) outlet air flow capacity(Q ) Fan flow(S ) 2 af Actual air flow rate in the Heating wire key parameters in the heating system. The heating power is the basic parameter of thelength( heatingL) Surface system, load(W) Motor calculation heat exchange tube(ω) N QQ− power( P ) 21< 0.05? m Q which is mainly related to theReserve building area, the thermal index of building1 heating and the heating coefficient Y time. The heating powerActualPh motor(kW) of the heating system can beHeat determined storage structure arrangement by Equation (1): power( P ) a Figure 3. Thermal calculation flow diagram24PfF of solid thermal storage system. Ph = (1) 1000t1η 2.2. Thermal Calculation of Key Parameters of EHSTSS Thermal calculation is mainly aimed at calculating the key parameters in the four subsystems of the EHSTSS. The following is a detailed investigation to the calculation of key parameters in the subsystem.

Energies 2020, 13, 5241 5 of 20

2 2 where F represents heating area, m ; Pf represents the value of heating index (W/m ) shown in Table1, t1 represents heating time, h; η thermal efficiency of the system, thermal efficiency is the ratio of the thermal Q1 generated by the heating element to the effective thermal Q released by the system, i.e., η = Q1/Q.

Table 1. Recommended value of heating index [18].

Type Energy Saving Non Energy Saving Type Energy Saving Non Energy Saving Residence 40~45 58~64 Oﬃce 50~70 60~80 Hospital 55~70 65~80 Hostel 50~60 60~70 Store 55~70 65~80 Canteen 100~130 115~140 Cinema 80~105 95~115 Gym 100~150 115~165

When the physical parameters and diameter of the heated wire are determined, the length of the heating wire used for a single heating element is the key parameter related to the parameters such as heating voltage and heating power. The length L of the heating wire can be expressed as follows:

3πU2D2 L = (2) 4kρΩNPh where U represents the rated voltage, V; ρ represents the resistivity, µΩ m, k represents a temperature Ω × coeﬃcient, N represents the heating element number, D represents the diameter of the heated wire, mm. The surface load of the heating wire is the parameter that aﬀects the service life of the heating element, so the rationality of the selection of the heating element can be judged according to the surface load. As the surface load increases, the temperature of heating element rises. In general, in high temperature heating applications, the surface load of heating elements can be controlled at 3~8 W/cm2. The surface load W of the heating wire can be expressed as follows:

P W = h (3) 3πLD

2.2.2. Thermal Calculation of Key Parameters in Thermal Storage System In a thermal storage system, the key thermal calculation parameters are mainly the number and arrangement of thermal storage units. At the end of the preheating of the TSS, the average temperature is recorded as T0 ◦C. When the thermal storage module is heated to a set temperature, the average temperature is T ◦C. When the physical and structural parameters of the thermal storage unit are known, the number n of thermal storage units can be expressed as follows:

mP t n = h 1 (4) ρsCV(T T ) − 0 where C represents the thermal storage unit speciﬁc heat, kJ/(kg C); ρs represents the thermal storage × ◦ unit density, kg/m3; V represents the heat storage unit volume, m3 and m represents the margin of thermal storage capacity. According to the number of thermal storage units, the arrangement of TSS is calculated. The horizontal row number a and the height row number d of the thermal storage unit are determined, and the longitudinal row number e can be expressed as follows:

Q e = [ + 0.5] (5) adρsCV(T T ) − 0 where Q represents thermal storage capacity, kWh; Q = mP1t1. Energies 2020, 13, 5241 6 of 20

2.2.3. Thermal Calculation of Key Parameters in Heat Exchange System The key parameters of the heat exchange system mainly include the heat transfer area, the number of heat exchange tubes and the air flow rate in the heat exchange tube. In the heat exchanger, the heat transfer area of the heat exchanger is related to the heat transfer coefficient, the temperature difference of the fluid and the maximum heat load. The calculation of heat transfer area A can refer to Equation (6):

Q A = (6) t α ∆T 2 × where α represents heat transfer coeﬃcient of heat exchanger; t2 represents the fastest heat release time, h; ∆t represents logarithmic temperature diﬀerence, ◦C; which can be referred to Equation (7) [19,20];

(T2 T3) (T1 T4) ∆T = − − − (7) ln[(T T )/T T ] 2 − 3 1 − 4 where T1, T2 represents air temperature at inlet and outlet of heat exchanger, ◦C; T3, T4 represents the supply and return water temperature of the heat exchanger, ◦C. In Equation (6), the heat transfer coeﬃcient α of heat exchanger can be expressed as:

!0.8 ε ωDdl 0.4 α = 0.023 Pr ctclψ (8) Ddl µ where ε is the thermal conductivity at the average temperature of the air, kW/(m ◦C);µ is the kinematic 2 × viscosity of air at average temperature, m /s; Ddl is the outer diameter of the heat exchange tube, m; Pr is the Prandtl number of air at average temperature; ct and cl are pipeline correction coefficients; ψ is thermal efficiency coefficient. The number of heat exchanger tubes is related to the heat transfer area, the length and diameter of the tubes. The length of the heat exchanger tube is suitable to the diameter, and the ratio of the tube length (L1) to the diameter (D1) is about 4~6 [19]. For the thermal calculation of the number of heat exchanger tubes we can refer to Equation (9):

A N1 = (9) πD1L1 where N1 represents the number of heat exchanger tubes. The flow resistance of heat exchanger accounts for a large proportion of the flow resistance of thermal storage system. The velocity of air in the tube is the key parameter affecting flow resistance. In the heat exchange tube, the velocity of air under standard state can be expressed as follows:

S ω = af1 (10) 0 A 3600 1.293 1 × × 2 where A1 represents cross-sectional area of heat exchange tubes, m ; Saf1 is the air mass ﬂow at the outlet of heat exchanger, kg/h; which can be expressed as:

P 3600 S = h × (11) af1 C T C T 1 1 − 2 2 where C , C the speciﬁc heat of the air at the outlet and inlet of the heat exchanger, kJ/(kg K). 1 2 × The actual velocity of air in heat exchanger tube can be calculated as (12):

ω = ω0[(T1 + T2) + 273]/273 (12) Energies 2020, 13, 5241 7 of 20

2.2.4. Thermal Calculation of Key Parameters in Fan-circulating System In a fan-circulating system, the motor power of the frequency conversion fan is an extremely important parameter. The motor power is determined by the required flow resistance and flow. In the EHSTSS, flow resistance consists of two parts. One part is the high temperature channel flow resistance ∆Pf1(Pa) and the back flow resistance ∆Pf2 (Pa) of heat exchange system, the other part is low temperature channel flow resistance∆Pf3(Pa), flow resistance ∆Pf4 (Pa)of the heat exchanger and the thermal storage channel flow resistance∆Pf5(Pa). The total resistance of the EHSTSS can be expressed as follows: ∆P = ∆Pf1 + ∆Pf2+∆Pf3 + ∆Pf4 + ∆Pf5 (13) where [21,22]: 2 2 λIω ρa 2 ∆P = ( + 1) (14) f1,2 0.5 2de (Tw/Tav) 2 λIω ρa ∆P = (15) f3,4,5 2de 3 where λ represents friction coefficient in channel; ρ2 represents medium density, kg/m ; ω represents air velocity, m/s; I represents length of channel, m; de represents channel section equivalent diameter, m; Tw represents thermal storage unit surface average temperature, ◦C; Tav represents air average temperature, ◦C. 3 The air volume flow Saf (m /h) required for the fan-circulating system can be referred to Equation (16): 10.20S (T2 + 273) S = af1 (16) af 273 1.293 × When the air pressure and air flow required for the fan-circulating system are determined, the motor power Pm(kW) can be calculated as follows:

1.1∆P S P = × af (17) m 3600 1020δ × where δ represents motor eﬃciency. In order to verify the rationality and correctness of the thermal calculation process and method of the EHSTSS, the case design is based on the system with heating power of 100 kW, daily thermal storage of 1MWh and heat transfer power of 125 kW. The case design is shown in AppendixA.

