WO2010103447A1

WO2010103447A1 - Illumination system with evaporative cooling

Info

Publication number: WO2010103447A1
Application number: PCT/IB2010/050973
Authority: WO
Inventors: Theodoor C. Treurniet; Omke J. Teerling
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-03-13
Filing date: 2010-03-08
Publication date: 2010-09-16
Also published as: TW201040458A

Abstract

An illumination device (2; 30) comprising at least one light-source (3; 31, 32); and a heat-sink member (4; 35). The heat-sink member is thermally connected to the at least one light-source (3; 31, 32), and the heat-sink member (4; 35) comprises a water retaining surface portion (6; 36, 37) arranged to be in contact with the surrounding atmosphere. By allowing liquid water to efficiently absorb heat generated by the light-source(s), very efficient cooling of the light-source(s) can be achieved.

Description

Illumination system with evaporative cooling

FIELD OF THE INVENTION

The present invention relates to an illumination system.

BACKGROUND OF THE INVENTION

Modern lighting systems based on light emitting diodes (LEDs) and other light-sources typically require cooling for reliably operating over a long period of time. To this end, various cooling techniques have been implemented, including passive convection cooling and active air cooling using a fan or similar. Additionally, cooling techniques have been suggested, in which the heat required for phase-change of a material is used for cooling light-sources in a lighting system.

JP6245651 discloses a plant culture device, in which lamps are cooled by spraying a water mist into the air and blowing the mist past the lamps with a fan. The heat from the lamps evaporates the water droplets in the mist, which increases the amount of water vapor in the air. After having passed the lamps, the air is cooled to increase the relative humidity of the air to a level that is advantageous for the plants.

Although appearing to serve its purpose, the cooling system disclosed in JP6245651 seems rather complex and requires a fan for its operation. Furthermore, there appears to be room for improvement with respect to the cooling efficiency of the system.

SUMMARY OF THE INVENTION

In view of the above-mentioned and other drawbacks of the prior art, a general object of the present invention is to provide an improved illumination system enabling simpler and/or more efficient cooling of the light-sources comprised therein.

According to a first aspect, the invention provides an illumination device comprising at least one light-source; and a heat-sink member being thermally connected to the at least one light-source, wherein the heat-sink member comprises a water retaining surface portion arranged to be in contact with the surrounding atmosphere. The illumination device may typically comprise a plurality of light-sources, such as LEDs, and the heat-sink member may typically be a part of the fixture by which the light-source(s) is/are supported.

By "water retaining surface portion" should, in the context of the present application, be understood a portion of the surface of the heat-sink member which is capable of holding liquid water, while allowing water vapor to escape from the heat-sink member.

The present invention is based on the realization that very efficient cooling of the light-source(s) in an illumination device can be achieved by allowing liquid water to efficiently absorb heat generated by the light-source(s).

The present inventors have further realized that such efficient absorption of heat by liquid water can be achieved by providing a heat-sink member which is in thermal contact with the light-source(s), and which has a water retaining surface portion which is arranged to be in contact with the surrounding atmosphere.

The heat-sink member may advantageously comprise a heat-dissipating structure, which may typically be made of a metal or other material having a high thermal conductivity. For efficient cooling, the water retaining surface portion may advantageously be configured in such a way that the liquid water is allowed to wet the heat-dissipating structure, enabling very efficient transfer of heat from the light-source(s) to the water in the water retaining surface portion of the heat-sink member.

Since the heat from the light-source(s) is thereby transferred directly to the liquid water, instead of being transferred through air, which is a relatively poor heat conductor, the cooling efficiency can be improved as compared to the prior art systems.

Furthermore, the cooling can be achieved without the need for fans or other active devices, whereby a simple and robust system can be provided.

In an exemplary embodiment, the illumination device may further comprises a carrier for providing electrical power to the at least one light-source, the at least one light- source being mounted on carrier. The carrier may be in direct contact with the light-source and the heat-sink member to provide thermal connection between the at least one light-source and the heat-sink member.

