WO2013108273A1

WO2013108273A1 - Improved performance of high flux leds (light emitting diodes) with thermoelectric management

Info

Publication number: WO2013108273A1
Application number: PCT/IS2012/000001
Authority: WO
Inventors: Jonsson STURLA; Jonsson THORGEIR; Kristjansson JON
Original assignee: Esjonsson, Ehf.
Priority date: 2012-01-19
Filing date: 2012-01-19
Publication date: 2013-07-25

Abstract

A light, comprising at least one high flux light emitting diode (LED), a heat sink and at least one thermoelectric module (TEM) having a first surface, thermally connected to the heat sink, and the second surface connected to the second heat sink. Thus creating a lower than the ambient temperature environment around the light emitting diode that changes the initial operating of the LED. The present invention relates to Light Emitting Diode (LED) based lamps, utilizing thermoelectric modules, improving the efficiency of the lamps and controlling several other variables s.a. frequency, color rendering index and lifespan of the light emitting diode. With heat conducting material, with a high surface area/volume ratio, improves the task of lowering the operating temperature of LEDs. The LED may be of any conventional type AC or DC, the invention however is particularly useful for devices using high-flux LEDs.

Description

Improved performance of high flux LEDs (light emitting diodes) with thermoelectric management.

DESCRIPTION

The present invention relates to high flux light emitting diodes and how to manage their thermal aspect by creating another ambient heat environment around the HB-LED. Further the present invention can utilize a higher current through the LED than presently is recommended resulting in a brighter LED and or fewer LEDs per light output. Practical design and application of Light Emitting Diodes (LED) type devices for use in Area Lighting and like schemes are limited by thermal energy-management issues. Therefore, by producing more light intensive LEDs more thermal energy is produced in the same unit volume of the device. Practical design of a LED light aims at finding the passive point and the optimal balance between the light output versus the entropy.

It is known that LEDs exhibit negative temperature coefficient aspects, at fixed power input, as the device's operating heat rises (entropy), the device's light output decreases. It is the entropy inside the LED chip and the semiconductors accompanied with it that determines the degradation level of the LED.

Attempts have been made in the prior art to solve the negative temperature coefficient issues. As an example, in LED highway traffic signal devices housings with ventilation configurations, both of passive (convection-type) and active (fan-driven-type) have been provided to prevent the LED-s from overheating. Present art LED traffic signal devices also address the inherent negative temperature coefficient nature via the electrical power supply. These approaches either increase power to the device to compensate for light output loss or address the form of the provided electrical power such as sine vs. square wave in an attempt to moderate the entropy. According to prior art, entropy in a closed system; as LEDs, will continue to increase until the LED no longer produces any light. All passive systems show similar development. The higher the current, the bigger portion of the power consumed becomes entropy. Entropy can not decrease in any system but it can at best, be constant. This is only possible in active cooling systems, where external work is needed to keep the entropy inside the system at acceptable levels.

Solid state thermoelectric modules (TEM) also referred to as thermoelectric coolers (TEC) or heat pumps have been used in various applications since the introduction of semiconductor thermocouple materials. Such devices convert electrical energy into a temperature gradient, known as the "Peltier" effect or convert thermal energy from a temperature gradient into electrical energy. By applying a current through a TEM a temperature gradient is created and heat is transferred from one side, the "cold" side of the TEM to the other side, the "hot" side. TEMs have been considered unsuitable in the art for cooling LED lighting devices as they have been ruled out for insufficient efficiency; that is, if configured and operated with conventional settings the energy cost of operating a TEM for cooling an LED device is more than the energy gained in operating the LED at a reduced current and a slower entropy incensement's.

The present inventors have now found that TEMs can surprisingly be used to cool and enhance the light output of LED lighting fixtures and particularly to maintain optimal light output from LED lighting fixtures. Further investigations have shown that besides

abovementioned benefits of using TEM they prolong the expected lifetime of LEDs, improve the CRI and prevent the LED structure from collapse through Droop.

The inventors have analyzed the behavior of conventional Hi-flux LEDs and such as used in traffic signals and area lighting devices and TEMs and developed models and prototypes for that are used to optimize the performance of the TEM-cooled LED devices of the present invention.

