Patent Publication Number: US-9885502-B2

Title: Methods and systems for solid state heat transfer

Description:
This application is a continuation of U.S. patent application Ser. No. 13/050,555, entitled METHODS AND SYSTEMS FOR SOLID STATE HEAT TRANSFER, filed on Mar. 17, 2011 and is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Many known thermoelectric devices utilize the Peltier effect for heating or cooling. Typically, a direct current is provided across a semiconductor material and/or across an array of interconnected semiconductor materials of different types (e.g., semiconductor materials having different doping properties). As a result of the current, the junction either generates heat or rejects heat (cooling). The heating or cooling effect of thermoelectric devices is used in a number of contexts ranging from superconductivity to microprocessor cooling devices to electric beverage coolers. 
     The Figure of Merit (FOM) describing the performance of typical thermoelectric devices is given by Equation (1) below:
 
 Z=S   e   2   k   e   /k   T   (1)
 
In Equation (1), Z is the FOM; S e  is the Seebeck coefficient of the device; k e  is the electrical conductivity of the device and k T  is the thermal conductivity. It can be seen that the performance of thermoelectric devices (Z) is proportional to electrical conductivity and inversely proportional to thermal conductivity. In other words, high electrical conductivity of the device material improves performance, while high thermal conductivity of the device material degrades performance. Accordingly, the ideal material for thermoelectric devices would be one that has a high electrical conductivity and a low thermal conductivity. Unfortunately, for most practical materials, electrical and thermal conductivity are proportional. As a result, existing thermoelectric devices suffer performance degradation due either to an excess of thermal conductivity or a lack of electrical conductivity.
 
    
    
     
       FIGURES 
       Various embodiments of the present invention are described here by way of example in conjunction with the following figures, wherein: 
         FIG. 1  illustrates a diagram showing one embodiment of a thermoelectric element. 
         FIG. 2  illustrates a diagram showing one embodiment of an alternate configuration of the thermoelectric element of  FIG. 1 . 
         FIGS. 3( a )-3( d )  illustrate a process flow showing the operation of one embodiment of the thermoelectric element of  FIG. 1 or 2 . 
         FIG. 4  is a chart illustrating a temperature-entropy (TS) cycle of the thermoelectric element, during implementation of the process flow of  FIGS. 3( a )-3( d ) . 
         FIGS. 5( a )-5( d )  illustrates a process flow showing the operation of one embodiment of a plurality of thermoelectric elements operating in parallel. 
         FIGS. 6( a )-6( b )  illustrate a process flow showing the operation of one embodiment of a compound thermoelectric element. 
         FIG. 7  illustrates a diagram of one embodiment of a heat switch. 
         FIG. 8  illustrates a diagram of another embodiment of a heat switch. 
     
    
    
     DESCRIPTION 
     Various embodiments are directed to thermoelectric devices for transmitting heat energy from one thermodynamic body to another utilizing heat switches and current provided in varying directions. The devices described herein may be used to cool a body (e.g., a waste-heat generating device such as a piece of electronic or mechanical equipment, a device to be cooled for operation such as a cooler, refrigerator or superconductive material, etc.). The devices described herein may also be used, in some embodiments, to heat bodies. 
     According to various embodiments, the devices described herein may comprise one or more thermoelectric elements thermally positioned between a thermal reservoir (e.g., to be heated) and a thermal load (e.g., to be cooled). Each thermoelectric element may comprise a first component made from a first material and a second component made from a second material. The first component and the second component may be in electrical contact with one another via an interface which may, for example, comprise an electrically conductive material, such as a metal. The first and second materials may have different Seebeck coefficients such that current across the interface in one direction tends to generate heat at the interface and, in some embodiments, draw heat to the interface while current across the interface in the opposite direction tends to push heat away from the interface, resulting in cooling. For example, in various embodiments the first material may be p-type semiconductor material and the second material may be n-type semiconductor material. The interface may be alternately thermally coupled and thermally insulated from the thermal reservoir and thermal load via one or more heat switches. For example, a heat switch may have a first terminal and a second terminal. When the temperature differential across the first and second terminals exceeds a heat switching threshold, the heat switch may close, creating a thermally conductive path between the first and second terminals. Similarly, when the temperature differential across the first and second terminals drops below the heat switching threshold, the heat switch may open, thermally insulating the first and second terminals. 
