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Thermoelectric Applications for Home Use: Thermostat and Green Barbecue 2.0
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 10283–10290
www.materialstoday.com/proceedings
ECT 2016
Thermoelectric Applications for Home Use: Thermostat and Green Barbecue 2.0 Marco Nesarajah*, Georg Frey Chair of Automation and Energy Systems, Saarland University, D-66123 Saarbrücken, Germany
Abstract This contribution presents two thermoelectric applications for home use. One is an energy harvesting system (EHS) at a heating to supply an electronic thermostat valve and the other is the revised green barbecue. In the case of the former one, the temperature difference between radiator surface and ambient temperature is used to produce electrical energy and supply the electronic thermostat valve. The green barbecue 2.0 is a thermoelectric EHS, which converts the thermal energy from a fire during barbecue to electrical energy. Hereby, it is possible to load a mobile phone, hear music or use a 12 V vehicle plug, beside the operation of the cooling fans. Both systems are described and measurement data are recorded, during the winter period of 2015/2016 for the thermostat and during different barbecues in 2016 for the green barbecue. These data are compared for both applications with simulation results from Modelica®/Dymola®. Thus, potential for improvement are identified and possible improvement opportunities are addressed. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS. Keywords: Energy Harvesting System; Green Barbecue; Modelica; Thermoelectric Applications; Thermoelectric Generator; Thermostat
1. Introduction Thermoelectric generators (TEG) can be used in energy harvesting systems (EHS) to generate electrical energy by applying a temperature difference over the device. Already today, working EHSs based on currently purchasable
* Corresponding author. Tel.: +49-(0)681-302-57566; fax: +49-(0)681-302-57599. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
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TEGs can be build-up and used meaningful, but this is mainly done in research projects; see for example [1-3]. Although, there are high amounts of waste heat in nearly every industrial process, TEG-based EHSs only eke out a marginal existence. The purchasable TEGs consist mainly of the thermoelectric material Bi2Te3 and its efficiency is still very low, but nevertheless acceptable systems can be build-up. One possibility to minimize the price depending on the situation is to use Peltier elements instead of TEGs. They consist of the same thermoelectric material but with some differences which make the price cheaper, see [4]. The goal of this contribution is to show that already today thermoelectric applications for home use may be applicable and affordable. With the growing popularity in the research of thermoelectricity, especially in thermoelectric materials, the number of useful application cases for TEG-based EHSs will increase. The main research focuses are on high figure of merit values of the materials and cost-efficient, eco-friendly and easy-to-make thermoelectric generators, [5, 6]. The two examples presented here are an EHS at a heating to supply an electronic thermostat valve and the revised green barbecue, firstly presented in [7]. For both application examples, a development process, which was already described in [8], was applied. In Sect. 2, the set-up of the EHS at the thermostat is explained as well as measurement data are shown and compared with simulation results. Section 3 provides the same information for the green barbecue following by the conclusions in Sect. 4. 2. Thermostat 2.1. Set-up There are some advantages to replace a standard thermostat at a heating with an electronic thermostatic valve. The most important one is the saving of energy by adjusting exact temperatures for explicit time intervals. Disadvantageously is the fact, that an electronic thermostatic valve needs electrical energy, which is normally delivered by a battery that has to be changed from time to time. The here developed thermoelectric EHS generates the electrical energy for the thermostat directly from the heating process and that is why a battery replacement is no longer necessary. The idea is that a small part of the heating energy will be converted by TEGs. Of course, this energy is lost for the room heating process, but it is so less that it will be of no consequence for the room temperature. The here presented EHS is installed in one office at our Chair and adapted to the working time; the heating mode is programmed from Monday to Friday between 7:30 and 17:30 o'clock. In the other time, the thermostat is programmed to the setback temperature. Consequently, this means that enough electrical energy has to be produced in the few hours of heating to supply the thermostat twenty-four-seven.
Fig. 1. Thermoelectric EHS at a heating to supply an electronic thermostat valve: (a) front view, (b) top view.
