WO2024251920A1

WO2024251920A1 - Indirect evaporative cooling system

Info

Publication number: WO2024251920A1
Application number: PCT/EP2024/065680
Authority: WO
Inventors: Muhammad Wakil SHAHZAD; Ben Bin XU
Original assignee: Northumbria University
Current assignee: Northumbria University
Priority date: 2023-06-09
Filing date: 2024-06-06
Publication date: 2024-12-12
Anticipated expiration: 2025-12-09
Also published as: GB2630806B; GB2630806A; GB202308625D0

Abstract

An indirect evaporative cooling system comprising a wet channel and a dry channel arranged in thermal contact along a thermally conductive contact interface which extends at least part of the respective lengths of the channels. The contact interface forms a heat exchanger between the channels such that, during operation, when humidified air in the wet channel evaporates, the heat exchanger is cooled, subsequently cooling input air passing through the dry channel.

Description

Indirect Evaporative Cooling System

Technical Field

The present invention relates to indirect evaporative cooling systems.

Background

Indirect evaporative cooling systems conventionally involve one or more “wet” channels arranged relative to one or more “dry” channels. In such systems, the dry channel is used to transport ambient air, while the wet channel is responsible for transporting the working fluid, which has been humidified to facilitate evaporative cooling. The cooling effect in these systems is achieved by heat exchange between the ambient air (in the dry channel) and humidified air (in the wet channel) across a heat exchange partition. The heat transported from the dry channel causes evaporation of water in wet channel, thus causing the air in the dry channel to cool down. These systems often employ certain materials to facilitate the humidification of the working fluid in the wet channel, with saturated felt being a commonly used material.

However, the conventional use of saturated felt in indirect evaporative cooling systems have a number of drawbacks. A first notable issue is that the felt material can degrade and detach over time, potentially obstructing air passages and reducing system efficiency. Further, the layered structure, comprising felt, adhesive, and a hydrophobic surface, creates additional thermal resistance, hampering the heat exchange process. Further still, the surface of the felt is conducive to microbial growth, posing potential health risks and adding to maintenance demands.

Water management in these systems is another challenging aspect. Ensuring the felt is optimally saturated for efficient cooling without causing waterlogging or excessive weight can be difficult. This issue, coupled with the inherent bulk of the felt and the added weight of water, contributes to such systems becoming heavy and cumbersome. Such bulkiness affects portability, ease of installation, and suitability for space-restricted locations.

An alternative approach to this issue is to humidify the air externally before introducing it into the wet channel. While this method offers a potential solution to the problems posed by the use of saturated felt, conventional implementations of this method also have their own drawbacks. A significant problem with this approach is that the evaporative potential of the humidified air (and hence its cooling potential) diminishes the further it travels along the wet channel from the humidifier. This leads to a decrease in cooling efficiency, particularly for larger spaces or longer channel lengths where the working fluid must travel a significant distance before reaching its destination. A proposed solution has been to introduce humidified air from multiple sources, each entering the wet channel at different points. However, this approach results in designs that are complex in their construction and unwieldy in terms of maintenance and operation. Summary of the Invention

In accordance with a first aspect of the invention, there is provided an indirect evaporative cooling system comprising a wet channel and a dry channel arranged in thermal contact along a thermally conductive contact interface which extends at least part of the respective lengths of the wet channel and a dry channel. The contact interface forms a heat exchanger between the wet channel and dry channel such that, during operation, when humidified air in the wet channel evaporates, the heat exchanger is cooled, subsequently cooling input air passing through the dry channel. The dry channel comprises an air intake at a first end and an air outlet at a second end such that, in use, the input air travels in a first flow direction. The wet channel is provided with a humidified air entry point positioned at an intermediate point along the length of the wet channel that is in thermal contact with the dry channel via the thermally conductive contact interface. The wet channel is further configured such that a first part of input humidified air travels from the humidified air entry point in a direction of the first flow direction and a second part of the input humidified air travels from humidified air entry point in a direction counter to the first flow direction.

