Capillary-film-evaporative-cooling heat exchanger and its associated thermodynamic processes

Abstract

A new type of heat transfer surface is disclosed. This heat transfer surface incorporates a capillary layer coated directly on typical heat transfer surfaces, such as fins of heat exchangers, such that water can be spread over the surface through capillary force, while evaporating to air. Heat exchangers made of such surfaces may be maintained even below the ambient temperature. These heat exchanging surfaces have many applications. For example, when used in the condenser of air-conditioning (AC) systems, these heat exchanging surfaces may significantly improve the performance of AC systems, including both conventional AC systems and solid-state AC based on thermoelectric effects. These heat exchanging surfaces may also be used in cooling towers of power plants, and in building envelopes.

Description

[0001]Capillary-Film-Evaporative-Cooling Heat Exchanger and Its Associated Thermodynamic Processes This application claims priority of U.S. Provisional Patent Application Serial No. 63 / 644,121, filed May 8, 2024, the disclosure of which is incorporated herein by reference in its entirety. Background Heat exchangers have been widely used to reject heat to ambient air, such as those used as condensers for air-conditioners and power plants, radiators for automobiles, and heat sinks for electronics. Heat exchangers typically use fins as an extended surface to increase the heat transfer area between the solid and air. In rejecting heat to air, the heat exchanger surface is usually at a temperature higher than the ambient air. The to-be- cooled fluid inside the heat exchanger must be at an even higher temperature than the heat exchanger surface due to thermal resistance. The larger the additional temperature difference between the to-be-cooled fluid and the external air, the less efficient the system is, which often results in higher energy consumption and / or a larger system size. Water evaporation cooling is often used to improve the performance of the normal convective air cooling (CAC). Evaporative cooling may occur as long as the air is not saturated, as it relies on the chemical potential difference between water and moisture air for mass transfer, rather than the temperature difference. Traditionally, there are two ways of using evaporative cooling, one is the spraying-evaporation-cooling heat exchanger (SEC) as shown in FIG. 1A; another is the falling-film-evaporation- cooling heat exchanger (FFEC) as shown in FIG 1B. The former firstly uses a natural evaporation cooling process to cool down the dry air 1 passing through an air duct 8 and then uses the chilled air 3 to cool down the to-be-cooled fluid 5 through the heat exchanger 6. A sprayer 2 connected to supply water 7 may be used to create the water droplets that create the chilled air 3. It may be adapted to all kinds of heat exchangers, such as the fin-tube air-liquid heat exchanger. The latter directly deposits a film 10 of water on the tube surface 11. The film 10 falls down by gravity. There are several limitations with FFEC exchangers. First, to prevent dry out, the film 10 is usually thick, over a few millimeters. Second, because the film 10 is thick, thermal resistance across the liquid film is high. Third, because the film 10 is thick, not all water can be evaporated. In fact, typically, only ~10% of the falling water is evaporated. The rest needs to be pumped back using a cycling pump. Fourth, the thick water film can be broken off by the air, entering air as droplets, which are lost. Fifth, because the film 10 is thick, it is not compatible with the small distance between the fins used in heat exchangers. Thus, it is often used to cool down the air-liquid heat exchanger made of horizontal bare tubes. With the same cooling capacity per unit volume of cooling air (enthalpy difference between the inlet and outlet states), the outlet states of the CAC, SEC and FFEC on a psychometric chart are at point 5, 4 and 2 in FIG. 1D, respectively. In FIG. 1D, point 1 is the ambient air state. In the case of SEC, point 3 is the state of the moist air after the water sprayer 2 and before the heat exchanger 6 in FIG. 1A, point 4 is the state of the moist air after the heat exchanger in FIG. 1A. Thus, for SEC, the air handling process is 1→3→4. For FFEC, the saturated moist air at the evaporation temperature is at state 0 and the air handling process is 1→2. Correspondingly, the temperatures of the to-be-cooled fluids inside the tube should be higher than the temperature at the state 5, 4 and 0 for CAC, SEC and FFEC, respectively. Thus, both the SEC and FFEC strategies can decrease the to-be-cooled fluid temperature compared to CAC. Although FFEC has a higher evaporation efficiency (defined by the evaporation water mass divided by the supply water mass), its evaporation area is normally smaller than the SEC. Meanwhile, due to the low thermal conductivity of water and the thin film, there is large temperature difference between the to-be-cooled fluid (usually liquids) inside the tube and the evaporation interface because of the high heat flux. Moreover, both SEC and FFEC have mist drift problem, i.e., water droplets entrapped into the air stream. Therefore, it would be beneficial if there were a system and method of heat exchange that overcame these issues. Summary A new type of heat transfer surface is disclosed. This heat transfer surface incorporates a capillary layer coated directly on typical heat transfer surfaces, such as fins of heat exchangers, such that water can be spread over the surface through capillary force, while evaporating to air. Heat exchangers made of these surfaces may be maintained even below the ambient temperature. These heat exchanging surfaces have many applications. For example, when used in the condenser of air-conditioning (AC) systems, these heat exchanging surfaces may significantly improve the performance of AC systems, including both conventional AC systems and solid- state AC based on thermoelectric effects. These heat exchanging surfaces may also be used in cooling towers of power plants, and in building envelopes. According to one embodiment, a heat exchanger is disclosed. The heat exchanger comprises an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action. In some embodiments, gravity is also used to spread the fluid. In some embodiments, the fluid comprises water. In some embodiments, the external heat exchange surface comprises one or more fins. In some embodiments, an anti-corrosion layer is disposed beneath the hydrophilic porous layer. In some embodiments, the hydrophilic porous layer comprises paper. In some embodiments, the hydrophilic porous layer comprises melamine foam. In some embodiments, a thickness of the hydrophilic porous layer is less than 500 microns. In some embodiments, a control unit is used to supply the fluid such that the hydrophilic porous layer is wetted. In some embodiments, the heat exchanger comprises a channel through which a fluid to be cooled passes. In some embodiments, the fluid is delivered through a channel having perforated holes, such that the fluid is spread to the hydrophilic porous layer via capillary action. In some embodiments, the heat exchanger comprises tubes containing a fluid to be cooled, wherein the external heat exchange surface is thermally connected to the tubes containing the fluid to be cooled. According to another embodiment, a cooling system is disclosed. The cooling