BACKGROUND OF THE INVENTION Thermoelectric devices have been known for many years. Such devices may be used for heating, cooling, temperature stabilization, power generation and temperature sensing. Modern thermoelectric coolers typically include an array of thermocouples which operate by using the Peltier effect.

A thermoelectric generator module is a semiconductor-based electronic component that functions as a small electric energy source, in which heat moves through the module from one side to the other. One face of the module, therefore, is relatively hot, while the opposite face is relatively cold, such that there exists a temperature differential which gives rise to a low DC power voltage across the faces.

This voltage is caused by the known Seebeck effect.

As known in the art, the voltage generated by the Seebeck effect may be represented by the expression V = S x (Th-Tc) in which V is the output voltage in volts, Th and Tc are the temperatures of the hot and cold faces of the thermoelectric module in degrees Kelvin, and S is the Seebeck coefficient in Volts/Kelvin Commonly used semiconductor materials are alloys of Bismuth Telluride (Bi2Te3) or Lead Telluride (PbTe) that have been suitably doped to provide individual distinct n and p characteristics. Thermoelectric materials most often are fabricated by either directional crystallization from a melt or pressed powder metallurgy.

Each of these materials may be characterized by the figure of merit"Z"which represents the efficiency of thermal conversion to electricity. By way of example, Fig. A illustrates the Z of Bi2Te. and PbTe over a range of temperatures. It can be seen

from this graph that the performance of (Bi2Te3) peaks within 50-250 °C, while the performance of PbTe peaks within 250-550 °C.

Z may be defined as S26/K, in which S is the Seebeck coefficient, 6 is the electrical conductivity, and K is the thermal conductivity, In accordance with the prior art, thermoelectric semiconductor materials are produced by crystallization from melt, and typically are fabricated in ingot form, thereafter being sliced into wafers of various thicknesses. After preparation of the surfaces of a wafer, it is then cut or diced into blocks that may be assembled into thermoelectric modules.

A typical thermoelectric module has two or more legs of semiconductor materials that are connected electrically in series and thermally in parallel, each pair including one n-leg and one p-leg. Each such module, together with electrical connector elements therebetween are typically mounted between two ceramic substrates, which, while having heat conduction properties, serve to electrically insulate the individual modules both from each other, and from external mounting surfaces.

When considering a thermoelectric module for generation of electrical power, it will be appreciated that the Seebeck effect, whereby voltage is produced by application of a thermal gradient to the module, is in direct proportion to the thermal conductivity of the module. Known thermoelectric modules, normally achieve an efficiency of no more than about 5-6%.

SUMMARY OF THE INVENTION The present invention seeks to provide a novel thermoelectric module, for use particularly in a thermoelectric generator, and which has an efficiency which considerably exceeds that attained by the known art.

There is thus provided, in accordance with a preferred embodiment of the invention, a thermoelectric module for use in a thermoelectric generator having a heat source and a heat sink, wherein the module includes a plurality of thermocouples each having an n-leg and a p-leg, wherein each leg has a hot end and a cold end; a plurality of electrical connectors arranged in electrically conductive contact with the hot ends of the n-legs and the p-legs, so as to connect the n-legs and the p-legs of the plurality of thermocouples in electrical series, and further, arranged in thermally conductive contact with the heat source, for permitting an input of thermal energy from the heat source into the plurality of thermocouples; a plurality of thermally and electrically conductive footings arranged in thermally and electrically conductive contact with the cold ends of the n-legs and the p-legs, and further, for connecting the n-legs and the p-legs of the plurality of thermocouples in parallel thermally, and in series electrically; and a thermally conductive electrical isolator element arranged in thermally conductive contact with the footings and with the heat sink, for permitting an output of thermal energy from the plurality of thermocouples to the heat sink.

Each thermocouple is operative to cause a thermal gradient between the hot ends and the cold ends, thereby to generate an electrical charge therein, each n-leg is formed of one or more semiconductor crystals doped so as to have n characteristics, and each p-leg is formed of one or more semiconductor crystals doped so as to have p characteristics.

Additionally in accordance with a preferred embodiment of the present invention, each of the n-and p-legs has at least first and second types of semiconductor crystals; the first and second types of the n-type semiconductor crystals are coupled in series between a selected one of the electrical connectors and a first of the footings; and each of the first and second types of the p-type semiconductor crystals is coupled in series between the selected electrical connector and a second of the footings.

Further in accordance with a preferred embodiment of the present invention, the first types of n-type and p-type semiconductor crystals have a first predetermined range of working temperatures at which an electrical charge is generated therein, and the second types of n-type and p-type semiconductor crystals have a second predetermined range of working temperatures at which an electrical charge is generated therein.