The present invention provides in a first aspect a light illuminating device that comprises at least one light emitting diode (LED), at least one thermoelectric module (TEM) having a first surface which is thermally connected to a heat sink thermally connected to a second surface of the at least one TEM,

The LED may be of any conventional type, the invention however is particularly useful for devices using hi-flux LEDs, in all areas where bright light is needed. It will be appreciated that the device of invention is able to produce more light per unit energy consumed, than corresponding LED-based lights without cooling, because the additional energy needed to operate the TEC is less than the energy and/or light output gained. Hence, in the most preferred embodiments of the invention, the device is configured such that the device produces more Illumination per unit consumed power when the TOC is applied to the TEM, than the illumination produced per unit consumed power when no TOC is applied to the TEM.

The thermal connection between the LED and TEM can be realized by an interface of thermally conducting material, e.g. a metal such as copper or aluminum. We have found that a small heat sink placed between the LED and the TEM where the heat sink has a large Surface area per volume will speed up the thermal transfer.

The surfaces of the TEM are typically referred to as the "hot side" and the "cold side", where the cold side is the first surface in contact with the first heat sink and the hot side the second surface in contact with the second heat sink or adjacent TEM optionally connected by a thermally conducting plate. However, it should be born in mind that the temperature gradient of the TEM can be reversed by reversing the current applied to the TEM. This is only used when stable temperature is needed around the LED in an extremely cold environment,

To realize the desired efficiency that makes TEM cooling worthwhile, at least one TEM is chosen and configured such that the device can be operated by running a TEM-operating current (referred to herein as TOC) through the TEM. The regulation of the current to the TEM does not follow the junction temperature of the LED but aims at maintaining constant flow of thermal energy through the device creating a sphere of lower level temperature zones around the whole LED increasing the proportion of light versus wasted heat. This will lower the thermal stress inside the semiconductor allowing more light to pass through the holes, further eliminating all heat development inside the chip of the LED, keeping the entropy constant. When LEDs are viewed in a heat sensitive camera it is observed that very soon after the activation of the LED light, heat starts to develop around the junction of the chip and only moves out to colder zones of a heat sink proportional to increased temperature inside the junction this is according to thermal laws.

It is not necessarily desired to obtain cooling of the LED substantially below the ambient temperature; on the contrary, the inventors have found that the desired efficiency and energy saving/light gain of the present invention is obtained by keeping the operating temperature of the LED close to or just below the ambient temperature. In some embodiments the operating temperature of the LED may even be slightly higher than the ambient temperature, but importantly, the LED operating temperature is prevented from raising much above ambient temperature, such as would be the case for an LED-lamp with passive cooling. If the ambient temperature is, e.g., about 20-25(30°).degree. C°, a non-cooled LED may be expected to warm up during operation and within a relatively brief period reach an operating temperature in the range of about 50-60.degree C°, at which point the illumination of the LED has decreased by about 30-40% or more due to the negative Illumination-temperature coefficient.

The heat sink is generally of a conventional type, i.e. with a flat surface that is in contact with the TEM's hot side, while the other side of the heat sink has an extensive surface area to efficiently dissipate the heat to the air in contact with the heat sink. The inventors have discovered the importance of the heat sink design, specially the surface area per volume unit heat sink ratio, for the thermal stability of the device. It is a prior art that turbulent air flow, and rough surface, increase the dissipation of heat through the device. By designing the first heat sink with a large surface area. High surface-area-to-volume ratio provides a strong "driving force" to speed up thermodynamic processes that minimize thermodynamic free energy.

From the above discussion and analysis it follows that particularly preferred embodiments of the invention relate to devices configured such that the device produces more illumination per unit consumed power when the TOC is applied to the TEM, than the Illumination produced per unit consumed power when no TOC is applied to the TEM. For example, if the TOC consumes 30% of the energy consumed by Leeds of a multi-LED lamp, the total energy consumption is 130% when the device is being cooled and 100% If the device is operated with no cooling; if this prevents the diodes from warming up and loosing 50% light output, the number of diodes in the lamp can be halved in the cooled lamp to obtain the same light intensity, reducing the LED energy to 50% and thus the overall energy consumed is 80%, i.e., a net energy gain of 20% can be obtained in this example by cooling the Leeds in accordance with the invention.