According to one embodiment, the illumination device may comprise a water retaining member arranged at the water retaining surface portion of the heat-sink member, the water retaining member being capable of retaining liquid water, while allowing water vapor to escape into the surrounding atmosphere. Through the provision of a water retaining member, the water retaining surface portion may be arranged practically anywhere on the heat-sink member, regardless of the orientation thereof when in use. Hereby, efficient evaporative cooling may be implemented with very few design constraints.

The water retaining member may advantageously comprise a liquid water absorbing material, such as a porous material. According to various embodiments, the water retaining member may thus, for example, comprise a textile material.

Various textile materials may advantageously be used because they are relatively easy to attach to a heat-dissipating structure and liquid water may be efficiently transported to a surface which is in direct thermal connection with the light-source(s), for example through so-called wick action.

By "textile material" should, in the context of the present application, be understood a material or product manufactured by textile fibers. The textile may, for example, be manufactured by means of weaving, braiding, knitting, crotcheting, quilting, or felting. In particular, a textile material may be woven or non-woven.

In an exemplary embodiment of the present invention, the at least one light- source may be arranged on a first side of the heat-sink member, and the water retaining surface portion may be arranged on a second side of the heat-sink member opposite the first side of the heat-sink member. Furthermore, the illumination device may comprise a plurality of light-sources, and one or several of these light-sources may be arranged on the second side of the heat-sink member.

This arrangement of the water retaining surface portion is particularly advantageous for applications in which the illumination device is intended to function as a down-light, since water can be retained on the water retaining portion without the need for any water retaining member. To this end, at least a portion of the heat-sink member should be configured in such a way that a film of water can remain thereon. This can, for example, be achieved by providing the water retaining surface portion with a suitable curvature, which depends on the surface conditions of the water retaining surface portion.

In various embodiments, the water retaining surface portion may, furthermore, be surrounded by a barrier for retaining liquid water in the water retaining surface portion. Such a barrier may be a mechanical barrier preventing flow of liquid water from the water retaining surface portion to surrounding areas, and/or the barrier may be formed by providing the water retaining surface portion with different surface properties as compared to surrounding portions of the illumination device. For example, at least a part of the water retaining portion may be hydrophilic, and surrounding portion may be hydrophobic.

Moreover, the illumination device according to various embodiments of the present invention may advantageously be comprised in an illumination system, further comprising a flow control device for controlling a supply of water to the water retaining surface portion of the heat-sink member; and processing circuitry connected to the flow control device and configured to control operation thereof.

The flow control device may be any device capable of controlling supply of water to the water retaining surface portion. For example, the flow control device may comprise passive means, such as a valve and/or active means, such as a pump.

The processing circuitry may be provided as either of a separate physical component, separate hardware blocks within a single component, or software executed by one or several microprocessors.

Additionally, the illumination system may comprise one or several sensors for sensing at least one operation-related parameter of the illumination device. Examples of useable sensors may include pressure sensors, humidity sensors, temperature sensors, optical sensors, etc.

According to an exemplary embodiment, the illumination system may include a pressure sensor being arranged in the water retaining surface portion to enable control of the flow control device based on a sensed water pressure. The sensed water pressure may be indicative of the pressure difference between the pressure exerted on the pressure sensor by the liquid water in the water retaining surface portion and the atmospheric pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention, wherein:

Fig. 1 schematically illustrates an illumination system comprising an exemplary embodiment of the illumination device according to the present invention;

Fig. 2 is a schematic block diagram illustrating the operation of the illumination system in fig 1 ; and

Fig. 3 schematically illustrates another exemplary embodiment of the illumination device according to the present invention. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

Fig 1 schematically illustrates an exemplary illumination system 1 installed in a green house environment. The illumination system in fig 1 includes an illumination device 2 having a plurality of light-emitting diodes (LEDs) 3 (only one of these is indicated with a reference numeral in the figure) being in thermal contact with a heat-sink member 4, which is here provided in the form of a metal structure. As is schematically indicated in the enlarged portion of fig 1, the LEDs 3 are electrically connected to a circuit board 5, which, in a manner well known to a person skilled in the art, places the LEDs 3 in good thermal contact with the heat-sink member 4.

