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Thermoelectric Power Generation from Waste Heat of Natural Gas Water Heater
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 110 (2017) 32 – 37
1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia
Thermoelectric power generation from waste heat of natural gas water heater Lai Chet Ding, Nathan Meyerheinrich, Lippong Tan*, Kawtar Rahaoui, Ravi Jain, Aliakbar Akbarzadeh School of Engineering, RMIT University, Melbourne 3000, Australia
Abstract Waste heat is a viable source of recoverable energy in both industrial and domestic scenario. One of the methods to convert waste heat to electrical power is by using thermoelectric cells. In this paper, a power generation unit consist of 60 TECs was designed, fabricated and tested with the aim to recover the waste heat from domestic natural gas water heater. In the conceptual design, the waste heat from the flue is transferred to the hot water reservoir by using heat pipes (with fins). The cold water is entering to the power generation unit and serving as coolant for the TECs and thus at the same time, the cold water is preheated prior entering to the burner and this unit as a cogeneration system. In the experiment, the power generation unit was tested in a hot water bath at varying hot water temperature (50 °C -100 °C) and different cold water flow rates, ranging from 11 LPM to 33 LPM. With 60 TECs, the power generation unit has generated the electric power at the minimum of 3.9 W and a maximum 42.4 W under maximum flow rate, with the aforementioned range for hot water temperature and incoming cold water at 19.5 °C, which corresponds to typical scenario for hot water heating system. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords: Thermoelectric generator; Power generation; Waste heat; Heat recovery; Hot water heater; Renewable energy
1. Introduction In recent years, sustainable resource for electricity generation has been something of which the society has undertaken to take charge on the emissions and to keep up with our rapidly growing economy. Yearly, significant
* Corresponding author. Tel.: +61 3 9925 6022. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.101
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
amount of waste heat from both commercial and residential appliances is lost into the atmosphere, from which the potential never gets harnessed. With limited technology of waste heat recovery systems around, an alternative green, reliable and cost effective method is by the use of thermoelectric technology, which utilizes a solid state device, capable of converting temperature gradients into electric power. Numerous research has been carried out to investigate the potential of thermoelectric system with various kinds of heat source such as automotive exhaust heat [1,2], thermal oil heater [3], cook stoves [4,5] and waste heat from industrial process [6]. In Australia, 48% of Australian households use natural gas to power their hot water heaters, plus another 4% in remote areas use LPG systems [7]. Thus, there is more than half of the Australian household’s have a perfectly viable heat source for the generation of electricity. By scavenging the hot exhaust gas exiting the flue and the use of cool water from mains, we are able to use the temperature gradient for power generation using TECs. Besides preheat the water entering the hot water service the system proposed is generating a small amount of usable electrical power. Nomenclature PGU TEC ܶ ܶ ܸሶ
power generation unit thermoelectric cell cold side temperature hot side temperature cold side volume flow rate
2. Operation of domestic natural gas heater and conceptual design of test rig The operation (the heating cycle) of the typical domestic natural gas heater can be illustrated in the diagram given in Fig. 1.
Fig. 1. Typical heating cycle of domestic natural gas heater.
Fig. 2 shows the schematic diagram of the domestic natural gas heater coupled with PGU. A loop heat pipe (1) is transferring the exhaust heat to the PGU. The natural gas or propane flame (11) heats the water inside the tank, in which then the exhaust gas also heating the heat pipe. From here, due to the thermosiphon effect the working fluid in
33
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Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
the heat pipe will flow upwards (2). From here, it will release the heat at the reservoir where the TEC hot surfaces of the TECs are located. After the heat transfer takes place across the TECs, it will then flow into the thermal reservoir (4), where the remaining vapor in the pipe (if any) cools down, and re-enter the cycle (5). This system will work as a passive closed loop thermosiphon and thus no external pumps required. Meanwhile, the cold water from the main will enter the system at (7) and circulate through the cold surface of the TECs (8). Due to the temperature gradient across the TECs, electric power will be generated and at the same time the cold water from the main is pre-heated for the hot water service. The preheated water will then flow through into boiler (10). From here onwards, the hot water is then used as per usual for house-hold appliances. The flow rate through the PGU (7, 8, 9, 10) is purely determined on the flow rate from the main water supply.The temperature difference between the hot side (3) and cold side (8) will dictate how much actual electric power is produced.
