WO1999044405A1

WO1999044405A1 - Thermoelectric generator for natural gas well head

Info

Publication number: WO1999044405A1
Application number: PCT/US1999/004354
Authority: WO
Inventors: Robert L. Farley; John C. Bass
Original assignee: Hi-Z Technology, Inc.
Priority date: 1998-02-27
Filing date: 1999-02-26
Publication date: 1999-09-02
Also published as: AU2795999A

Abstract

A natural gas well head thermoelectric generator system (28) taking advantage of waste heat produced at dehydration plants available at many natural gas well heads. Waste heat occurs at dehydration plants in the form of waste heat in hot glycol, waste heat from the dehydrator boiler's pilot light and waste heat from boiler stack emissions. The thermoelectric generator system has at least one hot side heat exchanger and at least one cold side heat exchanger. Sandwiched in-between the hot side heat exchanger (40) and the cold side heat exchanger (42) is a thermoelectric module (45) that contains a crate having the form of an eggcrate that defines a plurality of thermoelectric element spaces. Contained within the thermoelectric element spaces are pluralities of p-type and n-type thermoelectric elements. A metallized coating on both the hot and cold side surfaces connect the p-type thermoelectric elements to the n-type thermoelectric elements so that they are connected electrically in series.

Description

THERMOELECTRIC GENERATORFORNATURAL GASWELLHEAD

This invention relates to thermoelectric generators.

BACKGROUND OF THE INVENTION

Natural gas well heads are often located long distances from electric power grids, but electric power in usually needed for monitoring, measuring and communicating equipment. Also, a DC electric source is often needed for cathodic protection of well head equipment from corrosion. If connection to a power grid is not feasible, this electric power is normally provided with either solar cells, or an electric generator driven by a natural gas engine or in some cases by a natural gas fired thermoelectric generator.

High-pressure natural gas normally contains substantial amounts of water when it is extracted from the earth. This water content must be reduced to about 5 pounds per million cubic feet before the gas can be piped through pipelines to a gas processing plant. About 80 percent of all natural gas well heads require a dehydration plant to remove this water.

A typical prior art dehydration plant can be described by reference to FIGS. 20 and 21. This plant utilizes triethylyne glycol (hereinafter called glycol) to remove water from the natural gas. (Glycol is commonly used as an antifreeze agent in motor vehicle engines.) A property of glycol is that it will extract water from the gas as the gas is bubbled up through the glycol. Wet natural gas at a temperature of about 55 degrees F enters the tube side of heat exchanger 11 at inlet 30, where it is heated a few degrees to about 58 degrees F before it enters near the bottom of dehydrator 18. The natural gas is bubbled up through layers of glycol as shown in FIG. 20 and FIG. 21 where the water content in the gas is reduced to acceptable levels and the dry gas exits at 32 and is from there piped to a processing plant through gas pipe lines.

Dry glycol at a temperature of about 120 degrees F enters near the top of the dehydrator through pipe 16. As shown in FIG. 21 the dehydrator provides layers of glycol for the natural gas to bubble through. At each layer an ove.rflow pipe is provided through which glycol flows down to successively lower layers. The glycol becomes wetter and wetter as it flows downward through the successive layers and the natural gas becomes dryer and dryer as it bubbles up through the successive layers. The glycol in addition to becoming wetter is also cooled as it passes through the dehydrator and the temperature of the glycol that pools at the

1 bottom of dehydrator 18 is about 60 degrees F. The cool wet glycol is pumped by pump 15 to and through a preheat heat exchanger 8 (where it is heated to above 100 degrees F) to still column 4. In column 4 the glycol contacts super heated ceramic and/or stainless steel pall rings and much of the water contained in the glycol is flashed into steam. The remaining glycol-water solution falls into reboiler 1 where it is further heated to 375 degrees F by fire tube 6 which receives .an air/gas mixture under pressure from both a 350,000 btu/hr main burner nozzle and a pilot 20,000 btu/hr burner nozzle that are contained in an explosion proof housing 2. At this temperature of 375 degrees F water in the glycol is driven off as steam and the steam is vented through still pipe 4. Glycol is extracted through pipe 9 at about 375 degrees F and at a purity of about 99.9 percent glycol. The dry glycol is then cooled in preheat heat exchanger 8 and heat exchanger 11 to about 120 degrees before it is pumped up to the top of the dehydrator as discussed above.

