Optimized Design of Thermoelectric Energy Harvesting Systems for Waste Heat Recovery from Exhaust Pipes

applied sciences

Article Optimized Design of Thermoelectric Energy Harvesting Systems for Waste Heat Recovery from Exhaust Pipes

Marco Nesarajah * and Georg Frey

Chair of Automation and Energy Systems, Saarland University, Campus A5 1, 66123 Saarbruecken, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-681-3025-7566

Academic Editors: Min-Wook OH and Byung Jin Cho Received: 1 June 2017; Accepted: 15 June 2017; Published: 19 June 2017

Abstract: With the increasing interest in energy efficiency and resource protection, waste heat recovery processes have gained importance. Thereby, one possibility is the conversion of the heat energy into electrical energy by thermoelectric generators. Here, a thermoelectric energy harvesting system is developed to convert the waste heat from exhaust pipes, which are very often used to transport the heat, e.g., in automobiles, in industrial facilities or in heating systems. That is why a mockup of a heating is built-up, and the developed energy harvesting system is attached. To build-up this system, a model-based development process is used. The setup of the developed energy harvesting system is very flexible to test different variants and an optimized system can be found in order to increase the energy yield for concrete application examples. A corresponding simulation model is also presented, based on previously developed libraries in Modelica®/Dymola®. In the end, it can be shown—with measurement and simulation results—that a thermoelectric energy harvesting system on the exhaust pipe of a heating system delivers extra energy and thus delivers a contribution for a more efficient usage of the inserted primary energy carrier.

Keywords: energy efﬁciency; energy harvesting system; exhaust pipe; heating mockup; Modelica; thermoelectric generator; waste heat recovery

1. Introduction Growing environmental awareness and the increasing shortage of fossil fuels lead rethinking in politics and society. Today, a lot of effort and money are invested in environmentally friendly energy production. In addition to completely replacing fossil fuels, which is currently still not possible, a further focus is placed on the increase in efﬁciency of existing processes, which is a very important point and essential for a successful energy transition. As demonstrated in [1], nearly 35% of the inserted primary energy is already lost in the energy sector as waste heat or as transport losses. So, only 65% of the inserted energy reaches the end-user, whereas in this paper, high energy losses, mainly in the form of waste heat, are documented, e.g., in bulbs or in combustion engines of automobiles. Up to this point, only 34% of the primary energy has been used reasonably. In addition to this, the effective energy is not used meaningfully in all cases, e.g., the illumination of an empty room. In total, Ref. [1] comes to the conclusion that only 20% of the originally inserted primary energy is used wisely. In all of these three areas, namely in energy production, the energy conversion and the wise energy usage, big potential for energy saving exists. Regarding the ﬁrst two areas, thermoelectric energy harvesting systems (EHSs) can be used, amongst others, to slightly increase the energy yield. In general, energy harvesting is the transformation of ambient energy sources into small amounts of electrical energy, which are mostly buffered in an accumulator or a supercapacitor [2,3]. The energy

Appl. Sci. 2017, 7, 634; doi:10.3390/app7060634 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 634 2 of 12 sources include light, vibration, radio frequency, water flow or heat and a great advantage is the fact that the energy sources used are normally free of charge and freely available. That is why EHSs are often added to the sustainable energy sector depending on the used energy source, or at least as a system which enhances the energy efficiency of existing processes. If the energy comes from a non-exhaustible source, the EHS actually produces sustainable energy. However, if it is a manmade, exhaustible energy source, the waste energy would normally be staying unused and consequently be lost. So, it is definitely beneficial to use the waste energy with an EHS. A very common understanding of EHS is that the complete system from the transformation of the ambient energy source to the supply of an electrical device is taken into account [4]. Consequently, the EHS consists of an energy converter, electrical energy storage and very often an energy management system as well as the electronics which produce a regulated output voltage for the power supply of electrical devices. A special case of EHSs are thermoelectric EHSs. They use a temperature difference to generate electrical energy with thermoelectric generators (TEGs). The underlying physical effects are the thermoelectric effects, whereby the Seebeck Effect is the key factor for the functionality of TEGs. So far considerable efforts have been made toward enhancing the thermoelectric efficiency of various material classes, including tellurides [5–7], half-Heuslers [8], and silicides [9]. The advantages of the TEGs are that they are quiet, need neither working fluid and they do not have moving parts, are maintenance free and do not pollute the environment. One application is to use the temperature of the exhaust gas line in automobiles with TEG-based EHSs, see [10,11]. Another application is the more efficient use of solar energy, in addition to using only the visible light; the authors in [12] use the invisible, infrared light of the sun. As more than half of the energy coming from the sun is thermal energy, this seems reasonable. Other researchers consider thermoelectric EHSs in combination with phase change materials; see [13,14]. In [15] an experimental study of a thermoelectric generation system for application in exhaust heat of kilns is developed. In addition to this small-scale thermoelectric EHS (which produce only some watts of electrical energy), there are also large-scale thermoelectric EHSs producing a few hundred watts of electrical energy. They are mostly based on radioisotope thermoelectric generators and are applied in satellites, e.g., Apollo 14–17 (ca. 75 W), Galileo (287 W) or Voyager 1 and 2 (ca. 160 W), [16]. Here, a thermoelectric EHS at the exhaust pipe of a heating system is considered. In almost every German household, a heating system is installed; therefore, there is large potential for thermoelectric EHS, considering that 86.8% of heating energy used in Germany (2015) comes from fossil sources [17], and an estimated 71% of those heating systems (2013) have inadequate efficiencies [18]. That is why this contribution, in addition to the many other waste heat processes, deals with those of heating systems. The main idea is to generate electrical energy from the exhaust gas of the heating system. The solution should consist of commercially available components and be integrated as easily as possible into existing systems. In [19], a thermoelectric EHS at a heating system was also developed, but in contrast to this work, the system was directly integrated into a furnace, which would mean a major intervention in a real facility. The gained energy should be used as a primary goal for a variety of direct current (DC) home applications, e.g., to load mobile phones or accumulators. Moreover, as a secondary goal, it may supply the heating system, so it will be autarkical and operational during a blackout as well. According to [20], there was in Germany in 2015 an average power failure per customer (considering also force majeure) of 15.3 min; without force majeure of 11.9 min. This means that over the winter period, enough energy has to be produced by the thermoelectric EHS to overlap the time of blackouts. In the following, to investigate such an EHS, the mockup of a heating system is built up and a thermoelectric EHS is developed and optimized. Appl. Sci. 2017, 7, 634 3 of 12

