In this paper, we propose a method of improving the efficiency of thermoelectric generators (TEGs) by using a lens to concentrate heat on the heat source of a TEG. Initial experiments performed using discrete components show about 60mV increase in the amount of voltage generated when using a magnifying lens. Simulation results on the proposed TEG configuration exhibit up to 16% efficiency when the input heat flux is increased to 500 times that of the sun’s heat flux. The effects of varying the thermoelement length, width, and membrane diameter on the TEG’s performance are also characterized. Lastly, plans to fabricate the device on a SOI wafer in the future are presente

de Leon, Maria Theresa

Kraft, Michael

Taatizadeh, Pouya

English

Southampton (e-Prints Soton)

 IMPROVING THE EFFICIENCY OF THERMOELECTRIC GENERATORS BY USING SOLAR HEAT CONCENTRATORS  M. T. de Leon, P. Taatizadeh, and M. Kraft  University of Southampton; School of Electronics and Computer Science, United Kingdom  Abstract — In this paper, we propose a method of improving  the  efficiency  of  thermoelectric  genera-tors (TEGs) by using a lens to concentrate heat on the heat source of a TEG.  Initial experiments per-formed using discrete components show about 60mV increase  in  the  amount  of  voltage  generated  when using  a  magnifying lens. Simulation  results  on  the proposed  TEG  configuration  exhibit  up  to  16% efficiency  when  the  input  heat  flux is  increased  to 500 times that of the sun’s heat flux. The effects of varying the thermoelement length, width, and mem-brane diameter on the TEG’s performance are also characterized.  Lastly,  plans  to  fabricate the  device on a SOI wafer in the future are presented.  Keywords : Thermoelectric generator, Solar heat concentrator, Carnot efficiency  I - Introduction  The global energy crisis has paved the way for re-searchers  to  explore  alternative  means  of  generating power. One approach to providing electrical energy is by direct conversion of heat to electrical power with the use of thermoelectric generators (TEGs). It is attractive to  use  TEGs  because  they  have  no  mechanical  parts; hence resulting in an alternative  power system that is silent,  stable,  reliable,  environment-friendly,  and  pos-sess virtually unlimited lifetime [1, 2]. The basic principle behind the operation of a TEG is the Seebeck effect. The Seebeck effect produces voltage when a temperature difference is applied between the junctions  of  two  different  materials.  For  a  TEG  to supply a significant amount of power, several thermo-couples are connected electrically in series and thermal-ly in parallel.  Unfortunately, TEGs typically have low Carnot efficiency described by the equation below,      =     −          ⁄     (1) where Th and Tc are the temperatures at the hot and cold sides,  respectively.  A  typical  Peltier  device  exhibits only about 5-8% efficiency. By utilizing a solar heat concentrator in conjunction with  a  TEG,  the  temperature  difference  across  the thermoelements can be increased; subsequently improv-ing  the  TEG’s  efficiency.  Figure  1  illustrates  this innovation with the lens concentrating solar heat at the center  of the  TEG’s  membrane.  A  similar  concept  of focusing light onto a lens to provide the required actua-tion  energy  in  powering  up  a  micro-actuator  using photo-thermal-mechanical  energy  transformation  was proposed in [3] but their design was not fabricated.   Figure  1:  Illustration  of  proposed  TEG  with  solar  heat concentrator system. Suppose the sun uniformly irradiates an energy den-sity  qs  onto  the  lens  and  that  the  lens  has  a  convex surface area AL, then the heat power density qh of the incoming heat flux to the TEG’s membrane is given by:      =            (2) where Ah is the area of the heated surface at the TEG’s membrane.  With  this  approach,  we  can  generate  an input  heat  flux  in  the  order  of  hundreds  of  kW/m2. Based  on  the  general  heat  transfer  equation  [4],  an increase  in  the  input  heat  flux  would  translate  to  a corresponding  increase  in  the  temperature  difference across the thermoelements; also resulting in an effective increase in its output voltage given by:       =      −   Δ     (3) where N is the number of thermocouples and αp and αn are the Seebeck coefficients of the p- and n-type ther-moelements, respectively. The main objective of this work then is to present the feasibility of the proposed system by first conduct-ing  experiments  using  commercially-available  lenses and  a  TEG  module.  