3. Analysis of Heat Transfer Characteristics of EHSTSS The heat transfer process of EHSTSS is a complex process which includes heat conduction, convective heat-transfer and radiation heat-transfer [23]. There are different heat transfer modes between structures of the system, as shown in Figure4. The operation process of EHSTSS is an alternating process between thermal storage and thermal release. The temperature of the TSS is a key index to judge whether design parameters of the system are suitable. Therefore, the heat transfer characteristics analysis of the system is carried out to explore the temperature variation rule of the heat storage device. In the thermal storage process of an EHSTSS, heat transfer methods include convective heat-transfer between heating elements and cold air, radiation heat-transfer between heating elements and thermal storage module and heat conduction inside thermal storage module. In the thermal release process of the system, heat transfer methods include heat conduction inside thermal storage module and convective heat-transfer between cold air and thermal storage module. The heat transfer process in the system involves two regions of solid and fluid. Therefore, the analysis of the heat transfer characteristics between the two regions is of great significance to improve the heat storage efficiency of the heat storage module. Energies 2020, 13, 5241 8 of 20 Energies 2020, 13, 5241 8 of 20

Joule heat field

Fluid-solid coupling Flow field Solid Fluid Direct distribution coupling Fluid-solid interface

Temperature Temperature distribution distribution

FigureFigure 4.4. Flow-solid coupling diagram of solid thermal storagestorage system.system

3.1. HeatThe operation Transfer Analysis process in of Thermal EHSTSS Storage is an alternat Process ing process between thermal storage and thermal release.The The thermal temperature in the thermal of the TSS storage is a key module index isto derived judge whether from the design heating parameters element. of The the thermal system generatedare suitable. by Therefore, the heating the elementheat transfer interacts characteristics with the analysis heat storage of the module system throughis carried radiation out to explore heat the temperature variation rule of the heat storage device. exchange and convection heat exchange. The net heat Qs in the thermal storage module can be expressedIn the as thermal follows: storage process of an EHSTSS, heat transfer methods include convective heat- transfer between heating elements and cold air, radiation heat-transfer between heating elements and Qs = Qes + Qea Qa = CMs(T6 T5) (18) thermal storage module and heat conduction inside− thermal storage− module. In the thermal release whereprocessQ ofes therepresents system, radiantheat transfer heat methods between includ heatinge heat element conduction and thermal inside storagethermal module, storage module W; Qea representsand convective convective heat-transfer heat between between resistance cold air and element thermal and storage air, W; module.Qa represents The heat heat transfer carried process by air, W;in theMs systemis the weight involves of two the heatregions storage of solid module, and flui kg;d.T 5Therefore,and T6 represent the analysis initial of temperaturethe heat transfer and temperaturecharacteristics at between any time the of thermaltwo regions storage is of module great si duringgnificance heating, to improve◦C. the heat storage efficiency of theThe heat radiant storage heat module.Qes between the resistance element and the thermal storage module can be expressedThe operation as follows process [24]: of EHSTSS is an alternating process between thermal storage and thermal " 4 4# release. The temperature of the TSS is a key indexT7 to judgeT5 whether design parameters of the system Qes = β F1 (19) are suitable. Therefore, the heat transfer characteristics100 − analysis100 of the system is carried out to explore wherethe temperatureT7 represents variation heating rule element of the temperature,heat storage device.◦C; β represents radiation coefficient, F1 represents radiation surface area of heating element, m2. 3.1. Heat Transfer Analysis in Thermal Storage Process For the convective heat Qea between resistance element and air we can refer to Equation (20): The thermal in the thermal storage module is derived from the heating element. The thermal Qea = γ(T T )F (20) generated by the heating element interacts with the7 − heat1 2 storage module through radiation heat exchange and convection heat exchange. The net heat Qs in the thermal storage module can be where γ represents convective heat transfer coefficient, F represents heating wire area, m2. expressed as follows: 2 The thermal Qa carried by air can be written as: =+−= − QQQseseaa QCM（）s6 T T 5 (18) Qa = qm,hVp,h(T6 T2) (21) where Qes represents radiant heat between heating element− and thermal storage module, W; Qea whererepresentsqm,h convectiverepresents heat the mass between velocity resistance of the air,element m/s; Vandp,h representsair, W; Qa represents the constant heat pressure carried specific by air, volumeW; Ms is of the the weight air, m3 .of Applying the heat Equations storage module, (19)–(21) kg; into T5 Equation and T6 represent (18) gives: initial temperature and temperature at any time of thermal storage module during heating, °C. " 4 4# The radiant heat Qes between Tthe7 resistanceT5 element and the thermal storage module can be T6 CMs + qm,nVp,h = β + γF2T7 γF2T1 + qm,hVp,hT2 + CMsT5 (22) expressed as follows [24]: 100 − 100 −

44 The temperature T6 of the thermal storageTT module at a certain moment can be calculated by Q =−β 75F Equation (22). es 1 (19) 100 100 3.2. Heat Transfer Analysis in Thermal Release Process where T7 represents heating element temperature, °C; β represents radiation coefficient, F1 represents radiationIn the surface thermal area release of heating process, element, the thermal m2. source of the system is mainly the thermal storage module.For the The convective thermal released heat QeaQ betweenr by the thermalresistance storage element module and air is we equal can to refer the sumto Equation of the e ﬀ(20):ective =−γ Qea ()TTF712 (20) where γ represents convective heat transfer coefficient, F2 represents heating wire area, m2.

Energies 2020, 13, 5241 9 of 20

thermal Qc and the lost thermal of the thermal storage system. The thermal released Qr can be written as: Qr = Qc/η (23)

The thermal released Qr also can be written as Equation (24) [24]:

Qr = CMs(T T ) (24) 8 − 9 where T8 and T9 represent the initial temperature and the temperature at the moment of the thermal storage module during heat release, ◦C. The effective heat released by the system can be expressed as Equation (25): Qc = M C (T T ) (25) h p.h 3 − 4 where M represents the mass flow rate, kg/s; c represents the specific heat, kJ/ (kg K). h p.h × Applying Equations (24) and (25) into Equation (23) gives:

MhCp.h(T3 T4) T9 = T8 − (26) − CMcη

The temperature T9 of the thermal storage module at a certain moment can be solved by Equation (26).

3.3. Simulation Results and Discussion

3.3.1. Parameters of Geometry Model According to the thermal calculation process and method of the EHSTSS proposed in the paper, the physical model of the TSS is built. The physical model parameters are set as shown in Table2.

Table 2. Simulation parameters of the TSS.

Parameter Type Value Thermal storage unit length, width and height 240 mm 115 mm 53 mm × × Thermal storage capacity (MgO) 1000 kWh Thermal storage unit speciﬁc heat 960 J kg 1 K 1 · − · − Bulk density 2900 kg m 3 · − Heat transfer coeﬃcient 2.7 W m 1 K 1 · − · − Initial temperature of thermal storage module 380 K Final temperature of heat storage module 950 K

In order to measure the thermal storage module and ﬂuid temperature in the system, six sets thermocouples are arranged to monitor the temperature of the thermal storage module and air in the channel as shown in Figure5.

3.3.2. Mathematical Model According to the previous analysis, the heat storage system is mainly divided into the fluid region formed by the incompressible high-temperature air and the solid region formed by the heat storage module. In the fluid region, the fluid heat transfer process can be described by three equations: the mass conservation equation, the momentum conservation equation and the energy conservation equation [25]. Energies 2020, 13, 5241 10 of 20

In order to measure the thermal storage module and fluid temperature in the system, six sets Energiesthermocouples2020, 13, 5241 are arranged to monitor the temperature of the thermal storage module and air10 in of the 20 channel as shown in Figure 5.

