WO1991011029A1

WO1991011029A1 - Thermo-electric power generators

Info

Publication number: WO1991011029A1
Application number: PCT/GB1991/000057
Authority: WO
Inventors: David William Auckland; Roger Shuttleworth; Arthur Clarence Luff
Original assignee: BLAGDEN POWER GENERATION Ltd
Current assignee: BLAGDEN POWER GENERATION Ltd
Priority date: 1990-01-16
Filing date: 1991-01-16
Publication date: 1991-07-25
Anticipated expiration: 1992-07-16
Also published as: GB9000936D0

Abstract

The invention relates to thermo-electric power generators and especially to such generators adapted to produce power from pipelines. In a first aspect the invention provides a thermo-electric generation having first and second junctions of dissimilar materials in a circuit and adapted to be maintained at different temperatures so as to produce a direct current due to the Seebeck effect, chopper means (3) being provided to convert the direct current to an alternating current transformer means (4) being provided to transfer the alternating current from a first to a second level and output means (6) being provided to supply an electric current from the transformer means. In a second aspect the invention provides a thermo-electric power unit adapted for use with a pipe (81)comprising a thermo-electric generator (80), a heat sink (84) and means (86) and (87) for maintaining thermal contact between the heat sink (84) and a first surface of the thermo-electric generator (80) and for maintaining thermal contact between a second surface of the thermo-electric generator (80) and the pipe (81), at least one of the surfaces of the thermo-electric generator (80) being covered with a substance to enhance the transfer of heat to and from the surface.

Description

THERMO-ELECTRIC POWER GENERATORS

This invention relates to thermo-electric power generators, and especially but not exclusively, to such generators adapted to produce power from pipelines, such as sub-sea oil pipelines.

The creation of electrical power by thermoelectric means was discovered by T.J. Seebeck in 1821. He established that if two identical junctions of two dissimilar metals are joined in a single circuit then a minute current will flow around the circuit when one junction is held at a higher temperature than the other. The current flows because of a difference in electrical potential between the junctions. The product of the potential difference

(volts) and the current (amperes) determines the power derived (watts) . Since the potential produced per cell is very small many cells have to be connected in series in order to create a potential difference large enough to deliver useful quantities of power to an external load. The numbers are such as to render thermo-electric generation impractical in most applications (for example the number of junctions in series could be 50,000 or above). One well known use of the Seebeck effect is in thermocouple temperature sensing apparatus where thin wires of dissimilar metals are used to generate a voltage proportional to the termperature difference between the junctions of the wires. In recent years many proposals have been made to use the thermo-electric generating properties of semiconductor materials because semiconductor thermoelectric generators (TEG's) can produce about twenty times the voltage of a metal/metal TEG. We are aware of a proposal by GEC Avionics, published in

"Offshore Engineer - May 1989", to use existing solid

SUBSTITUTE SHEET state semiconductor TEG's (previously used to cool electric devices) to generate power from a sub-sea oil pipe. Hot oil in the pipe comprises a heat source and the cool sea a heat sink, the semiconductor TEG's being provided with associated cooling vanes which are surrounded by a mechanical shield. Sea water is intended to flow between the shield and the vanes to cool the vanes. Many thousands of TEG's are connected in series to produce a useful voltage. The TEG's are intended to power a "Christmas tree" control installation of an oil field.

Whilst GEC's proposal may produce electricity we believe that it has certain disadvantages which our own invention alleviates. According to a first aspect the invention consists in a thermo-electric power unit comprising first and second junctions of dissimilar materials in a circuit and adapted to be maintained at different temperatures so as to produce a direct current due to the Seebeck effect, chopper means adapted to convert the direct current to an alternating current, transformer means adapted to transform the alternating current from a first to a second level, and output means adapted to supply an electric current from the transformer means.

The invention allows us to make use of large currents at low voltage produced by the thermo-electric generators to produce useful power. Large currents at low voltages which are not suitable for powering electric apparatus can be converted to higher voltage currents using the chopper means and transformer means. This approach is directly against the trend in thermo-electric generators which is to use semiconductor devices in series to produce a high voltage in the first place.