system comprises the heat exchanger described above, wherein a gas flow passes over the external heat exchange surface. In some embodiments, the gas flow comprises air, which is cooled as it passes over the external heat exchange surface. In some embodiments, a data center is disclosed, which uses the cooling system described above. According to another embodiment, a cooling system is disclosed. The cooling system comprises a heat recovery exchanger, comprising a first channel having a first input and a first output and a second channel having a second input and a second output, wherein the first input receives outdoor air and wherein the second output delivers exhaust air and the outdoor air and the exhaust air exchange heat through walls of the heat recovery exchanger; and a heat exchanger comprising: a channel through which a fluid- to-be-cooled passes; an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, and wherein air from the first output passes over the external heat exchange surface and is cooled by the external heat exchange surface; and wherein the cooled air is provided to the second input. According to another embodiment, an air conditioning system is disclosed. The air conditioning system comprises a condenser; a compressor; an expansion valve; and an evaporator; wherein the condenser comprises one or more tubes to carry refrigerant, and an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action. In some embodiments, gravity is also used to spread the fluid. In some embodiments, the fluid comprises water. In some embodiments, a control unit is used to supply the fluid to wet the hydrophilic porous layer. In some embodiments, the air conditioning system comprises a heat recovery exchanger, comprising a first channel having a first input and a first output and a second channel having a second input and a second output, wherein the first input receives outdoor air and provides cooled outdoor air to the condenser, and wherein the second output delivers exhaust air that passes over the condenser. The outdoor air exchange heat with the exhaust air through walls of the heat recovery exchanger. According to another embodiment, a thermoelectric cooling system is disclosed. The thermoelectric cooling system comprises a plurality of thermoelectric devices creating a cold side and a hot side; a first heat spreader in communication with the cold side; a second heat spreader in communication with the hot side, wherein the second heat spreader is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action. In some embodiments, gravity is also used to spread the fluid. In some embodiments, the fluid comprises water. In some embodiments, a control unit is used to supply the fluid to wet the hydrophilic porous layer. According to another embodiment, an indoor refrigerator is disclosed. The thermoelectric cooling system described above is used to cool air inside the indoor refrigerator, eliminating a need for a compressor. According to another embodiment, a vapor condenser is disclosed. The vapor condenser comprises a vapor duct to deliver vapor to a heat exchanger; the heat exchanger comprising an internal channel through which the vapor flows; and an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action; wherein an output of the heat exchanger comprises condensate which is recirculated. In some embodiments, gravity is also used to spread the fluid. In some embodiments, the fluid comprises water. In some embodiments, a control unit is used to supply the fluid to wet the hydrophilic porous layer. According to another embodiment, a steam plant is disclosed. The steam plant comprises the vapor condenser described above, wherein the vapor comprises steam from the steam plant. According to another embodiment, an air conditioning system is disclosed. The air conditioning system comprises the vapor condenser described above, wherein the vapor is from a vapor compression cycle of the air conditioning system. According to another embodiment, a structure for use with a building envelope is disclosed. The structure comprises a metal extrusion profile, comprises one or more internal channels passing therethrough; wherein one or more of the one or more internal channels is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action. In some embodiments, gravity is also used to spread the fluid. In some embodiments, the fluid comprises water. In some embodiments, a control unit is used to supply the fluid to wet the hydrophilic porous layer. In some embodiments, air for outside the building flows through the one or more internal channels that are coated with the hydrophilic porous layer, and air from inside the building flows through different internal channels. Brief Description of the Drawings For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which: FIG. 1A shows a diagram of a traditional spraying- evaporation-cooling (SEC) heat exchanger; FIG. 1B shows a diagram of a traditional falling-film- evaporation-cooling (FFEC) heat exchanger; FIG. 1C shows a diagram of the disclosed capillary-film- evaporation-cooling (CFEC) heat exchanger according to one embodiment; FIG. 1D is the various cooling processes in a Psychrometric Chart; FIG. 1E shows a diagram of the disclosed capillary-film- evaporation-cooling (CFEC) heat exchanger according to another embodiment; FIG. 2A shows the air handling process associated with FIG. 2B shown in the psychrometric chart; FIG. 2B shows a diagram of the CFEC heat exchanger with heat recovery according to another embodiment; FIG. 3 shows a heat exchanger with a porous coating to spread the water or water incorporating anti-freezing agent; FIGs. 4A-4B are illustrations of the paper film coated fins at a dry state and an intermediate dynamic wetting state, respectively; FIG. 4C is a graph showing the heat transfer performance of the paper-coated CFEC heat exchanger; FIG. 4D shows a roll-to-roll process to fabricate the paper- coated fins according to one embodiment; FIGs. 5A-5E show the fabrication process to achieve a bifunctional coating for the aluminum heat exchange surface; FIG. 6 shows AC efficiency improvement with time; FIG. 7A shows a typical AC system; FIG. 7B shows the refrigerant cycle of FIG. 7A on an enthalpy- pressure diagram; FIG. 7C shows indoor (green dots) and outdoor air (red dots) states on a psychrometric chart for the system of FIG. 7A; FIG. 8A shows a AC system using the capillary-film- evaporation-cooling (CFEC) heat exchanger disclosed herein; FIG. 8B shows indoor and outdoor air states on a psychrometric chart for the system of FIG. 8A; FIG. 9A shows a diagram of a TEAC system according to one embodiment; FIG. 9B shows an illustration of the 3D structure of a single TEAC; FIG. 9C shows an assembly using a plurality of the TEACs shown in FIG. 9B; FIG. 10 shows the coefficient of performance (COP) of the TEAC using different thermal conduction resistance ratios between the heat spreader (Rs) and the fin (Ra) and different numbers of transfer units; FIG. 11A shows a traditional power plant condenser using a wet cooling tower; FIG. 11B shows a traditional power plant condenser using a dry air condenser; FIG. 11C shows a power plant condenser using the CFEC heat exchanger according to one embodiment; FIG. 11D shows a cooling tower using the CFEC heat exchanger in conjunction with an air conditioning system according to one embodiment; FIG. 11E shows a cooling tower using the CFEC heat exchanger in conjunction with a data center according to one embodiment; FIG. 12A