Demonstration of a low cost, high temperature elemental sulfur thermalbattery
Amey Barde⁎, Kaiyuan Jin, Mitchell Shinn, Karthik Nithyanandam, Richard E. WirzEnergy Innovation Lab, Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, United States
H I G H L I G H T S
• A low-cost, high-temperature (200–600 °C) TES system is demonstrated.
• SulfurTES temperatures are attractive for many TES applications.
• Safe and reliable operation of SulfurTES battery for over 10 thermal cycles.
• SulfurTES battery operated in hybrid thermal charging mode.
• High volumetric energy density.
A R T I C L E I N F O
Keywords:SulfurSulfurTES batteryThermal Energy Storage (TES)Volumetric energy densityHybrid energy storage
A B S T R A C T
Elemental sulfur is a low-cost energy storage media suitable for many medium to high temperature applications,including trough and tower concentrated solar power (CSP) and combined heat and power (CHP) systems. In thisstudy, we have demonstrated the viability of an elemental sulfur thermal energy storage (SulfurTES) systemusing a laboratory-scale thermal battery. The SulfurTES battery design uses a shell-and-tube thermal batteryconfiguration, wherein stationary elemental sulfur is isochorically stored in multiple stainless steel tubes and aheat transfer fluid (air) is passed over them through the surrounding shell. The safe and reliable operation wasdemonstrated for twelve thermal charge–discharge cycles in the temperature range of 200–600 °C, during whichthe SulfurTES battery stored up to 7.6 kW h of thermal energy with volumetric energy density range up to255 kW h/m3. Furthermore, the SulfurTES battery is operated in a hybrid thermal charging mode to demonstrateits ability to store surplus electrical energy. The present study establishes the feasibility of SulfurTES as a conceptthat could provide attractive system cost and volumetric energy density for a wide range of thermal energystorage applications.
1. Introduction
For renewable energy to contribute significantly to the overall en-ergy supply, low-cost storage options must be demonstrated and im-plemented. An effective implementation of a low-cost thermal energystorage (TES) system can achieve this goal by providing dispatchabilityto renewable energy resources including concentrated solar power(CSP) and can even be considered for storing surplus electric energyfrom photovoltaics (PV) and wind turbines during times of over-gen-eration. Moreover, studies have shown that integration of thermal en-ergy storage system with combined heat and power (CHP) system yieldsimproved thermal performance and reduces fossil fuel consumption,resulting in favorable economics [1,2].
The performance and value of a TES system are analyzed based on
three important parameters viz.; the cost of energy storage, operatingtemperature range, and thermal performance. Several thermal energystorage systems, operating on various physical principles have beendeveloped; most common being latent, sensible, and thermochemicalenergy storage systems. A review of latent [3–9] and thermochemical[10,11] energy storage systems shows that these technologies are in R&D phase and must overcome many practical challenges to be mature forindustrial applications.
State of the art TES system is a two-tank molten salt system oper-ating in the temperature range of 280–565 °C [12–14]. Many com-mercial CSP plants [15–19] have adopted this technology to provideenergy dispatchability. Despite their successful implementation, state ofthe art molten salt TES systems suffer from inherent drawbacks, in-cluding the higher cost of energy storage [3,20] and degradation at
https://doi.org/10.1016/j.applthermaleng.2018.02.094Received 26 October 2017; Received in revised form 12 February 2018; Accepted 26 February 2018
temperatures above (∼565 °C). Thus, there remains the need for low-cost, high-temperature thermal energy storage options that can over-come these challenges and achieve effective integration of renewableenergy resources.
