Refrigeration – Using electrical or magnetic effect – Thermoelectric; e.g. – peltier effect
Reexamination Certificate
2000-12-19
2002-03-05
Bennett, Henry (Department: 3744)
Refrigeration
Using electrical or magnetic effect
Thermoelectric; e.g., peltier effect
C062S383000, C165S096000
Reexamination Certificate
active
06351952
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a thermal bridge system and more particularly to such a thermal bridge system which can selectively either thermally isolate or thermally connect a warm object and a cool object without any immediate/short term or long-term degradation in thermal conductivity between the objects.
BACKGROUND OF THE INVENTION
Thermoelectric chips (“TECs”) chips are utilized in various cooling and heating applications. These TECs are actually miniature solid state heating/cooling devices which have no moving parts yet perform the function of drastically cooling one side of the chip while producing a proportionate increase in temperature on the other side of the chip. TECs function through what is known as the Peltier effect when current passes through the junction of two different types of conductors it results in a temperature change. Today, Bismuth Telluride is primarily used as the semiconductor material, heavily doped to create either an excess (N-type) or a deficiency (P-type) of electrons. Essentially, when a DC current passes through the junction of two wires made of dissimilar metals, the wire portions made of the first metal tend to heat up while the wire portions of the second metal tend to cool down. Correspondingly, if the current (polarity) is reversed, the heat is moved in the opposite direction. In other words, what was the hot face will become the cold face and vice a versa.
Very simply, a TEC consists of a number of P- and A-type pairs (couples) connected electrically in series and sandwiched between two ceramic plates. The cooling wire portions are all attached to a first ceramic plate (the cooling plate) and the warming wire portions are all attached to a second ceramic plate (the warming plate), where an air gap is kept between these two plates to act as an insulator. Precautionary measures are taken to insure that no water or condensation forms in between these two ceramic plates, as the water would act as a conductor and would short the heating/cooling wire portions.
When designed into systems, the warm ceramic plate of the TE chip is attached to a heat sink while the cool ceramic plate of the TE chip is attached to a device known as a cooling shoe, which absorbs latent heat from a medium. Typically, the cooling shoe is designed in a shape to accept or receive the shape of the object being cooled. For example, if the cooling shoe is designed to cool a can of soda, the cooling shoe would typically have a semicircular, concave shape so that the can of soda would fit into the cavity of the cooling shoe. This design feature is to effectively maximize surface contact, i.e. assist in cold transfer. Typical embodiments for these TE chip/heat sink/cooling shoe systems would be small-volume cooling systems, such as cooler chests or soda machines.
Thermodynamic principles mandate that the heat sink be spaced in optimal distance apart from the cooling shoe to prevent any convective heating of the cooling shoe. This optimal distance is typically two inches. Therefore, a spacer known as a bridge is typically placed between the cool ceramic plate of the TE chip and the cooling shoe. Further, rigid insulation or any other insulative material is utilized to insulate the bridge/TE chip structure so that convective heat transfer between the heat sink and the cooling shoe is minimized.
Please note that TE chips only function when a DC current is pumped through the heating/cooling wire portions within the chip. In the event of a power failure (or any other occurrence which interrupts current flow through the chip), the TE chip ceases to function as a heating/cooling device and, through conduction between the two ceramic plates via the heating/cooling wire portions, attempts to equalize the ceramic plate temperatures. Therefore, when no power is applied to the TE chip, the cooling shoe will warm up and the heat sink will cool down until they are at equal temperatures. Naturally, this is highly undesirable, as typical applications for TE chip-based cooling systems must maintain a specific temperature inside of the space being cooled. This situation is only aggravated by the fact that the power provided to these TE chips is typically cycled so that the temperature inside of the area being cooled is maintained within a predetermined range. In the event that the temperature within the area being cooled drops below the lower temperature of that predetermined range, power would then be cut to the TE chip. Unfortunately, this would result in the TE chip no longer functioning as a cooling device and actually (through conduction) equalizing the temperature of its plates and, therefore, the heat sink and cooling shoe. Accordingly, the temperature inside the cool space would immediately start to rise until that temperature exceeds the high temperature of the predetermined range. At that point in time, power to the TE chip would be cycled on and the cool space would immediately start to be cooled down. This system would continuously cycle, where the TE chip is either cooling tile space (through active cooling) or heating the space (through conductive heat transfer).
In an attempt to minimize or eliminate this undesirable situation, separation of the TE chip from either the heat sink or the bridge has been experimental and unfortunately there are several problems associated with this practice. When working with TE chips, it is imperative that a thermally efficient connection be made between the TE chip and any surface to which it is attached. Typically, a dielectric grease is utilized to connect the chip to the heat sink and the bridge. Unfortunately, by physically separating the TE chip from either the bridge or the heat sink, due to the viscous characteristics of the dielectric grease, the grease tends to stretch out in a string fashion to bridge the gap introduced between the TE chip and the body to which it is attached. Naturally, this results in a system in which the chip is not fully insulated (or isolated) from the object to which it is attached if the distance is limited. Therefore, the intended purpose of this gap (namely to thermally isolate the TE chip from either the bridge or the heat sink to prevent the equalizing of the temperatures of the cooling shoe and the heat sink) is frustrated as the thermal energy will merely transfer through these finger-like grease extrusions. Therefore, the temperature of the cooling shoe and heat sink will equalize.
Additionally, when the TE chip is placed back into position against either the bridge or the heat sink, the compression of the finger-like grease extrusions will result in the introduction of air pockets into the grease itself. These air pockets (or bubbles) act like little insulating bodies embedded within the grease, lowering the thermal efficiency of the conductive path of the heating/cooling device itself.
Another attempt to minimize the introduction of heat into the cooled area involved the use of an insulating cover placed over the heat sink, the cooling shoe, or both. If this insulating cover is placed over the heat sink, the only heat introduced into the cool area would be the latent heat stored in the heat sink itself. Alternatively, if this insulating cover is placed over the cooling shoe, limited heat gain would be introduced into the cool area. However, neither one of these situations really solves the problem at hand, as it is usually impossible to get to either the cooling shoe or heat shoe to install an insulating cover. Additionally, concerning covering either the heat sink or cold shoe with an insulating cover, this would tend to be a highly mechanical and complicated process and the net result would be insufficient.
SUMMARY OF THE INVENTION
The present invention provides a thermal bridge system comprising a first thermally conductive surface positioned proximate an object which absorbs energy and a second thermally conductive surface in thermal communication with the first conductive surface. The second surface is positioned proximate an object which dissipates energy. The thermal brid
Bennett Henry
Goodfaith Innovations, Inc.
Jiang Chen-Wen
Lenna Leo G.
LandOfFree
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