Optionally, the intermediate point is half way along the part of the length of the wet channel in thermal contact with the dry channel.

Optionally, the wet channel comprises a first humidified air exit point located at a first end and via which the first part of the input humidified air traveling in the first flow direction exits the wet channel, and a second humidified air exit point located at a second end and via which the second part of the input humidified air traveling in the second flow direction exits the wet channel.

Optionally, the system further comprises an airflow inducer located at the air intake of the dry channel and configured to drive input air through the dry channel.

Optionally, the airflow inducer comprises a fan.

Optionally, the airflow inducer comprises a forced air fan.

Optionally, the system further comprises a humidifier coupled to the humidified air entry point and configured to supply humidified air to the wet channel.

Optionally, an output of the dry channel is connected to the humidifier, thereby supplying the humidifier with air to be humidified.

Optionally, the output air of the dry channel is connected to the humidifier via a flow control device.

Optionally, the system further comprises a control unit coupled to the flow control device and configured to control a proportion of the output air from the dry channel which is input to the humidifier. Optionally, the control unit is configured to control the proportion of the output air from the dry channel input to the humidifier responsive to a temperature sensor signal received from a temperature sensor.

Optionally, the flow control device is configured so that the proportion of the output air input from the dry channel to the humidifier can be varied between 20% to 50%.

Optionally, the heat exchanger comprises one or more thin metal layers.

Optionally, the one or more thin metal layers are textured.

In accordance with embodiments of the invention, a modified indirect evaporative cooling system is provided. The system eliminates the need for internal humidification materials like saturated felt by humidifying working fluid (humidified air) in the wet channel by use of an external humidifier. However, unlike conventional arrangements which deploy this technique, in accordance with embodiments of the present invention, the wet channel of the system is provided with a humidified air entry point positioned at an intermediate point (typically substantially halfway along) the length of the wet channel where it is in thermal contact with the dry channel. Further, the wet channel is configured (typically by virtue of two working fluid exhaust points at opposite ends of the wet channel) such that, a first part of the working fluid travels from the humidified air entry point in substantially the same flow direction as air in the dry channel and a second (typically equal amount) of the working fluid travels in a direction counter to the flow direction of air in the dry channel. It has been found that two flows of working fluid traveling in opposite directions in the wet channel which are introduced in the wet channel at an intermediate point, give rise to a more effective distribution of evaporative potential (and thus enhanced cooling effect) compared to conventional systems where there is generally only a single direction of flow of the working fluid in the wet channel.

Various further features and aspects of the invention are defined in the claims.

Brief Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings where like parts are provided with corresponding reference numerals and in which:

Figure 1 provides a simplified schematic diagram depicting an indirect evaporative cooling system arranged in accordance with an example of the invention;

Figure 2, provides a simplified schematic diagram depicting an indirect evaporative cooling system as shown in Figure 1 and in which the direction of various airflows are depicted, and

Figure 3 provides a simplified schematic diagram depicting an alternative channel configuration in accordance with certain embodiments of the invention.

Detailed Description

Figure 1 provides a simplified schematic diagram depicting an indirect evaporative cooling system 101 arranged in accordance with a simple embodiment of the present invention and arranged in use, to receive ambient air, cool it, and supply it into be a designated space to cool the designated space.

The system includes a dry channel 102 and a wet channel 103. The system further comprises a humidifier 104 connected to the wet channel 103 via a humidified air conduit 105 which enters the wet channel 103 at a humidified air entry point 106. The wet channel 103 comprises a first humidified-air exit point 107 at one end, and a second humidified-air exit point 108 at the other end.

An air intake 109 is located at one end of the dry channel 102 and a cooled-air outlet 110 is located at the other end of the dry channel 102.

Located at the air intake end of the dry channel 102 is an airflow inducer provided by a forced air fan 112.

The output of the dry channel 102 is further coupled to the humidifier 104 via a humidifier air-feed conduit 113. A flow control device provided by a damper 114 is located at the entry point of the humidifier air-feed conduit 113.