Heat  transfer  simulations  of  the proposed  system  are  also  performed  to  validate  the improvement  in  the  efficiency  of  the  TEG.  Plans  to fabricate an integrated lens and TEG system for future work  are  also  presented.  This  microscale  system  has promising  applications  in  on-board  power  sources, sensor networks, and autonomous microsystems.  II – Preliminary Experiment and Results   As proof of concept, we first verified the functionali-ty of using a magnifying lens as the solar heat concen-trator  of  a  commercially-available  TEG  unit.  To  per-form  the  experiment,  a  Farnell  MCPE1-12707AC-S Peltier TEG unit is placed over the center of a Farnell 395-1AB  heat  sink.  A  5x  magnifying  lens  is  then  positioned over the TEG keeping a distance equivalent to its focal length to give the best solar heat concentra-tion  ratio.  To  monitor  the  temperature  changes,  one thermocouple wire is glued to the middle of the TEG unit and another wire is glued 1cm away.  The  output terminals of the TEG are connected to a digital multi-meter that displays the value of the generated voltage. Figure 2 shows a plot of the system’s output voltage with  and  without  the  use  of  a  magnifying  lens.  Two magnifying lenses having diameters of 6cm and 10cm are used. As expected, higher voltage is generated when a lens is used which can only be attributed to a greater temperature  difference  across  the  thermocouple  junc-tions of the TEG. Moreover, using the lens with a larger surface area resulted in a higher output voltage. Unfor-tunately, the thermocouple wires placed on the surface of the TEG unit only gives us an idea of how heat is distributed across the device but not the actual tempera-ture  difference  across  the  thermocouple  junctions. Nevertheless,  the  increase  in  the  amount  of  voltage generated is sufficient to show that the efficiency can be increased by using a heat concentrator.   Figure  2:  Open  circuit  output  voltage  measured  with  the experiment set-up with and without a magnifying lens.  III – TEG Design and Simulations  After verifying that the proposed system can be an effective way of improving the efficiency of thermoe-lectric generators using discrete components, its appli-cation  at  the  micro  level  is  investigated.  Micro-thermoelectric  generators  are  classified  based  on  the direction  of  heat  flow  through  the device  and  on  the layout orientation of the thermocouples during fabrica-tion [5]. We chose to focus on the lateral/lateral type of TEGs because it is the simplest to fabricate and has the most potential for CMOS integration. The configuration  of the proposed  TEG, shown in figure 3, is similar to the one proposed in [6] where the SOI wafer’s device layer is utilized for the suspended membrane and thermoelements. The membrane acts as the heat source of the TEG while the substrate acts as the heat sink. The  suspended  membrane is circular in geometry to insure optimum transfer of heat from the center  of  the  membrane  to  the  tip  of  the  thermoele-ments. The oxide and substrate layers below the mem-brane and the thermoelements are to be etched away to provide better thermal isolation and to optimize the heat   Figure 3: Conceptual design of the TEG device.  flux path so as to obtain the largest temperature differ-ence across the device. The  thermocouple  materials  are  heavily-doped  n-type  silicon  and  aluminum.  Heavily-doped  silicon  is seen to be a viable choice for thermoelement material since it has high Seebeck coefficients at doping levels between  3.5x1019cm-3  to  1.6x1020cm-3  [7,  8].  A  high doping  level  is  also  preferred  because  the  electrical resistivity  of silicon  decreases  with  increasing  dopant concentration; thus providing a smaller series resistance to the TEG device. It is preferred that both thermoele-ments use doped silicon but to minimize the number of masks needed during fabrication, aluminum is selected for the second thermoelement instead. The electrical and thermal  properties  of  the  two  thermocouple  materials are listed in table 1. The assumed dopant concentration of the n-type silicon thermoelement is 5x1019cm-3.  Table 1: Electrical and thermal properties of n-type silicon and aluminum.  Property  n-type silicon  aluminum  references Seebeck  coefficient (µV/K)  -400  -1.8  [7,9] Electrical  resistivity (Ω-cm)  2x10-3  2.65x10-6  [6,10] Thermal  conductivity (W/mK)  122  237  [6,11]  Heat transfer simulations of the proposed TEG de-vice  are  performed  using  COMSOL.  The  SOI  wafer used in the simulations has the following thicknesses:  500µm for the substrate layer, 5µm for the oxide layer, and  50µm  for  the  device  layer.  