Since the present invention can work with a

SUBSTITUTE SHEET low voltage we do not need to have an enormous number of thermo-electric generators in series to produce a useful voltage, we can have a relatively small number of thermo-electric generators arranged in series, or in parallel, or in series/parallel. Indeed, arranging the generators in parallel makes a power unit more reliable since the whole unit does not fail completely if one link in the chain of electrical path is broken. Preferably the chopper means is a solid state arrangement of, for example FETS. The output means may comprise rectifier means adapted to give a direct current output. The power unit can then take the place of a battery, supplement the power of a battery, or recharge a battery. A second major, subsidiary, feature of our proposals is that we may have the dissimilar metals forming the junctions comprising first and second relatively large blocks, or plates, of dissimilar metals either directly in contact with each other, or each in contact with an intermediary material so as to complete an electric circuit. Once again, this is against the current trend in thermo-electric generator technology which is to use semiconductor devices in the form of relatively thin plates. However, "heavy" thermo-electric generators (TEG's) can be made much more robust than semiconductor devices. Furthermore, relatively large blocks of metals with junctions at different temperatures produce a relatively large current, albeit at a small voltage, and the present invention can be used to convert the relatively large current at a relatively small voltage (in thermo-electric generating terms) to a relatively large, useful, voltage at a smaller current. Thus we do not need to have the TEG's themselves produce a usefully large voltage, but can create one from a suitably large current.

SUBSTITUTE SHEET An idea of what we mean by "relatively large blocks or plates" can be obtained from the dimensions given in Claims 5 to 7.

The thermo-electric junction may comprise two generally parallel blocks or plates of dissimilar metals meeting at an interface, the spaced apart opposite surfaces of the blocks or plates being at relatively higher and lower temperatures so that heat flows across the junction generally at right angles to the interface. Thus the TEG may be relatively thinner in its dimension- parallel to the direction of heat flow across the device, and relatively larger in one or both perpendicular directions transverse to the direction of heat flow. The thermo-electric power unit preferably comprises a series of adjacent thermo-electric generators having upper, lower and middle layers, the upper and lower layers being made of one metal and the middle layer of a different metal. The upper layer of a thermo-electric junction and the lower layer of an adjacent thermo-electric junction may be formed from a single piece of metal. This enables an electric circuit to be conveniently formed between adjacent TEG's. Alternatively adjacent thermo-electric generators may comprise upper and lower layers of electrically conductive material, and first and second blocks of dissimilar first and second metals extending between the upper and lower layers; the block of first metal of one thermo-electric generator extending from the upper layer of that generator to the lower layer of that generator, and the block of second metal of the one generator extending from one of the upper or lower layers of the one generator to the other layer of the adjacent generator, connecting the adjacent generators.

SUBSTITUTE SHEET Preferably the relative areas of contact between the first and second metals are such that in use the temperatures at the junctions of the two blocks of dissimilar metals with the upper layer are substantially the same as each other, and the temperatures at their junctions with the lower layer are substantially the same as each other. This maximises the thermo-electric current produced.

The lower layer is preferably adapted to extend around at least a portion of the circumference of a pipe and has an arcuate inner surface adapted to follow the external surface of the pipe. The inner layer may^' extend around in an arc which is a substantial portion of a circle. The inner layer may have a substantial depth in the direction of heat flow through the dissimilar metals so as to act as a gatherer of heat from an adjacent hot surface, and channel the heat to the first and second metals. The first and second metals may contact the lower layer over a smaller area than that over which the lower layer gathers heat.

The provision of a heat-gathering lower layer extending around a substantial portion of the circumference of a pipe and a smaller TEG in contact with the lower layer ensures that more heat is conducted through the TEG than will be the case if the lower layer were the same size as the TEG and ensures that useful heat from the pipe does not escape directly to the surrounding medium without influencing the TEG. By forcing the heat to pass through the TEG we increase the temperature difference across the TEG and thus increase the thermo-electric current produced by it. We may also provide thermal insulation between the lower, or inner, and upper, or outer, layers. One particular preferable feature of thermo-electric power units which we have discovered

SUBSTITUTE SHEET came as a great surprise to us. This feature is that in thermo-electric generators using metals when used to extract heat from a pipe the temperature drop between the thermo-electrically active junctions between the dissimilar metals should be arranged to be about half of the temperature difference between the hot material inside the pipe which is acting as a heat source and the surrounding temperature of water or air which is acting as a heat sink. We prefer to arrange the thermo-electric power unit so because we found in our models and experiments that such an arrangement produces the largest output current of the thermo-electric generators. This feature of having the temperature difference across the TEG being around half of the temperature difference between the heat source and the heat sink may be applicable to other applications, not just extracting heat from a pipe. The present invention also provides in a second aspect a thermo-electric power unit adapted for use with a pipe carrying a medium at a temperature different from the temperature of the medium surrounding the pipe comprising a thermo-electric generator, a heat sink and means for maintaining thermal contact between the heat sink and a first surface of the thermo-electric generator and for maintaining thermal contact between a second surface of the thermo-electric generator and the pipe, at least one of the surfaces of the thermo-electric generator being covered with a substance to enhance the transfer of heat to and from the surface.