A pilot light is a standard feature for a dehydrator heating system boiler. The pilot light operates to provide a continuous flame to ignite the main burner, which is operable intermittently on demand when heat is required. The pilot burner has a thermal output of 5.9 kilowatts. This relatively high thermal output is dictated because of some of the special requirements of boiler operation.

The boiler emissions stack of a dehydrator heating system boiler is also a considerable source of high temperature energy. Hot combustion gases leave the dehydrator boiler through the emissions stack. The energy of the gases can be from 66 kilowatt to 293 kilowatt. The temperature of the stack gases can vary over a wide range depending on boiler rating and whether only the pilot is burning or both the pilot and main burner are in operation.

What is needed is a better way to provide electricity at natural gas well head locations.

SUMMARY OF THE INVENTION

The present invention provides a natural gas well head thermoelectric generator system taking advantage of waste heat produced at dehydration plants available at many natural gas well heads. Waste heat occurs at dehydration plants in the form of waste heat in hot glycol, waste heat from the dehydrator boiler's pilot light and waste heat from boiler stack emissions. The thermoelectric generator system has at least one hot side heat exchanger and at least one cold side heat exchanger. Sandwiched in-between the hot side heat exchanger and the cold side heat exchanger is a thermoelectric module that contains a crate having the form of an eggcrate that defines a plurality of thermoelectric element spaces. Contained within the thermoelectric element spaces are pluralities of p-type and n-type thermoelectric elements. A metallized coating on both the hot and cold side surfaces connect the p-type thermoelectric elements to the n-type thermoelectric elements so that some or all of the thermoelectric elements are connected electrically in series.

In one embodiment, hot dry glycol and cold wet glycol are hot source and cold heat sink. This embodiment includes a hot side heat exchanger heated by hot dry glycol and a cold side heat exchanger cooled by cool wet glycol. In another embodiment, thermoelectric energy is produced from waste heat from the dehydrator boiler's pilot light. This embodiment includes a hot side heat exchanger heated by the dehydrator boiler pilot light and a cold side heat exchanger cooled by dehydrator boiler inlet draft air. In another embodiment, thermoelectric energy is produced from waste heat from the dehydrator boiler's emissions stack. This embodiment includes a hot side heat exchanger heated by the hot gases from the emissions stack and a cold side heat exchanger cooled by induced airflow through the cold side heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 A and B are two views of an eggcrate according to the present invention.

FIGS. 2 through 9 and 11 are section views of the above eggcrate.

FIG. 10 shows an end view.

FIG. 11 shows a section of the eggcrate.

FIGS. 12 and 14 show a preferred element arr.angement.

FIG. 13 shows an enlarged view of a portion of a cross section of a module showing parts of the eggcrate, the elements and metal coatings.

FIGS. 15 A and B and 16 A and B show a mold for making the eggcrate.

FIG. 17 shows our injection molding process.

FIG. 18 shows the mold fitted together.

FIG. 19 shows our metallizing process.

FIGS. 20 and 21 are drawings showing the principal equipment in prior art well head dehydration plant. FIG. 22 is an outline drawing of a preferred embodiment thermoelectric generator.

FIG. 23 shows where the thermoelectric generator shown in FIG. 22 fits in the prior art dehydration plant.

FIG. 24 is a detailed drawing of the thermoelectric generator shown in FIG. 22.

FIG. 25 is a section of FIG. 24.

FIG. 26 shows a first embodiment of the pilot light powered thermoelectric generator.

FIG. 27 is a side view of the carbon block sleeve attached to the pilot nozzle tube.

FIG. 28 is an end on view of the carbon block sleeve attached to the pilot nozzle tube.

FIG. 29 shows an end on view of the pilot light powered thermoelectric generator.

FIG. 30 shows the thermoelectric module on top of the heat transfer block.

FIG. 31 shows a top view of the pilot light powered thermoelectric generator.

FIG. 32 shows the installation of the heat shield pan hand support strap.

FIG. 33 shows a modified heat transfer block.

FIG. 34 shows pilot light flame directly in contact with the modified heat transfer block.

FIG. 35 shows a second embodiment of the pilot light powered thermoelectric generator.

FIG. 36 shows a stack generator installed on the emissions stack.