In certain embodiments, the device of the invention comprises a plurality of TEMs. These may arrange side by side, e.g., each arranged to cool a set of LEDs. Also, TEM may be arranged in a stacked fashion, such that two, three or more TEM form a "sandwich" wherein the TEM closest to the LEDs has its hot side thermally connected (either directly adjacent or connected with a thermally conducting material) to the cold side of a second TEM, which also may have its hot side connected to the cold side of a third TEM and so forth. The layers of the stacked TEMs may overlap or bridge two or more TEMs of the next layer so as to provide multiple routes for heat transfer. When using such stacked TEM, the heat sink can be seen as comprising the combination of the additional TEMs, any intermediate heat-conducting plates and the heat sink itself furthest away from the LED in the sandwich of components.

The device comprises in one embodiment a control unit for controlling and even reversing the TOC, and one or more sensors connected to the control unit for sensing one or more environmental parameters, wherein the control unit is configured to adjust the TOC based on parameters measured by the one or more sensors.

It may in some cases be beneficial to operate the TEM with an electronic controller able to maintain the temperature around the LED.

It may in some cases be beneficial to operate the device with (PWM) pulsed current to the one or more LEDs, e.g. such that current pulses alternate between different LEDs of the device and or for one or more thermo electric devices.

In certain embodiments the device may be operated with pulse with modulated current (PWM) to the one or more TEMs, e.g., in further embodiments, it may be beneficial to operate the device with pulsed current to the one or more LEDs and/or TEMs, e.g., such that current pulses alternate between different LEDs and/or TEMs of the device.

A related aspect of the invention provides a light illuminating device comprising at least one high flux light (HB-LED), emitting diode (LED) and at least one thermoelectric module (TEM) thermally connected to the first heat sink, and a second heat sink bigger then the first;

wherein the at least one TEM is selected and configured such that by running a TEM- operating current (TOC) through the TEM, the thermal power produced by the at least one

LED is transferred through the first heat sink already cooling the microclimatic temperature around the LED affecting the overall thermal strain from the light production, at least one TEM to the heat sink, thereby maintaining or lowering the temperature surrounding the LED and enhancing the light output from the LED, prolonging the expected lifetime of the LED, improving the CRI; the device thus consuming less overall power per amount of emitted light when the TEM is running as compared to the overall power per same amount of light when the device is operated without running an operating current through the TEM.

In a further aspect, the invention provides a method for enhancing the efficiency of an light illuminating device having one or more LEDs as a light source, comprising: providing the device with one or more thermoelectric module(s) (TEM) having a cold surface and a hot surface, such that the cold surface is thermally connected to a heat sink with a relative ratio between surface area and volume higher than 2 and the hot surface of the TEM is thermally connected to a bigger heat sink; applying a TEM-operating current (TOC) to the one or more

