Abstract:
The Seebeck effect is the generation of a voltage between two junctions of dissimilar materials, and this effect is used to convert heat to electricity using thermoelectric modules containing a plurality of junctions. The efficiency of power generation using these modules is typically very low and much lower than rotating machines such as gas turbines and steam turbines combined with rotating electrical generators. This disclosure presents a method for increasing the efficiency of these thermoelectric modules significantly by thermally switching the heat source to the thermoelectric elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 61/583,222, filed Jan. 5, 2012 and from U.S. Provisional Application Ser. No. 61/606,037, filed Mar. 2, 2012, the contents of which are incorporated hereby by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Thermoelectric devices are versatile in that they can cool, heat, and convert heat to electricity. A single solid state device can accomplish all three of these functions. These devices are not used in large scale application, however, because of their poor efficiency. Instead, rotating machines like compressors, gas turbines, steam turbines, and electrical generators are used for these functions. The desire to use silent, solid state devices with no moving parts is very strong and hence the need for highly efficient thermoelectric devices is also very strong. 
         [0003]    The understanding of the efficiency of thermoelectric devices has traditionally been defined for a static configuration of a constant temperature difference applied to either side of a semiconductor material. A voltage is generated in such a configuration that is proportional to the temperature difference, and this effect is called the Seebeck effect. Electrical power is generated from the temperature difference. Because semiconductor materials have high thermal conductivity, the conductive flow of heat from the hot side to the cold side dramatically reduces the energy conversion efficiency because this heat is wasted and not used to generate power. The traditional static configuration of temperatures applied to each side of the thermoelectric device results in conductive heat flow (loss) that is proportional to the temperature difference as described by the heat transfer equation. 
         [0004]    In the prior art, switching of thermoelectric devices has been employed for cooling purposes. For example, see “Efficient Switched Thermoelectric Refrigerators for Cold Storage Applications” by U. Ghoshal and A. Guha,  Journal of Electronic Materials  DOI: 10.1007/s11664-009-0725-3, March 2009. In this paper, the authors describe how using a thermal diode and an electrical switch may be combined with a thermoelectric device to increase its efficiency in cooling applications. US patent application 2011/0016886 describes an implementation of the switched thermoelectric cooling system. 
         [0005]    The prior art for cooling does not indicate how switching can increase the efficiency of a thermoelectric device when generating electricity from heat. An entirely different switching system is required to be combined with the thermoelectric device for power generation. In power generation mode, the thermoelectric module needs to be combined with a thermal switch and an electrical diode. In the prior art cooling mode, the additional components were a thermal diode and an electrical switch. 
         [0006]    Thermal switching of a thermoelectric module for purposes of matching a temperature-varying energy source has been disclosed and analyzed in “Enhancing Thermoelectric Energy via Modulations of Source Temperature for Cyclical Heat Loadings” by R. McCarty, K. P. Hallinan, B. Sanders, and T. Somephone,  Journal of Heat Transfer, Transactions of the ASME,  Volume 129, June 2007, but this paper does not mention the use of thermal switching for a constant energy source wherein the switching is designed to increase conversion efficiency from heat to electricity. 
         [0007]    Hence, the need exists for a more efficient configuration and use of thermoelectric devices for converting heat to electricity. 
       SUMMARY OF THE INVENTION 
       [0008]    In this invention, we allow the heat source to be coupled and decoupled dynamically in order to turn off the lossy conductive heat flow while still maintaining a temperature difference that can generate electricity for a period of time. The end result is electrical energy continues to be generated while the input heat is not being tapped, and the energy of the overall system is increased by several times. 
         [0009]    In one aspect of the invention there is provided an electrical generator characterized by comprising, in combination, a thermoelectric module, a heat source, a thermal switch, and an electrical diode. 
