WO2014114950A2

WO2014114950A2 - Thermoelectric generators

Info

Publication number: WO2014114950A2
Application number: PCT/GB2014/050199
Authority: WO
Inventors: Kevin Simpson; Andrew Knox; Andrea MONTECUCCO
Original assignee: European Thermodynamics Limited
Priority date: 2013-01-25
Filing date: 2014-01-27
Publication date: 2014-07-31
Also published as: GB2515446A; WO2014114950A3; GB201301335D0

Abstract

A thermoelectric generator (TEG) according to the present invention is described in which the TEG comprises at least two thermoelectric stages (TEG 'A' and TEG 'B') formed of different materials. The thermoelectric stages are thermally in series but electrically independent.

Description

Thermoelectric Generators

The invention relates to thermoelectric generators.

Thermoelectric generators (TEGs) produce a current flow in an external circuit by the imposition of a temperature difference (ΔΤ). The magnitude of the ΔΤ determines the magnitude of the voltage difference (AV) and the direction of heat flow determines the voltage polarity. The TEG can be electrically modelled as a voltage source in series with an internal resistance and the values for both vary with temperature. Figure 1 illustrates a basic prior art use of a thermoelectric generator 10. There is a voltage source 11, an internal resistance 12, and an external resistance 13.

The thermal resistance (or conductance, being the reciprocal of the resistance) of the TEG depends to some extent on the magnitude of the current flowing in the external circuit. This change in thermal conductance is due to the Peltier effect, which acts to pump heat from one side of the device to the other according to the current flowing through the device. The direction of heat pumping is determined by the polarity of the current flowing in the circuit as a result of the potential produced by voltage source 11. In a thermoelectric generator, the Peltier effect is considered to be parasitic and unwanted. Low electrical current will lead to a reduced thermal conductance (high thermal resistance; low heat pumping), and high electrical current will lead to an increased thermal conductance (low thermal resistance; high heat pumping). If the TEG is electrically short circuited, the TEG will have the highest possible thermal conductance. This condition is normally avoided accept when determining the Maximum Power Point, discussed below, because it leads to a very inefficient thermal circuit with a large amount of heat energy being transferred from the "hot" to the "cold" side with no benefit in electrical power generation. This is illustrated in Figure 2. There is a hot side 14, a cold side 15, a thermal resistance 16, and the flow of thermal energy Q is dependant to some extent upon the current flowing in the device and the device's intrinsic thermal conductance at open-circuit, i.e. no external electrical load connected

For a given thermal operating point the electrical power delivered by the TEG varies according to the current required by the electrical load. To maximise the power (or electrical energy) produced by the TEG, the electrical load impedance should equal the electrical source impedance (this is known as the "Maximum Power Transfer Theorem"). The source impedance corresponds to the internal electrical resistance of the TEG and the load impedance is the electrical resistance of the external circuit. The source electrical impedance varies with temperature and according to the type and construction of the individual TEG. The "Maximum Power Point" (the point at which the TEG delivers the maximum possible power to the external load for a given temperature) is given by one-half of the open circuit (load impedance =∞Ω) voltage, V_oc/2 , or by one-half of the short circuit (load impedance = 0Ω) current, I_sc/2. Such Maximum Power Point Trackers (MPPTs) work by periodically measuring the open circuit voltage (or short circuit current, although this is not preferred because of the decreased system thermal efficiency) and using the value obtained to adjust the load impedance such that the voltage seen at the TEG's terminals corresponds to one half of this value. In other words the MPPT adjusts RL to equal Rs. This is illustrated in Figure 3, the internal resistance 12 being the source resistance or Rs, and the external resistance 13 being the load resistance or RL. The open circuit voltage or V_oc is the electrical potential across the voltage source 11.

The normal operating mode for an electrical generator using a TEG is to ensure that RL > Rs, so that thermal conductance does not decrease the thermal to electrical conversion efficiency of the overall system.

The magnitude of the TEG voltage is determined by the Seebeck coefficient, expressed typically as a number of microvolts per Kelvin. For a material such as Bismuth Telluride, a value of 350μν/Κ is in the normal range. The Seebeck coefficient varies with absolute temperature. Referring to Figure 4, a plurality of "P" and "N" type semi-conductors 17 each have a Seebeck coefficient. If the P- and N- type materials are arranged electrically in series, and thermally in parallel, as shown in Figure 4, then the voltage from each element can be added such that a device comprising many such elements produces a usable voltage. Each P and N pair is referred to as a "couple", because they form a thermocouple. A typical known device is comprised of a large number of couples, e.g. 63, 127, 254, etc. The magnitude of the voltage also depends on the materials in use, and different materials may be optimised for different temperature regions. One material may be particularly preferred in a low temperature zone, with a maximum temperature (T_max of between 250°C - 300°C). Other materials may be preferred for high temperature use. Higher temperatures might range from 500°C - 1000°C. Higher temperature materials generally have a lower thermal to electrical conversion efficiency when operated in the temperature region associated with lower temperature materials.

