- Email: [email protected]
TAGS-85 based thermoelectric power generators
Accepted Manuscript Design and development of DC to DC voltage booster to integrate with PbTe/ TAGS-85 based thermoelectric power generators S.K. Sahu, Anil K. Bohra, P.G. Abichandani, Ajay Singh, Shovit Bhattacharya, Ranu Bhatt, Ranita Basu, Pritam Sarkar, S.K. Gupta, K.P. Muthe, S.C. Gadkari PII: DOI: Reference:
S2589-2991(18)30146-0 https://doi.org/10.1016/j.mset.2019.03.004 MSET 75
To appear in:
Materials Science for Energy Technologies
Received Date: Accepted Date:
6 November 2018 23 March 2019
Please cite this article as: S.K. Sahu, A.K. Bohra, P.G. Abichandani, A. Singh, S. Bhattacharya, R. Bhatt, R. Basu, P. Sarkar, S.K. Gupta, K.P. Muthe, S.C. Gadkari, Design and development of DC to DC voltage booster to integrate with PbTe/TAGS-85 based thermoelectric power generators, Materials Science for Energy Technologies (2019), doi: https://doi.org/10.1016/j.mset.2019.03.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design and development of DC to DC voltage booster to integrate with PbTe/TAGS-85 based thermoelectric power generators S. K. Sahu*, Anil K. Bohra, P. G. Abichandani, Ajay Singh*, Shovit Bhattacharya, Ranu Bhatt, Ranita Basu, Pritam Sarkar, S. K. Gupta, K. P. Muthe, S. C. Gadkari Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085
*Corresponding authors’ emails: Ajay Singh ([email protected]), S. K. Sahu ([email protected]),
Abstract Thermoelectric power generators (TEGs) based on Seebeck effect convert waste heat into electricity. However, the practical applications of such TEGs are limited by their low output DC voltage (10-300mV). Therefore, a self powered DC-DC voltage booster circuit is highly essential to integrate with TEGs for commercial use. In this work, we demonstrate a DC-DC voltage booster circuit to amplify the output voltage delivered by a PbTe/TAGS-85 based TEG which consists of 4 p-n couples alternately connected electrically in series and thermally in parallel. Design of the DC to DC booster circuit includes an oscillator and amplifier. Open circuit voltage of ~ 325 mV generated by the TEG for a hot side temperature (Th ) of 500°C and a temperature difference (ΔT ) of 410°C was amplified by the booster circuit to 10V. The designed DC-DC booster circuit exhibit the boosting efficiency of ~50%.
1. Introduction Rapid industrial evolution in last few years has increased the energy need and global warming problems, hence research and development has been promoted in the field of thermoelectric power generation as a means of recovering large amount of waste heat emitted by automobiles, factories etc [1, 2]. The thermoelectric power generators (TEGs) work on the principal of Seebeck effect and convert waste heat into the electrical energy. A TEG consists of several n- and p-type semiconductor thermoelements alternately connected electrically in series and thermally in parallel. Electrical connection between these n- and p-type thermoelements is usually made using metallic electrodes [2]. To make an efficient TEG, nand p-type thermoelectric materials with high figure-of-merit are required. It is also desirable that the joints between the thermoelements and the metallic electrodes should have low electrical contact resistance [2,3]. The open-circuit voltage of the TEG’s is directly proportional to the temperature difference and under the natural convection condition the temperature difference cannot be high due to finite thermal conductivity of thermoelectric materials. Therefore, in such circumstances of small temperature difference the output voltage of TEGs is usually small in range of 20–300 mV. Usually the current produced in a TEG is high (~ 1-10 A) due to very low electrical resistivity of the p- / n-type materials and their interfaces with metallic electrodes that are used to fabricate the TEG [2]. In order to make use of TEGs in practical applications requiring a higher voltage (such as powering a sensor, integrated circuits, mobile chargers etc) its integration with a self powered DC-DC voltage booster is highly required [4-10]. It may be noted that although various design of DC-DC booster circuits are previously reported [4-15] but for efficient transferring of power from TEG to the booster circuit there should be a good impedance matching between both. A scheme of integrating a TEG with a self powered DC-DC voltage booster for producing high output voltage is given in Figure 1. It may be noted that
commercially available DC-DC voltage booster either require a higher input voltage to operate or there is an impedance mismatch between TEG and DC-DC booster which leads to underutilization of electrical power generated by TEG. In this paper, we report the design and development of a DC-DC booster for harvesting the energy produced from an indigenous PbTe/TAGS-85 based TEG having 4 p-n couples alternately connected electrically in series and thermally in parallel. The output of TEG was connected to an indigenous, custom made DC-DC voltage booster consisting of an oscillator and amplifier stage. Finally the DC-DC booster delivered an output voltage of 10V and a load current of 4mA exhibiting conversion efficiency of ~50%.