TEMs to create a temperature gradient through the TEM; adjusting the TOC such that substantially all of the thermal energy created by the LED(s) when operated is transferred to the heat sinks, so that the smaller heat sink always has a lower temperature than the measured ambient temperature, thereby substantially maintaining the operating temperature of the LED(s) at ambient temperature or a lower temperature, wherein the TEM is configured and TOC adjusted such that the device consumes less overall power per amount of emitted light when the TEM is running as compared to the overall power per same amount of light when the device is operated without applying a TOC to the TEM. The preferred embodiment described herein is shown in FIG. 1 and comprises an High Brightness (HBLED) (1 ) LED and or (2) semiconductors and (2a)a lens to shape the light beam, (4) thermoelectric device integrated into two heat sinks, (3,5) called (3) The first heat sink and (5) the second heat sink. This combination creates a lower than ambient temperature zone around the (3) first heat sink creating lower thermal strain on the (2) origins and reflection of the illumination. (Fig. 2) This changes all calculations regarding thermal management of the LED by reducing the thermal energy and or withholding constant entropy level inside the semiconductor material. The combination of (3, 5) heat sinks and the placement of the (4) thermo electric device between them, creates a lower temperature zone around the (1 ) LED. The ratio between the surface area and the volume of the (3) heat sink must be higher or the same as per unit measurement. (5)The second heat sink must have a higher thermal capacity than (3) the first heat sink. Turbulent air flow and roughness of the surface of the (3, 5) heat sinks, further improves the stability of the thermal environment. By holding the entropy constant inside the semiconductor material creates a maximal photon output and or optimal computing capacity. This stability in temperature (Fig. 3) reduces LED degradation and or the performance of the semiconductors with prolonged life time of the device as a result, and or in groups in any application using HBLED s.a. critical outdoor applications for the purpose of light signaling and or wide area illumination type applications, as well as indoor spotlights and or in general lighting. Depending on the IP grading of the fixture, the apparatus can consists of a closed or open chamber (100) and it can be without. It is insulated; if used, with any high thermally resistant material s.a. aero gel type material (2) to prevent ambient heat from loading the total heat removal and creating a spatially larger cooling area around the origin of the illumination. It can be without any insulation and it can have vacuum. At least one LED is attached to a single heat sink (4) that must have a high ratio of surface area per volume heat sink and a low thermal resistance. Outside the chamber, if used is a thermo-electric module (3) with the cold side facing the (3) heat sink (4) and the hot side is attached to a larger volume heat sink (5). Said /3) heat sink is the outmost boundary between the chamber and the adjacent micro-environment. The (5) second heat sink is constructed according to presented formulas. It is connected to an external structure (6) for final removal of the total heat accumulated.

The LED (1) is a high brightness type of LED capable of producing 60-250 lumens per watt and above If the LEDs are arranged within a housing the thermoelectric module may also be attached to the housing, and/or be an integrated component of said housing.

The LEDs can be grouped in any geometrical order and attached to any curved and/or even surface. The angularity and alignment of LEDs (1) for the purpose of illumination and signaling is not an issue in this invention. It is the prolonged lifetime, better CRI and prevention of the Droop effect in LEDs that is the issue of this invention for the benefit of all LED based fixtures. The Peltier thermoelectric module (4) has one side hot and the other cold when activated with an electric current. The cold side is facing the first heat sink (3) The said heat sink having a ratio between the surface area and the volume of the heat sink. (3) is attached to the LED (1 ) or could be directly mounted on the LED. A thermally conducting metal plate (4) is attached to the hot side of the TEM (4). Insulating chamber can be constructed around the electrical components when humidity levels can harm the

components. It is important to have the right proportion between air/gas and insulation inside the chamber. Air in particular is a good insulator if the air has no movement and to prevent back flow of heat in the system air/gas has a very low thermal conductivity and even in very thin layers they are capable of insulating and or stopping the heat flow. The transition between the hot side of the TEM (3) and the heat sink (4) must be a vapor free material and able to withstand 1 bar pressure. The described embodiment can be constructed having two chambers. Chamber 1(A) is for the LEDs and the first heat sink. The chamber (1) can be filled with dry air or other gases (inert gases) to a higher air pressure than average ambient pressure to prevent the flow of gases (in particular ambient air carrying moisture) into the chamber. The chamber (1) can be filled with gases other than air, e.g. Nitrogen, Argon or Helium, to further prevent moisture inside the chamber. A second chamber can be constructed surrounding the space (B) to ensure more efficient movement of heat from the thermally conducting plate (4) to the final heat sink (7)-and then to the support structure (8). Chamber (2) is constructed around the 1 chamber with the peltier attached to the first heat sink and the second heat sink providing for a relative fast enough heat flow from the LED through chamber 1 over to the hot side of the peltier and finally the second heat sink. This flow is always faster than the heat flow through chamber 1. Therefore the first heat sink cools down below the ambient temperature. Thermally conductive materials are used near the light production inside the LED and through chamber 1 removing the heat from the air/gas faster than the ambient temperature is entering chamber 1 because the first heat sink has a lower temperature the heat flow comes naturally according to thermal laws from the LED to the second heat sink. The higher the delta T the faster the heat flow.

