GB2119169A - Thermoelectric systems - Google Patents

Abstract

Waste heat is converted into electrical energy by a thermoelectric system (10, 54) which includes a heat recovery unit (12, 56) having heat collecting fins (38, 64) and heat pipes (40, 66) for the collection and transmission of waste heat to one side of a thermoelectric device (24, 68) situated outside the flow of waste heat. The other side of the thermoelectric device (24, 68) is either water or air cooled. The thermoelectric devices (24, 68) utilized include elements (26, 28) having a thermal resistance matched to the thermal resistance of the heat exchanger to provide maximum power output. <IMAGE>

Description

SPECIFICATION Thermoelectric systems The present invention relates to thermoelectric systems and more efficient thermoelectric materials for use with same.

It has been recognized that the world supply of fossil fuels for the production of energy is being exhausted at ever increasing rates.

This realization has resulted in an energy crisis which impacts not only the world's economy, but threatens the peace and stability of the world. The solution to the energy crisis lies in the development of new fuels and more efficient techniques to utilize them. To that end, the present invention deals with energy conservation, power generation, pollution, and the generation of new business opportunities by the development of new thermoelectric systems which provide more electricity.

An important part of the solution with respect to the development of permanent, economical energy conversion lies in the field of thermoelectrics wherein electrical power is generated by heat. It has been estimated that more than two-thirds of all our-energy, for example, from automobile exhausts or power plants, is wasted and given off to the environment. Up until now, there has been no serious climatic effect from this thermal pollution. However, it has been predicted that as the world's energy consumption increases, the effects of thermal pollution will ultimately lead to a partial melting of the polar ice caps with an attendant increase in sea level.

Similarly the present invention provides a low cost, efficient and economical thermoelectric system to generate electrical energy from the waste generated by power plants, geothermal sites, automobiles, trucks and buses.

Therefore by the employment of waste heat from these and other sources, regeneration of electricity can provide a direct reduction in thermal pollution, while helping to conserve valuable finite energy sources.

The efficiency of a thermoelectric system is in part dependent upon the performance characteristics of the thermoelectric device or devices incorporated therein. The performance of a thermoelectric device can in turn be expressed in terms of a figure of merit (Z) for the material forming the device, herein Z is defined as: S2a Z= K Where: Z is expressed in units x 103 S is the Seebeck coefficient in V/'C K is the thermal conductivity in mW/cm-' C a is the electrical conductivity in (#- cm)~' From the above, one can see that in order for a material to be suitable for thermoelectric power conversion, it must have a large value for the thermoelectric power Seebeck coefficient (S), a high electrical conductivity (a), and a low thermal conductivity (K).Further, there are two components to the thermal conductivity (K) : Kl, the lattice component; and Ke, the electrical component. In non-metals, Kl dominates and it is this component which mainly determines the value of K.

Stated in another way, in order for a material to be efficient for thermoelectric power conversion, it is important to allow carriers to diffuse easily from the hot junction to the cold junction while maintaining the temperature gradient. Hence, high electrical conductivity is required along with low thermal conductivity.

Thermoelectric power conversion has not found wide usage in the past. The major reason for this is that prior art thermoelectric materials which are at all suitable for commercial applications have been crystalline in structure. Crystalline solids cannot attain large values of electrical conductivity while maintaining low thermal conductivity. Most importantly, because of crystalline symmetry, thermal conductivity cannot be controlled by modification.

In the case of the conventional polycrystalline approach, the problems of single crystalline materials still dominate. However, new problems are also encountered by virtue of the polycrystalline grain boundaries which cause these materials to have relatively low electrical conductivities. In addition, the fabrication of these materials is also difficult to control as a result of their more complex crystalline structure. The chemical modification or doping of these materials, because of the above problems are especially difficult.