It is known to stack different materials so that they are thermally in series, for example as shown in Figure 5. In the situation illustrated in Figure 5 there is a low temperature material layer 18 and a high temperature material layer 19. A thermocouple 20 is located between the zones to measure the mid-temperature. The important features of such a known device are: (a) The thermoelectric materials are thermally and electrically in series.

(b) The device allows temperature sharing between layers of materials.

(c) The combination of materials is optimised for a particular maximum temperature.

(d) "Standard" MPPT techniques can be used. A problem with such devices is that they are hard to design because of the need to match heat flux and temperature gradient through different layers. In a typical example of an application area for such a device, for example power generation from a vehicle's internal combustion engine exhaust gases, there are specific issues in implementing such a system, including:

(a) A desire to produce usable amounts of power very soon after the vehicle engine is started, whilst the exhaust is still warming up.

(b) A desire to maximise power production over a wide range of driving conditions, in other words a desire to cope with variable exhaust pipe temperatures.

(c) A desire to be able to offer some form of thermal protection to the thermoelectric elements in the event of an over temperature condition, for example caused by excessively lean fuel burn, or unusually high engine load for an extended period of time. The invention seeks to provide a thermoelectric generator which is more efficient than known thermoelectric generators and more amenable to meeting the above mentioned design challenges. The invention provides a thermoelectric generator (TEG) comprising at least two thermoelectric stages formed of different thermoelectric materials, the stages being thermally in series, but electrically independent. This makes it possible to control the mid temperature between the different materials, i.e. the layer separating the different material stages, and hence the temperature gradient across each stage, such as to improve the thermal to electrical conversion efficiency of one or each stage, under different operating conditions, thus maximising power output over a wider temperature range. It is also possible by this technique to provide a degree of thermal protection from overheating. There may be means to vary the mid temperature using Maximum Power Point Tracker (MPPT) technology.

Each stage may be provided with its own MPPT converter. When there are more than two stages, mid temperature between any pair of adjacent stages may be controlled.

One thermoelectric stage may utilise a material particularly suitable for lower temperatures e.g. below 300 degrees C.

Such a preferred material is bismuth telluride.

Another thermoelectric stage may utilise a material particularly suitable for intermediate temperatures e.g. from 300 to 600 degrees C.

Such a preferred material is lead telluride.

Yet another thermoelectric stage may comprise a material particularly suitable for high temperatures e.g. from 600 to 1000 degrees C.

Such a preferred material is silicon-germanium. There may be a controller arranged to receive information about temperatures, including at least one mid temperature, and control at least one MPPT converter using that information. The converter may also use voltage and current information relating to the operating conditions of the thermoelectric materials on one or both sides of the intermediate layer.

In a preferred embodiment a plurality of TEGs each as defined above may be positioned along a heat path, e.g. an engine exhaust, to combine together to convert heat from the path into electric power.

In such an embodiment each generator may experience a different overall temperature gradient, as temperature may change, e.g. decrease, along the path, but the controller may be used to adjust the MPPT converters as desired for each generator, independently.

The controller may be arranged to adjust the MPPT converters such that the optimum power is extracted from at least one TEG.

The arrangement may be such that the thermal energy flow through each TEG is regulated to some extent.

The regulation may be such that the profile of the temperature gradient along an exhaust gas heat exchanger is controlled to some extent. This may make it possible, for example, to reduce the formation of condensates on part e.g. fins, of the heat exchanger.

By way of example, specific embodiments of the invention will now be described, with reference to the accompanying drawings, in which: Figures 1 - 5 illustrate the background and the prior art, as already described;

Figure 6 illustrates a basic embodiment of the invention;

Figure 7 illustrates a practical embodiment of the invention. Figure 6 shows a source of heat 21 (for example exhaust gases) and a source of cold 22 (for example cooling water). Between these lie a first fixed thermal resistance Rhot, and a second fixed thermal resistance R_coid. Between these resistances there is a first thermoelectric generator TEG A and a second thermoelectric generator TEG B. TEG A is associated with a first MPPT converter A and TEG B is associated with a second MPPT converter B. There is a thermocouple 23 to measure the mid temperature between TEG A and TEG B. The device operates as follows:

(a) During initial warm up, converter A short-circuits the high temperature TEG A (which typically has lower efficiency than the low temperature TEG B) so that the minimum ΔΤ is present across TEG A. This places the maximum ΔΤ across TEG B, thereby maximising the available power from the complete system.