More particularly, the first predetermined range of working temperatures extends between a first maximum temperature and a first minimum temperature, and the second predetermined range of working temperatures extends between a second maximum temperature and a second minimum temperature, and the following apply: the first maximum temperature is equal to a maximum temperature sustainable at the hot end of the thermocouple legs, the second minimum temperature is equal to a minimum temperature sustainable at the cold end of the thermocouple legs, the first minimum temperature and the second maximum temperature are approximately equal, and the difference between the first maximum temperature and the second minimum temperature equals a maximum thermal gradient sustainable between the hot and cold ends of the legs.

Additionally in accordance with a preferred embodiment of the present <BR> <BR> <BR> <BR> invention, in each of the n-and p-legs, the first and second types of semiconductor crystals have metallized surfaces which are electrically and thermally coupled by a soldered connection.

Further in accordance with a preferred embodiment of the present invention, the hot ends of the n-and p-legs are arranged in non-joining, touching contact with one of the electrical connectors across an interface, thereby to permit lateral thermal expansion of the hot ends relative to the connector, while maintaining contact therewith.

Additionally in accordance with a preferred embodiment of the present invention, each footing is a resilient, compressible footing, which is operative, in response to a compression force applied across the module, between the heat source and the heat sink, to provide additional compression together of the legs with the

electrical connectors, thereby to improve thermal and electrical conduction across the interface.

Further in accordance with a preferred embodiment of the present invention, the compressible footing is associated with a predetermined one of the cold ends of the n-legs and the p-legs, and includes: a flexible housing formed of an electrically and thermally conductive material, having a roof portion joined to the predetermined cold end, a base portion joined to the isolator element, and a flexible side wall between the roof portion and the base portion; and a resilient compression element arranged within the flexible housing between the base portion and the roof portion, and operative to urge apart the base portion and the roof portion, such that, in the presence of a compression force applied across the module between the heat source and the heat sink, the compression element is operative to urge the hot end of the predetermined leg into increased thermal and electrical conductive contact with the electrical connector associated therewith.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully understood and appreciated from the following detailed description, taken in conjunction with the drawings, in which: Fig. 1 is a graph showing comparative figure of merit Z for semiconductor materials based on Bi2Te3 and PbTe, over a range of temperatures; Fig. 2 is a diagrammatic representation of a thermoelectric generator, employing a thermoelectric module constructed in accordance with the present invention; Fig. 3 is a pictorial illustration of a thermoelectric module useful in the thermoelectric generator of Fig. 2, incorporating an array of interconnected thermocouples, constructed in accordance with a preferred embodiment of the present invention ; Fig. 4 is an enlarged more detailed pictorial illustration of an exemplary thermocouple employed in the module of Fig. 3, indicated at 3 therein; and Fig. 5 is a partially cut away side view of the p-leg of the thermocouple illustrated in Fig. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Fig. 2, there is seen a thermoelectric generator, referenced generally 10, constructed in accordance with a preferred embodiment of the present invention. Thermoelectric generator 10 includes a thermoelectric module 12 which has a"hot"face 14 and a"cold"face 16, and which is enclosed between a heat source 18, having a heat transfer plate 19, and a heat sink 20. The arrangement of these elements is such that the hot face 14 is in heat exchanging, touching contact with heat source 18, which may be any suitable heat source, and which may be a waste or by-product heat source, such as found in vehicles, industrial plant, and the like, or it may provided by a principal heat source, such as solar energy or the like. Furthermore, cold face 16 of thermoelectric module 12 is arranged so as to be in heat exchanging, touching contact with heat sink 20, which may be any suitable heat dissipation or heat exchange apparats.

It will of course be appreciated, however, that an aim of the present invention is to provide a highly efficient system wherein very little excess heat is conducted to the cold face 16.

As will be understood from the following description, the thermoelectric module 12 of the present invention is constructed, inter alia, so that the thermal conductivity across the hot interface, defined by hot face 14 and heat source 18, and the cold interface, defined by cold face 16 and heat sink 20, is maximized. Some of the connections within the thermoelectric module 12 can be thermal joints, and are preferably formed by soldering, typically with an alloy of Pb, Sn and Ge. These connections may alternatively be provided by use of an appropriate electrically and thermally conductive adhesive.

