Patent Application
20180351068
The present invention relates to thermoelectric modules. In particular, the present invention provides thermoelectric modules having improved fault tolerance and output characteristics.
Thermoelectric modules may be used in devices for cooling, heating or power generation. Thermoelectric modules are all solid-state, with no moving parts, and so they are usually lighter, smaller and acoustically quieter than equivalent mechanical systems. Thermoelectric modules are often used in applications which require a high reliability and long lifetime. For example, thermoelectric generators comprising one or more thermoelectric modules can be used to provide a low maintenance power supply to a device in a remote or inaccessible location. As such, thermoelectric devices are also environmentally friendly and relatively inexpensive, e.g. by eliminating the need to frequently replace batteries.
Generally, standard thermoelectric modules have little tolerance to failure of a single component within the module. For example, since the thermoelectric elements in standard thermoelectric modules are connected in series, an open circuit failure of any one element or any of the electrical interconnects which connect a pair of elements, causes complete failure of the whole module. A more fault resistant module would lead to an improved reliability and lifetime.
In order to improve the fault tolerance of a thermoelectric module, it is known to provide one or more bypass circuits within the module to bypass one or more failed or faulty thermoelectric elements. For example, WO9522188 discloses a fault tolerant thermoelectric device circuit for a thermoelectric cooler. A primary or secondary electrical bypass circuit is activated using a sensor and a switch within the bypass circuit in response to a failure of one or more thermoelectric elements.
U.S. Pat. No. 3,632,451 discloses a thermoelectric device having parallel circuits interconnected at equal potential points, each parallel circuit comprising a number of thermoelectric couples (thermocouples) connected electrically in series. This provides tolerance to failure of any one component of the circuit.
However, known thermoelectric modules with improved fault tolerance typically provide significantly reduced output or performance characteristics, even before a fault has occurred, compared with standard thermoelectric modules. For example, known thermoelectric generators with improved fault tolerance often produce significantly lower output voltages, which can make it difficult to interface the device efficiently with the required electrical system, or lower output powers which are insufficient for their purpose.
In a first aspect of the present invention, there is provided a thermoelectric module comprising:
Thus, the at least one shunt electrical path of the present invention provides an alternative electrical path for current to follow in the event of a failure of a component (e.g. a thermoelectric element) within the portion of the main electrical path with which the shunt electrical path is in parallel. This improves the fault tolerance of the thermoelectric module.
Advantageously, by specifically selecting the resistance of the shunt electrical path based on the resistance of the main electrical path and the resistance of the portion of the main electrical path with which the given shunt electrical path is in parallel, the thermoelectric module of the present invention ensures that there is only a small decrease in the outputs of the module (e.g. output power) before a fault occurs, and only a small further drop in performance after one or more faults occur.
For example, if the resistance of the shunt electrical path were lower than the resistance of the portion of the main electrical path with which the given shunt electrical path is in parallel, then current would predominantly flow through the shunt electrical path rather than the main electrical path, affecting the performance of the module.
Additionally, if the resistance of the shunt electrical path were too high, then, in the event of a fault, the performance of the module could be significantly affected, as the current through the shunt electrical path would be very low. The inventors of the present invention have discovered that if the resistance of the shunt electrical path is less than the resistance of the main electrical path (i.e. less than the total resistance through the thermoelectric module), then the performance of the module is not significantly limited in the event of a failure. Thus, the present invention provides a fault tolerant thermoelectric module with improved performance characteristics.
In the event of a failure of a component within the portion of the main electrical path with which the at least one shunt electrical path is in parallel, the resistance of the portion of the main electrical path tends to infinity and so the current will automatically be diverted through the shunt electrical path which now has a lower resistance than the portion of the main electrical path.
The present invention may therefore be more cost effective and simple to manufacture compared to known fault tolerant thermoelectric modules, as it does not require any switches or intricate electrical circuits within the shunt electrical path. Moreover, since the shunt electrical path is a passive element requiring no threshold voltages, it can provide fault tolerance for a range of module designs under any operating conditions with a high reliability.