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Figure 1 shows the construction of the EHS from the front and the top. Visible in the front view are the both cooling elements at the radiator and two switching boxes. The smaller one (switching box, No. 1) containing an accumulator and a maximum power point tracker (MPPT), whereas the additional, bigger switching box (switching box, No. 2) is necessary for measurement purposes. It contains different sensors and a micro-controller and has a direct connection with a network database. The EHS is also working without the additional box (No. 2). The top view shows the electronic thermostat and the TEGs between the aluminum half-shells, surrounding the heating tubes, and the cooling elements. The two TEGs, which are here Peltier elements, see [4], are connected in series. An attempt to commercialize such a project was done by Micropelt, [9]. However, the heat for the hot TEG side is not coming from the heating tubes like in this case, but directly extracted from the heat in the thermostat valve with especially developed TEGs. In contrast to this, standard TEGs and mainly standard components are used here. 2.2. Measurement and Simulation For measurement reasons, a second switching box is needed as shown in Fig. 1. A schematic layout of the complete system with measuring devices is shown in Fig. 2 (for a clearly arranged overview, switching box No. 2 is not shown, only the sensors which are located inside the box No. 2). Temperature sensors are directly attached at the aluminum half-shell and the cooling element and one room temperature sensor is also available, presented with thermometer symbols in Fig. 2. Additionally, the voltage and the current coming from the TEGs—equal with the produced electrical power—and the voltage and the current going from the accumulator to the thermostat—equal with the consumption—are measured. This is done in the second switching box. The measuring points are marked with an S for sensor. Thereby, it is clear that the conversion losses through the MPPT, the intermediate storage in a supercapacitor and the low-dropout regulator are not considered, as they are located behind the measuring device. All data are collected and evaluated by a micro-controller and are sent to a database.
Fig. 2. Schematic layout of the EHS at the heating. Visible are the TEGs in series and the thermostat as well as the content of the first switching box—MPPT, supercapacitor (SC), low-dropout regulator (LDO) and accumulator (ACCU)—and the sensors.
As an example week for the winter period 2015/2016, the different temperature profiles for room temperature, the temperature of the aluminum half-shell and the cooling element temperature are shown in the upper diagram of Fig. 3 for the week from March 21st, 2016 to March 28th, 2016. As it is the record of an office room, the heating periods on working day at working time are visible. The maximum temperature difference between the aluminum half-shell and the cooling element is about 5 K, whereas the average value for the heating phase is between 3 and 4 K. The produced electrical power is between 3 and 4 mW on average during the heating period and is depicted in the lower diagram of Fig. 3. In stand-by mode the consumption of the thermostat is 0.3 mW and during switching operation it can consume up to 65 mW. It is also obviously that the thermostat switches not only to the stored times, but also moves the valve very often during the heating phase and even inexplicably in the night mode or at the weekend. The right axis in the lower diagram of Fig. 3 shows the consumed respectively generated electrical energy during this example week. From Monday to Friday, the energies are almost balanced, but then moves over the
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weekend in favor of consumption. There are even weeks during this winter period with a positive energy balance at Friday afternoon, but the not explainable high consumption of the thermostat during the weekend always leaves a negative balance at the end. Also visible in the lower diagram of Fig. 3 is the simulation result for the produced electrical power and energy. The measurement temperatures are given to a simulation model of the EHS in Modelica/Dymola. The simulation gives a slightly higher power output and thus, a slightly higher energy after one week. In the simulation, the EHS delivers after one week 734.26 J, whereas the real measurement data gives 707.3 J as the produced energy at the end of the week. It can be noted, that the EHS at the heating is at the moment not able to supply the thermostat autarkic under the current conditions. Of course, there were calculations in advance to analyze the profitableness of an EHS and with the knowledge of the stand-by power of the thermostat and the necessary power to switch the valve, the planned EHS should be sufficient enough. Unfortunately, there are more switching cycles than expected.
