Patent Description:
Electronic devices produce excess heat in use, and require thermal management in order to maintain their optimal operational temperature levels. Operating such a device in temperatures above (or below) the optimal temperature range will negatively impact the reliability, operational lifespan and/or efficiency of the device. Therefore thermal management systems work to maintain the optimal operational temperature of the device to maximise reliability, operational lifespan and/or efficiency.

Thermal management systems are designed to regulate and/or control the temperature of operational devices. An example component is a heat exchanger, which is designed to transfer heat away from a heat source, e.g. from an operational electronic device. Thermal energy may be extracted directly from the device by a heat exchanger, or by using a fluid coolant to transfer thermal energy from the device to the heat exchanger. Coolants typically have a high thermal capacity and therefore can hold large amounts of thermal energy. An example of a heat exchanger is found in an internal combustion engine in which engine coolant flows through the heat source (the engine itself). The coolant transfers heat away from the engine, heating up as it does so, and subsequently cooling down the engine. The heated coolant passes through radiator coils, and as air flows past the coils, thermal energy is transferred from the coolant, cooling it, to the incoming air. The thermal energy is carried away by the heated air.

The present invention seeks to provide an improved thermal management system for a thermal load, i.e. heat source, such as an electronic device. Such a thermal management system comprising the features of the preamble of claim <NUM> is known from <CIT>.

According to one aspect of the invention there is provided a thermal management system, for a heat source, comprising a fluid coolant, a pump, a heat exchanger; and a temperature control unit (TCU), as set out in claim <NUM>.

The TCU is adapted to heat the fluid coolant to a pre-determined temperature. This ensures that the fluid coolant is most efficiently able to absorb heat from a heat source, and transfer it away, thus cooling a heat source to the optimal operating temperature.

In one example, the TCU comprises a thermometer, a TCU heat source, and a controller. The controller activates the TCU heat source and heats the fluid coolant to a pre-determined temperature if the coolant temperature, measured by the thermometer, is below a pre-determined threshold. Therefore, if the fluid coolant is too cold, i.e. below its optimal temperature, the TCU heats it up before it is passed back past the heat source.

According to the invention, the fluid coolant comprises a carrier fluid and encapsulated phase change material (PCM) particles suspended in the carrier fluid. This improves the efficiency of the coolant to transfer energy away from the heat source. Preferably, the pre-determined temperature to which TCU is adapted to heat the fluid coolant to is below the melting point of the PCM.

In another example, the heat exchanger comprises encapsulated PCM particles suspended in the fluid coolant flow, and a porous mesh. In this example, the porous mesh gap size is smaller than the PCM particle size which constrains the PCM particles in a containment area, but through which the fluid coolant may pass. Therefore the PCM particles are contained within the heat exchanger, and may not infiltrate the wider cooling system, where they may cause damage. Preferably, the encapsulated PCM is formed of two or more PCM particles combined together.

In another example, the heat exchanger comprises a porous foam, matrix of tube cavities or mesh, any or each incorporating encapsulated PCM particles. The coolant fluid will transfer thermal energy to the PCM particles as it flows through or past the porous foam, matrix of tube cavities or mesh.

In another example, where the system comprises encapsulated PCMs, more than one type of PCM may be used, each different type of PCM having a different melting point. This allows the thermal management system to customise the thermal response of the PCM, i.e. the heat capacity profile, and the performance of the thermal management system.

In another example, the thermal management system is coupled to a wider fuel system or another device; and the fluid coolant is a fuel source for the other device. This allows the broader system as a whole to save weight by using fuel as both fuel and a co0lant.

According to one aspect of the invention there is provided a method as defined in claim <NUM>, comprising the steps of pre-heating a fluid coolant to a pre-determined temperature, pumping the fluid coolant to a heat source, and extracting thermal energy from the heat source to the fluid coolant. The pre-heating of the coolant (to a pre-determined temperature) improves the efficiency of the coolant to absorb thermal energy from the heat source.