2. SystemAppl. Sci. Description 2017, 7, 634 3 of 12

2.1. System2. System Analysis Description

As2.1. describedSystem Analysis previously, a thermoelectric EHS for the exhaust pipe of a heating system is here developed. That is why a normal heating system has to be analyzed. However, the exhaust pipe As described previously, a thermoelectric EHS for the exhaust pipe of a heating system is here of thedeveloped. heating systemThat is why can bea normal representative heating system for other has to possible be analyzed. exhaust However, pipes, the e.g., exhaust in industry pipe of or in automobiles.the heating Related system test can benches be representative on heating for systems other possible are already exhaust described pipes, e.g., in [21in ,22industry]. or in Inautomobiles. general, the Related main test electrical benches consumers on heating insystems gas- or are oil-fired already heating described systems in [21,22]. for space heating and domesticIn hot general, water the preparation main electrical are the consumers blower in of gas- the boiler,or oil-fired the electronicsheating systems and for the space different heating pumps. In theseand systems,domestic therehot water are generally preparation two are main the pumps.blower of One the toboiler, supply the theelectronics heat emission and the system different (in this case radiators),pumps. In these which systems, is now calledthere are ‘space generally heating tw pump’,o main pumps. as well asOne the to pump supply for the charging heat emission a domestic hot watersystem storage, (in this calledcase radiators), ‘domestic which hot is water now pump’.called ‘space Additionally, heating pump’, for comfort as well andas the to pump ensure for water charging a domestic hot water storage, called ‘domestic hot water pump’. Additionally, for comfort hygiene and legionella prophylaxis, in many systems an additional ‘hot water circulation pump’ is and to ensure water hygiene and legionella prophylaxis, in many systems an additional ‘hot water installed, which circulates the heated tapping water in the circulation circuit permanently or with circulation pump’ is installed, which circulates the heated tapping water in the circulation circuit definedpermanently control and or with e.g., defined night shut-off. control and e.g., night shut-off. Moreover,Moreover, data data has has to beto be collected collected from from heating heating systemssystems to calculate calculate previously, previously, if ifsuch such an anEHS EHS is promisingis promising or not. or That not. is That why is a measurementwhy a measuremen systemt system was installed was installed in one in building one building at our at university, our whereuniversity, an oil-fired where heating an oil-fired system heating is available.system is avai Tolable. avoid To avoid disturbing disturbing the the real real heating heating system, system, only temperatureonly temperature sensors aresensors attached are attached and the and following the following temperatures temperatures are measured: are measured: room room and and ambient temperature,ambient temperature, outside pipe outside temperature pipe temperature at the beginning at the beginning and in and the in middle the middle of the of exhaustthe exhaust gas gas pipe as well aspipe inside as well pipe as inside temperature pipe temp (equalerature to (equal medium to medium temperature) temperature) at the at beginning the beginning and and in the in the middle. middle. The data are stored in a database and can be visualized via a web interface. An exemplary The data are stored in a database and can be visualized via a web interface. An exemplary typical typical curve progression is shown for the 15th of November 2016 between 08:42 and 10:22 o’clock curve progression is shown for the 15th of November 2016 between 08:42 and 10:22 o’clock in Figure1. in Figure 1.