Practical application to drive: sensor, ICs, electronic devices, mobile chargers etc
5-10 V
DC to DC voltage booster 10-300 mV
p Hot
n
p
n
p
n
p
n
TEG
Cold
Thermal conductor & electrical insulator Source of waste heat (e.g. Radioisotope, automobile exhaust, power plant etc)
Figure 1: Schematic showing integration scheme of TEG with the DC to DC voltage booster
2. Details of thermoelectric power generator to be integrated with DC to DC converter circuit
Depending on the operating temperature of TEG, different kinds of n and p-type semiconductor materials are currently being used for fabrication of such devices. Semiconductor alloys like n-type PbTe and p-type (AgSbTe2)0.15(GeTe)0.85 (TAGS-85) are well established for electrical power generation at a hot end temperature of ~500°C. A few years back our group reported the fabrication and characterization of n-type PbTe and p-type TAGS-85 thermoelements with Ag shoes at the ends and subsequently the fabrication of TEG using few numbers p-n couples alternately connected in series [3]. A typical schematic of a 4 p-n legs based TEG is shown in Figure 2(a). In order to fabricate this TEG, first single-phase polycrystalline n-type PbTe and p-type TAGS-85 material was synthesized by high temperature vacuum melt process. In brief stochiometric powders were taken, grinded in agate mortar pestle for 1 hour. The homogenously mixed powder was filled in a pre-cleaned quartz ampoule, after filling the materials the ampoule is sealed under vacuum (10-6mbar). The sealed ampoule was placed in a furnace and heated to ~ 900ºC and then rocked for 1 hour. The rocking eliminates the gravitational segregation of the different component materials and yield high homogeneity in the melt. Finally the molten material is cooled to the room temperature and grinded into fine powders. In order to fabricate n-type PbTe element (Size: 7.5 mm diameter and 8 mm height) with metallic shoes at ends, fine powders of Fe, (50%PbTe+ 50%Fe) and PbTe were filled in a stainless steel ( S.S.) die having thin graphite liner to avoid reaction between S.S. and thermoelectric materials. This layered structure was then pressed in a vacuum hot press under the conditions i.e. base vacuum ~10-6 Torr, temperature 600ºC and load of 700 Kgs. The fabrication of p-type TAGS-85 thermoelement was done in a similar way as that of n-type PbTe. Fine powders of Fe, SnTe, and TAGS-85 were filled in a S.S. die with graphite liner. SnTe acts like an diffusion barrier between Fe
and TAGS-85, otherwise the diffusion of Fe into the TAGS-85 results in the rapid degradation of the contacts. In order to design TEG, n-type PbTe and p-type TAGS-85 thermoelements were joined by a silver metal strip. The TEG was fabricated in the following steps: Initially 4 n-type and 4 p-type thermoelments were alternately packed in a zircar housing. The zircar housing has two advantages (a) it provides an enclosure to hold the thermoelements. (b) due to poor thermal conductivity of zircar it blocks the direct flow of heat between hot and cold surface. After placing the thermoelements in asbestos, silvers stripes were placed on the top and bottom side the thermoelements. The entire assembly is than vacuum hot pressed at a load 100 Kg. The optimized bonding temperature of 400˚C was found suitable for joining the silver stripes with the thermoelements. The photograph of the fabricated 4 p-n legs TEG is shown in Figure 2(b). Typical open circuit voltage and current at matched load of 10 mΩ of a four p-n legs TEG as a function of hot end temperature (Th) are shown in Figure 2(c-d). For a Th of 500°C the TEG showed an open circuit voltage of 325 mV and a current of 16 Amp with matched load of 10 mΩ. In this TEG, the contribution of contacts to total device resistance was found to be 4 %. The working efficiency of the devices was estimated by taking ratio of output electrical power and heat input given to the TEG, and it found to be ~6 % [3].
Zirconia based housing
n
p
Top view of actual 4 p-n legs TEG
Silver interconects
p
n
n
p
p
n n
+
-
Output terminals of TEG
(b) 16
Current (A) at 10 m
Open circuit voltage (mV)
(a)
300
12
225 150 75 0
(c) 100
200
300 0
400
500
8
4
0
(d) 100
Th( C)
200
300 0
400
500
Th ( C)
Figure 2: (a) Schematic of a 8 p-n legs device (b) Photograph (top view) of a 4 p-n legs TEG (c) Open circuit voltage (d) current at matched load of 10mΩ of the 4 p-n legs TEG as a function of hot end temperature (cold end maintained at 80°C).