Preferably, the method also comprises pumping the fluid coolant from the heat source to a heat exchanger, transferring thermal energy from the fluid coolant to a heat dump via the heat exchanger, and pumping the fluid coolant from the heat exchanger to a temperature control unit (TCU). The TCU works to ensure the coolant is at the optimal temperature.

Preferably, transferring thermal energy from the fluid coolant causes it to cool to below the pre-determined temperature.

In one example, the method further comprises transferring thermal energy from the fluid coolant to an encapsulated phase change material (PCM), and transferring thermal energy from the encapsulated PCM to the heat dump. The incorporation of a PCM helps improve the coolant heat capacity.

In another example, the method further comprises measuring the temperature of the fluid coolant, and if the fluid coolant temperature is below a pre-determined threshold, then activating a heat-source to heat the fluid coolant to a pre-determined temperature. This ensures that if the coolant is too cold, it is pre-heated to a pre-determined temperature before being passed past the heat source.

The invention may be performed in various ways and specific examples will now be described with reference to the accompanying drawings, in which:.

<FIG> shows a schematic view of an example thermal management system <NUM> known to the art. During operation, a thermal load or heat source <NUM>, for example an electronic device, generates excess heat. Coolant, typically a fluid i.e. liquid or gas, is used to reduce or regulate the temperature of the system <NUM> and the heat source <NUM>. Ideally, the coolant used has high thermal capacity so as to be able to absorb and transfer large amounts of thermal energy from the heat source <NUM>. A common example of a coolant is water. Other examples include water-glycol and oil-based fluids. A coolant may maintain its state of matter (e.g. stay liquid or gas) during the cooling process, or the coolant can undergo a phase transition, i.e. changing from one state of matter to another, with latent heat adding to the cooling efficiency. Latent heat is thermal energy which allows a substance, e.g. the coolant, to change state without changing the temperature of the substance. In contrast to latent heat, "sensible heat" involves an energy transfer that results in a temperature change of the system, and is the most common form of heat storage. In an example where the coolant is water, transferring heat from a heat source may cause the initially liquid coolant to transition into a gas, i.e. steam.

During operation, coolant is transferred around the closed-loop system <NUM> along coolant pipes <NUM> by a pump <NUM>. The coolant flows past or through the heat source <NUM>, which in the example shown is an electronic device. The coolant extracts/absorbs thermal energy from the device <NUM>, cooling the device <NUM>, and subsequently the coolant carries the thermal energy away from the device <NUM> to the heat exchanger <NUM>. The heat exchanger <NUM> extracts the thermal energy from the coolant, and the coolant exits the heat exchanger <NUM> as cooled fluid, whilst the thermal energy is transferred to a heat dump, i.e. expelled through an exhaust <NUM>.

<FIG> shows an example of a thermal management system <NUM> adapted to control the temperature of a heat source <NUM>, such as an electronic device. During operation of the device <NUM>, excess heat is generated. Coolant is pumped around the closed-loop system <NUM> though a system of coolant pipes <NUM> by a pump <NUM>. The coolant flows past, around or through the electronic device <NUM> and extracts/absorbs thermal energy from the device <NUM>. The thermal energy generated by the device <NUM> is transferred away in the (heated) fluid coolant to a heat exchanger <NUM>, thus cooling the device <NUM>. The cooling pipes <NUM> may be surrounded by, abutted to or placed near the heat exchanger <NUM> to allow the thermal energy gathered as the cooling fluid passed the heat source <NUM>, to be transferred away from the fluid coolant (thus cooling the fluid coolant). The heat exchanger <NUM> expels the extracted thermal energy through an exhaust <NUM>. The (cooled) fluid coolant exits the heat exchanger <NUM> ready to be pumped back round the closed loop system <NUM> to the device <NUM> again. In one example, the fluid coolant may be pumped to a different cooling system.

