An internal microfluidic heat-exchange system for a thermoelectric module

Abstract

A microfluidic internal heat-exchange system for a thermoelectric module. A thermoelectric module with an electrically and thermally insulating internal separator creating internal hot chambers and cold chambers to allow microfluidic heat transfer. Fluids are passed through the chambers using microfluidic pumps and controllers. The module may also be further encased in a structure to allow direct fluid heat-transfer with the external surfaces of the module. Grooves or channels may also be etched on external and internal surfaces to aid heat-transfer between fluids ad surfaces. The modules may also be connected in a patchwork with centralised control of individual local temperatures.

Description

AN INTERNAL MICROFLUIDIC HEAT-EXCHANGE SYSTEM FOR A THERMOELECTRIC MODULEDESCRIPTION

[0001] This invention relates to a thermoelectric module with a microfluidic heat exchange system built internally into the module itself.

[0002] Thermoelectric modules, often called Peltier modules are a frequently used heat pump solution. We refer to them as TEMs throughout this document. A conventional TEM construction is shown in Figure 1. It comprises of an array of alternate positive and negatively doped thermoelectric semiconductor dice that are electrically connected in series. This is then sandwiched between two opposing ceramic substrates so that they are thermally in parallel. Their structure and operation is well documented in prior art. When a current is passed through the semiconductor dice, a thermal differential is created at the junction between the positively and negatively doped semiconductors. Depending on the arrangement of the positive-negative junctions of the dice and the direction of current flow, one of the substrates absorbs heat and is referred to as the “cold face” and the other dissipates heat and is the “hot face”. It can be used as a cooling device by linking the cold face to a surface being cooled via a heat transfer system or alternatively as a heating device by linking the hot face to a surface requiring heating via a heat transfer system. As it is a heat pump, heat would also need to be evacuated from or supplied to the other opposite face as the case may be.

[0003] A key advantage of the TEM is its small footprint, weight and form-factor compared to other commonly used heat pumps such as conventional vapour compression air-conditioning. For example, common TEMs are just a few centimetres square on its planar surfaces with a thickness of between 3.5 to 10 millimetres ie comparable to just two or three credit cards stacked on each other. This makes it attractive for use in situations where there is limited space available eg for cooling in small electronics enclosures or where mobility is required such as in portable refrigerators. It is also useful in cases where air-conditioning is not practical such as outdoor stadiums, events and restaurants where individual or very localised solutions may be more appropriate.

[0004] There are however a number of problems when practically implementing TEMS for heat transfer as further described below. Two problems in particular are highlighted below which the present invention solves. They are referred to as problem Alpha and problem Beta for ease of reference.

[0005] Problem Alpha in TEMs is the need for additional bulky external heat exchange mechanisms to provide or dissipate heat from the cold-face or hot-face respectively.

[0006] Consider the example of using the TEM to cool a very small enclosure. In this case there are three heat-exchange options for the cold face. Option 1 , called passive, is to directly attach the flat planar cold face to a flat thermally conductive surface on the enclosure or object being cooled. The second is to affix a heatsink and fan to the cold face to blow cold air from the cold face within the enclosure to be cooled. The third option is to attach a liquid circulation system between the cold face and the enclosure. Similarly heat is required to be evacuated from the opposite hot face. This is important not just to maintain the cooling cycle but because excessive heating of the hot face can result in malfunction of the TEM. In practice, Option 1 is used infrequently due to the low heat flux rates that the passive approach supports. Further it may not be practical to bring the two planar surfaces in direct thermally efficient contact. Most uses therefore depend on active methods such as Option 2 or Option 3 with forced convection or conduction.

[0007] All these options, and in particular Options 2 and 3, undermine the attraction of the small footprint, weight and form factor of the TEM. While the TEM can be a planar structure of a few centimetres in length and width and just millimetres in thicknesses, the external heat exchange devices described above can be orders of magnitude bigger in size eg tens of centimetres in height, width and depth and weigh hundreds of grams, if not kilograms. T o provide an idea of scale, while the TEM itself may have the dimensions comparable to a thick credit card, the complete system with the external heat-exchange mechanism can become the size of a shoebox. Circulation tubing systems for liquids can also take up more space and lead to design complexity.

