US20100058778A1

US20100058778A1 - Thermoelectrically powered indirect evaporative cooling system with desiccant dehumidification

Info

Publication number: US20100058778A1
Application number: US12/231,716
Authority: US
Inventors: Mohinder S. Bhatti; Ilya Reyzin; John F. O'Brien
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2008-09-05
Filing date: 2008-09-05
Publication date: 2010-03-11

Abstract

Disclosed is a system for cooling using indirect evaporative cooling, dehumidification using desiccant, and a thermoelectrically powered heat exchanger. The thermoelectric heat exchanger pulls heat from the working air exiting the indirect evaporative cooler and injects that heat energy into ambient air that is then forced through a portion of the rotating desiccant wheel to regenerate the desiccant by removing water molecules from the desiccant material. Liquid water that is condensed by cooling the working air with the thermoelectric heat exchanger is saved and provided to the indirect evaporative cooler for use to cool via evaporation.

Description

TECHNICAL FIELD

This invention relates generally to evaporative cooling and, more particularly, to an evaporative cooling system that is indirectly powered via thermoelectric effect.

BACKGROUND INFORMATION

The subject invention pertains to an indirect evaporative cooling system to abstract heat from the ambient air. Examples of an indirect evaporative cooler can be found in the U.S. Pat. Nos. 6,497,107; 4,977,753 and 4,976,113. In an indirect evaporative cooler, the dry and wet air streams do not come in direct contact with each other and as such the absolute humidity of the dry air does not change during its passage through the dry channels of the evaporative cooler. However, its dry bulb temperature drops, which is the desired effect in summer time. For proper comfort cooling, the absolute humidity of the cold air stream exiting the dry channels of the evaporative cooler must remain low. Thus an indirect evaporative cooler is superior to a direct evaporative cooler (colloquially referred to as “swamp” or “sump” cooler) since in the latter the dry and wet air streams mix resulting in higher absolute humidity of the conditioned air, which often is characterized by an unpleasant odor like that prevailing in a swamp. Direct evaporative coolers are effective for building cooling in environments where the prevailing relative humidity is quite low. They are very popular in the desert southwest of the United States and in Iran, for example.
An indirect evaporative cooler is incapable of dehumidifying the air and as such a desiccant wheel is incorporated in the air conditioning system to dehumidify the ambient air preparatory to its entry into the evaporative cooler. Examples of the air conditioning systems incorporating desiccant wheels to dehumidify the ambient air can be found in the U.S. Pat. Nos. 5,660,048; 5,727,394; 5,758,508; 5,860,284; 5,890,372; 6,003,327; 6,018,953; and 6,050,100.
A shortcoming of the desiccant-assisted evaporative cooler system is that it consumes excess amount of water to effect evaporative cooling. Moreover, it requires large amount of thermal energy to regenerate the desiccant material. What is needed is an evaporative cooler that can provide substantial cooling in a humid ambient environment with minimum consumption of water and thermal energy.

SUMMARY OF THE INVENTION

In general terms, this invention provides an indirect evaporative cooler that pre-treats working air by first thermoelectrically cooling it and then dehumidifying it before it is input to an indirect evaporative cooler. Moisture laden working air is looped back from the wet side of the evaporative cooler to start the pre-treat process over again. Ambient air is thermoelectrically heated and the heated air is used to dehumidify a desiccant wheel that becomes water laden when being used to dehumidify the working air preparatory to entering the evaporative cooler. The thermoelectric cooling for pre-treating the working air produces condensate, which is gathered and sent to the indirect evaporative cooler to be used for evaporation. Heat rejected via the thermoelectric cooling for pre-treating the working air returned from the wet side of the evaporative cooler is reused to heat the ambient air that is used to dehumidify the desiccant wheel.
A thermoelectric module is configured as a heat exchanger that pulls heat from the working air exiting the wet side of the indirect evaporative cooler and injects that heat energy into ambient air that is then forced through a portion of the rotating desiccant wheel to regenerate the desiccant by removing water molecules from the desiccant material. The thermoelectric module is configured as plural Peltier cells with the hot plates of the cells in thermal communication with the hot side of the exchanger and the cold plates in thermal communication with the cold side of the exchanger. Water that is condensed in the thermoelectric module heat exchanger is gathered and plumbed to be provided for use in wetting absorbent media lining the interior of the wet side of the indirect evaporative cooler.
One aspect of the indirect evaporative cooler is the recovery of spent water in the thermoelectrically powered heat exchanger.
Another aspect of the indirect evaporative cooler is the use of the thermoelectrically powered heat exchanger to effect the dual goals of condensing water vapor from the working air returned from the wet side of the indirect evaporative cooler and regenerating a desiccant wheel.
These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment. The drawings that accompany the detailed description are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an indirect evaporative cooling system according to an embodiment of the present invention.