The at least one shunt electrical path may comprise an elongate electrically conductive element such as a wire. Optionally, the at least one shunt electrical path may comprise at least one resistor. In some embodiments, the shunt electrical path may comprise additional circuit components.
Optionally, the resistance of a given shunt electrical path, Rs, may satisfy the equation:
½(Rportion+(Rportion+Rmain)0.5)<Rs<5(Rportion+(Rportion+Rmain)0.5)
wherein Rmain is the resistance of the main electrical path through the module and Rportion is the resistance of the portion of the main electrical path with which the given shunt electrical path is in parallel.
Optionally, at least one shunt electrical path may be connected across one of the n-type and/or p-type thermoelectric elements, thereby protecting against failure of the thermoelectric element and/or its electrical connection to the main path.
In some embodiments, at least one shunt electrical path may be connected in parallel across one of the plurality of thermocouples. This may be more cost effective and easy to manufacture than providing a shunt electrical path across individual thermoelectric elements.
Optionally, a shunt electrical path may be connected in parallel across each of the plurality of thermocouples. This may be advantageous as if any one of the thermocouples in the thermoelectric module fail, then the corresponding shunt electrical path may operate to maintain performance of the module.
Optionally, at least one shunt electrical path may be connected in parallel to a portion of the main electrical path, the portion of the main electrical path comprising at least two thermocouples. Thus, a single shunt electrical path may be connected in parallel across multiple thermocouples. This may be particularly advantageous in parts of the main electrical path that are prone to failure. For example, these shunt electrical paths may be used to provide secondary shunt electrical paths in the event of a failure of a primary shunt electrical path connected across a single thermocouple.
The thermoelectric module may comprise at least one substrate. The plurality of thermocouples and the at least one shunt electrical path may be disposed on the at least one substrate. The use of at least one substrate may be advantageous as the substrate(s) may provide mechanical support to the thermoelectric module and/or may electrically insulate the thermocouples from any external surfaces.
Optionally, the at least one substrate may comprise, or consist essentially of, a ceramic or a polymeric material. The substrate(s) may comprise a thin film substrate. The at least one substrate may comprise, or consist essentially of, a polymeric material, e.g. a polyimide such as Kapton®. The substrate(s) may be flexible or rigid.
The at least one shunt electrical path may be disposed on or in the substrate(s) using any suitable means. In some embodiments, the at least one shunt electrical path may be at least partially printed onto the at least one substrate, or disposed thereon or therein via other physical or chemical deposition techniques. Alternatively, the at least one shunt electrical path may be soldered onto the at least one substrate.
Optionally, the thermoelectric module may comprise a bottom substrate and a top substrate. The n-type and p-type thermoelectric elements may be disposed between the bottom and top substrates. In some embodiments, the at least one shunt electrical path may be disposed on the bottom substrate and/or the top substrate.
The plurality of thermocouples, and the n-type and p-type thermoelectric elements of each thermocouple, may be electrically connected in series by a plurality of electrical interconnects. Optionally, the plurality of electrical interconnects may comprise a plurality of wires and/or electrical connections printed on, or otherwise disposed on or in, the at least one substrate.
In some embodiments, the electrical interconnects may be disposed on or in the bottom and/or top substrates.
Optionally, the at least one shunt electrical path may have a lower resistivity than the plurality of electrical interconnects. Additionally or alternatively, the at least one shunt electrical path may have a smaller cross-sectional area than the plurality of electrical interconnects.
In some embodiments, the n-type and p-type thermoelectric elements may be directly electrically connected in series (i.e. without the need for any electrical interconnects).
In a second aspect of the present invention there is provided a thermoelectric generator comprising at least one thermoelectric module according to any embodiment of the first aspect of the invention.
In a third aspect of the present invention, there is provided a thermoelectric cooler or a thermoelectric heater comprising at least one thermoelectric module according to any embodiment of the first aspect of the invention.