Among the best known currently existing polycrystalline thermoelectric materials are (Bi,Sb)2Te3, PbTe, and Si-Ge. The (Bi,Sb)2Te3 materials are best suited for applications inthe - 1 0'C + 1 50 C range with its best Z appearing at around 30 C. (Bi,Sb)2Te3 represents a continuous solid solution system in which the relative amounts of Bi and Sb are from 0 to 100%. The Si-Ge material is best suited for high temperature applications in the 600to to 1 000 C range with a satisfactory Z appearing at above 700 C. The PbTe polycrystalline material exhibits its best figure of merit in the 300 C to 500 C range.None of these materials is well suited for applications in the 1 00 C to 300 C range. This is indeed unfortunate, because it is in this temperature range where a wide variety of waste heat applications are found; Among such applications are geothermal waste heat and waste heat from internal combustion in, for example, trucks, buses, and automobiles. Applications of this kind are important because the heat is truly waste heat. Heat in the higher temperature ranges must be intentionally generated with other fuels and therefore is not truly waste heat.

New and improved thermoelectric alloy materials have been discovered for use in the aforesaid temperature ranges. These materials are disclosed and claimed in copending U.S.

Application Serial No. 341,864, Case 1049, filed January 22, 1982 corresponding to our copending British Patent Application No.

8300997.

The thermoelectric materials there disclosed can be utilized in the systems herein. These materials are not single phase crystalline materials, but instead, are disordered materials.

Further, these materials are multiphase materials having both amorphous and multiple crystalline phases. Materials of this type are good thermal insulators. They include grain boundaries of various transitional phases varying in composition from the composition of matrix crystallites to the compo#ition of the various phases in the grain boundary regions.

The grain boundaries are highly disordered with the transitional phases including phases of high thermal resistivity to provide high resistance to thermal conduction. Contrary to conventional materials, the material is designed such the grain boundaries define regions including conductive phases therein pfO- viding numerous electrical conduction paths through the bulk material for increasing electrical conductivity without substantially effecting the thermal conductivity. In essence, these materials have all of the advantages of polycrystalline materials in desirably low thermal conductivities and crystalline bulk Seebeck properties. However, unlike the conventional polycrystalline materials, these disordered multiphase materials also have desirably high electrical conductivities.Hence, as disclosed in the aforesaid referenced application, the S2a product for the figure of merit of these materials can be independently maximized with desirably low thermal conductivities for thermoelectric power generation.

Amorphous materials, representing the highest degree of disorder, have been made for thermoelectric applications. The materials and methods for making the same are fully disclosed and claimed, for example, in U.S.

Patents 4,177,473, 4,177,474 and 4,178,415 which issued in the name of Stanford R. Ovshinsky. The materials disclosed in these patents are formed in a solid amorphous host matrix having structural configurations which have local rather than long-range order and electronic configurations which have an energy gap and an electrical activation energy. Added to the amorphous host matrix is a modifier material having orbitals which interact with the amorphous host matrix as well as themselves to form electronic states in the energy gap. This interaction substantially modifies the electronic configurations of the amorphous host matrix to substantially reduce the activation energy and hence, increase sub stantially the electrical conductivity of the material. The resulting electrical conductivity can be controlled by the amount of modifier material added to the host matrix.The amor phous host matrix is normally of intrinsic-like conduction and the modified material changes the same to extrinsic-like conduction.

As also disclosed therein, the amorphous host matrix can have lone-pairs having orbitals wherein the-orbitals of the modifier material interact therewith to form the new electronic states in the energy gap. In another form, the host matrix can have primarily tetrahedral bonding wherein the modifier material is added primarily in# a non-substitutional man ner with its orbitals interacting with the host matrix. Both d and f band materials as well as boron and carbon, which add multiorbital pos sibilities can be used as modifiers to form the new electironic states in the energy gap.

As a result of the foregoing, these amor phous thermoelectric materials have substantially increased electrical conductivity. How ever, because they remain amorphous after modification, they retain their low thermal conductivities making them well suited for thermoelectric applications, especially in high temperature ranges above 400 C.

These materials are modified on an atomic or microscopic level with the atomic configu rations thereof substantially changed to pro vide the heretofore mentioned independently increased electrical conductivities. In contrast, the materials disclosed in the aforesaid referenced application are not atomically modified.