     In use, a control circuit may cause electric current to flow, across the interface in a first direction, causing the interface to heat. This may raise the temperature differential between the interface and the thermal reservoir, causing the first heat switch to close, thereby allowing heat from the interface to be dissipated to the thermal reservoir (e.g., ambient air or other material). When enough heat is dissipated to reduce the differential between the interface and the thermal reservoir, the first heat switch may re-open. The control circuit may alternately cause electric current to flow across the interface in a second direction, opposite the first, causing the interface to cool. Cooling the interface may increase the temperature differential between the interface and the thermal load and may, in some embodiments, generate heat on its own. This may, in turn, cause the second heat switch to close, allowing heat from the thermal load to be transferred to the interface until the temperature differential between the interface and the thermal load drops below a threshold for the second heat switch, at which point the second heat switch may close, thermally isolating the interface from the thermal load. In this way, the thermoelectric element may operate as a heat pump to pump heat away from the thermal load and towards the thermal reservoir. Also, in some embodiments, the operation of the heat switches may serve to prevent undesirable heat conduction in the thermoelectric element, mitigating efficiency losses due to thermal conductivity. 
       FIG. 1  illustrates a diagram showing one embodiment of a thermoelectric device  100 . The device  101  may comprise a thermoelectric element  100 . The thermoelectric element  100  may comprise a first component  102  and a second component  104  in electrical contact with one another at an interface  106 . The first and second components  102 ,  104  may be made from and/or may contain materials having different Seebeck coefficients such that current across the first and second components  102 ,  104  in a first direction causes cooling at the interface  106  and current across the first and second components  102 ,  104  in a second direction causes heating at the interface  106 . For example, in various embodiments, the first component  102  may be made from and/or contain p-type semiconductor while the second component  104  may be made from and/or contain n-type semiconductor. In embodiments where the components  102 ,  104  are made from different kinds of semiconductors, the element  100  may be configured such that the p-type and n-type materials do not create a rectifying pn junction. For example, it may be possible to pass current from p-to-n as well as from n-to-p. This may be achieved in any suitable manner. For example, the interface  106  may be made from and/or include a metal or other conductive material that may form an ohmic connection with one or both of the components  102 ,  104 . In various embodiments, the interface  106  may electrically separate the components  102 ,  104  by a distance exceeding the thickness of a hypothetical depletion region that would result from joining the first and second components  102 ,  104 . 
     The thermoelectric device  101  may also comprise first and second heat switches  108 ,  110  alternately creating and blocking thermal connections between the interface  106  and an ambient thermal reservoir  112  on the one hand, and a thermal load  114  on the other hand. The heat switch  108  may have a first terminal  116  in thermal contact with the interface  106  and a second terminal  118  in thermal contact with the thermal reservoir or ambient  112 . The thermal reservoir or ambient  112  may be a thermal mass that may receive heat from the element  100 . For example, the thermal reservoir or ambient  112  may be the ambient air or other material surrounding the element  100 . The heat switch  110  may have a first terminal  120  in thermal contact with the interface  106  and a second terminal  120  in thermal contact with a thermal load  114 . The thermal load  114 , for example, may be an object to be cooled such as a sensor, microprocessor, etc. Each of the heat switches  108 ,  110  may have an open state, where the respective heat switch terminals  116 ,  118 ,  120 ,  122  are thermally isolated from one another and a closed state where the respective terminals are in thermal contact with one another. The heat switches  108 ,  110  may be actuated in any suitable manner. For example, in some embodiments, the heat switches may be actuated between closed and open states by a control circuit  130 , as described below. Further, in some embodiments, the heat switches  108 ,  110  may be thermal switches that are actuated between closed and open states, for example, based on a temperature differential across the respective heat switches  108 ,  110 . For example, when the temperature differential exceeds a closing threshold, a thermal switch may close. When the temperature differential drops below an opening threshold, the thermal switch may open. According to various embodiments, one-way thermal switches may be used. One-way thermal switches may open or close only in response to temperature differentials in one direction. For example, switch  108  may be a one-way switch that closes in response to the interface  106  becoming warmer relative to the ambient  112  and opens in response to the interface  106  becoming cooler relative to the ambient  112 . 