shows the basic structure that may be used with building envelopes; and FIG. 12B-12C show two implementations of the structure shown in FIG. 12A. Detailed Description To address the above issues, a thermodynamic process that enables the heat exchanger to operate at a much lower temperature difference between the to-be-cooled fluid and air is disclosed. Importantly, the fin-tube heat exchanger surface is covered with a thin porous layer to spread out a water film by capillary force, which evaporates all the supply water directly into air without drift. This is shown in FIG. 1C, and is referred to throughout this disclosure as a capillary-film-evaporation-cooling (CFEC) heat exchanger. The CFEC heat exchanger includes a channel or tube 20 through which the fluid-to-be-cooled passes, and an external heat exchange surface, such as fin 21. A major difference compared to FFEC is that there is no flooding water on the heat exchanger surface, as the water will spread across the coated porous layer mainly by capillary action to form a thin film 22 of water. In other words, unlike FFEC, which relies on gravity to spread the water, the present system utilizes capillary action. As noted above, in SEC, some of the sprayed water can also fall onto fins and tube surfaces. Due to lack of control, the water film can be thick and in some cases, the water films even block the air passage between fins. This lack of control reduces the heat transfer effectiveness. In comparison, the CFEC heat exchanges avoid such issues by controlling the water film thickness. This thin film 22 allows most of the supply water to evaporate. Thus, the film 22 is much thinner than the falling film 10 of FIG. 1B, and may be applied directly to the fins typically used in most heat exchangers. In some embodiments, this thin porous layer may be less than 10% of the pitch between adjacent fins. For example, the thin porous layer may be less than 1 mm. In certain embodiments, the thin porous layer may be less than 500 microns. The thinner film also means a much smaller heat transfer resistance between the to-be-cooled fluid and the cooling air. Referring to FIG. 1D, point 0 is the saturated moist air at the evaporation temperature, which is nearly equal to the temperature of the to- be-cooled fluid through the proposed CFEC heat exchanger. The basic thermodynamic processes of the air handling in such a CFEC is the same as the FFEC, 1→2. However, point 2 is at lower temperature in CFEC than FFEC, with water evaporation at the state 0. However, as explained above, due to larger evaporation area of the fins and small heat transfer resistance, the heat transfer efficiency (defined by the temperature difference between the inlet and outlet of the to-be-cooled fluid divided by the temperature difference between the inlet to-be-cooled fluid and the wet-bulb temperature of the inlet cooling air) of the proposed CFEC is much better than FFEC and SEC. For example, when the air is at 35oC with 60% humidity, the wet-bulb temperature is 28oC, which means water can be cooled to this temperature. In this case, the to-be-cooled fluid can be cooled to near 28oC, below the ambient temperature. With this evaporative cooling, heat seemingly goes from low temperature water surface to the hotter air, because evaporation is driven by the chemical potential difference of water and air. It should be recognized that even if the air is fully saturated, the CFEC still provides working advantage over conventional fins used without any water, or SEC and FFEC. In this case, the evaporation of water can still happen if the surface is above the external air temperature. The reduced water film thickness and evaporative heat transfer reduces the temperature difference between the wall and the external air, making heat transfer more efficient. In addition to spreading water via capillary action through all the surfaces, one can also deliver water from an internal channel 25 with perforated holes 26 along the channel 25, as shown in FIG. 1E. The metallic casing of these channels 25 can serve as fins 27, thermally connected with the tubes 28 containing the to- be-cooled liquid. Water can permeate out through the perforated holes, and spread uniformly to hydrophilic external surfaces covered with porous materials to form the film 29. In another implementation, since the temperature of the exit air after the evaporative heat exchanger is even lower than the ambient temperature as shown in FIG. 1D, an additional air-air heat exchanger may be used between the outdoor air and the air exiting the CFEC to precool the outdoor air before it enters the CFEC. FIG. 2B shows a schematic of the two heat exchangers contained within an air duct 36. This process is shown in FIG. 2A, where ei and eo represent the states of the cooling air at the inlet and outlet of the CFEC, respectively. Point 1 represents the outdoor air 30, point 5 represents the exhaust air 31. The heat recovery exchanger 32 has a first channel into which outdoor air 30 enters. The air to be passed over the CFEC heat exchanger 33 with a water film 34 exits from this first channel. The CFEC heat exchanger 33 may include a refrigerant tube 35 through which the fluid to be cooled passes. The second channel receives the exhaust air from the CFEC heat exchanger 33 (which is at a lower temperature that the air passing through the first channel) as its input and expels the exhaust air 31. Without the additional heat recovery exchanger 32, air entering CFEC heat exchanger 33 is at state 1 and exits at state 2. With the additional heat recovery exchanger 32, air entering the CFEC heat exchanger 33 is at ei and cooled to eo, which is lower than the temperature at state 2. To achieve the principle described above, a steady water film 34 is coated directly on the fins of the CFEC heat exchanger 33 rather than gravity-driven falling film as is done in FFEC or sprayed water as in SEC. This layer of water may be achieved by coating a porous layer on the heat exchanger surface of a condenser which can hold water for direct evaporative cooling of the condenser, as shown in FIG. 3. FIG. 3 shows an aluminum fin 40 in thermal contact with the fluid-to-be-cooled 45. The exterior surface of the aluminum fin 40 may be first coated with a nonporous layer, such as an anticorrosive layer 41 to minimize corrosion of the fins 40. The outermost layer may be a porous layer 42. A binder layer 43 may be disposed between the anticorrosive layer 41 and the porous layer 42. The porous layer 42 is hydrophilic to quickly soak and spread the water all over the fins 40 and tubes. Unlike the traditional falling-film evaporation technologies where water falls by gravity, in this embodiment, water 48 spreads via both capillary action and gravity and nearly all the supply water will be evaporated into air with minimal or no drainage. This technology may not only greatly enhance the air-side heat transfer performance, but also reduce the temperature of the to-be-cooled fluid with the same heat flux and inlet cooling air temperature, and minimize the water consumption. Water 48 will be distributed from the top of the heat exchanger in the vertical direction and spread inside the film driven by the capillary force and gravity. The control unit 46 will be designed to control the amount of water flow. This can be done by using sensors 49, such as temperature and humidity sensors to sense the water amount in the capillary film, which is used as a feedback to the control unit 46, which adjusts the