Figure 1. Measured temperatures at an oil-fired heating on the campus, visualized by the web interface. Figure 1. Measured temperatures at an oil-ﬁred heating on the campus, visualized by the web interface. Visible are the aforementioned temperatures and an approx. 20/80 on/off behavior of the heating. VisibleThe outside are thetemperatures aforementioned of the pipe temperatures have their andmaximum an approx. at about 20/80 74 °C, on/off whereas behavior the inside of the pipe heating. The outsidetemperatures temperatures reach a temperature of the pipe ofhave 163 °C. their For maximumthe time being, at about the temperature 74 ◦C, whereas levels theseem inside to be pipe temperaturesappropriate reach to install a temperature a TEG-based of EHS. 163 ◦C. For the time being, the temperature levels seem to be A second measurement system was installed in a private house. This building represents a appropriate to install a TEG-based EHS. normal one-family house and besides different temperatures, the energy consumptions of the pumps A second measurement system was installed in a private house. This building represents a normal as well as the temperatures of a cockle stove in the living room are measured. Figure 2 shows a typical one-familybehavior house for the and 23rd besides of January different 2016 temperatures, between 00:00 theand energy23:59 o’clock. consumptions The measured of the data pumps are again as well as the temperaturesstored in a database of a cockle and can stove be visualized in the living by the room web are interface. measured. Figure2 shows a typical behavior for the 23rd of January 2016 between 00:00 and 23:59 o’clock. The measured data are again stored in a database and can be visualized by the web interface. Appl. Sci. 2017, 7, 634 4 of 12 Appl. Sci. 2017, 7, 634 4 of 12

InIn the the upper upper diagram diagram of of Figure Figure2 ,2, the the temperatures temperatures areare depicted and and in in the the lower lower diagram diagram the the powerpower consumptions consumptions are are depicted.depicted. Th Thee central central gas-fired gas-fired condensing condensing boiler boiler recovers recovers the latent the latent heat of heat ◦ ofvaporization, vaporization, which which is is reflected reflected in in the the low low maxi maximummum exhaust exhaust gas gas temperatures temperatures of ofabout about 75 75°C. C. Therefore,Therefore, a TEG-baseda TEG-based EHS EHS is is not not suitable suitable for for new new condensing condensing boilers boilers as as the the exhaust exhaust temperatures temperatures are tooare low. too Nevertheless, low. Nevertheless, the on/off the on/off behavior behavior of the of boilerthe boiler is also is also recognizable recognizable for for this this heating heating system. system. The powerThe power consumptions consumptions of the of pumps the pump are,s on are, average, on average, 45 W 45 for W thefor domesticthe domestic hot hot water water pump pump (runs (runs only a fewonly times a few a day),times 6a Wday), for 6 the W space for the heating space pumpheating (runs pump all (runs day) andall day) 2 W and for the2 W hot for water the hot circulation water pumpcirculation (runs almostpump (runs continuously almost continuously between 5:30between and 5:30 23:30 and o’clock). 23:30 o’clock). The consumption The consumption of the of the other componentsother components (like blower (like blower and the and heating the heating electronics) electronics) is about is about 23 W, 23 which W, which results results from from the overall the consumptionoverall consumption minus the minus pump the consumptions. pump consumptions. In addition In addition to that, to the that, gas the consumption gas consumption of the of heating the is alsoheating measured is also measured (diagram is(diagram not shown is not in shown Figure 2in) andFigure during 2) and the during on phase the on of thephase boiler, of the there boiler, is an averagethere is gas an consumptionaverage gas consumption of about 30 of kW about (the 30 gas kW volume (the gas consumed volume consumed is measured is measured and then and multiplied then multiplied by the calorific value for natural gas, 10 kWh/m3). An investigation based on the German by the calorific value for natural gas, 10 kWh/m3). An investigation based on the German BImSchV BImSchV regulation has yielded an efficiency of 93.4%, after which 6.6% of the energy used is lost as regulation has yielded an efficiency of 93.4%, after which 6.6% of the energy used is lost as waste heat. waste heat. If the measured gas consumption is taken as the basis, the exhaust gas has a residual If the measured gas consumption is taken as the basis, the exhaust gas has a residual energy of 1.98 kW. energy of 1.98 kW.

(a)

(b)

Figure 2. Measurement in a private house: (a) Measured temperatures at the heating and the cockle Figure 2. Measurement in a private house: (a) Measured temperatures at the heating and the cockle stove; (b) Power consumptions of the pumps in the heating. stove; (b) Power consumptions of the pumps in the heating. Appl. Sci. 2017, 7, 634 5 of 12

Appl. Sci. 2017, 7, 634 5 of 12 2.2. Heating Mockup 2.2. Heating Mockup To avoid disturbing a real heating system, a heating mockup was planned and ﬁnally built-up in To avoid disturbing a real heating system, a heating mockup was planned and finally built-up our laboratory. As a heat source for the heating mockup, an industrial hot air blower is used, at which in our laboratory. As a heat source for the heating mockup, an industrial hot air blower is used, the temperature and the velocity of the air can be controlled. The dimensions of the exhaust gas pipe at which the temperature and the velocity of the air can be controlled. The dimensions of the exhaust correspondgas pipe correspond to reality. Theto reality. real heating The real system heating on system the campus on the campus is shown is shown in Figure in Figure3a and 3a the and heating the mockupheating is mockup shown inis shown Figure 3inb. Figure 3b. Appl. Sci. 2017, 7, 634 5 of 12

2.2. Heating Mockup To avoid disturbing a real heating system, a heating mockup was planned and finally built-up in our laboratory. As a heat source for the heating mockup, an industrial hot air blower is used, at which the temperature and the velocity of the air can be controlled. The dimensions of the exhaust gas pipe correspond to reality. The real heating system on the campus is shown in Figure 3a and the heating mockup is shown in Figure 3b.