Fig. 3. Measurement and simulation data of the week of March 21st, 2016. Above: temperature profiles during the week for the room temperature, the aluminum half shell temperature and the temperature at the cooling element; below: power and energy consumption of the thermostat as well as power and energy production of the EHS and the simulation results for the produced power and energy.
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3. Green Barbecue 3.1. Set-up This thermoelectric EHS is attached on a common used fireplace for barbecue. The purpose is to use the heat of the fire, which will be lost over the outside wall of the fireplace, to generate electrical energy. Therefore, copper insets are integrated in the four walls and serve as heat sources for the TEGs. The reasons for the insets are the better thermal conductivity of copper compared to steel and the thickness of the insets which prevents a deformation under thermal stress. TEGs are directly attached at the copper insets and on the cold side CPU cooling elements are mounted to ensure a high temperature difference over the thermoelectric devices. In contrast to the electrical thermostat, active cooling elements are used here meaning that the EHS has to deliver at least the energy consumption of the CPU fans to run self-sufficiently. There are two TEGs and cooling elements attached at each wall side, and in total there are eight TEGs at the fireplace. Additionally, on each wall side, the hot and cold TEG side temperature is measured with Pt1000 temperature sensors, which are used for controlling purposes. Figure 4 shows the different development stages of the Green Barbecue. As load, there is an amplifier to hear music during barbecue, a USB-plug to load mobile devices and a 12 V vehicle plug to supply for example an ice box. In contrast to the first version presented in [7], the Green Barbecue 2.0 is more stable against malfunction TEGs, due to a substitution of the former TEGs and another connection, and delivers more usable power, due to the integration of an MPPT and a temperature dependent fan controlling. All fans are connected in parallel and with the calculation of an average hot side temperature, the velocity of the fans is adjusted. Respectively both TEGs of one wall side are connected in parallel, as they work under almost the same temperature conditions, and the four parallel TEG blocks are connected in series to increase the voltage level. In a switching box, the power line of the TEGs is connected to an MPPT, which delivers a constant 12 V power output and load an accumulator supplying directly the fans of the cooling elements, the 12 V vehicle plug and the amplifier, as well as a DC/DC converter which provides the 5 V level for the USB plug. A picture of the overall system is shown in Fig. 4e.
Fig. 4. Development stages of the Green Barbecue 2.0: (a) unmodified and purchasable fireplace, (b) copper insets for a better heat transfer to the TEGs, (c) TEGs and cooling elements are attached at each side wall, (d) temperature sensors and splashbacks are mounted, (e) overall system view including the switching box.
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3.2. Measurement and Simulation Measurement data of the Green Barbecue 2.0 can be directly logged with a laptop. They contain a time stamp, the four hot and the four cold side temperatures, the voltage of the TEG power line, the current coming from the TEGs into the MPPT as well as the voltage level of the accumulator, the current going from the accumulator to the loads and the duty cycle of the fans, which can be a controlled parameter by the intern microcontroller or a manual set parameter. The schematic layout is shown in Fig. 5. Again, the measuring points are marked with an S and the temperature measuring points are presented with thermometer symbols. Figure 6 shows the measurement results for an exemplary barbecue, which took place on March 22nd, 2016. The upper diagram shows the different temperature curves, whereat the hot and cold temperatures on each wall side were measured. However, it is to note that one temperature sensor (Cold 2) did not work and that there were temporary signal errors for a second temperature sensor (Hot 4). The maximum reached temperature is about 350 °C at the hot wall side 1 and it is clearly visible that the fire is not uniform in the fireplace as there are high differences between the hot side temperatures. The maximum temperature difference is temporary 200 K on the wall side 1 at 12:50 o’clock. The lower diagram of Fig. 6 shows the produced and consumed power and energy. In total the produced and consumed energy is nearly equal (about 70 kJ). The high power consumption at the beginning is traced back to the supplying of the fans and the amplifier, which are switched on directly, although there is yet no energy delivered from the TEGs. This energy comes from the integrated accumulator. The short drop of the power consumption results from the signal errors of the temperature sensor (Hot 4) as the controlling of the fans is done by the set-up of the fans duty cycle which are a function of the average hot side temperature.
