WO2005008319A1

WO2005008319A1 - Micro-chanel cooling for video projectors (“beamers”)

Info

Publication number: WO2005008319A1
Application number: PCT/IB2004/051191
Authority: WO
Inventors: Paulus B. J. Schaareman; Wilhelmus H. M. M. Evers; Jeoren Gijzen; Celine C. S. Nicole; Marcellus T. Van Mil; Clemens J. M. Lasance
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-07-17
Filing date: 2004-07-12
Publication date: 2005-01-27

Abstract

The present invention relates to micro-chanel cooling of liquid crystal display devices (200) comprised in beamers (e.g. projectors). Micro-chanels (201) carrying cooling agent are thus integrated into substrates (202) encapsulating the liquid crystal (203), and interconnected with a heat sink. The micro-channels can be arranged along the black mask (205) and (205) or transistors and thus entirely outside the addressable portions (i.e. the pixel areas) of the display. Alternatively, even more efficient cooling can be provided by means of microchannels arranged across the addressable areas. The cooling provided by the inventive micro-channels are sufficient to eliminate any air cooling arrangements, alternatively micro-channel cooling can be supplemented by fan arrangements. Furthermore, the polariser and the analyser can be arranged in direct contact with the liquid crystal cell, thus eliminating the need for expensive anti-dust plates.

Description

MICRO-CHANEL COOLING FOR VIDEO PROJECTORS ( ' ' BEAMERS ' ' )

The present invention relates to beamers comprising liquid crystal display (LCD) modules, and in particular to the cooling of such LCD modules.

A liquid crystal beamer (projector) generally comprises a light source generating white light which is directed through various lenses and split up by wavelength selective mirrors reflecting red, blue, and green light, respectively, and subsequently directed towards one out of a red, a blue and a green LCD module. The LCD modules are transmissive and comprise a polariser, a liquid crystal cell and an analyser. Typically, the polariser and the analyser are spatially separated from the actual liquid crystal cell, in order to provide for efficient cooling of the respective parts. Each LCD module provides for red, blue, or green image portions, respectively, and the image portions are subsequently synthesized in a recombination unit and focused in a projection lens arranged at a distance from the recombination unit. Light from the light source impinges the LCD module on the polariser side, and the analyser side faces the recombination unit. A large portion of the light impinging the LCD modules is absorbed in the modules and converted into heat. The light source therefore needs to have a power substantially higher than the specified output power of the beamer. The generation of heat in the LCD module is a big problem, since the liquid crystal layers degrade and loose contrast when heated. Heat also affects the reliability of the display, regarding the liquid crystal cell (i.a. due to degradation of polyimide layers) as well as the polarisers and analysers (i.a. due to degradation of TAC (triacetyl cellulose) layers). Furthermore, non-uniform heat distribution in the liquid crystal cell generally results in a non-uniform image. Therefore, efficient and uniform cooling of the liquid crystal cell is critical. Conventional beamers utilizes air cooling provided by fan arrangements. However, such designs have two main drawbacks. First, the fan needs to be powerful and is therefore quite noisy. Second, the components in the beamer (e.g. the liquid crystal cell, the polariser, the various analyser, the lenses and the recombination unit etc.) need to be sufficiently spaced apart to allow necessary air flows. Therefore, such solutions are limited in respect of compactness as well as noise levels. As an alternative to air cooling, US 5,170,195 (Masayuki et al) proposes the use of liquid cooling. Liquid outperforms air both in regard of absorption and conduction of heat, and therefore has obvious advantages. The liquid cooling proposed by Masayuki et al is provided by means of containers filled with a transparent cooling agent and functioning as a cooling element. The cooling agent is thus pumped in a closed cycle between the container and a heat radiator radiating excess heat. The container is provided with two transparent windows on opposite sides, and the polariser of the respective LCD module is mounted in direct contact with one of the windows and the respective liquid crystal cell is mounted in direct contact with the other window. Light impinging the LCD module is thus transmitted through the polariser, via the cooling container to the liquid crystal cell. Excess heat developing in the polariser and the liquid crystal cell is conducted through the windows and into the cooling agent. This design indeed has some advantages in comparison with conventional air cooling: the noise level can be reduced since the pump fs more silent than the corresponding air fan, and the LCDs and lenses can be mounted without considering air flow. However, the liquid cooling disclosed by Masayuki et al is not as effective as desired for cooling the liquid crystal cell, and the cooling elements are bulky and thus restrict the compactness of the design. Furthermore, the transparency requirements are high on the windows as well as on the cooling agent not to affect the brightness of the beamer. Therefore there is a need for improved cooling systems in LCD beamers. In fact, next generation beamers is expected to require a cooling system fulfilling the following requirements: - Capacity of removing at least 10 W/cm² of heat from the LCD module. - Uniform and silent cooling. - Compact design of the LCD module as well as the recombination unit (thus providing for reduced manufacturing costs). - Uniform transmission of light. - Low overall manufacturing cost.