         [0010]    In one embodiment of the invention, the generator may include one or more of the following features:
       (a) further including a capacitor for storing electrical energy;   (b) wherein the thermoelectric module preferably includes a semiconductor material; wherein the semiconductor material includes elements of both n and p types connected electrically in series;   (e) wherein the thermoelectric module contains one or more thermo-tunneling elements;   (d) wherein the heat source comprises a pipe with fluid flowing inside;   (e) wherein the heat source comprises sunlight collected onto a bulk material;   (f) wherein the heat source comprises flames or other hot gases;   (g) wherein the thermal switch comprises a motorized iris mechanism pushing one or more thermoelectric modules periodically against and periodically pulling away from the heat source;   (h) wherein the thermal switch is comprised of a memory metal whose shape changes with temperature adapted to periodically push the thermoelectric module against and periodically pull it away from the heat source;   (i) wherein the heat source comprises collected sunlight and the thermal switch is comprised of a concentrator that shifts the sunlight periodically to and periodically not to the thermoelectric module, wherein the shifting is accomplished by an actuator or by rotation of the earth or a combination thereof;   (j) wherein the thermoelectric modules are mounted on a linear tube which slides between a heat source and a cold source; wherein the tube preferably is motorized in a reciprocal fashion which causes the thermoelectric modules periodically to make contact with the heat source and periodically to remove them from the heat source; or wherein the tube is motorized in a rotary motion which causes the thermoelectric modules periodically to make contact with the heat source and periodically to remove them from the heat source;   (k) further including a voice coil motor which provides periodic forces for causing the thermoelectric module to make and break contact with the heat source; and   (l) wherein the thermoelectric module is encased in a vacuum enclosure.       
 
         [0023]    In one embodiment, the generator may be characterized by further including a boundary material attached to the heat source. 
         [0024]    In another embodiment, the generator may be characterized by one or more of the following features;
       (a) wherein the thermoelectric module periodically makes contact with the boundary layer;   (b) wherein the boundary layer is made from a high thermal conductivity and high heat capacity material selected from the group consisting of copper, gold and silver; and   (c) wherein the boundary layer is optimized to rapidly raise the temperature of another material coming in contact with it; and wherein the boundary layer preferably is comprised of soft flexible graphite or metal to allow surface matching with one side of the thermoelectric module over a period of time.       
 
         [0028]    In one embodiment of the invention the generator is characterized in that electrical power of a periodically varying voltage is collected over time and stored as electrical energy. 
         [0029]    In another embodiment of the invention the generator may be characterized by one or more of the following features:
       (a) further including a DC voltage converter to match the voltage of the generator with that of the load;   (b) including a synchronized inverter to match the AC voltage of the load;   (c) comprising multiple thermoelectric modules whose thermal switches are out of phase so as to provide a more constant voltage level over time; and   (d) wherein multiple thermoelectric modules are employed together with series and parallel electrical connections to achieve a desired voltage output level.       
 
         [0034]    In one embodiment of the invention, the generator is characterized in that the thermal switch is a material whose thermal conductivity can change or be changed. 
         [0035]    In another embodiment of the invention, the generator may be characterized by one or more of the following features:
       (a) wherein the thermal switch comprises a material that changes state from crystalline to amorphous;   (b) wherein the thermal switch comprises carbon black;   (c) wherein the thermal switch comprises a material that changes phase from solid to liquid;   (d) wherein the change in thermal conductivity is activated by temperatures naturally occurring in the generator; and   (e) wherein the change in thermal conductivity is activated by an applied voltage by a voltage driver that is synchronized with the desired thermal switching.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]      FIG. 1  shows a basic thermoelectric element and how the Seebeck effect is employed to generate electricity from heat that is manifest as a temperature difference. 
           [0042]      FIG. 2  shows the basic configuration of the invention wherein a thermoelectric module with a few elements is combined with a thermal switch and an electrical diode. 