     A control circuit  130  may control operation of the device  101 . For example, the control circuit  130  may control the presence or absence, as well as the direction, of current passed through the components  102 ,  104  and interface  106  of the element. For example, the control circuit  130  may comprise and/or be in communication with one or more power supplies for generating current. Further, in some embodiments, the control circuit  130  may control actuation of the heat switches  108 ,  110 , as described herein. According to various embodiments, the control circuit  130  may be in communication with various sensors for sensing the state of device  101  parts. For example, temperature sensors  132 ,  134 ,  136  may be in communication with the control circuit  130  to provide temperature information describing temperatures of the interface  106 , the ambient  112  and the thermal load  114 , respectively. Also, in some embodiments, the control circuit  130  may be in communication with sensors  138 ,  140  for sensing the position of the heat switches  110 ,  108 , respectively. The control circuit  130  itself may be any suitable form of control device including, for example, an analog circuit, a digital circuit, a mixed analog and digital circuit, etc. In some embodiments, the control circuit  130  may comprise a microprocessor. 
       FIG. 2  illustrates a diagram showing one embodiment of an alternate configuration of the thermoelectric device  101 . The device  101 , as shown in  FIG. 2 , demonstrates that many different spatial configurations are possible. For example, in  FIG. 2 , the interface  106  is illustrated connecting non-adjacent sides of the components  102 ,  104 . It will be appreciated that any suitable configuration may be used. 
       FIGS. 3 and 4  illustrate the operation of one embodiment of the thermoelectric device  101 .  FIG. 3( a )-3( d )  illustrate a process flow showing the operation of one embodiment of the thermoelectric device  101 .  FIG. 4  is a chart  400  illustrating a temperature-entropy (TS) cycle of the thermoelectric device  101  during implementation of the process flow of  FIGS. 3( a )-3( d ) . In  FIG. 4 , temperature, or T is indicated on the vertical y-axis, while entropy or S is indicated on the horizontal x-axis. 
     Referring now to  FIG. 3( a ) , the control circuit  130  may cause a current (indicated by arrow  302 ) to flow across the interface  106  from the component  102  to the component  104  (e.g., from p-type material to n-type material). Both of the heat switches  108  and  110  may be open, preventing heat transfer between the interface  106  and either the ambient  112  or the thermal load  114 . The current  302  from p-type to n-type may cause the interface  106  to heat. The TS characteristics of the thermoelectric element  100  in the configuration shown in  FIG. 3( a )  are illustrated by curve  402  of the TS chart  400 . As illustrated, the entropy of the element  100  may decrease slightly, while the temperature increases as the interface  106  heats. 
     As the interface  106  heats, a temperature differential between the interface  106  and the ambient  112  may increase. When the temperature differential exceeds a closing threshold of the heat switch  108 , the heat switch  108  may close, creating a thermally conductive path between the interface  106  and the ambient  112 . At or about the closing of the heat switch  108 , the control circuit  130  may cause the current  302  to cease. This configuration is illustrated in  FIG. 3( b ) . With the heat switch  108  closed and the current  302  turned off, heat from the interface  106  may be released to the ambient. This process is illustrated in  FIG. 4  by curve  404 . As shown, the temperature of the element  100  may stay roughly constant, while the entropy of the element decreases as heat is lost to the ambient  112 . In embodiments where the heat switch  108  is controlled by the control circuit  130 , the control circuit  130  may close the heat switch based a signal received from one or more sensors  132 ,  134  indicating the temperature differential between the interface  106  and the ambient  112  or upon the occurrence of any other suitable condition (e.g., the passage of a predetermined time from the initiation of the current  302 , the passage of a predetermined level of charge through the components  102 ,  104  and interface  106  from the current  302 , etc.). 
     As heat is released to the ambient  112 , the temperature differential between the ambient  112  and the interface  106  may decline. When the temperature differential between the ambient  112  and the interface  106  declines below an opening threshold of the heat switch  108 , the heat switch  108  may open, causing substantial thermal isolation between the interface and the ambient  112 . In embodiments where the heat switch  108  is controlled by the control circuit  130 , the control circuit  130  may open the switch  108 , for example, based on a determination of the temperature differential between the ambient  112  and the interface  106  received from the sensors  132 ,  134  and/or any other suitable criteria (e.g., the passage of a predetermined amount of time from the closing of the switch  108 ). 
     At or about the opening of the heat switch  108 , the control circuit  130  may cause a current, indicated by arrow  304 , to flow across the interface  106  from the component  104  to the component  102  (e.g., from n-type to p-type). The current  304  may cause cooling at the interface  106 . For example, the current  304  may drive heat away from the interface  106 . The configuration of the element  100  with the heat switch  108  open and the current  304  flowing from n-type to p-type is shown in  FIG. 3( c ) . The TS properties of the element  100 , as illustrated in  FIG. 3( c )  are shown by curve  406  of the chart  400 . As illustrated, the temperature of the element  100  may decline, while the entropy increases. 