FIG. 37 shows detail of the stack generator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention may be described by reference to the drawings.

HOT AND COLD GLYCOL

In one preferred embodiment of the present invention a specially designed thermoelectric generating unit 28 is inserted as shown in FIG. 23 in the dehydration plant described in FIGS. 20 and 21 and in the background section of this specification. This thermoelectric generator utilizes as its heat source the hot (375 degrees F) dry glycol exiting reboiler 1 through pipe 9 and utilizes as its cold sink the cool (60 degrees F) wet glycol entering preheat heat exchanger 8 through pipe 14. Thermoelectric Generator Unit

A schematic version of generator 28 is shown in FIG. 22 and a detailed version of generator is shown in FIGS. 24 and 25. The unit comprises of one hot side heat exchanger 40, a first cold side heat exchangers 42, connecting hose 43 and a second cold side heat exchanger 44, eight thermoelectric modules 45 electrically connected in series and four spring loaded compression elements 46. The hot glycol enters hot side heat exchanger 40 at A and exits at B as shown in FIG. 22. The cool glycol enters one of the cold side heat exchangers 42 at C, passes through it and connecting hose 43 (shown in FIG. 24) and then passes through the other cold side heat exchanger 44 and exits at D, as shown in FIG. 4.

Heat Exchangers

The hot side heat exchanger 40 is in this embodiment a welded steel structure. The body of the heat exchanger is 14 1/2 inches long and 3 inches wide, it is basically constructed of two identical machined finned sections 40 A and 40 B. These two sections are welded together and nipples 40 C and 40 D are welded at opposite ends of the heat exchanger as shown in FIG. 24 to form a fi ned passage for the hot glycol. Each of the cold side heat exchangers is also a welded steel structure. They each comprise a fined section 42 A and 44 A and a cover plate 42 B and 44 B which are welded together and to nipples 42 C and 44 C to forai a finned passage for the cool glycol.

Compression Elements

Compression elements 46 comprise a steel frame 46 A which is basically a four sided rectangular frame with outside dimensions of 5.64 inches high, 3 3/4 inches wide and 1 3/4 inches thick. The frame provides a rectangular space 3J2 inches wide and 5.02 inches high for the heat exchangers and the thermoelectric modules 45 to fit into. Each frame also comprises a thrust button 46 A 1 and a nut plate 46 A 2 containing a hexagonal nut matching space. Compression elements 46 each provide about 1000 pound compression on the heat exchangers and the thermoelectric modules. This is accomplished with a Belleville spring stack 46 B each of which are centered over two thermoelectric modules 45 each of which in turn are sandwiched between two thin (0.01 inch thick) alumina (Al O₃) wafers 46 C as shown in FIG. 25. The load is provided by tightening adjustment screw 46 D through nut 46 E to compress spring stack 46 B. Torque produced by screw 46 D on nut 46 E is resisted by nut plate 46 A 2. Upward thnist produced by screw 46 D is absorbed on the top side by adjusting nut 46 E and then support frame 46 A to thrust button 46 A 1 located on the opposite side of compression element 46. Downward thrust from screw 46 D is absorbed on the bottom side by load washer 46 H and the spring stack 46 B . Thus, the two thermoelectric modules 45 are held in tight compression between the heat exchangers by opposite forces of about 1000 pounds provided by thrust button 46 A 1 and spring stack 46 B.

PILOT POWER THERMOELECTRIC GENERATOR

In .another preferred embodiment of the present invention, the pilot light contained within the combustion chamber of a dehydrator of a natural gas dehydration plant is used as the heat source for the hot side of the heat exchanger.

First Pilot Light Powered Embodiment

In a first pilot light powered embodiment, thermoelectric module 45 is inserted into explosive proof housing 2 as shown in FIG. 26. A 350,000 btu hr main burner 101 and a 20,000 btu/hr pilot burner 102 are contained within explosion proof housing 2 and serve the primary function of supplying fire tube 6 with heat to further heat the glycol-water solution in reboiler 1 to 375 degrees F. The specific details of operation of main burner 101 and pilot burner 102 are explained thoroughly in United States Patent No. 4,597,733, the disclosure of which is herein incorporated by reference.