Wirz et al. [21] proposed a novel concept to use elemental fluids,including sulfur, as the primary fluid for thermal energy storage. Sulfuris a molten medium over much of the temperature ranges important toCSP and therefore has many of the heat transfer and overall systemperformance advantages of molten salt while avoiding the poor heattransfer performance of solid-liquid phase change materials (PCMs). Incomparison to state of the art molten salt, sulfur has several char-acteristics that make it attractive for low-cost TES. First of all, it is anabundant and low-cost material (0.07–0.12 $/kg) [22]. Sulfur does notdegrade with temperature due to its elemental nature and can be usedas a thermal storage fluid at temperatures up to 1200 °C [23]. Thepressure-temperature characteristics of sulfur show moderate sulfurvapor pressures [24,25], resulting in a low containment cost. Further,the experimental and numerical analyses have revealed a superior heattransfer performance of sulfur resulting in a thermally responsivethermal storage system with charge/discharge rates that are 3–14 timeshigher than PCM-TES systems [25–30]. Moreover, a comprehensivesystem-cost analysis showed that elemental sulfur thermal energy sto-rage systems can demonstrate a superior thermal performance at athermal storage cost that is well- below DOE cost target ($15/kW h)[31–33]. Thus, previous thermodynamic, heat transfer, and system costanalyses have shown that elemental sulfur thermal energy storage
(SulfurTES) systems promise excellent performance as a low-cost, high-temperature thermal energy storage technology that can be readilyscaled for CSP and CHP [32] applications. For state of the art com-mercial CSP (power tower and parabolic trough) and CHP systems,200–600 °C is the preferred operating temperature range, and thus,these temperatures are of interest for the application of the SulfurTESsystems. However, SulfurTES systems can be operated at temperatureshigher than 600 °C, due to the chemical stability of the elemental sulfurat these temperatures.
The objective of this research effort is to demonstrate the viability ofSulfurTES system for medium to high temperature applications(200–600 °C). To achieve this objective, we designed, fabricated, andoperated an ‘elemental sulfur thermal battery’ (SulfurTES battery)comprised of multiple sulfur-filled tubes in a shell-and-tube thermalbattery configuration. The liquid sulfur is isochorically contained ineach steel tube, each of which serves as an individual thermal storageelement. Similar to molten salt, the molten sulfur stores thermal energyprimarily via sensible energy. In this study, we have demonstrated thatthe SulfurTES battery can be operated safely and reliably from 200 to600 °C for multiple thermal cycles. The performance of the SulfurTESbattery is attractive to many TES applications based on its wide oper-ating temperature range, high thermal energy storage capacity, andhigh volumetric energy density. Moreover, we also operated theSulfurTES battery in a hybrid thermal charging mode to demonstrate itsability to integrate with both thermal (e.g., solar, industrial heat, ex-haust gas) and electrical (e.g., PV, wind turbine) energy sources. The
Nomenclature
CSP Concentrated Solar PowerCHP Combined Heat and PowerTES Thermal Energy StorageHTF Heat Transfer FluidPCM Phase Change MaterialLCOE Levelized Cost Of ElectricitySCFM Standard Cubic Feet per MinuteTEMA Tubular Exchanger Manufacturers Association
DAQ Data Acquisition SystemSFD Sulfur Filling DeviceCI Charging InletCO Charging OutletDCI Discharging InletDCO Discharging OutletCS heat capacity of sulfurCSS heat capacity of SS316TS sulfur temperatureTSS SS316 tube temperature
Fig. 1. Schematic of the SulfurTES battery and open-loop charge-discharge HTF(air) system.
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outcome of this study is an important step in demonstrating the via-bility of SulfurTES systems for industrial and commercial applications.
2. Experimental set-up
The experimental set-up comprises a SulfurTES battery integratedwith an air-based heat transfer fluid (HTF) loop for thermal cycling. Theconfiguration and function of the HTF loop and SulfurTES battery aredescribed in this section.
2.1. HTF loop
Figs. 1 and 2 show a schematic and pictures of the air loop setup forthermal cycling of the SulfurTES battery. A high pressure (80 psig) aircompressor coupled with pressure regulator supplied the air at a re-quired flow rate and pressure. A coalescing air filter was installed toremove the oil and oil mist down to 0.2 ppm, and particles larger than5 µm to ensure dry and clean air for thermal cycling experiments. Aflowmeter, coupled with pressure gauge was installed to measure themass flow rate of air entering the system.
