利用热能储存的热电发电机外文文献

Experimental investigation of solar reversible power generation in Thermoelectric Generator TEG using thermal energy storage Krishnadass Karthick a b S Suresha Grashin C Joya R Dhanuskodib a Department of Mechanical Engineering National Institute of Technology Tiruchirappalli Tamil Nadu India b Research Zhao et al 2005 Tang et al 2007 Poudel et al 2008 The applications of TEG ranges from autonomous sensor application Elefsiniotis Kokorakis Becker Hsu Huang Chu Yu Tzeng Jeng Liu et al 2014 heat pipes He Su Riffat Hou Remeli et al 2016 and power generation of a commercialavailablethermoelectric cooler TEC module are used to generate electric power Chen Liao Wang Kiziroglou et al 2017 Agbossoua Zhanga Sebald Zhanga Agbossoua Feng Jaworski Ednarczyk Rodr guez Garc a Bay n Rojas 2016 reportedcyclingstability ofD Mannitol under inert conditions Literature suggests that the thermal cycles performed at a melt freeze cycle at 10 C min exhibited an enthalpy decrease from 285 kJ kg to 281 kJ kg Neumann Niedermaier Gschwander Schossig 2018 Theliteraturealsoreports the supercooling nature ofD Mannitol at a temperature of 128 C and interpreted the thermal degradation of theD Mannitol Bay n Rojas 2017 which hypothesized that the heating ofD Mannitol produces 2 5 Anhydro D Mannitol and water as major by products The study concluded that the formation of the by product is indepen dent of the atmosphere The present study focuses on the application ofD Mannitol as PCM integrated with TEG modules for solar power generation Since experi mentswereevaluatedfordayandnightsolarpowergeneration heating and cooling of TEG modules were performed and discussed The revers ible operation of TEG modules is enabled by storing thermal energy in PCM and later this stored heat is used as a heat source The thermal and electrical parameters of TEG is presented in this experimental study Experimental investigations Experimental setup The schematic diagram for heating and cooling is shown in Fig 1 a and b respectively An electrical heater supplies heat input to the experimental setup and heat fl uxwas controlled by an autotransformer for regulating power supply A heater was placed on top of the heat spreader plate made up of aluminum for uniform heat distribution to the TEG Above the heater a holding plate mild steel was placed such that the heater is held at a constant position which reduces the in terface gap between the thermal contact layers The experiments were conducted with eight number of TEGs and were connected in series The dimensions of individual TEG was 40 40 4 mm The TEGs were placed in an array of two rows containing four TEGs in each row The PCM D Mannitol container had a shape of a cuboid with a dimen sionL200 W100 H35mm Foreffectiveheattransfer thecontainer had internal fi ns The base plate of the fi n has 8 mm thickness 25 mm height and 90 mm width The fi n base plate consists of 9 individual fi ns with a thickness of 7 mm each The PCM D mannitol was pur chased from spectrum reagents with a purity of 98 and used without further purifi cation About 0 82 kg ofD Mannitol was melted and poured into the PCM container The fi ns were placed immediately in the container such that they were completely immersed in melted D Mannitol Silicone grease purchased from Dow Coming TC 5121 with a thermal conductivity of 2 5 W mK was applied to the interfaces between TEG modules and aluminum fi n heat spreader plate for 0 20 40 60 80 100 120 0100200300400500 Mass Temperature C Fig 4 TGA curve ofD Mannitol 70 20 30 80 130 180 20406080100120140160180200 Heat flow mW Temperature C Fig 3 DSC curve ofD Mannitol Table 1 Thermophysical properties ofD Mannitol Sl NoPropertiesValue unit 1 Melting point solidifi cation point166 38 120 18 C 2Density ofD Mannitol1520 kg m3 3Latent heat ofD Mannitol during melting281 89 kJ kg 3Latent heat ofD Mannitol during solidifi cation219 52 kJ kg Fig 2 Experimental setup 14 12 10 8 6 4 2 0 2 4 6 0 50 100 150 200 050100150200250300350400 Open circuit voltage Voc V Temperature C Time min TEG hot side temperatureTEG cold side temperature PCM top temperaturePCM bottom temperature Open circuit voltage Fig 5 Thermal and electrical characteristics in an open circuit voltage for 90 W 4 5 kW m2 109K Karthick et al Energy for Sustainable Development 48 2019 107 114 effective heat transfer Karthick Suresh Singh Joy and Dhanuskodi 2018 The whole experimental setup was insulated with silica aerogel Cooling of the thermoelectric modules