Fig. 2. The conceptual design of the power generation system.
In this paper, only the performance of the power generation unit (which is the grey shaded region in Fig. 2) was tested and presented. Fig. 3 depicts the construction of the unit. In (a), the cold water inlet is connected to the supply from the main passing through a 3 mm gap in order to create a high convection heat transfer at the cold side. Then then, commercially available TECs are mounted on the surface of the mild steel (b & c) by using Artic Silver thermal adhesive, and the opposite attached surface of the TECs are directly exposed to the hot side of the fluid (as in (a)). The TECs at the same height are connected in series and across all the series connection, they are eventually connected in parallel so that any destructive cell will not causing the system cease to operate. Eventually, the PGU was tested by submerge the unit into a boiler and connected to the cold water supply from the main to simulate its actual operating condition.
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
Fig. 3. The conceptual design of the power generation system.
3. Testing and result 3.1. Single TEC testing Before testing the performance of the performance of the PGU, the TEC used in the unit was tested and the performance of a TEC used in this experiment is presented in Fig. 4. The TEC was tested at a range of οܶ varying between 14 㽅C to 59 㽅C, at the cold side temperature, ܶ of 22 㽅C. Over the aforementioned temperature difference, single TEC is generating 0.08 W and 1.23 W, respectively.
Fig. 4. (a) Test setup for single TEC testing; (b) Performance of single TEC.
3.2. Testing of the prototype The performance of the PGU at incoming cold water flow rate, ܸሶ =0.55 L/s (ܶǡ of 19.5 㽅C) with varying hot side temperature is presented in Fig. 5. It shows that the unit fabricated is able to generate a minimum of 3.9 W at ܶ =50㻌㽅 C and up to a maximum of 42.4W (average power of 0.71W per cell). At ܶ =80㻌㽅C, the system is producing an electric output of 21.3 W. On the other hand, the conversion efficiency of this system is defined as ݊௧ ൌ ܹሶ Ȁܳሶ, which is the electric power generated over the rate of heat transfer across the PGU. The conversion efficiency of the PGU is shown
35
36
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
in Fig. 6. The increase ܶ will increase the Carnot efficiency of the system and eventually it enables the PGU to operate at higher conversion efficiency. As demonstrated in Fig. 6, this system possesses the conversion efficiency in the range of 0.37% to 1.03 %. In the other words, the maximum heat exchange capacity of this unit is about 4.12 kW.
Fig. 5. Electric power output of the PGU under ܸሶ =0.55 L/s at varying ܶ .
Fig. 6. Conversion efficiency of the PGU under ܸሶ =0.55 L/s at different ܶ .
The performance of the PGU under varying cold water flow rate, ܸሶ is depicted in Fig. 7 and it suggested that reducing the incoming cold water flow rate by 50% will reduce the electric output power by 43%. Also, from Fig. 7, the power output generated at ܸሶ of 0.37 L/s is higher than the power output generated under the flow rate of 0.55 L/s. This might caused by a poor flow distribution across different surfaces of the cold side in the heat exchanger at high flow rate.
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
Fig. 7. Electric power output of the PGU under ܶ =98㽅C at varyingܸሶ .