In a fourth aspect of the present invention there is provided a method of manufacturing a thermoelectric module, e.g. a thermoelectric module according to any embodiment of the first aspect of the invention, the method including the steps of:
The method may also include disposing the at least one shunt electrical path on or in a substrate, e.g. a bottom and/or a top substrate.
Optionally, the method may include the step of disposing the plurality of thermocouples between the bottom substrate and the top substrate.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
A side view of a standard thermoelectric module 100 as is well known in the prior art is shown in
The thermoelectric module 100 may be electrically connected to an external power source to provide electrical cooling or heating, or be thermally connected across a temperature difference to generate electrical power.
An electrical schematic of the thermoelectric modules in
If an additional electrical shunt path is provided, as in the present invention, the failure of one component still maintains the majority of the performance of the rest of the module by providing an alternative continuous electrical path for the current to follow.
An electrical schematic of a thermoelectric module 300 according to an embodiment of the present invention is shown in
In
In practice, failure may be most often seen in the thermoelectric elements 303, 304 or in the interfaces to the electrical interconnects in a thermoelectric module. If one of these components fails in
An additional shunt electrical path 405′ with resistance Rs′ is connected in parallel to two thermocouples, as opposed to a single thermocouple as in
Optionally, shunt electrical path 405′ may be connected in parallel to more than two thermocouples. In other embodiments, shunt electrical path 405′ may be connected in parallel to one or more shunt electrical paths 405, thereby forming a secondary shunt electrical path in the event of a failure of a primary shunt electrical path.
The introduction of the shunt electrical paths 305, 405, 405′ results in a drop in performance of the thermoelectric module. For example, if the thermoelectric module in
Therefore, assuming Rt=0 and using Ohm's law (V=IR) the electrical current flowing through the thermocouple, without a fault, is approximately given by:
Ic≈(Vn+Vp)/2(Rn+Rp)
While the electrical current through the shunt electrical path is approximately given by
Is≈(Vn+Vp)/2Rs
The power outputted to the load is proportional to the voltage across it (assumed constant) and the current through it, so the output power of the thermoelectric module, P, is proportionally given by:
P α Ic−Is
P α (Rs−(Rn+Rp))/Rs/(Rn+Rp)
Therefore, in the example where the shunt electrical path is in parallel to a single thermocouple, in order to maximise the output power of the module before a fault occurs, the term (Rs−(Rn+Rp)) must be maximised. This leads to the requirement that Rs>Rn+Rp, i.e. the shunt electrical path has a resistance greater than the resistance of the n-type and p-type thermoelectric elements, which form a thermocouple, in order for the introduction of the shunt electrical path to not significantly affect the performance of the module before failure occurs.
In some embodiments, the shunt electrical path resistance Rs may preferably be greater than twice the resistance of the thermocouple (Rn+Rp). In some embodiments, the shunt electrical path resistance may preferably be greater than 10 times the thermocouple resistance (Rn+Rp).
If a thermoelectric element or electrical interconnect fails for which a shunt electrical path is connected in parallel (i.e. any of the thermoelectric elements 303, 304 in
For example, if there are A thermocouples in the module, and the impact of the electrical interconnects (i.e. Rt) is again neglected the fraction of power output after failure of one thermocouple compared to the power output before failure is approximately given by:
A (Rn+Rp)/(A (Rn+Rp)+Rs−(Rn+Rp))
due to the resistance increase.
In order to limit this power loss the resistance of the shunt electrical path must satisfy the condition:
Rs<A (Rn+Rp).
Combined with the condition above, this leads to the equation:
Rn+Rp<Rs<A(Rn+Rp) (1)
Both these conditions can be satisfied in a module with multiple thermocouples (i.e. A>1).
As shown in
B(Rn+Rp)<Rs<A(Rn+Rp) (2)
To ensure high performance in a thermoelectric module used for heating or cooling a similar condition will apply. The conditions on the shunt electrical path can be generalised to include the shunt electrical path providing an alternative electrical path bypassing any numbers of thermocouples or thermoelectric elements, and including the case where the electrical interconnect resistance cannot be neglected (i.e. Rt is not small enough to be approximated to 0).