Modern electronic devices are increasingly sensitive and even slight temperature variations either above or below the optimal operational thermal threshold can adversely affect the device performance. Therefore it is imperative that such devices are maintained within an acceptable operational thermal margin. As discussed, coolants can be used to help reduce or regulate the temperate of a device. However, the effectiveness of a coolant, e.g. its ability to absorb/transmit thermal energy, is influenced by the temperature of the coolant itself and therefore it is also important to maintain the coolant within an optimal temperature range so as to achieve efficient cooling of a heat source. If the coolant is too hot, or too cold, the thermal conductivity is reduced, thus decreasing the coolant's ability to absorb thermal energy from a heat source, and the device being cooled may overheat.

According to the invention, the thermal management system <NUM> also comprises a temperature control unit (TCU) <NUM>. The TCU <NUM> is located on the closed loop at a position after the heat exchanger <NUM> and before the heat source <NUM>, e.g. an electric device. As shown in <FIG>, the TCU <NUM> comprises a temperature monitoring device <NUM> (e.g. a thermometer), a controller <NUM>, and a TCU heat source <NUM>. During operation of the thermal management system <NUM>, after extracting thermal energy from the heat source <NUM>, the coolant is cooled by the heat exchanger <NUM> to a temperature deliberately below the optimal temperature for the coolant. The coolant is pumped around the system <NUM> through the coolant pipes <NUM>, and the coolant enters the TCU <NUM>, the TCU measures the coolant temperature with the temperature monitoring device <NUM>. If the coolant temperature is below the optimal operational temperature for the coolant, the controller <NUM> activates the TCU heat source <NUM> to heat up the coolant to the desired temperature, i.e. the optimal temperature for cooling the electronic device <NUM>. It is easier to heat the coolant to a desired temperature, i.e. the optimal temperature for cooling the electronic device <NUM>, than it would be to cool it down to the desired temperature. Therefore the coolant temperature can be more finely controlled, and the efficiency of the thermal management system <NUM> is improved. Accurate control of the temperature of the coolant is also important when the coolant incorporates phase change materials, as described in more detail below.

Further according to the invention, the thermal management system incorporates a phase change material (PCM). PCMs melt and solidify (i.e. change state) at a certain temperature, and are capable of: storing thermal energy as the PCM transforms from a solid to a liquid state; and releasing energy as the PCM transforms from a liquid to a solid state. PCMs store latent heat, i.e. thermal energy released or absorbed during a constant-temperature process, e.g. such as a first-order phase transition. Latent heat is thermal energy which allows the change of state of a substance without changing its temperature. In contrast to latent heat, "sensible heat" involves a thermal energy transfer that results in a temperature change of the system, and is the most common form of heat storage. Examples of such PCMS include ice/water (which melts/solidifies at <NUM>), wax (e.g. paraffin wax) and salt hydrides (also known as ionic or saline hydrides). Waxes can be formulated with a range of melting points (approximately between -<NUM> and +<NUM>).

<FIG> is a graph showing the relationship between the temperature of a sensible heat storage material and the thermal energy supplied to it. Any thermal energy transferred to the sensible heat storage medium results in a temperature increase of the storage medium. An example of a sensible heat storage medium is water, wherein transferring thermal energy to water results in the temperature of the water increasing. Transferring thermal energy away from water would result in the water cooling.

<FIG> is a graph showing the relationship between the temperature of a latent heat storage material, such as a PCM, and the thermal energy supplied to it. As thermal energy is transferred to the PCM, the temperature of the PCM increases until the PCM reaches its melting/freezing temperature (MP) (point LH1). At this point, the PCM experiences a change in state from a solid to a liquid. In some examples, the MP of the PCM is approximately <NUM> to <NUM>. During the phase transition from a solid to a liquid (between points LH1 and LH2), latent heat (i.e. thermal energy) is stored by the PCM, but the temperature of the PCM does not increase and remains at the melting point (MP). Once the PCM has fully changed state from a solid to a liquid (point LH2), no more latent heat is stored, and sensible heat storage begins again, i.e. the increase in PCM temperature resumes as thermal energy is supplied to the PCM.

