# Patent

## Title
Heater and method of operating

## Abstract

A heater includes a heater housing with a fuel cell stack assembly disposed therein. The fuel cell stack assembly includes a plurality of fuel cells which convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. The fuel cell stack assembly includes a fuel cell manifold for receiving the fuel within a fuel inlet and the oxidizing agent within an oxidizing agent inlet and distributing the fuel and oxidizing agent to the fuel cells. A fuel supply conduit supplies the fuel to the fuel inlet and an oxidizing agent supply conduit supplies the oxidizing agent to the oxidizing agent inlet. A sonic orifice is disposed between the fuel supply conduit and the fuel inlet or between the oxidizing agent supply conduit and the oxidizing agent inlet, thereby limiting the velocity of the fuel or the oxidizing agent through the sonic orifice.

## Background
TECHNICAL FIELD OF INVENTION
The present invention relates to a heater which uses fuel cell stack assemblies as a source of heat; more particularly to such a heater which is positioned within a bore hole of an oil containing geological formation in order to liberate oil therefrom; and even more particularly to such a heater which includes a sonic orifice which limits the velocity of fuel or oxidizing agent supplied to the fuel cell stack assemblies.
BACKGROUND OF INVENTION
Subterranean heaters have been used to heat subterranean geological formations in oil production, remediation of contaminated soils, accelerating digestion of landfills, thawing of permafrost, gasification of coal, as well as other uses. Some examples of subterranean heater arrangements include placing and operating electrical resistance heaters, microwave electrodes, gas-fired heaters or catalytic heaters in a bore hole of the formation to be heated. Other examples of subterranean heater arrangements include circulating hot gases or liquids through the formation to be heated, whereby the hot gases or liquids have been heated by a burner located on the surface of the earth. While these examples may be effective for heating the subterranean geological formation, they may be energy intensive to operate.
U.S. Pat. Nos. 6,684,948 and 7,182,132 propose subterranean heaters which use fuel cells as a more energy efficient source of heat. The fuel cells are disposed in a heater housing which is positioned within the bore hole of the formation to be heated. The fuel cells convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. U.S. Pat. Nos. 6,684,948 and 7,182,132 illustrate strings of fuel cells that may be several hundred feet in length. Operation of the fuel cells requires fuel and air to be supplied to each of the fuel cells and spent fuel (anode exhaust) and spent air (cathode exhaust) must be exhausted from each of the fuel cells. In order to do this, a fuel supply conduit and an air supply conduit are provided such that each extends the entire length of the string of fuel cells to supply fuel and air to each of the fuel cells. Homogeneous distribution of fuel and air to each of the fuel cells may be problematic due to the length of the heaters which may be hundreds of feet long to in excess of one thousand feet, thereby resulting in pressure differentials from fuel cell to fuel cell along the length of the heater. The pressure differentials may result in variations in fuel and air flow to the fuel cells which may not be compatible with the desired operation of the heater.
What is needed is a heater which minimizes or eliminates one of more of the shortcomings as set forth above.
SUMMARY OF THE INVENTION
A heater includes a heater housing extending along a heater axis. A fuel cell stack assembly is disposed within the heater housing and includes a plurality of fuel cells which convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. The fuel cell stack assembly includes a fuel cell manifold for 1) receiving the fuel within a fuel inlet of the fuel cell manifold and distributing the fuel to the plurality of fuel cells and 2) receiving the oxidizing agent within an oxidizing agent inlet of the fuel cell manifold and distributing the oxidizing agent to the plurality of fuel cells. A fuel supply conduit is provided in fluid communication with the fuel cell manifold for communicating the fuel to the fuel inlet of the fuel cell manifold and an oxidizing agent supply conduit is provided in fluid communication with the fuel cell manifold for communicating the oxidizing agent to the oxidizing agent inlet of the fuel cell manifold. A sonic orifice is disposed between the fuel supply conduit and the fuel inlet or between the oxidizing agent supply conduit and the oxidizing agent inlet and adapted to achieve a critical pressure ratio, thereby limiting the velocity of the fuel or the oxidizing agent through the sonic orifice.