     As the temperature of the interface  106  declines, the temperature differential between the interface  106  and the thermal load  114  may increase. When this temperature differential exceeds a closing threshold of the heat switch  110 , the heat switch  110  may close, creating a thermally conductive path between the interface  106  and the thermal load  114 . In embodiments where the heat switch  110  is actuated by the control circuit  130 , the control circuit  130  may determine when to close the heat switch  110  based, for example, on temperature readings received from sensors  132  and  136  indicating the temperature differential between the interface  106  and the thermal load  114  and/or any other suitable criteria (e.g., the passage of a predetermined time from the initiation of the current  302 , the passage of a predetermined level of charge through the components  102 ,  104  and interface  106  from the current  304 , etc.). 
     At or about the time that the heat switch  110  closes, the control circuit  130  may cause the current  304  to cease, resulting in the configuration of the element  100  shown in  FIG. 3( d ) . Thermal energy generated by the thermal load  114  may propagate from the relatively hot thermal load  114  to the relatively cold interface  106 . The TC properties of the element  100 , as illustrated in  FIG. 3( d ) , are shown by the curve  408 . As illustrated, the temperature of the element  100  may remain about constant, while the entropy of the element  100  may increase. 
     As heat energy from the thermal load  114  is transferred to the interface  106 , the temperature differential between the thermal load  114  and the interface  106  may decline until it reaches an opening threshold of the heat switch  110 . At this point, the heat switch  110  may open causing substantial thermal isolation between the thermal load  114  and the interface  106 . At or about the opening of the heat switch  110 , the control circuit  130  may cause the current  302  to flow, as illustrated in  FIG. 3( a ) . In embodiments where the heat switch  110  is controlled by the control circuit  130 , the control circuit  130  may open the switch  110 , for example, based on a determination of the temperature differential between the thermal load  114  and the interface  106  received from the sensors  132 ,  134  and/or any other suitable criteria (e.g., the passage of a predetermined amount of time from the closing of the switch  110 ). The process may continue in this manner as long as desired. In this way, the element  100  may pump heat from the thermal load  114  to the ambient  112 . 
     According to various embodiments, the control circuit  130  may utilize the sensors  132 ,  134 ,  136 ,  138 ,  140  to control the various components of the element  100  to implement the process flow shown in  FIGS. 3( a )-3( d ) . For example, the control circuit  130  may receive a signal indicating that the switch  108  is either open or about to open, indicating that the current  302  should be stopped. The signal may, for example, originate from sensor  140 , which may indicate a state of the switch  108 . In some embodiments, the signal may be received from one or both of the sensors  132 ,  134 , which (e.g., collectively) may indicate a temperature differential between the ambient  112  and the interface  106 . The control circuit  130  may initiate the current  304  upon receiving a signal indicating that the switch  108  has closed or is about to close (e.g., from one or more of sensors  140 ,  132 ,  134 ). Similarly, the control circuit  130  may receive information about the state of the switch  110  and/or the temperature of the interface  106  and thermal load  114  via sensors  138 ,  132 , and  136 . This information may be used by the control circuit  110  to initiate and stop the current  304 , as described above. 
     As described above, the control circuit  130  may, in some embodiments, also control the actuation of the heat switches  108 ,  110 . For example, the control circuit  130  may sense the temperature(s) of the ambient  112 , thermal load  114  and interface  106  (e.g., via sensors  134 ,  136  and  132 ). When the opening and/or closing thresholds of the heat switches  108 ,  110 , are reached, the control circuit  130  may cause the appropriate switch  108 ,  110  to open or close, for example, as described above. For example, the switches  108 ,  110  may comprise components that may be brought into physical contact, and therefore thermal contact, with one another on command of the control circuit  130  (e.g., through the use of one or more stepper motors, solenoids, magnetic fields, etc.). Other example heat switches  108 ,  110  may comprise microelectronic machines (MEMS) and/or nano components actuated piezoelectrically and/or electrostatically. In other embodiments, the switches  108 ,  110  may comprise voids that may be alternately filled and emptied of a conductive gas, such as, for example, helium. 
