TE heat pumps are well known and offer an efficient means for pumping or moving small amounts of heat, generally less than 200 watts. Well known uses include temperature controlling (cooling or heating) small enclosures that contain such items as food, wine, medicine or electronics, and direct temperature control of electronic chips and laser diode devices by direct placement of a TE heat pump on a device. A less well-known application for TE technology is to pump heat through the TE unit to produce electrical power, known as a TE generator.
A conventional TE device, chip, or module 20 is shown in FIG. 1 with the typical upper alumina substrate 14 removed. The module consists of an array of negatively (N) and positively (P) doped thermoelectric semiconductor elements, 10a and 10b, arranged in pairs or thermocouples with TE elements 10a and 10b connected electrically in series and thermally in parallel. The TE elements are connected electrically by copper connectors such as tabs or bus bars 12. The TE elements and their connectors are in turn sandwiched between the top alumina substrate 14 and a bottom alumina substrate 16. The substrates provide electrical insulation and mechanical support and protection for the TE elements 10a and 10b. 
When DC electrical power is applied to such an array of thermocouples via the positive connection wire 18a and the negative connection wire 18b, heat is absorbed on one side (cold side), conducted through the elements by charge carriers, and deposited on the opposite side (hot side). Switching the direction of the DC current will reverse the hot and cold sides. Conversely, putting a temperature differential across the two sides will produce DC electrical power, often called a generator. In FIG. 1, for the current direction shown, substrate 14 is on the cold side of the module where heat is absorbed, and substrate 16 is on the hot side of the module from which heat is rejected.
FIGS. 2 and 2A show a perspective and a cross-sectional view of a typical TE assembly 22 incorporating TE module 20 from FIG. 1, found in a temperature controlled enclosure such as a cooled portable picnic box. One side is exposed to the interior air and the opposite side is exposed to the ambient air. On the interior side, generally called the cold side, a heat exchanger or cold sink 24 is attached to the top substrate or cold side 14 of module 20 to increase heat transfer to the cold side from the interior air. Similarly a heat exchanger or hot sink 26 is attached to the hot side 16 of the module to increase heat transfer between the hot side 16 and the exterior air. The heat exchangers are generally extended surfaces made of highly heat-conductive material, such as aluminum-finned heat sinks. The hot sink 26 is normally larger than the cold sink 24 as it transfers both the heat pumped from the cold side and the heat generated by the power input into module 20. A means of moving air or other heat-transferring fluid over or across cold sink 24 and especially hot sink 26, such as a fan (not shown) is typically used to improve thermal efficiency. A metal spacer 28 is often placed between the cold side 14 and cold sink 24 to increase the distance between the hot sink 26 and cold sink 24 and thereby reduce parasitic heat transfer between the hot and cold sinks. Insulating foam 30 is often placed or sprayed into this area around the spacer and module as shown to further reduce this unwanted heat transfer. The heat sinks 24 and 26 are generally held against the cold and hot sides or surfaces 14 and 16 of module 20 by use of stainless steel screws 32. A series of insulating, fastening, and Belleville washers 34, 36, 38 are used to insulate and keep the assembly under compression. Interface materials such as thermal grease or compliant thermal pads (not shown) are placed between cold side 14 and spacer 28, between hot side 16 and hot sink 26, and between spacer 28 and cold sink 24 to increase heat transfer by removing or filling small air gaps.
The assembly 22 is typically fit into a hole cut into the insulating walls 31 of the cooled enclosure.
While the above assembly works adequately, those skilled in the art recognize that it has a number of disadvantages.
One disadvantage is the use of the insulating substrates 14 and 16, typically alumina ceramic. The substrates are costly and reduce thermal efficiency by adding resistance to the flow of heat; this resistance to heat flow is called thermal resistance. The alumina substrates are still used in small-scale TE heat pumps for consumer appliances sold today, despite decades of trying to minimize thermal resistance by removing electrical insulators from the TE module and connecting the TE elements directly to the heat sinks and thereby combine the heat sink and electrical connector functions. For example, U.S. Pat. No. 2,903,857 to Lindenblad (1959) discloses an extremely large TE heat pump, intended for regulating room temperature in homes, in which each thermoelectric pair of TE elements is bonded directly to an individual or discrete heat sink in the form of a large, generally U-shaped channel member on one side, and to individual, spaced cold sink plates on the other. Multiple channel members are then bolted together in series for increased heating and cooling capacity, and to define an elongated airflow path through the series-connected channels. The disadvantages to this design are that it does not lend itself to moisture sealing, has poor structural support for the huge hot sinks, and relies on flexible electrical connections between the spaced cold sink plates in each TE pair to protect the TE elements from thermal stresses. Lindenblad's assembly also appears to rely on an inadequate natural, fanless convection to remove heat from the interconnected hot sinks, possibly due to a widely held belief at the time that the efficiency of the TE materials would improve by orders of magnitude, which large improvement never occurred.