3. Details of DC to DC voltage booster The Figure 3 shows the block diagram of DC-DC booster. It consists of two main components, first one an oscillator & driver circuit followed by a voltage amplifier stage
Oscillator and driver stage
Step up transformer & switching MOSFET (voltage amplifier stage)
Double rectifier & filter circuit
Voltage amplifier stage Figure 3. Block diagram of DC-DC voltage booster
Output 10V@ 4 mA
TEG output 300 mV
(shown in the dotted block).
The main function of oscillator & driver circuit is to convert the DC output of TEG into an AC signal for driving and switching MOSFET of voltage amplifier circuit. Subsequently a pulse train generated by voltage amplifier stage is rectified and filtered to produce a DC output.
Now we discuss each stage of the of DC-DC booster in detail
i. Oscillator and driver stage: Oscillator and driver circuit is the heart of DC-DC booster which coverts DC output voltage of TEG into an AC signal. It is based on Colpitts oscillator along with a clamping circuit. It takes input power from the low DC voltage generated by TEG source and it produces a 3V sinusoidal signal at frequency of around 250 kHz. The minimum start-up DC voltage required to activate this circuit is around 100mV. The output of oscillator and driver circuit is coupled to the power stage of voltage booster for driving the switching MOSFET.
ii. Power stage of DC-DC booster: Figure 4 (a) shows basic diagram of voltage amplifier stage of DC-DC voltage booster. It consists of a step-up transformer, a switching MOSFET, rectifier and a filter circuit (in doubler configuration). Oscillator and driver stage’s output (i.e. an AC signal around frequency ~ 250 kHz) switches the MOSFET “ON & OFF”, i.e. it chops the DC output voltage from TEG at 250kHz. When the MOSFET is in ON condition, non-dot end of the transformer’s primary coil brought to ground potential and dot end of transformer becomes positive. At the secondary coil of transformer, a voltage equivalent to the turns ratio times the input voltage is produced and in this condition diode D1 conducts and charges the capacitor C1. When MOSFET is turned OFF, the magnetizing energy of the transformer reverses the polarity of the transformer and the non dot end of transformer’s coils becomes
positive, diode D2 conducts. Thus stored energy into the transformer is transferred to the capacitor C2 through diode D2. Finally the voltage across the capacitor C1 and C2 get added and it produces an amplified DC output.
(a)
Output 10V@4mA D1
C1
L2 L1
R1 C3 D2
Load
C2
D3
TEG Source's Output
Oscillator and Driver Circuit
M1
0
0
(b)
Output of DC-DC booster
TEG assembly Output of TEG
DC -DC booster circuit
Figure 4. (a) Exploded view of the power stage block of DC-DC voltage booster circuit (a) Actual photograph of TEG assemble and DC-DC voltage booster unit
Figure 4(b) shows the actual photograph of the TEG and DC-DC booster assembly. In this assembly a 4 p-n legs based TEG is sandwiched between an electrical heater (hot source) and water cooled copper block (heat sink). The temperature of heater can be controlled using a PID controller within ±1°C of set temperature value. The temperature of heater and water cooled block is monitored using thin K-type thermocouple. The input power given to DC-DC booster circuit is obtained by measuring the voltage drop at the input terminal of the circuit. Current drawn by booster circuit is estimated by placing a known resistor in series with the input terminal of the booster circuit.
4. Performance of the of DC-DC voltage booster Finally the performance of the DC-DC voltage booster was tested with varying the input voltage supplied through TEG and obtained results are plotted in Figure 5. The output voltage of the TEG can be varied by changing its hot end temperature. The voltage booster unit turned ON only after it receives an input signal of 100 mV from TEG. From Figure 5, it may be seen that above 10 mV of input the output of the booster increases linearly with the input voltage. We have also estimated the boosting efficiency of the developed circuit by taking the ratio of input electrical power to the output electrical power. For an input electrical power of 80 mW (voltage ~ 300 mV & current ~ 267mA) the booster unit delivered a power of 40 mW (voltage ~ 10 V & current ~ 4mA) at load of 2.7 kΩ, implying that the developed booster has en efficiency of 50%.