Preferably at least one surface of the thermo-electric generator is covered with a grease which enhances the transfer of heat to or from the surface and the grease is preferably a silicon grease. Preferably the thermo-electric power unit of the second aspect of the invention comprises a first

SUBSTITUTE SHEET layer of electrical insulation between the heat sink and the first surface of the thermo-electric generator and a second layer of electrical insulation between the second surface of thermo-electrical generator and the pipe and preferably the first and second layers of electrical insulation are layers of polytetrafluorethylene.

In the second aspect the means for maintaining thermal contact between the first surface of the thermo-electrical generator and the heat sink and for maintaining thermal contact between the second surface of the thermo-electric generator and the pipe preferably comprises a mechanical means to apply a force to the heat sink to urge the heat sink towards the pipe, thereby applying a compressive force on the thermo-electric generator between the heat sink and the pipe.

According to a third aspect the invention consists in a pipeline carrying a hot material and having a thermo-electric power unit in accordance with the first aspect of the invention.

The pipeline may have signalling means adapted to indicate the position of the pipeline, the signalling means receiving power directly or indirectly from the thermo-electric power unit.

According to a fourth aspect the invention consists in an add-on unit adapted to be secured around an existing pipeline, the add-on unit incorporating a thermo-electric power unit in accordance with the first aspect of the invention.

The add-on unit preferably includes signalling means adapted to indicate the position of the pipeline.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings of which:-

SUBSTITUTE SHEET Figures 1 and 2 illustrate a thermo-electric power unit in accordance with the first aspect present invention;

Figures 3 and 4 illustrate a second embodiment of the invention;

Figures 5 shows schematically the electrical circuit of the thermo-electric generators of the embodiments of Figures 1 to 4;

Figure 6 is a cross-section through a pipe in accordance with the second aspect of the invention; and

Figure 7 is a section on line VIII-VIII of Figure 6;

Figures 8 and 9 illustrate a third embodiment of the invention; Figure 10 is a cross-section through a pipe in accordance with the third embodiment of the invention; and

Figure 11 is a plan view of a pipe in accordance with the third embodiment of the invention. A thermo-electric power unit 1 is shown in

Figures 1 and 2 and comprises a series of thermo-electric generators (TEG's) 2 (only two of which are shown as 2a and 2b in the drawings) , a chopper 3, transformer 4, rectifier 5 and output lead 6.

The chopper 3 is an arrangement of Field Effect transistors of a known kind and is a compact solid-state component.

The TEG's 2 each comprise a copper/constantan/copper sandwich having upper, middle and lower layers 7, 8, and 9. The TEG's are very large (in TEG terms) , the area of contact between copper layers and the constantan layer being 50mm x 50mm, the copper layers are each about 10mm thick and the constantan layer is about 8mm thick. The upper layer 7 of TEG 2a and the lower layer 9 of adjacent

SUBSTITUTE SHEET TEG 2b are formed from a single block or thick plate of copper which is kinked in the region between the TEG's. The TEG's 2a and 2b are thus electrically connected in series. The thermo-electric power unit 1 is mounted on the outside surface of a pipe 10 which is hot by virtue of containing hot fluid, such as oil or water. The pipe 10 is a steel pipe of 300mm diameter. The pipe 10 is under the sea and so the outside of the pipe and the upper layers 7 of the TEG's are cooled by water. A "potting compound" 11 is provided between the TEG's to provide electrical insulation and to contribute to a small degree to the mechanical strength of the unit 1. Typically there are ten or so TEG's 2 connected in series.

Hot oil in the pipe 10 heats the underside of lower layer 9 and the junction between layer 9 and layer 8, junction 12, constitutes the "hot" junction of the TEG. The upper layer 7 is cooled by the sea and the junction between the layer 7 and the layer 8, junction 13, constitutes the "cold" junction of the TEG. The TEG is arranged such that the thermal characteristics of the pipe and the TEG are such that the temperature difference between junctions 12 and 13 is about half of the temperature difference between the hot oil in the pipe and the sea temperature. We have found that this, surprisingly, gives us the best power output from the unit 1.