As will be appreciated from the description below, however, due to differential thermal expansion, at least one of the connections must be provided by application of a compressive force to the thermoelectric module 12. Accordingly, as seen schematically in the drawing, the heat transfer plate 19 of heat source 18, and the heat sink 20 are attached direct, as via tie members 22, so as to apply a predetermined pressure to the thermoelectric module 12, thereby to achieve a desired rate of thermal conductivity within the thermoelectric module 12.

Referring now briefly to Fig. 3, thermoelectric module 12 is formed of an array of thermocouples 24. Each thermocouple 24 has an n-leg and a p-leg, respectively referenced 26n and 26p. The legs 26n and 26p are connected in series electrically, via connectors 28 on the hot side of thermoelectric module 12, and in parallel thermally, via compressible footings 30. Footings 30 are connected to an isolator plate 31, which defines the cold face 16 (Fig. 2) of thermoelectric module 12. Electrical power is generated by application of a thermal gradient across the thermoelectric module 12, and the well-known Seebeck effect, and is thus not described herein in detail.

Connectors 28 are preferably made of a material that has very good thermal and electrical conduction properties, and which also has a melting temperature higher than the working temperature in its vicinity. Accordingly, while copper would otherwise be a suitable material, as the temperature at the hot face 14, and thus also at connectors 28, may reach as much as 550-600 °C, nickel is preferred.

Referring now to Fig. 4, an exemplary thermocouple 24, such as that indicated in the region denoted"3"in Fig. 3, constructed in accordance with a preferred embodiment of the present invention. The thermocouple has n-and p-legs 26n and 26p, the p-leg 26p, together with a portion of connector 28 and footing 30", also being shown in an elevational view in Fig. 5, to which reference is also made herein.

Apart from the fact that the legs are formed either as n-or p-legs, they are constructed in an otherwise identical manner. Accordingly, reference is now made to a leg 26 and its surrounding structure, it being appreciated that description of a leg, without specifying the leg type, refers to both of legs 26n and 26p. This is also apparent from the drawings, wherein components which are common to both legs are denoted by the numerical reference indication mentioned in the description herein, but with the addition of an n or p suffix, as appropriate.

Each leg 26 is formed from at least one semiconductor crystal, suitably doped so as to have the necessary n-type or p-type characteristics, which are coupled in series. In the illustrated example, there is shown a pair of"cascaded"semiconductor crystals, identified herein as first crystal 32 and second crystal 34. As seen particularly in Fig. 4, the first and second cascaded crystals 32n and 34n of the n-leg are coupled to an electrical connector 28, and a first footing 30' ; and the first and second cascaded crystals 32p and 34p of the p-leg are coupled to the same electrical connector 28, and to a second footing 30".

The first crystals 32 are located at the hot end of each leg, and thus work at relatively elevated temperatures, as described below. The second crystals 34, however, are located at the cold end of each leg, and thus work at relatively low temperatures, also as described below.

The first and second crystals 32 and 34 have metallized, opposing contact surfaces, respectively referenced 36 and 38, typically of nickel, which are soldered together, typically with an alloy of Pb, Sn and Ge, thereby to provide an interface 40 with good electrical and thermal conductivity.

First crystals 32 have a metallized"hot"surface 42, also formed typically of nickel, via which it is thermally and electrically coupled to connector 28. The metallized surface 42 is required not for purposes of connection, but in order to

prevent diffusion of the material of the first crystals 32 to the connector 28, which would cause a degradation in their properties.

During operation, however, as will be better understood from the description below, the temperature at hot surface 42 can reach up to 500-600 °C. At these temperatures, there occurs differential lateral expansion between the semiconductor crystal 32 and the nickel connector 28. Accordingly, while it is necessary to ensure optimal thermal and electrical coupling between first crystals 32 and connector 28, this is done by means other by than forming a direct joint therebetween.

The required connection between first crystals 32 and connector 28 is provided by applying a compressive force across thermoelectric module 12, as described above in conjunction with Fig. 2. In order to ensure at least a minimal force, predetermined to ensure required electrical and thermal conductivities across the interface between first crystals 32 and connector 28, footings 30 are provided as resilient, compressible footings.

In more detail, each of footings 30, associated with the cold ends of n-leg 26n and p-leg 26p, is formed of a flexible housing 44 and a resilient compression element 46, such as a suitable spring. Due to the poor electrical and thermal conductivity of the spring construction, it is necessary for housing 44 to be formed of an electrically and thermally conductive material, such as copper. Housing 44 has a roof portion 48 which is joined as by soldering, typically with an alloy of Pb, Sn and Ge, to a preferably nickel, metallized cold end 50 of second crystal 34. The roof portion 48 is connected via thin flexible side walls 52 to a base portion 54, preferably also formed of a conductive material such as copper, which, in turn, is joined as by soldering, typically with an alloy of Pb, Sn and Ge, to the isolator plate 31.