In general, the desired resistance of the shunt electrical path, Rs, can be related to the resistance of the main electrical path through the module, Rmain, and the resistance of the portion of the main electrical path with which the shunt electrical path is connected in parallel, Rportion. The desired resistance must fulfil the conditions:
R
portion
<Rs<R
main (3)
To ensure optimal performance of the whole thermoelectric module both before failure and after a single failure, both of these conditions should be fulfilled as strongly as possible. Hence, it may be most advantageous when Rmain/Rportion is large, for example when each shunt electrical path is connected in parallel to a single thermocouple in a module comprising a large number of thermocouples.
In preferred embodiments, to simultaneously achieve best performance of the module before and after failure, the range of Rs may be smaller, given by:
0.5(Rportion+(Rportion+Rmain)0.5)<Rs<5 (Rportion+(Rportion+Rmain)0.5) (4)
The inventors of the present invention have determined that equation 4 helps to minimise the loss of module performance both before and after failure of a component. In some embodiments it may be advantageous to have a larger loss of performance (e.g. output power) after a failure than before failure, as a failure is not guaranteed to occur.
The n-type 504 and p-type 503 thermoelectric elements are electrically connected in series to form a main electrical path through the module by a plurality of electrical interconnects 502 disposed on the bottom 501 and top 511 substrates. In this example, a thermocouple is defined as an n-type 504 and p-type 503 thermoelectric element connected by an electrical interconnect 502 disposed on the top substrate.
The electrical interconnects 502 may be formed of any electrically conductive material. Optionally, the electrical interconnects 502 may be printed on the bottom 501 and top 511 substrates.
In
The shunt electrical paths 505 may be disposed on or in the bottom substrate 501 at the same time as the electrical interconnects 502. Optionally, the shunt electrical paths 505 may be formed of the same material as, or a similar material to, the electrical interconnects 502. The shunt electrical path 505 may not be the same thickness as the electrical interconnects 502.
The shunt electrical path 505 may be formed by patterning a direct bonded metal on the bottom substrate 501, including but not limited to copper, silver or aluminium. Optionally, the shunt electrical path 505 may have a protective surface layer to avoid diffusion, degradation or oxidation, for example a nickel layer. The shunt electrical path may be disposed on the bottom substrate by a printing process, or other form of physical or chemical deposition.
In some embodiments, the shunt electrical path(s) may only provide an alternative (e.g. bypass) electrical path in areas of the module most prone to failure. For example, as shown in the example bottom substrate 801 of
In some embodiments, shunt electrical paths may be present in or on one or both substrates (e.g. the bottom and/or the top substrate). In other embodiments, the shunt electrical paths may not be disposed in or on a substrate. Optionally, shunt electrical paths may be present in or on the substrate that is least likely to have a failure of the electrical interconnects disposed thereon, for example the substrate that is nearer, in use, the ambient temperature.
If, as in the example in
In one embodiment of the invention, the shunt electrical path is made of an electrically conductive material with an electrical resistivity lower than the electrical resistivity of the electrical interconnects.
However, it may be advantageous to manufacture the shunt electrical path using an electrically conducting material with a resistivity similar to the electrical resistivity of the electrical interconnects. For example, this may simplify processing or allow fewer manufacturing steps. Therefore, the required difference in resistance between the shunt electrical path and the electrical interconnects can be produced due to a difference in the geometry of the components.
The shunt electrical path(s) may have a significantly smaller cross-sectional area perpendicular to the current flow compared to the electrical interconnects. For example,
Additionally or alternatively, the shunt electrical path(s) 1005 may not be connected across the shortest distance between the electrical interconnects 1002, as shown in
In some embodiments, the electrical resistivity of the shunt electrical path(s) may be different from (e.g. higher than) the electrical resistivity of the electrical interconnects and the cross sectional area of the shunt electrical path(s) may be lower than the cross sectional area of the electrical interconnects.
The above embodiments are described by way of example only. Many variations are possible without departing from the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1520950.5 | Nov 2015 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2016/053699 | 11/24/2016 | WO | 00 |