In this preferred embodiment, heat is transferred from the hot end of resilient stainless steel pilot nozzle tube 103 to deformable copper block sleeve 104, which is attached perpendicular to pilot nozzle tube 103, as shown in FIG. 27. Copper block sleeve 104 has the following dimensions: thickness = 1 inch, length = 1.875 inches, and width = 2.5 inches. A hole is drilled through the center of copper block sleeve 104 so as to encompass the 1.5-inch outside diameter of pilot nozzle tube 103. Copper block sleeve 104 is machined to a close tolerance, thereby allowing for a close fit with pilot nozzle tube 103 against the inside diameter of copper block sleeve 104. After copper block sleeve 104 is slid over pilot nozzle tube 103, machined set screw 105 can then be tightened to hold the copper block sleeve 104 in place, as shown in FIG. 28. The lower portion of copper block sleeve 104 rests securely against the 6- inch inner fire tube 6, as shown in FIG 26. As pilot nozzle tube 103 is heated, it expands against deformable copper block sleeve 104 allowing for an efficient transfer of heat. Conversely, as the pilot operation is terminated, resilient pilot nozzle tube 103 retracts to a loose fit, thereby allowing for easy adjustment, maintenance or inspection.

Attached to copper block sleeve 104 is heat transfer block 106, as shown in FIG. 28. Heat transfer block 106 is made from a solid copper bar and has the following dimensions: thickness = .375 inch, length = 5.0 inches, and width = 4.250 inches. Heat transfer block 106 is bolted to copper block sleeve 104 with two V* inch x % inch length silicon bronze bolts 108. The underside of heat transfer block 106 is machined to .001 inch flatness to allow close fit with copper block sleeve 104. The top surface of heat transfer block 106 is also machined to .001 inch flatness to allow for thermoelectric module 45 placement. There are four .213 inch holes 118 drilled through heat transfer block 106 that accept ^lA inch x 28 UNF helicoil receiving threads so that four Belleville disc spring compression stack studs 107 can be mounted, as shown in FIGS. 29, 30 and 31.

The cold side of the heat exchanger is aluminum bonded finned heat sink 110 that is available from Wakefield Engineering with offices in Wakefield, MA, as well as other sources. Finned heat sink 110 is split into two-finned heat sink halves as shown in FIG. 29. Each heat sink halve of heat sink 110 is machined from a larger original piece that has the following dimensions: length = 6 inches, width = 4 ³/ inch, .and height = 3 inches. Furthermore, heat sink 110 is machined such as to facilitate placement inside 6-inch fire tube 6, as shown if FIG. 29. A thin (.01 inch thick) alumina (Al₂O₃) wafer 111 is placed between thermoelectric module 45 and finned heat sink 110 to electrically insulate thermoelectric module 45 from electrically conductive heat sink 110. As shown in FIG. 31, finned heat sink 110 has four holes drilled into it to permit the installation of four Belleville disc spring compression stacks 119 over studs 107. Each spring compression stack provides about 1000 pound compression on the thermoelectric sandwich consisting of heat transfer block 106, finned heat sink 110 and thermoelectric module 45.

As shown in FIG. 32 and FIG. 26, a heat shield pan 112 and support strap 113 are installed. Also, thermoelectric module power leads exit explosive proof housing 2 via NEMA type explosive proof conduit fitting 116. During normal operation, pilot nozzle tube 103 will heat up to a temperature of 1200 degrees Fahrenheit at its exterior surface. This produces an equilibrium heat transfer temperature to be maintained in copper block sleeve 104 of 750 degrees Fahrenheit. This heat transfers to heat transfer block 106 so that it maintains a temperature of about 450 degrees Fahrenheit. This heat transfers through thermoelectric module 45. On the cold side of the heat exchanger, draft air as high as 100 cubic feet per minute flows through finned heat sink 110. This allows for a temperature differential between hot and cold side of 360 degrees Fahrenheit. This temperature difference is sufficient to permit thermoelectric module 45 to produce an output of 14 watts and 1.65 volts at matched load. This system can be coupled with a DC/DC boost voltage converter and used to charge a battery and provide over 200 watt-hr per day of energy. When main burner 101 is activated, the air draft is as high as 350 cubic feet per minute. However, this does not effect the performance of thermoelectric module 45 in that heat shield pan 112 is designed to prevent excess cooling of copper block sleeve 104 and heat transfer block 106. Also, when main burner 101 is activated, there is adequate draw through exhaust stack 114 to prevent excess heating of thermoelectric module 45.