A 10 kW air heater was installed at the inlet of the SulfurTES batteryto heat the air up to 650 °C for flow rates up to 35 SCFM. A plate heatexchanger was installed at the SulfurTES battery outlet to cool the hotair before releasing to the ambient. The path of air flowing throughSulfurTES battery was reversed when the operation was switched fromcharging to discharging mode, as discussed in Section 3.2.
2.2. SulfurTES battery
The SulfurTES battery design is based on a shell and tube heat ex-changer concept, and it was fabricated using SS316 (stainless steel 316)to enable its operation at a maximum operating temperature (600 °C)and pressure (200 psig) [25]. SS316 is a cost-effective, high-strengthalloy suitable for high-temperature (∼650 °C) thermal applications. Acomprehensive material compatibility analysis showed that SS316 of-fers corrosion resistance to sulfur at high operating temperatures for along period of operation [25]. These features make SS316 a suitablematerial for the SulfurTES battery. Fig. 3 shows the salient features ofthe SulfurTES battery. A rectangular shell, 41″×14.5″×11″ in size,encloses ten stainless steel tubes that are arranged in a triangular (30°)manner as per TEMA standards [34]. The steel tubes are 2″ NPS, Sch. 40in diameter, 1 m in length, and sealed by 0.5″ thick caps at either end.Individual steel tube contains 3.2 kg of sulfur, thus total sulfur contentin SulfurTES battery amounting to 32 kg. Each steel tube is equippedwith a 3″ long instrumentation port and a 13″ long thermo-well. Theinstrumentation port was used to fill a steel tube with sulfur, and then aK-type thermocouple was installed through this port to measure the
sulfur temperature during thermal cycling. The thermo-well houses acartridge heater (120 V, 200W) that serves as a secondary source ofheat and allows the SulfurTES battery to operate in a ‘hybrid thermalcharging mode’, details of which are discussed in Section 4.3. Thecartridge heater is located below the axis to improve heat transfer to thesulfur [35].
The SulfurTES battery has 21 baffles; 2 end baffles (0.25″ thick) thatsupport the steel tubes and 19 central baffles (0.0625″ thick) thatsupport the tubes and provide a tortuous path to the air to improve heattransfer.
The shell is equipped with eight access ports for thermocouples tomeasure the tube temperature along its length. In addition, twelveSS316 pipes were welded to shell body that serve as inlet/outlet portsfor the heat transfer fluid. There is one inlet and one outlet port foreach, charging and discharging process. Additional ports were providedin the midspan region to provide redundancy as well as flexibility (fore.g., attachment of additional air heater) of the SulfurTES battery;however, these were not used for the test. The shell is closed by endplates, bolted to the flanges with a high-temperature gasket inter-stitially placed between them. The SulfurTES battery is insulated with ahigh temperature (∼1000 °C) ceramic insulation to minimize the heatloss and ensure safe operation [36,37].
Design of the SulfurTES battery is informed by the heat transferbehavior of sulfur isochorically stored in a steel tube. The details of thisinvestigation are presented elsewhere [30], however key results of thevalidated CFD model are briefly discussed in this section. Fig. 4(a)shows the buoyancy-driven flow and temperature contours withinsulfur during charge and discharge thermal cycles. During thermalcharging, buoyancy-driven flow transfers heat from the tube wall to thesulfur mass. This flow is reversed during thermal discharging resultingin the extraction of stored heat from sulfur. The heat transfer coefficientof sulfur natural convection is significantly high resulting in rapidcharge-discharge rates as shown in Fig. 4(b).
The computational tool was also used to determine the location of acartridge heater within sulfur mass to achieve superior heat transferperformance. Fig. 5(a) shows the flow and temperature contours withinsulfur for two different cartridge heater locations: central and bottom.For central cartridge heater, the buoyancy-driven currents are restrictedto the top region, resulting in a non-uniform temperature distributionand undesirable hot spots near the heater surface. In the case of bottomcartridge heater, thermal energy is distributed throughout the sulfurmass via buoyancy driven flow resulting in a uniform temperaturedistribution. Fig. 5(b) shows that the charge time for the bottom heateris lower compared to the central heater, which indicates superior per-formance of the bottom heater configuration.