was performed by a passive cooling system The cooling setup consists of a heat exchanger copper tube with an outer diameter of 3 mm silicone connecting tubes and an overhead water tank or container The cooling of modules is based on the principle of thermosyphon The working medium of the heat exchanger is water The ends of the copper tube were connected to the water tank placed at a higher elevation than the experimental setup in a closed loop manner Experimental procedure A 3 D model of the experimental setup is shown in Fig 1 c for heating and Fig 1 d for cooling The experimental setup for the same is shown in Fig 2 Heat from the heater is uniformly distributed on the top surface of the TEG by means of heat spreader plate The PCM con tainer withD Mannitol is placed on a heat sink The heat rejected by the TEG system is stored as thermal energy in the PCM Thus a temper ature difference is maintained between TEG hot side and TEG cold side during heating Thus reversible operation in TEG is performed using heat energy stored inD Mannitol After melting ofD Mannitol heater and insulation are removed from the heat spreader plate The copper tube with water as working fl uid is kept on the top of heat spreader plate The copper tube is connected to an overhead water container by silicone tubes The temperature is measured using K type thermocouples Liquid temperature calibrator Sansel TCAL 1501 250 having an accuracy of 0 3 C was used to calibrate the thermocouples The temperature readings are measured between the TEG module and aluminum heat spreader plate Th base fi n plate Tc as shown in Fig 1 The thermo couples are placed on the internal surfaces by creating a slot of 1 5 mm on the surface of the spreader plate and the fi n base plate Two thermocouples are placed in the PCM container to record the tem perature distribution of the PCM The top temperature of PCM Tt is measured at a distance of 10 mm below the fi n base plate Similarly the PCM bottom temperature Tb is measured at a distance of 30 mm below the fi n base plate The representation of Thand Tcare used as the temperature of the hot side top side of TEG and the temperature of the cold side bottom side of TEG in pictorial representation for both heating and cooling However it should be noted that during the cooling process the reversible operation of TEG modules is started due to PCM discharge and cooling system The bottom side of TEG is main tained at a higher temperature during cooling process Since the process is naturally reversed for conventional purpose Thand Tcare represented as the same Therefore the Tctemperature is high when compared to Th SincetheTcisgreaterthantheTh thevoltagegainsanegativevalueasthe voltage produced depends upon temperature difference A total of eigh teen thermocouples are placed Sixteen thermocouples are placed on topandbottomofeightTEGs AnaverageofeighttopTEGsareconsidered asTEGhotsidetemperatureandanaverageofeightbottomTEGsarecon sidered as the TEG cold side temperature in the graphical representation The TEG modules are connected in series with an external variable resistor and a switch When the switch is OFF open circuit voltages are measured and similarly when the switch is ON closed circuit voltages are measured The temperature volatge and current readings were recorded using a data acquistion device Keysight 34970A for every 5 s Thermal analysis ofD Mannitol D Mannitol is a sugar based Phase Change Material PCM with me dium range melting temperature and high latent heat capacity The thermophysical properties ofD Mannitol are summarized in Table 1 The melting temperature and heat of fusion ofD Mannitol were deter mined by using Differential Scanning Calorimeter DSC Perkin Elmer USA From the DSC curve as shown in Fig 3 it can be analyzed that the onset temperature during melting of the PCM is 165 C and attains a peak at 166 38 C and completely melts at 172 6 C During cooling the solidifi cation was initiated at an onset temperature of 114 36 C The thermophysical properties were studied using Thermogravimet ric Analysis TGA Perkin Elmer USA The TGA analysis test was con ducted to fi nd the mass degradation temperature of the PCM The TGA analysis is performed to ensure that the degradation temperature should be much above the working temperature range of experimentation The TEGsystem is designed inthetemperature rangefrom ambienttempera ture to 200 C From the TGA curve as shown in Fig 4 it is observed that there is one step loss at 290 C The