valves 47 to control the amount of water 48 supplied to fins 40. Alternatively or additionally, the sensors 49 may include a camera, which may be used to monitor the amount of water 48 in the capillary film. This visual indication may also be used as a feedback to the control unit. Numerical and / or machine learning can also be used by the control unit 46 to more accurately monitor the amount of water in the film. The control unit 46, valves 47 and sensors 49 described above with respect to FIG. 3 may be used with any of the embodiments described herein. For winter usage as a heat pump, a liquid absorbent, such as LiCl, may be added into the porous layer 42. There are a few advantages using this approach. First, such an adsorbent may serve as anti-freezing agent to prevent freezing in water. Second, the adsorbent can also adsorb water from the outside side, which has large latent heat released during condensation. This approach can reduce the temperature drop of the external air between the inlet and the outlet for the same amount of heat pumping capacity, thus improving the heat pumping efficiency. Having described the operation of the disclosed heat exchanger surface, techniques to form this porous coating will be discussed. There are different methods that may be used to coat a porous film for spreading water. The first method comprises coating a premade porous film onto the heat exchange surface. In this embodiment, the porous layer is formed and then anchored to the heat exchange surface. This porous layer may be paper film, as an example. FIG. 4D shows the roll- to-roll process to fabricate the paper-coated fins according to one embodiment. An aluminum sheet 50 is first coated with an anti- corrosion layer via dip coating. After exiting the anti-corrosion tank 51, the aluminum sheet 50 then enters a second tank 52 where a binder layer is applied using dip coating. Paper 53 is then applied to both surfaces of the aluminum sheet 50. The paper 53 may have a water spreading velocity of greater than 15 cm / min. Finally, the paper-coated aluminum sheet may enter a punching machine 54 and be punched as required in the heat exchanger making process. This coating process may totally prevent wrinkles after the film dries or the formation of air bubbles after the film get hydrated. This is very important to have a good heat transfer. While paper is noted above, it is understood that other materials may be used as well. For example, other fibrous metals or particulate materials that can form hydrophilic porous channels for water to evaporate may be used. FIG. 4A-4B show illustrations of the paper film coated fins at a dry state and an intermediate dynamic wetting state, respectively. Additionally, FIG. 4C shows the heat transfer performance of the paper-coated CFEC heat exchanger under different evaporation temperatures. The ambient air is at 29oC / 23%RH. The heat flux is determined by measuring the amount of evaporated water, while the temperature is set by an electrical heater. The figure shows that evaporation carries a large heat flux away from the surface, even when the surface is below the ambient air temperature. A second method of creating porous capillary layer on the heat exchanger surface is to coat the surface with a hydrophilic layer followed by pore formation. Established methods in the past to coat porous materials onto a surface may be adapted to make these coatings. Alternatively, the metal may be directly etched to form porous layer on metal surface such as by using an anodization process. Melamine foam is used as an illustration of this approach. One such material is porous melamine foam. A method to fabricate the fins with a thin layer of porous melamine foam is described herein. First, a melamine solution is prepared. This comprises the following steps. A primary polymer of waterborne melamine formaldehyde resin is prepared. Next, the primary polymer of the water-borne melamine formaldehyde resin, a polar solvent, a surfactant and an additive are mixed in a thermostatic mixing tank proportionally to obtain mixture A. Mixture A is then uniformly mixed with solid methylol melamine in a thermostatic single screw extruder or twin-screw extruder proportionally to obtain a mixture B. Mixture B is then uniformly mixed with a foaming agent proportionally to obtain a mixture C. Finally, mixture C is uniformly mixed with a curing agent in a thermostatic high-speed mixer proportionally to obtain a mixture D. This completes the preparation of the melamine solution. Next, the melamine solution is coated on the heat exchange surface. First, the heat exchange surface is dip-coated with mixture D to obtain a melamine solution coating layer with uniform thickness. After dip-coating, the heat exchange surface is transferred to an oven to reduce the water content. The water content may be reduced to about 10~25%wt at the temperature 35~60oC. This completes the coating process. Lastly, the melamine coating is foamed. This includes foaming, curing and drying the mixture D by using microwave radiation and quenching the mixture D in a microwave foaming device to obtain the porous melamine film. The specific components used of this sequence are described below. Hydrophilic and porous melamine film comprises the following raw materials in parts by weight: 0-99%wt primary polymer of waterborne melamine formaldehyde resin, 1-100%wt of solid methylol melamine, 0-43%wt of polar solvent, 0.5-5%wt of surfactant, 1-20%wt of foaming agent, 0.1-10%wt of curing agent and 0-10%wt of additive. Preferably, the high-density melamine foam comprises the following raw materials in parts by weight: 50- 90%wt of primary polymer of waterborne melamine formaldehyde resin, 10-50%wt of solid methylol melamine, 0-10%wt of polar solvent, 1-3%wt of surfactant, 5-15%wt of foaming agent, 1-5%wt of curing and 0-5%wt of additive. Specifically, the molar ratio of formaldehyde to melamine in the raw materials used for preparing the primary polymer of the waterborne melamine formaldehyde resin is 6:1 to 1:1, and the solid content is 50% to 80%. The solid methylol melamine is powder or particles of methylol melamine monomer or primary polymer, or partially etherified methylol melamine, or a mixture of the previous two materials. The polar solvent is water or alcohol with a boiling point of 40°C. to 120°C. The surfactant is an anionic surfactant, a nonionic surfactant or a cationic surfactant with the functions of emulsification, dispersion and bubbling. The foaming agent is a physical foaming agent with a low boiling point or a chemical foaming agent which is easily decomposed / reacts with acid to produce gas. The curing agent is organic acid or inorganic acid. The additive is one or more of a light stabilizer, a heat stabilizer, an antiager, a formaldehyde scavenger, a colorant and a filler. More preferably, the molar ratio of formaldehyde to melamine is 4:1 to 2:1, and the solid content is 70% to 80%. The solid methylol melamine is one or more of a monomer or primary polymer of solid trimethylol melamine, solid tetramethylol melamine, solid hexamethylol melamine, partially etherified methylol melamine or partially butylated methylol melamine. The polar solvent is desalted water or distilled water. The surfactant is one or more of aqueous or solvent-free alkyl sodium sulfonate or potassium salt, sodium alkyl benzene sultanate or potassium salt, alkylnaphthalene sulfonate, alkyl sulfate sodium salt or potassium salt, alkyl carboxylate, polyoxyethylene ether