The row of ten or so TEG's 2 connected in series with the sea temperature at, say, 4°C and the oil at, say, 80°C produces about 30 amperes at 120 millivolts. This TEG output is at a very low voltage indeed compared with conventional TEG assemblies and hitherto would have been considered useless. However, our unit 1 feeds this low voltage, high current, supply to the chopper 3 which converts the D.C. supply

SUBSTITUTE SHEET into A.C. which is fed to transformer 4 which outputs a higher voltage A.C.

The rectifier then converts the A.C. to D.C. and the output lead 6 feeds power to any suitable electrical device.

The electrical device may be a signalling device adapted to indicate the position of the undersea pipeline. Such a device powered by a reliable robust, power unit which extracts power from the hot oil could be very useful. At present signalling devices on pipelines are powered by batteries which last up to two years and are very expensive to buy and expensive and dangerous to replace. The present invention is far cheaper and does not need replacing. (It will of course have a finite life which we estimate to be about 30 years) .

The power unit l may be provided on pipeline as it is laid, or it may be added to existing pipelines. It will probably be necessary to clean an existing pipeline before fitting the unit 1 in order to improve the thermal contact between the pipeline and the unit.

Cooling fins, not shown, may be provided at the outer layer 7. Figures 3 and 4 illustrate a second embodiment of the invention and show a steel pipeline 30 which is near to the well-head of an oil well and carries hot oil emerging from the ground. The pipeline 30 has four "spokes" 31 extending axially, each spoke 31 comprising an array of TEG's 32 connected in series. The four spokes are connected in parallel.

Figure 4 shows the arrangement of a spoke 31 which comprises a lower, or inner, layer 33 of copper brazed or otherwise connected to the pipeline 30, an upper, or outer, layer of copper 34, first blocks 35

SUBSTITUTE SHEET of a first metal, bismuth, extending between the layers 33 and 34, and second blocks 36 of a second ^• metal, Ni-Mo alloy (Ni 84%, Mo 16%) also extending between layers 33 and 34. Adjacent TEG's 32a and 32b are connected in series. TEG 32a has its Ni/Mo block 36a at its right-hand end (as seen in Figure 4) joining its outer layer 34a to the inner layer 33b of the adjacent TEG 32b. The bismuth layer 35a of the TEG 32a joins the inner and outer layers 33a and 34a of the TEG 32a. The inner layer 33a is connected to the outer layer of the TEG to the left of TEG 32a by the Ni/Mo block 36 of the adjacent TEG. The bismuth and Ni/Mo blocks 35 and 36 of each TEG are electrically separated from each other by gaps 37, and from the adjacent block of the other thermo-electric metal of the adjacent TEG by gaps 38. The gaps 37 also divide the inner layer 33 into segments, and the gaps 38 divide the outer layer 34 into segments. The gaps 37 and 38 could be replaced by insulators, for example mica, and would be if the device were used in an electrically conductive medium.

The TEG's 32 behave as though there were two bismuth - Ni/Mo junctions at the inner and outer layers provided that junctions 39 and 40 between the bismuth/copper layer 34 and Ni/Mo/copper layer 36 are at the same temperature as each other, and junctions 41 and 42 between the bismuth/copper layer 33 and Ni/Mo/copper layer 36 are at the same temperature as each other. The TEG's 32 are constructed by matching of the thermal conductivities of the metals and heat flow cross-sectional areas such that the temperatures at junctions 39 and 40 are the same, and those at junctions 41 and 42 are the same. This is why there is more bismuth than Ni/Mo. The outer layer 34 forms a cylindrical sheath connecting the spokes in parallel. A smooth outer

SUBSTITUTE SHEET sheath has the advantage in the underwater environment that it reduces the tendency for organisms to grow on an object. The space inside of the sheath, referenced 44, may be filled with insulating material. A chopper, transformer, and rectifier module

43 is provided in a similar manner to the embodiment of Figures 1 and 2. The power unit of Figures 3 and 4 may be used for the same purposes as that of Figures 1 and 2. Figure 5 illustrates schematically the electrical circuit of the embodiments of Figures 1 to 4. There is some current lost- from the useful output of the power unit due to conduction between TEG's through the steel of the pipeline, the steel providing a short circuit to the TEG output voltage VT. The resistance of the steel pathway limits the useful power which can be obtained from the TEG's - the maximum power is achieved when the internal and external resistances are matched, that is to say when the resistance of the load over which VT acts is equal to or greater than the resistance of the steel.