These requirements constitute a complicated thermal problem not solvable by means of conventional techniques. The present invention proposes a cooling technology based on micro-channels integrated in the LCD modules, and thereby allow higher compactness and higher light output from the beamers. Micro-channel structures can be manufactured behind or in front of the black mask of the LCD module, but could alternatively be placed anywhere in the LCD module allowing direct, localised and efficient cooling by liquid forced convection. The coolant could be in a single-phase flow mode or two-phase flow mode. Dissipating 10 W/cm² in a compact set-up is not easy with air, in particular if the level of fan noise must be limited. Therefore, liquid is considered because of its extremely good properties for heat dissipation. Single-phase liquid cooling has a capacity of removing several hundreds of W/cm², and two-phase liquid cooling provides even higher capacity. Thereby a small amount of liquid is sufficient and a compact design is facilitated. Micro-channel cooling using heat sinks are already available for cooling of electronics. However, due to optical difficulties none of these products could be used in a transmissive optical set-up. Therefore, the present invention proposes an "invisible" micro- channel design allowing use of liquid cooling capabilities in a situation otherwise not possible. Thus, according to one aspect of the present invention, a beamer is provided that comprises at least one liquid crystal display module. The liquid crystal display module comprises a liquid crystal cell and a liquid cooling system. The liquid cooling system comprises a heat sink, a liquid cooling agent, and a micro -channel system interconnected with the heat sink and integrated inside said liquid crystal cell, whereby the liquid cooling agent is movable in a loop between said heat sink and said micro-channel system. The micro -channels are thus integrated in the interior of the liquid crystal cell, such that the material delimiting the channels is part of the liquid crystal cell. The inventive beamer is advantageous in that efficient cooling is provided for the liquid crystal cell without the need for bulky cooling elements and without considering air passages in the arrangement. The micro -channels can be formed in the substrates during the manufacturing process. For example, in case the substrates are made out of glass the micro-channels can be defined by means of etching. Alternatively, the micro-channels could be defined by means of sand blasting or powder blasting. Of course, the substrates are not restricted to glass but could be replaced by any other conventional material. An advantage using micro-channels is that the cooling acts were the heat is actually generated. In addition, increasing the number of cooling channels increases the total surface contact with the material that has to be cooled, and thus results in even higher cooling efficiency. Thereby sufficient cooling is always available. According to one embodiment, the liquid crystal cell is an active matrix display unit and thus comprises a set of transistors and a black mask, arranged on opposite sides of a liquid crystal layer and defining pixel areas spaced apart by passive areas. In such case the micro -channels are preferably arranged along said passive areas. This design is advantageous in that the optical light paths in the liquid crystal cell are free from micro- channels. In case the micro-channels are arranged across the light path, special requirements regarding transparency and refractive index have to be put on the cooling agent and also regarding the surface smoothness in the micro-channels not to affect the optical properties of the display. These considerations are eliminated in case the micro -channels are arranged only along the passive areas, and thus not across the addressable display portions (the pixels). According to one embodiment, the micro-channel system is integrated in the liquid crystal cell on the same side as and essentially overlaps the black mask. According to this embodiment, the cooling agent might instead be light absorbing (black). This is advantageous because the cooling agent will then absorb unwanted reflections from the black mask. Conventionally, the black mask reflects light (40% for Cr-masks and 20% for Al- masks) back to the polariser. The degradation of the polariser is related to the amount of incident light, and reducing reflections therefore reduces the degradation. This embodiment is particularly advantageous for avoiding iterative reflections when a reflective polariser is used.lt is even possible to substitute the black mask for correspondingly designed micro- channel system comprising black cooling agent, thus eliminating the need for a black matrix and thereby reducing the manufacturing cost. Conventional LCD modules are provided with glass plates mounted at a distance on each side of the cell and carrying the analyser and polariser, respectively. If dust sticks to these plates, the module is put out of focus due to the small depth of focus of the projection lens. Therefore these plates are often called anti-dust plates. The anti-dust plates are typically formed out of quartz or sapphire, and are quite expensive. The separation of the polariser and analyser from the liquid crystal cell is needed in order to allow sufficient air flow for cooling the device. According to one embodiment, the polariser and the analyser is instead arranged in direct contact with the liquid crystal cell. Excess heat in the polariser and analyser is thereby transported directly into the cooling agent via the micro -channels. This embodiment is advantageous both because the design can be made more compact but also because the dust free plates otherwise needed to carry the polariser and analyser can be omitted resulting in easier and cheaper manufacturing. According to one embodiment, the beamer is a full colour beamer and thus comprises