Rather, they are fabricated in a manner which introciuces disorder into the material on a macroscopic level. This disorder allows vari ous phases including conductive phases to be intoduced into the material much in the same manner as modification atomically in pure amorphous phase materials to provide con trolled high electrical conductivity while the disorder in the other phases provides low thermal conductivity. These materials therefore are intermediate in terms of their thermal conductivity between amorphous and regular polycrystalline materials.

A thermoelectric device generates electricity by the establishment of a temperature differ ential across the materials contained therein.

The thermoelectric devices generally include elements of both p-type and n-type material.

In the p-type material the temperature differ ential drives positively charged carriers from the hot side to the cold side of the elements, while in the n-type material the temperature differential drives negatively charged carriers from the hot side to the cold side of the elements.

The conventional heat exchangers utilized to transfer heat to the thermoelectric device have been large, heavy and inefficient. They include many, closely spaced heat collecting surfaces which define passages that become readily clogged by the flow of a heated fluid therein. Also, conventional heat exchangers are designed such that the thermoelectric devices are an integral and inseparable part thereof. Due to this inseparability from the thermoelectric devices, it is difficult, if not impossible to clean and maintain them.

Conventional heat exchangers are also generally constructed from large amounts of copper, aluminum, or stainless steel for example. Hence, they can only be manufactured at high cost. They also exert a high back pressure in the exhaust lines of the internal combustion engines in which they are used.

This makes it difficult to establish and maintain proper operation of the engines. Lastly, because the thermoelectric devices are an integral part of the heat exchangers, the thermoelectric devices are exposed to potential contamination from the exhaust gases in the exhaust lines.

The present invention provides a thermoelectric system for generating electrical energy from a flow of fluid which has been heated to an elevated temperature, said system comprising: at least one thermoelectric device for generating said electrical energy responsive to a temperature differential applied thereto; first heat transfer means including at least one heat pipe disposed within said fluid flow, said first heat transfer means extending externally from said fluid flow and being thermally coupled to said at least one thermoelectric device for transferring at least a portion of the heat of said fluid to said at least one device; and second heat transfer means thermally coupled to said at least one thermoelectric device to establish with said first heat transfer means said temperature differential at said at least one thermoelectric device.

The present invention further provides a thermoelectric system for generating electrical energy from a flow of waste heat, said system comprising: thermoelectric device means for generating said electrical energy responsive to a temperature differential applied thereto; first heat transfer means including a plurality of heat pipes having portions within the flow of waste heat and being coupled to said thermoelectric device means for transferring a portion of the waste heat to said thermoelectric device means; and second heat transfer means coupled to said thermoelectric device means for applying a temperature to said thermoelectric device means which is lower than the temperature transferred to said thermoelectric device means by said first heat transfer means to thereby apply said temperature differential to said thermoelectric device means.

The present invention further provides a thermoelectric system for generating electrical energy from a flow of fluid which has been heated to an elevated temperature, said system comprising: at least one thermoelectric device for generating said electrical energy responsive to a temperature differential applied thereto remote from said heated fluid flow; flow heat transfer means disposed within said fluid flow, said first heat transfer means extending externally from said fluid flow and being thermally coupled to said at least one thermoelectric device for transferring at least a portion of the heat of said fluid flow to said at least one device; and second heat transfer means thermally coupled to said at least one thermoelectric device to establish with said first heat transfer means said temperature differential at said at least one thermoelectric device.

The present invention provides thermoelectric systems to generate electrical energy from waste heat. Preferably the systems are compact in size and have no moving parts. Further the systems may be adapted to utilize waste heat from many different sources of waste heat, including the waste heat from internal combustion engines.