Temperature Heating element measuring point

1 2

3 4

FigureFigure 5. 5.Thermocouple Thermocouple arrangementarrangement diagramdiagram (left (left view). view)

3.3.2.The Mathematical mass conservation Model equation is: According to the previous analysis, the heat storage system is mainly divided into the fluid ∂ρa region formed by the incompressible high-temperatu+ div ρaure =air0 and the solid region formed by the (27)heat ∂t j storage module. In the fluid region, the fluid heat transfer process can be described by three equations:The momentum the mass conservation equationequation, is: the momentum conservation equation and the energy conservation equation [25]. dv 2 The mass conservation equation is:ρa = F p + ξ v (28) dt − ∇ ∇ ∂ρ The energy conservation equation is: a +=div()ρ u 0 (27) ∂t aj

dTa 2 The momentum conservationρ equationaC is:= (λaTa) + φ + S (29) p.h dt ∇ i dv ρξ=−∇+∇2 where Ta is the air temperature in the thermala storageF pv channel, K; v is the air velocity in the thermal(28) 1 dt 1 1 storage channel, m s ; λa is the thermal conductivity of air, W m K ; F is the mass force on the · − · − · − fluid,The N; p energyis the fluid conservation pressure, equation Pa; ζ is the is: dynamic viscosity of air, kg m 1 s 1; φ is the loss equation; t · − · − is the time, s; Si is the energy source term. Ind thisT study, the energy source term is not considered, so Si ρλφa =∇2 + + is set as 0. a.hCTSp () aa i (29) dt The heat storage module transfers energy by heat conduction. The heat conduction problem of the heatwhere storage Ta is the module air temperature can be simplified in the thermal as a steady-state storage channel, heat conduction K; v is the problem. air velocity and in its the governing thermal equationstorage channel, can be expressed m·s−1; λa is as: the thermal conductivity of air, W·m−1·K−1; F is the mass force on the fluid, N; p is the fluid pressure, Pa; ζ is the dynamic viscosity of air, kg·m−1·s−1; ϕ is the loss equation; t is the 2 2 2 time, s; Si is the energy source term.∂Ts In this ∂study,Ts the∂ T energys ∂ T sources term is not considered, so Si is ρsC = λs( + + ) + Sj (30) set as 0. ∂t ∂2x ∂2 y ∂2z The heat storage module transfers energy by heat conduction. The heat conduction problem of 1 1 where λs is the thermal conductivity of the thermal storage unit, W m K ; Ts is the temperature of the heat storage module can be simplified as a steady-state heat· − conduction· − problem. and its thegoverning thermal equation storage module, can be expressed K; x, y and as:z are the coordinate values, m; Sj is the heating power per unit volume of the thermal storage module, W. ∂∂∂∂TTTT222 On the thermal exchange interfaceρλCSs between=+++() sssthe high-temperature air and the thermal storage ss∂∂∂∂22 2 j (30) channel, the directly coupled fluid-solidtxyz interface must meet the energy continuity condition, that is, the temperature and heat flux density of the air and the heat storage channel are equal. It can be expressed as:   Ta = Ts   ∂Ta ∂Ts (31)  qa = λa = λs = qs − ∂n − ∂n Energies 2020, 13, 5241 11 of 20 where λs is the thermal conductivity of the thermal storage unit, W·m−1·K−1; Ts is the temperature of the thermal storage module, K; x, y and z are the coordinate values, m; Sj is the heating power per unit volume of the thermal storage module, W. On the thermal exchange interface between the high-temperature air and the thermal storage channel, the directly coupled fluid-solid interface must meet the energy continuity condition, that is, the temperature and heat flux density of the air and the heat storage channel are equal. It can be expressed as: = TTas   ∂∂TT  (31) Energies 2020, 13, 5241 qq=−λλas =− = 11 of 20  aa s  s  ∂∂nn 

−22 −11 where qqaa is the heat fluxflux densitydensity onon thethe fluidfluid side,side, J J·mm ·ss ; ;qqs sisis the the heat heat flux flux density density on on the the solid solid side, side, · − · − JJ·mm−22·ss−1;1 n; nisis the the fluid-solid fluid-solid interface interface normal normal vector. vector. · − · − 3.3.3. Mesh Generation and Boundary Conditions The modelmodel was was calculated calculated using using the commercial the commerc simulationial simulation software FLUENT15.software FLUENT15. The tetrahedral The structuredtetrahedral gridstructured is used grid to divide is used the to heatdivide storage the heat system. storage The system. grid division The grid of division heat storage of heat system storage is shownsystem inis shown the Figure in the6. The Figure mesh 6. The irrelevance mesh irreleva test isnce implemented test is implemented under the under 4 types the of 4 grid types numbers. of grid Afternumbers. the simulationAfter the simulation process reaches process the reaches steady the state, steady the results state, the are shownresults inare the shown Table in3. the Compared Table 3. withCompared the high with number the high of grids,number there of isgrids, a certain there di isff aerence certain in difference the temperature in the simulationtemperature results simulation when theresults number when of the grids number is less of than grids 1.06 is million, less than and 1.06 the million, simulation and results the simulation change little results when change the number little ofwhen grids the exceeds number 1.06 of million. grids Therefore,exceeds 1.06 the calculationmillion. Therefore, speed and th timee calculation is considered speed comprehensively, and time is andconsidered the number comprehensively, of meshes is determinedand the number to be of 1.06 meshes million. is determined to be 1.06 million.

FigureFigure 6.6. Grid division of heat storage system.system

Table 3. Simulation results of the grid independence veriﬁcation.verification.

NumberNumber of of OutletOutlet Temperature Temperature of of CoreCore Temperature Temperature of PorosityPorosity Grid Type Grid Type Grids/Grids Million/ Million Regenerator/KRegenerator /K ofRegenerator/K Regenerator/K 0.9 0.9648 648 688 StructuredStructured 1.06 1.06667 667 702 15% 15% hexagonalhexagonal grid grid 1.17 1.17669 669 704 1.28 1.28670 670 705

In thethe numericalnumerical simulation, simulation, time time step step is 10.is 10. The Th aire air flowing flowing in the in heatthe heat storage storage channel channel and itsand front its andfront rear and cavities rear cavities is a turbulent is a turbulent flow, and flow, the andk-ε turbulence the k-ε turbulence model is model applied. is Theapplied. unsteady The Reynoldsunsteady time-averagedReynolds time-averagedN-S equation N-S isequation used in simulation.is used in simulation The coupling. The between coupling velocity between and velocity pressure and is realizedpressure by is SIMPLICrealized algorithm.by SIMPLIC The algorithm. finite volume The method finite isvolume used to method discretize is the used governing to discretize equations. the Thegoverning convection equations. term diThefference convection scheme term adopts difference the second-order scheme adopts upwind the scheme. second-order Dimensionless upwind −3 residualsscheme. Dimensionless of continuous equationsresiduals reducedof contin touous less thanequations 1 10 reduced3 are the to convergence less than criteria.1 × 10 are the × − convergenceThermal criteria. storage: Heating power is 270 kW. Absorption coefficient is 0.7. Cold air flow rate is 1 m/s.Thermal The outlet storage: is set Heating to the pressure power outletis 270 kW. and theAbsorption boundary coefficient condition is of 0.7. the Cold wall air is adiabaticflow rate andis 1 nom/s. slip. The The outlet interface is set to between the pressure the fluid outlet region and and the solid boundary region condition is set as coupling of the wall interface. is adiabatic The totaland thermal storage time is 26.190 s. Thermal release: The heating power is set to 0 kW. The inlet of cold air speed is 28 m/s and air temperature is given according to field test data. Thermal release time is 41.130 s.