Output of DC to DC voltage booster (V)
10 8 6 4 2 turn ON voltage
0
0
50
100
150
200
250
300
Input voltage (in mV) supplied to booster by TEG
Figure 5. Output voltage delivered by DC to DC voltage circuit as a function of input voltage supplied by TEG, load 2.7 kΩ
It may be noted that in the present work, the maximum power delivered by the TEG at the matched load of 10 mΩ is ~ 2.6 W, while due to impedance mismatch the DC to DC converter (impedance of TEG << input impedance of DC-DC booster) is only extracting a only a small fraction (~80 mW) of maximum power generated by TEG. Hence the further extension of this work will be to place many numbers of DC-DC booster circuit will be parallel connected to the output of a single TEG as a result much higher power can be extracted from TEG. With such a combination of single TEG with many numbers of parallely connected DC-DC boosters can deliver high voltage (~ 10 V) and high current (~ 50-100 mA) at the load.
5. Conclusion In this paper, we have demonstrated a prototype DC-DC booster for amplifying the output voltage produced by a thermoelectric power generator. The booster has a turn on DC voltage
of around 100 mV. For a typical input voltage of 300mV from TEG, the DC –DC voltage booster delivered an output of 10 V/ 4 mA at load resistance of 2.7 kΩ. This prototype DCDC booster has an efficiency of around 50%. The present experimental results suggest that the proposed design and development of DC-DC booster is able to extract the energy from ultra low voltage sources and produce the desired output voltage for powering other electronic devices. Further higher output power can be delivered by the use of multiple power stages (connected in parallel to oscillator and driver stage) of voltage amplifier circuit of DCDC converter.
References 1. D. M. Rowe, CRC handbook of thermoelectrics, CRC press, 1995 2. D. K. Aswal, R. Basu, A. Singh, Key issues in development of thermoelectric power generators: High figure-of-merit materials and their highly conducting interfaces with metallic interconnects. Energy Conversion & Management 114 (2016) 50. 3. A. Singh, S. Bhattacharya, C. Thinaharan, D. Aswal, S. Gupta, J. Yakhmi and K. Bhanumurthy,Development of low resistance electrical contacts for thermoelectric devices based on n-type PbTe and p-type TAGS-85 ((AgSbTe2)0.15(GeTe)0.85), Journal of Physics D: Applied Physics 42 (2008) 015502. 4. R. Hazli, A. H. Hamidon, Design of DC-DC boost converter with thermoelectric power source, International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering 2(2013) 4170. 5. B. Shen, R. Hendry, J. Cancheevaram, C. Watkins, M. Mantini, R.Venkatasubramanian, DCDC Converter Suitable for Thermoelectric Generator, International Conference on Thermoelectrics (2005)517. 6. B. Shen, R. Hendry, C. Watkins, R.Venkatasubramanian, DC-DC converter for low voltage power source, US Patent US 7,706,152 B2, Date of patent April 27, 2010. 7. A. Richelli, S. Comensoli, Z. M. Kovács-Vajna, A DC/DC Boosting Technique and Power Management for Ultralow-Voltage Energy Harvesting Applications, IEEE Transaction on Industrial Electronics 59(2012)2701. 8. H. Mamur, R. Ahishka, Application of a DC–DC boost converter with maximum power point tracking for low power thermoelectric generators, Energy Conversion & Management 97 (2015)265
9. M. Guan , K. Wang, D. Xu , Wei-Hsin Liao, Design and experimental investigation of a lowvoltage thermoelectric energy harvesting system for wireless sensor nodes, Energy Conversion and Management 138(2017)30 10. E. J. Carlson, K. Strunz, B. P. Otis, A 20 mV input boost converter with efficient digital control for thermoelectric energy harvesting. IEEE J Solid-State Circuits 45 (2010) 741. 11. A. Paraskevas, E. Koutroulis, A simple maximum power point tracker for thermoelectric generators. Energy Convers Manage 108 (2016) 355. 12. M. Guan, K. Wang, Q. Zhu, W. H. Liao, A high efficiency boost converter with MPPT scheme for low voltage thermoelectric energy harvesting. J Electron Mater 45 (2016) 5514. 13. Ramadass YK, Chandrakasan AP. A battery-less thermoelectric energy harvesting interface circuit with 35 mV startup voltage. IEEE J Solid-State Circuits 46 (2011) 333. 14. J. P. Im, S. W. Wang, S. T. Ryu, G. H. Cho, A 40 mV transformer-reuse self-startup boost converter with MPPT control for thermoelectric energy harvesting. IEEE J Solid-State Circuits 47(2012) 3055. 15. Y. K. Teh, K. T. Mok,
Design of transformer-based boost converter for high internal
resistance energy harvesting sources with 21 mV self-startup voltage and 74% power efficiency. IEEE J Solid-State Circuits 49 (2014) 2694.