[0008] All of these combine to reduce the range of applications and situations where TEMs can be utilised. For example, one use suggested for TEMs is for an active cooling system in clothing for extreme climates. However, conventional implementations on the market require several bulky external fans or pumps for heat-exchange to be worn or carried by the wearer which limit its practical use, comfort and convenience. Eliminating the need for such bulky apparatus or providing a more attractive form factor would significantly encourage the adoption of such devices.

[0009] One of the causes of problem Alpha is also problem Beta described in this section. Problem Beta in existing TEMs is that while the surfaces on the semiconductor junctions that produce the thermoelectric effect are inside the module, the forced heat-exchange devices are attached to the outside of the module. Therefore heat has to travel from the semiconductor junction on the inside of the module, through the body of the ceramic substrate and then to the outer surface of the ceramic substrate where the heat exchange apparatus is attached. The heat flux is hence reduced by the joint between the semiconductor junction and the ceramic substrate and the thermal resistance of the body of the ceramic substrate. The temperature at the external surface of the hot-face is therefore lower than at the hot thermoelectric junction and likewise, the temperature on the external surface of the cold face is higher that at the cold junction. This results in the thermal differential between the heat exchange surfaces being smaller than if the heat exchange apparatuses where connected directly to the thermoelectric junction themselves. As the supportable heat flux across a heat exchange surface is related to the temperature differential, this lower temperature differential results in a lower rate of heat exchange than otherwise possible. A heat exchange mechanism that operated within the TEM itself would be highly beneficial as it would have direct contact to the semiconductor junction where the thermal differential is produced as well as the conductive strips at the junction which are both electrical and thermal conductors. There is however no identified prior art. One of the challenges in designing such a solution before the present invention was the need to deal with both hot and cold heat-exchange fluid flows in juxtaposition to each other in a microscopic chamber such as that within a TEM module. Additionally such fluid flows needs to be done without electrical interference.

[0010] Problem Beta has a bearing on problem Alpha as a more efficient heattransfer mechanism would eliminate or reduce problem Alpha described. In that sense, providing an internal heat transfer mechanism is key to unlocking the true potential for TEMs.

[0011] To overcome these problems and challenges, the present invention proposes a microfluidic heat transfer mechanism that sits directly inside the TEM itself. A microscale thermally and electrically insulating rigid planar separator is built into the internal space between the two ceramic substrates. The separator divides the internal space into a separate hot chamber and cold chamber within the TEM. Separate hot and cold fluid flows are directed within the respective hot and cold chambers. In a preferred implementation the separator is constructed using a rigid aerogel but other materials may be used. A microfluidic pump and controller are used to pump dielectric heat-exchange fluids into the hot and cold chambers respectively to provide the required temperature differential. The hot and cold fluids are in direct contact with the semiconductor junctions as well as the conductor strips between them which are also thermal conductors, In a preferred implementation the hot and cold heat-exchange fluids are also pumped in opposite directions to each other to minimise the time available for unwanted heat-transfer between the two fluid flows. The heat-transfer fluids are then pumped out and evacuated to a reservoir or heatsink that does not need to be in the immediate vicinity of the TEM.

[0012] The separator may be coated with different films in order to optimise its thermal and electrical performance.

[0013] The semiconductor dies may be shaped to optimise microfluidic flows in say an elliptoid state.

[0014] Multiple such TEMs may be connected to create a patchwork quilt of modules that can number into the hundreds or thousands.

[0015] Multiple pumps and controllers may be used to control multiple chambers, modules and channels.

[0016] Multiple separators may be used to create multiple hot and cold chambers to provide more granular control of flows. In other implementations, the separators are modelled into channels rather than chambers in order to get even finer control of the fluid flows.

[0017] Pressure, temperature and other data from the pumps, controllers and other parts of the system using sensors may be communicated back to a central device which optimises the fluid flows through multiple chambers, modules and pumps to achieve even and dynamic temperature control over, say, a human body using expert systems, machine learning or other forms of artificial intelligence.