           [0043]      FIG. 3   a  is similar to  FIG. 2 , with the addition of a boundary layer to improve efficiency, and  FIGS. 3   b - 3   e  are graphs showing the prior art ( FIG. 3   b ) and examples of the present invention ( FIGS. 3   c - 3   e ). showing the generation of electrical power over time as the heat source is switched on and then off. 
           [0044]      FIGS. 4   a - 4   c  show three different embodiments for the thermal switching using mechanical motion. 
           [0045]      FIG. 5  shows another embodiment of the invention where a tube with thermoelectric devices mounted on the outside slides into alternating contact with a hot source and then a cold source. 
           [0046]      FIG. 6  shows another embodiment where the tube rotates instead of slides. 
           [0047]      FIG. 7  shows another embodiment wherein a voice coil actuates the thermoelectric module in and out of contact with the heat source. 
           [0048]      FIG. 8  is an apparatus used to measure the increased efficiency of the invention vs. the prior art static thermal environment. 
           [0049]      FIG. 9  shows the voltage generated by the apparatus of  FIG. 8  displayed on an oscilloscope. 
           [0050]      FIG. 10  illustrates the calculations used to demonstrate the increased electrical energy that is generated with the invention switched thermal environment vs. the prior art static thermal environment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]      FIG. 1  illustrates the basic Seebeck effect for converting heat to electricity. Two materials A  101  and B  102  are joined at junctions AB  103  and BA  104 . Typically material A  101  is a metal and material B  102  is a semiconductor. The voltage generated is proportional to the temperature difference T 2 -T 1  and the constant of proportionality is the Seebeck coefficient S AB  of the two materials. In prior art implementations, a constant temperature difference is applied between the two junctions. The very low efficiency of this effect, even for optimized material selection, is due the high thermal conductivity of material B  102  causing much of the heat from the heat source to flow to the cold side. This flow of heat represents a loss for the module because it is not converted to electricity. 
         [0052]    Heat flow through a material takes time, and the time constant of heat flow in  FIG. 1  is the heat capacity of material B  102  times the thermal conductivity of material B  102 . The Seebeck effect is immediate, however, and the voltage appearing across the junctions AB  103  and BA  104  is instantaneously equal to S AB *(T 2 −T 1 ), even prior to any heat flowing into material B. In this invention, the instantaneity of the Seebeck effect (power generation) in contrast with the delayed heat flow effect (loss) is exploited to achieve higher efficiency. 
         [0053]      FIG. 2  illustrates the invention of switching the heat source  201  against the hot side  202  of a thermoelectric module  203 . The thermoelectric module  203  consists of a plurality of junctions as illustrated in  FIG. 1  connected electrically in series and thermally in parallel. The semiconductor material  204  alternates between n type and p type, which causes all of element voltages to sum together to produce the module voltage. In the prior art implementations, the heat source  201  would be in contact with one side of the module continuously. In this invention, the heat source is  201  in contact momentarily, and raises the temperature of the upper junctions to a high temperature. The electricity generated from this momentary contact is captured and stored in the capacitor  205 . Before much of the heat from the heat source  201  flows into the semiconductor elements  204 , the heat source  201  is pulled away from the upper junctions  207 . As a result, the full Seebeck voltage is captured in the capacitor prior to the large losses from heat flow to the cold side  207  are able to occur. 
         [0054]    The diode  206  in  FIG. 2  prevents the electricity stored in the capacitor  205  from being delivered back to the thermoelectric module  203  when heat source  201  is not in contact. 
         [0055]      FIGS. 3   b - 3   e  show graphs of the behavior of the prior art ( FIG. 3   b ) as well as the switched thermoelectric configuration of  FIG. 2  with the addition of a boundary layer  309  ( FIG. 3   a ) to improve efficiency further. For both the prior art ( FIG. 3   a ) and the invention cases in  FIGS. 3   b - 3   e,  the following assumptions are made: (1) the same thermoelectric module is used, (2) the heat source has the same temperature, and (3) the cold side has the same temperature. 