(a) (b)

Figure 3. Real system versus mockup: (a) Real oil-fired heating on the campus; (b) Heating mockup Figure 3. Real system versus mockup: (a) Real oil-fired heating on the campus; (b) Heating mockup in in the laboratory. the laboratory. Real heating data are measured for two different heating systems, as mentioned in the previous (a) (b) subchapter,Real heating and dataare stored are measured in a database. for two With different a self-written heating LabVIEW systems, asprogram mentioned (Version in the 2015 previous SP1 subchapter,(32 bit), National andFigure are 3.Instruments, storedReal system in versus a database.Au stin,mockup: TX, ( Witha )USA), Real oil-fired a self-writtenit is heatingnow possible on LabVIEWthe campus; to control (b program) Heating the mockup heating (Version mockup. 2015 SP1 (32For bit), the National heatin source, the Instruments,laboratory. there are Austin,two main TX, options: USA), to it iscontrol now possiblethe airflow to (t controlemperature the heating and velocity) mockup. Formanually the heat or source,Real automatically. heating there data are are The twomeasured automatic main for options: two mode different is to again controlheating divided systems, the airflow into as mentioned three (temperature options: in the previous to andcontrol velocity) the manuallyheating subchapter,mockup or automatically. according and are stored Theto the in automatic alive database. data modeWithof the a self-written isoil-fired again dividedheating LabVIEW on into program the three campus, (Version options: to 2015 give toSP1 controla special the heatingtime range mockup(32 forbit), the accordingNational stored Instruments, data to the in livethe Au database, datastin, TX, of theUSA), which oil-fired it iswill now then heating possible be reproduced on to control the campus, the by heating the to heating give mockup. a specialmockup, time For the heat source, there are two main options: to control the airflow (temperature and velocity) rangeor to for give the a txt-file stored as data temperature in the database, profile. which With th willis mockup, then be it reproduced is now possible by the to heatingtest a thermoelectric mockup, or to manually or automatically. The automatic mode is again divided into three options: to control the EHS at the exhaust pipe without interrupting a real system. give a txt-fileheating as temperaturemockup according profile. to the With live data this of mockup, the oil-fired it isheating now on possible the campus, to test to give a thermoelectric a special EHS at the exhausttime range pipe for without the stored interrupting data in the database, a real which system. will then be reproduced by the heating mockup, 3. Developmentor to give a of txt-file the Energyas temperature Harvesting profile. WithSystem this mockup, it is now possible to test a thermoelectric EHS at the exhaust pipe without interrupting a real system. 3. DevelopmentTo develop of the the EHS Energy for the Harvesting exhaust Systempipe of the heating, a development process, which has alreadyTo develop 3.been Development presented the EHS of inthe for [23] Energy the is exhaust used.Harvesting The pipe Systemmodi offied the scheme heating, is ashown development in Figure process,4 and the which EHS is has developed according to this guideline. The steps in the red frame can also be realized within a already beenTo presented develop the in EHS [23 ]for is the used. exhaust The pipe modified of the heating, scheme a development is shown process, in Figure which4 and has the EHS ismultiphysics developedalready according simulation. been presented to this in guideline.[23] is used. The The modi stepsfied inscheme the redis shown frame in Figure can also 4 and be the realized EHS is within a multiphysicsdeveloped simulation. according to this guideline. The steps in the red frame can also be realized within a multiphysics simulation.

Figure 4. Model-based development process for thermoelectric energy harvesting systems [23]. FigureFigure 4. 4.Model-based Model-based development development process for thermo thermoelectricelectric energy energy harvesting harvesting systems systems [23]. [23 ]. Appl. Sci. 2017, 7, 634 6 of 12 Appl. Sci. 2017, 7, 634 6 of 12

Regarding the development process, the selection of the heat source and the settings of the objectivesRegarding have already the development been done in process, the previous the selection section. The of the other heat steps source can be and summarized the settings to of five the mainobjectives steps, havewhich already are the been ‘Thermal done Design’, in the previous the ‘Sel section.ection of The Thermoelectric other stepscan Device’, be summarized the ‘Mechanical to five Design’,main steps, the ‘Electrical which are Desi the ‘Thermalgn’ and ‘Control Design’, Concepts’. the ‘Selection of Thermoelectric Device’, the ‘Mechanical Design’,Concerning the ‘Electrical the thermal Design’ design, and ‘Controla segment Concepts’. cooling aggregate, [24], which is originally designed to coolConcerning down a flowing the thermal medium, design, is installed a segment in the cooling exhaust aggregate, gas pipe. [24 It], is which built up is originallyof cooling designed elements, to whichcool down penetrate a flowing into the medium, medium, is installed collect the in the heat exhaust and transport gas pipe. it It to is builtthe outer up of side cooling of the elements, pipe, seewhich Figure penetrate 5a. There, into the the TEGs medium, and collectthe correspondi the heatng and cooling transport units it tocan the be outereasily side attached. of the This pipe, componentsee Figure is5a. used There, to collect the TEGs as much and heat the as corresponding possible from cooling the exhaust units pipe can (the be easilyheat of attached. the medium). This Thiscomponent is, of course, is used an tointervention collect as much in the heat real as process possible and from that the is exhaustwhy a heating pipe (the mockup heat of was the planned medium). andThis finally is, of course,built-up an in interventionour laboratory. in the Consequently, real process all and following that is why results a heating are valid mockup for a washot flowing planned mediumand finally in a built-uppipe, but in cannot our laboratory. be transferred Consequently, directly to alla real following system, results e.g., a real are validheating, for before a hot flowing doing moremedium studies in a on pipe, the but influence cannot befor transferred the real proce directlyss. toFirst a real measurements system, e.g., aon real the heating, mockup before with doing the installedmore studies clean on segment the influence cooling for theaggregate real process. and a First real measurements heating behavior on the are mockup shown with in theFigure installed 5b. Theclean temperatures segment cooling for aggregateeach section and aof real the heating segment behavior cooling are aggregate—in shown in Figure total5b. The there temperatures are four sections—arefor each section measured. of the segment cooling aggregate—in total there are four sections—are measured.