The heat-sink member 4 has a water retaining surface portion 6, which, in the presently illustrated case is defined by providing the water retaining surface portion 6 with different wetting properties than a bounding surface portion 7 of the heat-sink member 4 which surrounds the water retaining surface portion 6. For example, the bounding surface portion 7 may be provided with a layer of a hydrophobic material, such as PTFE or similar.

As is also schematically illustrated in fig 1 , the illumination device 2 further comprises suspension structures 8a-b for suspending the illumination device 2 in the green house. In the presently illustrated example, the illumination device 2 is suspended in a water supply pipe 10 for supplying water to the illumination devices 2 in the green house (of course, the illumination system 1 may comprise several illumination devices 2).

As can be seen in fig 1 , the illumination system 1 further comprises a conduit 11 branching away from the water supply pipe 10. Along the conduit 11, a flow control device 12 in the form of a controllable valve is arranged to enable a controlled supply of water to the water retaining surface portion 6.

The valve 12 may advantageously be controlled by processing circuitry (not shown in fig 1 , see fig 2) which may be centralized or distributed in the illumination system 1.

Through the provision of a heat-sink member 4 having a water retaining surface portion 6, a film of liquid water 13 is placed in thermal connection with the LEDs 3 in a very efficient manner. When the water 13 absorbs heat generated by the LEDs 3, the temperature of the water increases, which leads to evaporation from the surface of the water film 13. When the water thus transitions from its liquid phase to its gaseous phase, heat is efficiently dissipated from the LEDs 3.

Below, a simple example for the configuration of fig 1 is provided: Assumptions:

Ambient temperature: 30⁰C

Relative humidity: 90 % (typical value for a green house)

Size of water retaining surface: 500 mm x 250 mm

Light sources: 200 LEDs with 30% WPE (Wall Plug Efficiency), each operating at 3 W

Total thermal heat power = 20Ox 3 W x (100- 30%) = 420 W

Temperature of heat-sink member: 80⁰C

Given the above assumptions, and based on a given Nusselt number for free convection at a horizontal surface and relation between heat and mass transfer, the evaporative cooling capacity is ~ 400 W/illumination device (= 3200 W/m²). With the same assumptions, the required water flow for maintained cooling becomes ~ 4 1 /h to each illumination device. (For an illumination system comprising 50 illumination devices, 200 1/h is required)

As a comparison, a rough calculation yields that conventional active water cooling using water circulating past the light-sources would require a total water flow of -340 1 /h to and from the 50 illumination devices in the exemplary illumination system. In this case, additional cooling of the heated up cooling water is required.

Fig 2 is a block diagram schematically illustrating the operation of the illumination system in fig 1. With reference to fig 2, the illumination system 1 comprises an illumination device 2, a water supply pipe 10, and a branch conduit 11 leading water from the water supply pipe 10 to the illumination device 2. The illumination system 1 further comprises a flow control device 12 arranged to enable control of the flow of water through the branch conduit 11. The flow control device 12 is controlled by processing circuitry 20, which may control the flow control device 12 based on acquired values of various parameters, such as the relative humidity and/or the temperature of the atmosphere surrounding the illumination device 2, the thickness of the water film 13, the temperature of the water in the water film 13, etc. Such values of the various parameters may be acquired using various suitable sensors 21, 22. The processing circuitry 20 may, furthermore, be configured to communicate externally as indicated by the double arrow 23 in fig 2 to receive instructions and/or transmit status data etc.