[0018] The invention will now be described solely by way of example and with reference to the accompanying drawings in which:Figure 1 represents prior art and shows a 3-dimensional representation of a conventional TEM with semiconductor dice sandwiched inside ceramic substrates. Two external heat-exchange devices are also shown that are typically deployed with a conventional TEM.Figure 2 shows a 3-dimensional TEM module with an implementation of the present invention incorporating the rigid planar separator. The side walls are not shown as the array is not limited to this size.Figure 3 shows a front view of an implementation of the present invention with the rigid planar separator and the hot and cold chambers. Again the side walls are not shown as the array is not limited to the size depicted. It is preferred that that planar separator is rigid but a flexible material may also be used.Figure 4 shows a plan view of the cross section taken across the plane identified by the dotted line A in Figure 3Figure 5 is an implementation with side walls shown of the module and the position of inlet and outlet manifolds to the hot and cold chambers. The sidewalls have been shown so that the working of the whole system can be depicted but are not to be considered a limitation of the size of the semiconductor arrays.Figure 6 shows how a plurality of such TEMs may be connected together to form a patchwork quiltFigure 7 shows an implementation of the present invention where micro channels are etched on the outside or inside of the ceramic substrates to further improve heat exchange.Figure 8 shows an implementation of the invention where the present TEM or conventional TEM is further encased in a further heat exchange module which permits direct fluid contact with he outside of the ceramic substrate

[0019] The exemplar implementation is now described in more detail.

[0020] Figure 1 shows a 3-dimensional representation of a conventional TEM. Positive semiconductor dice 1 are connected in series to negative semiconductor dice 2 by electrical conducting strips 3. The electrical conducting strips are made of a material such as copper. It may be noted that the semiconductor dies are cut from blocks or wafers and usually have a cuboid shape. They are sandwiched between two thermally conductive but electrically insulating encasing surfaces 4 and 5. Encasements 4 and 5 are typically ceramic substrates but can be of any other material meeting the thermal and electrical requirements. A DC voltage source is connected via the wires 6 and 7. The polarity of the DC source is such that a hot face is created on substrate 5 and a cold face is created on substrate 4. An external air heatsink 50 is also shown that attaches to the hot-face 5 which would typically be cooled through a forced conventional device such as an additional fan that is not shown. A water-cooled heat sink 51 is also shown which may be fixed to the coldface 4 with a heat exchange fluid such as water being pumped through the manifolds 52 by an external pump that is not shown. It can be seen that the the heat-chance devices 50 and 51 together with the fan and water pump add considerably to the bulk of the solution compared to just a TEM on its own.

[0021] Figure 2 shows a 3-dimensional representation of a TEM with an implementation of the present invention. Items 1 through to 7 remain the same as in Figure 1. It should be noted that the electrical conductors 3 are also good thermal conductors which are relevant to this implementation. A rigid planar separator structure 8 is inserted between the two outer substrates 4 and 5. The separator 8 lies on a parallel plane with substrate planes 4 and 5 to create two new chambers10 and 11. Chamber 10 is the hot chamber as it is bounded by the hot face 5 and chamber 11 is the cold face as it is bounded by the cold face 4. Separator 8 is fabricated from a material that is rigid and is both a thermal and electrical insulator. One such material would be an aerogel but it may be made of any other material that has these qualities. It can also be a composite material or fabricated by laying down different films of substances with different electrical and thermal properties. In this preferred implementation, the separator 8 does not cut through the thermoelectric dies which remain fully intact. Separator 8 is constructed with gaps in the planar structure to allow the thermoelectric dies to pass through. Manufacture of such a material and structure are well understood in semiconductor arts. In other implementations, the semiconductor dies may be cut through so that there are two separate halves on each side of separator connected by an electrical conductor. The semiconductor dies in this implementation retain their cuboid or rectangular cuboid shape but they may be manufactured in other shapes that optimise fluid flow and heat transfer while maintaining their thermal performance.

[0022] Figure 3 shows the front view of the TEM shown in Figure 2. The position of the separator 8, the hot chamber 10 and cold chamber 11 can be seen more clearly.

[0023] Figure 4 shows a cross sectional view of the TEM along the plane shown by the dotted line A-A in Figure 3. The cross section is taken across the middle of the planar separator 8. The shaded areas show the individual semiconductor dicel and 2 surrounded by the separator 8. The two by four array size of thermoelectric dice is shown as an example only. Other implementations may include any number of thermoelectric dice in arrays of any size. The dice are shown in a cuboid shape but may also be in an elliptic or other shape that allows for more efficient fluid flow or heat transfer.

[0024] Figure 5 shows such a module described above with a thermally and electrically insulating closed wall structure 12. The hot chamber 10 has an inlet manifold 14 and an outlet manifold 16. The cold chamber 11 has an inlet manifold 15 and outlet manifold 13. Dielectric heat exchange fluids can be passed through chambers 10 and 11 via the manifolds 13, 14, 15, 16 using microfluidic pumps and controllers. The separator 8 protects the fluids from thermal leakage betweenthemselves. The fluids are in direct contact with the joints and junctions of the dies 1 and 2 as well as the thermally conductive electrical conductive strips 3. The hot and cold fluids respectively are also pumped in contra-flow to each other to ensure that individual fluid elements spend minimal time in juxtaposition with each other. The number of thermoelectric dies shown in the figure is for example purposes only and in practice could be an array of any size.