An example of the scale of the thermo-electric power unit of Figures 3 and 4 is that in the preferred embodiment for a pipeline of 30 cm diameter the outer and inner copper layers 33 and 34 are 58mm thick, the Bismuth and Ni/Mo blocks are 94mm thick, the axial length of the Bismuth blocks 35 is 280mm, the axial length of the Ni/Mo blocks is 26mm, and the axial length of the gaps 37 and 38 is 11mm. Such an arrangement with one TEG in each spoke gave an output of 60 amperes at 4.8 millivolts for a temperature difference of 40°C between the heat source inside the pipeline and heat sink outside it.

Figures 6 and 7 show an improved pipeline 50 incorporating thermo-electric power units 51. A 320mm outside diameter steel pipe 52, which may be an

SUBSTITUTE SHEET existing pipeline, is surrounded by four columns 51a, 51b, 51c, 51d each having an array of TEG's 57 connected in series. Each power unit 51 comprises an array of arcuate copper heat gatherers 58 extending through substantially one-quarter of a circle, each heat gatherer 58 having an associated TEG 57 disposed at the central position of its outer arcuate surface. The TEG's 57 extend for only about one-eighth of the arcuate length of the heat gatherers 58. A cylindrical outer copper sheath 59 of 51cm diameter surrounds the pipeline 50 and the TEG's 57. There is a layer of insulating material 60 between the heat gatherers 58 and the sheath 59, except for where the TEG's 57 are located. Dividing walls 61 of an insulator, typically mica, extend axially along the pipeline and separate the power units 51 from each other electrically. A thin layerof electrical (but not thermal) insulation 62 extends between the heat gatherers 58 and the pipe 52. In the embodiment shown the copper heat gatherers 58 are about 54mm thick in the radial direction, the copper sheath 59 about 3mm thick, the insulation 60 about 36mm thick, and the dividing walls 61 about 5mm thick in the circumferential direction. The TEG's are best shown in Figure 7 and comprise a relatively large bismuth block 70 extending between the heat gatherer 58 and the outer sheath 59, a smaller Ni/Mo block 71 extending between the heat gatherer 58 and outer sheath 59, mica divider 72 separating the bismuth block 70 from the Ni/Mo block 71, and mica divider 73 separating the bismuth and Ni/Mo blocks of adjacent TEG's. The arrangement of the bismuth, Ni/Mo and dividers is similar to the structure of Figures 3 and 4: Ni/Mo 71a of one TEG 57a connects the copper 58b of the adjacent TEG 57b to the copper 59 of the TEG 57a, whilst bismuth 70a of

SUBSTITUTE SHEET TEG 57a connects the copper 58a to the copper 59a of the one TEG. Thus the TEG's of each power unit 51 are in series.

The mica 72 separating the bismuth and Ni/Mo blocks of a TEG also extends to the insulation layer 62 and separates axially adjacent heat gatherers 58. Thus the mica 72 has a shape in the cross-section of Figure 6 which is equivalent to the projection of heat gatherer 58 and TEG 57. The four power units 51 can be connected in series or in parallel at the discretion of the user, we prefer to connect them in series. A chopper, transformer, and rectifier are also provided, but are not shown. The embodiment of Figures 6 and 7 is so arranged that the temperature drop betwen junctions 73 between the metal blocks 70 and 71 and copper heat gatherer 58 and junctions 74 between the metal blocks and the copper sheath 59 is about half of the temperature difference between the hot oil in the pipe and the sea temperature. This is surprisingly found to give the best power output.

For the embodiment described a guide to the dimensions of the TEG's is as follows: the radial depth of bismuth and Ni/Mo blocks is 36mm, the axial length of the bismuth blocks is 20mm, the axial length of the Ni/Mo blocks is 2mm and the axial length of the mica dividers is 1.5mm.

We found that the embodiment illustrated produced a current of amperes at volts which was fed to the chopper, transformer, and rectifier means to produce a current of amperes at volts per metre length of pipeline. Thus the pipeline produces about 6W/metre from four columns of TEG's as shown. The layer of insulation 62 isolates the steel pipe 52 from the electric circuit (but still allows

SUBSTITUTE SHEET thermal heat flow outwards) , which improves the efficiency of the TEG's.

The heat gatherers 58 and insulation 60 ensure that much of the heat lost by the pipe is conducted to the junctions 73 and so passes through the TEG's. Heat from about one quarter of the pipe passes through each TEG - if the copper 58 were the same circumferential extent as the TEG's 58 we estimate that up to 95% of the heat which reaches the junction 73 would escape directly to the sea and the efficiency of the device would accordingly be about twenty times worse. Thus the heat gatherers are significant. In arrangements where more of the circumference of the pipe is covered by TEG's the need for heat gatherers falls. However, by having TEG's which are relatively small compared with the circumference of the pipe and channelling heat gathered from a wider region through them the temperature at junctions 73 is increased in comparison to what it would otherwise be.