3.3.4. Simulation Results Analysis In the thermal storage process, the temperature variation of the TSS is shown in Figure6. The temperature near the heating element is higher, and the temperature of the outlet section is more uniform. At the initial moment of heating, as shown in Figure7a–c, the temperature at the inlet section emerges a peak, and the direction of the peak points to the inside of the thermal storage channel. It is caused by the diﬀerence of air velocity due to the accelerated ﬂow of cold air inside the device Energies 2020, 13, 5241 12 of 20 no slip. The interface between the fluid region and solid region is set as coupling interface. The total thermal storage time is 26.190 s. Thermal release: The heating power is set to 0 kW. The inlet of cold air speed is 28 m/s and air temperature is given according to field test data. Thermal release time is 41.130 s.

3.3.4. Simulation Results Analysis In the thermal storage process, the temperature variation of the TSS is shown in Figure 6 The temperature near the heating element is higher, and the temperature of the outlet section is more uniform. At the initial moment of heating, as shown in Figure 7a–c, the temperature at the inlet Energiessection2020 emerges, 13, 5241 a peak, and the direction of the peak points to the inside of the thermal storage12 of 20 channel. It is caused by the difference of air velocity due to the accelerated flow of cold air inside the device and the uneven temperature distribution at the inlet of the heat storage channel. Because of and the uneven temperature distribution at the inlet of the heat storage channel. Because of the the turbulence at the outlet of the thermal fluid, the temperature of the thermal fluid at the outlet of turbulence at the outlet of the thermal fluid, the temperature of the thermal fluid at the outlet of is is lower. When the heating time reaches the set value, as shown in Figure 7d, the temperature of the lower. When the heating time reaches the set value, as shown in Figure7d, the temperature of the thermal storage module is around 900 K, and it reaches the preset temperature. Meanwhile, the thermal storage module is around 900 K, and it reaches the preset temperature. Meanwhile, the heating heating element temperature is approximately 1100 K, which is lower than the maximum element temperature is approximately 1100 K, which is lower than the maximum temperature of temperature of the element. In conclusion, the temperature distribution of the thermal storage the element. In conclusion, the temperature distribution of the thermal storage module is relatively module is relatively uniform. The design and arrangement of the module and heating elements are uniform. The design and arrangement of the module and heating elements are reasonable, which reasonable, which verifies correctness of the thermal calculation of the system. verifies correctness of the thermal calculation of the system.

T/K T/K 400 400 450 450 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 1050 1050 1100 1100

(a) (b)

T/K 400 T/K 400 450 450 500 500 550 550 600 600 650 650 700 700 750 750 800 800 850 850 900 900 950 950 1000 1000 1050 1050 1100 1100

(c) (d) Figure 7. Cloud profile of temperature distribution at section during heating. (a) heat for 2 h; (b) Figure 7. Cloud proﬁle of temperature distribution at section during heating. (a) heat for 2 h; (b) heat heat for 4 h; (c) heat for 6 h; (d) heat for 7.2 h for 4 h; (c) heat for 6 h; (d) heat for 7.2 h.

In the heat releaserelease process,process, the the temperature temperature distribution distribution of of the the thermal thermal storage storage module module is shownis shown in inFigure Figure8. At 8. theAt endthe end of the of thermal the thermal release, release, the temperature the temperature of module of module is uniformly is uniformly distributed distributed around around380 K, which 380 K, iswhich consistent is consistent with the with preset the preset temperature. temperature. The temperature The temperature near the near inlet the isinlet lower is lower than thanother other locations, locations, which which is caused is caused by the by rapid theheat rapid release heat release due to thedue low to the air temperaturelow air temperature and fast ﬂowand fastrate flow at the rate entrance. at the Inentrance. conclusion, In conclusion, the TSS has th ae goodTSS has match a good with match air ﬂow with and air velocity flow and provided velocity by providedthe fan-circulating by the fan-circulating system, which system, makes the which heat ma storagekes the module heat completestorage module the heat complete release within the heat the releasesetting time.within The the correctness setting time. and rationalityThe correctness of the thermaland rationality calculation of andthe guidancethermal calculation for the EHSTSS and guidanceis proved. for the EHSTSS is proved. Energies 2020, 13, 5241 13 of 20

T/K 370 380 390 400 410 420 430 440 450 460

Figure 8. The temperature distribution of the systemsystem after the end of the thermal release.release

4. Experimental Experimental Results Results and and Discussion Discussion In the paper, the operating data data of of the the EHSTSS EHSTSS with with a a rated rated power power of of 125 125 kW kW in in Anshan Anshan City, City, (Liaoning Province, Province, China) China) are are extracted extracted and and processed. processed. The The external external and and internal internal structure structure of the of experimental equipment is shown in Figure 9. The heating area of the equipment is 1000 m2. The average outdoor temperature in winter at the location of the equipment is −0.2 °C and the heating time is 114 days. In order to make the temperature of the measuring point more accurate at the testing time, the temperature at the sampling time and the temperature at 20 s before and after the sampling are measured, and the average of the three values is taken as the temperature at this time. The control parameters of the device are adjusted to stabilize the various operating data of the equipment. The operation data of the device under stable conditions was used.

Heating element

Thermal Storage Structure Thermal Storage Controller Blower module

Duct

Figure 9. Experimental facility

In the experiment, thermocouples are used to measure the temperature at different positions of the thermal storage module, and the positions the arrangement of thermocouples refer to in Figure 5.

4.1. Analysis of Experimental Results Under Thermal Storage In the thermal storage process, temperature variation at each measuring point is shown in Figure 10. At the end of thermal storage, the temperature of measuring point 2 is the highest, and the temperature value is 715 °C. The lowest temperature was measured at point 4, with a temperature value of 683 °C. The maximum temperature difference of the measuring point is 33 °C. The temperature standard deviation of each temperature measurement point at each moment is calculated, as shown by the dotted line in Figure 10a. In the middle of thermal storage, the temperature deviation at each point is relatively large, the maximum is 28.3 °C. At the beginning and end of thermal storage, the temperature deviation of the each point is small, only about 10 °C. Therefore, the temperature of the thermal storage module is relatively uniform, and the thermal stress of the thermal storage unit is smaller. The TSS design is reasonable.

Energies 2020, 13, 5241 13 of 20

T/K 370 380 390 400 410 420 430 440 450 460 Figure 8. The temperature distribution of the system after the end of the thermal release

4. Experimental Results and Discussion Energies 13 In 2020the, paper,, 5241 the operating data of the EHSTSS with a rated power of 125 kW in Anshan13 City, of 20 (Liaoning Province, China) are extracted and processed. The external and internal structure of the experimentalthe experimental equipment equipment is shown is shown in Figure in Figure 9. The9. Theheating heating area areaof the of equipment the equipment is 1000 is 1000 m2. The m 2 . averageThe average outdoor outdoor temperature temperature in winter in winter at the atlocation the location of the equipment of the equipment is −0.2 °C is and0.2 theC andheating the − ◦ timeheating is 114 time days. is 114 In order days. to In make order the to temperature make the temperature of the measuring of the measuring point more point accurate more at accuratethe testing at time,the testing the temperature time, the temperature at the sampling at the time sampling and the timetemperature and the at temperature 20 s before atand 20 after s before the sampling and after arethe measured, sampling areand measured, the average and of the the three average values of theis taken three as values the temperature is taken as at the this temperature time. The control at this parameterstime. The control of the parametersdevice are adjusted of the device to stabiliz are adjustede the various to stabilize operating the various data of operating the equipment. data of The the operationequipment. data The of operation the device data under of thestable device conditions under stable was used. conditions was used.