Fig. 2. SulfurTES battery and open-loop charge-discharge HTF(air) system.
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2.3. Instrumentation system
The instrumentation system mainly comprised of K-type thermo-couples, installed throughout the SulfurTES battery, to measure tem-peratures of different components, including sulfur, steel tubes, shell,and insulation. Thermocouples were connected to DAQ system coupledwith LabVIEW to record temperature measurements. As shown inFig. 6, ‘tube thermocouples’, T1 to T8, were installed to record the axialtemperature gradient along the length of a steel tube. In addition, thesetemperatures are used to estimate the thermal energy stored in steeltubes (i.e. stainless-steel mass). Sulfur temperatures were recordedusing 19 ‘sulfur thermocouples’; TS1 to TS9 are in direct contact withsulfur while TH1-TH10 are integrated with the cartridge heaters em-bedded in steel tubes. It is acknowledged that the thermocouples TH1-
TH10 are not in a direct contact with sulfur (see Fig. 3(b)), and a finitetemperature difference between cartridge heater temperature (TH) andcorresponding sulfur temperature may exist. The cartridge heaterthermocouple (TH) and the location of interest for sulfur temperatureare very close to each other and separated by highly conductive, thinthermowell wall, as shown in Fig. 7. Due to this fact and analysis pre-sented at the end of Section 2.2, it is assumed that the cartridge heatertemperature (TH) represents the sulfur temperature at a tip of thethermowell. It is an industrially established practice to use thermowell-thermocouple assembly for temperature measurement of hot fluids, anda similar approach is adopted here. These sulfur temperatures wereused to monitor the heat transfer behavior and thermal performance.
Shell temperature was recorded at 10 different locations to estimatethe energy stored in the shell. In addition, thermocouples were installed
Fig. 3. (a) Schematic of the SulfurTES battery, (b) Salient features of the steel tube.
Fig. 4. (a) Buoyancy driven flow and temperature contours within sulfur during thermal cycle, (b) period for thermal charging-discharging process in the tem-perature range of 200–600 °C.
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at various key locations including the insulation surfaces, SulfurTESbattery inlet/outlet ports, heat exchanger, and inlet/outlet valves. Inaddition to the estimation of thermal energy, these temperature mea-surements are used to study the thermal charge-discharge character-istics of the SulfurTES battery via analytical and numerical tools. Asshown in Fig. 6, a high-temperature pressure gauge (PG) was installedin a representative steel tube to record the pressure for safety purpose.
3. Experimental procedure
In this section, the thermal charge-discharge procedure for theSulfurTES battery is discussed. This procedure was used for multiplethermal cycles to investigate the thermal performance of the SulfurTESbattery. In addition, a sulfur filling process is briefly discussed.
3.1. Sulfur filling process
The steel tubes were filled with molten sulfur using the sulfur fillingdevice (SFD) to ensure safe and fast operation [25]. In the beginning,3.2 kg of sulfur powder was loaded in the SFD and system was heated tomelt the sulfur powder. Then, under gravity, the molten sulfur wastransferred into the steel tube via access port to fill 80% of the tubevolume. This operation was conducted under an inert atmosphere toavoid generation of unwanted gases that increase the system pressure[25].
3.2. Thermal cycling of the SulfurTES battery
The performance of SulfurTES battery was analyzed for multiplethermal cycles. At the beginning of thermal charging, a steady flow ofair was set along the required flow path as shown in Fig. 8. A supply ofcooling water to the heat exchanger was used to ensure effective
cooling of hot air exiting the SulfurTES battery. The air heater was usedto provide high temperature (650 °C) air to the SulfurTES battery,which serves as a primary source of thermal energy. The thermalcharging using hot air alone continued until the sulfur temperature, as
Fig. 5. (a) Buoyancy driven flow and temperature contours within sulfur for different cartridge heater locations, (b) thermal charging period for cartridge heater withdifferent locations in the temperature range of 200–600 °C.