     According to various embodiments, multiple thermoelectric elements may be utilized in parallel. Figure s  5 ( a )- 5 ( d ) illustrates a process flow showing the operation of one embodiment of a thermoelectric device  501  comprising a plurality of thermoelectric elements  500  operating in parallel. The thermoelectric elements  500  may be elements, such as elements  100  of  FIGS. 1 and 2 , described above. For example, each of the thermoelectric elements  500  may comprise first and second components in electrical contact with one another at an interface, paralleling the first and second components  102 ,  104  and interface  106  described above. The elements  500  may be connected in parallel. For example, the interfaces of each of the elements  500  may be thermally connected to a common ambient or thermal reservoir  502  via heat switches, paralleling the heat switch  108  described herein. Further, the interfaces of the elements  500  may also be thermally connected to a common thermal load  504  via heat switches paralleling the heat switch  110  described herein. A control circuit  506  may be constituted and operate similar to the control circuit  130  described above. According to various embodiments, the control circuit  506  may control each of the thermoelectric elements  500  collectively. Also, in some embodiments, the control circuit  506  may comprise multiple portions that may or may not be in communication with one another, with each portion independently controlling one or more of the thermoelectric elements  500 . 
     As illustrated in  FIG. 5( a ) , the control circuit  506  may cause a current  508  to flow across the interfaces of the elements  500  in a direction tending to cause the interfaces to cool (e.g., from p-type material to n-type material). In this way, each of the elements  500 , as depicted in  FIG. 5( a )  may operate in a manner similar to the element  100  as depicted in  FIG. 3( a ) . The interfaces of the elements  500  may heat, while the heat switches connecting the interfaces of the elements  500  to the common ambient  502  and the common thermal load  504  may be open, thermally isolating the elements from the ambient  502  and thermal load  504 . As the temperature of the interfaces of the elements  500  increases, the temperature differential between the interfaces of the elements  500  and the common ambient  502  may increase to or above a closing threshold of the heat switches separating the elements  500  from the common ambient  502 . This may cause these heat switches to close, resulting in a thermal connection between the interfaces of the elements  500  and the common ambient  502 . Alternatively, the control circuit  506  may close the switches based on any suitable criteria. At or about the closing of the switches between the elements  500  and the common ambient  502 , the control circuit  506  may cause the current  508  to cease. An example configuration of the elements  500  in this state is shown by  FIG. 5( b ) . As shown in  FIG. 5( b ) , each individual element may be configured, and may operate, in a manner similar to that described above with respect to  FIG. 3( b ) . For example, heat energy concentrated at the interfaces of the elements  500  may be conducted and/or dissipated to the common ambient  502 . 
     As the temperature differential between the elements  500  and the common ambient  502  decreases, the switches separating the interfaces of the elements  500  from the common ambient  502  may open, causing substantial thermal isolation between the elements  500  and the common ambient  502 . (E.g., the switches may open when the temperature differential between the elements  500  and the common ambient  502  reaches and/or exceeds an opening threshold, or as determined by the control circuit  506 .) At or about the opening of the switches between the elements  500  and the common ambient, the control circuit  506  may cause a current  510  to flow through the elements  500  in a direction that causes cooling of the respective interfaces (e.g., from n-type material to p-type material). This configuration is illustrated in  FIG. 5( c ) . The configuration and operation of each individual thermoelectric element  500  shown in  FIG. 5( c )  may be similar to the operation of the element  100 , as illustrated in FIG.  3 ( c ). For example, the interfaces of the elements  500  may cool as heat is driven away from the interfaces. As the elements  500  cool, the temperature differential between the interfaces of the elements  500  and the common thermal load  504  may increase until reaching or exceeding a closing threshold of the heat switches between the interfaces of the elements  500  and the common thermal load  504 . When the differential reaches or exceeds the closing threshold of the heat switches between the interfaces of the switches between the interfaces of the elements  500  and the common thermal load  504 . At or about the closing of these switches, the control circuit  506  may cause the current  510  to cease. 
       FIG. 5( d )  shows the device  501  after the cessation of the current  510  and after the switches between the interfaces of the elements  500  and the common thermal load  504  have closed (e.g., in response to temperature differentials between the interfaces and common thermal load  504  or by the control circuit  506 ). The configuration and operation of each individual thermoelectric element  500  shown in  FIG. 5( d )  may be similar to the operation of the element  100 , as illustrated in  FIG. 3( d ) . For example, heat from the common thermal load  504  may flow to the elements  500  until the temperature differential between the interfaces of the elements  500  is reduced to or below an opening threshold of the relevant switches, at which point the switches may close. At or about the closing of the switches (e.g., the switches between the interfaces of the elements  500  and the common thermal load  504 ), the control circuit  506  may begin the current  508 , putting the elements  500  in the configuration shown in  FIG. 5( a ) . The process may continue as long as desired and may have the effect of transferring heat from the common thermal load  504  to the common ambient  502 . The use of multiple thermoelectric elements  500  in the device may increase the capacity of the system compared to the use of only a single element  100  in the device  101 . 