The thermoelectric systems of the present invention may incorporate heat collecting means in the form of heat collecting fins arranged within the flow of a fluid providing waste heat generated from any source and a heat transfer means. The heat transfer means may be adapted to extend externally from the flow of the heated fluid to at least one thermoelectric device which is entirely separated from the flow of heated fluid. This allows the transfer of heat to one side- of the at least one thermoelectric device while maintaining separation of the device from possible contaminates in the heated flow of fluid. The other side of the thermoelectric device may be exposed to a cooling medium to establish a temperature differential across the thermoelectric device to thereby enable the generation of electrical energy.

The heat transfer means utilized to establish the hot side of the thermoelectric device preferably takes the form of a heat exchanger which includes one or more heat pipes. The heat pipes are hollow, sealed cylinders having a a working fluid contained therein. The working acts to efficiently convey the heat collected from the heated fluid to the hot side of the thermoelectric device. This is accomplished by taking advantage of the thermodynamics of vaporization and condensation of the working fluid. Further, since the heat pipes are sealed, they provide a continuously cycling contaminant free system.

The utilization of the heat pipes herein described coupled with the heat collecting fins can provide a low cost, compact, high heat transfer device which exhibits a low back pressure to the heated fluid flow. The heat transfer means can have a longer life and is easier to clean and maintain than those conventional heat exchangers due to the# relative ease in separating the heat transfer means from the thermoelectric devices.

Preferably the system of the present invention cools the cold side of the thermoelectric device by maintaining a flow of water or other fluids thereabout. Alternatively, the cold side of the thermoelectric device can be cooled by exposing the same to ambient air.

The systems of the present invention preferably utilize the materials disclosed in co-pending application, U.S. Serial Number 341,864 corresponding to our copending British Patent Application No. 8300997. These materials are particularly useful as the p-llype elements of the thermoelectric devices.

The elements of the thermoelectric devices may be thermally coupled in parallel and electrically coupled in series. Also, and in accordance with a further- aspeqt of the present invention, the thermal resistance (RTE) of the thermoelectric elements can be matched to the thermal resistance (RHx) of the heat transfer means or heat exchanger to maximize the power output for a given amount of thermoelectric material required for the devices.

Embodiments of this invention will now be described by way of example, with reference to the drawings accompanying this specification in which: Figure 1 is a side plan view of a thermoelectric system configured in accordance with a first embodiment of the present invention; Figure 2 is a cross-sectional view taken along line 2-2 of Fig. 1; Figure 3 is a cross-sectional view taken along line 3-3 of Fig. 2; Figure 4 is a cross-sectional view taken along line 4-4 of Fig. 3; Figure 5 is a side plan view of a thermoelectric system configured in accordance with an alternative embodiment of the present invention; Figure 6 is a cross-sectional view taken along line 6-6 of Fig. 5; Figure 7 is a side view partly in crosssection of a thermoelectric device adapted for use in the systems of the present invention; Figure 8 is a cross-sectional view taken along line 8-8 of Fig. 7;; Figure 9 is a cross-sectional view taken along line 9-9 of Fig. 7; Figure 10 is a schematic diagram of an electrical analog of a portion of the system of the present invention; and Figure 11 is a schematic diagram of a portion of the system of the present invention.

Referring# now to Figs. 1 and 2, there is shown a thermoelectric system 10 structured in accordance with a first embodiment of the present invention. The thermoelectric system 10 includes a heat recovery unit 12 divided into a heat recovery chamber 14 and cooling chamber 16 by partition wall 18. Fastened to the heat recovery unit 12 are a pair of duct means 20 and 22. Duct means 20 comprise ducts 46 and 48 for directing the flow of a fluid, heated by waste heat, through the heat recovery chamber 14. Duct means 22 comprise ducts 50 and 52 for directing the flow of a cooling fluid through the cooling chamber 16.

The- heat recovered from the fluid in the heat recovery chamber 14 is transferred from the heat recovery chamber 14 to one side of a number of thermoelectric devices 24 disposed in the cooling chamber 16. The heat thus transferred maintains the one side of the thermoelectric devices at an elevated temperature.