160 140 120 Temp1 100 Temp2 80 Temp3 60 Temp4

temperature [°C] temperature 40 20 0 15:26 15:36 15:46 15:56 16:06 16:16 16:26 th 5 of February, 2016 (a) (b)

FigureFigure 5. 5. ThermalThermal design totocollect collect the the heat: heat: (a )( segmenta) segment cooling cooling aggregate aggregate to extract to extract heat from heat ﬂowing from flowingmedium; medium; (b) Temperature (b) Temperature measurement measurement of each of section each sectio of then segmentof the segment cooling cooling aggregate aggregate for a 20/80 for a heating20/80 heating load. load.

At each section, four thermoelectric devices can be attached, which are consequently under the At each section, four thermoelectric devices can be attached, which are consequently under the same temperature conditions. In [25], it was shown that in some cases expensive TEGs can be replaced same temperature conditions. In [25], it was shown that in some cases expensive TEGs can be replaced by cheap thermoelectric coolers (TEC, Peltier element). According to [25], for the first two sections, by cheap thermoelectric coolers (TEC, Peltier element). According to [25], for the first two sections, TEGs are used and TECs are used for the third and fourth sections. TEGs are used and TECs are used for the third and fourth sections. The next step of the development process, the heat transfer of the waste heat, will be done with The next step of the development process, the heat transfer of the waste heat, will be done with controlled air forced cooling elements. These are more suitable for real heating with an on/off scenario controlled air forced cooling elements. These are more suitable for real heating with an on/off scenario of 20/80 than liquid cooling elements, as described in [26]. Due to the lack of space, special small of 20/80 than liquid cooling elements, as described in [26]. Due to the lack of space, special small central processing unit (CPU) cooling elements are attached and the fans of the cooling elements are central processing unit (CPU) cooling elements are attached and the fans of the cooling elements are only mounted at the outside on the first and the fourth section. The fans on the first section blow air only mounted at the outside on the first and the fourth section. The fans on the first section blow air through the heat sinks and the fans on the last section pull the air from the heat sinks. So, there is through the heat sinks and the fans on the last section pull the air from the heat sinks. So, there is airflow from the first section to the fourth. The mechanical setup of the heating mockup is shown in airflow from the first section to the fourth. The mechanical setup of the heating mockup is shown in Figure 6. There, the four sections and the temperature sensors are visible for these sections; the inside Figure6. There, the four sections and the temperature sensors are visible for these sections; the inside pipe temperature sensor at the beginning, the cooling elements and the fans for Sections 1 and 4 are pipe temperature sensor at the beginning, the cooling elements and the fans for sections 1 and 4 are also visible. The blue arrow in Figure 6b shows the air flow direction through the cooling elements. also visible. The blue arrow in Figure6b shows the air flow direction through the cooling elements. Regarding the last hardware point of the development process, the electrical design, the prospective Regarding the last hardware point of the development process, the electrical design, the prospective was to be very flexible. That is why a switch cabinet with a front panel was built up (see Figure 7a). was to be very flexible. That is why a switch cabinet with a front panel was built up (see Figure7a). As the thermoelectric devices of one section have the same temperature differences, they are As the thermoelectric devices of one section have the same temperature differences, they are connected connected in parallel. The individual blocks can then be individually interconnected (see front panel in parallel. The individual blocks can then be individually interconnected (see front panel in Figure7b). in Figure 7b). There are different connection possibilities, e.g., TEG blocks in series or in parallel or a There are different connection possibilities, e.g., TEG blocks in series or in parallel or a mixture of both, mixture of both, connecting them to one or two MPPTs, connecting some TEG blocks with the fans Appl. Sci. 2017, 7, 634 7 of 12