sulfate sodium salt or potassium salt, alkyl olefin sulfonate, poly- oxyethylene ether, alkanolamide, amine oxide, a silicon surfactant or a fluorocarbon surfactant. The physical foaming agent is one or more of alkane, halohydrocarbon, alcohol, ether, ester or ketone with the low boiling point. The chemical foaming agent is one or more of carbonate or bicarbonate, azodicarbamide, isocyanate or animonium chloride. The organic acid is one or more of acetic acid, propanoic acid, oxalic acid, citric acid, palmitic acid, ben- zoic acid, sulfamic acid or glutemic acid. The inorganic acid is one or more of hydrochloric acid, phosphoric acid, sulfuric acid, nitric acid, sulfurous acid, pyrophosphoric acid or boric acid. In certain embodiments, the heat exchange surface may be aluminum, such as aluminum fins. Similar process applies to other typical metals used in heat exchangers such as copper and steel. In certain embodiments, an anti-corrosion treatment is used. Herein, a method to achieve a bifunctional coating working as anti-corrosion and binder at the same time is described and shown in FIGs. 5A-5E. FIG. 5A shows that the aluminum heat exchange surface 60 is activated with dilute hydrochloric acid (0.5M). The tetraethyl orthosilicate (TEOS) is subject to hydrolysis in ethyl alcohol, as shown in FIG. 5B. In order to obtain a gelatinous precursor of coatings, the hydrolytic condensation reaction was carried out in a water bath with the temperature controlled at 35oC. TEOS is diluted with absolute ethyl alcohol (EtOH) in the beaker at a molar ratio EtOH / Si = 3:1, and this was followed by introduction of a dilute solution of formic acid with a molar ratio of water / Si = 4:1. The reaction was progressed in three steps. First, the reaction was carried out for 5 days with the beaker sealed with a plastic film, and 2 days more with several small holes of a needle size that were perforated on the film. Finally, the film was removed and the reaction was continued for another 3 days. After completion of reaction, the solution becomes viscous forming a gelatinous state. Next, as shown in FIG. 5C, the prepared aluminum heat exchange surface is immersed in the prepared TEOS solution. This process is progressed in two steps. First, the clean heat exchanger is dip- coated with TEOS solution. Second, the coated heat exchanger is disposed in the air to dry the coating layer. Additionally, these two steps are repeated 3~5 times to make sure the final TEOS coating layer is thick enough (10~50 um). The TEOS-coated aluminum heat exchange surface was then dried in an oven. Finally, before becoming totally dry, the aluminum heat exchange surface then dipped into a prepared (3- Glycidyloxypropyl)trimethoxysilane (GPTMS) solution for several minutes (see FIG. 5D). The molar ratio of the components was GPTMS: H2O: NaOH = 1:5:0.1671. In a typical preparation GPTMS (20 cc) was added dropwise to a NaOH aqueous solution (8 cc, 1.85 M, pH >14) under stirring at 25 °C; the solution became transparent in a few minutes after the addition of GPTMS. This GPTMS layer may work as the binder between TEOS and the porous layer on top (see FIG. 5E), such as the paper layer described above. Having described the CFEC heat exchanger and techniques to prepare the porous layer, its use in various systems will be described next. Air Conditioning Systems Air-conditioning (AC) accounts for 70% of peak residential electricity consumption and ~4% of global greenhouse gas emissions. Since Carrier invented modern AC system in 1902, the progress in improving the AC efficiency has been slow, ~1.7% per year, as shown in FIG. 6. Global warming will force deployment of more AC systems, further accelerating climate change unless the efficiency of AC systems can be drastically improved. The AC industry expects that the AC market will expand 3 times by 2050 and 5 times in developing countries. The CFEC heat exchanger described above may be used to dramatically improve the efficiency of these air conditioning systems. Before described the improvement afforded by the present heat exchanger, a discussion of the operation of a typical AC system is provided. FIG. 7A illustrates the key components of an AC system, the flow paths of the refrigerant, and the indoor and outdoor air. The refrigerant thermodynamic cycle is shown in FIG. 7B, wherein pressure is displayed on the vertical axis and enthalpy is on the horizontal axis. Referring to FIGs. 7A-7B, refrigerant is expanded (9” to 5) by the expansion valve 70 to temperatures which are sometimes as low as 5oC. The refrigerant then passes through the evaporator 71 to absorb heat from air 75 (5 to 5’). This low temperature is needed to condense out water in the moist air. The evaporated refrigerant is then compressed to a high temperature by a compressor 72 situated outdoor (5’ to 9’). The compressed refrigerant is cooled down through a heat exchanger (condenser 73) by the outside air 76 to subcooled liquid state at high pressure (9’-9”). To transfer heat from the refrigerant to the outside air 76 in the condenser 73, the refrigerant needs to be compressed to a high temperature, such as 55oC in the illustrated example (the AHRI standard for outdoor air at 35oC specifies that the condenser work at 50-55oC). The high-pressure refrigerant then expands through an expansion valve 70 (9”-5) back to 5oC to complete the cycle. Hence, the refrigeration cycle must work over a wide temperature range, 5-55oC in this example, to condition room air typically desired at 22oC. To understand the reason for this wide range of temperatures, the thermodynamic processes of indoor air (cross hatched dots) and the outdoor air (solid dots) are illustrated in the psychrometric chart in FIG. 7C. The shaded region on the chart indicates the condition of air that humans feel comfortable. A portion of room air (called return air) at State 1 is sent to the evaporator, where it cools down to State 2, ~ 8oC, which is 3oC higher than the condenser surface temperature at 5oC (State 5) to allow heat transfer from the air to the refrigerant. Moisture in air at 8oC is supersaturated and condenses on the evaporator. After water is condensed out, the conditioned air will mix with some of the bypass return air to State 3, and then supplied to the room. If the dehumidification load is heavy, the evaporator-outlet air at State 3 needs reheat (sometimes electrically) to State 4 to satisfy the requirement of the comfortable supply air, before it re-enters the air-conditioned space. The outside ambient air at 35oC (State 6) enters the condenser with a surface temperature of 55oC (State 9), and exits the condenser at 50oC (State 8). Note that although the temperature excursion of the indoor air (1-2) is comparable to that the outdoor air (6-8), the enthalpy change of the outdoor air is only ~50% of indoor air, due to the latent heat release of water moisture during condensation. Hence, the outdoor air flow rate is ~2 times that of the indoor air. A large temperature difference is needed between the condenser surface and the outside air so that the size of the condenser is reasonable. For such an example, ^் ଶ^଼the Carnot cycle coefficient of performance is ^^^^^^ ൌ ^் ି ^ ൌ=5.6. ^^்ହ^ The best commercial AC using R410A as refrigerant already exhibits a COP of 4.7 (SEER 16), which is 84% of the theoretical maximum, although practical AC systems have an average COP of 3.5 only. The room for improvement is limited unless the working temperature range can be fundamentally changed. On the