The flow of cooling fluid through the cooling chamber 16 maintains the other side of the thermoelectric devices 24 at a somewhat lower. temperature. This establishes a temperature differential across the devices and thereby enables the generation of electricity.

A thermoelectric device 24 adapted to generate electricity is shown in Figs. 7, 8, and 9.

The device 24 includes n-type and p-type thermoelectric elements, 26 and 28 respec tively. - The n-type and p-type elements, 26 and 28, bre thermally connected in parallel and electrically connected in series in alternating relation.

The p-type elements 28 are preferably the new and improved materials disclosed in copending U.S. application Serial Number 341,864 corresponding to our co-pending British Patent Application No. 8300997. An alloy described in the aforesaid co-pending application that exhibits a high figure of merit (Z) over the temperature range of 1 00'C to 300 C includes about 10 to 20 percent bismuth, about 20 to 30 percent antimony, about 60 percent tellurium and less than 1 percent silver and preferably is (Bi10Sb30Te6O) 99% + (Ag2sSb2sTe50) 1%. Further, the aforesaid alloy (Bi,OSb30Te3O) 99% + (Ag25Sb2#Te50) 1 % is made p-type by including a dopant material, such as telurium iodide (Tel4), of about .2 percent therein. The n-type elements 26 can comprise conventional materials such as material containing bismuth (Bi), tellurium (Te) and selenium (Se), in the proportion of Bi40Te54Se6.

These n-type and p-type elements 26 and 28 are soldered onto a substrate 30 which has a copper lead matrix or pattern 32 screen printed or otherwise applied thereon. Another substrate 34. having a copper lead matrix or pattern 36 screen printed or otherwise applied thereon is sweated onto the elements. The copper lead patterns 32 and 34 are arranged to connect the n-type and p-type elements in electrical series in alternating relation. It can also be seen that the n-type and p-type elements 26 and 28 respectively are arranged in thermally parallel relation between the substrates 30 and 34.

The substrates 30 and 34 have a high thermal conductivity in order to maintain the temperature differential across the elements 26 and 28 and have a low electrical conductivity to act as an electrical insulator and provide electrical isolation between the lead patterns. The substrates 30 and 34 are a ceramic material, such as alumina or the like.

Waste heat in the form of exhaust gases from the operation of internal combusion engines can establish a 200 C temperature differential across the thermoelectric devices 24.

If the elements 26 and 28 of the device 24 have a Seeback coefficient (S) of 0.15 mV/ C then the voltage which can be produced from each element can be determined from the expression Vte = SATte. For a ATte of 200 C, Vte is .15mV/C X 200 C or 30mV. The number of elements 26 and 28 needed to produce 14V, the voltage utilized in automobiles and trucks can be determined as follows: 14V n= =467 30mV Thereafter, any number of elements in series groups of 467 elements can be connected in parallel to obtain the required current for the system at 14V. Of course, each thermoelectric device 24 will include less than the required 467 elements.The number of devices to be connected in series relation to provide the 14V is equal to the total number of required elements divided by the number of elements in each device. For example if each device includes 32 elements, then 467 divided by 32 will be required. In this example, 467 divided by 32 equals 14.6 Hence 15 devices must be connected in series to assure an output voltage of at least 14V.

As best seen in Figs. 2, 3 and 4 waste heat is collected in heat recovery chamber 14 by a plurality of substantially parallel and horizontally spaced heat collecting fins 38. The collecting fins 38 are connected perpendicularly to heat pipes 40. The heat pipes 40, are formed from a good thermal conductor such as for example copper, stainless steel, aluminum, or the like. They extend from within the heat recovery chamber 14, through the partition wall 18 into the cooling chamber 16.

The heat pipes 40 are generally cylindrical, hollow and sealed at each end. Approximately 5-10% of the interior volume of the heat pipes 40 is occupied by a working fluid 42 such as water, for example. It has been found that this construction of heat pipe transfers heat from the recovery chamber 14 to the cooling chamber 16 more efficiently than solid pipes or any other known construction.