The smooth external surface of the power unit hinders the growth of marine life on the power unit. The unit may have an external coating of Titanium or another growth inhibiting material. A third embodiment of the present invention is shown in Figures 8, 9, 10 and 11. The third embodiment of the invention uses a thermo-electric generator 80 in a module form. The thermo-electric generator 80 is formed in a similar manner to the thermo-electric power unit 57 of the second embodiment shown in Figures 6 and 7. However, the thermo-electric generator modules 80 are provided in separate units, each unit comprising a series of junctions. Each unit is characteristically formed as a block of 52mm by 52mm by 3mm.

The thermo-electric generating modules 80 are

SUBSTITUTE SHEET positioned on a portion 81 of an oil pipeline that is shaped as a polygon of eleven sides. The pipeline cross-section can be seen in Figure 10.

A layer of polytetrafluorethyline (P.T.F.E.) is provided between the thermo-electric generator module 80 and the pipe 81. The layer of P.T.F.E. is typically 0.04mm thick. Another layer of P.T.F.E., also typically 0.04mm thick, is also provided between the thermo-electric generating module 80 and a heat sink 84.

The heat sink 84 is typically a copper U-shaped channel, typically formed of copper sheet 5mm thick. The copper channel acts as a heat sink to the medium surrounding the pipeline. Bolt holes such as the bolt hole 85 shown in

Figure 9 are provided in the copper channel 84. Bolts such as stainless steel bolt 86 are welded to a flat surface of the polygonal pipe 81 and the copper channel 84 is secured to the polygonal pipe 81 by nuts 87 such as seen in Figure 9. Normally a nylon shouldered washer, numbered 88 in Figure 9, is provided in the arrangement.

Three thermo-electric generator modules 80 can be seen in Figure 10 secured to a polygonal tube 81. A plan view of the arrangement can be seen in Figure 11. It will be seen from Figure 11 that two bolts are provided for each section of copper channel to secure the thermo-electric generating module 80 in thermal contact with the pipe 81. Polystyrene in-fills 89 are provided between each section of copper channel to insulate the pipe 81.

In operation of the third embodiment heat flows from the hot fluid within the pipe 81 through the pipe 81 to the thermo-electric generator modules 80 and therethrough to the heat sinks provided by the copper channels 84. The layers of P.T.F.E. film 82

SUBSTITUTE SHEET and 83 electrically insulate the thermo-electric generator modules 80 whilst allowing good heat transfer.

The plurality of thermo-electric generating modules 80 are connected together in series/parallel and then connected to chopper means and transformer means as previously described for the first and second embodiments. The thermo-electric generator modules generate the current as a function of the square of the temperature difference across their thickness.

The applicants have appreciated that it is very beneficial to the efficiency of the third embodiment to provide the coating of silicon grease along all material interfaces. Therefore a layer is provided between the P.T.F.E. film 82 and the surface of the pipe 81 and a layer is further provided between the other surface of the P.T.F.E. film 82 and the thermo-electric generator module 80. Silicon grease is similarly provided in layers between the thermo-electric generator module 80 and the P.T.F.E. film 83 and also the P.T.F.E. film 83 and the U-shaped copper channel 84.

The silicon grease enhances heat transfer since it fills any cavities defined between adjacent surfaces when they are brought together.

In manufacture, the interfaces between the materials are covered with silicon grease and then the two films of P.T.F.E. 83 and 82 and the thermo-electric generator 80 are compressed together by applying a force to the copper channel 80 using nuts 87 co-operating with the bolts 86.

The efficiency of operation is greatly enhanced by providing the layers of silicon grease and by applying a compressive force to urge the layers of P.T.F.E. film the copper channel and thermo-electric generator 80 together and also to urge the lower layer

SUBSTITUTE SHEET of P.T.F.E. film 82 into contact with the surface of the pipe 81. The arrangement of the third embodiment has been found to provide thermo-electric generating unit of greatly enhanced efficiency. The power units described could be used to power position locating signalling devices, to power a subsea completion, or a control mechanism for a subsea completion.

One use we foresee is that the power units and pipelines described could be used to produce power for its own sake - to be fed into the National Grid, or to power a land based facility such as an oil refinery or town.