other hand, evaporative cooling is a widely known phenomenon and used in developing and underdeveloped regions. It is known that due to evaporative cooling, water can be maintained at a temperature below the ambient temperature, with the limit being the wet bulb temperature. Evaporative cooling has been integrated in different air conditioning systems to cool outside air by spraying water directly into air, before it goes through the condenser of an AC system. The lower incoming air temperature means that the condenser surface temperature will be reduced correspondingly. For example, assume the condenser surface temperature originally is at 55oC when the outside temperature is 35oC. In SEC system, the incoming air can be cooled to the wet bulb temperature of 24oC, the condenser surface can be correspondingly reduced to 47oC. The reduced condenser surface temperature translates into higher efficiency for the refrigerant cycle. Using the disclosed CFEC air-side heat exchanger, the condenser surface can be maintained close to the wet-bulb temperature, ~24oC in the above cited example, because evaporative cooling relies on the chemical potential difference of water and moist air, and because evaporation involves phase change that is efficient in taking away the heat. FIG. 8A shows an AC system using the CFEC heat exchanger outside. In this figure, the condenser 80 includes tubes 81 through which the refrigerant flows, and external heat exchange surfaces. These external heat exchange surfaces may include fins 82 with a water film 83. The evaporator 84, compressor 85 and the expansion valve 86 are as described above in FIG. 7A. FIG. 8B shows the indoor and outdoor air on a psychrometric chart with heat recovery as shown in FIG. 2B included. In this figure, RA represents the return air from the inside room, SA represents the supply air to the room, DP represents the dew point, OA represents the outside air, EA represents the exhaust air. ei and eo represent the inlet and outlet air states of the CFEC condenser, respectively. Subscripts evap and cond refer to the evaporator and condenser, respectively. Subscript CFEC / w HR refers to the CFEC condenser with heat recovery. Noted that the cooling air process through the CFEC condenser with heat recovery follows the sequence OA→ei→eo→EACFEC W / HR, where the path will go through the heat recovery heat exchanger (OA→ei), the CFEC condenser (ei→eo) and the heat recovery heat exchanger (eo→EACFEC W / HR) in turn. The actual process of air may slightly deviate from what is drawn in the figure; for example, OA may be directly connected to eo via a dashed line due to nonequilibrium state of the air. In this case, without any change of the indoor air cycle, the AC will work between 5oC to 24oC, with a maximum COP of 12.6, which more than doubles the maximum COP of existing systems. This number just illustrates one example for one working condition, and those experienced in thermodynamics can readily design different conditions. Solid State Thermoelectric Coolers Another possible use of the capillary-film-evaporation- cooling (CFEC) heat exchanger is in solid state thermoelectric coolers (TEC). Solid-state thermoelectric coolers (TEC) are attractive because they do not use refrigerant. However, commercial thermoelectric cooling module has a very low efficiency when they are used for air conditioning because of the large temperature difference at its two ends and the low figure of merit (ZT) of thermoelectric materials. Using CFEC heat exchangers, the thermoelectric cooling based air-conditioning systems may be improved to have significantly higher efficiency. The basic idea of thermoelectric-enhanced indirect evaporation cooler (TEAC) is to use the TEC to make the indirect evaporation cooling more efficient. As shown in FIG. 9A, the processing air 90 will be cooled at the cold end of the TEC module 91 and the rejected heat will be carried away by CFEC heat exchanger at the hot end of the TEC module 91. The TEC module 91 includes a cold side, in communication with a first heat spreader 92 and optionally fins 93 to cool the processing air 90. The TEC module 91 also includes a hot side, which includes a second heat spreader 94, which is coated by the water layer 95, which represents the CFEC heat exchange surface 99 described above and heats the exhaust air 98. The hot and cold side are generated by semiconductor devices, such as Peltier coolers. These semiconductor devices may be alternating N-type devices 96 and P- type devices 97. To maximize the TEC efficiency, the evaporation temperature at the hot side of each TEC module 91 should be maintained within 10 K of the local processing air temperature at the cold side of the same TEC module. To minimize water consumption, the heat recovery as shown in FIG. 2B may be applied between the incoming outdoor air and the exiting exhaust air at the hot end of TEC module 91. This air conditioning strategy is shown in FIG. 9A-9C, wherein FIG. 9B shows the structure of a single TEAC and FIG. 9C shows an assembly that includes a plurality of TEAC modules. Modeling shows, even for the commercial TE materials (ZT~0.6 at 27oC), the TEC efficiency that is achieved, which is about 3.0 to 4.0, is comparable to traditional vapor compression refrigeration technologies. This system was modelled assuming the following parameters and the use of a louver fin. The air-side convection heat transfer coefficient is assumed to be h=120 W / m2K, the fin to TEC area ratio is Afin / ATEC=20 , the fin effectiveness is ηfin=0.9, coupled to a TEC module with a ZT value of 0.6, and a size of 4cm^4cm (or ATEC=16 cm2). The water film thickness is δw=0.1mm, thermal conductivity is kw=0.6 W / mK. The temperature of the inlet processing air is 26oC, and the required supply air temperature is Tsa=18oC. As shown in FIG. 10, the TEAC performance is simulated with different thermal conduction resistance ratios between the heat spreader (Rs) and the fin (Ra) under different numbers of transfer unit at the cold side. The result shows when the ratio Rs / Rais smaller than 0.2 and NTU is larger than 3.2, the energy efficiency of the proposed TEAC (COPTEAC) will be greater than 3.0. One 4cm^4cm TEC module can achieve a cooling capacity of about 10 W. Thus, the TEAC is an efficient, affordable but more compact green cooling technology. TEAC may also be integrated with refrigerators. In a typical household condition of 25 °C and 50% relative humidity, the wet- bulb temperature is 18.6 °C. Air inside a refrigerator is ~3 °C. Hence, a TEAC may be designed to work between 0-20 °C. With a nondimensional thermoelectric figure of merit of 0.8, the COP of such a TEAC may be 1.56. Although this COP is not very high, the TEAC does not use refrigerants that cause global warming. Thus, the TEAC may replace the compressor in an indoor refrigerator. Cooling Towers and Power Plant Condenser Steam cycle power plants are equipped with condensers where exhaust steam is condensed back into water which is recirculated to the power plant. Two different steam condensers are used in two fundamentally different cooling systems: the surface condenser with wet cooling tower and the air-cooled condenser. While steam is disclosed herein, it is understood that this concept applies to any vapor. In a surface condenser with wet cooling tower 100, cold water 101 coming from wet cooling tower 100 is circulated to condense the steam in a surface condenser 102, as shown in FIG. 11A. The warm water coming out of the surface