BRIEF DESCRIPTION OF DRAWINGS
This invention will be further described with reference to the accompanying drawings in which:
FIG. 1 is an isometric partial cross-sectional view of a heater in accordance with the present invention;
FIG. 2 is view of a plurality of heaters of FIG. 1 shown in a bore hole of a geological formation;
FIG. 3 is an end view of the heater of FIG. 1;
FIG. 4 is an axial cross-sectional view of the heater of FIGS. 1 and 3 taken through section line 4-4;
FIG. 5 is an axial cross-sectional view of the heater of FIGS. 1 and 3 taken through section line 5-5;
FIG. 6 is an axial cross-sectional view of a fuel cell stack assembly of the heater of FIGS. 1 and 3 taken through section line 6-6;
FIG. 7 is an elevation view of a fuel cell of the fuel cell stack assembly of FIG. 6;
FIG. 8 is an enlargement of a portion of FIG. 7;
FIG. 9 is an enlargement of a portion of FIG. 8;
FIG. 10 is an isometric view of a flow director of a combustor of the heater of FIG. 1;
FIG. 11 is a radial cross-section view the heater of FIG. 1 taken through section line 11-11;
FIG. 12 is an isometric view of a baffle of the heater of FIG. 1;
FIG. 13 is an enlargement of a portion of FIG. 4 showing adjacent fuel cell assemblies;
FIG. 14 is an enlargement of a portion of FIG. 5 showing adjacent fuel cell assemblies;
FIG. 15 is an enlargement of a portion of FIG. 13;
FIG. 16 is an enlargement of a portion of FIG. 14; and
FIG. 17 is an alternative arrangement of FIG. 14.



DETAILED DESCRIPTION OF INVENTION
Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, a heater 10 extending along a heater axis 12 is shown in accordance with the present invention. A plurality of heaters 101, 102, . . . 10n-1, 10n, where n is the total number of heaters 10, may be connected together end to end within a bore hole 14 of a formation 16, for example, an oil containing geological formation, as shown in FIG. 2. Bore hole 14 may be only a few feet deep; however, may typically be several hundred feet deep to in excess of one thousand feet deep. Consequently, the number of heaters 10 needed may range from 1 to several hundred. It should be noted that the oil containing geological formation may begin as deep as one thousand feet below the surface and consequently, heater 101 may be located sufficiently deep within bore hole 14 to be positioned near the beginning of the oil containing geological formation. When this is the case, units without active heating components may be positioned from the surface to heater 101 in order to provide plumbing, power leads, and instrumentation leads to support and supply fuel and air to heaters 101 to 10n, as will be discussed later.
Heater 10 generally includes a heater housing 18 extending along heater axis 12, a plurality of fuel cell stack assemblies 20 located within said heater housing 18 such that each fuel cell stack assembly 20 is spaced axially apart from each other fuel cell stack assembly 20, a first fuel supply conduit 22 and a second fuel supply conduit 24 for supplying fuel to fuel cell stack assemblies 20, a first oxidizing agent supply conduit 26 and a second oxidizing agent supply conduit 28; hereinafter referred to as first air supply conduit 26 and second air supply conduit 28; for supplying an oxidizing agent, for example air, to fuel cell stack assemblies 20, and a plurality of combustors 30 for combusting exhaust constituents produced by fuel cell stack assemblies 20. While heater 10 is illustrated with 3 fuel cell stack assemblies 20 within heater housing 18, it should be understood that a lesser number or a greater number of fuel cell stack assemblies 20 may be included. The number of fuel cell stack assemblies 20 within heater housing 18 may be determined, for example only, by one or more of the following considerations: the length of heater housing 18, the heat output capacity of each fuel cell stack assembly 20, the desired density of fuel cell stack assemblies 20 (i.e. the number of fuel cell stack assemblies 20 per unit of length), and the desired heat output of heater 10. The number of heaters 10 within bore hole 14 may be determined, for example only, by one or more of the following considerations: the depth of formation 16 which is desired to be heated, the location of oil within formation 16, and the length of each heater 10.
Heater housing 18 may be substantially cylindrical and hollow. Heater housing 18 may support fuel cell stack assemblies 20 within heater housing 18 as will be described in greater detail later. Heater housing 18 of heater 10x, where x is from 1 to n where n is the number of heaters 10 within bore hole 14, may support heaters 10x+1 to 10n by heaters 10x+1 to 10n hanging from heater 10x. Consequently, heater housing 18 may be made of a material that is substantially strong to accommodate the weight of fuel cell stack assemblies 20 and heaters 10x+1 to 10n. The material of heater housing 18 may also have properties to withstand the elevated temperatures, for example 600° C. to 900° C., as a result of the operation of fuel cell stack assemblies 20 and combustors 30. For example only, heater housing 18 may be made of a 300 series stainless steel with a wall thickness of 3/16 of an inch.