a red, a green, and a blue liquid crystal display module which are optically interconnected by a recombination unit. According to this embodiment, the micro-channel systems of each display module are interconnected into one single system having a common heat sink. Thereby, a very compact, robust, and cost effective display module can be provided for beamers. The efficient cooling of the micro -channels thus facilitate integration of three LCD modules into one single assembly, avoiding carrier plates and otherwise needed anti-reflective coatings (e.g. Ar-coatings). Moreover, the distance between the recombination prism and projection lens can be reduced due to improved cooling of the recombination provided by the micro -channels in the LCD modules. Thereby the projection lens can be made less expensive. The micro -channels are preferably arranged in a web-like configuration having vertical and horizontal channels crossing and interconnecting each other. Thereby the cooling effect can be uniformly spread across the display. It is also possible to use micro-channels having different dimensions in different portions of the display, thereby facilitating further optimisation of the cooling effect. The proposed invention is attractive because of its simplicity and efficiency in solving a complicated thermal problem. The design can be implemented for cooling of three sets of LCD modules in the recombination unit of a beamer. The total power to dissipate in such an arrangement will typically be below 30 W/cm². If required, it is however possible to cool only one LCD module, for example in a black and white beamer, or to provide even higher cooling capacities. In essence, micro -channel cooling allows the transport and dissipation of large heat densities. Small amounts of liquid are forced through a cooling circuit in the respective LCD module, and heat transfer is made via a heat exchanger or by evaporation. The present invention thus provides for direct cooling of the liquid crystal display module, as opposed to the cited prior art which provides for indirect cooling via the separate cooling element. The indirect cooling is not as effective due to conduction and convection resistances associated with separating the cooling agent from the actual display module. The cooling agent might be water, different dielectric liquids, alcohol, oil, mixtures thereof, or any other suitable liquid. Water in particular has a high heat capacity in combination with optical properties suitable for use in cooling of optical elements. In case the micro-channels are arranged across the addressable portions of the display (i.e. the pixel areas), it is important for the cooling agent to be transparent and preferably to have a refractive index matching the surrounding material. In case the micro-channels are arranged solely along the black mask, transparency and refractive index are not critical. In fact, the cooling agent is preferably dark (e.g. black ink) in order to reduce unwanted reflections or even to substitute the entire black mask. Either single-phase cooling or two-phase cooling is employed. Single-phase cooling can be provided either via natural convection or via forced convection. Two-phase cooling is accomplished via evaporating boiling. Heat in the display module then causes the cooling agent to change state from liquid to gas (i.e. boils the liquid), and the gas is carried off to the heat sink where it is condensed back to liquid phase. The cooling agent transport could be provided by means of natural convection or by means of a pump. Natural convection can be provided by means of evaporation/condensation and capillary force cycles. In case a pump is employed, the pump could be a mechanical pump or an electrokinetic pump. Electrokinetic pumps have the advantage of small dimensions, but are not yet commercially available for higher flow rates. Using a pump instead of or in addition to natural convection provides for flexibility in the design, since the natural convection is thereby made non-critical. The heat sink typically comprises radiator elements in order to radiate excess heat. In case of two -phase cooling, the heat sink further comprises a condensation unit wherein gas phase cooling agent is condensed to liquid phase. In order to increase the efficiency of the heat sink, it can be cooled by a fan arrangement blowing air on the heat sink. Even though the inventive micro-channel cooling indeed is very powerful, the cooling can be further improved using complementary air cooling. Complementary air cooling can be directed towards lens systems, the heat sink and/or any other part benefiting from additional cooling. The present invention thus has several advantages: - The compactness of the total system provides substantial advantage as such and facilitates reduced manufacturing costs. - A design with liquid channels containing dark liquid coolant or light absorbing coolant, provides for the possible removal of the black mask. - Having liquid micro-channels superposed to the black mask, reflections of light towards the polariser are suppressed. - In general, reflections are minimized because the index of refraction of liquid (>1) is closer to the refractive index of the various substrates. Thereby, anti-reflective coating otherwise needed at the glass/air interfaces between the different parts (e.g. polariser and liquid crystal cell) are eliminated. - The anti-dust glass can be eliminated. - Micro-channels have higher heat transfer coefficient as compared to indirect cooling arrangements, and less cooling agent flow is thus needed. Furthermore, a large number of micro -channels can be employed thereby increasing the total contact surface area resulting in additionally improved cooling. - Better temperature spreading is obtained using crossed flow liquid coolant movement (horizontally and vertically). - The width of the micro -channels can be locally optimised in order to better spread the heat uniformly in the LC module.