condenser 102 is sprayed into air 106 in a cooling tower and evaporatively cooled down, where air 106 is forced through the tower by either natural draft or mechanical fans 107. The exhaust steam 103 from the turbine 104 is condensed at the outside of the surface condenser tubes. Only part of the cycled cooling water is evaporated in the cooling tower (approximate 1% to 1.5% of the water flow). This leads a low evaporation efficiency and high pump power consumption. And also, some cooling water is lost due to drift (air entrainment) and blow- down, which increases the water consumption. Thus, a continuous source of fresh water (make-up water 105) is required to operate a wet cooling tower. Fog and ice often form around the wet-cooling tower, which have negative impacts on environment. Another form of condensing system is the air-cooled condenser, which is shown in FIG. 11B. The process is similar to that of a radiator. Exhaust steam 110 from the low-pressure section of a steam turbine 111 runs through the condensing tubes. Heat transfers from the process steam to the cooling air via extended surfaces or tubes 112. The tubes 112 are usually finned and ambient air is pushed through the fins with the help of a large fan 113. The steam condenses to water 114 to be reused in the water-steam cycle. Air-cooled condensers are more environmentally acceptable forms of condensing steam, as there is no water usage or fog problem. However, the performance of the air-cooled condenser is less efficient than wet cooling tower design because the ambient dry air temperature is higher than its wet air temperature. As shown in FIG. 11C, the CFEC heat exchanger may be used with the air-cooled condenser of FIG. 11B. The exhaust steam 120 enters the finned tubes 121 shown in FIG. 11B. The porous layer 122 described above is applied to the finned tubes 121. As described, a fan 123 may be used to push ambient air through the fins. The heat is extracted from the steam, resulting in condensate 124, which can be recirculated to the power plant. Using the CFEC heat exchanger as the condenser will significantly reduce the size of the heat exchanger compared to dry air cooling. Further, the CFEC heat exchanger will reduce the amount of water used compared to wet-cooling tower. The lower steam condensate temperature using CFEC technology also means that the steam exit temperature at the steam turbine can be even lower, enhancing the power plant efficiency as a whole. The CFEC heat exchanger as in FIG. 11C, in the form of a cooling tower, can also be coupled to the vapor 130 from a vapor compression cycle air-conditioning system to cool the vapor to create refrigerant 134, as shown in FIG. 11D. The vapor 130 enters the finned tubes 131 and the porous layer 132 described above is applied to the finned tubes 131. As described above, a fan 123 may be used to push ambient air through the fins. This configuration in principle is similar to the conventional split-type air- conditioners. Such cooling towers are used often in cooling large buildings such as data centers. For data centers, as the humidity is not of a major concern, it is possible to just use such capillary heat transfer surfaces to cool outside air directly, which is currently done by some direct spray cooling towers. FIG. 11E illustrates an implementation. Water is passed through channels 140, which may include walls 141 having perforated holes 142. The water passes through the perforated holes 142 and forms a water film 143 on the surface of the fins, creating water film coated fins 144. Fan 145 may be used to push a gas stream, which may be ambient air, through the water film coated fins 144. This may be used to cool the data center to a desired temperature. Building Envelope with Evaporative Cooling Evaporative cooling for buildings is a technology that utilizes water evaporation and blowing air to cool buildings. Traditionally, in an evaporative cooling system, hot outside air is forced through wet cooling pads by means of a motor-driven fan. There are three kinds of evaporative cooling systems found in buildings: (1) direct evaporative cooling, where the process air directly exchanges heat with the cold water. This results in more deep cooling but humidifies the process air; (2) indirect evaporative cooling, where the process air indirectly exchanges heat with the cold water / air through a heat exchanger. This results in reduced cooling capacity but there is no need for additional dehumidification; and (3) direct / indirect evaporative cooling, which is also referred to as dew point evaporative cooling. This combines the benefits of the direct and indirect evaporative coolers. However, all evaporative cooling products, where air and water come into direct contact, experience some level of drift due to the exiting flooding water at the evaporative interface. Drift is defined as liquid water droplets from the recirculating flow entrained in the discharge air stream. Water evaporative cooling walls represent a very powerful technology for summer cooling of buildings and can preserve high insulation levels, which make them high-performing in cold seasons as well. The “wet-wall” solution is available both in ceramic or cement or pottery bricks, or aluminum and PVC pads where the vertical walls are cooled by a liquid film falling in a plane channel, along which air mass is allowed to flow down while reducing temperatures and draining heat from the wall. These existing cooling wall uses falling water film, which suffers from similar problems as the above-mentioned falling film heat exchangers, as described with respect to FIG. 1B. A CFEC wet wall may be used in this application. FIG. 12A shows the basic structure of the proposed building envelope with evaporative cooling. This includes a metal extrusion profile 150 with one or more channels 151 having internal surfaces, such as fins 152, coated with a water film 153. The metal extrusion may be aluminum, copper, steel or another metal. In other words, the internal surfaces serve as the CFEC heat exchange surfaces. There are two basic configurations, as shown in FIG. 12B and FIG. 12C. In FIG. 12B, all capillary-drawn water films (which are referred to as wet channels 156) are in contact with the fresh air 155 that is going to be used indoors. This fresh air 155 may be pushed through the wet channels 156 using fans 157. This fresh air 155 may be further mixed with drier outdoor or return air as the building supply air. In FIG. 12C, the fresh indoor air 160 passes through dry channels 162. This fresh indoor air 160 is separated from the wet channels 161, which is exhausted to the outside as exhaust air 163. In this way, the indoor air has lower humidity. As described above, a fan 164 may be used to push air through the wet channels 161. The present system has many advantages. As explained above, evaporation cooling is more efficient. However, current systems have drawbacks. By disposing a hydrophilic porous layer on the heat exchange surface, the new capillary film evaporative cooling technique is able to provide improved heat transfers. Further, the thickness of the porous layer does not preclude its use in any traditional applications. In some modeling, it was determined that the coefficient of performance of an traditional AC system may be doubled using this approach. Further, this technique is applicable to thermoelectric coolers, steam plant condensers and building envelopes. The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Claims