Fig. 5. Schematic layout of the EHS at the fireplace: Visible are the connection of the TEGs and the structure of the switching box as well as the loads which are supplied.
Also shown in Fig. 6 are the simulation results for the energy and power production of the EHS at the fireplace. The hot side temperatures are given to a simulation model in Modelica/Dymola, whereas the values for the area of the signal errors of temperature sensor Hot 4 are interpolated. The simulation results fit qualitative to the measurement data, but are however slightly better than in reality (about 73.5 kJ). Finally, it can be said that the EHS at the fireplace runs autarkical and is capable to supply the extra loads and thus represents a successful thermoelectric EHS for home use.
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4. Conclusions Two thermoelectric applications for home use are here presented and it is shown that in principal thermoelectric EHSs are already today applicable and also affordable, as the setups are mainly done by standard components. The Green Barbecue 2.0 runs completely autarkical whereas this is currently not valid for the thermostat. In the case of the Green Barbecue, the simulation has shown that with the current setup the maximum energy yield is almost reached. To increase beyond the produced power, the construction has to be changed, which means a better connection especially of the cooling element on the cold TEG side, which is now the performancedetermining factor. A further possibility to increase the energy yield is a separate control of the fans, as each TEG block has its own temperature levels. Consequently, there is an optimal duty cycle for the fans on each side. The usage of four MPPTs, one per each TEG block, would also slightly increase the power output, but in contrast significantly increase the costs of the system and is therefore not recommended. The second developed thermoelectric EHS for home use, the thermostat, delivers not enough energy to run selfsufficient. Previous calculations act on the assumption that the thermostat consumes generously 700 J per week, due to the stand-by consumption of 0.3 mW and the consumption of valve movement of 65 mW, which is necessary two times a day. Comparing this value with the real measured and simulation data, the EHS should work fine. But not taken into account are the many inexplicably movements of the valve during especially the night and the weekend. So, the consumption of the thermostat exceeds the energy productions of the EHS. A possible solution of this problem is to intervene in the microcontroller of the bought thermostat and to modify the switching behavior. At least, the time to exchange the batteries will be extended in its current state. Hot 2
Hot 3 Cold 3
Hot 4 Cold 4
300 200 100 0
power consumption energy consumption
power produced energy produced
power prod sim energy prod sim
power [W]
15
100 80 60
10
40 5 0 11:45
energy [kJ]
temperature [°C]
Hot 1 Cold 1
20 0 12:00
12:15
12:30
12:45
13:00
13:15
13:30
time, March 22nd, 2016 Fig. 6. Measurement and simulation data for the Green Barbecue at a barbecue on March 21st, 2016 between 11:45 and 13:39 o’clock. Above: temperature curves of each wall side for hot and cold temperature, below: power and energy consumption of the Green Barbecue as well as power and energy production of the EHS and the simulation results for the produced power and energy.