4. Conclusion This work has demonstrated the potential of the electric power generation using the TECs. The heat source proposed in this study is the exhaust waste heat from the domestically available natural gas water heater since with a typical efficiency of 60-70%, significant amount of energy from the combustion is wasted. The extraction of the waste heat from the flue gas is subjected to the heating cycle of the natural gas water heater. For a continuous heat extraction, the commercial version of the natural gas water heater such as for the hot water supply in the hotel might be another viable option. Through the testing conducted, a PGU with 60 TECs will able to produce 42.4 W when the TECs are exposed to the hot water around boiling temperature and at the hot side temperature of 80 ϨC, the PGU generates 21.3 W of electricity. The thermal to electric conversion efficiency of this system is around 0.37% to 1.03%. References [1] Frobenius F, Gaiser G, Rusche U, Weller B. Thermoelectric generators for the integration into automotive exhaust systems for passenger cars and commercial vehicles. J Electron Mater 2016;45(3):1433-40. [2] Yu S, Du Q, Diao H, Shu G, Jiao K. Start-up modes of thermoelectric generator based on vehicle exhaust waste heat recovery. Appl Energ 2015;138:276-90. [3] Barma MC, Riaz M, Saidur R, Long BD. Estimation of thermoelectric power generation by recovering waste heat from biomass fired thermal oil heater. Energ Convers Manage 2015;98:303-13. [4] Gao HB, Huang GH, Li HJ, Qu ZG, Zhang YJ. Development of stove-powered thermoelectric generators: a review. Appl Therm Eng 2016;96:297-310. [5] O’Shaughnessy SM, Deasy MJ, Doyle JV, Robinson AJ. Performance analysis of a prototype small scale electricity-producing biomass cooking stove. Appl Energ 2015;156:566-76. [6] Ebling DG, Krumm A, Pfeiffelmann B, Gottschald J, Bruchmann J, Benim AC, Adam M, Herbertz RR, Stunz A. development of a system for thermoelectric heat recovery from stationary industrial processes. J Electron Mater 2016;45(7):3433-9. [7] Hot water service | YourHome [Internet]. Yourhome.gov.au. 2016 [cited 14 July 2016]. Available from: http://www.yourhome.gov.au/energy/hot-water-service
37
ScienceDirect Energy Procedia 110 (2017) 32 – 37
1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia
Thermoelectric power generation from waste heat of natural gas water heater Lai Chet Ding, Nathan Meyerheinrich, Lippong Tan*, Kawtar Rahaoui, Ravi Jain, Aliakbar Akbarzadeh School of Engineering, RMIT University, Melbourne 3000, Australia
Abstract Waste heat is a viable source of recoverable energy in both industrial and domestic scenario. One of the methods to convert waste heat to electrical power is by using thermoelectric cells. In this paper, a power generation unit consist of 60 TECs was designed, fabricated and tested with the aim to recover the waste heat from domestic natural gas water heater. In the conceptual design, the waste heat from the flue is transferred to the hot water reservoir by using heat pipes (with fins). The cold water is entering to the power generation unit and serving as coolant for the TECs and thus at the same time, the cold water is preheated prior entering to the burner and this unit as a cogeneration system. In the experiment, the power generation unit was tested in a hot water bath at varying hot water temperature (50 °C -100 °C) and different cold water flow rates, ranging from 11 LPM to 33 LPM. With 60 TECs, the power generation unit has generated the electric power at the minimum of 3.9 W and a maximum 42.4 W under maximum flow rate, with the aforementioned range for hot water temperature and incoming cold water at 19.5 °C, which corresponds to typical scenario for hot water heating system. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords: Thermoelectric generator; Power generation; Waste heat; Heat recovery; Hot water heater; Renewable energy
1. Introduction In recent years, sustainable resource for electricity generation has been something of which the society has undertaken to take charge on the emissions and to keep up with our rapidly growing economy. Yearly, significant
* Corresponding author. Tel.: +61 3 9925 6022. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.101
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
amount of waste heat from both commercial and residential appliances is lost into the atmosphere, from which the potential never gets harnessed. With limited technology of waste heat recovery systems around, an alternative green, reliable and cost effective method is by the use of thermoelectric technology, which utilizes a solid state device, capable of converting temperature gradients into electric power. Numerous research has been carried out to investigate the potential of thermoelectric system with various kinds of heat source such as automotive exhaust heat [1,2], thermal oil heater [3], cook stoves [4,5] and waste heat from industrial process [6]. In Australia, 48% of Australian households use natural gas to power their hot water heaters, plus another 4% in remote areas use LPG systems [7]. Thus, there is more than half of the Australian household’s have a perfectly viable heat source for the generation of electricity. By scavenging the hot exhaust gas exiting the flue and the use of cool water from mains, we are able to use the temperature gradient for power generation using TECs. Besides preheat the water entering the hot water service the system proposed is generating a small amount of usable electrical power. Nomenclature PGU TEC ܶ ܶ ܸሶ
power generation unit thermoelectric cell cold side temperature hot side temperature cold side volume flow rate
2. Operation of domestic natural gas heater and conceptual design of test rig The operation (the heating cycle) of the typical domestic natural gas heater can be illustrated in the diagram given in Fig. 1.