With continued reference to all of the Figs. but now with emphasis on FIGS. 6 and 7, fuel cell stack assemblies 20 may be, for example only, solid oxide fuel cells which generally include a fuel cell manifold 32, a plurality of fuel cell cassettes 34 (for clarity, only select fuel cell cassettes 34 have been labeled), and a fuel cell end cap 36. Fuel cell cassettes 34 are stacked together between fuel cell manifold 32 and fuel cell end cap 36 and are held therebetween in compression with tie rods 38. Each fuel cell stack assembly 20 may include, for example only, 20 to 50 fuel cell cassettes 34.
Each fuel cell cassette 34 includes a fuel cell 40 having an anode 42 and a cathode 44 separated by a ceramic electrolyte 46. Each fuel cell 40 converts chemical energy from a fuel supplied to anode 42 into heat and electricity through a chemical reaction with air supplied to cathode 44. Further features of fuel cell cassettes 34 and fuel cells 40 are disclosed in United States Patent Application Publication No. US 2012/0094201 to Haltiner, Jr. et al. which is incorporated herein by reference in its entirety.
Fuel cell manifold 32 receives fuel, e.g. a hydrogen rich reformate which may be supplied from a fuel reformer 48, through a fuel inlet 50 from one or both of first fuel supply conduit 22 and second fuel supply conduit 24 and distributes the fuel to each of the fuel cell cassettes 34. Fuel cell manifold 32 also receives an oxidizing agent, for example, air from an air supply 54, through an air inlet 52 from one or both of first air supply conduit 26 and second air supply conduit 28. Fuel cell manifold 32 also receives anode exhaust, i.e. spent fuel and excess fuel from fuel cells 40 which may comprise H2, CO, H2O, CO2, and N2, and discharges the anode exhaust from fuel cell manifold 32 through an anode exhaust outlet 56 which is in fluid communication with a respective combustor 30. Similarly, fuel cell manifold 32 also receives cathode exhaust, i.e. spent air and excess air from fuel cells 40 which may comprise O2 (depleted compared to the air supplied through first air supply conduit 26 and second air supply conduit 28) and N2, and discharges the cathode exhaust from fuel cell manifold 32 through a cathode exhaust outlet 58 which is in fluid communication with a respective combustor 30.
With continued reference to all of the Figs. but now with emphasis on FIGS. 6, 8, and 9; combustor 30 may include an anode exhaust chamber 60 which receives anode exhaust from anode exhaust outlet 56 of fuel cell manifold 32, a cathode exhaust chamber 62 which receives cathode exhaust from cathode exhaust outlet 58 of fuel cell manifold 32, and a combustion chamber 64 which receives anode exhaust from anode exhaust chamber 60 and also receives cathode exhaust from cathode exhaust chamber 62. Anode exhaust chamber 60 may be substantially cylindrical and connected to anode exhaust outlet 56 through an anode exhaust passage 66 which is coaxial with anode exhaust chamber 60. Anode exhaust chamber 60 includes a plurality of anode exhaust mixing passages 68 which extend radially outward therefrom into combustion chamber 64. Cathode exhaust chamber 62 may be substantially annular in shape and radially surrounding anode exhaust passage 66 in a coaxial relationship. Cathode exhaust chamber 62 includes a plurality of cathode exhaust mixing passages 70 extending axially therefrom into combustion chamber 64. Cathode exhaust mixing passages 70 are located proximal to anode exhaust mixing passages 68 in order to allow anode exhaust gas exiting anode exhaust chamber 60 to impinge and mix with cathode exhaust exiting cathode exhaust chamber 62. Combustion of the mixture of anode exhaust and cathode exhaust may occur naturally due to the temperature within combustion chamber 64 being equal to or greater than the autoignition temperature of the mixture of anode exhaust and cathode exhaust due to the operation of fuel cell stack assemblies 20 or the operation of a plurality of electric resistive heating elements (not shown) that may be used to begin operation of fuel cell stack assemblies 20. In this way, anode exhaust is mixed with cathode exhaust within combustion chamber 64 and combusted therein to form a heated combustor exhaust comprising CO2, N2, O2, and H2O. Combustor 30 includes a combustor exhaust outlet 72 at the end of combustion chamber 64 for communicating the heated combustor exhaust from the combustor 30 to the interior volume of heater housing 18 thereby heating heater housing 18 and subsequently formation 16. Using combustor 30 to generate heat for heating formation 16 allows fuel cell stack assemblies 20 to be operated is such a way that promotes long service life of fuel cell stack assemblies 20 while allowing heaters 10 to generate the necessary heat for heating formation 16.