In transferring heat from the heat recovery chamber 14 to the cooling chamber 16, the working fluid 42 is vaporized in that portion of the heat pipe 40 within the heat recovery chamber 14. The vaporized working fluid 42 then flows to that portion of the heat pipe 40 within the cooling chamber 16 where it gives up its heat to the thermoelectric devices 24.

The working fluid 42 then condenses and returns to that portion of the heat pipe 40 within the heat recovery chamber 14 to repeat the cycle of heat transfer.

Mounting members 44 are connected to the heat pipes 40 within the cooling chamber 16. They extend vertically within the cooling chamber 16 and are disposed longitudinally of the heat recovery unit 1 2 in good thermal contact with the hot side of the thermoelectrc devices 24. The mounting members 44 are also formed from a good thermal conductor to enable efficient heat transfer from the heat pipes to the hot side of thermoelectric devices 24 mounted thereon.

The cold side of the thermoelectric devices 24 have plate members 45 connected, in good thermal contact, thereto. The plate members 45 extend vertically within the cooling chamber 16, are disposed longitudinally of the heat recovery unit 12 and are substantially parallel to the mounting members 44.

Adjacent plate members 45 provide passages 47 for the direction of the cooling medium through the cooling chamber to cool the cold side of the thermoelectric devices 24. Further, the mounting members 44 and the plate members 45 form a casing for the thermoelectric devices 24 to isolate them from the cooling medium.

In operation of the thermoelectric system 10, hot waste exhaust gases from the operation of internal combustion engines are directed through the heat recovery chamber 14 through ducts 46 and 48 of duct means 20.

Therein heat is collected by the heat collector fins 38 and transmitted to the heat pipes 40.

The working fluid 42 is vaporized and trans- fers its heat to the hot side of the thermoelectric devices 24 mounted on mounting members 44 within the cooling chamber 16.

The cold side of each thermoelectric device 24 is cooled by a cooling medium to establish a temperature differential across each device.

In this embodiment, the cooling medium is water. The water is directed through the passages 47 of the cooling chamber 16 by ducts 50 and 52 of duct means 22. The passages 47 are exposed to the cold side of each device 24. As a result, the water contacts and cools the cold side of the devices 24.

The utilization of waste heat to establish a temperature differential across the thermoelectric devices 24 requires a different approach in design than previously employed for obtaining a low cost device that maximizes electrical output. Where the heat source is free or relatively inexpensive, the design philosophy should be to maximize the electrical power output for a minimum amount of thermoelectric material used to minimize system cost.

As shown in electrical analog fashion in Fig.

10, in order to maximize power output across resistor (R2) in a series circuit containing a power source (V), resistor (R,) and resistor (R2) the resistance of R2 should equal the resistance of Rt. In a similar fashion, Fig 11 depicts the thermal schematic diagram of a thermoe!ectric system. For a given temperature differential (A), the maximum electrical power output will occur for a given amount of thermoelectric material when the thermal resistance of the thermoelectric device (rum) is equal to the thermal resistance of the heat exchanging device (RHx).

Thermal resistance is expressed by the following: R= KA where R is the thermal resistance I is the thickness of the material A is the area of the material K is the thermal conductivity of the material Therefore to maximize electrical output as described above the following must hold: ITE RHX = RTE = ATEKTE RHX can be calculated or measured. Since K.rE is capable of being measured, various values for ITE and ATE can be choosen.

Preferably, the value for RHX should be as low as possible to maximize heat transfer.

Since RHX is low and should equal RTE for maximum electrical power output as discussed above, ITE should be small and ATE large.

Turning to Figs. 5 and 6 there is shown a thermoelectric system 54 structured in accordance with another embodiment of the present invention. The thermoelectric system 54 includes a heat recovery unit 56 comprising a heat recovery chamber 58. Fastened to the heat recovery unit 56 are ducts 60 and 62 for directing the flow of a fluid heated by waste heat through the heat recovery chamber 58.