Further research made discoveries that improved the thermal management of LEDs.

LEDs have developed since 2001. The LED can use higher currents, produce less waste heat (entropy) and can withstand a higher junction temperature. Light producers minimize the thermal problem by using less current per LED than it is rated for. Thus producing less waste heat. Our new discoveries are:

1. The importance of creating a constant sub ambient micro climate around the whole LED, the bigger the better. Thus lowering the thermal strain that elevated ambient temperature creates around the LED generally. Only active cooling can do that.

2. A better results are created by moving a part of the heat sink and attach it to the cold side of the TEM. The moved heat sink having as big surface area as possible and made of a high thermal conductivity material.

3. Dynamic electronic control can improve performance compared with static models

4. Cooling LEDs affects many aspects of The LEDs performance. The frequency of the light waves can be fine tuned. The performance lowering droop effect is eliminated. Degradation is slowed by decades, producing longer lasting LEDs

5. Importance of marinating a temperature gradient big enough to move the heat

(entropy) a few millimeters away from Fig 1 the (1 , 2, 2.a) heat source thus lowering the thermal strain on the LED further lowering the junction temperature. The entropy remains constant during the operation of the light and or other semiconductors providing for a stable operating values and a prolonged operating lifetime of the light and or other semiconductors.

6. Common High flux LEDs improves their lumen performance such as the color

rendering index, while having lower operating temperature. DRAWINGS FIG. 1 A picture of the preferred embodiment. Elevation from the vertical side. Comprising (1) at least one LED light source, High flux LED and or electronic semiconductor device. (2)A lens and or reflective mirrors for the LED. (3) A first heat sink having a high volume and surface ratio between. (4) A pettier device facing the cold side to the (3) first heat sink and the hot side to the (5) second heat sink.

FIG. 2 A thermal picture of the preferred embodiment (fig.1 ) during operation, showing the essence of this invention. (3) The first heat sink is colder than (5) the second heat sink. This creates colder than or the same as the ambient temperature around the (1 , 2) light production and or other semiconductors.

FIG. 3 A temperature measurement of the preferred embodiment showing: (10) the

temperature at the (3) first heat sink and (20) the temperature at the (5) second heat sink as a function of time. FIG. 4 the preferred embodiment of the (3) first and (5) second heat sink, in plan view.

Utilizing a high conductivity metal namely, but not limited to extruded, cut and or carved aluminum. The heat sink having as large circumference as possible.

According to prior art, this will provide for a relative large surface area/volume ratio, producing stability in temperature during the operation of the light and or other charged semiconductors.

FIG. 5 Diagram of a thermal development in the device over time when operated with

passive (3, 5) heat sinks only. Only the TEM is turned off. (10) The

temperature at the (3) first heat sink and (20) the temperature at the (5) second heat sink as a function of time. References Cited [Referenced By] U.S. Patent Documents

6252154 June 2001 Kamada et al.

6832849 December 2004 Yoneda et al. 6902291 June 2005 Rizkin et al.

6964501 November 2005 Ryan

2002/0070360 June 2002 Machi

2004/0120156 June 2004 Ryan

Foreign Patent Documents 1 067 332 Jan., 2001 EP

WO 00/37314 Jun., 2000 WO

Other References

International Search Report. Cited by other.

Claims

CLAIMS 1. A light comprising at least one high flux light emitting diode (HB-LED), and a heat sink with as large surface to volume ratio, valid for heat sinks thicker than 10 mm, at least one thermoelectric module (TEM) having a first surface which is thermally connected to the heat sink, and the second surface connected to the TEM, a second heat sink thermally connected to the TEM. Thus creating a thermal environment around the at least one high flux light emitting diode that is lower than the ambient temperature changing the initial operating temperature of the HB-LED. Thus preventing a Droop, diverting the Thermal stress from the junction area to the back (hot side) of the TEM and increasing the ratio of emitted light energy per dissipated heat energy. The Entropy (S) remains constant inside the

semiconductor material.