Although  a  thinner device layer is preferred to obtain a larger temperature drop across the thermocouple, a 50µm device layer is selected to ensure mechanical stability of the TEG. Each thermoelectric leg is set to have a length of 500µm and a width of 20µm. Heat transfer by convection is mod-eled  by  setting  the  top  and  bottom  surfaces  of  the membrane, top surface of the rim, and bottom surface of the substrate to have heat transfer  coefficients corres-ponding to natural external convection with air.   The ambient temperature is set to 20˚C. The membrane to be heated located in the middle of the device has a diame-  Figure 4: Temperature distribution after 10min for a 1mm x 1mm device with 8 thermocouples. Input heat flux is 20kW/m2.  ter  of  5mm.  The  above-mentioned  dimensions  allow 300  thermocouples  to  be  placed  uniformly  along  the circumference  of  the  membrane  with  at  least  6µm distance  between  thermoelements.  The  temperature distribution of a smaller TEG device (1mm x 1mm) is shown in figure 4 to illustrate the heat transfer simula-tions performed.  For the 10mm x 10mm device simulations, only 16 thermocouples  are  distributed  uniformly  along  the circumference  of  the  membrane  so  as  to  minimize simulation time. We expect that having more thermo-couples  would  decrease  the  temperature  difference across each thermocouple as there is an increase in the area of the heat flux path from the membrane to the rim of  the  device  layer.  However,  trends  in  efficiency improvement  indicated  by  changes  in  ∆T  should  still hold. To demonstrate the effect of using a heat concentra-tor, a constant heat flux occupying 1mm2 is applied at the center of the membrane. The value of the input heat flux is  varied  from 50kW/m2 to 500kW/m2. With the solar heat flux equivalent to 1 kW/m2 [12] and based on equation  2,  the  surface  area  of  the  lens  is  effectively varied  from 50mm2 to 500mm2.  The results obtained, listed in table 2, clearly shows that the efficiency of the TEG  improves  with  increasing  input  heat  flux.  This means  that  by  using  a  convex  lens,  the  temperature difference  across  a  thermocouple  can  be  increased; resulting in an increase on the TEG’s efficiency.  Table 2: Temperature difference and efficiency of 10mm x 10mm TEG with 16 thermocouples for varying input heat flux Q. Lens surface area is equivalent to Q/1000 mm2.  Q (kW/m2)  Tc (K)  Th (K)  ∆T (K)  ηc (%) 50  321.482  330.838  9.356  2.828 100  342.356  360.926  18.570  5.145 150  361.222  388.947  27.725  7.128 200  378.943  415.789  36.846  8.862 250  395.894  441.835  45.941  10.398 300  412.279  467.297  55.018  11.774 350  428.227  492.308  64.081  13.016 400  443.823  516.957  73.134  14.147 450  459.132  541.309  82.177  15.181 500  474.199  565.413  91.214  16.132 Simulations  are  also  performed  with  variations  on the dimensions of the thermocouple and on the diameter of the membrane. Higher efficiencies can be achieved by  using  TEGs  with  longer  lengths  and  narrower widths. However, the mechanical stability of the TEG after  etching  out  the  oxide  and  substrate  layers  must also be considered. It is targeted to fabricate different geometries of the device to explore this tradeoff. Cha-racterizations of these variations are exhibited in figures 5-7.     Figure 5: Open circuit output voltage and output power to a matched  load for different thermoelement (TE) lengths.  TE width is 20µm and membrane diameter is 5mm.   Figure 6: Open circuit output voltage and output power to a matched  load  for  different  thermoelement  (TE)  widths.  TE length is 500µm and membrane diameter is 5mm.   Figure 7: Open circuit output voltage and output power to a matched load for different membrane diameters. Thermoele-ment length is 500µm and width is 20µm.  Figure 5 shows that the open circuit output voltage versus  temperature  plot  does  not  change  with  ther-moelement length. It should be noted, though, that the change in temperature increases with the thermoelement length so  a  longer  thermocouple would  have  a  larger temperature  difference across  it,  resulting  in  a  higher output  voltage.    Meanwhile,  a  longer  thermocouple  results in a higher series resistance, which translates to a lower output power under matched load conditions. Variations in the thermoelement width shown in fig-ure 6 also imply  variations in the number of thermo-couples.  A  wider  thermoelement  would  have  lesser number  of  thermocouples  around  the  5mm-diameter membrane.  