Oil rigs will eventually have to be carefully dismantled when their oil reservoirs run out. This is a requirement of international law. It has been estimated that the cost of decommissioning the existing oil rigs in the U.K. sector of the North Sea will be around five thousand million pounds. We envisage an alternative use for spent oil rigs and that is as off-shore power stations. Even though the oil may have run out the oil rig will still be a source of hot liquid (the replacement water which is pumped in in order to extract the oil) heated in underground reservoirs by geothermal energy. The present invention can be used to generate electricity from that source (or indeed a functioning oil producing oil pipeline) . At 6W per metre of pipeline it is easy to see that 1000km of pipeline would give 6MW, which is a significant output.

We also envision the embodiment of Figures 6 and 7 and the embodiment of Figures 8, 9, 10 and 11 modified by replacing the large blocks of bismuth and Ni/Mo with semiconductor TEG devices. This could improve the efficiency by a factor of twenty, but would have the drawback of being less robust. If the

SUBSTITUTE SHEET semiconductor devices are connected in parallel this may not be so important.

The power units described may be provided on new pipes as they are laid, or as add-on, or retrofit, units which are fitted to existing pipes.

The power units described may be provided in combination with battery energy storage means. One advantage of this is that an electrical device can then be operated intermittantly at higher power than the TEG's can provide, the TEG's re-charging the batteries between operations.

SUBSTITUTE SHEET

Claims

1. A thermo-electric power unit comprising a thermo-electric generator having first and second junctions of dissimilar materials in a circuit and adapted to be maintained at different temperatures so as to produce a direct current due to the Seebeck effect; chopper means adapted to convert the direct current to an alternating current; transformer means adapted to transform the alternating current from a first to a second level; and output means adapted to ' supply an electric current from the transformer means.

2. A thermo-electric power unit according to Claim 1 in which the chopper means comprises a solid state arrangement of transistors.

3. A thermo-electric power unit according to Claim 1 or Claim 2 in which the output means comprises rectifier means adapted to give a direct current output.

4. A thermo-electric power unit according to any preceding claim in which the dissimilar materials forming the junctions comprise first and second relatively large blocks or plates of dissimilar metals either directly in contact with each other or each in contact with an intermediary material so as to complete an electric circuit.

5. A thermo-electric power unit according to Claim 4 in which the area of contact between the dissimilar metals, or between each metal and the intermediary material, is at least one square centimetre.

6. A thermo-electric power unit according to Claim 4 or Claim 5, in which the area of contact between the dissimilar metals, or between each metal and the intermediary materialise in the range 1 square

BSTITUTE SHEET centimetre to 50 square centimetres.

7. A thermo-electric power unit according to any one of Claims 4 to 6 in which the thickness of at least one of the dissimilar metals measured in a direction normal to the plane of contact between the dissimilar metals, or the dissimilar metal and the intermediary metal, is at least 1mm.

8. A thermo-electric power unit according to any one of Claims 4 to 7 in which the thermo-electric junction comprises two generally parallel blocks or plates of dissimilar metals meeting at an interface and in which the spaced apart opposite surfaces of the blocks or plates are at relatively higher and lower temperatures so that heat flows across the junction generally at right angles to the interface.

9. A thermo-electric power unit according to any one of Claims 4 to 8 in which a thermo-electric junction comprises a generally square area of interface of about 5cm x 5cm between dissimilar metals.

10. A thermo-electric power unit according to any preceding claim which comprises a series of adjacent thermo-electric generators having upper, lower and middle layers, the upper and lower layers being made of one metal and the middle layer of a different metal.

11. A thermo-electric power unit according to Claim 10 in which the upper layer of a thermoelectric junction and the lower layer of an adjacent thermo-electric junction are formed from a single piece of metal.

12. A thermo-electric power unit according to any one of Claims l to 10 in which adjacent thermo-electric generators comprise upper and lower layers of electrically conductive material, and first and second blocks of dissimilar first and second metals extending between the upper and lower layers;

SUBSTITUTE SHEET the block of first metal of one thermo-electric generator extending from the upper layer of that generator to the lower layer of that generator, and the block of second metal of the one generator extending from one of the upper or lower layers of the one generator to the other layer of the adjacent generator, connecting the adjacent generators.

13. A thermo-electric power unit according to Claim 12 in which the relative areas of contact between the first and second metals are such that in use the temperatures at the junctions of the two blocks of dissimilar metals with the upper layer are substantially the same as each other and the temperatures at their junctions with the lower layer are substantially the same as each other.

14. A thermo-electric power unit according to any one of Claims 10 to 13 in which the lower layer is adapted to extend around at least a portion of the circumference of a pipe and has an arcuate inner surface adapted to follow the external surface of the pipe.