Second Pilot Light Powered Embodiment

In a second pilot light powered embodiment, copper block sleeve 104 and heat transfer block 106 (which were bolted together in the first pilot light powered embodiment) are now machined to form one modified heat transfer block 120, as shown in FIG. 33. By making modified heat transfer block 107 one piece, heat loss that occurs at the junction of copper block sleeve 104 and heat transfer block 106 is eliminated.

This second pilot light powered modification is further illustrated in FIGS. 34 and 35. Pilot nozzle tube 103 is cut off just after pilot nozzle 100. Modified heat transfer block 120 is then slip-fitted over the end of shortened pilot nozzle tube 103. Finally, the cut off portion 103 A of pilot nozzle tube 103 is slip-fitted over the opposite end of modified heat transfer block 120. Pilot flame 121 now directly contacts modified heat transfer block 120. This contrasts with the first pilot light powered embodiment in that heat loss is minimized and efficiency increased because heat no longer has to travel through pilot nozzle tube 103 before it gets to the hot side of the heat exchanger. STACK GENERATOR

Boiler emissions stack 200 as shown in FIG. 36 is also a considerable source of available energy to convert into electricity. Stack generator 203 consists of thermoelectric module 45, a finned hot heat exchanger 201 and a finned cold heat exchanger 202, as shown in FIG. 37.

Generator 203 obtains its energy from hot combustion gases leaving boiler 1, which can range from 66 kilowatt to 293 kilowatt depending on boiler 1 rating and whether only the pilot light is burning or if both the pilot light and the main burner are in operation. To handle this wide range in gas temperature, thermostat-actuated gas diverting valve 204 is mounted upstream of hot heat exchanger 201 to divert combustion gases into and through finned hot heat exchanger 201 when only the pilot burner is operating and around hot heat exchanger 201 when both burners are in operation. Valve 204 is controlled by pneumatic cylinder 205.

Generator 203 uses induced airflow to cool finned cold heat exchanger 202. Air duct 206 is connected between finned cold heat exchanger 202 and venturi section 207. The lower pressure in stack 200 at the throat of venturi section 207 provides adequate suction of the airflow from finned cold heat exchanger 202 to cool cold heat exchanger 202 without consuming any energy. Stack generator 203 is capable of producing 28 watts of power at 3.3 volts.

THERMOELECTRIC MODULES

Following is a description to a preferred process for fabricating an eggcrate type thermoelectric modules 45 for use in the thermoelectric generator discussed above.

Injection Molded Eggcrate

The eggcrate for this preferred embodiment is injection molded using the mold pattern shown in FIGS. 15 A and B and 16 A and B. FIGS. 15 A and B show the bottom 20 of the mold pattern and FIGS 16 A and B show the top 22 of the pattern. The top and bottom are shown in their molding position in FIG. 18. A high temperature thermo plastic, such as the liquid crystal polymer resin, Dupont Zenite, is injected through sprue 24 using well known plastic molding techniques in an injection molding machine 26 as depicted in FIG. 17. The Dupont Zenite plastic is dried at 275 F and the barrel temperatures of the molding machine range from 625 F at the rear to 640 F near the nozzle. Both the bottom mold and the top mold are maintained at a temperature of about 200 F. Zenite melts at about 550 °F. In the usual manner the fluid plastic passes through sprue 24, runner 28, and gate 30 into the mold cavity. The vent is shown at 34 in FIG. 18. The finished part is ejected by injection pins 32 as shown in FIG. 17. Initial production runs made by applicant's supplier have produced excellent eggcrates at a rate of about 50 eggcrates per hour. This rate can easily be increased to 200 eggcrates per hour for one worker and ultimately the process can be completely automated. This compares to a one-worker production rate of about 3 eggcrates per hour with the prior art method of assembling thermoelectric module eggcrates from appropriately slotted layers of insulating materials.