Fig. 6. The position of thermocouples within SulfurTES battery for measurement of sulfur and steel tube temperatures. (Note: Thermocouples T1-T8 are installed ontube2 along its upper surface.)
Fig. 7. Position of cartridge heater thermocouple and location of interest forsulfur temperature measurement.
Fig. 8. HTF path during thermal charging and discharging.
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recorded by TH1-TH10 reached 350 °C. As this temperature was in-creased beyond 350 °C, cartridge heaters were activated and the Sul-furTES battery was switched to a hybrid thermal charging mode. Thethermal energy stored in the system is tracked in real time using thetemperature measurements of thermal masses (sulfur and stainlesssteel) recorded in the LabView. The goal of the thermal charging pro-cess was to store> 5 kWh of the thermal energy, and thermal chargingwas continued until this goal was achieved. (The details of the thermalenergy storage calculations are provided in Section 4.2.) Then, airheater and cartridge heaters were switched off to terminate the thermalcharging process. At the end of thermal charging, an axial thermalgradient develops along the SulfurTES battery, as indicated in Fig. 8.After charging, cold air was supplied to the SulfurTES battery to startthe thermal discharging process as shown in Fig. 8. The path of HTF wasreversed during discharge to reduce the local temperature differencebetween the HTF and tube wall to improve exergetic efficiency and toensure the HTF exits the SulfurTES battery at a higher temperature toimprove energetic efficiency. The thermal discharging was continueduntil the average temperature of the SulfurTES battery was reduced to200 °C.
4. Results and discussion
Using the procedure described in the previous section, the SulfurTESbattery was operated for twelve thermal cycles over a temperaturerange of 200–600 °C. As discussed in this section, the performance ofthe SulfurTES battery was then analyzed based on operating tempera-ture, thermal storage capacity, volumetric energy density, and hybridthermal charging ability.
4.1. Thermal map of SulfurTES battery
As discussed in Section 1, the temperature range of 200–600 °C is of
Fig. 9. Thermal map within SulfurTES battery during the thermal charge-discharge process.
Fig. 10. Thermal energy storage and volumetric energy density of theSulfurTES battery for twelve charge-discharge cycles.
Table 1Thermal energy stored in sulfur for various pipe size of steel tube.
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interest for many commercial applications. Thus, the ability of theSulfurTES battery to operate in this temperature range for multiplethermal cycles is an important performance criterion.
Fig. 9(a) shows SulfurTES battery inlet temperature for twelvecharge-discharge thermal cycles over a temperature range of∼200–600 °C with peak temperatures of 575–620 °C as recorded at theinlet. For first few cycles, the SulfurTES battery was thermally chargedcarefully in a stepwise manner, and to a lower maximum temperatureof 575 °C to ensure the safety of the system. As a result, the step changesin the SulfurTES inlet temperature are observed for these cycles.Afterwards, the system was thermally charged directly up to 620 °C,above the target maximum temperature (600 °C). The many tempera-ture measurements acquired from the thermocouples were used to de-velop a thermal map within the SulfurTES battery. The temporal trendsof these measurements for a representative cycle are presented inFig. 9(b)–(d). The axial temperature distribution along the steel tubewas captured using thermocouples T1 to T8 and the temperature evo-lution with time is shown in Fig. 9(b). These results show that a thermalgradient is set in the SulfurTES battery along its axial length. For ex-ample, at the end of thermal charging, the temperature of the steel tuberanges from T1 at 567 °C to T8 at 480 °C, with an average temperatureof 512 °C. The sulfur temperature was measured at two locations withina steel tube as shown in Fig. 6. Sulfur temperatures, as recorded byTH1-TH10 in a relatively hot section of the SulfurTES battery, are in therange of 550–570 °C at the end of thermal charging (Fig. 9(c)). Thesulfur temperatures, as recorded by TS1-TS9 in a relatively cold sectionof the SulfurTES battery are in the range of 495–515 °C (Fig. 9(d)). Thisthermal map of the SulfurTES battery shows that various thermal sto-rage masses were thermally charged to significantly high temperaturesin the range of 500–570 °C.