         [0056]    On the right side of  FIG. 3  are graphs of power output for several different types of boundary materials. The area under the curve of a power graph represents energy. The top graph  301  ( FIG. 3   b ) shows the case for the prior art wherein the heat source  201  had been applied continuously and the junction temperatures have reached steady state. In this case, the area A  305  represents the total energy generated by the prior art approach with a static heat source. The remaining graphs show different cases of boundary layers attached to the heat source with the switching of the invention applied. 
         [0057]    The second graph  302  ( FIG. 3   c ) shows the case for a boundary layer  309  that has similar thermal and geometric properties as the thermoelectric semiconductor (low thermal conductivity and low heat capacity). In this case, the temperature (and hence the voltage generated) of the hot side  202  rises exponentially with a time constant of the boundary material  309 . When the heat source  201  is removed, the voltage drops exponentially with a time constant of the thermoelectric material  204 . In this case, the energy produced in this process is B+D which is approximately equal to area A=B+C, so not much gain over the prior art. 
         [0058]    The third graph  303  ( FIG. 3   d ) shows a case where the boundary layer  309  is chosen to have thermal properties opposite of the semiconductor  204 , i.e. high heat capacity and high thermal conductivity. In this case, the temperature of the upper junctions  202  rises much faster, and so does the voltage as shown in the graph  303 . Now, the total energy generated is B+D which is greater than the energy of the prior art A=B+C. 
         [0059]    The fourth graph  304  ( FIG. 3   e ) shows another case with the optimized boundary layer  202 , but the contact time of the heat source  201  is reduced. In this case B+D&gt;&gt;B+C indicating an even greater benefit over the prior art ( FIG. 3   a ). 
         [0060]    As  FIGS. 3   b - 3   e  illustrate, the benefit of the invention is maximized when the boundary layer material  201  is has the highest possible heat capacity and the highest possibly thermal conductivity. In this case, the momentary contact produces the fastest temperature rise in the upper junctions  202  and approaches the temperature of the heat source  201  with a minimal temperature gradient between the heat source  201  and the upper junctions  202 . 
         [0061]    Without limitation, in configuring the entire system for the invention, the heat source material is its original container, which could be water in a power plant, a selective surface for solar heat, a silicon chip for scavenging electronics heat, or whatever material happens to be the container of the heat. The thermoelectric module should be made from the highest ZT material that is practically available. The boundary layer is optimized to raise the junction temperature as fast as possible for the given heat source and the given thermoelectric module. 
         [0062]      FIGS. 4   a - 4   c  show several embodiments for implementing the thermal switching portion of the invention. In all cases, it is assumed that the electrical output of the thermoelectric modules  402  is connected through a diode to an electrical load that receives the power generated, as illustrated in  FIG. 2 . 
         [0063]      FIG. 4   a  shows an iris mechanism  401  used to push multiple thermoelectric modules  402  into a pipe or other heat source with a pentagonal cross-section. The thermoelectric modules  402  are shown at the ends of the iris mechanism  401 , and the heat source is not shown but intended to be in the center. The iris mechanism  401  works similarly to that used to regulate the amount of light through a camera lens. As the iris segments  407  are rotated, the hole in the center becomes smaller thereby pushing one side of the thermoelectric modules temporarily against a heat source. The iris segments  407  are rotated by a motor, which is not shown in  FIG. 4   a , but said motor operates to achieve periodic momentary contact of the modules  402  to the heat source. 
         [0064]      FIG. 4   b  shows another mechanism wherein a wire  403  made of nitinol or similar material changes its shape in response to temperature. The wire  403  is pre-programmed to have higher curvature when cold and lower curvature when hot. Then, it will pull the thermoelectric module  402  away from the heat source  201  when enough heat has traversed through the module to the nitinol  403 , and will push the module  402  toward the heat source  201  when enough heat has dissipated from the module. A repetitive motion of contact and no contact can be achieved with the proper pre-programming of the nitinol wire  403 . 