With continued reference to all of the Figs. and now with emphasis on FIGS. 6, 10, 11, and 12; each combustor 30 may include a flow director 74 and heater 10 may include a baffle 76 positioned radially between fuel cell stack assemblies 20/combustors 30 and heater housing 18 in order increase the effectiveness of transferring heat from the heated combustor exhaust to heater housing 18 and subsequently to formation 16. Baffle 76 is substantially cylindrical and coaxial with heater housing 18, thereby defining a heat transfer channel 78, which may be substantially annular in shape, radially between heater housing 18 and baffle 76. As shown most clearly in FIG. 12, baffle 76 may be made of multiple baffle panels 80 (for clarity, only select baffle panels 80 have been labeled) in order to ease assembly of heater 10. Baffle panels 80 may be loosely joined together in order to prevent a pressure differential between heat transfer channel 78 and the volume that is radially inward of baffle 76. Baffle 76 includes a plurality of baffle apertures 82 (for clarity, only select baffle apertures 82 have been labeled) extending radially through baffle 76 to provide fluid communication from flow director 74 to heat transfer channel 78.
Flow director 74 includes a central portion 84 which is connected to combustor exhaust outlet 72 and receives the heated combustor exhaust therefrom. Flow director 74 also includes flow director outlets 86 which extend radially outward from central portion 84. Each flow director outlet 86 communicates with a respective baffle aperture 82 to communicate heated combustor exhaust to heat transfer channel 78. After being communicated to heat transfer channel 78, the heated combustor exhaust may pass upward through each heater 10 until reaching the top of bore hole 14. Each flow director outlet 86 defines a flow director cleft 88 with an adjacent flow director outlet 86. Flow director clefts 88 allow various elements, e.g. first fuel supply conduit 22, second fuel supply conduit 24, first air supply conduit 26, second air supply conduit 28, and electrical conductors, to extend axially uninterrupted through heater housing 18. Flow director 74 may be made of a material that has good oxidation resistance, for example, stainless steel or ceramic coated metal due to the high temperatures and corrosive conditions flow director 74 may experience in use. In addition to flow director 74 and baffle 76 providing the benefit of placing the heated combustor exhaust where heat can be most effectively be transferred to formation 16, flow director 74 and baffle 76 provide the benefit of segregating fuel cell stack assemblies 20 from the heated combustor exhaust because fuel cell stack assemblies 20 may be sensitive to the temperature of the heated combustor exhaust. In order to further thermally isolate fuel cell stack assemblies 20 from the heated combustor exhaust, baffle 76 may be made of a thermally insulative material or have a thermally isolative layer to inhibit transfer of thermal energy from heat transfer channel 78 to fuel cell stack assemblies 20.
With continued reference to all of the Figs. but now with emphasis on FIGS. 4, 5, 13, 14, 15, and 16; in addition to first fuel supply conduit 22, second fuel supply conduit 24, first air supply conduit 26, and second air supply conduit 28 supplying fuel and air to fuel cell stack assemblies 20, first fuel supply conduit 22, second fuel supply conduit 24, first air supply conduit 26, and second air supply conduit 28 also provide structural support to fuel cell stack assemblies 20 within heater 10. The lower end of heater housing 18 includes a support plate 90 therein. Support plate 90 is of sufficient strength and securely fastened to heater housing 18 in order support the weight of fuel cell stack assemblies 20, combustors 30 first fuel supply conduit 22, second fuel supply conduit 24, first air supply conduit 26, second air supply conduit 28 and baffle 76 that are located within heater 10. Support plate 90 is arranged to allow the heated combustor exhaust from lower heaters 10 to rise through each heater housing 18, much like a chimney, ultimately allowing the heated combustor exhaust to pass to the surface of formation 16.
First fuel supply conduit 22 and second fuel supply conduits 24 are comprised of first fuel supply conduit sections 22S and second fuel supply conduit sections 24S respectively which are positioned between support plate 90 and the lowermost fuel cell stack assembly 20 within heater 10, between adjacent fuel cell stack assemblies 20 within a heater 10, and between the uppermost fuel cell stack assembly 20 within a heater 10 and support plate 90 of the next adjacent heater 10. Similarly, first air supply conduit 26 and second air supply conduits 28 are comprised of first air supply conduit sections 26S and second air supply conduit sections 28S respectively which are positioned between support plate 90 and the lowermost fuel cell stack assembly 20 within heater 10, between adjacent fuel cell stack assemblies 20 within a heater 10, and between the uppermost fuel cell stack assembly 20 within a heater 10 and support plate 90 of the next adjacent heater 10.