The thermal map of the SulfurTES battery shows a spatial tem-perature distribution within SulfurTES battery. As shown in Fig. 9(b),the rate of temperature rise recorded by T1 is higher compared to T8.As air heater is turned off to switch the SulfurTES battery from thermalcharging to discharging mode, T1 shows a noticeable drop in tem-perature which is not observed in T8. This drop is a result of the sen-sitivity of the T1 thermocouple to the air heater operation due to itsproximity to the thermal charging inlet. As thermal discharging pro-ceeds, the rate of temperature drop recorded by T8 is much higher thanT1. Similarly, this behavior was observed due to the proximity of T8 tothe discharging inlet rendering it more sensitive to the dischargingoperating conditions (for e.g., HTF flow rate) as compared to T1. Si-milar behavior is exhibited by sulfur temperatures in hot (TH1-TH10)and cold sections (TS1-TS10) of the SulfurTES battery. In a future study,we will use these data to study the effects of operating conditions on thethermal charge/discharge characteristic of the SulfurTES battery.
4.2. Thermal energy storage
Sulfur and stainless steel are primary thermal storage masses of theSulfurTES battery. For 200–600 °C temperature range, the specific heatcapacity of sulfur and SS 316 varies as 1.12–1.275 kJ/kg-K [38] and0.5–0.6 kJ/kg-K [39] respectively. The thermal energy stored in sulfurand steel is calculated as,
= −E M C T( 200)Sulfur S S S avg, (1)
= −E M C T( 200)Steel SS SS SS avg, (2)
The average temperature of sulfur and stainless steel were calcu-lated using temperature measurements by thermocouples installedthroughout the SulfurTES battery. Fig. 10 shows the thermal energystored in SulfurTES battery for twelve thermal cycles, which varies inthe range of 6.0–7.6 kW h. It is observed that sulfur contributes ap-proximately 47% of the total energy stored in the SulfurTES battery.The contribution of sulfur in the total energy storage is calculated asfollows.
⎜ ⎟= ⎛⎝ +
⎞⎠
∗contribution of sulfur energyE
E E% 100Sulfur
Sulfur Steel (3)
It is preferable to store maximum thermal energy in a low-costsulfur to minimize the cost of energy storage. However, steel tubes wereoverdesigned and fabricated using Sch. 40 pipes to ensure safe andreliable operation for this demonstration effort. Therefore, the mass ofstainless steel in the SulfurTES battery is significantly higher thanwould be required in a commercial system, leading to greater con-tribution towards thermal energy storage capacity for the results re-ported herein. The investigation of pressure-temperature characteristics[25] shows that the sulfur vapor pressure is moderate even at 600 °C,and thinner pipes can be used to safely contain the sulfur. For example,for Sch. 10 pipes, the contribution of energy stored in the sulfur can besignificantly higher, especially for large diameter steel pipes. A first-order analysis was conducted to predict the relative contribution ofsulfur and steel in the thermal energy storage for different tube con-figurations. Table 1 shows that contribution of sulfur in the thermalenergy storage can be simply increased by using larger tubes (i.e., from53% for 2″ NPS, Sch. 40 pipes to 84% for 8″ NPS, Sch. 10 pipes). In theprevious study, we showed that tube sizes up to 8″ in diameter providedesirable heat transfer performance for sulfur-based TES [30].
These analyses show that a large-scale commercial SulfurTES bat-tery will be able to exploit the low-cost of sulfur as the primary thermalstorage material. This sulfur contribution in the total energy storage isestimated based on the thermal energy stored in steel tube wall andsulfur only, without accounting for the contribution from tube acces-sories, including the end caps, thermo-well, and instrumentation port.However, tubes used in the SulfurTES battery are fitted with these
Fig. 11. Hybrid thermal charging of SulfurTES battery using high temperature HTF and cartridge heaters.