       FIGS. 6( a )-6( b )  illustrate a process flow showing the operation of one embodiment of a compound thermoelectric device  601 . The compound thermoelectric element may comprise one or more thermoelectric elements  605 ,  603  connected to each other, to an ambient or thermal reservoir  604 , and to a thermal load  606  by heat switches  620 ,  622 ,  624 . Element  605  may comprise a first component  612  and a second component  614  electrically connected by an interface  608 . Similarly, element  603  may comprise a first component  616  and a second component  618  electrically connected by an interface  610 . The respective first components  612 ,  616 , second components  614 ,  618  and interfaces  608 ,  610  of the elements  605 ,  603  may be connected in a manner similar to that illustrated and described above with respect to the thermoelectric element  100 . For example, the first components  612 ,  616  and the second components  614 ,  618  may be made from materials (e.g., p-type and n-type semiconductor material, respectively) that have different Seebeck coefficients such that current across the first components  612 ,  616  and second components  614 ,  618  causes cooling at the interfaces  608 ,  610  in a first direction and heating at the interfaces  608 ,  610  in second direction. Also, the electrical connections between the respective first components  612 ,  616  and second components  614 ,  618  may not create rectifying pn junctions, as described herein above. 
     In a first state of operation, as illustrated by  FIG. 6( a ) , the control circuit may cause a first current  650  to flow across the respective elements  605 ,  603  in the direction shown. For example, the current  650  may flow across the element  605  from component  612  to component  614  (e.g., from p-type to n-type). Accordingly, the element  605  (e.g., at the interface  608 ) may heat. The current  650  may also flow across the element  603  from component  618  to component  616  (e.g., from n-type to p-type). This may cause the element  603  (e.g., at the interface  610 ) to cool. Although the current  650  is illustrated and described as a single current, it will be appreciated that current across the first element  605  and current across the element  603  may be separate currents (e.g., separately generated, separately switched, etc.). 
     Heat at the interface  608  may cause a temperature differential between the interface  608  and the ambient  604  to exceeding a closing threshold of the heat switch  620 , causing it to close and allowing heat from the interface  608  to be conducted from the element  605  and interface  608  to the ambient  604 . Similarly, a lack of heat (e.g., cool) at the interface  610  may cause a temperature differential between the interface  610  and the thermal load  606  exceeding a closing threshold of the heat switch  624 , causing it to close and allowing heat from the thermal load  606  to be conducted to the element  603  (e.g., the interface  610 ). As shown in  FIG. 6( a ) , the heat switch  622  may be open creating substantial thermal isolation between the elements  605 ,  603 . Alternatively, the positions of the respective switches  620 ,  622 ,  624  may be controlled by the control circuit  602 , according to any suitable method. 
     According to various embodiments, the heat switch  622  may be a one-way heat switch. For example, in the configuration illustrated in  FIG. 6( a ) , the interface  608  is heated while the interface  610  is cooled. Accordingly, there may be a significant temperature differential from hot to cold between the interface  608 ,  610 . The heat switch  622 , in various embodiments, may not close in response to a temperature differential in this direction. For example, the heat switch  622  may only close in response to a temperature differential when the interface  610  is hotter than the interface  608 . In various embodiments, the other heat switches  620 ,  624  may be similarly one-way. This may prevent the switches  620 ,  624  from closing, and thus disrupting the operation of the thermoelectric device  601  due to extreme heat at the thermal load  606  and/or extreme cold at the ambient  604 . 