U.S. Pat. No. 2,997,514 to Roeder (1961) discloses an assembly that again bonds discrete heat sinks to the TE elements, but addresses the sealing and structural issues with expensive and thermally resistive spring contact members and impractical packaging of the assembly.
U.S. Pat. No. 3,076,051 to Haba (1963) discloses a different approach that minimizes the thermal resistance of the electrical insulator rather than eliminating it. Haba does this by making the alumina insulating layer extremely thin by means of anodized aluminum plates. One problem with this approach is that the aluminum has three to four times the thermal expansion of alumina ceramic and causes stress fractures of the solder bonds that connect the TE elements to the copper connectors upon heating and cooling.
Several types of structure for the so-called “Direct Transfer Method” have been proposed in patents such as U.S. Pat. No. 3,213,630 to Mole (1963), U.S. Pat. No. 4,730,459 to Schicklin et al. (1988), the published article in International Thermoelectric Society proceedings entitled “Application of Thermoelectric Technology to Naval Submarine Cooling” by Blankenship et al. (1988), and U.S. Pat. No. 6,385,976 to Yamamura et al. (2002). The Direct Transfer Method bonds a heat sink to the TE elements utilizing the walls of metal conduits as the electrical connectors, while the conduit interior conducts heat transfer fluid. Although used successfully in large-scale applications such as the cooling of trains and submarines, this technique is prohibitively expensive and complex in small-scale applications, particularly in consumer products.
U.S. Pat. No. 6,226,994 to Yamada et al. (2001) also discloses a basic “double skeleton” thermoelectric structure without the conventional substrates. The structure consists of TE elements protruding through a partition, the connecting electrodes on one side having an inverted, fluid-cooled T-shape, the structure believed to be especially intended for use with liquid-cooled systems. This basic structure is believed to be relatively fragile and difficult to ship and handle for assembly. While Yamada et al. focuses on liquid-cooled arrangements, the less-detailed air-cooled embodiments replace the T-shaped electrodes with conventional flat electrodes bonded directly to a conventional unitary hot sink with a conventional insulating layer, similar to the Haba structure described above and believed to have similar stress problems. Sealing of the TE elements and their electrodes against moisture and corrosion is not adequately addressed. Air cooling is not believed to be practical with any of Yamada et al's disclosed structures, since the cooling flow path and electrode orientation would reduce efficiency by causing pressure drop, by raising the delta temperature of both the TE elements and their electrodes across the array, and by presenting an inefficient fan interface.
A second important disadvantage of known TE module construction is the corrosion and corresponding thermal performance degradation resulting from moisture condensing on the cold side elements. Water vapor is a relatively small molecule and permeates most module packaging materials other than glass or metal. As small voids or spaces around the cold-side surfaces cool, the water vapor in the spaces condenses to liquid water and lowers the vapor pressure. This creates a vapor pressure gradient where the high-pressure vapor on the exterior of the hot side of the module is pushed toward the lower pressure vapor on the cold side, and then condenses. Vapor thereby moves through interstitial areas in screw threads, connector wires, and insulation foams and condenses on the cold side. The liquid water is then trapped in the open spaces of the module, causing corrosion and parasitic heat losses.
One typical solution to the condensation/corrosion problem is to seal the perimeter of the TE module or its foam packaging assembly and all ingress points such as screws or wires with sealant materials such as epoxy or silicone. This solution is time-consuming, expensive, and prone to leaks. A second solution is to let the vapor in, but provide an exit for condensed water via weep holes or wicking materials. This solution is hard to implement with conventional TE modules, since the elements are close together and sandwiched between the two alumina substrates, such that surface tension tends to keep the water inside the module.
Another disadvantage with known TE module construction is the thermal expansion and contraction of materials. TE heat pump materials, like other electronic materials, have different rates of thermal expansion upon heating or cooling. This difference creates stresses that reduce the life of the module by breaking the solder bonds that connect the elements to the copper conductors. This problem is compounded by the fact that one side is heating and expanding while the other side is cooling and contracting. This difference in expansion and contraction is particularly troublesome in applications that experience wide and/or fast variations in temperature.
Yet another disadvantage with known TE module construction is that the process of combining the TE modules and heat sinks is time-consuming and error prone. Time-consuming procedures must be used to guarantee that uneven pressure from the screw assembly does not damage the brittle TE module. Processes must be implemented to insure the flatness and cleanliness of the mating surfaces, since the thermal performance is extremely sensitive to any dirt, dust, debris, bumps, irregularities, or non-flat areas on or between the mating surfaces of the TE module and heat sink.