The resilient compression element 46 is seated within base portion 54 so as to be disposed also in contact with roof portion 48, and is operative to urge apart the base portion 54 and roof portion 48. Accordingly, in the presence of a compression force applied across thermoelectric module 12 between the heat source 18 and the heat sink 20, the resilient element 46 urges the hot end of the leg associated therewith into increased thermal and electrical conductive contact with its associated electrical connector 28. Preferably, a pressure of 10 kg/cm2 is applied across the interface between legs 26 and connector 28.

As seen in the drawings, there are provided clips, referenced 56, which are affixe, as by soldering, typically with an alloy of Pb, Sn and Ge, to the connector 28, and which are configure so as to generally maintain alignment of the legs 26 with connector 28 while, at the same time, not preventing the differential thermal expansion described above.

It will be appreciated by persons skilled in the art that the use of crystals as a thermoelectric medium is particularly advantageous, as they are both electrically and thermally anisotropic.

Furthermore, the use of two or more types of crystals in a cascaded series, as in accordance with a preferred embodiment of the present invention, enables the use of semiconductor crystals which are thermoelectrically operative in different, but complimentary, temperature ranges.

It has been found by the Inventor that particularly suitable materials, for the first and second crystals of the n-leg, 32n and 34n, and for the first and second crystals ofthep-leg, 32p and 34p, are as follows: First crvstals 32n : PbTe doped with In, having an operating range of 250-550 °C First crystals 32p: Pbl,-ySn, GeyTe, doped with Na, having an operating range of 250-550 °C Second crystals 34n: Bi2Te, Se3-,, doped with a compound of Bi, Se, and Cl, having an operating range of 0-250 °C Second crystals 34p: Bi2 xSbxTe3 ySey doped with Pb, having an operating range of 0- 250 °C Notwithstanding the above example, it will be appreciated that there also exist other combinations of suitable semiconductor materials, and that the above combination is thus for demonstrative purposes only, and is not meant to exclude other suitable materials.

A particular advantage of employing two types of semiconductor crystals which have different working ranges, in a cascaded series, is that this arrangement increases the working range of the thermoelectric generator, when compared with that employing a single type of crystal only, and thus markedly increases its power rating, accordingly.

The above examples illustrate the principle of using first types of n-type and p-type semiconductor crystals which have a first predetermined range of working temperatures at which an electrical charge is generated therein; and second types of n-type and p-type semiconductor crystals which have a second predetermined range of working temperatures at which an electrical charge is generated therein.

It will also be appreciated that the most efficient cascaded system is one in which: the maximum operating temperature (in the present example, 550 °C) of the first crystals 32n and 32p is equal to or greater than a maximum temperature sustainable at the hot end of the thermocouple legs 26n and 26p; the minimum operating temperature (in the present example, 0 °C) of the second crystals 34n and 34p is equal to or less than a minimum temperature sustainable at the cold end of the thermocouple legs 26n and 26p ; the minimum operating temperature (in the present example, 250 °C) of the first crystals 32n and 32p, and the maximum operating temperature of the second crystals 34n and 34p, are approximately equal; and the difference between the maximum operating temperature of the first crystals 32n and 32p and the minimum operating temperature of the second crystals 34n and 34p, equals or is greater than a maximum thermal gradient sustainable between the hot and cold ends of the thermocouple legs.

Referring now once again to Fig. 3, while any preferred number of the thermocouples 24 of the present invention may be employed in thermoelectric module 12, and while the dimensions of the thermocouple legs 26 may also be varied, an exemplary thermoelectric module constructed in accordance with the present invention, may have the following specifications:

Number of thermocouples in module 35 Overall module dimensions (mm) 60 x 60 x 15 Leg dimensions (mm) 4 x 4 x 5 Operating temperatures (°C) at hot face 550 at cold face 50 Maximum temperature of first crystal (°C) 600 Average figure of merit Z, K''>1.3 x 10-3 Thermal power input (w) 120 Electrical power output (w) at least 10 Load voltage (v) 1.8-2 Open circuit voltage (v) 3. 5 Efficiency (%) at least 8 % It will thus be appreciated by persons skilled in the art that, the present invention provides a thermoelectric module which not only has a novel and improved construction than previously known, but which is significantly more efficient than previously known in the art.

It will further be appreciated by persons skilled in the art that the scope of the present invention is not limited to what has been specifically shown and described hereinabove, merely by way of example. Rather the scope of the invention is limited solely by the claims, which follow.