Each fuel cell manifold 32 includes a first fuel supply boss 92 and a second fuel supply boss 94. First fuel supply boss 92 and second fuel supply boss 94 extend radially outward from fuel cell manifold 32 and include an upper fuel supply recesses 100 and a lower fuel supply recess 102 which extend axially thereinto from opposite sides for receiving an end of one first fuel supply conduit section 22S or one second fuel supply conduit section 24S in a sealing manner. Upper fuel supply recess 100 and lower fuel supply recess 102 of each first fuel supply boss 92 and second fuel supply boss 94 are fluidly connected by a fuel supply through passage 104 which extends axially between upper fuel supply recess 100 and lower fuel supply recess 102. An upper fuel supply shoulder 106 is defined at the bottom of upper fuel supply recess 100 while a lower fuel supply shoulder 108 is defined at the bottom of upper fuel supply recess 100. In this way, first fuel supply conduit sections 22S form a support column with first fuel supply bosses 92, thereby supporting fuel cell stack assemblies 20 and combustors 30 on support plate 90 within heater housing 18. Similarly, second fuel supply conduit sections 24S, form a support column with second fuel supply bosses 94, thereby supporting fuel cell stack assemblies 20 and combustors 30 on support plate 90 within heater housing 18. First fuel supply conduit sections 22S and second fuel supply conduit sections 24S may be made of a material that is substantially strong to accommodate the weight of fuel cell stack assemblies 20 and combustors 30 within heater 10. The material of first fuel supply conduit sections 22S and second fuel supply conduit sections 24S may also have properties to withstand the elevated temperatures within heater housing 18 as a result of the operation of fuel cell stack assemblies 20 and combustors 30. For example only, first fuel supply conduit sections 22S and second fuel supply conduit sections 24S may be made of a 300 series stainless steel with a wall thickness of 1/16 of an inch.
Fuel passing through first fuel supply conduit 22 and second fuel supply conduit 24 may be communicated to fuel inlet 50 of fuel cell manifold 32 via a fuel flow connection passage 110 extending between fuel supply pass through passage 104 and fuel inlet 50. As shown, in FIG. 13, each fuel cell manifold 32 may include only one fuel flow connecting passage 110 which connects pass through passage 104 of either first fuel supply boss 92 or second fuel supply boss 94 to fuel inlet 50. Also as shown, fuel cell manifolds 32 of adjacent fuel cell stack assemblies 20 may include fuel flow connecting passage 110 in opposite first and second fuel supply bosses 92, 94 such that every other fuel cell manifold 32 receives fuel from first fuel supply conduit 22 while the remaining fuel cell manifolds 32 receive fuel from second fuel supply conduit 24. However; it should be understood that, alternatively, both first fuel supply boss 92 and second fuel supply boss 94 of some or all of fuel cell manifolds 32 may include fuel flow connection passage 110 in order to supply fuel to fuel inlet 50 from both first fuel supply conduit 22 and second fuel supply conduit 24.
Each fuel cell manifold 32 includes a first air supply boss 112 and a second air supply boss 114. First air supply boss 112 and second air supply boss 114 extend radially outward from fuel cell manifold 32 and include an upper air supply recesses 116 and a lower air supply recess 118 which extend axially thereinto from opposite sides for receiving an end of one first air supply conduit section 26S, or one second air supply conduit section 28S in a sealing manner. Upper air supply recess 116 and lower air supply recess 118 of each first air supply boss 112 and second air supply boss 114 are fluidly connected by an air supply through passage 120 which extends axially between upper air supply recess 116 and lower air supply recess 118. An upper air supply shoulder 122 is defined at the bottom of upper air supply recess 116 while a lower fuel supply shoulder 124 is defined at the bottom of lower air supply recess 118. In this way, first air supply conduit sections 26S form a support column with first air supply bosses 112, thereby supporting fuel cell stack assemblies 20 and combustors 30 on support plate 90 within heater housing 18. Similarly, second air supply conduit sections 28S, form a support column with second air supply bosses 114, thereby supporting fuel cell stack assemblies 20 and combustors 30 on support plate 90 within heater housing 18. First air supply conduit sections 26S and second air supply conduit sections 28S may be made of a material that is substantially strong to accommodate the weight of fuel cell stack assemblies 20 and combustors 30 within heater 10. The material of first air supply conduit sections 26S and second air supply conduit sections 28S may also have properties to withstand the elevated temperatures within heater housing 18 as a result of the operation of fuel cell stack assemblies 20 and combustors 30. For example only, first air supply conduit sections 26S and second air supply conduit sections 28S may be made of a 300 series stainless steel with a wall thickness of 1/16 of an inch.