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References [1] L.I. Anatychuk, R.V. Kuz, Materials for Vehicular Thermoelectric Generators, Journal of Electronic Materials, 2012, 41, 1778-1784. [2] M.E. Kiziroglou, S.W. Wright, T.T. Toh, P.D. Mitcheson, T. Becker, E.M. Yeatman, Design and Fabrication of Heat Storage Thermoelectric Harvesting Devices, IEEE Transactions on Industrial Electronics, 2014, 61, 302-309. [3] V. Leonov, Thermoelectric Energy Harvesting of Human Body Heat for Wearable Sensors, IEEE Sensors Journal, 2013, 13, 2284-2291. [4] M. Nesarajah, G. Frey, Thermoelectric Power Generation: Peltier Element versus Thermoelectric Generator, 42nd Conf. IEEE IECON2016, Oct. 2016. [accepted] [5] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nature Materials, 2008, 105-114. [6] J.D. König, K. Bartholomé, H. Böttner, D. Jänisch, M. Klein Altstedde, M. Köhne, J. Nurnus, A. Roch, K. Tarantik, Thermoelectrics: power from waste heat, BINE-Themeninfo, 2016. [7] M. Nesarajah, G. Frey, Energy Harvesting from Open Fireplaces, Springer Proceedings in Energy, 2nd ENEFM2014, 2015, 525-531. [8] M. Nesarajah, G. Frey, Multiphysics Simulation in the Development of Thermoelectric Energy Harvesting Systems, Journal of Electronic Materials, 2016, 1408-1411. [9] Micropelt: Der energieautarke Heizkörperstellantrieb (lit.: The self-powered radiator actuator). http://www.micropelt.de/downloads/itrv_heft.pdf
ScienceDirect Materials Today: Proceedings 5 (2018) 10283–10290
www.materialstoday.com/proceedings
ECT 2016
Thermoelectric Applications for Home Use: Thermostat and Green Barbecue 2.0 Marco Nesarajah*, Georg Frey Chair of Automation and Energy Systems, Saarland University, D-66123 Saarbrücken, Germany
Abstract This contribution presents two thermoelectric applications for home use. One is an energy harvesting system (EHS) at a heating to supply an electronic thermostat valve and the other is the revised green barbecue. In the case of the former one, the temperature difference between radiator surface and ambient temperature is used to produce electrical energy and supply the electronic thermostat valve. The green barbecue 2.0 is a thermoelectric EHS, which converts the thermal energy from a fire during barbecue to electrical energy. Hereby, it is possible to load a mobile phone, hear music or use a 12 V vehicle plug, beside the operation of the cooling fans. Both systems are described and measurement data are recorded, during the winter period of 2015/2016 for the thermostat and during different barbecues in 2016 for the green barbecue. These data are compared for both applications with simulation results from Modelica®/Dymola®. Thus, potential for improvement are identified and possible improvement opportunities are addressed. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS. Keywords: Energy Harvesting System; Green Barbecue; Modelica; Thermoelectric Applications; Thermoelectric Generator; Thermostat
1. Introduction Thermoelectric generators (TEG) can be used in energy harvesting systems (EHS) to generate electrical energy by applying a temperature difference over the device. Already today, working EHSs based on currently purchasable
* Corresponding author. Tel.: +49-(0)681-302-57566; fax: +49-(0)681-302-57599. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.
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Marco Nesarajah / Materials Today: Proceedings 5 (2018) 10283–10290
TEGs can be build-up and used meaningful, but this is mainly done in research projects; see for example [1-3]. Although, there are high amounts of waste heat in nearly every industrial process, TEG-based EHSs only eke out a marginal existence. The purchasable TEGs consist mainly of the thermoelectric material Bi2Te3 and its efficiency is still very low, but nevertheless acceptable systems can be build-up. One possibility to minimize the price depending on the situation is to use Peltier elements instead of TEGs. They consist of the same thermoelectric material but with some differences which make the price cheaper, see [4]. The goal of this contribution is to show that already today thermoelectric applications for home use may be applicable and affordable. With the growing popularity in the research of thermoelectricity, especially in thermoelectric materials, the number of useful application cases for TEG-based EHSs will increase. The main research focuses are on high figure of merit values of the materials and cost-efficient, eco-friendly and easy-to-make thermoelectric generators, [5, 6]. The two examples presented here are an EHS at a heating to supply an electronic thermostat valve and the revised green barbecue, firstly presented in [7]. For both application examples, a development process, which was already described in [8], was applied. In Sect. 2, the set-up of the EHS at the thermostat is explained as well as measurement data are shown and compared with simulation results. Section 3 provides the same information for the green barbecue following by the conclusions in Sect. 4. 2. Thermostat 2.1. Set-up There are some advantages to replace a standard thermostat at a heating with an electronic thermostatic valve. The most important one is the saving of energy by adjusting exact temperatures for explicit time intervals. Disadvantageously is the fact, that an electronic thermostatic valve needs electrical energy, which is normally delivered by a battery that has to be changed from time to time. The here developed thermoelectric EHS generates the electrical energy for the thermostat directly from the heating process and that is why a battery replacement is no longer necessary. The idea is that a small part of the heating energy will be converted by TEGs. Of course, this energy is lost for the room heating process, but it is so less that it will be of no consequence for the room temperature. The here presented EHS is installed in one office at our Chair and adapted to the working time; the heating mode is programmed from Monday to Friday between 7:30 and 17:30 o'clock. In the other time, the thermostat is programmed to the setback temperature. Consequently, this means that enough electrical energy has to be produced in the few hours of heating to supply the thermostat twenty-four-seven.
