Refrigeration – With indicator or tester – Condition sensing
Reexamination Certificate
2001-10-10
2002-12-31
Jiang, Chen-Wen (Department: 3744)
Refrigeration
With indicator or tester
Condition sensing
C062S003700, C062S003300
Reexamination Certificate
active
06499306
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the field of thermoelectric heat exchangers, which may function either as a heater or a cooler. More particularly, the present invention relates to an improved design of a thermoelectric cooler (TEC), including power system design and packaging design, which result in compactness, increased efficiency, and increased reliability.
BACKGROUND OF THE INVENTION
TEC's perform the same cooling function as freon-based vapor compression or absorption refrigerators and air conditioners. In all such units, thermal energy is extracted from a region, thereby reducing its temperature, then rejected to a “heat sink” region of higher temperature. While freon based systems utilize the gas vaporization and compression cycle to perform cooling, thermoelectric coolers utilize the temperature difference that is created across a semiconductor thermocouple when voltage is applied.
A conventional cooling system contains three fundamental parts—the evaporator, compressor and condenser. The evaporator or cold section is the part where the pressurized refrigerant is allowed to expand, boil and evaporate. During this change of state from liquid to gas, energy (heat) is absorbed. The compressor acts as the refrigerant pump and recompresses the gas to a liquid. The condenser expels the heat absorbed at the evaporator plus the heat produced during compression, into the environment or ambient. Vapor-cycle devices have moving mechanical parts and require a working fluid, while thermoelectric elements are totally solid state.
Solid state heat pumps have been known since the discovery of the Peltier effect in 1834. In the Peltier effect, a voltage applied to the junction between two dissimilar metals creates a temperature difference between the two metals. This temperature differential can be used for cooling or for heating.
The devices became practical only recently, however, with the development of semiconductor thermocouple materials. TEC thermocouples are made from two elements of semiconductor, primarily Bismuth Telluride. The semiconductor is heavily doped to create an excess (n-type) and a deficiency (p-type) of electrons. The junction between the n-type and the p-type is a semiconductor thermocouple. At the cold side, energy (heat) is absorbed by electrons as they pass from a low energy level in the p-type semiconductor element, to a higher energy level in the n-type semiconductor element. The power supply provides the energy to move the electrons through the system. At the hot side, energy is expelled to a heat sink as electrons move from a high energy level element (n-type) to a lower energy level element (p-type). Heat absorbed at the cold side is pumped to the hot side at a rate proportional to current passing through the circuit and the number of couples.
These thermocouples, connected in series electrically and in parallel thermally, are integrated into thermoelectric modules. The thermoelectric modules are packaged between metallized ceramic plates to afford optimum electrical insulation and thermal conduction with high mechanical strength in compression. Thermoelectric modules can be mounted in parallel to increase the heat transfer effect or can be stacked in multistage cascades to achieve high differential temperatures. Solid state cooling is relatively simple compared to some of the classical technique using a compressor because there are no moving parts. These devices have the capability to be either heating systems or cooling systems depending on the direction of the current. Thermoelectric modules are divided into a hot side and a cold side, and are typically attached to heat sinks, creating a heat exchanger for use in a TEC.
Development of TECs has enabled the production of commercial miniature solid state air conditioners for cooling enclosures for devices such as electronics lasers, computers, scientific and medical equipment, as well as other similar equipment. Conventional cooling systems for enclosures remove the heat from one place (usually termed a hot spot) and blow the heat somewhere else in the enclosure until it is eventually vented or otherwise conducted/radiated outside. A common technique for cooling is through the use of an exhaust fan that draws outside air (often through filters) through the enclosure. However, certain electronics applications are sealed in an enclosure from the outside environment. This typically dictates the use of a heat exchanger for cooling because a heat exchanger can control the internal temperature of the enclosure without exchanging air between the enclosure and the outside environment. A TEC works well in many of these cooling applications.
However, these TEC's have some disadvantages. Moisture reaching the thermoelectric modules or the electrical components can reduce reliability. The cooling surface of the TEC often condenses out moisture from the air. The presence of even small droplets of water can cause damage to the thermoelectric modules and this may reduce the operational life of the device and the efficiency of the system. Also, in commercial applications of TECs, the units may be exposed to dust, dirt and water (rain or deliberate wash-down water from cleaning purposes). This exposure to dust, dirt, and water may decrease the reliability and efficiency of the system. In some cases, the units are exposed to acid or chemical attack. Other units require protection from explosive chemicals. Therefore, a TEC, which seals (so as to be highly moisture resistant) the thermoelectric modules would be very desirable. Additionally, a TEC which seals electrical components would be very desirable.
Also, moisture travelling between the hot side and cold side of the TEC may reduce system efficiency by allowing heat to transfer between the hot side and the cold side of the TEC. Also, any moisture placed on the hot side of the TEC (for example by wash-downs, etc.) may penetrate into the cold side of the TEC. This may lead to damage of the devices contained in the enclosure or potentially damage the TEC itself. Therefore, a TEC, which seals (so as to be highly moisture resistant) between the hot side and cold side of the TEC would be very desirable.
Another disadvantage of conventional TEC's is that they are typically designed with a relatively small cooling capacity. Because of this relatively small cooling capacity, it is important to maximize the thermal isolation between the hot side and cold side of the TEC.
Any transfer of heat from the hot side to the cold side will reduce system performance and efficiency. Any thermal load on the TEC may affect its efficiency. There are generally two, but not limited to two, broad classifications of heat that must be removed from the enclosure. The first is the real, sensible, or active heat load. This is the load that is intended to be cooled. This load could be the I
2
R load of an electrical component, the load of dehumidifying air, or the load of cooling objects.
The other kind of load is often referred to as the parasitic load. This is the load due to the fact that the object is cooler than the surrounding environment. This load can be comprised of conduction and convection of the surrounding gas, thermal leak through insulation, conduction through wires, waste heat generated from the TEC's own internal electrical components, condensation of water, and in some cases formation of ice. Regardless of the source of these parasitic loads, they all have potential to affect TEC efficiency.
Thermal loads from the energy dissipation of the TEC's electrical components may become important and effect operational efficiency if not properly designed. Any airflow or moisture flow between the hot side and the cold side of the cooling system may also reduce overall performance. Therefore, a TEC with improved thermal isolation, improved sealing between the hot and cold side, and/or improved design regarding parasitic loads would be desirable.
Another disadvantage of conventional TEC's is the size. TEC's may utilize numerous
Electrografics International Corporation
Jiang Chen-Wen
Woodcock & Washburn LLP
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