     Referring now to  FIG. 6( b ) , the control circuit  602  may cause the current  650  to cease and cause a current  652  to flow in a direction opposite to that of the current  650 . This current may heat the interface  610  of the element  603  and cool the interface of the element  605 . Cooling of the interface  608  may cause the temperature differential between the interface  608  and the ambient  604  to drop below an opening threshold of the heat switch  620 , causing the heat switch  620  to open and resulting in substantial thermal isolation between the element  605  and the ambient  604 . Similarly, heating of the interface  610  may cause the temperature differential between the interface  610  and the thermal load  606  to drop below an opening threshold of the heat switch  624 , causing the heat switch  624  to open and resulting in substantial thermal isolation between the element  603  and the thermal load  606 . At the same time, heating of the interface  610  and cooling of the interface  608  may cause a temperature differential from the interface  608  to the interface  610  to meet or exceed a closing threshold of the heat switch  622 , causing it to close and create a thermally conductive path between the interfaces  608 ,  610  and, thereby, between the elements  605 ,  603  themselves. In this way, heat drawn to the interface  610  may be conducted to the interface  608 . The thermoelectric element may operate as described, alternating between the configuration of  FIG. 6( a )  and the configuration of  FIG. 6( b )  for as long as desired. In effect, the operation described may serve to pump heat from the thermal load  606  to the ambient  604 . Although the current  652  is illustrated and described as a single current, it will be appreciated that current across the first element  605  and current across the element  603  may be separate currents (e.g., separately generated, separately switched, etc.). As described herein, the switches  620 ,  622 ,  624  may alternately be actuated by the control circuit  602 . 
       FIG. 7  illustrates a diagram of one embodiment of a-heat switch  700 . The heat switch  700  may be utilized as any of the switches  108 ,  110 ,  620 ,  622 ,  624  described herein. As illustrated, the heat switch  700  comprises a first terminal  702  mechanically and thermally coupled to a first switch element  706  and a second terminal mechanically coupled to a second switch element  708 . The first switch element  706  may be received within a cavity  710  of the second switch element  708 . When the switch  700  is in an open position, a gap  712  may exist between the first element  706  and the second element  708 , preventing physical contact between the two. In some embodiments, the switch elements  706 ,  708  may be maintained in a vacuum to prevent heat conduction across the gap  712  by air. 
     The switch  700  may be opened and/or closed based on a temperature differential between its terminals  702 ,  704 . Mechanically, the switch  700  may be closed by eliminating the gap  712  and bringing the elements  706 ,  708  into physical contact with one another. This may occur, for example, by some combination of thermal contraction of the element  708  and/or thermal expansion of the element  706 . This combination may be brought about by a temperature differential between the terminals  702 ,  704 . For example, when the terminal  702 , and thus the element  706 , is at a higher temperature than the terminal  704 , and thus the element  708 , the element  706  may thermally expand relative to the element  708 , narrowing the gap  712 . This operation could alternately be viewed as the element  708  thermally contracting relative to the element  706 . Any combination of thermal expansion by the element  706  and/or thermal contraction by the element  708  may occur. The temperature differential at which the gap  712  disappears and the elements  706 ,  708  enter thermal contact with one another may be referred to as a closing threshold differential. According to various embodiments, the switch  700  may be considered a one-way heat switch. For example, a negative temperature differential from the terminal  704  to the terminal  702  would not cause the switch to close. Heating the terminal  704  would cause the element  708  to expand, thus tending to increase the gap  712 . Cooling the terminal  702  would also tend to increase the gap  712  by thermally contracting the element  706 . 
     Once closed, the switch  700  may be re-opened according to any suitable mechanism. For example, reducing or reversing the temperature differential between the terminals  702  and  704  may cause differential expansion of the elements  706 ,  708  sufficient to recreate the gap  712  despite the fact that the elements  706 ,  708  are in thermal contact with one another when the switch  700  is closed. For example, while there is a temperature differential between the terminals  702 ,  704 , there may be a temperature gradient between the terminals  702 ,  704 , across the elements  706 ,  708 . Accordingly, the elements  706 ,  708  may not expand or contract uniformly (e.g., because one or both of the elements  706 ,  708  may not have a single, uniform temperature). When the temperature differential between the terminals  702 ,  704  reaches a value that causes a large enough portion of the element  706  to thermally contract, and/or a large enough portion of the element  708  to expand, contact between the elements  706 ,  708  may be broken. This may recreate the gap  712 , causing substantial thermal isolation between the elements  706 ,  708  and the terminals  702 ,  704 . The temperature differential which causes the switch  700  to open may be referred to as the opening temperature differential threshold. The opening temperature differential threshold may, or may not, be equal to the closing temperature differential threshold. 