Heating element

Thermal Storage Structure Thermal Storage Controller Blower module

Duct

Figure 9. ExperimentalExperimental facility facility.

InIn the the experiment, experiment, thermocouples thermocouples are are used used to to me measureasure the the temperature temperature at at different diﬀerent positions of of thethe thermal storage module,module, andand thethe positionspositions thethe arrangementarrangement of of thermocouples thermocouples refer refer to to in in Figure Figure5. 5. 4.1. Analysis of Experimental Results Under Thermal Storage 4.1. AnalysisIn the thermal of Experimental storage process, Results temperatureUnder Thermal variation Storage at each measuring point is shown in Figure 10. At theInend the ofthermal thermal storage storage, process, the temperature temperature of variat measuringion at point each 2measuring is the highest, point and is shown the temperature in Figure 10.value At isthe 715 end◦C. of The thermal lowest temperaturestorage, the wastemperatur measurede of at measuring point 4, with point a temperature 2 is the highest, value ofand 683 the◦C. temperatureThe maximum value temperature is 715 °C. diTheﬀerence lowest of temperatur the measuringe was pointmeasured is 33 ◦atC. point The temperature4, with a temperature standard valuedeviation of 683 of each°C. temperatureThe maximum measurement temperature point difference at each of moment the measuring is calculated, point as is shown 33 °C. by The the temperaturedotted line in standard Figure 10 a.deviation In the middle of each of thermal temperature storage, measurement the temperature point deviation at each at eachmoment point is is calculated,relatively large, as shown the maximum by the isdotted 28.3 ◦C. line At thein beginningFigure 10a. and In end the of middle thermal of storage, thermal the storage, temperature the temperaturedeviation of thedeviation each point at each is small, point only is relatively about 10 large,◦C. Therefore, the maximum the temperature is 28.3 °C. At of thethe thermalbeginning storage and endmodule of thermal is relatively storage, uniform, the temperature and the thermal deviation stress of of the thermaleach point storage is small, unit isonly smaller. about The 10 TSS°C. design is reasonable. Therefore,Energies 2020 the, 13, temperature5241 of the thermal storage module is relatively uniform, and the thermal14 stress of 20 of the thermal storage unit is smaller. The TSS design is reasonable. 800 point 1 point 2 32 650 test value in point 1 simulation value in point 1 700 point 3 point 4 test value in point 3 simulation value in point 3 piont 5 point 6 28 600 test value in point 6 simulation value in point 6 600 temperature deviation 24 500 550 20 400 16 500 300 Temperature/℃

12 Temperature/℃ 450

200 deviation/℃ Temperature 8 400 100 4 350 0 100 200 300 400 500 600 200 240 280 320 360 400 Time/min Time/min (a) (b) Figure 10. Temperature variation curves of measuring points of thermal storage module in the thermal storage Figure 10. Temperature variation curves of measuring points of thermal storage module in the thermal process (a) experimental value and deviation of temperature at measuring point; (b) curve of test point storage process (a) experimental value and deviation of temperature at measuring point; (b) curve of temperature test value and simulation value. test point temperature test value and simulation value. At the same time, the test value and simulation value of the temperature at measuring points 1, 3, and 6 are compared and analyzed, as shown in Figure 10b. The standard deviation statistics of the simulation value and the test value error rate of each measuring point are investigated, and the error deviations of the simulation value and the test value of the measuring points 1, 3, and 6 in the whole thermal storage process are respectively 0.796%, 0.925% and 0.805%. The simulation value is close to the experimental value, so it can be seen that the EHSTSS designed in the paper has good heat transfer performance. The average temperature curve of the thermal storage module in different typical working days is shown in Figure 11. The average temperature of the thermal storage module rises from 130 °C to about 700 °C during the heating time of 10 h. The average temperature variation of the typical day has a small fluctuation, the temperature difference of the thermal storage module at the same time is small. The maximum temperature deviation is only 33.2 °C. It can be seen that the EHSTSS has good operational stability, and it can meet the design requirements.

800 40 The temperature in Nov The temperature in Dec 700 35 The temperature in Jan Temperature deviation 600 30

25 500 20 400

Temperature/℃ 15 300 10 deviation/℃ Temperature 200 5 100 0 60 120 180 240 300 360 420 480 540 Time/min Figure 11. Temperature curve of the heat storage module under the heating.

The water supply temperature is constant as the control target. The average temperature of the TSS and frequency curve of the fan are shown in Figure 12. With the average temperature rising gradually in the preset range, the frequency of the fan declines, and the water supply temperature is maintained at the setting temperature about 50 °C. It can be obtained that the parameters of heating system, thermal storage system, heat exchange system and fan-circulating system have good adaptability and compatibility. They ensure that the temperature change of the thermal storage module and the supply water can be stabilized in the set range.

Energies 2020, 13, 5241 14 of 20

800 point 1 point 2 32 650 test value in point 1 simulation value in point 1 700 point 3 point 4 test value in point 3 simulation value in point 3 piont 5 point 6 28 600 test value in point 6 simulation value in point 6 600 temperature deviation 24 500 550 20 400 16 500 300 Temperature/℃

12 Temperature/℃ 450

200 deviation/℃ Temperature 8 400 100 4 350 0 100 200 300 400 500 600 200 240 280 320 360 400 Time/min Time/min (a) (b) Figure 10. Temperature variation curves of measuring points of thermal storage module in the thermal storage Energiesprocess2020 (a) , 13experimental, 5241 value and deviation of temperature at measuring point; (b) curve of test14 point of 20 temperature test value and simulation value.

At the same time, the the test test value value and and simulation simulation value value of of the the temperature temperature at atmeasuring measuring points points 1, 1,3, 3,and and 6 are 6 are compared compared and and analyzed, analyzed, as asshown shown in Fi ingure Figure 10b. 10 Theb. The standard standard deviation deviation statistics statistics of the of thesimulation simulation value value and andthe test the value test value error error rate of rate each of eachmeasuring measuring point point are investigated, are investigated, and the and error the errordeviations deviations of the ofsimulation the simulation value valueand the and test the value test valueof the ofmeasuring the measuring points points1, 3, and 1, 3,6 in and the 6 whole in the wholethermal thermal storage storage process process are respectively are respectively 0.796%, 0.796%, 0.925% 0.925%and 0.805%. and 0.805%. The simulation The simulation value is valueclose to is closethe experimental to the experimental value, so value, it can so be it seen can that be seen the EHSTSS that the EHSTSSdesigned designed in the paper in the has paper good has heat good transfer heat transferperformance. performance. The average temperature curve of the thermal storagestorage module in didifferentfferent typical working days isis shownshown inin FigureFigure 1111.. The averageaverage temperature of the thermalthermal storagestorage module rises from 130130 ◦°CC to about 700 ◦°CC during the heating time of 1010 h.h. The average temperature variation of the typical day has a small fluctuation,fluctuation, thethe temperature temperature difference difference of the thermal storage module at the same time is small. The maximum temperature deviation is only 33.233.2 ◦°C.C. It can be seen that the EHSTSS has good operational stability, andand itit cancan meetmeet thethe designdesign requirements.requirements.

800 40 The temperature in Nov The temperature in Dec 700 35 The temperature in Jan Temperature deviation 600 30

25 500 20 400

Temperature/℃ 15 300 10 deviation/℃ Temperature 200 5 100 0 60 120 180 240 300 360 420 480 540 Time/min Figure 11. Temperature curve of the heat storagestorage module underunder thethe heating.heating.