(b) During variable exhaust temperature, where the temperature available is sufficient to provide the maximum allowable ΔΤ across TEG B, converter B runs in MPPT mode and converter A controls ΔΤ across TEG A to keep TEG B at T_max by reducing the voltage across TEG A to less than its MPPT voltage.

(c) The critical temperature is defined as the temperature at which both TEG A and TEG B are operating at AT_max and both converters are running in MPPT mode. (d) Above ATmax TEG A and/or TEG B is loaded by converters A and/or B such that electrical resistances RL < Rs. This increases the thermal conductivity of the TEG cascade and causes a larger temperature difference across the hot fixed thermal resistance Rhot. The choice of which TEG to load in this way will be determined by the mid temperature not exceeding T_max for TEG B and by the relative efficiencies of TEGs A and B for the operating temperature. Increased loading is variable and progressive, to the point where the overall TEG cascade is protected from an over- temperature condition by short-circuiting both TEGs A and B using the power converters. In this extreme case, the highest ΔΤ will be across Rhot. In this embodiment TEG A utilises lead telluride and TEG B utilises bismuth telluride.

The invention is not limited to the features of the device illustrated in Figure 6. For example there could be additional TEGs, each with their own converter. All TEGs can be optimised for operation in a particular temperature range. Similarly, different thermal resistances for each thermoelectric layer may be used to allow for different ranges of temperatures on the TEGs and the mid temperatures associated therewith.

For the arrangement shown in Figure 6, there are nine easily defined operating points (although in practise there is a continuum of ranges) and these are defined in Table 1 below:

Table 1

Defined operating points for a two TEG cascade

The above table assumes that at open circuit, the thermal resistance of a TEG = 2x Rhot and at short circuit, the thermal resistance of a TEG = ½ x Rfjot. It is also assumed that Rfjot = Rcoid and RTEG @ MPP = R_Hot.

Figure 7 illustrates a practical example related to the exhaust and cooling systems of a vehicle. Heat from the exhaust pipe 24 is utilised to generate electricity which is connected to the + 12v supply 29 in the vehicle. For example it may be simply connected across the vehicle's battery. Whatever electrical loads are present in the vehicle (air conditioning, power steering, headlights, fuel pump, etc.) will draw power from the + 12v supply. If the total load is, say, 500W, and the TEGs are supplying 200W, then the vehicle's alternator would only have to provide 300W. The torque required to drive an alternator is therefore lower than would be required for 500W, and therefore the engine does not have to work so hard. This in turns means that less fuel is burnt and there is therefore a reduction in fuel consumption.

Exhaust gases in the exhaust pipe 24 pass through an exhaust gas heat exchanger 25 which is coupled via one or more thermoelectric module(s) 30 to a fluid heat exchanger 26 through which flows the coolant. This therefore brings about a temperature gradient across a plurality of thermoelectric modules, marked 30a, 30b, 30c and 30d in Figure 7, each module being similar to that shown in Figure 6. Connected between the thermocouples of the modules, and the MPPT converters 27 associated respectively with the modules, is a controller 28. Hot exhaust gas enters the heat exchanger 25 on the left hand side. The section of the heat exchanger for TEG 30a collects a portion of the thermal energy in the gas and this flows through the different thermoelectric layers and materials forming TEG 30a to the fluid heat exchanger on the cold side. The high and low temperature materials each experience a different temperature gradient and they are each connected to their own MPPT converter, these converters having their outputs connected to the + 12v and Ov supply rails 29 in the vehicle. The percentage of the total energy flow into TEG 30a which is not converted into electricity is removed by the fluid heat exchanger. TEG 30b also collects a portion of the thermal energy from the exhaust gas, but the gas is at a different temperature to that which TEG 30a was exposed to, and therefore TEG 30b, although of an essentially identical construction to TEG 30a, experiences a different overall temperature gradient and therefore a different operating point. Hence MPPT's 3 and 4 will have different input voltages and therefore operate at different points to MPPT's 1 and 2, but would produce the same output voltage for use on the vehicle +12v supply. Likewise for TEGs 30c and 30d, each having successively lower input temperatures, and different operating points, for the same output voltages. The embodiment shown in Figure 7 utilises a counter flow heat exchanger topology.