What is claimed is:

1. A heat exchanger, comprising: an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action.

2. The heat exchanger of claim 1, wherein gravity is also used to spread the fluid.

3. The heat exchanger of claim 1, wherein the fluid comprises water.

4. The heat exchanger of claim 1, wherein the external heat exchange surface comprises one or more fins.

5. The heat exchanger of claim 1, wherein an anti-corrosion layer is disposed beneath the hydrophilic porous layer.

6. The heat exchanger of claim 1, wherein the hydrophilic porous layer comprises paper.

7. The heat exchanger of claim 1, wherein the hydrophilic porous layer comprises melamine foam.

8. The heat exchanger of claim 1, wherein a thickness of the hydrophilic porous layer is less than 500 microns.

9. The heat exchanger of claim 1, wherein a control unit is used to supply the fluid such that the hydrophilic porous layer is wetted.

10. The heat exchanger of claim 1, further comprising a channel through which a fluid to be cooled passes.

11. The heat exchanger of claim 1, wherein the fluid is delivered through a channel having perforated holes, such that the fluid is spread to the hydrophilic porous layer via capillary action.

12. The heat exchanger of claim 1, further comprising tubes containing a fluid to be cooled, wherein the external heat exchange surface is thermally connected to the tubes containing the fluid to be cooled.

13. A cooling system, comprising: the heat exchanger of claim 1; wherein a gas flow passes over the external heat exchange surface.

14. The cooling system of claim 13, wherein the gas flow comprises air, which is cooled as it passes over the external heat exchange surface.

15. A data center comprising the cooling system of claim 14.

16. A cooling system, comprising: a heat recovery exchanger, comprising a first channel having a first input and a first output and a second channel having a second input and a second output, wherein the first input receives outdoor air and wherein the second output delivers exhaust air and wherein the outdoor air and the exhaust air exchange heat through walls of the heat recovery exchanger; and a heat exchanger comprising: a channel through which a fluid-to-be-cooled passes; an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, and wherein air from the first output passes over the external heat exchange surface and is cooled by the external heat exchange surface; and wherein the cooled air is provided to the second input.

17. An air conditioning system, comprising: a condenser; a compressor;an expansion valve; and an evaporator; wherein the condenser comprises one or more tubes to carry refrigerant, and an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action.

18. The air conditioning system of claim 17, wherein gravity is also used to spread the fluid.

19. The air conditioning system of claim 17, wherein the fluid comprises water.

20. The air conditioning system of claim 17, wherein a control unit is used to supply the fluid to wet the hydrophilic porous layer.

21. The air conditioning system of claim 17, further comprising a heat recovery exchanger, comprising a first channel having a first input and a first output and a second channel having a second input and a second output, wherein the first input receives outdoor air and provides cooled outdoor air to the condenser, and wherein the second output delivers exhaust air that passes over the condenser, and wherein the outdoor air exchanges heat with the exhaust air though walls of the heat recovery exchanger.

22. A thermoelectric cooling system, comprising: a plurality of thermoelectric devices creating a cold side and a hot side; a first heat spreader in communication with the cold side; and a second heat spreader in communication with the hot side, wherein the second heat spreader is coated with ahydrophilic porous layer, which is adapted to spread a fluid using capillary action.

23. The thermoelectric cooling system of claim 22, wherein gravity is also used to spread the fluid.

24. The thermoelectric cooling system of claim 22, wherein the fluid comprises water.

25. The thermoelectric cooling system of claim 22, wherein a control unit is used to supply the fluid to wet the hydrophilic porous layer.

26. An indoor refrigerator, wherein the thermoelectric cooling system of claim 22 is used to cool air inside the indoor refrigerator, eliminating a need for a compressor.

27. A vapor condenser, comprising: a vapor duct to deliver vapor to a heat exchanger; the heat exchanger comprising an internal channel through which the vapor flows; and an external heat exchange surface, wherein the external heat exchange surface is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action; wherein an output of the heat exchanger comprises condensate which is recirculated.

28. The vapor condenser of claim 27, wherein gravity is also used to spread the fluid.

29. The vapor condenser of claim 27, wherein the fluid comprises water.

30. The vapor condenser of claim 27, wherein a control unit is used to supply the fluid to wet the hydrophilic porous layer.

31. A steam plant, comprising the vapor condenser of claim 27, wherein the vapor comprises steam from the steam plant and the condensate is recirculated to the steam plant.

32. An air conditioning system, comprising the vapor condenser of claim 27, wherein the vapor is from a vapor compression cycle of the air conditioning system and the condensate is recirculated to the air conditioning system.

33. A structure for use with a building envelope, comprising: a metal extrusion profile, comprises one or more internal channels passing therethrough; wherein one or more of the one or more internal channels is coated with a hydrophilic porous layer, which is adapted to spread a fluid using capillary action.

34. The structure of claim 33, wherein gravity is also used to spread the fluid.

35. The structure of claim 33, wherein the fluid comprises water.

36. The structure of claim 33, wherein a control unit is used to supply the fluid to wet the hydrophilic porous layer.

37. The structure of claim 33, wherein air for outside the building flows through the one or more internal channels that are coated with the hydrophilic porous layer, and air from inside the building flows through different internal channels.

Images