The invention will now be further described with reference to the accompanying, exemplifying drawings, on which: Figure 1 illustrates a cross section of a prior art LCD module for a beamer. Figure 2, 3 and 4 illustrates cross sections of various LCD modules according to the present invention. Figure 5 illustrates schematically a full colour display module for a beamer, comprising three LCD modules and a recombination unit. Figure 6 illustrates a micro-channel cooling system comprising micro- channels arranged in a web-like configuration inside a liquid crystal cell, a heat sink, and a pump.

Figure 1 thus illustrates a cross section of a prior art LCD module 100 comprising a polariser 101, an analyser 102, and a liquid crystal cell 103-109. The cell comprises a liquid crystal layer 105 sandwiched between substrates 104, 105. A black mask 108 is arranged on one side of the liquid crystal layer and transistor units 109 are arranged on the other side of the liquid crystal layer. Furthermore, outer substrates 103, 107 are covering the liquid crystal cell. The analyser and polariser are arranged at a distance from the liquid crystal cell in order to facilitate necessary air cooling of the device. Figure 2 illustrates a cross section of an inventive liquid crystal cell 200, simplified as 5 plates, wherein cooling is provided by means of micro-channels 201. The LCD module generally comprises a liquid crystal layer 203 sandwiched between a first and a second substrate 202, 204. According to this design, the micro-channels 201 are arranged in the first substrate. The substrates can for example be formed out of glass, and the micro- channels can in such case be manufactured by means of etching micro-channel patterned grooves into a first glass plate and subsequently gluing or bonding a second glass plate onto the first plate thus covering the groves and defining the micro-channels. It is also possible to have both plates grooved and subsequently glued together. The inventive liquid crystal cell 200 further comprises a black mask 205 (of which cross sections are shown in the figure), and transistors 206 aligned with the black mask lines. It is even possible for the liquid micro- channels to replace the mask, provided that the liquid used absorbs the light in order to protect the transistors. The number of black mask lines, and thus the number of micro- channels, depend on the number of pixels in that particular LCD. For clarity, only eight mask lines are shown. An alternative embodiment is illustrated in Figure 3. Similar to the previous embodiment this display module comprises a liquid crystal layer sandwiched between substrates 302, 304, 305, a black mask 309, and transistors 310. The carriers for the polariser 301 and the analyser 306 are however removed and the polariser and analyser foils are instead mounted directly on the liquid crystal cell providing a compact design. Furthermore, the inventive micro-channels 308, 311 are arranged on both sides of the liquid crystal layer, thus providing for improved cooling effect. In general, when the polariser and the analyser are arranged directly on the liquid crystal cell it might be appropriate to have two-sided micro-channels in order to provide sufficient cooling due to the additional heat formed. Here, as well as in the previous embodiment, the micro-channels are arranged along the black mask, i.e. along the passive portions of the addressable area. Figure 4 illustrates yet another embodiment, comprising a liquid crystal layer 403, substrates 402, 404, 405, a black mask 410, and transistors 411. Similar to the Figure 3 embodiment, the polariser 401 and analyser 406 are arranged directly on the liquid crystal cell and micro-channels 408, 411 are arranged on both sides of the liquid crystal layer. However, according to this embodiment the micro-channels are arranged along the addressable portions of the display. Particular care is needed here not to affect the brightness of the display. Preferably the cooling agent should be fully transparent and have a similar refractive index as the surrounding substrate. Furthermore, the micro-channels should preferably be formed with smooth surfaces, not to disturb the light paths. This embodiment is advantageous in that the micro-channels can be made wider. The black mask lines are typically between 2 and 5 microns wide, whereas the addressable areas (i.e. the pixels) could be about 20 microns wide (depending on the display resolution). Of course, it is easier to provide efficient cooling using 20 micron wide micro-channels than using channels less than 5 microns wide. In addition, such wide micro -channels could more easily be defined by means of sand blasting, and a less powerful pump is needed in order to circulate the cooling agent. Thereby, cooling is made more efficient and manufacturing is less expensive. Here as well as in the previous embodiment two series or sets of micro- channels are provided, one on each side of the liquid crystal cell. It is however also envisaged to arrange the micro -channels across the addressable areas but only on one side of the liquid crystal cell. In order to increase the cooling efficience, additional layers of parallel micro - channels could be added in case increased heat transfer is required (e.g. two layers on each side of the liquid crystal layer). According to still one embodiment, illustrated in Figure 5, a highly compact display module 500 for a beamer is achieved by joining three colour LCD modules 501, 502, 503 together with a recombination prism 504 into one single module which is internally cooled by means of micro-channels. Figure 6 schematically illustrates a micro-channel system comprising a liquid crystal cell 601 having integrated micro-channels, a pump 603, and a heat sink 602. The components are