2. A light illuminating device, comprising: a. at least one high flux light emitting diode (HB- LED), and or at least one light emitting diode (LED); b. at least one thermoelectric module (TEM) having a first surface which is thermally connected to a heat sink with a

surface/volume ratio above 2; c. a heat sink with a surface/volume ratio above 2, thermally connected to at least one high flux light emitting diode (HB-LED) creating a lower ambient temperature zone around the junction area of the LED. Reducing the initial thermal stress in the LED improving the lifespan of the LED, improving the color rendering index and higher relative light output per consumed Watt.

3. The device of claim 2, wherein the at least one TEM is configured such that the device is operated by running a controlled current through the TEM. The controller can have 3 modes, stop, and pause and run all working in a predetermined temperature range and or with a dynamic PWM of the current.

4. A light illuminating device, comprising: a. at least one light emitting diode (LED) and or one high flux light emitting diode (HB-LED),; b. at least one thermoelectric module (TEM) having a first surface which is thermally connected to the heat sink; c. a heat sink thermally connected to a second surface of the at least one HB-LED; wherein the TEM is configured such by the controller that the operating temperature of the LED(s) is lower than the ambient temperature

5. A light illuminating device, comprising: a. at least one light emitting diode (LED); b. at least one heat sink with a surface/volume ratio above 2; c a thermoelectric module (TEM) having a first surface which is thermally connected to the said heat sink and the second surface connected to a heat sink with a lover surface/volume ration than 2 but with a higher heat capacity.

6. The device of claim 5, wherein the TOC for each of said at least one TEM is controlled by frequency of PWM current and amount of current applied. In a on/off mode controlled by temperature

7. A light illuminating device, comprising: a. at least one high flux light emitting diode (LED); b. at least one thermoelectric module (TEM) having a first surface which is thermally connected to the heat sink; c. a heat sink thermally connected to a second surface of the at least one TEM; wherein the TEM has a coefficient of performance (COP) during normal operation in the range of about 2-6.

8. A light illuminating device, comprising: a. at least one (high flux, and or HB-Led), light emitting diode (LED); b. at least one thermoelectric module (TEM) having a first surface which is thermally connected to the LED; c. a heat sink thermally connected to a second surface of the at least one TEM; wherein the device is configured to produce more illumination per unit consumed power when the TOC is applied to the TEM, than the illumination produced per unit consumed power when no TOC is applied to the TEM.

9. The device of claim 8, comprising a plurality of LEDs.

10. The device of claim 8, comprising a plurality of TEMS.

11. The device of claim 8, comprising a plurality of TEMs thermally connected in a stacked fashion.

12. A light illuminating device, comprising: a. at least one (and/or, high flux, and or HB-Led), light emitting diode (LED); b. one heat sink with a surface/volume ratio above 20; c. at least one thermoelectric module (TEM) having a first surface which is thermally connected to the heat sink; c. a heat sink thermally connected to a second surface of the at least one TEM; and d. a thermally insulating cover with a enclosed air or gas creating an enclosed chamber substantially insulating the LED from ambient air, wherein the enclosed chamber has a higher pressure than ambient pressure during normal operation, thus preventing humidity and dust.

A method for changing the ambient temperature around the LED and or groups of LEDs where by creating a new ambient micro climate with a lower temperature than nearby space. By moving a portion of the heat sink having first contact with the cold side of the TEM and the LED and or LEDs. The said moved heat sink having as large surface area as possible. This method creates desired ambient temperature around the LED/LEDs as long as the TEM is controlled (ATH) to maintain as constant temperature grade as possible. This method changes the thermal calculations by using lower temperature than general ambient is from time to time.

The method of claim 13, wherein the TOC is dynamic within limits to create temperature stable environment on both sides of the TEM. The dynamic range depends on the LED type used and the level of current applied to the LED/LEDs

The method of claim 13, wherein the heat sink moved to the cold side of the TEM has as high thermal conductivity as possible.

The method of claim 13, wherein the created temperature grade between ambient and outward facing surface of the LED/LEDs is maintained with calibration of the and dimensioning of the heat sinks