         [0065]      FIG. 4   c  shows a third mechanism wherein the heat source is from concentrated sunlight  404 . The sunlight  404  is concentrated on a selective surface  405  on one side of the module  402 , heating it up. Later, the concentrated sunlight  404  is removed from this module  402  and, without limitation, shifted to another module. This movement of the concentrated light  404  may be achieved, without limitation, by physically moving the optics or by the rotation of the earth or a combination of these. 
         [0066]    In all cases of  FIGS. 4   a - 4   c,  the thermoelectric module  402  may be encased in a vacuum enclosure  406 , as illustrated in  FIG. 4   c , to prevent premature oxidation or other degradation of the module parts from the intense heat. 
         [0067]    Another thermal switching mechanism is shown in  FIG. 5 . Here, a linear square pipe  502  in the center carries a cold fluid and a spiral hot-fluid pipe  504  has surfaces parallel to the central cold pipe  502 . A linear, hollow, square tube  501  has thermoelectric devices  503  mounted on the sides. This tube slides in between the fluid-carrying pipes  502 ,  504 , and  505 . The inner sides of the thermoelectric modules  503  are always in thermal contact with the central cold pipe  502 . The outer sides are either in thermal contact with a hot pipe  504  or, when the linear position of the tube is shifted, in thermal contact with another pipe  505 . The second spiral pipe  505  is optional, but provides a means to remove, store, and recover heat from prior contacts with the hot spiral pipe  504 . 
         [0068]    In  FIG. 5 , a motorized or other mechanism (not shown) periodically shifts the tube  501  linearly to apply heat to the outer side of the thermoelectric modules  503  momentarily, then shifts back to stop drawing heat from the hot spiral pipe  504 . By reciprocating the linear tube  501  back and forth, the thermal switching is accomplished to achieve the behavior and the gain in efficiency illustrated in  FIGS. 3   c - 3   e.    
         [0069]    The reciprocating motion of the tube in  FIG. 5  above might be difficult to achieve with inexpensive hardware. And, typically reciprocating motions require more energy than continuous rotary motion because of the momentum reversals.  FIG. 6  illustrates a similar implementation as  FIG. 5  but using rotary motion to accomplish the thermal switching. 
         [0070]    In  FIG. 6 , the hollow tube  601  has a round cross section with curved thermoelectric devices mounted on it. Also, the spacing between the cold central pipe  605  and the linear outer pipes  603  and  604  has a round cross section that snugly accommodates the tube  601 . By rotating the tube  601  inside the pipes, the outer sides of the thermoelectric modules  602  are placed in periodic momentary thermal contact with the hot pipe  604  while the inner side of the modules is always in contact with a cold pipe  605 . The mechanism of  FIG. 6  could also be reciprocating to avoid wrapping of wires or electrical brush contacts. The tube  601  with the thermoelectric modules  602  would rotate  90  degrees, and then rotate back - 90  degrees in each cycle. 
         [0071]      FIG. 7  shows another embodiment of the invention. A voice coil  701 , which is commonly used in loudspeakers, is the actuating mechanism for pushing the thermoelectric module  703  into contact with the heat source  201 , and then pulling it away. In this implementation, one watt of electrical power generated more than enough force in the voice coil  701  to lift the 256-element thermoelectric module  703 . Without limitation, the contact side of the heat source  201  may include a layer of flexible, soft graphite film  702 . These graphite films are available from GrafTech International of Parma, Ohio, USA, and they have thermal conductivity greater than 100 watts per meter per degree Kelvin, which is comparable to hard metals. Because of the softness of these graphite films, the surface will automatically conform to the irregularities on the hot side surface of the thermoelectric module  703 , thereby making good thermal contact. 