Conflict of interest
Authors do not have any conflict of interest with anyone
Practical application to drive: sensor, ICs, electronic devices, mobile chargers etc
5-10 V
DC to DC voltage booster 10-300 mV
p Hot
n
p
n
p
n
p
n
Thermal conductor & electrical insulator Source of waste heat (e.g. Radioisotope, automobile exhaust, power plant etc)
TEG
Cold
S2589-2991(18)30146-0 https://doi.org/10.1016/j.mset.2019.03.004 MSET 75
To appear in:
Materials Science for Energy Technologies
Received Date: Accepted Date:
6 November 2018 23 March 2019
Please cite this article as: S.K. Sahu, A.K. Bohra, P.G. Abichandani, A. Singh, S. Bhattacharya, R. Bhatt, R. Basu, P. Sarkar, S.K. Gupta, K.P. Muthe, S.C. Gadkari, Design and development of DC to DC voltage booster to integrate with PbTe/TAGS-85 based thermoelectric power generators, Materials Science for Energy Technologies (2019), doi: https://doi.org/10.1016/j.mset.2019.03.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design and development of DC to DC voltage booster to integrate with PbTe/TAGS-85 based thermoelectric power generators S. K. Sahu*, Anil K. Bohra, P. G. Abichandani, Ajay Singh*, Shovit Bhattacharya, Ranu Bhatt, Ranita Basu, Pritam Sarkar, S. K. Gupta, K. P. Muthe, S. C. Gadkari Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085
*Corresponding authors’ emails: Ajay Singh ([email protected]), S. K. Sahu ([email protected]),
Abstract Thermoelectric power generators (TEGs) based on Seebeck effect convert waste heat into electricity. However, the practical applications of such TEGs are limited by their low output DC voltage (10-300mV). Therefore, a self powered DC-DC voltage booster circuit is highly essential to integrate with TEGs for commercial use. In this work, we demonstrate a DC-DC voltage booster circuit to amplify the output voltage delivered by a PbTe/TAGS-85 based TEG which consists of 4 p-n couples alternately connected electrically in series and thermally in parallel. Design of the DC to DC booster circuit includes an oscillator and amplifier. Open circuit voltage of ~ 325 mV generated by the TEG for a hot side temperature (Th ) of 500°C and a temperature difference (ΔT ) of 410°C was amplified by the booster circuit to 10V. The designed DC-DC booster circuit exhibit the boosting efficiency of ~50%.
1. Introduction Rapid industrial evolution in last few years has increased the energy need and global warming problems, hence research and development has been promoted in the field of thermoelectric power generation as a means of recovering large amount of waste heat emitted by automobiles, factories etc [1, 2]. The thermoelectric power generators (TEGs) work on the principal of Seebeck effect and convert waste heat into the electrical energy. A TEG consists of several n- and p-type semiconductor thermoelements alternately connected electrically in series and thermally in parallel. Electrical connection between these n- and p-type thermoelements is usually made using metallic electrodes [2]. To make an efficient TEG, nand p-type thermoelectric materials with high figure-of-merit are required. It is also desirable that the joints between the thermoelements and the metallic electrodes should have low electrical contact resistance [2,3]. The open-circuit voltage of the TEG’s is directly proportional to the temperature difference and under the natural convection condition the temperature difference cannot be high due to finite thermal conductivity of thermoelectric materials. Therefore, in such circumstances of small temperature difference the output voltage of TEGs is usually small in range of 20–300 mV. Usually the current produced in a TEG is high (~ 1-10 A) due to very low electrical resistivity of the p- / n-type materials and their interfaces with metallic electrodes that are used to fabricate the TEG [2]. In order to make use of TEGs in practical applications requiring a higher voltage (such as powering a sensor, integrated circuits, mobile chargers etc) its integration with a self powered DC-DC voltage booster is highly required [4-10]. It may be noted that although various design of DC-DC booster circuits are previously reported [4-15] but for efficient transferring of power from TEG to the booster circuit there should be a good impedance matching between both. A scheme of integrating a TEG with a self powered DC-DC voltage booster for producing high output voltage is given in Figure 1. It may be noted that
commercially available DC-DC voltage booster either require a higher input voltage to operate or there is an impedance mismatch between TEG and DC-DC booster which leads to underutilization of electrical power generated by TEG. In this paper, we report the design and development of a DC-DC booster for harvesting the energy produced from an indigenous PbTe/TAGS-85 based TEG having 4 p-n couples alternately connected electrically in series and thermally in parallel. The output of TEG was connected to an indigenous, custom made DC-DC voltage booster consisting of an oscillator and amplifier stage. Finally the DC-DC booster delivered an output voltage of 10V and a load current of 4mA exhibiting conversion efficiency of ~50%.