The cooled-air outlet 110 is positioned within a designated space 115 to be cooled by the indirect evaporative cooling system 101 . The system further comprises a control unit 116 which is connected via a signal line 117 to the damper 114 and a temperature sensor 118 located in the designated space 115.

The dry channel 102 and wet channel 103 are positioned in contact with each other along part of their respective lengths. Along the length where the dry channel 102 and wet channel 103 are in contact is located a thermally conductive contact interface 119. As will be described in more detail below, this contact interface 119 forms a heat exchanger such that the dry channel 102 and wet channel 103 are in thermal contact with each other.

Operation of the indirect evaporative cooling system 101 will now be described with reference to Figure 2 which corresponds to Figure 1 except that arrows are provided indicating air flow within the system.

Air to be cooled is driven into the air intake 109 of the dry channel 102 by the forced air fan 112. Typically, the forced air fan 1 12 is positioned relative to the dry channel 102 to minimise the degree to which the motor of the forced air fan 112 has a heating effect on air entering the dry channel 102

This air travels in a first flow direction, depicted in Figure 2 as from left to right. Meanwhile, humidified air (working fluid) from the humidifier 104 enters the wet channel 103 at the humidified air entry point 106 and a first proportion of the humidified air travels in the same direction as the first flow direction and exits the wet channel 103 at the first humidified-air exit point 107 and a second proportion of the humidified air travels in a counter direction to the first flow direction (shown from right-to-left in Figure 2) and exits the wet channel 103 at second humidified-air exit point 108.

Water droplets in the humidified air in the wet channel 103 undergo evaporation, a process which consumes thermal energy, or heat, from its immediate surroundings. This phenomenon is driven by the principle of latent heat of evaporation, where the air molecules at the liquid-air interface absorb the required latent heat from the wet channel's surroundings and transform from a liquid state to a gaseous state. This evaporation process consequently causes a drop in temperature within the wet channel 103.

The decrease in temperature in the wet channel 103 thereby cools the contact interface 119. As the contact interface 119 is thermally conductive, it absorbs heat from the air traveling through the dry channel 102, guided by the natural movement of heat from a region of higher temperature to a region of lower temperature. This heat transfer from the dry channel 102 to the contact interface 119 effectively reduces the temperature of the air in the dry channel 102.

This cooled air, now at a lower temperature than when it entered, then exits the dry channel 102 at the cooled-air outlet 110. This mechanism provides a continuous supply of cooled air to the designated space 115, thereby effectively reducing its ambient temperature without introducing additional humidity, as the air in the dry channel does not come into direct contact with the water in the wet channel 103.

Via the pressure generated by the forced air fan 112, a proportion of the cooled air is diverted via the damper 114 into the humidifier air-feed conduit 113 and into the humidifier 104. In this way, the working fluid of the system (i.e., the humidified air in the wet channel 103), is already cooled relative to the air entering the dry channel 102 therefore offering a relatively high cooling potential as working fluid.

Moreover, such an arrangement provides a relatively simple configuration which requires only a single air flow inducer (the forced air fan 112) and a single flow control device (the damper 114) to direct both the input air being cooled and the working fluid through the dry and wet channels 102, 103, respectively.

The proportion of the cooled air diverted to the humidifier 104 is dependent on the degree to which the damper 114 is opened. This is controlled, via the signal line 117, by the control unit 1 16.

As can be seen from Figures 1 and 2, the humidified air entry point 106 is located at an intermediate point - typically approximately half way - along the part of the length of the wet channel 103 that is in thermal contact with the dry channel 102 via the thermally conductive contact interface 119.

Consequently, by virtue of the provision of the first humidified-air exit point 107 and second humidified- air exit point 108 located at either end of the wet channel 103, approximately half of the humidified air travels in the first flow direction and exits the wet channel 103 via the first humidified-air exit point 107 and approximately half of the humidified air travels in a direction counter to the first flow direction and exits the wet channel 103 via second humidified-air exit point 108.