Fig. 1. Typical heating cycle of domestic natural gas heater.
Fig. 2 shows the schematic diagram of the domestic natural gas heater coupled with PGU. A loop heat pipe (1) is transferring the exhaust heat to the PGU. The natural gas or propane flame (11) heats the water inside the tank, in which then the exhaust gas also heating the heat pipe. From here, due to the thermosiphon effect the working fluid in
33
34
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
the heat pipe will flow upwards (2). From here, it will release the heat at the reservoir where the TEC hot surfaces of the TECs are located. After the heat transfer takes place across the TECs, it will then flow into the thermal reservoir (4), where the remaining vapor in the pipe (if any) cools down, and re-enter the cycle (5). This system will work as a passive closed loop thermosiphon and thus no external pumps required. Meanwhile, the cold water from the main will enter the system at (7) and circulate through the cold surface of the TECs (8). Due to the temperature gradient across the TECs, electric power will be generated and at the same time the cold water from the main is pre-heated for the hot water service. The preheated water will then flow through into boiler (10). From here onwards, the hot water is then used as per usual for house-hold appliances. The flow rate through the PGU (7, 8, 9, 10) is purely determined on the flow rate from the main water supply.The temperature difference between the hot side (3) and cold side (8) will dictate how much actual electric power is produced.
Fig. 2. The conceptual design of the power generation system.
In this paper, only the performance of the power generation unit (which is the grey shaded region in Fig. 2) was tested and presented. Fig. 3 depicts the construction of the unit. In (a), the cold water inlet is connected to the supply from the main passing through a 3 mm gap in order to create a high convection heat transfer at the cold side. Then then, commercially available TECs are mounted on the surface of the mild steel (b & c) by using Artic Silver thermal adhesive, and the opposite attached surface of the TECs are directly exposed to the hot side of the fluid (as in (a)). The TECs at the same height are connected in series and across all the series connection, they are eventually connected in parallel so that any destructive cell will not causing the system cease to operate. Eventually, the PGU was tested by submerge the unit into a boiler and connected to the cold water supply from the main to simulate its actual operating condition.
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
Fig. 3. The conceptual design of the power generation system.
3. Testing and result 3.1. Single TEC testing Before testing the performance of the performance of the PGU, the TEC used in the unit was tested and the performance of a TEC used in this experiment is presented in Fig. 4. The TEC was tested at a range of οܶ varying between 14 㽅C to 59 㽅C, at the cold side temperature, ܶ of 22 㽅C. Over the aforementioned temperature difference, single TEC is generating 0.08 W and 1.23 W, respectively.
Fig. 4. (a) Test setup for single TEC testing; (b) Performance of single TEC.