Practical application to drive: sensor, ICs, electronic devices, mobile chargers etc
5-10 V
DC to DC voltage booster 10-300 mV
p Hot
n
p
n
p
n
p
n
TEG
Cold
Thermal conductor & electrical insulator Source of waste heat (e.g. Radioisotope, automobile exhaust, power plant etc)
Figure 1: Schematic showing integration scheme of TEG with the DC to DC voltage booster
2. Details of thermoelectric power generator to be integrated with DC to DC converter circuit
Depending on the operating temperature of TEG, different kinds of n and p-type semiconductor materials are currently being used for fabrication of such devices. Semiconductor alloys like n-type PbTe and p-type (AgSbTe2)0.15(GeTe)0.85 (TAGS-85) are well established for electrical power generation at a hot end temperature of ~500°C. A few years back our group reported the fabrication and characterization of n-type PbTe and p-type TAGS-85 thermoelements with Ag shoes at the ends and subsequently the fabrication of TEG using few numbers p-n couples alternately connected in series [3]. A typical schematic of a 4 p-n legs based TEG is shown in Figure 2(a). In order to fabricate this TEG, first single-phase polycrystalline n-type PbTe and p-type TAGS-85 material was synthesized by high temperature vacuum melt process. In brief stochiometric powders were taken, grinded in agate mortar pestle for 1 hour. The homogenously mixed powder was filled in a pre-cleaned quartz ampoule, after filling the materials the ampoule is sealed under vacuum (10-6mbar). The sealed ampoule was placed in a furnace and heated to ~ 900ºC and then rocked for 1 hour. The rocking eliminates the gravitational segregation of the different component materials and yield high homogeneity in the melt. Finally the molten material is cooled to the room temperature and grinded into fine powders. In order to fabricate n-type PbTe element (Size: 7.5 mm diameter and 8 mm height) with metallic shoes at ends, fine powders of Fe, (50%PbTe+ 50%Fe) and PbTe were filled in a stainless steel ( S.S.) die having thin graphite liner to avoid reaction between S.S. and thermoelectric materials. This layered structure was then pressed in a vacuum hot press under the conditions i.e. base vacuum ~10-6 Torr, temperature 600ºC and load of 700 Kgs. The fabrication of p-type TAGS-85 thermoelement was done in a similar way as that of n-type PbTe. Fine powders of Fe, SnTe, and TAGS-85 were filled in a S.S. die with graphite liner. SnTe acts like an diffusion barrier between Fe
and TAGS-85, otherwise the diffusion of Fe into the TAGS-85 results in the rapid degradation of the contacts. In order to design TEG, n-type PbTe and p-type TAGS-85 thermoelements were joined by a silver metal strip. The TEG was fabricated in the following steps: Initially 4 n-type and 4 p-type thermoelments were alternately packed in a zircar housing. The zircar housing has two advantages (a) it provides an enclosure to hold the thermoelements. (b) due to poor thermal conductivity of zircar it blocks the direct flow of heat between hot and cold surface. After placing the thermoelements in asbestos, silvers stripes were placed on the top and bottom side the thermoelements. The entire assembly is than vacuum hot pressed at a load 100 Kg. The optimized bonding temperature of 400˚C was found suitable for joining the silver stripes with the thermoelements. The photograph of the fabricated 4 p-n legs TEG is shown in Figure 2(b). Typical open circuit voltage and current at matched load of 10 mΩ of a four p-n legs TEG as a function of hot end temperature (Th) are shown in Figure 2(c-d). For a Th of 500°C the TEG showed an open circuit voltage of 325 mV and a current of 16 Amp with matched load of 10 mΩ. In this TEG, the contribution of contacts to total device resistance was found to be 4 %. The working efficiency of the devices was estimated by taking ratio of output electrical power and heat input given to the TEG, and it found to be ~6 % [3].
Zirconia based housing
n
p
Top view of actual 4 p-n legs TEG
Silver interconects
p
n
n
p
p
n n
+
-
Output terminals of TEG
(b) 16
Current (A) at 10 m
Open circuit voltage (mV)
(a)
300
12
225 150 75 0
(c) 100
200
300 0
400
500
8
4
0
(d) 100
Th( C)
200
300 0
400
500
Th ( C)
Figure 2: (a) Schematic of a 8 p-n legs device (b) Photograph (top view) of a 4 p-n legs TEG (c) Open circuit voltage (d) current at matched load of 10mΩ of the 4 p-n legs TEG as a function of hot end temperature (cold end maintained at 80°C).