The completed injection molded eggcrate is shown in FIG. 1A. This embodiment contains boxes (spaces) for 100 thermoelectric elements. The dimensions of the elements are 5J mm X 5J mm X 3.0 mm. The dimensions of the spaces at the bottom of the eggcrate are 5J mm X 5J mm. A top view of the eggcrate is shown in FIGJ. FIGS 2 through 9 show various sections through the eggcrate. FIG. 10 is a side view and FIG. 11 is a sectional view that shows an expanded view of one of the boxes created by the eggcrate. Note that the upper part of the walls of the box is tapered 5 degrees as shown at Y in FIG. 11. In this embodiment the straight part of the walls of the box forms a 0.2 inch square as shown at X in FIG. 11. This dimension is held to a tolerance of plus 0.001 inch to provide a tight fit for thermoelectric elements that are 0.200 inch square with a tolerance of minus 0.001. Note that a support ridge 62 as shown in FIGS. 11 and 12 is provided around the boundary of the eggcrate at the midplane between the two surface planes of the eggcrate . This support ridge provides extra strength for the eggcrate and is utilized during subsequent stages of module fabrication and can be useful in mounting the completed module for use.

FIG. 12 shows a top view of the eggcrate with the locations indicated for the P and N elements. The elements are placed in these locations with the installer assuring that each element rest firmly against stops 10 as shown in FIGS. IB and 11. Conductor material is then sprayed on the top and bottom of the eggcrate as shown in FIGS 19A and 19B and then the conductor material at the tops and bottoms is ground down until the tops of all insulator

10 surfaces are cleared of conductor material. A preferred procedure for loading the eggcrate is discussed in detail below. FIGS 13 and 14 show examples of sections of the finished product at location 13-13 and 14-14 as shown in FIG. 12. Note in FIG. 14 how the effect is to connect all the thermoelectric elements in series electrically. In this particular section the hot surface is on the top and the electron flow is from left to right.

Thermoelectric Elements

Thermoelectric elements with dimensions of 5J mm X 5J mm X 3.0 mm are prepared using any of several well known techniques such as those described in Thermoelectric Materials edited by Sittig, published in 1970 by Noyes Data Corporation, Park Ridge, New Jersey. Preferred materials are Lead Telluride for high temperature applications and Bismuth Telluride for low temperature applications. These elements may also be purchased commercially from suppliers such as Melcor Corporation with offices in Treton, New Jersey. One half of the elements should be "n" elements and one half "p" elements.

Loading the Eggcrates

The "p" elements are positioned in the appropriate boxes of the egg create as shown in FIG. 12. The element should be snug against the stop. The "n" elements are also positioned in the appropriate boxes of the egg crate as shown in FIG. 12. Each element should be snug against the stop. A 2 inch long 1/8 inch wide copper mesh wire lead is inserted at positions 61 and 63 as shown in FIG. 12. At the location of the junction of the leads to the module we provide two "p" elements and two "n" elements side by side and electrically in parallel for extra support for the leads to reduce the likelihood that the leads would break loose.

Metallizing the Hot and Cold Surfaces

Using spring-loaded clamps, we clamp a number of modules to a rotatable mandrel. In FIGS. 19 A and B we show 20 modules clamped to such a mandrel. We then grit blast the module/element surface with 180-240 grit A12O3 to a uniform matte finish with the mandrel rotating at 55 rpm. Then we use compressed air to blow the module/element surface clean. Next we apply a metal thermal spray coating to the exposed surface using a thermal spray coating system as shown in FIGS. 19 A and B. These spray techniques are well known. Further specific details are provided in Metals Handbook, Ninth Edition, published by the American Society for Metals. A variety of metals can be used to coat the surface. Our

11 preferred coating is a two-layer coating comprising a first 0.006 inch thick coating of molybdenum and a second 0.06 inch thick coating of aluminum. Both coatings are applied using the system shown in FIGS. 19 A and B with the mandrel rotating at 55 rpm and the spray gun running back and forth at speeds of about 0.2 inch per second. After the first surface is coated we remount the modules to expose the unsprayed surface and repeat the above-described process with the second coating.

Grind the Module Surfaces

The surfaces must be ground down to expose the eggcrate walls. To do this we position a sprayed module in the mounting chuck of a surface finishing machine. We reduce the surface of the module to the appropriate height as measured from eggcrate tab 62 shown in FIGS. 11 and 12. We then remove the module from the chuck, reverse the module and reduce the opposite face of the module until the module surface is the appropriate height from the egg crate tab.