Appl. Sci. 2017, 7, 634 7 of 12 connectingAppl. Sci. them2017, 7, to634 one or two MPPTs, connecting some TEG blocks with the fans or supplying7 of 12 the or supplying the fans over a 12 V power supply. Beneath the front panel, the switch cabinet also fans over a 12 V power supply. Beneath the front panel, the switch cabinet also includes an industrial includesor supplying an industrial the fans PC over to controla 12 V thepower hardware supply. setup Beneath via thethe frontLabVIEW panel, program, the switch an cabinet accumulator, also PC to control the hardware setup via the LabVIEW program, an accumulator, the two MPPTs, the 12 V theincludes two MPPTs, an industrial the 12 PCV power to control supply, the hardware some relays setup (for via systemthe LabVIEW reconfiguration program, an during accumulator, operation powermode;the supply, two condition: MPPTs, some all the relays TEG 12 Vblocks(for power system in supply, series reconﬁguration and some between relays (forthe during systemrelays), operation reconfiguration a vehicle mode;plug toduring condition:connect operation different all TEG blocksloadsmode; in as series condition:well andas different betweenall TEG measurementblocks the relays), in series a devices.and vehicle between Al plugl thethe to measurementrelays), connect a vehicle different data plug loadsare to alsoconnect as wellwritten different as differentin the measurementdatabaseloads as and well devices. can as be different visualized All the measurement measurement by the previously devices. data mentioned are Al alsol the writtenmeasurement web interface. in the databasedata The are following also and written can measurements be in visualized the byare thedatabase carried previously andout: can mentioned be visualized web by interface.the previously The mentioned following web measurements interface. The arefollowing carried measurements out: are carried out: • • Temperatures:Temperatures: at at ﬁrst/second/third/fourth first/second/third/fourth sect section,ion, inside inside pipe pipe beginni beginning,ng, inside inside pipe pipe end end • Temperatures: at first/second/third/fourth section, inside pipe beginning, inside pipe end • • Voltages:Voltages: Input Input MPPT1/MPPT2, MPPT1/MPPT2, OutputOutput MPPTs (equ (equalal to to accumulator accumulator voltage), voltage), hot hot air air blower blower • Voltages: Input MPPT1/MPPT2, Output MPPTs (equal to accumulator voltage), hot air blower • Current: Input MPPT1/MPPT2, Output MPPTs, hot air blower • Current:• Current: Input Input MPPT1/MPPT2, MPPT1/MPPT2, Output Output MPPTs, hot hot air air blower blower

Figure 6. Heating mockup with attached thermoelectric energy harvesting systems (EHS); (a) shows Figure 6. Heating mockup with attached thermoelectric energy harvesting systems (EHS); (a) shows Figurethe complete 6. Heating setup; mockup (b) shows with anattached enlargement thermoelec of a partialtric energy excerpt harvesting of a; (c) shows systems an enlargement(EHS); (a) shows of the complete setup; (b) shows an enlargement of a partial excerpt of (a); (c) shows an enlargement of thea completepartial excerpt setup; of (b.b ) shows an enlargement of a partial excerpt of a; (c) shows an enlargement of b a partiala partial excerpt excerpt of of ( b.).

(a) (b)

Figure 7. Switch cabinet of the(a Heating) Mockup: (a) complete switch cabinet; (b) Enlargement(b) of the front panel to realize different wiring structures. Figure 7. Switch cabinet of the Heating Mockup: (a) complete switch cabinet; (b) Enlargement of the Figure 7. Switch cabinet of the Heating Mockup: (a) complete switch cabinet; (b) Enlargement of the front panel to realize different wiring structures. front panel to realize different wiring structures. Appl. Sci. 2017, 7, 634 8 of 12 Appl. Sci. 2017, 7, 634 8 of 12

4.4. Simulation Simulation Model Model ToTo simulate simulate thermoelectric thermoelectric EHSs, EHSs, a a Modelica Modelica lib libraryrary called called EHSTEG EHSTEG (Energy (Energy Harvesting Harvesting Systems Systems basedbased on on Thermoelectric Thermoelectric Generators) Generators) was wasdeveloped developed and applied; and applied; more information more information about the aboutmodeling the ofmodeling TEGs can of be TEGs found can in be [27,28]. found With in [27 this,28]. library, With this a complete library, asystem complete can systembe reproduced can be reproduced and this is alsoand done this isfor also the doneheating for mockup the heating with mockup the attach withed EHS the attached(see Figure EHS 8). (seeThe advantage Figure8). The with advantage Modelica iswith thatModelica graphically, is thatit looks graphically, very similar it looks to the very real similarsystem, towhich the realmakes system, it easily which usable makes and ensures it easily thatusable different and ensures physical that principals different can physical be simulated principals in one can simulation be simulated environment in one simulation (electrical, environment thermal, and(electrical, so on). Visible thermal, in andFigure so on).8 is the Visible heating in Figuremockup8 is consisting the heating of the mockup hot air consisting blower, the of exhaust the hot gas air pipeblower, and the the exhaust segment gas cooling pipe andaggregate the segment components cooling. The aggregate EHS is components.presented in the The simulation EHS is presented model byin heat the simulation transfer paste model (grey by components), heat transfer air paste gaps (grey (pale components), blue components), air gaps the (paleTEGs/TECs blue components), represented bythe the TEGs/TECs yellow/green represented components by the as yellow/green well as the cooling components elements as well with as theand cooling without elements fans. The with blue and connectionwithout fans. lines The present blue connection the electrical lines present wiring the be electricaltween the wiring thermoelectric between the devices, thermoelectric which devices,can be adaptedwhich can to bethe adapted real wiring to the at real the wiring front panel. at the frontIn the panel. simulation In the simulationmodel, the model,measured the measuredhot medium hot temperaturesmedium temperatures of the real of heater, the real which heater, are which stored are in storedthe database, in the database, act as simulation act as simulation input. input.