FIG. 2 shows a cross sectional schematic view of a thermoelectrically powered heat exchanger assembly for use in the indirect evaporative cooling system of FIG. 1.

FIG. 3 shows a flow chart of operation of an indirect evaporative cooler.

DETAILED DESCRIPTION

Evaporative cooling is used to condition air of an enclosed space. Evaporative coolers lower the dry bulb temperature of air by using latent heat of evaporation, changing liquid water to water vapor. In this process, the overall energy in the air remains substantially constant. Heat in the air is used to evaporate water. In evaporative coolers of the indirect type, some kind of heat exchanger is used to isolate working air on the wet side from conditioned air on the dry side; the cooled moist air on the wet side never comes in direct contact with the conditioned air on the dry side.
Referring to FIG. 1, an indirect evaporative cooling system 100 according to an embodiment of the present invention is schematically illustrated. In the mixing valve 110, fresh ambient air is blended with cool, dry air from the thermoelectric heat exchanger 120 and the blended air is sent to the desiccant wheel 130 where it is dehumidified with elevation in its dry bulb temperature.
The working air is passed through the desiccant wheel 130 to remove moisture therefrom to provide a moisture-depleted stream of working air exiting the rotating desiccant wheel 130. The desiccant wheel 130 is regenerated by passing hot gases therethrough to remove moisture from the desiccant wheel 130.
An indirect evaporative cooler 140 is provided having a dry side and a wet side separated by a moisture-impervious wall wherein heat is extracted from said dry side through an internal heat exchange process on the wet side. Cooling in the dry side is achieved by evaporation of water into air passing through the wet side. The evaporating water is provided to the absorbent media lining the interior of the wet side.
The dehumidified blend air then enters the indirect evaporative cooler 140 where it is split into two streams. Approximately 20 to 40 percent of air enters the wet side of the evaporative cooler as the working air and the remaining 60 to 80 percent of the air enters the dry side of the evaporative cooler 140 emerging as the conditioned air 150 for providing comfort cooling in the conditioned space 154.
The moisture-laden working air 142 exiting the wet side of the evaporative cooler 120 is sent to the cold side 122 of the thermoelectrically energized heat exchanger 120 where the moisture is condensed from the working air 142. Relatively cool dry air from the thermoelectrically energized heat exchanger 120 is sent to the mixing valve 110 for blending with the make up ambient air for supply to the dehumidification portion of the desiccant wheel 130.
The heat extracted from the working air 142 is carried away by the ambient air supplied to the hot side 124 of the thermoelectrically energized heat exchanger 120. Relatively hot air from the hot side 124 of the thermoelectrically energized heat exchanger 120 is sent to the regeneration portion of the desiccant wheel 130 where it is utilized to drive the moisture from the desiccant wheel 130 thereby regenerating the slowly turning wheel. The desiccant wheel 130 revolves at a rate of about 3 to 4 revolutions per minute. The liquid water condensed from the moisture laden working air 142 in the thermoelectrically energized heat exchanger 120 is gathered by a moisture recovery assembly (160) and supplied (as indicated by the broken line) to a water reservoir of the indirect evaporative cooler.
FIG. 2 shows detailed construction of the thermoelectrically powered heat exchanger 120. Thermoelectric modules (TEM's) 210, 212, 214, 216, 218, 220, for example Peltier cells, are arranged in a stack as plural opposed pairs with their respective hot plates 222, 224 and cold plates 226, 228 facing one another. Sandwiched in between each pair of facing opposed cold plates 226, 228 (or attached to a single cold plate at the ends of the stack) is a heat transfer conduit 230 divided into plural lumens by convoluted louvered fins 232. The convoluted louvered fins 232 help guide the working air as it flows through the cold side of the heat exchanger 120 and, importantly, serves to provide abundant surface area for easy transfer of heat energy from the working air to the TEM cold plates 226, 228.
Sandwiched in between each pair of facing opposed hot plates 222, 224 is a flat tube 240 heat transfer conduit. Ambient air is drawn into the hot side inlet tank 242 flows through the flat tubes 240, drawing heat energy from the TEM hot plates 222, 224. The heated ambient air flows from the ends of the flat tubes 240 into the outlet tank 244 and is drawn out of the heat exchanger by a pump 250 to sent onward to regenerate the desiccant wheel.
A switch 260 and a DC power supply 262 are connected to the stack of TEM's in a parallel circuit according to the appropriate polarities to cause each cold plate to draw in heat and each hot plate to emit heat.
Referring to FIG. 3, a flow diagram illustrates operation of the thermoelectrically power indirect evaporative cooler with desiccant dehumidification. This method of conditioning a space using indirect evaporative cooling has aspects that save energy and save water. A flow of hot and humid working air is cooled 310 by being passed through a cold side of a thermoelectrically powered heat exchanger. This provides a flow of cooled partially dry working air 320. The cooled working air is mixed 330 with a first incoming flow of ambient air, providing 340 a blended flow of air. The blended flow of air is dehumidified 350 by flowing through a dry portion of a rotating desiccant wheel, resulting in a flow 360 of dry air. Thermal energy is transferred from a first portion of the dry air selected for conditioning to the remaining second portion of the dry air using an indirect evaporative cooler. The first portion of the dry air becomes the flow 370 of hot and humid working air that is returned to the thermoelectrically powered heat exchanger. The second portion of the dry air becomes a flow 380 of conditioned air distributed 390 to the space to be conditioned. A second incoming flow of ambient air is heated 400 using a hot side of the thermoelectrically powered heat exchanger, to yield a flow 410 of heated ambient air. The rotating desiccant wheel is regenerated 420 using the flow of heated ambient air to drive moisture from a water laden portion of the rotating desiccant wheel. Water condensate is recovered 430 from cooling of the flow of hot and humid working air at the cold side of the thermoelectrically powered heat exchanger. The recovered water is supplied 440 to the indirect evaporative cooler for use in evaporating into the working air.
The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.