As in the previous embodiment, the heat recovery chamber 58 includes heat collecting fins 64, connected perpendicularly to heat pipes 66. The heat recovered by the fines 64 is transferred to the heat pipes 66 which in turn is transferred to an area external of the heat recovery chamber 58. There, thermoelectric devices 68, of the kind described above, are coupled to mounting members 70. The devices 68 are heated on one side thereof by the heat conducted through the heat pipes 66 and mounting members 70.

Cooler ambient air is used to cool the other side of the thermoelectric devices 68. To aid in the utilization of ambient air to cool the thermoelectric devices, horizontally disposed and vertically spaced cooling fins 72 are attached perpendicularly to plate members 74 which are in good thermal contact with the thermoelectric devices 68.

The operation of this embodiment parallels that of the previous embodiment except that ambient air is utilized to cool the cold side of the thermoelectric devices. The design and material considerations discussed above are the same in both embodiments except that in this alternative embodiment, a higher operating temperature is required since the cold side of the devices will be at a higher temperature.

Claims (41)

1. A thermoelectric system for generating electrical energy from a flow of fluid which has been heated to an elevated temperature, said system comprising: at least one thermoelectric device for generating said electrical energy responsive to a temperature differential applied thereto; first heat transfer means including at least one heat pipe disposed within said fluid flow, said first heat transfer means extending externally from said fluid flow and being thermally coupled to said at least one thermoelectric device for transferring at least a portion of the heat of said fluid flow to said at least one device; and second heat transfer means thermally coupled to said at least one thermoelectric device to establish with said first heat transfer means said temperature differential at said at least one thermoelectric device.

2. A system according to claim 1 wherein said first heat transfer means comprises a plurality of heat collectors disposed within said fluid flow and thermally coupled to said at least one heat pipe.

3. A system according to claim 2 wherein each said heat collector is substantially planar and lies in a plane substantially parallel to the direction of the flow of said fluid.

4. A system according to claim 2 wherein said first heat transfer means comprises a plurality of said heat pipes.

5. A system according to claim 4 wherein each said heat pipe is thermally coupled to each said heat collector

6. A system according to claim 1 wherein said at least one heat pipe is formed from a material having good thermal conductivity.

7. A system according to any one of claims 1 to 6 wherein said heat pipe is formed from copper, stainless steel or aluminum.

8. A system according to claim 6 wherein said at least one heat pipe is substantially cylindrical in shape.

9. A system according to any one of claims 1 to 8 wherein said heat pipe includes a working fluid therein, said fluid occupying between 5 to 10 percent of the inner volume of said heat pipe when said fluid is fully condensed.

10. A system according to any one of claims 1 to 9 wherein said second heat transfer means includes air flow cooling means for establishing at said at least one device a lower temperature than that established by said first heat transfer means.

11. A system according to any one of claims 1 to 9 wherein said second heat transfer means includes water flow cooling means for establishing at said at least one device a lower temperature than that established by said first heat transfer means.

12. A system according to any one of claims 1 to 11 wherein said fluid flow is isolated from said second heat transfer means.

13. A thermoelectric system for generating electrical energy from a flow of waste heat, said system comprising: thermoelectric device means for generating said electrical energy responsive to a temperature differential applied thereto; first heat transfer means including a plurality of heat pipes having portions within the flow of waste heat and being coupled to said thermoelectric device means for transferring a portion of the waste heat to said thermoelectric device means; and second heat transfer means coupled to said thermoelectric device means for applying a temperature to said thermoelectric device means which is lower than the temperature transferred to said thermoelectric device means by said first heat transfer means to thereby apply said temperature differential to said thermoelectric device means.

14. A system according to claim 13 further comprising a first chamber for conducting said waste heat and a second chamber sealed from said first chamber for containing said second heat transfer means and said thermoelectric device means.

15. A system according to claim 14 wherein said heat pipes extend from said first chamber into said second chamber.

16. A system according to claim 13 wherein said first heat transfer means comprises a plurality of heat collectors coupled to said heat pipes.