The waterwater supplysupply temperature temperature is is constant constant as as the the control control target. target. The The average average temperature temperature of the of TSSthe andTSS frequencyand frequency curve curve of the of fan the are fan shown are inshown Figure in 12 Figure. With 12. the With average the temperatureaverage temperature rising gradually rising ingradually the preset in range,the preset the frequencyrange, the offrequency the fan declines, of the fan and declines, the water and supply the water temperature supply temperature is maintained is atmaintained the setting at temperaturethe setting temperature about 50 ◦ C.about It can 50 be°C. obtainedIt can be thatobtained the parametersthat the parameters of heating of system,heating thermalsystem, storagethermal system, storage heat system, exchange heat system exchange and fan-circulatingsystem and fan-circulating system have good system adaptability have good and compatibility.adaptability and They compatibility. ensure that the They temperature ensure that change the oftemperature the thermal change storage of module the thermal and the storage supply Energieswatermodule can2020 and be, 13 the, stabilized 5241 supply in water the set can range. be stabilized in the set range. 15 of 20

700 Frequency/Hz 16.5 Water temperature/℃ 600 Regenerative temperature/℃ 15.0

℃ 500 13.5 400 12.0 300

Temperature/ 10.5 200 /Hz frequency Fan

100 9.0 0 0 100 200 300 400 500 600 Time/min

FigureFigure 12. 12.Supply Supply water water temperature, temperature, thermal thermal storage stor moduleage module average average temperature temperature and fan and frequency fan variationfrequency curve variation under curve thermal under storage. thermal storage

4.2. Analysis of Experimental Results Under Thermal Release In the thermal release process, temperature at each measuring point of the module is shown in Figure 13. At the end of thermal release, the temperature of measuring point 5 is the highest, and the temperature value is 121 °C. The lowest temperature was measured at point 3, with a temperature value of 106 °C. The maximum temperature difference is 15 °C. The temperature standard deviation of each measuring point of the system at different times is analyzed. The maximum standard deviation of the system is 19.2 °C, and the temperature standard deviation of the system is maintained at about 10 °C for most of the time. The thermal storage module of the system is proved to have good temperature uniformity, and the structure of thermal storage module is reasonable. The coordination between air circulation system and thermal storage system is good.

The test value and simulation value of the temperature at measuring points 1, 3, and 6 are compared and analyzed in the thermal release process, as shown in Figure 13b. The standard deviation statistics of the simulation value and the test value error rate of each measuring point are analyzed, and the error deviations of the simulation value and the test value of the measuring points 1, 3, and 6 in the whole thermal storage process are respectively 8.6%, 6.7% and 3.34%. The simulated value has a certain deviation from the experimental value. This is mainly caused by the unavoidable heat leakage in the thermal insulation layer design during the experiment of the EHSTSS. Therefore, more attention should be paid to thermal insulation design in the system. The average temperature variation curve of the thermal storage module on different typical working days is shown in Figure 14. The average temperature of thermal storage module on the typical daily varies from 700℃ to 130 °C during the thermal release time of 14 h. At the same time, the standard temperature deviation changes relatively large between each day, and the maximum temperature deviation reaches 59 °C. Therefore, the operating fluctuation of the system during the

Energies 2020, 13, 5241 15 of 20

700 Frequency/Hz 16.5 Water temperature/℃ 600 Regenerative temperature/℃ 15.0

℃ 500 13.5 400 12.0 300

Temperature/ 10.5 200 /Hz frequency Fan

100 9.0 0 0 100 200 300 400 500 600 Time/min Figure 12. Supply water temperature, thermal storage module average temperature and fan Energiesfrequency2020, 13, 5241 variation curve under thermal storage 15 of 20

4.2. Analysis of Experimental Results Under Thermal Release 4.2. Analysis of Experimental Results Under Thermal Release In the thermal release process, temperature at each measuring point of the module is shown in In the thermal release process, temperature at each measuring point of the module is shown in Figure 13. At the end of thermal release, the temperature of measuring point 5 is the highest, and the Figure 13. At the end of thermal release, the temperature of measuring point 5 is the highest, and the temperature value is 121 °C. The lowest temperature was measured at point 3, with a temperature temperature value is 121 ◦C. The lowest temperature was measured at point 3, with a temperature value value of 106 °C. The maximum temperature difference is 15 °C. The temperature standard deviation of 106 ◦C. The maximum temperature diﬀerence is 15 ◦C. The temperature standard deviation of each of each measuring point of the system at different times is analyzed. The maximum standard measuring point of the system at diﬀerent times is analyzed. The maximum standard deviation of the deviation of the system is 19.2 °C, and the temperature standard deviation of the system is maintained system is 19.2 C, and the temperature standard deviation of the system is maintained at about 10 C at about 10 °C ◦for most of the time. The thermal storage module of the system is proved to have good◦ for most of the time. The thermal storage module of the system is proved to have good temperature temperature uniformity, and the structure of thermal storage module is reasonable. The coordination uniformity, and the structure of thermal storage module is reasonable. The coordination between air between air circulation system and thermal storage system is good. circulation system and thermal storage system is good.

450 800 21 test value in point 1 point1 point2 400 simulation value in point 1 700 point3 point4 18 test value in point 3 simulation value in point 3 600 point5 point6 350 15 test value in point 6 ℃ temperature deviation 500 300 simulation value in point 6 12 400 250 9 300 Temperature/℃ Temperature/ 200 200 6 Temperaturedeviation/℃ 150 100 3 200 250 300 350 400 450 500 550 600 0 100 200 300 400 500 600 700 800 Time/min Time/min (a) (b) Figure 13. Temperature variation curves of measuring points of thermal storage module in the thermal release Figure 13. Temperature variation curves of measuring points of thermal storage module in the thermal process (a) experimental value and deviation of temperature at measuring point; (b) curve of test point release process (a) experimental value and deviation of temperature at measuring point; (b) curve of temperature test value and simulation value. test point temperature test value and simulation value. The test value and simulation value of the temperature at measuring points 1, 3, and 6 are The test value and simulation value of the temperature at measuring points 1, 3, and 6 are compared and analyzed in the thermal release process, as shown in Figure 13b. The standard compared and analyzed in the thermal release process, as shown in Figure 13b. The standard deviation deviation statistics of the simulation value and the test value error rate of each measuring point are statistics of the simulation value and the test value error rate of each measuring point are analyzed, analyzed, and the error deviations of the simulation value and the test value of the measuring points and the error deviations of the simulation value and the test value of the measuring points 1, 3, and 6 1, 3, and 6 in the whole thermal storage process are respectively 8.6%, 6.7% and 3.34%. The simulated in the whole thermal storage process are respectively 8.6%, 6.7% and 3.34%. The simulated value has a value has a certain deviation from the experimental value. This is mainly caused by the unavoidable certain deviation from the experimental value. This is mainly caused by the unavoidable heat leakage heat leakage in the thermal insulation layer design during the experiment of the EHSTSS. Therefore, in the thermal insulation layer design during the experiment of the EHSTSS. Therefore, more attention more attention should be paid to thermal insulation design in the system. should be paid to thermal insulation design in the system. The average temperature variation curve of the thermal storage module on different typical The average temperature variation curve of the thermal storage module on diﬀerent typical working days is shown in Figure 14. The average temperature of thermal storage module on the working days is shown in Figure 14. The average temperature of thermal storage module on the typical daily varies from 700℃ to 130 °C during the thermal release time of 14 h. At the same time, typical daily varies from 700°C to 130 ◦C during the thermal release time of 14 h. At the same time, the standard temperature deviation changes relatively large between each day, and the maximum the standard temperature deviation changes relatively large between each day, and the maximum temperature deviation reaches 59 °C. Therefore, the operating fluctuation of the system during the temperature deviation reaches 59 ◦C. Therefore, the operating ﬂuctuation of the system during the heat release process is relatively large, and the system can be optimized from the angle of thermal release. The water supply temperature is constant as the control target. The average temperature of the TSS and frequency curve of the fan are shown in Figure 15. The average temperature gradually decreases in the preset range. Meanwhile, the frequency of the blow rises. The temperature of the supply water still remains at a preset temperature of about 50 ◦C. It can be concluded that the parameters of subsystem in the EHSTSS are reasonable, and each part has good compatibility with each other. Therefore, supply water temperature can be maintained stability by adjusting frequency of the fan to control the heat release. Energies 2020, 13, 5241 16 of 20

heat release process is relatively large, and the system can be optimized from the angle of thermal release.