Claims

1. A method of indirect evaporative cooling a space, the method comprising:

cooling a flow of hot and humid working air via a cold side of a thermoelectrically powered heat exchanger, to provide a flow of cooled working air;

mixing the cooled working air with a first incoming flow of ambient air, to provide a combined flow of air;

dehumidifying the combined flow of air via a dry portion of a rotating desiccant wheel, to provide a flow of cooled and dehumidified working air;

transferring thermal energy from a first portion of the cooled and dehumidified working air to a second portion of the flow of cooled and dehumidified working air via an indirect evaporative cooler, so that the second portion becomes the flow of hot and humid air, and so that the first portion becomes a flow of conditioned air;

distributing the flow of conditioned air from the indirect evaporative cooler to the space to be cooled;

heating a second incoming flow of ambient air via a hot side of the thermoelectrically powered heat exchanger, to provide a flow of heated ambient air;

regenerating the rotating desiccant wheel using the flow of heated ambient air to drive moisture from a water laden portion of the rotating desiccant wheel;

recovering, at the cold side of the thermoelectrically powered heat exchanger, water condensed via cooling of the flow of hot and humid working air; and

supplying the recovered water to the indirect evaporative cooler for use in evaporating into the second portion of cooled and dehumidified working air.

2. An air conditioning system for conditioning air in an interior space, the system comprising:

a thermoelectric heat exchanger module connected to receive a flow of hot and humid working air into a cold side fluid passage so that a flow of cooled humid working air emerges from the cold side fluid passage;

a rotating desiccant wheel connected to dry the flow of cooled humid working air receive from the thermoelectric heat exchanger module so that a flow of cooled dried working air emerges from the rotating desiccant wheel;

an indirect evaporative cooler having a wet channel and a dry channel, the wet channel being connected to receive a first portion of the flow of cooled dried working air from the rotating desiccant wheel, the dry channel being connected to receive a second portion of the flow of cooled dried working air and to supply conditioned air for the interior space, and the wet channel producing the hot and humid working air;

a return conduit connected to route the hot and humid working air emerging from the wet channel of the indirect evaporative cooler to the cold side fluid passage of the thermoelectric heat exchanger module; and

a regeneration conduit connected to provide heated ambient air emerging from a hot side fluid passage of the thermoelectric heat exchanger module to a moisture laden portion of the rotating desiccant wheel to remove moisture from the desiccant wheel; and

a moisture recovery assembly connected to gather liquid water condensed in the cold side fluid passage of the thermoelectric heat exchanger and supply the gathered liquid water to the indirect evaporative cooler to be evaporated in the wet channel.

3. A thermoelectric heat exchanger module comprising:

plural thermoelectric devices, each of the devices having a hot plate, a cold plate, a positive electrode, and a negative electrode;

a cold side fluid passage having plural cold side lumens, each of the cold side lumens being in thermal contact with the cold plate of at least one of the thermoelectric devices;

a hot side fluid passage having plural hot side lumens, each of the hot side lumens being in thermal contact with the hot plate of at least one of the thermoelectric devices;

wherein the plural thermoelectric devices are connected electrically in a parallel circuit with one another and are physically disposed so that adjacent pairs of the thermoelectric devices either sandwich one of the cold side lumens between their respective cold plates, or sandwich one of the hot side lumens between their respective hot plates.

4. The thermoelectric heat exchanger module of claim 3, wherein each thermoelectric device comprises a solid-state active heat pump which transfers heat from the cold plate to the hot plate.

5. The thermoelectric heat exchanger module of claim 3, wherein each thermoelectric device comprises a Peltier effect cell.