As will be apparent to the skilled person, the water retaining portion of the heat-sink member may be configured in various ways. In addition to the configuration of the water retaining surface portion described above in connection with fig 1 , the water retaining surface portion may be realized by providing a water retaining member being capable of retaining liquid water, while allowing water vapor to escape into the surrounding atmosphere.

Fig 3 schematically illustrates an exemplary illumination device 30, having light-sources 31, 32 mounted on opposite sides thereof. As in the illumination device 2 of fig 1, the light-sources 31, 32 are provided in the form of LEDs which are mounted on respective carriers 33, 34, for enabling supply of electrical power to the light-sources 31, 32.

The light-sources 31, 32 are in good thermal contact with a heat-sink member 35 via the carriers, as is schematically illustrated in the enlarged portion of fig 3.

The heat-sink member 35 is provided with fins 36, 37 on both sides of the illumination device 30 to increase the area of the heat-sink member 35 that is in contact with the surrounding atmosphere.

In the exemplary embodiment schematically illustrated in fig 3, the surface of the fins 36, 37 is the water retaining surface portion of the illumination device 30. To provide the desired water retaining capability, a water retaining member 40 in the form of a porous material, such as a textile, is provided on the fins. The water retaining member 40 ensures that liquid water supplied thereto migrates towards to heat-sink member 40 to make direct contact with the heat-sink member. This may, for example, be achieved through so-called wick action. As in the embodiment described above with reference to fig 1, heat generated by the light-sources 31, 32 and absorbed by the water retained by the water retaining member 40 results in evaporation of the water, whereby the illumination device 30 is cooled.

The person skilled in the art will realize that the present invention is by no means limited to the preferred embodiments. For example, various other arrangements of the water retaining surface portion may be achieved, such as by providing a protrusion defining a shallow basin structure. Furthermore, the water retaining surface portion may be achieved by providing the surface of the heat-sink member with a suitable surface roughness.

Claims

CLAIMS:

1. An illumination device (2; 30) comprising: at least one light-source (3; 31, 32); and a heat-sink member (4; 35) being thermally connected to said at least one light-source (3; 31, 32), wherein said heat-sink member (4; 35) comprises a water retaining surface portion (6; 36, 37) arranged to be in contact with the surrounding atmosphere.

2. The illumination device (30) according to claim 1, comprising a water retaining member (40) arranged at said water retaining surface portion (36, 37), said water retaining member (40) being capable of retaining liquid water, while allowing water vapor to escape into said surrounding atmosphere.

3. The illumination device (30) according to claim 2, wherein said water retaining member (40) comprises a porous material.

4. The illumination device (30) according to claim 3, wherein said water retaining member (40) comprises a textile material.

5. The illumination device (2) according to any one of the preceding claims, wherein said at least one light-source (3) is arranged on a first side of said heat-sink member (4), and said water retaining surface portion (6) is arranged on a second side of said heat-sink member (4) opposite said first side of the heat-sink member.

6. The illumination device (2) according to any one of the preceding claims, wherein said water retaining surface portion (6) is surrounded by a barrier (7).

7. An illumination system (1), comprising: an illumination device (2; 30) according to any one of the preceding claims; a flow control device (12) for controlling a supply of water to the water retaining surface portion (6; 36, 37) of said heat-sink member (4; 35); and processing circuitry (20) connected to said flow control device (12) and configured to control operation thereof.

8. The illumination system (1) according to claim 7, further comprising a sensor (21, 22) for sensing at least one operation-related parameter of said illumination device

(2; 30), said sensor (21, 22) being connected to said processing circuitry (20), said processing circuitry (20) being configured to control operation of said flow control device (12) based on a value of said operation related parameter being sensed by said sensor (21, 22).

9. The illumination system (1) according to claim 8, wherein said sensor is a pressure sensor being arranged in said water retaining surface portion (6; 21, 22) to enable control of said flow control device (12) based on a sensed water pressure.

10. The illumination system (1) according to any one of claims 7 to 9, further comprising a air- flow device arranged to enable flow of air past said water retaining surface portion (6; 36, 37).