According to the invention, PCMs are incorporated into the fluid coolant, wherein the coolant comprises a carrier fluid. The carrier fluid may be water based (e.g. water, or water glycol (ethylene or propylene)) or oil based (e.g. polyalphaolefin (PA0) or silicate esters). PCM particles or capsules are suspended in the carrier fluid. In some examples, the particles are each approximately <NUM> - <NUM> diameter. The preferable size range may be limited due to stability thresholds encountered during the standard production process. The carrier fluid works with the PCMs to provide enhanced heat transfer capabilities of the resulting coolant when compared to a simple fluid alone, since the latent heat storage of the PCM allows the coolant to extract, store and more thermal energy from a heat source.

In one example, the PCM may be encapsulated in an outer resin or shell to ensure that the PCM maintains its shape and/or location during its change of state. Examples of such encapsulating materials include thermosetting plastics, such as melamine formaldehyde (MF) or polyurethane (PU).

Further according to the invention, encapsulated PCMs are suspended in a carrier fluid, and free to flow without confinement throughout the coolant pipes <NUM> of a thermal management system, such as that shown in <FIG>. Both the suspended PCM and carrier fluid absorb thermal energy directly from the heat source <NUM>, increasing the cooling effect compared to a simple fluid coolant alone. Like standard fluid coolants, the effectiveness of a PCM to absorb/transmit thermal energy is influenced by the temperature of the PCM. In order to maximise the thermal efficiency of the PCM to absorb thermal energy, it is important to maintain the PCM within an optimal temperature range, preferably just below the melting point of the PCM (e.g. MP in <FIG>), for example within <NUM> of the MP, and preferably within <NUM> of the MP, so as to achieve efficient cooling of a heat source. If the PCM is too cold, the thermal conductivity is reduced, thus decreasing the PCM's ability to absorb thermal energy from a heat source. In one example, the PCM-laced coolant may be cooled by a heat exchanger <NUM> to below the optimal operational temperature for thermal absorption (e.g. well below the MP of the PCM). The PCM-laced coolant may then be brought back up to an optimal temperature by a TCU <NUM> so as to provide optimal thermal extraction from a heat source <NUM>.

<FIG> shows an example of an encapsulated PCM <NUM> suspended in a carrier fluid <NUM> within a coolant pipe <NUM>. In some examples, the coolant pipe <NUM> could be a cooling tank, or other storage container or conduit suitable for thermal energy transfer. In this example, the encapsulated PCM <NUM> is confined by at least one porous mesh or membrane <NUM> that allows the fluid <NUM> to pass through or along the coolant pipe <NUM>, but retains the particles <NUM> in a "PCM containment zone". In other words, the PCM particles are not able to pass through the membrane/mesh, but the fluid coolant <NUM> is. In <FIG>, the fluid <NUM> enters the PCM containment zone carrying thermal energy. The thermal energy is transferred to the encapsulated PCM particles <NUM> as the fluid <NUM> passes through, and the cooled coolant exits the PCM containment zone. The membrane or mesh <NUM> ensure that the PCM particles <NUM> are contained within a predetermined space within the coolant pipe <NUM>, and the fluid <NUM> may flow freely through. The PCM particles <NUM> absorb the thermal energy contained in the fluid <NUM> by changing state from solid to liquid, within the encapsulation shell. The thermal energy is then expelled from the heat exchanger to an exhaust.

<FIG> shows another example of a heat exchanger arrangement <NUM>, wherein at least one encapsulated PCM is incorporated into a porous mesh, membrane, matrix of cavities, or foam <NUM> material. In some examples, the porous mesh, membrane, matrix of cavities, or foam <NUM> is a polymer, such as polyester, polyether, or polyurethane, or a metal, such as aluminium, an alloy of aluminium, titanium, or steel. The coolant <NUM> is channelled via piping <NUM> through the PCM porous mesh, membrane, matrix of cavities, or foam <NUM>, transferring thermal energy to the PCM suspended in the material, which is then expelled from the heat exchanger to an exhaust. The cooled coolant <NUM> then exits the heat exchanger <NUM>.