Hence,  wider  thermoelements  result  in  a drop in the output voltage. The supposed reduction in output power due to the drop in the output voltage is offset  by  the  lower  series  resistance  of  wider  ther-moelements, showing minimal variations of the output power  to  a  matched  load  at  different  thermoelement widths especially at ∆T less than 50K. Similarly,  changing  the  membrane  diameter  also means a change in the number of thermocouples of the device.  A  smaller  membrane  diameter  implies  lesser number of thermocouples. Figure 7 shows that a larger membrane diameter results in a higher  output  voltage and  output  power.  The  only  limiting  factor  for  the membrane  diameter  is  the  structural  stability  of  the device, which is better for smaller membrane diameters.  IV – Fabrication  Several 10mm x 10mm TEGs are to be fabricated at the  Southampton  Nanofabrication  Center.  The  device layer  of  the  SOI  wafer  should  have  resistivity  in  the order of a few kΩ-cm to avoid parasitic resistive coupl-ing  between  adjacent  thermocouples.  The  fabrication begins by RCA cleaning of the SOI wafer. The next step is  doping  of  the  n-type  thermoelements  by  diffusion. Then, the silicon areas of the device layer to be occu-pied by aluminum as the second thermoelement and for electrical contacts are to be patterned using a negative photoresist  and removed  by deep reactive ion etching (DRIE).  Next,  deposition through  e-beam  evaporation and liftoff  of  aluminum  follow to form the aluminum thermoelements  and  the  electrical  connections  of  the thermocouples. After which, patterning and removal of silicon between thermoelements must be performed by DRIE, forming the gaps between thermoelements.   In this step, holes on the membrane are also etched out in preparation of oxide etching. The last step of the TEG fabrication  process  involves  back  etching  of  the  sub-strate and oxide etching by HF vapor phase to release the membrane and the thermoelements.  For the solar heat concentrator, a potential technique for the fabrication of spherical convex lens molds using isotropic  wet  etching  of  silicon  is  presented  in  [13].  These molds can be subsequently used to fabricate the lenses using hot embossing or UV-molding processes. The  lenses  to  be  fabricated  are  estimated  to  have  a diameter of about 10-15mm to maximize the lens area to heated surface area ratio. It is also envisioned that an array of these TEGs may be connected in series to boost the open circuit output voltage.  Inter-TEG  connections  in  this  case  may  be done  internally  by  including  inter-TEG  routing in  the mask  design  for  aluminum  or  externally  using  bond wires. With a TEG array, accurate design of an array of lenses is also critical for proper heat concentration on each TEG’s membrane.   V - Conclusions  This work proposes a method of improving the effi-ciency of thermoelectric generators with the use of solar heat concentrators. Experiments consisting of commer-cially-available magnifying lenses and a thermoelectric module  proved  that  the  use  of  a  heat  concentrator generates higher output voltage. Simulation results of a 10mmx10mm  TEG  device  with  increasing  input  heat flux  demonstrate  an  increase  in  the  device’s  Carnot efficiency. Characterization plots of the output voltage and output power for different thermoelement lengths, widths, and membrane diameters are also discussed as a guide in determining the geometry of several TEGs for fabrication.  Fabrication  processes  under  investigation for both TEG and convex lens are also presented.  VI - Acknowledgements  M. T. de Leon is supported by the University of the Philippines and the Department of Science and  Tech-nology, Philippines.  References  [1]  Y.  Deng  and  J.  Liu,  Journal  of  Renewable  and Sustainable Energy 1, 052701, 2009. [2] M. Strasser, R. Aigner, C. Lauterbach, T.F. Sturm, M. Franosch and G. Wachutka, Sensors and Actua-tors A 114, pp. 362-370, 2004.  [3] S. Baglio, S. Castorina, L. Fortuna, and N. Savalli, Sensors and Actuators A 101, pp. 185-193, 2002. [4]  A.  Bejan  and  A.  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Heat Transfer Handbook,

The Physics Hypertextbook, http://physics.info, Accessed last

Thermoelectric Effects in Metals: Thermocouples, Web-Materials,

Improving the efficiency of thermoelectric generators by using solar heat concentrators

De Leon, M. T.

Taatizadeh, P.

Kraft, Michaël

Open Repository and Bibliography - Liège

Improving the efficiency of thermoelectric generators by using solar heat concentrators

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