3. Details of DC to DC voltage booster The Figure 3 shows the block diagram of DC-DC booster. It consists of two main components, first one an oscillator & driver circuit followed by a voltage amplifier stage
Oscillator and driver stage
Step up transformer & switching MOSFET (voltage amplifier stage)
Double rectifier & filter circuit
Voltage amplifier stage Figure 3. Block diagram of DC-DC voltage booster
Output 10V@ 4 mA
TEG output 300 mV
(shown in the dotted block).
The main function of oscillator & driver circuit is to convert the DC output of TEG into an AC signal for driving and switching MOSFET of voltage amplifier circuit. Subsequently a pulse train generated by voltage amplifier stage is rectified and filtered to produce a DC output.
Now we discuss each stage of the of DC-DC booster in detail
i. Oscillator and driver stage: Oscillator and driver circuit is the heart of DC-DC booster which coverts DC output voltage of TEG into an AC signal. It is based on Colpitts oscillator along with a clamping circuit. It takes input power from the low DC voltage generated by TEG source and it produces a 3V sinusoidal signal at frequency of around 250 kHz. The minimum start-up DC voltage required to activate this circuit is around 100mV. The output of oscillator and driver circuit is coupled to the power stage of voltage booster for driving the switching MOSFET.
ii. Power stage of DC-DC booster: Figure 4 (a) shows basic diagram of voltage amplifier stage of DC-DC voltage booster. It consists of a step-up transformer, a switching MOSFET, rectifier and a filter circuit (in doubler configuration). Oscillator and driver stage’s output (i.e. an AC signal around frequency ~ 250 kHz) switches the MOSFET “ON & OFF”, i.e. it chops the DC output voltage from TEG at 250kHz. When the MOSFET is in ON condition, non-dot end of the transformer’s primary coil brought to ground potential and dot end of transformer becomes positive. At the secondary coil of transformer, a voltage equivalent to the turns ratio times the input voltage is produced and in this condition diode D1 conducts and charges the capacitor C1. When MOSFET is turned OFF, the magnetizing energy of the transformer reverses the polarity of the transformer and the non dot end of transformer’s coils becomes
positive, diode D2 conducts. Thus stored energy into the transformer is transferred to the capacitor C2 through diode D2. Finally the voltage across the capacitor C1 and C2 get added and it produces an amplified DC output.
(a)
Output 10V@4mA D1
C1
L2 L1
R1 C3 D2
Load
C2
D3
TEG Source's Output
Oscillator and Driver Circuit
M1
0
0
(b)
Output of DC-DC booster
TEG assembly Output of TEG
DC -DC booster circuit
Figure 4. (a) Exploded view of the power stage block of DC-DC voltage booster circuit (a) Actual photograph of TEG assemble and DC-DC voltage booster unit
Figure 4(b) shows the actual photograph of the TEG and DC-DC booster assembly. In this assembly a 4 p-n legs based TEG is sandwiched between an electrical heater (hot source) and water cooled copper block (heat sink). The temperature of heater can be controlled using a PID controller within ±1°C of set temperature value. The temperature of heater and water cooled block is monitored using thin K-type thermocouple. The input power given to DC-DC booster circuit is obtained by measuring the voltage drop at the input terminal of the circuit. Current drawn by booster circuit is estimated by placing a known resistor in series with the input terminal of the booster circuit.
4. Performance of the of DC-DC voltage booster Finally the performance of the DC-DC voltage booster was tested with varying the input voltage supplied through TEG and obtained results are plotted in Figure 5. The output voltage of the TEG can be varied by changing its hot end temperature. The voltage booster unit turned ON only after it receives an input signal of 100 mV from TEG. From Figure 5, it may be seen that above 10 mV of input the output of the booster increases linearly with the input voltage. We have also estimated the boosting efficiency of the developed circuit by taking the ratio of input electrical power to the output electrical power. For an input electrical power of 80 mW (voltage ~ 300 mV & current ~ 267mA) the booster unit delivered a power of 40 mW (voltage ~ 10 V & current ~ 4mA) at load of 2.7 kΩ, implying that the developed booster has en efficiency of 50%.
Output of DC to DC voltage booster (V)
10 8 6 4 2 turn ON voltage
0
0
50
100
150
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300
Input voltage (in mV) supplied to booster by TEG
Figure 5. Output voltage delivered by DC to DC voltage circuit as a function of input voltage supplied by TEG, load 2.7 kΩ
It may be noted that in the present work, the maximum power delivered by the TEG at the matched load of 10 mΩ is ~ 2.6 W, while due to impedance mismatch the DC to DC converter (impedance of TEG << input impedance of DC-DC booster) is only extracting a only a small fraction (~80 mW) of maximum power generated by TEG. Hence the further extension of this work will be to place many numbers of DC-DC booster circuit will be parallel connected to the output of a single TEG as a result much higher power can be extracted from TEG. With such a combination of single TEG with many numbers of parallely connected DC-DC boosters can deliver high voltage (~ 10 V) and high current (~ 50-100 mA) at the load.