Fig. 1. Thermoelectric EHS at a heating to supply an electronic thermostat valve: (a) front view, (b) top view.
Marco Nesarajah / Materials Today: Proceedings 5 (2018) 10283–10290
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Figure 1 shows the construction of the EHS from the front and the top. Visible in the front view are the both cooling elements at the radiator and two switching boxes. The smaller one (switching box, No. 1) containing an accumulator and a maximum power point tracker (MPPT), whereas the additional, bigger switching box (switching box, No. 2) is necessary for measurement purposes. It contains different sensors and a micro-controller and has a direct connection with a network database. The EHS is also working without the additional box (No. 2). The top view shows the electronic thermostat and the TEGs between the aluminum half-shells, surrounding the heating tubes, and the cooling elements. The two TEGs, which are here Peltier elements, see [4], are connected in series. An attempt to commercialize such a project was done by Micropelt, [9]. However, the heat for the hot TEG side is not coming from the heating tubes like in this case, but directly extracted from the heat in the thermostat valve with especially developed TEGs. In contrast to this, standard TEGs and mainly standard components are used here. 2.2. Measurement and Simulation For measurement reasons, a second switching box is needed as shown in Fig. 1. A schematic layout of the complete system with measuring devices is shown in Fig. 2 (for a clearly arranged overview, switching box No. 2 is not shown, only the sensors which are located inside the box No. 2). Temperature sensors are directly attached at the aluminum half-shell and the cooling element and one room temperature sensor is also available, presented with thermometer symbols in Fig. 2. Additionally, the voltage and the current coming from the TEGs—equal with the produced electrical power—and the voltage and the current going from the accumulator to the thermostat—equal with the consumption—are measured. This is done in the second switching box. The measuring points are marked with an S for sensor. Thereby, it is clear that the conversion losses through the MPPT, the intermediate storage in a supercapacitor and the low-dropout regulator are not considered, as they are located behind the measuring device. All data are collected and evaluated by a micro-controller and are sent to a database.
Fig. 2. Schematic layout of the EHS at the heating. Visible are the TEGs in series and the thermostat as well as the content of the first switching box—MPPT, supercapacitor (SC), low-dropout regulator (LDO) and accumulator (ACCU)—and the sensors.