Supporting fuel cell stack assemblies 20 and combustors 30 from the bottom of heater housing 18 on support plate 90 results in the weight being supported by first air supply conduit sections 26S, second air supply conduit sections 28S, first air supply conduit sections 26S, and second air supply conduit sections 28S in compression which maximizes the strength of first air supply conduit sections 26S, second air supply conduit sections 28S, first air supply conduit sections 26S, and second air supply conduit sections 28S and requires minimal strength of connection fasteners which join first air supply conduit sections 26S, second air supply conduit sections 28S, first air supply conduit sections 26S, and second air supply conduit sections 28S. This also tends to promote sealing first air supply conduit sections 26S, second air supply conduit sections 28S, first air supply conduit sections 26S, and second air supply conduit sections 28S with fuel cell manifolds 32. Combining the structural support of fuel cell stack assemblies 20 and combustors 30 by supply conduit sections 26S, second air supply conduit sections 28S, first air supply conduit sections 26S, and second air supply conduit sections 28S provides the further advantage of avoiding additional structural components. Furthermore, supply conduit sections 26S, second air supply conduit sections 28S, first air supply conduit sections 26S, and second air supply conduit sections 28S of a given heater 10x are independent of all other heaters 10 in the sense that they only need to support fuel cell stack assemblies 20 and combustors 30 of heater 10x, thereby relying on heater housings 18 of heaters 10 as the principal support for heaters 10.
Fuel passing through first air supply conduit 26 and second air supply conduit 28 may be communicated to air inlet 52 of fuel cell manifold 32 via an air flow connection passage 126 extending between air supply pass through passage 120 and air inlet 52. As shown, in FIG. 14, each fuel cell manifold 32 may include only one air flow connecting passage 126 which connects air supply through passage 120 of either first air supply boss 112 or second air supply boss 114 to air inlet 52. Also as shown, fuel cell manifolds 32 of adjacent fuel cell stack assemblies 20 may include air flow connection passage 126 in opposite first and second air supply bosses 112, 114 such that every other fuel cell manifold 32 receives air from first air supply conduit 26 while the remaining fuel cell manifolds 32 receive air from second air supply conduit 28. However; it should be understood that, alternatively, both first air supply boss 112 and second air supply boss 114 of some or all of fuel cell manifolds 32 may include air flow connection passage 126 in order to supply air to air inlet 52 from both first air supply conduit 26 and second air supply conduit 28.
When heaters 101, 102, . . . 10n-1, 10n are connected together in sufficient number and over a sufficient distance, the pressure of fuel at fuel cell stack assemblies 20 may vary along the length of heaters 101, 102, . . . 10n-1, 10n. This variation in the pressure of fuel may lead to varying fuel flow to fuel cell stack assemblies 20 that may not be compatible with desired operation of each fuel cell stack assembly 20. In order to obtain a sufficiently uniform flow of fuel to each fuel cell stack assembly 20, fuel flow connection passages 110 may include a sonic fuel orifice 128 therein. Sonic fuel orifice 128 is sized to create a pressure differential between the fuel pressure within fuel supply through passage 104 and the fuel pressure within fuel inlet 50 such that the ratio of the fuel pressure within fuel supply through passage 104 to the fuel pressure within fuel inlet 50 is at least 1.85:1 which is known as the critical pressure ratio. When the critical pressure ratio is achieved at each sonic fuel orifice 128, the velocity of fuel through each sonic fuel orifice 128 will be the same and will be held constant as long as the ratio of the fuel pressure within fuel supply through passage 104 to the fuel pressure within fuel inlet 50 is at least 1.85:1. Since the velocity of fuel through each sonic fuel orifice 128 is equal, the flow of fuel to each fuel cell stack assembly 20 will be sufficiently the same for desired operation of each fuel cell stack assembly 20. The density of the fuel may vary along the length of heaters 101, 102, . . . 10n-1, 10n due to pressure variation within first fuel supply conduit 22 and second fuel supply conduit 24, thereby varying the mass flow of fuel to each fuel cell stack assembly 20; however, the variation in pressure within first fuel supply conduit 22 and second fuel supply conduit 24 is not sufficient to vary the mass flow of fuel to each fuel cell stack assembly 20 to an extent that would not be compatible with desired operation of each fuel cell stack assembly 20.
