Refrigeration – Processes – Evaporation induced by sorption
Reexamination Certificate
2001-08-07
2002-08-20
Jiang, Chen-Wen (Department: 3744)
Refrigeration
Processes
Evaporation induced by sorption
C062S144000, C062S480000, C062S003300
Reexamination Certificate
active
06434955
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrically driven cooling cycle that makes use of the symbiotic effects of adsorption and thermoelectric cooling cycles to produce a useful cooling effect at an evaporator.
2. Description of Background Art
A central challenge in cooling science today is the development of miniaturized chillers, in particular for microelectronic appliances such as personal computers. The general aim is to develop a device that is: (1) compact; (2) virtually free of moving parts and reliable; (3) efficient in converting input to cooling power (i.e., a high Coefficient Of Performance or COP for short); (4) capable of high cooling densities typically measured in Watt per square centimeter (W/cm
2
); and (5) affordable. (COP is defined as the ratio of useful cooling power to input power).
Various types of cooling devices have been, hitherto, proposed or commercialised for the above-mentioned purposes. The simplest is forced air convection with the option of an extended heat sink that effectively increases the heat source surface area for heat exchange and/or the possibility of introducing ribs or barriers on the surfaces to be cooled to increase air turbulence so as to realize better heat dissipation. This method is adequate for many types of current microelectronic cooling applications; however, the current methods might cease to satisfy the compactness constraints of future generations of microelectronic cooling applications that will require a cooling density at least an order of magnitude higher than presently required.
Thermoelectric chillers are also in use, but suffer from inherently low COP (typically in the range of 0.1-0.5 for the temperature ranges characteristic of many microelectronic applications) and high cost. The low COP means that major increases in cooling density will require unacceptably high levels of electrical power input and rates of heat rejection to the environment that will be difficult to satisfy in a compact package, all at increased cost.
Passive thermo-syphons have been proposed. These devices involve virtually no moving parts, except with the possibility of one or more cooling fans at the condenser. Such a device; however, is highly orientation dependent, since it relies on gravity to feed condensate from a condenser located at a higher elevation so as to provide liquid flush back to the evaporator, which is located at a lower elevation.
Thermo-syphons equipped with one or more mini pumps have also been proposed [
1
]. Instead of relying on gravity, condensate is pumped from the condenser back to the evaporator. This scheme is orientation independent and also allows for the possibilities of forced convective boiling, spraying of condensate or jet-impingement of condensate at the evaporator, which will effectively enhance boiling characteristics and therefore cooling performance.
Laid-out heat pipes [
2
,
3
] have found applications especially in laptop computers. The evaporating ends of the heat pipes are judiciously arranged over the CPU while the condensing ends of the same are laid out so as to effectively increase the surface area of the heat sink.
Mini vapour compression chillers [
4
] have also found applications. In one design, the evaporator is arranged over the heat source surface while the mini condensing unit is positioned away from the heat source. The advantage of such a system lies in its higher COP. However, many moving parts are involved in the compressor and they have to be highly reliable. Further scaling down of the compressor for miniaturized cooling applications may also be a technical challenge, and this may lead to a sizable loss of compressor efficiency due to high flow leakages and in turn the low chiller COP.
Thermoelectric chillers [
5
] satisfy the requirement of compactness, the absence of moving parts except for the possibility of one or more cooling fans, and an insensitivity to scale (since energy transfers derive from electron flows). Typically, commercial thermoelectric devices comprise semiconductors, most commonly Bismuth Telluride. The semiconductor is doped to produce an excess of electrons in one element (n-type), and a dearth of electrons in the other element (p-type). Electrical power input drives electrons through the device. At the cold end, electrons absorb heat as they move from a low energy level in the p-type semiconductor to a higher energy level in the n-type element. At the hot side, electrons pass from a high energy level in the n-type element to a lower energy level in the p-type material, and heat is rejected to a reservoir.
Thermoelectric devices have found niche applications for small-scale cooling. When substantial temperature differences are needed, thermoelectric devices inherently suffer from low COP, with the concomitant drawbacks of relatively high power input, accommodating even greater heat rejection, and an appreciable cost per watt of cooling power.
Adsorption chillers have been proposed to cool electronic devices in space capsules [
1
]. The advantage of such devices is that they are virtually free of moving parts, except for the on-off valves that separately connect the reactors to the evaporator and condenser (therefore these units are highly reliable). Adsorption chillers are also capable of being miniaturized [
6
], since adsorption of refrigerant into and desorption of refrigerant from the solid adsorbent are primarily surface, rather than bulk processes [
7
-
13
]. A refrigerant such as water is exothermically adsorbed, and endothermically desorbed, from the porous adsorbent, which is usually packed in a reactor having good heat transfer characteristics.
Many adsorbent-adsorbate pairs are available, such as silica gel-water, silica gel-methanol, zeolite-water, activated carbon-nitrogen, activated carbon-methanol, etc. Silica gel-water has been the preferred pair in commercial adsorption chiller development targeted for process cooling or air-conditioning owing to: (a) silica gel's comparatively large uptake capacity for water; (b) the high latent heat of evaporation of water; (c) the relatively low temperatures for desorption; and (d) the harmless nature of the chemicals.
However the COP of commercial adsorption chiller driven by low temperature waste heat (typically less than 85° C.) is low, typically in the range of 0.1-0.6 for typical air-conditioning and process cooling uses. The intrinsically low COP is related to: (i) small temperature differences among the reservoirs; and (ii) the batch-wise system operating characteristics.
The technology of coupling a thermoelectric device (often referred to as a Peltier device), to an adsorber and a desorber is not new [
14
]. It is typically applied to humidification, dehumidification, gas purification, and gas detection. Its application in an integral chiller system—i.e., to produce a thermodynamic cooling cycle—so as to realize the above-mentioned virtues has, hitherto, not been proposed.
In one version, a thermoelectric device is connected to one reactor [
15
;
16
;
17
]. Since one junction of the thermoelectric device is able to act either as the cooling end (with the other junction concomitantly acting as a heating end) or the heating end (with the other junction concomitantly acting as a cooling end) simply by means of switching the direction of direct current, the same junction is attached to the reactor in a thermally conductive but electrically non-conductive manner. If the reactor is designated to be an adsorber or absorber, direct current will be applied through the thermoelectric device in a manner such that the junction acts as the cooling end so that the heat generated by the adsorber or absorber is removed by the thermoelectric device to the environment. Conversely, if the reactor is designated to be a desorber or generator, the direction of flow of direct current through the thermoelectric device is reversed so that the junction acts as the heating
Chakraborty Anutosh
Chua Hui Tong
Gordon Jeffrey M.
Ng Kim Choon
Birch & Stewart Kolasch & Birch, LLP
Jiang Chen-Wen
The National University of Singapore
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