As an example week for the winter period 2015/2016, the different temperature profiles for room temperature, the temperature of the aluminum half-shell and the cooling element temperature are shown in the upper diagram of Fig. 3 for the week from March 21st, 2016 to March 28th, 2016. As it is the record of an office room, the heating periods on working day at working time are visible. The maximum temperature difference between the aluminum half-shell and the cooling element is about 5 K, whereas the average value for the heating phase is between 3 and 4 K. The produced electrical power is between 3 and 4 mW on average during the heating period and is depicted in the lower diagram of Fig. 3. In stand-by mode the consumption of the thermostat is 0.3 mW and during switching operation it can consume up to 65 mW. It is also obviously that the thermostat switches not only to the stored times, but also moves the valve very often during the heating phase and even inexplicably in the night mode or at the weekend. The right axis in the lower diagram of Fig. 3 shows the consumed respectively generated electrical energy during this example week. From Monday to Friday, the energies are almost balanced, but then moves over the
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weekend in favor of consumption. There are even weeks during this winter period with a positive energy balance at Friday afternoon, but the not explainable high consumption of the thermostat during the weekend always leaves a negative balance at the end. Also visible in the lower diagram of Fig. 3 is the simulation result for the produced electrical power and energy. The measurement temperatures are given to a simulation model of the EHS in Modelica/Dymola. The simulation gives a slightly higher power output and thus, a slightly higher energy after one week. In the simulation, the EHS delivers after one week 734.26 J, whereas the real measurement data gives 707.3 J as the produced energy at the end of the week. It can be noted, that the EHS at the heating is at the moment not able to supply the thermostat autarkic under the current conditions. Of course, there were calculations in advance to analyze the profitableness of an EHS and with the knowledge of the stand-by power of the thermostat and the necessary power to switch the valve, the planned EHS should be sufficient enough. Unfortunately, there are more switching cycles than expected.
Fig. 3. Measurement and simulation data of the week of March 21st, 2016. Above: temperature profiles during the week for the room temperature, the aluminum half shell temperature and the temperature at the cooling element; below: power and energy consumption of the thermostat as well as power and energy production of the EHS and the simulation results for the produced power and energy.
Marco Nesarajah / Materials Today: Proceedings 5 (2018) 10283–10290
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3. Green Barbecue 3.1. Set-up This thermoelectric EHS is attached on a common used fireplace for barbecue. The purpose is to use the heat of the fire, which will be lost over the outside wall of the fireplace, to generate electrical energy. Therefore, copper insets are integrated in the four walls and serve as heat sources for the TEGs. The reasons for the insets are the better thermal conductivity of copper compared to steel and the thickness of the insets which prevents a deformation under thermal stress. TEGs are directly attached at the copper insets and on the cold side CPU cooling elements are mounted to ensure a high temperature difference over the thermoelectric devices. In contrast to the electrical thermostat, active cooling elements are used here meaning that the EHS has to deliver at least the energy consumption of the CPU fans to run self-sufficiently. There are two TEGs and cooling elements attached at each wall side, and in total there are eight TEGs at the fireplace. Additionally, on each wall side, the hot and cold TEG side temperature is measured with Pt1000 temperature sensors, which are used for controlling purposes. Figure 4 shows the different development stages of the Green Barbecue. As load, there is an amplifier to hear music during barbecue, a USB-plug to load mobile devices and a 12 V vehicle plug to supply for example an ice box. In contrast to the first version presented in [7], the Green Barbecue 2.0 is more stable against malfunction TEGs, due to a substitution of the former TEGs and another connection, and delivers more usable power, due to the integration of an MPPT and a temperature dependent fan controlling. All fans are connected in parallel and with the calculation of an average hot side temperature, the velocity of the fans is adjusted. Respectively both TEGs of one wall side are connected in parallel, as they work under almost the same temperature conditions, and the four parallel TEG blocks are connected in series to increase the voltage level. In a switching box, the power line of the TEGs is connected to an MPPT, which delivers a constant 12 V power output and load an accumulator supplying directly the fans of the cooling elements, the 12 V vehicle plug and the amplifier, as well as a DC/DC converter which provides the 5 V level for the USB plug. A picture of the overall system is shown in Fig. 4e.
Fig. 4. Development stages of the Green Barbecue 2.0: (a) unmodified and purchasable fireplace, (b) copper insets for a better heat transfer to the TEGs, (c) TEGs and cooling elements are attached at each side wall, (d) temperature sensors and splashbacks are mounted, (e) overall system view including the switching box.