In some examples, the thermal management system is coupled to a wider fuel system for another device, and the fluid coolant pumped around the system may be a fuel for the other device. In this example, it is important that any suspended PCM particles are constrained within a PCM containment zone, and not allowed to flow into the device engine or the wider system outside of the thermal management system, as this could cause damage to the wider system and/or device.

In another example, and as shown in <FIG>, the encapsulated PCM particles <NUM> may be combined or incorporated into a polynuclear PCM, or PCM "molecule" <NUM>. The resultant PCM molecule <NUM> is larger than a standard (single) particle, thus allowing for a coarser mesh or membrane <NUM> to be used in restraining the PCM within the carrier flow. In some examples, the polynuclear particles have a diameter in the <NUM> - <NUM> diameter range, allowing for a mesh with an example gap size of up to <NUM> - <NUM> in size (depending on the polynuclear particle size). The coarser mesh or membrane <NUM> provides less resistance to the coolant flow, and a smaller pump required in order to achieve the same rate of flow. In one example, the polynuclear PCM is formed through flocculation. In another example, the individual PCM particles may be bonded together by forming them into dies.

In some examples, the different types of heat exchanger as described above may be combined, e.g. comprising both a porous mesh <NUM> or membrane to constrain suspended encapsulated PCM particles <NUM>, and a porous mesh, membrane, matrix of cavities, or foam <NUM> comprising encapsulated PCM, through which the fluid coolant <NUM> may flow. In one example, the fluid coolant <NUM> may comprise suspended PCM particles small enough to pass through the membrane or mesh etc. within the heat exchanger, wherein larger PCM particles (e.g. a polynuclear PCM) incorporated within the heat exchanger are contained.

In another example, the encapsulated PCM incorporated either in the heat exchanger, or within the carrier fluid as suspended particles, may comprise a blend or range of different PCMs of varying melting points. In one example, a single encapsulated particle comprises a single PCM having a set MP. Different particles within the suspended particles or incorporated into the heat exchanger may have different MPs to other encapsulated PCMs. Alternatively, in another example, a single encapsulated particle PCM particle may comprise a mix of PCMs within a single particle. In either example, a blend or range of PCMs incorporated into the heat exchanger or suspended in the carrier fluid provides the opportunity to customise the thermal response of the PCM, i.e. the heat capacity profile. The different PCMs (having different MPs) may be incorporated into a polynuclear PCM molecule <NUM>, or individually within any of the examples outlined above, e.g. free flowing suspended particles, or incorporated into a mesh, membrane or matrix. The blend of PCMs provide a customised response to the thermal energy transfer and can be used to provide feedback about how much latent thermal capacity is left, e.g. providing warning levels.

<FIG> is a graph showing the relationship between the temperature of an example blended PCM material and the thermal energy supplied to it. As thermal energy is transferred to the blended PCM, the temperature increases until the blended PCM reaches its first melting/freezing temperature (MP1). At this point, the volume of the PCM with the lowest melting point experiences a change in state from a solid to a liquid, during which the temperature of the blended PCM does not increase and remains at the first melting point (MP1), between points LH1 and LH2. Once the first volume of PCM has fully changed state from a solid to a liquid, no more latent heat is stored, and sensible heat storage begins again, after LH2, i.e. the temperature of the PCM blend continues to increase as thermal energy is supplied to it. When the temperature of the blended PCM reaches the second melting point (MP2), latent heat storage begins again, at point LH3, when the second volume of PCM with the higher melting point than the first volume, begins to change from a solid to a liquid. During this time, the second temperature (MP2) is maintained until the second volume of PCM is fully changed from a solid to a liquid, at point LH4, whereupon sensible heat storage begins again. The temperature of the blended PCM increases again as thermal energy is supplied to it, until the upper melting point is reached (MP3), at which point (LH5) the remaining PCM begins to melt, during which the temperature (MP3) is maintained. Once all of the PCM is melted, at point LH6, sensible heating begins again.