By inputting humidified air at an intermediate point along the length of the wet channel 103 which is adjacent to the contact interface 119 (rather than at one end), it has been found that a more effective distribution of humidity can be achieved along the length of the wet channel 103. Consequently, a greater length of the contact interface 119 is exposed to humidified air with a higher evaporative cooling potential thereby providing more efficient heat transfer from the air in the dry channel 102 to that in the wet channel 103. In turn this provides an enhanced cooling effect on the air that passes through the dry channel 102.

The control unit 116 is configured to regulate operation of the indirect evaporative cooling system 101 shown in Figure 1 by adjusting (varying) the degree to which the damper 114 is opened thereby controlling the fraction of the cooled air from the dry channel 102 which is diverted into the humidifier air-feed conduit 113.

The temperature sensor 1 18 is configured to detect the temperature of the air in the designated space

115 and send a corresponding sensor signal to the control unit 116 responsive to which the control unit

116 controls the degree to which the damper 114 is opened.

In particular, as the temperature within the designated space 115 increases, the control unit 116 controls the damper 114 to increase the rate of flow of air diverted into the humidifier 104 thereby increasing the rate of flow of the working fluid (the humidified air) in the wet channel 103 thereby increasing the cooling potential of the system.

Similarly, as the temperature within the designated space 115 decreases, the control unit 116 controls the damper 1 14 to decrease the rate of flow of air diverted into the humidifier 104 thereby reducing the rate of flow of the working fluid (the humidified air) in the wet channel 103 thereby decreasing the cooling potential of the system.

In typical examples the control unit 116 is configured to control the damper 1 14 so that the fraction of cooled air from the dry channel 102 diverted into the humidifier air-feed conduit 113 remains within a predetermined range.

In certain examples, an optimal range has been found to be a range between 20% and 50% of the air from the dry channel 102. Such a range has been found to be a good compromise between maintaining a useful volume of cooled air into a designated space being cooled, whilst adapting for common fluctuation ranges in temperature in designated spaces such as rooms of buildings due, for example, to the effects of heating from sunlight through windows, heating due to the use of electrical equipment, changes in the number of occupants and so on.

In such examples, to achieve maximum cooling of the designated space 115, 50% of the air from the dry channel 102 is diverted into the humidifier air-feed conduit 113, and to achieve a minimum degree of cooling, 20% of the air from the dry channel 102 is diverted into the humidifier air-feed conduit 113.

As is depicted in Figure 1 , the contact interface 119 which thermally couples the dry channel 102 and wet channel 103 is formed from adjacent sections of the walls of the dry channel 102 and wet channel 103. These sections of wall typically comprise a thermally conductive material, such as thin metal sheeting. A metal such as aluminium can be used due to its thermal conductivity and its lightweight, corrosion-resistant properties.

In typical examples, these sections ofthin metal sheeting are textured, for example corrugated, dimpled or similar. Such texturing increases the turbulence of the air streams within the respective channels and expands the surface area of the interface 119, thereby enhancing the heat transfer between the channels. Further, this texturing, in particular corrugation, improves the structural strength of the channels, enabling them to withstand the operational pressures and stresses within the system.

As would be apparent to the skilled person indirect evaporative cooling systems, in accordance with examples of the invention, can be constructed using a variety of suitable materials, including but not limited to metals, plastics, composites, and ceramics, or any combination thereof. The selection of these materials should take into account factors such as durability, cost, thermal conductivity, and resistance to corrosion.

The construction of these systems can be accomplished using manufacturing techniques that will be familiar to a person skilled in the art. These techniques can include, for instance, moulding, casting, welding, 3D printing, or any other fabrication methods appropriate for the chosen materials and design requirements.

Moreover, the deployment of these systems can occur in a variety of settings that a skilled person will be familiarwith. These settings could range from residential homes, commercial buildings, and industrial facilities to data centres and other environments requiring efficient cooling solutions. The suitability of a specific setting will depend on factors such as the size and layout of the space, ambient conditions, and specific cooling needs."