         [0072]      FIG. 8  shows a two-pellet embodiment of the invention wherein one element  801  is n-type and the other  802  is p-type. The bottoms of the elements are soldered to copper pads  803  on a circuit board  805 . A thin copper foil bridge  804  is soldered to the tops of the elements. This copper bridge  804  is thick enough to have a small electrical resistance as compared to the two elements, but otherwise is as thin as possible to have minimal thermal mass. That is to say, the copper thickness is chosen to optimally trade off the energy losses of electrical resistance of the copper with the thermal mass of the copper. The small thermal mass allows for a fast temperature rise when the copper bridge  804  contacts the heat source. Because the generated electricity (Seebeck) is related to the temperature, a fast rise in temperature results in the most electrical energy generated. 
         [0073]    To measure the performance of the two-element embodiment of  FIG. 8 , a heat source with a flat surface (in this case a soldering pencil with a flat tip with an attached graphite pad) was brought downward and placed momentarily in contact with the copper bridge  804  in  FIG. 8 . The oscilloscope picture  901  in  FIG. 9  shows the voltage produced  902 . When the heat source was physically applied, the voltage ramped up quickly  903  as the temperature of the copper bridge  804  rose. When the heat source was physically removed, the voltage generated exhibited an exponential decrease  904  back to zero as shown in the trace of  FIG. 9 . 
         [0074]    The rise time  903  in  FIG. 9  was about 0.5 seconds, and this voltage rise is normalized and re-represented in the first 0.5 seconds of the blue-lined graph  1001  in  FIG. 10 . The exponential decay  904  after the heat was removed is copied to the rest of the blue line  1002  in  FIG. 10 . The flat portion  905  of the oscilloscope trace was taken out, simulating the removal of the heat source after 0.5 seconds. 
         [0075]    In thermoelectric power generation, the electrical power generated is proportional to V 2 , where V is the voltage if the load is resistive. The red line  1003  in  FIG. 10  represents the square of the normalized voltage values in the blue line  1001  and  1002 . 
         [0076]    Energy is the integral of power over time. Graphically, energy is the area under the curve of power as a function of time. In  FIG. 10 , the area under the red line  1003  indicates the electrical energy that can be produced from the invention device if the heat source is in contact from time 0 to time 0.5 seconds. In prior art thermoelectric implementations, the heat source is connected in steady state with the hot side of the thermoelectric device. The voltage generated in steady state is a constant, and, after normalization, stays at a level of 1. The square of 1 is 1, so the normalized power produced is also 1 for the prior art implementation. 
         [0077]    If we compare normalized electrical energy produced by the invention device (the area under the red line  1003  in  FIG. 10 ) with the normalized electrical energy produced by the prior art thermoelectric device (the shaded area  1004  in  FIG. 10 ), we see that the invention produced more electrical energy than the traditional thermoelectric device when the amount of heat input is the amount of heat drawn from the source between time 0 and time 0.5 seconds in  FIG. 10 . 
         [0078]    The electrical energy generated may be compared quantitatively by computing the area under the red curve  1003  and comparing it to the shaded area  1004 . The area under the red curve  1003 , assuming the energy harvesting is stopped at time 3.5 seconds to be ready for the next cycle, is 1.55 normalized units. The shaded area  1004  representing the prior art thermoelectric device is 0.5 normalized units. Hence, the invention device produced three times as much electrical energy as the prior art for the same heat energy input. 
         [0079]    In the embodiments described, the thermal switch was always shown as a physical mechanism that brought the hot side of the thermoelectric module in contact with the heat source momentarily and periodically. Without limitation, the thermal switch also could be accomplished by a layer of special material that changes its thermal conductivity momentarily and periodically. Phase change materials that have much greater thermal conductivity in the crystalline state and lower thermal conductivity in the amorphous state are an example of materials for this purpose. Carbon black materials that are used in resettable fuses also could serve this purpose. The material changes its state from crystalline when cold to amorphous when hot. Liquid crystal materials change their phase in response to an electrical potential, allowing for the thermal switch to be electrically activated and de-activated.