[0025] Figure 6 shows another embodiment where a single such TEM 20 can be connected into a set of much larger network of TEMs that resemble a patchwork quilt. 23 represents microfluidic connectors such as pipes or channels for the flow of hot chamber fluids to or from chamber 10. 24 represents fluid connectors for flow of cold chamber fluids to or from chamber chamber 11. The flow is regulated by pumps 21 which may be of microfluidic or other type. Sensors may also be incorporated into the pumps or TEMs or fluid connectors to transmit temperature, flow and other relevant date to a controller where human, electronic, mechanical or software based mechanisms can be used to further regulate or optimise the fluid flows to achieve the targeted temperature distribution.

[0026] Figure 7 shows a further variation of the implementation where microchannels 32 may also be etched on the inside of the ceramic substrate to improve heat exchange with fluids in chambers 10 and 11 identified previously. In another implementation, microchannels 31 are etched on the outside of the ceramic substrate to allow direct heat exchange with fluids as in Figure 8 on the outer surface of the TEM.

[0027] Figure 8 shows a TEM where the heat exchange fluids are directly applied to the outside of the TEM rather than the inside. The TEM is enclosed in an enclosure 30. The enclosure is electrically insulating but may be a thermal conductor or insulator as the situation requires. A thermally and electrically insulating planar separator 38 is attached to the mid-plane of the TEM 20 to separate the enclosure 38 into a separate hot outer chamber 39 and cold outer chamber 40 respectively. The separator 38 is analogous to separator 8 and may be made of the same or other material. A hot-side heat exchange fluid is then passed through the hot outer chamber 39 via manifolds 36 and 38 to transfer heat away from the module to an exhaust or an object being heated. A cold- side heat transfer fluid is passed throughthe cold outer chamber 40 via manifolds 34 and 35 respectively to provide heat input to the TEM from a heat source or object being cooled.

Claims

CLAIMS1. A thermoelectric module with a single or plurality of intra-module planar separators and internal hot and cold chambers through which different heat exchange fluids may be passed.

2. A thermoelctric module that is in an enclosure that creates either or both of outer hot and cold chambers to either of which a fluid may be directly applied for heat transfer with the TEM.

3. A thermoelectrical module of claim 1 or 2 in which any of the substrates, casings or separators have microscale grooves or channels etched on any of their surfaces.

4. A thermoelectric module of Claims 1 , 2 or 3 above through which different heat-exchange fluids at different rates may be passed in any direction in the hot and cold chambers respectively by microfluidic or other type of pumps or controllers.

5. A thermoelectric module of any of claims 1 to 4 above where the internal semiconductor dice are shaped to optimise heat exchange with heat-exchange fluids such as an elliptoid or other shape.

6. A thermoelectric module according to any of claims 1 to 5 above where the planar separators are made of a thermally insulating and electrically insulating material.

7. A thermoelectric module of any of Claims 1 to 6 above where the planar separator is made of an aerogel or a composite aerogel or a mixture of an aerogel and other substance.

8. A thermoelectric module of any of the Claims 1 to 7 above where additional microfluidic channels are constructed in the hot and cold chambers.

9. A thermoelectric module of any of the Claims 1 to 8 where the hot and cold heat exchange fluids are flowed in opposite directions to each other in the hot and cold chambers.

10. A thermoelectric module of any of the Claims 1 to 9 above where there is only fluid flow in the hot chambers or cold chambers but not both.

11. A thermoelectric module of any of Claims 1 to 10 above where the heat exchange fluids flow in any direction relative to each other.

12. A plurality of thermoelectric modules in any of claims 1 to 11 above that are connected by any structure allowing the flow of heat-exchange fluids between different thermoelectric modules.

13. Thermoelectric modules of Claim 12 that are connected by flexible or any other tubes.

14. A plurality of thermoelectric modules in any of the claims 1 to 13 above that are connected via sensors to a control system that regulates and optimises the relative flow of heat-exchange fluids to regulate localised temperatures across the system according to single or multiple dynamically evolving target states.

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