The implementation of the invention in the system depicted in Figure 1 is a simple example of the invention and the skilled person will appreciate that in other embodiments, more complex configurations may be implemented. For example, a modular approach may be taken where an arrangement of a wet channel and dry channel generally taking the form as depicted in Figure 1 may be repeated in a modular fashion, thereby creating a system comprising multiple wet and dry channels. In such examples, the wet channels may be fed with humidified air from one or more humidifiers.

In alternative embodiments of the invention, various different structural configurations may be taken. For example, in the embodiment described above, the indirect evaporative cooling system 101 is provided with a single airflow inducer (the forced air fan 1 12) which is configured to drive input air into the dry channel 102 and also provide the means by which air is driven into the humidifier air-feed conduit 113. However, in alternative embodiments, one or more additional air flow inducers maybe deployed, for example one or more separate air flow inducers (e.g., one or more further forced air fans) may be provided to drive air into the humidifier air-feed conduit 113.

Further, in the example of the invention described above, the wet channel 103 is provided by a substantially continuous conduit terminated at either end by the first humidified-air exit point 107 and second humidified-air exit point 108, and positioned at a substantially intermediate point is the humidified air entry point 106. This configuration ensures that the humidified air travelling through the wet channel splits into two approximately equal flows: a first flow in the direction of the flow of the air in the dry channel 102 and a second flow which is counter to the direction of the flow in the dry channel 102. However, alternative configurations are possible where the wet channel is divided into separate conduits, each conduit configured to direct either humidified air in the first flow direction or in a direction counter to the first flow direction. An example of this is depicted in Figure 3.

Figure 3 provides a simplified schematic diagram depicting an alternative channel configuration 301 (other parts of the system are omitted for clarity) in which the dry channel 302 is adjacent a wet channel which is formed from a first wet-channel conduit 303a and a second wet-channel conduit 303b.

The first wet-channel conduit 303a receives humidified air from the humidifier 304 via a first humidified air conduit 305a and is terminated by a first humidified-air exit point 306. The first wet-channel conduit 303a of the wet channel is configured to direct humidified air from the humidifier 304 in a first flow direction which is the same as the flow direction of the air in the dry channel 302.

The second wet-channel conduit 303b receives humidified air from the humidifier 304 via a second humidified air conduit 305b and is terminated by a second humidified-air exit point 307. The second wet-channel conduit 303b of the wet channel is configured to direct humidified air from the humidifier 304 in a second flow direction which is counter to the flow direction of the air in the dry channel 302.

As will be understood, in this example, the humidified air entry point of the wet channel is formed in the region 308 the first humidified air conduit 305a enters the first wet-channel conduit 303a and the second humidified air conduit 305b enters the second wet-channel conduit 303b. by the positioned at an intermediate point along the length of the wet channel and is configured such that a first part of input humidified air travels from the humidified air entry point in a direction of the first flow direction and a second part of the input humidified air travels from humidified air entry point in a direction counter to the first flow direction.

In examples of the invention, the dimensions of the wet channel and dry channel (or wet channels and dry channels in a modular arrangement) can be adapted to suit a variety of factors depending on the particular needs and constraints of the application. The appropriate dimensions might be influenced by the desired level of cooling, the available physical space for installation, the volume of air to be cooled, the environmental conditions, and the efficiency of the heat exchanger material. These factors can all play a part in determining the optimal size and configuration of the channels.

For instance, in locations with high ambient temperatures, larger channels might be beneficial to maximise the surface area for evaporation and heat exchange. Conversely, in more confined spaces, smaller, more compact channels may be necessary.

In one purely illustrative example, the wet channel and dry channel take a substantially planar configuration with a width of approximately 350mm to 400mm, length 1000mm to 1200mm and a channel gap of 5mm to 7mm. This configuration is not prescriptive, but rather serves to demonstrate one potential embodiment of the invention in a given set of circumstances.

In certain embodiments of the invention, the working fluid purged from the system, typically at a temperature of 27-28°C and a humidity level of 80-90%, can be repurposed for other applications, thereby enhancing the overall efficiency and sustainability of the system. This uses the principle of waste heat recovery, transforming what would be wasted heat into a valuable resource.