interconnected by means of piping 604, mounted at the inlet 605 and outlet 606 of the liquid crystal cell by means of liquid distributor and liquid collector means, respectively. According to this embodiment, the collector and distributor means are interconnected with a plurality of inlet and outlet openings, respectively, in order to improve the cooling agent distribution inside the liquid crystal cell. Micro -channel structures can be built-in during the process of manufacturing of the LCD module. According to the invention, the micro-channels can be placed directly on top of the mask lines during manufacturing of the mask. The dimensions of the micro- channels depend on the cooling efficiency required as well as on desired resolution and aspect ratio. The width of the micro-channels is typically between 0.5 and 20 microns, and the height is typically between 1 and 20 microns. Smaller dimensions might be difficult to achieve using sand or powder blasting, and is therefore preferably formed using etching techniques. A given LC stack typically includes glass plates and silicon nitrate layers. The micro-channels could be provided in either of these layers or plates. In case etching is utilized, the particular etching technique of course has to be adopted to the material at hand. For example, micro-channels could be chemically etched into a first silicon nitrate layer during the manufacturing process of a LC stack. Thereafter a second layer of silicon nitride could be added on top of the first layer thus covering the grooves and forming the micro- channels. After bonding or gluing, the assembly is sealed leaving only an inlet opening and an outlet opening opened. The remaining manufacturing steps could then be completed in conventional manner. The layer that incorporates the micro-channels is preferably made a little bit larger than the other layers, thus extending somewhat out from the LC stack. Thereby conventional junctions (e.g. plenum chambers, collectors, or orifices) could be bonded or glued in front of the micro-channel openings operative to collect and distribute the coolant fluid into and from the micro-channels. In case micro -channels are formed in a glass substrate, the micro-channels are preferably formed as grooves in a first glass substrate which is subsequently glued or chemically bonded to a second glass substrate thus forming a single substrate having interior micro -channels. The grooves could, for example, be etched or sandblasted into the first substrate. In order to even out the cooling effect, the micro-channels are preferably interconnected into a web-like micro-channel circuitry. Thereby the risk for temperature gradients across the liquid crystal cell is reduced. The cooling agent preferably has a high heat capacity. Examples of feasible cooling agents include water, dielectric coolants, or mixtures of oil and alcohol. The technique used is thus based on single-phase or two -phase micro -channel flow convection in order to operate high heat transfer. In case single-phase cooling is utilized, single-phase coolant liquid is provided to the inlet of the micro-channels having a temperature of T_mi_et- Having travelled through the micro-channels in the LC stack, the temperature increases to T_outiet- The coolant fluid is then transported to a standard heat exchanger outside the display module by means of piping. The heat exchanger exchanges heat by radiation and/or natural convection. In case a more compact design is desired, a smaller heat exchanger in combination with a fan could be used to increase the heat exchanging process. The fluid outlet temperature is decreased to Tmi_et in the heat exchanger and is subsequently sent back to the micro-channels. In a three colour module, the same heat exchanger could be used for all thee liquid crystal cells. In order to improve the circulation of the cooling agent, a pump (e.g. mechanical, magnetic, Piezo, or electrokinetic) could be arranged in the circuit, e.gt. along the piping. In case two-phase cooling is utilized, the micro-channel output opening should have a cross section large enough to handle the vapour phase flow. The vapour is preferably lead through a metallic structure (or a structure formed out of some other, heat radiating material). Heat exchange with the environment could be made via natural convection and optionally by means of a small fan. Vapour is then condensed to liquid phase and brought back to the micro-channel structure by means of capillary forces. Optionally, additional sub- structures could be optimised to increase the capillary forces transporting the cooling liquid back to the micro-channels. When filling the cooling circuit with cooling agent, it is important to eliminate any residual air from the circuit, otherwise the two-phase cooling will not be totally efficient. This fact is the same as for any conventional heat pipe. In essence, the present invention relates to micro-channel cooling of liquid crystal display devices comprised in beamers (e.g. projectors). Micro-channels carrying cooling agent are thus integrated into substrates encapsulating the liquid crystal, and interconnected with a heat sink. The micro-channels can be arranged along the black mask and or transistors and thus entirely outside the addressable portions (i.e. the pixel areas) of the display. Alternatively, even more efficient cooling can be provided by means of micro- channels arranged across the addressable areas. The cooling provided by the inventive micro- channels is sufficient to eliminate any air cooling arrangements, alternatively micro-channel cooling can be supplemented by fan arrangements. Furthermore, the polariser and the analyser can be arranged in direct contact with the liquid crystal cell, thus eliminating the need for expensive anti-dust plates.