3.2. Testing of the prototype The performance of the PGU at incoming cold water flow rate, ܸሶ =0.55 L/s (ܶǡ of 19.5 㽅C) with varying hot side temperature is presented in Fig. 5. It shows that the unit fabricated is able to generate a minimum of 3.9 W at ܶ =50㻌㽅 C and up to a maximum of 42.4W (average power of 0.71W per cell). At ܶ =80㻌㽅C, the system is producing an electric output of 21.3 W. On the other hand, the conversion efficiency of this system is defined as ݊௧ ൌ ܹሶ Ȁܳሶ, which is the electric power generated over the rate of heat transfer across the PGU. The conversion efficiency of the PGU is shown
35
36
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
in Fig. 6. The increase ܶ will increase the Carnot efficiency of the system and eventually it enables the PGU to operate at higher conversion efficiency. As demonstrated in Fig. 6, this system possesses the conversion efficiency in the range of 0.37% to 1.03 %. In the other words, the maximum heat exchange capacity of this unit is about 4.12 kW.
Fig. 5. Electric power output of the PGU under ܸሶ =0.55 L/s at varying ܶ .
Fig. 6. Conversion efficiency of the PGU under ܸሶ =0.55 L/s at different ܶ .
The performance of the PGU under varying cold water flow rate, ܸሶ is depicted in Fig. 7 and it suggested that reducing the incoming cold water flow rate by 50% will reduce the electric output power by 43%. Also, from Fig. 7, the power output generated at ܸሶ of 0.37 L/s is higher than the power output generated under the flow rate of 0.55 L/s. This might caused by a poor flow distribution across different surfaces of the cold side in the heat exchanger at high flow rate.
Lai Chet Ding et al. / Energy Procedia 110 (2017) 32 – 37
Fig. 7. Electric power output of the PGU under ܶ =98㽅C at varyingܸሶ .
4. Conclusion This work has demonstrated the potential of the electric power generation using the TECs. The heat source proposed in this study is the exhaust waste heat from the domestically available natural gas water heater since with a typical efficiency of 60-70%, significant amount of energy from the combustion is wasted. The extraction of the waste heat from the flue gas is subjected to the heating cycle of the natural gas water heater. For a continuous heat extraction, the commercial version of the natural gas water heater such as for the hot water supply in the hotel might be another viable option. Through the testing conducted, a PGU with 60 TECs will able to produce 42.4 W when the TECs are exposed to the hot water around boiling temperature and at the hot side temperature of 80 ϨC, the PGU generates 21.3 W of electricity. The thermal to electric conversion efficiency of this system is around 0.37% to 1.03%. References [1] Frobenius F, Gaiser G, Rusche U, Weller B. Thermoelectric generators for the integration into automotive exhaust systems for passenger cars and commercial vehicles. J Electron Mater 2016;45(3):1433-40. [2] Yu S, Du Q, Diao H, Shu G, Jiao K. Start-up modes of thermoelectric generator based on vehicle exhaust waste heat recovery. Appl Energ 2015;138:276-90. [3] Barma MC, Riaz M, Saidur R, Long BD. Estimation of thermoelectric power generation by recovering waste heat from biomass fired thermal oil heater. Energ Convers Manage 2015;98:303-13. [4] Gao HB, Huang GH, Li HJ, Qu ZG, Zhang YJ. Development of stove-powered thermoelectric generators: a review. Appl Therm Eng 2016;96:297-310. [5] O’Shaughnessy SM, Deasy MJ, Doyle JV, Robinson AJ. Performance analysis of a prototype small scale electricity-producing biomass cooking stove. Appl Energ 2015;156:566-76. [6] Ebling DG, Krumm A, Pfeiffelmann B, Gottschald J, Bruchmann J, Benim AC, Adam M, Herbertz RR, Stunz A. development of a system for thermoelectric heat recovery from stationary industrial processes. J Electron Mater 2016;45(7):3433-9. [7] Hot water service | YourHome [Internet]. Yourhome.gov.au. 2016 [cited 14 July 2016]. Available from: http://www.yourhome.gov.au/energy/hot-water-service
37