15. A thermo-electric power unit according to Claim 14 in which the lower layer extends around in an arc which is a substantial portion of a circle.

16. A thermo-electric power unit according to any one of Claims 10 to 15 in which the lower layer has a substantial depth in the direction of heat flow through the dissimilar metals so as to act as a gatherer of heat from an adjacent hot surface, and channels the heat to the first and second metals.

17. A thermo-electric power unit according to Claim 16 in which the first and second metals contact the lower layer over a smaller area than that over which the lower layer gathers heat.

18. A thermo-electric power unit according to Claim 16 or Claim 17 in which thermal insulation is

SUBSTITUTE SHEET provided between the lower, or inner, and upper, or outer, layers so as to cause heat gathered by the inner layer to pass through the first and second metals in order to reach the outer layer.

19. A thermo-electric power unit according to Claim 10 or Claim 11 which is so constructed that the temperature drop across the middle layer is about half of the temperature difference between a heat source and a heat sink.

20. A thermo-electric power unit according to any one of Claims 12 to 18 in which the temperature difference between opposite ends of the first and second metals is arranged to be about half of the temperature difference between the heat source and the heat sink.

21. A thermo-electric power unit adapted for use with a pipe carrying a medium at a temperature different from the temperature of the medium surrounding the pipe comprising a thermo-electric generator, a heat sink and means for maintaining thermal contact between the heat sink and a first surface of the thermo-electric generator and for maintaining thermal contact between a second surface of the thermo-electric generator and the pipe, at least one of the surfaces of the thermo-electric generator being covered with a substance to enhance the transfer of heat to and from the surface.

22. A thermo-electric power unit as claimed in Claim 21 wherein at least one surface of the thermo-electric generator is covered with a grease which enhances the transfer of heat to or from the surface.

23. A thermo-electric power unit as claimed in Claim 22 wherein the grease is a silicon grease.

24. A thermo-electric power unit as claimed in any one of claims 21, 22 or 23 comprising a first

SUBSTITUTE SHEET layer of electrical insulation between the heat sink and the first surface of the thermo-electric generator and a second layer of electrical insulation between the second surface of thermo-electric generator and the pipe.

25. A thermo-electric power unit as claimed in Claim 24 wherein the first and second layers of electrical insulation are layers of polytetrafluorethylene.

26. A thermo-electric power circuit as claimed in any one of the claims 21 to 25 wherein the means for maintaining thermal contact between the first surface of the thermo-electric generator and the heat sink and for maintaining thermal contact between the second surface of the thermo-electric generator and the pipe comprises a mechanical means to apply a force to the heat sink to urge the heat sink towards the pipe, thereby applying a compressive force on the thermo-electric generator between the heat sink and the pipe.

27. A thermo-electric power unit as claimed in any one of claims 21 to 26 wherein the thermo-electric generator produces a direct current and a chopper means is provided to convert the direct current to an alternating current, a transformer means is provided to transform the alternating current produced by the chopper means from a first to a second level and an output means is provided to supply an electric current from the transformer means.

28. A thermo-electric power unit as claimed in any one of claims 21 to 27 wherein the thermo-electric generator and heat sink are arranged such that the temperature difference between the first and second surfaces of the thermo-electric generator is half of the temperature difference between the medium within the pipe and the medium surrounding the

SUBSTITUTE SHEET pipe.

29. A thermo-electric power unit substantially as herein described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

30. A pipeline incorporating a thermo-electric power unit substantially as herein described with reference to and as shown in Figures 3 to 5 of the accompanying drawings.

31. A pipeline incorporating a thermo-electric power unit substantially as herein described with reference to and as shown in Figures 6 and 7 of the accompanying drawings.

32. A thermo-electric power unit substantially as herein described with reference to and as shown in Figures 8, 9, 10 and 11.

33. A pipeline carrying a hot or cold material and having associated therewith a thermo-electric power unit in accordance with any of the preceding claims.

34. A pipeline according to claim 33 which has signalling means adapted to indicate the position of the pipeline, the signalling means receiving power directly or indirectly from the thermo-electric power unit.

35. An add-on, or retrofit, unit adapted to be secured around an existing pipeline, the retrofit unit incorporating a thermo-electric power unit in accordance with any one of Claims 1 to 29.

36. A retrofit unit according to Claim 35 which includes signalling means adapted to indicate the position of the pipeline, the signalling means receiving power directly or indirectly from the thermo-electric power unit.

SUBSTITUTE SHEET