Inspection

We heat the hot surface of the module to 250 C and cool the cold side of the module to 50 C. We then measure the open circuit voltage of the module. It should be about 3.2 volts with bismuth telluride elements. We then apply an electrical load to the module until the voltage drops to 1.6 volts and measure the current. We calculate the power produced by the module as P=I X V. The power level should be at least 13 watts for the bismuth telluride elements.

Performance of the Unit

The total output of the eight thermoelectric modules connected in series, with a hot side temperature of 375 degrees F and a cold side temperature of about 65 degrees F, will be about 62 watts at about 12 volts. Additional power and higher voltages can be obtained by adding additional thermoelectric generator units. As indicated only a very small percentage of available waste energy of the dehydration plant was utilized in the above-described arrangement.

FIG. 26 shows curves of power output as a function of glycol flow rate for several hot glycol temperatures. For flow rates greater than 10 gpm, it is feasible to connect more than one unit in series to provide increased power. For flow rates above 30 gpm generators should

12 preferably be connected in parallel. For flow rates outside these ranges a redesign of the unit may be preferably making it longer or wider.

Engineers installing the generator in the field may elect to incorporate bypass lines and valves to allow them to bypass the generator. This would allow the generator to be removed from service if not needed or for repair. A flow control valve could be provided if desired although in most cases it would not be needed. If the generator is to be used to provide cathodic protection, the generator would normally be connected to a constant current regulator which will automatically vary the system impedance to match system requirements. If the generator is to be used to provide power for lighting, instruments or communication, the generator would normally be connected to a constant voltage shunt regulator. A battery is not required to make the system operate. However, if the generator is to be used to provide power for a system, such as communication, where the short term peak power requirements is higher than the normal power output from the generator, then a battery and a battery regulator would be included to carry the short term high peak loads. All of this extra equipment can be obtained "off the shelf.

The foregoing description of the present invention has been presented for the purpose of illustration and is not intended to limit the invention to the precise form disclosed. It is understood that many modifications and changes may be effected by those skilled in the art. For example, thermoelectric modules other than the one described could be utilized in which case details of the heat exchanger and compression elements would probably need to be modified appropriately. It is feasible to make modules with many more thermoelectric elements in which case a single thermoelectric module may be sufficient to provide the required voltage. Other well known methods of holding the thermoelectric elements in good thermal contact with the heat exchangers could be used. There may be advantages of tapping into hot and cold glycol lines at locations other than those described. The heat exchangers were described as welded carbon steel units. These heat exchangers could be made with other well-known techniques utilizing teachings of this specification. When sales are high enough to justify it, Applicants plan to manufacture the heat exchangers using aluminum castings. This should greatly reduce the cost. As to the thermoelectric modules, many other materials besides Zenite can be used for injection molded eggcrates. These include Xydar

13 (manufactured by Amaco, which is substantially equivalent to Zenite), polyethylene, silicones, teflons, and many others. Zenite was primarily selected because of its superior properties (i.e., melting point, thermal stability, etc.) at higher temperatures. Also it should be possible to use a ceramic material in the form of a "slip". (This is the term used for describing a fine ceramic material suspended in a liquid.) After molding, the liquid is removed by drying and /or the mold (typically plaster of paris) absorbing the liquid. The components are then sintered to give them strength. Zenite, in fact, contains a fine glass powder filler to reduce material costs and control other material properties. This filler could be some other material such as carbon or chopped fibers made from fiberglass, graphite fibers, etc. Other moldable materials that could be used are organic precursors that transform from the organic to the inorganic state when heated. Materials of this nature would be very desirable for higher temperature eggcrates that would be used with high temperature thermoelectric materials such as PbTe and SiGe which operate at temperatures greater than 350 °C which is typically an upper limit on most organic materials. These materials would allow the eggcrate to be loaded to higher values at temperatures where organic materials typically lose their strength. Phosphate and silicate pastes and cements might also be used for the eggcrate material for high temperature applications. These materials could be formed into eqqcrates using silk-screening techniques used in the electronics industry. Instead of dumping the heat from the thermoelectric modules into the cool wet glyclo, the heat could be dumped into any other cool sink at the well head site, such as cool natural gas. Alternatively, the sink could be another fluid cooled by the natural gas. Accordingly it is intended by the appended claims to cover all modifications and changes as fall within the true spirit and scope of the invention.