Claims

CLAIMS:

1. A beamer comprising at least one liquid crystal display module (501, 502, 503), said liquid crystal display module comprising a liquid crystal cell (200), characterised in that said liquid crystal display module further comprises a liquid cooling system (600), said liquid cooling system comprising a heat sink (602), a liquid cooling agent, and a micro- channel system (201, 601) interconnected with said heat sink and integrated inside said liquid crystal cell, whereby said liquid cooling system is operative to move said liquid cooling agent in a loop between said heat sink and said micro-channel system.

2. A beamer according to claim 1, wherein said liquid crystal cell (200) is an active matrix display cell comprising a set of transistors (206) and a black mask (201), arranged on opposite sides of a liquid crystal layer and defining pixel areas spaced apart by passive areas, and wherein said micro-channel system (201) is arranged along said passive areas.

3. A beamer according to claim 2, wherein said micro-channel system is integrated in the liquid crystal cell on the same side as and essentially overlapping said black mask.

4. A beamer according to claim 1, wherein said liquid crystal display module further comprises an analyser and a polariser arranged on opposite sides of and in direct contact with the liquid crystal cell.

5. A beamer according to claim 1, wherein said cooling system comprises two sets of micro -channels, arranged on opposite sides of the liquid crystal layers.

6. A beamer according to claim 1, said beamer being a RGB beamer comprising a red, a green, and a blue liquid crystal display module optically interconnected by a recombination unit, and wherein the micro-channel systems of each display module is interconnected with and a common heat sink.

7. A beamer according to claim 1, wherein said liquid cooling system comprises a pump operative to pump cooling agent between the micro-channels and the heat sink.

8. A beamer according to claim 7, wherein said pump is an electrokinetic pump.