total mass degradation reached a maximum at 330 C with a mass reduction of 25 1 The reduction of the material was drastic within the range of 290 C to 330 C Further 14 12 10 8 6 4 2 0 2 4 6 0 50 100 150 200 050100150200250300350 Open circuit Voltage Voc V Temperature C Time min TEG hot side temperatureTEG cold side temperature PCM top temperaturePCM bottom temperature Open circuit voltage Fig 7 Thermal and electrical characteristics in an open circuit voltage for 110 W 5 5 kW m2 14 12 10 8 6 4 2 0 2 4 6 0 50 100 150 200 050100150200250300350 Open circuit voltage Voc V Temprature C Time min TEG hot side temperatureTEG cold side temperature PCM top temperaturePCM bottom temperature Open circuit voltage Fig 6 Thermal and electrical characteristics in an open circuit voltage for 100 W 5 kW m2 Table 2 Performance characteristics of TEG for heating in open circuit Sl NoHeat input W Heat fl ux kW m2 Time taken for phase change min Time taken for heating min Average voltage V Peak voltage V 1904 545 52663 874 33 21005422394 374 84 31105 5352085 395 88 110K Karthick et al Energy for Sustainable Development 48 2019 107 114 increase in temperature resulted in the degradation of the material Hence D Mannitol can be used in the temperature range of up to 200 C for high temperature thermal energy storage applications In TGA analy sis theD Mannitol indicated a mass loss of the sample from 290 4 C Therefore D Mannitolisconsideredasthephasechangematerialforther malstorageinthisexperimentwithaworkingtemperatureofambientto 200 C Results and discussion Thermal and electrical performance in an open circuit Fig 5 6 and 7 showthe performance of TEG systemduringopen cir cuit condition for a heat input of 90 W 4 5 kW m2 100 W 5 kW m2 and 110 W 5 5 kW m2 respectively The performance characteristics of TEG during heating is also shown in Table 2 The heat input supplied by heater to TEG through the heat spreader plate resulted in the in crease of temperature in hot side and cold side of TEG gradually During phase change period ofD Mannitol the thermal characteristics of the system remained at a constant indicating the heat energy absorbed by theD Mannitol to undergo phase transition process From Fig 5 6 and 7 it was observed that the time taken for phase change during heating varieddependingonthedifferentheatinputsindicatingthetimeperiod of phase transition and heat fl ux when the heat fl ux reaching the sur face is reduced the time period of phase transition increased resulting the increase in time during heating When meltingof PCM started a small but clear increase in voltage is observed Increase in voltage indicates the stabilization of cold side temperature by PCM undergoingisothermalphase change The melting ofD Mannitolstartedat163 Cand meltedcompletelyat168 C During this period the increase in temperature was very slow at all points and the voltage produced at a constant rate The fi nned structure located inside the PCM container accounts for the effi cient heat transfer The PCM zone exhibited a uniform temperature providing better perfor mance of TEG as PCM temperature is stable and subjected to meager temperature rise during phase change Theheatingprocess wascontinued until thePCM reached a temper ature of 172 C After this temperature the cooling process started with a passive coolingsystem Due togravity waterentered intotheheatex changer As cooling continued the heat exchanger started to exchange heat between the copper tube and top aluminum plate by the means of conduction This heat was further transported to water by natural convection The hot water rises up as the hot water has less density This resulted in the temperature drop in the top aluminum plate whereas the fi n base plate was maintained at a higher temperature Therefore the fl ow of heat transfer in TEG modules was reversed and themagnitude of temperature difference between thermoelectric mod ulesattainednegativevalues Thus temperaturedifferencereversedthe polarity of the voltage produced and the net voltage generated during the cooling phase gained a negative value by a sudden drop in positive voltage while cooling started The thermal and electrical characteristics were almost constant as the cooling process is independent of the heat input Fig 8 shows the comparison of open circuit voltage for different heat fl uxes during heating and cooling phases During cooling phase as shown in Fig 8 0 0 0 3 0 6 0 9 1 2 1 5 1 8 2 1 0 50 100 150 200 050100150200250300 Electric output power Po