<FIG> shows a flowchart of an example method <NUM> to be carried out in conjunction with the temperature control system <NUM> as described above. As discussed previously, if a coolant is too hot, or too cold, the thermal conductivity is reduced, thus decreasing the coolant's ability to absorb thermal energy from a heat source. It is easier to heat a coolant to a desired temperature than it is to cool it down to the desired temperature. Therefore, in one example, a coolant may be cooled by a heat exchanger to below the optimal operational temperature for thermal absorption (e.g. well below the MP of a PCM in a PCM-laced coolant). At step <NUM> of the example method, the fluid coolant is (pre)heated (from below a desired temperature) to a pre-determined temperature - i.e. the desired temperature for optimal thermal energy transfer from a heat source. At step <NUM>, the fluid coolant is pumped to a heat source, and at step <NUM> the fluid coolant extracts/transfers thermal energy from the heat source.

<FIG> shows a flowchart of another example method <NUM>, incorporating the steps of the method <NUM> shown in <FIG>. After extracting thermal energy from the heat source in step <NUM>, the method <NUM> further comprises pumping the fluid coolant from the heat source to a heat exchanger at step <NUM>. The thermal energy is transferred from the fluid coolant to a heat dump (i.e. an exhaust) at step <NUM>, and where possible, it is desirable to reduce the heat of the fluid coolant to below that of the desired level, i.e. the optimal temperature for cooling. The fluid coolant is then pumped from the heat exchanger to a temperature control unit at step <NUM>. Whether the fluid coolant is subsequently re-heated to a desired pre-determined temperature by the temperature control unit, or is pumped straight to the heat source, is determined in step <NUM>, as shown in <FIG>.

At step <NUM> of the method step <NUM>, the temperature monitoring device measures the temperature of the fluid coolant. The controller then determines whether the fluid coolant temperature is above or below a pre-determined threshold temperature at step <NUM>. If the fluid coolant is above a pre-determined threshold temperature, i.e. at the optimal temperature or above, then the fluid coolant is pumped onto the heat source. If the fluid coolant is below the optimal temperature then at step <NUM> the controller activates a heat source in the TCU to heat the fluid coolant up to the desired (i.e. optimal temperature). Considering that the fluid coolant comprises a PCM, as encapsulated particles suspended in a carrier fluid, then the desired temperature for the fluid coolant is just below the melting point of the PCM. If there is a blend of multiple PCMs in the fluid coolant, then the desired temperature is just below the lowest melting point of the blend of PCMs.

In one example, the threshold temperature below which the TCU activates the TCU heat source to heat the fluid coolant is a different value to the desired temperature of the fluid coolant. In another example, the fluid coolant, having green pumped from one heat source, to a heat exchanger and a TCU, may be pumped through a different heat source subsequently.

In one implementation of the invention, the heat source is a high-load electrical device that is only operational for short bursts, and requires down-time between operating cycles. In this example, the electrical device requires efficient cooling to counter the generation of a large amount of thermal energy in a very short amount of time. The thermal management system must keep the electrical device cool during its short operational burst, and the thermal energy can be expelled in the periods between the operational bursts of the electrical device.

Claim 1:
A thermal management system (<NUM>) for a heat source (<NUM>), the system comprising a closed loop:
a fluid coolant to extract thermal energy from the heat source,
a pump (<NUM>) to pump the fluid coolant around the system;
a heat exchanger (<NUM>) to extract thermal energy from the fluid coolant; and
a temperature control unit (TCU) (<NUM>) on the closed loop, the TCU located after the heat exchanger and before the heat source, wherein the TCU is adapted to heat the fluid coolant to a pre-determined temperature, characterized in that the fluid coolant comprises: a carrier fluid (<NUM>); and encapsulated phase change material (PCM) (<NUM>) particles suspended in the carrier fluid.