800 80 The temperature in Nov 700 The temperature in Dec 70 The temperature in Jan 600 60 Energies 2020, 13, 5241 Temperature deviation 16 of 20 500 50 Energiesheat release2020, 13 ,process 5241 is relatively400 large, and the system can be optimized from the angle of thermal16 of 20 40 release. 300 Temperature/℃ 30 200 800 80 Temperature deviation/℃ 100 20 The temperature in Nov 700 70 0 The temperature in Dec 10 0 120 240 360 480 600 720 600 The temperature in Jan Time/min Temperature deviation 60 500 Figure 14. Average temperature curve of the module under thermal50 release process 400 40 The water supply temperature300 is constant as the control target. The average temperature of the Temperature/℃ 30 TSS and frequency curve of200 the fan are shown in Figure 15. The average temperature gradually Temperature deviation/℃ decreases in the preset range.100 Meanwhile, the frequency of the blow20 rises. The temperature of the supply water still remains at a preset temperature of about 50 °C. It can be concluded that the 0 10 parameters of subsystem in the0 EHSTSS 120 are 240 reason 360able, 480 and 600 each 720 part has good compatibility with Time/min each other. Therefore, supply water temperature can be maintained stability by adjusting frequency of the fan toFigure Figurecontrol 14. 14. theAverage Average heat release. temperaturetemperature curvecurve ofof the modulemodule under thermal thermal release release process process.

The water supply temperature is constant as the control target. The average temperature of the 700 Fan frequency/Hz 16.5 TSS and frequency curve of the fan are Watershown temperature/℃ in Figure 15. The average temperature gradually decreases in the preset range.600 Meanwhile, Regenerative the freq temperature/℃uency of the blow 15.0rises. The temperature of the supply water still remains 500at a preset temperature of about 50 °C. It can be concluded that the 13.5 parameters of subsystem in400 the EHSTSS are reasonable, and each part has good compatibility with

each other. Therefore, supply water temperature can be maintained stability12.0 by adjusting frequency 300 of the fan to control the heat release. Temperature/℃ 200 10.5 Fan frequency/Hz Fan Fan frequency/Hz 700100 16.59.0 Water temperature/℃ 6000 Regenerative temperature/℃ 0 100 200 300 400 500 600 700 800 15.0 500 Time/min 13.5 Figure 15. Supply water400 temperature, thermal storage module average temperature and fan Figure 15. Supply water temperature, thermal storage module average temperature and fan frequency 12.0 variationfrequency curve variation under thermalcurv300e under release. thermal release

Temperature/℃ 200 10.5 4.3.4.3. EHSTSS EHSTSS Electric Electric HeatingHeating CharacteristicsCharacteristics and Pressure Drop Analysis frequency/Hz Fan 100 TheThe electrothermalelectrothermal conversion process process of ofEHST EHSTSSSS under under operation operation9.0 condition condition is analyzed. is analyzed. The 0 Theaverage average temperature temperature of each of measuring each0 100 measuring 200 point 300 for 400point the 500 heat for 600 storage the 700 heat 800module storage and module the power and of the EHSTSS power ofare EHSTSS shown arein Figure shown 16 in within Figure 72 16h. Aswithin shown 72 inTime/min h. Figure As shown 16 before in Figure the temperature 16 before of the the temperature heat storage of themodule heatFigure storage reaches 15. moduletheSupply set value,water reaches temperature,the the EHSTSS set value, thermaloperates the storag EHSTSS at fulle modulepower operates toaverage ensure at full temperature that power the tosystem ensureand fancan that store the systemheat while canfrequency store outputting heatvariation while curvheat. outputtinge underAfter thermal reaching heat. release After the reaching set temperature, the set temperature, in order into order ensure to ensurethat the that thetemperature temperature of ofthe the system system is ismaintained maintained at atthe the set set temperature temperature before before pure pure heat heat release, release, EHSTSS EHSTSS operates4.3.operates EHSTSS at at half half Electric power. power. Heating WhenWhen Characteristics thethe systemsystem is andonly Pressure exothermic, exothermic, Drop theAnalysis the power power output output of of EHSTSS EHSTSS is isstopped. stopped. TheThe power power output output of of EHSTSS EHSTSS changeschanges periodicallyperiodically with the the temperature temperature of of the the heat heat storage storage module, module, The electrothermal conversion process of EHSTSS under operation condition is analyzed. The whichwhich veriﬁes verifies that that the the system system has has goodgood heatheat exchangeexchange and heat transfer performance. performance. averageEnergies 2020 temperature, 13, 5241 of each measuring point for the heat storage module and the power of EHSTSS17 of 20 are shown in Figure 16 within 72 h. As shown in Figure 16 before the temperature of the heat storage module reaches the set value, 800the EHSTSS operates Energy at storage full system power temperature to ensure140 that the system can store Energy storage system power heat while outputting heat. 700After reaching the set temperature,120 in order to ensure that the

temperature of the system is 600maintained at the set temperature before100 pure heat release, EHSTSS operates at half power. When the system is only exothermic, the power output of EHSTSS is stopped. 500 80 The power output of EHSTSS changes periodically with the temperature of the heat storage module, 400 60 which verifies that the system has good heat exchange and heat transfer performance. 300 40

200 20 Energy storage system power/kW

Energy storage system temperature/℃ 100 0 0 10203040506070 Time/h

FigureFigure 16.16. EHSTSSEHSTSS power-temperature curve curve in in 72 72 h h

The pressure drop in the heat storage system under different wind speeds is tested by adjusting the speed of the hot air fan. The change curve is shown in Figure 17.

50 Calculated value of hydrodynamic loss Experimental value of hydrodynamic loss 40

10 Hydrodynamicloss/Pa 0 4567891011 -1 Air velocity/m▪s Figure 17. Variation curve of aerodynamic loss in the thermal storage system

As shown in Figure 17, the pressure drop of the heat storage system is 42.4 Pa at the rated wind speed of the fan, which is close to the calculated value 38.7 Pa. At the same time, the calculated value of the pressure drop of the heat storage system at other wind speeds is also close to the test value. The rationality of fan selection and channel design is verified.

5. Conclusions In the paper, the thermal calculation method and heat transfer characteristics of an EHSTSS is studied. The key parameters of the subsystems in the EHSTSS were investigated, and the feasibility of the proposed method was verified by a case design. The following conclusions are drawn through the actual case design, simulation analysis and experimental verification of the EHSTSS. Systematic verification shows that within the rated heating and thermal release time, the internal thermal of the thermal storage module can be fully stored and released. The maximum temperature of the thermal storage module is controlled at about 700 °C, and the outlet water temperature is stabilized at about 50 °C. The design system can fully meet the design requirements. During the heating process, the temperature gradient of each measuring point of the system is small. The maximum temperature standard deviation of each temperature measurement point at each moment is 28.3 °C. The simulation value is consistent with the test value, and error deviations of the temperature is only 0.796%, 0.925% and 0.805%. The good heat transfer performance of the system is proved. In the thermal release process, the temperature distribution of the system is uniform, and the temperature standard deviation of the system in each measuring point is maintained at about 10 °C for most of the time. However, the standard temperature deviation changes relatively large between typical day in each month, and the maximum temperature deviation reaches 59 °C. This phenomenon indicates that there are some defects in the long-term operation stability of the system, which needs

Energies 2020, 13, 5241 17 of 20

800 Energy storage system temperature 140 Energy storage system power 700 120

600 100

500 80

400 60

300 40

200 20 Energy storage system power/kW

Energy storage system temperature/℃ 100 0 0 10203040506070 Time/h Energies 2020, 13, 5241 17 of 20 Figure 16. EHSTSS power-temperature curve in 72 h

The pressure dropdrop inin thethe heatheat storagestorage system underunder didifferentﬀerent windwind speedsspeeds isis testedtested byby adjusting thethe speedspeed ofof thethe hothot airair fan.fan. The change curve isis shownshown inin FigureFigure 1717..