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accessories (refer Section 2.2), which increases the steel mass and henceits contribution to the total energy storage. Hence, for the current study,% contribution of the sulfur in the thermal energy storage for 2″ NPS,Sch. 40 tube is lower (47%, realistic conditions) than reported inTable 1 (53%, ideal conditions).
The performance of a TES system is also analyzed based on thevolumetric energy density. The volumetric energy density is re-presented as,
=Volumetric energy densityTotal thermal energy
Volume of the storage components (4)
Fig. 10 shows the volumetric energy density of the SulfurTES bat-tery for twelve thermal charge-discharge cycles conducted for thepresent study. It is observed that a volumetric energy density of200–255 kW h/m3 can be achieved resulting in compact sulfur-TESsystems. For these tests, the thermal energy stored in the battery andthe volumetric energy density range from 6 to 7.6 kW h and200–255 kW h/m3, respectively. The variation in these values is basedon the operating parameters during each cycle, most notably thethermal charging duration and HTF flow rates.
4.3. Hybrid thermal charging
Fig. 11(a) shows sulfur temperatures (TH) before and after Sul-furTES battery was switched to a hybrid thermal charging mode. Asharp change in temperature ramp rate can be observed as cartridgeheaters were activated. As discussed in Section 2.3, TH is the cartridgeheater temperature, but it is used to represent the sulfur temperature ata given axial position. In addition to sulfur temperatures (TH), the ef-fect of secondary heat source was analyzed by examining tube tem-peratures. As expected, the closest thermocouples, T1-T4 most readilyobserved the cartridge heater activity. Fig. 11(b) compares sulfur tem-perature with tube temperatures (T1-T4) during thermal charging.After a few minutes of cartridge heater operation, tube temperaturesincrease with higher ramp rate. These effects can be clearly seen in theinset plot of Fig. 11(b), which shows that the effect of hybrid charging is
not localized near the cartridge heater but distributed throughout thesteel tube via convective heat transfer within sulfur [35]. Thus, we havedemonstrated the hybrid thermal charging of SulfurTES battery whichis extremely important for practical applications.
5. Conclusion
We have demonstrated an elemental sulfur thermal energy storage(SulfurTES) battery as a viable energy storage technology for high-temperature industrial and commercial applications. The SulfurTESbattery was safely and reliably operated for twelve thermal cycles in thetemperature range of 200–600 °C. During multiple thermal cycles, theSulfurTES battery stored up to 7.6 kW h of thermal energy. Moreover, asignificantly high volumetric energy density of 200–255 kWh/m3 wasachieved, thus demonstrating a compact footprint. For this initial de-monstration, the contribution of sulfur in the total thermal energystored in the SulfurTES battery was 47% but can be increased to 84% bysimply increasing the tube diameter. The SulfurTES battery was alsooperated in a hybrid thermal charging mode, which allows energy to bestored from a wide range of potential energy sources, including thermal(e.g., solar, process heat, turbine exhaust) and electrical (e.g., PV, wind)sources. Beyond demonstrating this new capability, this study has es-tablished a framework for system level performance analyses of larger-scale SulfurTES systems, thus allowing future integration with the manyapplications that require high temperature energy storage, includingCSP, CHP, and CCHP.
Acknowledgement
This research effort was supported by the ARPA-E Award DE-AR0000140, Southern California Gas Company Grant Nos.5660042510, 5660042538, and California Energy Commission ContractNo. EPC-14-003. We would also like to thank Prof. Vijay Dhir and Prof.Adrienne Lavine for their support of this research effort.
Appendix A. Technical specifications of SulfurTES components
In this section, key technical specifications of important equipment and components are provided. These components include, (a) air heater, (b)cartridge heater, and (c) insulation (see Table A1–A3).
Table A1Technical specification of the air heater.
Technical parameter Value
Make Sylvania SUPERHEAT MAXOperating temperature range 25–650 (°C)Air flow rate 12–35 SCFMPower 10 kWThermocouple K-type
Table A2Technical specification of the cartridge heater.
Technical parameter Value
Make TempcoOperating temperature range 25–600 °CVoltage 110 VPower 200WThermocouple K-type
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