5. Conclusion In this paper, we have demonstrated a prototype DC-DC booster for amplifying the output voltage produced by a thermoelectric power generator. The booster has a turn on DC voltage
of around 100 mV. For a typical input voltage of 300mV from TEG, the DC –DC voltage booster delivered an output of 10 V/ 4 mA at load resistance of 2.7 kΩ. This prototype DCDC booster has an efficiency of around 50%. The present experimental results suggest that the proposed design and development of DC-DC booster is able to extract the energy from ultra low voltage sources and produce the desired output voltage for powering other electronic devices. Further higher output power can be delivered by the use of multiple power stages (connected in parallel to oscillator and driver stage) of voltage amplifier circuit of DCDC converter.
References 1. D. M. Rowe, CRC handbook of thermoelectrics, CRC press, 1995 2. D. K. Aswal, R. Basu, A. Singh, Key issues in development of thermoelectric power generators: High figure-of-merit materials and their highly conducting interfaces with metallic interconnects. Energy Conversion & Management 114 (2016) 50. 3. A. Singh, S. Bhattacharya, C. Thinaharan, D. Aswal, S. Gupta, J. Yakhmi and K. Bhanumurthy,Development of low resistance electrical contacts for thermoelectric devices based on n-type PbTe and p-type TAGS-85 ((AgSbTe2)0.15(GeTe)0.85), Journal of Physics D: Applied Physics 42 (2008) 015502. 4. R. Hazli, A. H. Hamidon, Design of DC-DC boost converter with thermoelectric power source, International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering 2(2013) 4170. 5. B. Shen, R. Hendry, J. Cancheevaram, C. Watkins, M. Mantini, R.Venkatasubramanian, DCDC Converter Suitable for Thermoelectric Generator, International Conference on Thermoelectrics (2005)517. 6. B. Shen, R. Hendry, C. Watkins, R.Venkatasubramanian, DC-DC converter for low voltage power source, US Patent US 7,706,152 B2, Date of patent April 27, 2010. 7. A. Richelli, S. Comensoli, Z. M. Kovács-Vajna, A DC/DC Boosting Technique and Power Management for Ultralow-Voltage Energy Harvesting Applications, IEEE Transaction on Industrial Electronics 59(2012)2701. 8. H. Mamur, R. Ahishka, Application of a DC–DC boost converter with maximum power point tracking for low power thermoelectric generators, Energy Conversion & Management 97 (2015)265
9. M. Guan , K. Wang, D. Xu , Wei-Hsin Liao, Design and experimental investigation of a lowvoltage thermoelectric energy harvesting system for wireless sensor nodes, Energy Conversion and Management 138(2017)30 10. E. J. Carlson, K. Strunz, B. P. Otis, A 20 mV input boost converter with efficient digital control for thermoelectric energy harvesting. IEEE J Solid-State Circuits 45 (2010) 741. 11. A. Paraskevas, E. Koutroulis, A simple maximum power point tracker for thermoelectric generators. Energy Convers Manage 108 (2016) 355. 12. M. Guan, K. Wang, Q. Zhu, W. H. Liao, A high efficiency boost converter with MPPT scheme for low voltage thermoelectric energy harvesting. J Electron Mater 45 (2016) 5514. 13. Ramadass YK, Chandrakasan AP. A battery-less thermoelectric energy harvesting interface circuit with 35 mV startup voltage. IEEE J Solid-State Circuits 46 (2011) 333. 14. J. P. Im, S. W. Wang, S. T. Ryu, G. H. Cho, A 40 mV transformer-reuse self-startup boost converter with MPPT control for thermoelectric energy harvesting. IEEE J Solid-State Circuits 47(2012) 3055. 15. Y. K. Teh, K. T. Mok,
Design of transformer-based boost converter for high internal
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Conflict of interest
Authors do not have any conflict of interest with anyone
Practical application to drive: sensor, ICs, electronic devices, mobile chargers etc
5-10 V
DC to DC voltage booster 10-300 mV
p Hot
n
p
n
p
n
p
n
Thermal conductor & electrical insulator Source of waste heat (e.g. Radioisotope, automobile exhaust, power plant etc)
TEG
Cold