One significant application of this repurposed air can be in vertical farming, where both the temperature and humidity levels of the expelled air can be beneficial for plant growth. Vertical farming, characterised by the practice of growing crops in vertically stacked layers, often requires controlled climate conditions, which the purged air from this cooling system can readily provide. Moreover, systems in accordance with the present invention might typically be located on the roof or side walls of buildings, which complements the typical location of vertical farming installations within or on the exterior of structures. Vertical farming systems are often integrated into buildings, making use of indoor spaces, external walls, or even rooftops to maximise space utilisation and sunlight exposure.

Given these common location characteristics, the integration of cooling systems in accordance with embodiments of the invention with vertical farming can be particularly advantageous. The proximity between the two systems can facilitate an efficient transfer of purged air from the wet channel or channels. Such co-location can minimise the need for extensive ducting or transportation of conditioned air, thereby saving energy and reducing operational costs. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations).

It will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope being indicated by the following claims.

Claims

1 . An indirect evaporative cooling system comprising a wet channel and a dry channel arranged in thermal contact along a thermally conductive contact interface which extends at least part of their respective lengths, said contact interface forming a heat exchanger between the wet channel and the dry channel such that, during operation, when humidified air in the wet channel evaporates, the heat exchanger is cooled, subsequently cooling input air passing through the dry channel, wherein the dry channel comprises an air intake at a first end and an air outlet at a second end such that, in use, the input air travels in a first flow direction, and the wet channel is provided with a humidified air entry point positioned at an intermediate point along a part of the length of the wet channel that is in thermal contact with the dry channel via the thermally conductive contact interface, the wet channel being further configured such that a first part of input humidified air travels from the humidified air entry point in a direction of the first flow direction and a second part of the input humidified air travels from the humidified air entry point in a direction counter to the first flow direction.

2. An indirect evaporative cooling system according to claim 1 , wherein the intermediate point is half way along the part of the length of the wet channel in thermal contact with the dry channel.

3. An indirect evaporative cooling system according to claim 1 or 2, wherein the wet channel comprises a first humidified air exit point located at a first end and via which the first part of the input humidified air traveling in the first flow direction exits the wet channel, and a second humidified air exit point located at a second end and via which the second part of the input humidified air traveling in the second flow direction exits the wet channel.

4. An indirect evaporative cooling system according to claims 1 , 2 or 3, further comprising an airflow inducer located at the air intake of the dry channel and configured to drive input air through the dry channel.

5. An indirect evaporative cooling system according to claim 4, wherein the airflow inducer comprises a fan.

6. An indirect evaporative cooling system according to claim 5, wherein the airflow inducer comprises a forced air fan.

7. An indirect evaporative cooling system according to any of claims 1 to 6, further comprising a humidifier coupled to the humidified air entry point, and configured to supply humidified air to the wet channel.

8. An indirect evaporative cooling system according to claim 7, wherein an output of the dry channel is connected to the humidifier, thereby supplying the humidifier with air to be humidified.

9. An indirect evaporative cooling system according to claim 8, wherein the output air of the dry channel is connected to the humidifier via a flow control device.

10. An indirect evaporative cooling system according to claim 9, further comprising a control unit coupled to the flow control device and configured to control a proportion of the output air from the dry channel which is input to the humidifier.

11. An indirect evaporative cooling system according to claim 10, wherein the control unit is configured to control the proportion of the output air from the dry channel input to the humidifier responsive to a temperature sensor signal received from a temperature sensor.

12. An indirect evaporative cooling system according to claim 10 or 11 , wherein the flow control device is configured so that the proportion of the output air input from the dry channel to the humidifier can be varied between 20% to 50%.

13. An indirect evaporative cooling system according to any of claims 1 to 12, wherein the heat exchanger comprises one or more thin metal layers.

14. An indirect evaporative cooling system according to claim 13, wherein the one or more thin metal layers are textured.