The controller 28 measures the temperatures of the hot and cold sides of each TEG and of the mid temperature, and uses these temperature values to adjust the MPPT converters such that the optimum power can be extracted from the TEG. This is illustrated at the right hand side of Figure 7 in Figure 7 A for one of the TEGs, by way of example. It should be noted that the energy flow through each TEG can be regulated to some extent and thus the profile of the temperature gradient along the exhaust gas heat exchanger can also be controlled to some extent.

The exhaust pipe 24 will generally be cylindrical although this is not essential. Fins of the heat exchanger may be arranged to produce a variety of cross sections, for example triangular, rectangular, square, hexagonal or octagonal, or less geometrically regular shapes. A requirement is that the external surface(s) of the exchanger presents a set of flat faces on which to mount the TEGs. In alternative constructions, one or both faces of the thermoelectric materials may be directly attached to heat exchanger components without the need for the flat faces. The invention is not restricted to the features of the arrangement shown in Figure 7. For example a TEG array could simply be connected to the + 12v supply via a diode so that when voltage exceeds 12v, energy is dumped on to the + 12v line in the vehicle. Alternatively, one or more converters may be used which is not an MPPT converter, although in this case the energy produced by a given TEG array might be less than would be possible utilising MPPT converter technology. Other vehicles voltages, e.g. +6v or +24v may be preferred for the particular embodiment.

Claims

Claims:

1. A thermoelectric generator (TEG) comprising at least two thermoelectric stages formed of different thermoelectric materials, the stages being thermally in series, but electrically independent.

2. The thermoelectric generator of claim 1, comprising means to vary at least one mid temperature.

3. The thermoelectric generator of claim 2, wherein the means to vary the mid temperature is arranged to adjust the current flow through at least one of the thermoelectric stages.

4. The thermoelectric generator of claim 3, wherein the thermoelectric generator comprises a power converter arranged to adjust the current flow.

5. The thermoelectric generator of claim 4, wherein the power converter is a Maximum Power Point Tracker (MPPT) power converter arranged to use Maximum Power Point Tracker (MPPT) technology.

6. The thermoelectric generator of claim 4 or claim 5, wherein each stage is provided with its own power converter.

7. The thermoelectric generator of any preceding claim, wherein, when there are more than two stages, mid temperature between any pair of adjacent stages is controllable.

8. The thermoelectric generator of any preceding claim, wherein one thermoelectric stage may utilise a material particularly suitable for lower temperatures e.g. below 300 degrees C.

9. The thermoelectric generator of claim 8, wherein the material is bismuth telluride.

10. The thermoelectric generator of claim 8 or claim 9, wherein another thermoelectric stage may utilise a material particularly suitable for intermediate temperatures e.g. from 300 to 600 degrees C.

11. The thermoelectric generator of claim 10, wherein the material is lead telluride.

12. The thermoelectric generator of claim 10 or claim 11, wherein yet another thermoelectric stage may comprise a material particularly suitable for high temperatures e.g. from 600 to 1000 degrees C.

13. The thermoelectric generator of claim 12, wherein the material is silicon- germanium.

14. The thermoelectric generator of any preceding claim, wherein there is a controller arranged to receive information about temperatures, including at least one mid temperature, and control at least one power converter using that information.

15. The thermoelectric generator of claim 14, wherein the converter further uses voltage and current information relating to the operating conditions of the thermoelectric materials on one or both sides of an intermediate layer between thermoelectric layers.

16. The thermoelectric generator of claim 15, wherein the controller is arranged to adjust the at least one power converter such that the optimum power is extracted from at least one TEG.

17. The thermoelectric generator of any preceding claim, wherein a plurality of the TEGs are positioned along a heat path, e.g. an engine exhaust, to combine together to convert heat from the path into electric power.

18. The thermoelectric generator of any preceding claim, wherein each generator is configured to experience a different overall temperature gradient, as temperature changes.

19. The thermoelectric generator of any preceding claim, wherein the arrangement is such that the thermal gradient across each TEG is regulated.

20. The thermoelectric generator of claim 19, wherein the regulation may be such that the profile of the temperature gradient along an exhaust gas heat exchanger is controlled.

21. The thermoelectric generator of claim 2, wherein the means to vary the mid temperature is a diode.