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3.2. Measurement and Simulation Measurement data of the Green Barbecue 2.0 can be directly logged with a laptop. They contain a time stamp, the four hot and the four cold side temperatures, the voltage of the TEG power line, the current coming from the TEGs into the MPPT as well as the voltage level of the accumulator, the current going from the accumulator to the loads and the duty cycle of the fans, which can be a controlled parameter by the intern microcontroller or a manual set parameter. The schematic layout is shown in Fig. 5. Again, the measuring points are marked with an S and the temperature measuring points are presented with thermometer symbols. Figure 6 shows the measurement results for an exemplary barbecue, which took place on March 22nd, 2016. The upper diagram shows the different temperature curves, whereat the hot and cold temperatures on each wall side were measured. However, it is to note that one temperature sensor (Cold 2) did not work and that there were temporary signal errors for a second temperature sensor (Hot 4). The maximum reached temperature is about 350 °C at the hot wall side 1 and it is clearly visible that the fire is not uniform in the fireplace as there are high differences between the hot side temperatures. The maximum temperature difference is temporary 200 K on the wall side 1 at 12:50 o’clock. The lower diagram of Fig. 6 shows the produced and consumed power and energy. In total the produced and consumed energy is nearly equal (about 70 kJ). The high power consumption at the beginning is traced back to the supplying of the fans and the amplifier, which are switched on directly, although there is yet no energy delivered from the TEGs. This energy comes from the integrated accumulator. The short drop of the power consumption results from the signal errors of the temperature sensor (Hot 4) as the controlling of the fans is done by the set-up of the fans duty cycle which are a function of the average hot side temperature.
Fig. 5. Schematic layout of the EHS at the fireplace: Visible are the connection of the TEGs and the structure of the switching box as well as the loads which are supplied.
Also shown in Fig. 6 are the simulation results for the energy and power production of the EHS at the fireplace. The hot side temperatures are given to a simulation model in Modelica/Dymola, whereas the values for the area of the signal errors of temperature sensor Hot 4 are interpolated. The simulation results fit qualitative to the measurement data, but are however slightly better than in reality (about 73.5 kJ). Finally, it can be said that the EHS at the fireplace runs autarkical and is capable to supply the extra loads and thus represents a successful thermoelectric EHS for home use.
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4. Conclusions Two thermoelectric applications for home use are here presented and it is shown that in principal thermoelectric EHSs are already today applicable and also affordable, as the setups are mainly done by standard components. The Green Barbecue 2.0 runs completely autarkical whereas this is currently not valid for the thermostat. In the case of the Green Barbecue, the simulation has shown that with the current setup the maximum energy yield is almost reached. To increase beyond the produced power, the construction has to be changed, which means a better connection especially of the cooling element on the cold TEG side, which is now the performancedetermining factor. A further possibility to increase the energy yield is a separate control of the fans, as each TEG block has its own temperature levels. Consequently, there is an optimal duty cycle for the fans on each side. The usage of four MPPTs, one per each TEG block, would also slightly increase the power output, but in contrast significantly increase the costs of the system and is therefore not recommended. The second developed thermoelectric EHS for home use, the thermostat, delivers not enough energy to run selfsufficient. Previous calculations act on the assumption that the thermostat consumes generously 700 J per week, due to the stand-by consumption of 0.3 mW and the consumption of valve movement of 65 mW, which is necessary two times a day. Comparing this value with the real measured and simulation data, the EHS should work fine. But not taken into account are the many inexplicably movements of the valve during especially the night and the weekend. So, the consumption of the thermostat exceeds the energy productions of the EHS. A possible solution of this problem is to intervene in the microcontroller of the bought thermostat and to modify the switching behavior. At least, the time to exchange the batteries will be extended in its current state. Hot 2
Hot 3 Cold 3
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power consumption energy consumption
power produced energy produced
power prod sim energy prod sim
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temperature [°C]
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time, March 22nd, 2016 Fig. 6. Measurement and simulation data for the Green Barbecue at a barbecue on March 21st, 2016 between 11:45 and 13:39 o’clock. Above: temperature curves of each wall side for hot and cold temperature, below: power and energy consumption of the Green Barbecue as well as power and energy production of the EHS and the simulation results for the produced power and energy.
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