FigureFigure 8. 8. SimulationSimulation model model of of the the thermoel thermoelectricectric EHS EHS at at the the heating heating mockup. mockup.

5.5. Results Results and and Discussion Discussion ToTo test test the the prospective prospective EHS EHS for for a a real real heating heating sc scenario,enario, the the real real heating heating data data for for the the oil-fired oil-ﬁred heatingheating on on the the campus campus of of the the 15 15 November November 2016 2016 between between 08:42 08:42 o’clock o’clock and and 10:22 10:22 o’clock o’clock are are given given to theto theHeating Heating Mockup, Mockup, cf. cf.Figure Figure 1. 1Thereby,. Thereby, the the following following was was selected selected as as the the connection connection variant, becausebecause it it gives gives the the best best result: result: TEG TEG block block 1 1 and and 2 2 are are connected connected in in series series and and then then to to MPPT1, MPPT1, whereas whereas TECTEC block block 3 3 and and 4 4 are are also also connected connected in in series series and and suppl supplyy the the fans fans of of Section Section 1.1. Thus, in in this this scenario, scenario, aa fan fan control control over over the the duty duty cycle cycle is is not not necessary, necessary, as as the the fan fan velocity velocity will will be be self-controlled self-controlled by by the the energyenergy supported supported from TEC block 3 and 4. Consequently, thethe powerpower delivereddelivered from from TEG TEG block block 1 1 and and 2 2can can be be completely completely considered considered gained gained power power of the of system. the system. The measurements The measurements as well as as the well simulation as the simulationresults are results shown are in Figure shown9 ,in where Figure the 9, upperwhere diagramthe upper shows diagram the shows measured the measured temperatures temperatures as well as asthe well set temperatureas the set temperature inside the inside pipe and the the pipe lower and diagramthe lower shows diagram the measuredshows the gainedmeasured power gained and powerenergy and as well energy as the as well simulated as the ones.simulated As there ones. are As no there additional are no consumersadditional consumers connectedand connected the fans and are thedirectly fans suppliedare directly by TECsupplied block by 3 and TEC 4, block there is3 noand direct 4, there power is no consumption. direct power In consumption. the upper diagram In the of upperFigure diagram9, it is visible of Figure that the 9, reproducedit is visible that inside the pipe reproduced temperature inside of thepipe medium temperature ﬁts very of wellthe medium with the fitsset very temperature well with of the the set medium temperature (which of is the also medium the real (which measured is also temperature) the real measured for temperatures temperature) over for80 ◦temperaturesC. However, over for lower 80 °C. temperatures However, for there lower are temperatures some overshoots, there are which some are overshoots, related to thewhich setting are relatedoptions to of the the setting hot air blower—itoptions of canthe onlyhot air be adjustedblower—it for can temperatures only be adjusted over 80 for◦C andtemperatures due to that over fact, 80 °C and due to that fact, it turns on briefly and then off again to control lower temperatures. Moreover, it is obvious that the temperatures of the single sections (now, with the attached EHS) are Appl. Sci. 2017, 7, 634 9 of 12

Appl. Sci. 2017, 7, 634 9 of 12 it turns on brieﬂy and then off again to control lower temperatures. Moreover, it is obvious that the ◦ temperaturesall under 90 °C, of theand single for the sections last section (now, the with temper theatures attached are EHS)approximately are all under 50 °C. 90 Consequently,C, and for the the last ◦ sectionproduced the temperaturespower is very arelow; approximately this is visible 50 in theC. Consequently,lower diagram the of Figure produced 9. The power maximum is very gained low; this ispower visible is in around the lower 2 W, diagramwhich also of matches Figure9 with. The the maximum simulation gained result. power Since the is around oil-fired 2 heating W, which starts also matchesabout 39 with times the a simulation day, the daily result. energy Since output the oil-ﬁred is approx. heating 36.4 starts kJ, which about is 39 about times 10 a day,Wh thea day. daily energyConcerning output that is approx. the number 36.4 of kJ, heating which isdays about for 10this Wh region a day. is Concerningon average about that the 263 number days according of heating daysto [29]; for this will region be 2.63 is on kWh average per year. about 263 days according to [29]; this will be 2.63 kWh per year.

set inside pipe first section second section

180 inside pipe third section fourth section 160 140 120 100 80 60

temperature [°C] 40 20 power produced power produced sim energy produced energy produced sim 2.5 3

2.0 2 1.5

1.0 1 power [W] energy [kJ] 0.5

0.0 0 02:13 PM 02:33 PM 02:53 PM 03:13 PM 03:33 PM 03:53 PM st time, December 21 , 2016

FigureFigure 9. 9.Measurement Measurement andand simulationsimulation data for for the the heat heatinging mockup: mockup: heating heating scenario scenario of ofthe the oil-fired oil-ﬁred heatingheating on on campus campus onon 1515 NovemberNovember 2016 between 8:42 8:42 a.m. a.m. and and 10:22 10:22 a.m., a.m., executed executed on on 21 21December December 20162016 at at the the Heating Heating Mockup.Mockup. Above: temperature temperature curves curves of of each each section section asas well well as asthe the set set and and real real insideinside pipe pipe temperature temperature atat thethe beginning;beginning; below: below: produced produced power power and and energy energy of ofthe the EHS EHS and and the the corresponding simulation results. corresponding simulation results.