Since sonic fuel orifices 128 substantially fix the flow of fuel to fuel cell stack assemblies 20, the electricity and/or thermal output of fuel cell stack assemblies 20 may not be able to be substantially varied by varying the flow of fuel to fuel cell stack assemblies 20. In order to vary the electricity and/or thermal output of fuel cell stack assemblies 20, the composition of the fuel may be varied in order to achieve the desired electricity and/or thermal output of fuel cell stack assemblies 20. As described previously, fuel is supplied to fuel cell stack assemblies 20 by fuel reformer 48. Fuel reformer 48 may reform a hydrocarbon fuel, for example CH4, from a hydrocarbon fuel source 130 to produce a blend of H2, CO, H2O, CO2, N2, CH4. The portion of the blend which is used by fuel cell stack assemblies 20 to generate electricity and heat is H2, CO, and CH4 which may be from about 10% to about 90% of the blend. Fuel reformer 48 may be operated to yield a concentration of H2, CO, and CH4 that will result in the desired electricity and/or thermal output of fuel cell stack assemblies 20. Furthermore, a dilutant such as excess H2O or N2 may be added downstream of fuel reformer 48 from a dilutant source 131 to further dilute the fuel. In this way, the fuel composition supplied to fuel cell stack assemblies 20 may be varied to achieve a desired electricity and/or thermal output of fuel cell stack assemblies 20.
Similarly, when heaters 101, 102, . . . 10n-1, 10n are connected together in sufficient number and over a sufficient distance, the pressure of air at fuel cell stack assemblies 20 may vary along the length of heaters 101, 102, . . . 10n-1, 10n. This variation in the pressure of air may lead to varying air flow to fuel cell stack assemblies 20 that may not be compatible with desired operation of each fuel cell stack assembly 20. In order to obtain a sufficiently uniform flow of air to each fuel cell stack assembly 20, air flow connection passages 126 may include a sonic air orifice 132 therein. Sonic air orifice 132 is sized to create a pressure differential between the air pressure within air supply through passage 120 and the air pressure within air inlet 52 such that the ratio of the air pressure within air supply through passage 120 to the air pressure within air inlet 52 is at least 1.85:1 which is known as the critical pressure ratio. When the critical pressure ratio is achieved at each sonic air orifice 132, the velocity of air through each sonic air orifice 132 will be the same and will be held constant as long as the ratio of the air pressure within air supply through passage 120 to the air pressure within air inlet 52 is at least 1.85:1. Since the velocity of air through each sonic air orifice 132 is equal, the flow of air to each fuel cell stack assembly 20 will be sufficiently the same for desired operation of each fuel cell stack assembly 20. The density of the air may vary along the length of heaters 101, 102, . . . 10n-1, 10n due to pressure variation within first air supply conduit 26 and second air supply conduit 28, thereby varying the mass flow of air to each fuel cell stack assembly 20; however, the variation in pressure within first air supply conduit 26 and second air supply conduit 28 is not sufficient to vary the mass flow of air to each fuel cell stack assembly 20 to an extent that would not be compatible with desired operation of each fuel cell stack assembly 20.
Since sonic air orifices 132 substantially fix the flow of fuel to fuel cell stack assemblies 20, the electricity and/or thermal output of fuel cell stack assemblies 20 may not be able to be substantially varied by varying the flow of fuel to fuel cell stack assemblies 20. There are multiple strategies that may be utilized for supplying a sufficient amount of air in order to vary the electricity and/or thermal output of fuel cell stack assemblies 20. In a first strategy, sonic air orifices 132 may be sized to supply a sufficient amount of air needed to operate fuel cell stack assemblies 20 at maximum output. In this strategy, excess air will be supplied to fuel cell stack assemblies 20 when fuel cell stack assemblies 20 are operated below maximum output. The excess air supplied to fuel cell stack assemblies 20 will simply be passed to combustors 30 where it will be used to produce the heated combustor exhaust as described previously.
In a second strategy, sonic air orifices 132 may be sized to supply a sufficient amount of air needed to operate fuel cell stack assemblies 20 at medium output. When fuel cell stack assemblies 20 are desired to operate above medium output, additional hydrocarbon fuel, for example CH4, may be supplied to first fuel supply conduit 22 and second fuel supply conduit 24 downstream of fuel reformer 48. The additional CH4 that is added downstream of fuel reformer 48 may be supplied by hydrocarbon fuel source 130 or from another source. The un-reformed CH4 will be supplied to fuel cell stack assemblies 20 where the CH4 will be reformed within fuel cell stack assemblies 20 through an endothermic reaction which absorbs additional heat that would otherwise require additional air. In this way, fuel cell stack assemblies 20 may be operated at maximum output while requiring lesser amounts of air.