1 7. A system according to claim 16 wherein said heat collectors are substantially planar and lie in a plane substantially parallel to the flow of said waste heat.

18. A system according to claim 13 wherein said second heat transfer means includes air cooling means for applying said lower temperature to said device means.

19. A system according to claim 13 wherein said second heat transfer means includes water cooling means for applying said lower temperature to said device means.

20. A system according to claim 13 wherein said heat pipes are formed from a material having good thermal conductivity.

21. A system according to claim 20 wherein said heat pipes are formed from copper, stainless steel or aluminum.

22. A system according to claim 20 wherein said heat pipes are cylindrical in shape.

23. A system according to claim 22 wherein each said heat pipe includes a working fluid therein and wherein said working fluid occupies from 5 to 10 percent of the inner volume of said heat pipes when fully condensed.

24. A system according to claim 23 wherein said working fluid is water.

25. A system according to any one of claims 13 to 24 further including means for isolating said waste from said thermoelectric device means.

26. A system according to any one of claims 1 to 25 wherein said at least one thermoelectric device comprises at least one n-type thermoelectric element and at least one p-type thermoelectric element, and said n and p type elements being connected electrically in series and thermally in parallel.

27. A system according to claim 26 wherein said n and p type thermoelectric elements are dimensioned for substantially matching their thermal resistance to the thermal resistance of said first and second heat transfer means for generating the maximum amount of electrical power out of said thermoelectric device.

28. A system according to claim 27 wherein said thermoelectric device further comprises a pair of substantially planar plates formed from a material having a high thermal conductivity and low electrical conductivity sandwiching said n and p type elements.

29. A system according to claim 28 wherein said plates have a lead pattern for connecting said elements electrically in series.

30. A system according to claim 29 wherein said lead pattern is formed from copper.

31. A system according to claim 29 wherein said plates have inner plate surfaces, and said lead pattern is screen printed onto said inner plate surfaces.

32. A system according to any one of claims 26 to 31 wherein said p-type thermoelectric element is comprised of about 10 to 20 percent bismuth, about 20 to 30 percent antimony, about 60 percent tellurium, and less than 1 percent silver.

33. A system according to any one of claims 26 to 31 wherein said p-type thermoelectric element is comprised of about 10 to 20 percent bismuth, about 20 to 30 percent antimony, about 60 percent tellurium, less than 1 percent silver and about two tenths of one percent tellurium iodide (Tel4) as a p-type dopant.

34. A system according to any one of claims 26 to 31 wherein said n-type thermoelectric element is comprised of about 40 percent bismuth, about 54 percent tellurium, and about 6 percent selenium.

35. A thermoelectric system for generating electrical energy from a flow of fluid which has been heated to an elevated temperature, said system comprising: at least one thermoelectric device for generating said electrical energy responsive to a temperature differential applied thereto remote from said heated fluid flow; first heat transfer means disposed within said fluid flow, said first heat transfer means extending externally from said fluid flow and being thermally coupled to said at least one thermoelectric device for transferring at least a portion of the heat of said fluid flow to said at least one device; and second heat transfer means thermally coupled to said at least one thermoelectric device to establish with said first heat transfer means said temperature differential at said at least one thermoelectric device.

36. A system according to claim 35 wherein said first heat transfer means includes a plurality of heat collectors disposed within said fluid flow.

37. A system according to any one of claims 35 or 36 wherein said first heat transfer means includes at least one heat pipe.

38. A system according to claim 37 wherein each said heat collector is substantially planar and lies in a plane substantially parallel to the direction of the flow of said fluid and is thermally coupled to said at least one heat pipe.

39. A system according to any one of claims 35 or 36 wherein said first heat transfer means comprises a plurality of said heat pipes.

40. A thermoelectric system substantially as hereinbefore described with reference to and as illustrated in either Figs. 1 to 4 or Figs. 5 to 6 when taken in conjunction with Figs. 10 and 11.

41. A thermoelectric device for use in a thermoelectric system substantially as hereinbefore described with reference to and as illustrated in Figs. 7 to 9.