     According to various embodiments, the elements  706 ,  708  may be made from different materials having different coefficients of thermal expansion. In this way, the closing and/or closing temperature differential threshold may be dependent on absolute temperatures of the terminals  702 ,  704 .  FIG. 8  illustrates a diagram of another embodiment of a heat switch  800  actuated by thermal differentials. The heat switch  800  may operate in a manner similar to that of the switch  700  and may have similar properties. A terminal  802  may be in thermal communication with a roughly cylindrical element  802  that may be received within a hollow element  806 , in thermal communication with a terminal  804 . A gap  812  between the elements  806 ,  808  may ensure substantial thermal isolation. Some combination of thermal expansion by the element  808  and/or thermal contraction by the element  806  may eliminate the gap  812 , bring the elements  806 ,  808  (and the terminals  802 ,  804 ) into thermal contact with one another. The switch  800  may have a closing temperature differential threshold and an opening temperature differential threshold, for example, as described herein with respect to switch  700 . 
     The examples presented herein are intended to illustrate potential and specific implementations of the present invention. It can be appreciated that the examples are intended primarily for purposes of illustration of the invention for those skilled in the art. No particular aspect or aspects of the examples are necessarily intended to limit the scope of the present invention. For example, no particular aspect or aspects of the examples of system architectures, methods described herein are necessarily intended to limit the scope of the invention. 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements. Those of ordinary skill in the art will recognize, however, that these sorts of focused descriptions would not facilitate a better understanding of the present invention, and therefore, a more detailed description of such elements is not provided herein. 
     Moreover, the processes associated with the present example embodiments (e.g., process flows shown by  FIGS. 3( a )-3( d ), 5( a )-5( d ) and 6( a )-6( b ) ) may be executed by programmable equipment (e.g., control circuits  130 ,  506 ,  602 , etc.), such as computers. Software or other sets of instructions that may be employed to cause programmable equipment to execute the processes. The processes may be stored in any storage device, such as; for example, a computer system (non-volatile) memory, an optical disk, magnetic tape, or magnetic disk. Furthermore, some of the processes may be programmed when the computer system is manufactured or via a computer-readable memory medium. 
     It can also be appreciated that certain process aspects described herein may be performed using instructions stored on a computer-readable memory medium or media that direct a computer or computer system (e.g., control circuits  130 ,  506 ,  602 , etc.) to perform process steps. A computer-readable medium may include, for example, any non-transitory media such as, for example, memory devices such as diskettes, compact discs of both read-only and read/write varieties, optical disk drives, and hard disk drives. A computer-readable medium may also include memory storage that may be physical, virtual, permanent, temporary, semi-permanent and/or semi-temporary. It will be appreciated that the term non-transitory refers to the medium and not to any data stored thereon. For example, a random access memory (RAM) is non-transitory, although the data stored thereon may change regularly. 
     A “computer,” “machine,” “computer device,” “host,” “engine,” or “processor” may be, for example and without limitation, a processor, microcomputer, minicomputer, server, mainframe, laptop, personal data assistant (PDA), wireless e-mail device, cellular phone, pager, processor, fax machine, scanner, or any other programmable device configured to transmit and/or receive data over a network. Computer systems and computer-based devices disclosed herein may include memory for storing certain software applications used in obtaining, processing, and communicating information. It can be appreciated that such memory may be internal or external with respect to operation of the disclosed example embodiments. The memory may also include any means for storing software, including a hard disk, an optical disk, floppy disk, ROM (read only memory), RAM (random access memory), PROM (programmable ROM), EEPROM (electrically erasable PROM) and/or other computer-readable memory media. 
     In various example embodiments of the present invention, a single component may be replaced by multiple components, and multiple components may be replaced by a single component, to perform a given function or functions. Except where such substitution would not be operative to practice embodiments of the present invention, such substitution is within the scope of the present invention. Any of the servers or computer systems described herein, for example, may be replaced by a “server farm” or other grouping of networked servers (e.g., a group of server blades) that are located and configured for cooperative functions. It can be appreciated that a server farm may serve to distribute workload between/among individual components of the farm and may expedite computing processes by harnessing the collective and cooperative power of multiple servers. Such server farms may employ load-balancing software that accomplishes tasks such as, for example, tracking demand for processing power from different machines, prioritizing and scheduling tasks based on network demand, and/or providing backup contingency in the event of component failure or reduction in operability. 
     While various embodiments have been described herein, it should be apparent that various modifications, alterations, and adaptations to those embodiments may occur to persons skilled in the art with attainment of at least some of the advantages. The disclosed embodiments are therefore intended to include all such modifications, alterations, and adaptations without departing from the scope of the embodiments as set forth herein.