50 Calculated value of hydrodynamic loss Experimental value of hydrodynamic loss 40

10 Hydrodynamicloss/Pa 0 4567891011 -1 Air velocity/m▪s

Figure 17. Variation curve of aerodynamic lossloss inin thethe thermalthermal storagestorage systemsystem

As shown in Figure 1717,, the pressure dropdrop ofof thethe heatheat storagestorage systemsystem isis 42.442.4 PaPa at the rated wind speedspeed ofof thethe fan,fan, whichwhich isis closeclose toto thethe calculatedcalculated valuevalue 38.738.7 Pa.Pa. At the same time, the calculated value of thethe pressurepressure dropdrop ofof thethe heatheat storage system at other wind speeds is also close to the test value. TheThe rationalityrationality ofof fanfan selectionselection andand channelchannel designdesign isis veriﬁed.verified.

5. Conclusions InIn thethe paper,paper, thethe thermalthermal calculationcalculation methodmethod andand heatheat transfertransfer characteristicscharacteristics of anan EHSTSSEHSTSS isis studied.studied. TheThe keykey parametersparameters ofof the the subsystems subsystems in in the the EHSTSS EHSTSS were were investigated, investigated, and and the the feasibility feasibility of theof the proposed proposed method method was was verified verified by aby case a case design. design. The The following following conclusions conclusions are are drawn drawn through through the actualthe actual case case design, design, simulation simulation analysis analysis and and experimental experimental verification verification of the of EHSTSS.the EHSTSS. Systematic verificationverification showsshows thatthat withinwithin thethe ratedrated heating and thermal releaserelease time, the internal thermalthermal ofof thethe thermalthermal storage storage module module can can be be fully fully stored stored and and released. released. The The maximum maximum temperature temperature of theof the thermal thermal storage storage module module is controlled is controlled at about at 700about◦C, 700 and °C, the and outlet the water outlet temperature water temperature is stabilized is atstabilized about 50 at◦ C.about The 50 design °C. The system design can syst fullyem meet can fully the design meet the requirements. design requirements. During the heatingheating process, the temperaturetemperature gradientgradient of each measuring point of the systemsystem isis small.small. TheThe maximummaximum temperature temperature standard standard deviation deviation of eachof each temperature temperature measurement measurement point point at each at momenteach moment is 28.3 is◦ C.28.3 The °C. simulation The simulation value value is consistent is consistent with thewith test the value, test value, and error and deviationserror deviations of the temperatureof the temperature is only is 0.796%, only 0.796%, 0.925% 0.925% and 0.805%. and 0.805%. The good The heat good transfer heat transfer performance performance of the system of the issystem proved. is proved. InIn thethe thermalthermal releaserelease process,process, thethe temperaturetemperature distributiondistribution of thethe systemsystem isis uniform,uniform, andand thethe temperaturetemperature standardstandard deviationdeviation ofof thethe systemsystem inin eacheach measuringmeasuring pointpoint isis maintainedmaintained atat aboutabout 1010 ◦°CC forfor mostmost ofof thethe time.time. However, the standard temperaturetemperature deviation changes relatively large betweenbetween typicaltypical dayday inin each month, and the maximum temperaturetemperature deviation reaches 5959 ◦°C.C. This phenomenon indicatesindicates thatthat therethere areare some some defects defects in in the the long-term long-term operation operation stability stability of of the the system, system, which which needs needs to be further improved. Corresponding parameter optimization can be made in terms of the number of channels of the heat storage module, the channel structure, and the location of the air inlet and outlet of the device. From the perspective of case design, simulation analysis and experimental verification, the thermal calculation method, process and heat transfer characteristics analysis proposed in this paper are suitable for the design of an EHSTSS and it can provide a reference for the design and verification method of EHSTSS.

Author Contributions: All authors conceived and designed the study. H.Z. performed the experiment veriﬁcation, the analysis of results and wrote the manuscript with guidance from N.Y., Z.X. and L.J., L.C. performed the simulation of the study. All authors have read and agreed to the published version of the manuscript. Energies 2020, 13, 5241 18 of 20

Funding: This work was supported by “The National Key Research and Development Program of China” (2017YFB0902100). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A

Table A1. Parameter design results of EHSTSS case.

Name Value Name Value Configuration of heating Enthalpy value of return 100 189 power/kW water/kJ kg 1 · − Enthalpy value of supply Basic parameter Heating time/h 10 231 water/kJ kg 1 calculation · − Maximum heat release time/h 8 Pipe diameter/mm 50 Inlet temperature of water/°C 45 Maximum heating load/ kW 125 Outlet temperature of water/°C 55 Total thermal storage capacity/kW h 1000 · Number of single phase heating Design power/kW 100 11 elements Three-phase voltage/V 400 Single-phase voltage/ V 231 Calculation of Resistance of heating element Total number of heating elements 33 0.57 heating element (single)/Ω parameter Diameter of heating elements/ mm 3 Heating power(single)/ kW 0.74 Temperature coefficient 1.08 Length of heating element/ mm 2474 2 1 Surface load of heating element/ Resistivity/Ω mm− m− 4.79 2 3.18 · · W cm− · Thermal storage margin 1.1 Total thermal storage capacity /kW h 1100 · Specific heat capacity of thermal Calculation of 1.064 Volume of thermal storage unit/ m3 0.0041 storage unit/ kJ kg 1 °C 1 thermal storage · − · − Final average temperature of module 700 Weight of thermal storage unit/ kg 11.55 thermal storage module/°C structure Initial average temperature of Design quantity of thermal storage design 150 716 thermal storage module/°C unit Horizontal row number of 6 Longitudinal row number 7 thermal storage unit Height row number of thermal 20 Actual number of thermal storage unit 724 storage unit Thermal storage unit 2800 Weight of thermal storage module/ kg 8357 density/kg m 3 · − Thermal storage capacity of 1.71 Real thermal storage capacity/ kW h 1111 thermal storage unit/ kW h · · Outlet air flow of heat exchanger/ Design heat transfer load/ kW 125 605 m3 h 1 · − Calculation of Inlet air temperature of heat Inlet air flow of heat exchanger/ 650 1967 heat exchanger exchanger/°C m3 h 1 · − selection Outlet air temperature of heat 105 Heat transfer area/m2 115 exchanger/°C Heat transfer Heat transfer area of heat exchanger 1.1 0.503 coefficient/kW m 2 k 1 pipe/m2 · − · − Length of heat exchanger 1000 Number of heat exchanger tubes 5 tube/mm Diameter of heat exchanger Real air velocity in heat exchanger 25 27.26 tube/mm tube/ m s 1 · − Flow resistance of thermal storage 6 Total flow resistance/ Pa 20177 Calculation of channel/Pa frequency Flow resistance of heat Outlet air flow of circulating fan/ 19688 1969 conversion fan exchanger/Pa m3 h 1 · − selection Flow resistance of low 11 Motor efficiency 0.75 temperature duct/Pa Flow resistance of high Calculation power of circulating 22 6.5 temperature duct/Pa fan/kW Flow resistance after heat 450 Real power of circulating fan/kW 7.5 exchanger/Pa Energies 2020, 13, 5241 19 of 20

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