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Claims

We claim:

1. A natural gas well head thermoelectric generator system comprising:

A) a natural gas dehydration plant comprising a glycol based dehydration system in which water is separated from water laden natural gas by contacting the water laden natural gas with relatively dry glycol during which process glycol is cooled to produce cool wet glycol and by heating of the glycol to remove water absorbed from the gas by the glycol thus producing hot dry glycol,

B) a thermoelectric generator incorporated in said dehydration plant for generating electric power, said generator comprising:

1) at least one hot side heat exchanger,

2) at least one cold side heat exchanger,

3) at least one thermoelectric module defining a cold surface and a hot surface sandwiched between said at least one hot side heat exchanger and said at least one cold side heat exchanger said at least one thermoelectric module comprising: a) a crate having the form of .an eggcrate defining a plurality of thermoelectric element spaces, b) a plurality of p-type thermoelectric elements, c) a plurality of n-type thermoelectric elements, said p-type and said n-type thermoelectric elements being positioned in said thermoelectric element spaces, d) a metallized coating on said cold surface connecting p-type thermoelectric elements to n-type thermoelectric elements on said cold surface, e) a metallized coating on said hot surface connecting p-type thermoelectric elements to n-type thermoelectric elements on said hot surface, wherein the position of said p-type and said n-type elements, the configuration of said eggcrate and said metallized coatings being effective to cause a plurality of said thermoelectric elements to be electrically connected in series.

2. A natural gas well head thermoelectric generating system as in Claim 1, wherein said at least one hot side heat exchanger is heated by hot dry glycol.

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3. A natural gas well head thermoelectric generating system as in Claim 1, wherein said cold side heat exchanger is cooled by cool wet glycol.

4. A natural gas well head thermoelectric generating system as in Claim 1, wherein said cold side heat exchanger is cooled by cool natural gas.

5. A natural gas well head thermoelectric generating system as in Claim 1 and further comprising a compression means for maintaining close contact between said module and said heat exchangers.

6. A natural gas well head thermoelectric generating system as in Claim 5 wherein said compression means comprises at least one Belleville spring stack.

7. A well head thermoelectric generating system as in Claim 1 wherein said eggcrate is injection molded.

8. A well head thermoelectric generating system as in Claim 7 wherein said injection molded crate is comprised of high temperature plastic.

9. A well head thermoelectric generating system as in Claim 8 wherein said high temperature plastic is a liquid crystal plastic.

10. A well head thermoelectric generating system as in Claim 8 wherein said high temperature liquid crystal plastic is Dupont Zenite.

11. A well head thermoelectric generating system as in Claim 8 wherein said high temperature plastic is a silicone plastic.

12. A well head thermoelectric generating system as in Claim 7 wherein said injection molded eggcrate is comprised of an electrically insulating ceramic material.

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13. A well head thermoelectric generating system as in Claim 1 wherein said metallized coatings comprise a layer of molybdenum and a layer of aluminum.

14. A well head thermoelectric generating system as in Claim 1 wherein said metallized coatings comprise a layer of a nickel-aluminum alloy.

15. A well head thermoelectric generating system as in Claim 1, wherein said hot side heat exchanger is heated by a dehydrator boiler pilot light burner, wherein said cold side heat exchanger is cooled by boiler draft air.

16. A natural gas well head thermoelectric generating system as in Claim 15 and further comprising a compression means for maintaining close contact between said module and said heat exchangers.

17. A natural gas well head thermoelectric generating system as in Claim 16 wherein said compression means comprises at least one Belleville spring stack.

18. A natural gas well head thermoelectric generating system as in Claim 1 wherein said at least one hot side heat exchanger is heated by heat from a dehydrator boiler emissions stack, wherein said at least one cold side heat exchanger is cooled by an induced airflow means.

19. A natural gas well head thermoelectric generating system as in Claim 18 and further comprising a compression means for maintaining close contact between said module and said heat exchangers.

20. A natural gas well head thermoelectric generating system as in Claim 19 wherein said compression means comprises at least one Belleville spring stack.

21. A natural gas well head thermoelectric generating system as in Claim 18 wherein said induced airflow means comprises an air duct leading from said cold side heat exchanger to a venturi section located within said emissions stack.

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22. A natural gas well head thermoelectric generating system as in Claim 18 wherein said heat from a dehydrator boiler emissions stack is directed through said hot side heat exchanger by a thermostat-actuated gas diverging valve.

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