To calculate the efficiency of the EHS, Figure 10 represents the system consideration relating to the Toheat calculate flow rates. the The efficiency energy ofbalance the EHS, is given Figure by 10 represents the system consideration relating to the heat flow rates. The energy balance is given by 0= − −use −lost , (1) . . . . where is the inflow heat rate, 0 =theQ 1outflow− Q2 − heatQuse rate,− Q lostuse, the useful heat rate and lost the (1) lost heat. rate. With the assumption. of no heat losses, the useful. heat rate can be calculated .by where Q1 is the inflow heat rate, Q2 the outflow heat rate, Quse the useful heat rate and Qlost the lost = − = ⋅ ⋅ − , heat rate. With the assumption of nouse heat losses, the useful heat rate can be calculated by (2) The temperatures and as well as the mass flow rate are measured or calculated values, . . . . whereas the specific heat capacityQuse = ofQ 1the− Qmedium2 = m · cancp · (beT1 taken− T2) from, the simulation. Thus, the (2)

useful heat rate here is approx. 560 W at the peaks, cf. Figure. 9. The produced power is 2 W and so Thethe temperatures efficiency is aboutT1 and 0.36%,T2 as acco wellrding as the to mass the following flow rate equation:m are measured or calculated values, whereas the specific heat capacity c of the medium can be taken from the simulation. Thus, the useful heat p el TEG = , (3) rate here is approx. 560 W at the peaks, cf. Figure9. Theuse produced power is 2 W and so the efficiency is about 0.36%, according to the following equation: where TEG is the efficiency of the TEG-based EHS and el the produced electrical power.

Pel ηTEG = . , (3) Quse where ηTEG is the efﬁciency of the TEG-based EHS and Pel the produced electrical power. Appl. Sci. 2017, 7, 634 10 of 12 Appl. Sci. 2017, 7, 634 10 of 12

FigureFigure 10. 10. SchematicSchematic system system consideration consideration of of the the heat heat flow ﬂow rates rates for for the the EHS EHS at at the the heating heating mockup.

To conclude, it can be said that the presented EHS can harvest electrical energy from the exhaust To conclude, it can be said that the presented EHS can harvest electrical energy from the exhaust gas of a heating system and thus it delivers a contribution for a more efficient usage of the inserted gas of a heating system and thus it delivers a contribution for a more efficient usage of the inserted primary energy carrier. The efficiency of the entire system is currently not very high; however, since primary energy carrier. The efficiency of the entire system is currently not very high; however, since the the waste heat is normally unused, such systems can be worthwhile. The energy amount is high waste heat is normally unused, such systems can be worthwhile. The energy amount is high enough enough to supply small DC applications, e.g., to charge a mobile phone. Also, it is possible to supply to supply small DC applications, e.g., to charge a mobile phone. Also, it is possible to supply the the heating and their components during a short blackout, but for this purpose, an accumulator is heating and their components during a short blackout, but for this purpose, an accumulator is needed needed to store the electrical energy over the year—which, it has to be admitted, could also be loaded to store the electrical energy over the year—which, it has to be admitted, could also be loaded by the by the electrical grid. electrical grid. To improve the EHS and to increase the efficiency of the heating, the water to be heated could To improve the EHS and to increase the efficiency of the heating, the water to be heated could be be used as a cooling medium and thus, the water will be preheated (less energy input of the heating used as a cooling medium and thus, the water will be preheated (less energy input of the heating is is necessary) and the thermoelectric devices will produce more electrical power. necessary) and the thermoelectric devices will produce more electrical power. It can be summarized that especially if there is a need for a robust, maintenance free and reliable It can be summarized that especially if there is a need for a robust, maintenance free and reliable power supply and the investment costs do not matter, thermoelectric EHS are highly recommended, power supply and the investment costs do not matter, thermoelectric EHS are highly recommended, e.g. for usage in space applications. To increase the energy yield, the number of TEGs or the e.g., for usage in space applications. To increase the energy yield, the number of TEGs or the temperature difference applied over the thermoelectric devices has to be enhanced. temperature difference applied over the thermoelectric devices has to be enhanced. Author Contributions: Marco Nesarajah conceived, designed and performed the experiments, analyzed the data Authorand wrote Contributions: the paper. GeorgMarco Frey Nesarajah supported conceived, the work designed through and its expertise, performed knowle the experiments,dge, feedback analyzed and input. the data and wrote the paper. Georg Frey supported the work through its expertise, knowledge, feedback and input. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Quaschning, V. 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