W Temperature C Time min TEG hot side temperatureTEG cold side temperature PCM top temperaturePCM bottom temperature Electric power output Fig 11 Thermal and electrical characteristics in a closed circuit voltage for 110 W 5 5 kW m2 0 0 0 3 0 6 0 9 1 2 1 5 1 8 0 50 100 150 200 050100150200250300350 Electric output power Po W Temperature C Time min TEG cold side temperatureTEG hot side temperature PCM top temperaturePCM bottom temperature Electric power output Fig 10 Thermal and electrical characteristics in a closed circuit voltage for 100 W 5 kW m2 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6 1 8 2 0 0 50 100 150 200 050100150200250300350 Electric output power Po W Temperature C Time min TEG hot side temperatureTEG cold side temperature PCM top temperaturePCM bottom temperature Electric power output Fig 9 Thermal and electrical characteristics in a closed circuit voltage for 90 W 4 5 kW m2 12 10 8 6 4 2 0 2 4 6 050100150200250300350 V egat l ovt iucr i cnepO oc V Time min 90 W100 W110 W Fig 8 Comparison of open circuit voltage for different heat fl uxes 111K Karthick et al Energy for Sustainable Development 48 2019 107 114 the open circuit voltage produced and reached a peak value of around 11 49 V with a time period of 5 min in all cases of heat inputs as shown in Fig 8 The time taken to attain peak voltage was around 7 to 8minafterthecoolinginitiatedasthetemperaturedifferencewasmax imumwhenthecooingprocessprogressed Afteronehour theopencir cuit voltage remains to be constant as the voltage reached a value of 0 2Vduetothenegligibletemperaturedifferenceofaround1 C During cooling phase the average open circuit voltage produced was around 2 90 V for a time period of 1 h and 55 min The average and peak values of open circuit voltage were almost similar in all cases of heat inputs heat fl uxes for cooling phase Thermal and electrical performances in a closed circuit To study the performance characteristics of a closed circuit tests were performed by connecting an external load of 14 in series for all experiments Figs 9 10 and 11 show the experimental results of thermal and electrical chracteristics for 90 W 4 5 kW m2 100 W 5 kW m2 and 110 W 5 5 kW m2 respectively in a closed circuit con dition The experimental results of performance characteristics for heating phase are shown in Table 3 As expected the melting point of theD Mannitol was started from 163 C to 168 C From Figs 9 10 and 11 it was observed that the thermal characteristics were almost similar when compared to the open circuit voltage condition Figs 12 and 13 show the comparison of closed circuit voltage and electrical power output for different heat fl uxes From Figs 12 and 13 the peak closed circuit voltage obtained during cooling phase was 5 09 V with a current value of 0 36 A for which the peak power output was 1 85 W for 90 W 4 5 kW m2 Similarly in the case of 100 W 5 kW m2 the peak power generated during cooling was 1 98 W For the heat input of 110 W 5 5 kW m2 the peak power generated during coolingwas1 86W Sincethevoltagegenerateddependsuponthetem perature difference higher temperature difference was obtained due to sudden cooling and thus the voltage generated reached a peak value The time period for this process was around 8 to 10 min In the case of cooling the average power output was around 0 31 W for a time period of 1 h 45 min as shown in Fig 13 From Fig 13 the cooling phase was observed similar in all cases independent of heat input From the experimental values the average power output was equalormoreinthecoolingprocesswhencomparedtotheheatingpro cess for 90 W and 100 W due to high melting temperature PCM was taken for study The results performed for closed circuit voltage show a slight variation in the time taken for heating and cooling when com pared to the open circuit voltage but almost the similar trend was ob served The total energy generated during heating and cooling in kWhr is shown in Table 4 Effect of supercooling in experimental study The cooling process showed a variation when the material showed the supercooling properties D Mannitol expelled its stored heat at around 157 C resulting in the increase of temperature ofD Mannitol from 156 C to 160 C and the progress in cooling process happened as normal This process took place for about 4 min for 90 W 4 5 kW m2 similar to other power input cases also This similarity for all cases of heat fl ux during the cooling process was because the heatfl uxhas a direct infl uence onheatingcycle an