In a third strategy, each fuel cell stack assembly 20 may be in fluid communication with both first air supply conduit 26 and second air supply conduit 28 as shown in FIG. 15. However, sonic air orifice 132 which receives air from first air supply conduit 26 may be sized to supply a sufficient amount of air needed to operate fuel cell stack assemblies 20 at a low output level while sonic air orifice 132 which receives air from second air supply conduit 28 may be sized to supply a sufficient amount of air needed to operate fuel cell stack assemblies 20 at a medium output level. When fuel cell stack assemblies 20 are desired to be operated at the low output level, air may supplied to fuel cell stack assemblies 20 only through first air supply conduit 26. When fuel cell stack assemblies 20 are desired to be operated at the medium output, air may be supplied to fuel cell stack assemblies 20 only through second air supply conduit 28. When fuel cell stack assemblies 20 are desired to be operated above the medium output, for example, the maximum output, air may be supplied to fuel cell stack assemblies 20 through both first air supply conduit 26 and second air supply conduit 28. In this way, variable amounts of air can be supplied to fuel cell stack assemblies 20, thereby increasing efficiency by supplying less air at lower output levels of fuel cell stack assemblies 20.
In use, heaters 101, 102, . . . 10n-1, 10n are operated by supplying fuel and air to fuel cell stack assemblies 20 which are located within heater housing 18. Fuel cell stack assemblies 20 carry out a chemical reaction between the fuel and air, causing fuel cell stack assemblies 20 to be elevated in temperature, for example, about 600° C. to about 900° C. The anode exhaust and cathode exhaust of fuel cell stack assemblies 20 is mixed and combusted within respective combustors 30 to produce a heated combustor exhaust which is discharged within heater housing 18. Consequently, fuel cell stack assemblies 20 together with the heated combustor exhaust elevate the temperature of heater housing 18 with subsequently elevates the temperature of formation 16.
While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.

## Claims

1. A method of operating a heater having 1) a heater housing extending along a heater axis; 2) a fuel cell stack assembly disposed within said heater housing and having a plurality of fuel cells which convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent, said fuel cell stack assembly having a fuel cell manifold for a) receiving said fuel within a fuel inlet of said fuel cell manifold and distributing said fuel to said plurality of fuel cells and b) receiving said oxidizing agent within an oxidizing agent inlet of said fuel cell manifold and distributing said oxidizing agent to said plurality of fuel cells; 3) a fuel supply conduit in fluid communication with said fuel cell manifold for communicating said fuel to said fuel inlet of said fuel cell manifold; 4) an oxidizing agent supply conduit in fluid communication with said fuel cell manifold for communicating said oxidizing agent to said oxidizing agent inlet of said fuel cell manifold; and 5) a sonic orifice disposed between said fuel supply conduit and said fuel inlet or between said oxidizing agent supply conduit and said oxidizing agent inlet; said method comprising:
operating said heater to achieve a first pressure upstream of said sonic orifice; and
operating said heater to achieve a second pressure downstream of said sonic orifice;
wherein the ratio of said first pressure to said second pressure is at least 1.85:1, thereby limiting the velocity of said fuel or said oxidizing agent through said sonic orifice.

2. A method as in claim 1 wherein said fuel cell stack assembly is one of a plurality of fuel cell stack assemblies disposed within said heater housing, said sonic orifice is a sonic fuel orifice disposed between said fuel supply conduit and said fuel inlet, said first pressure is a first fuel pressure, and said second pressure is a second fuel pressure; said heater further comprises:
a sonic oxidizing agent orifice disposed between said oxidizing agent supply conduit and said oxidizing agent inlet of any one of said plurality of fuel cell stack assemblies; and
said method further comprises:

operating said heater to achieve a first oxidizing agent pressure upstream of said sonic oxidizing agent orifice; and
operating said heater to achieve a second oxidizing agent pressure downstream of said sonic oxidizing agent orifice;
wherein the ratio of said first oxidizing agent pressure to said second oxidizing agent pressure is at least 1.85:1 thereby establishing substantially uniform flow of said oxidizing agent through each said sonic oxidizing agent orifice.

3. A method as in claim 2 wherein said fuel is a reformed fuel and said method further comprises:
supplying an unreformed fuel to a fuel reformer from a fuel source; and
using said fuel reformer to produce said reformed fuel.

4. A method as in claim 3 wherein said reformed fuel is a blend comprising H2 and CO and said method further comprises:
varying the proportion of H2 and CO in said blend produced by said fuel reformer to vary one of the heat and electric output of said plurality of fuel cell stack assemblies.

5. A method as in claim 3 further comprising adding said unreformed fuel to said fuel supply conduit downstream of said fuel reformer to vary one of the heat and electric output of said plurality of fuel cell stack assemblies.

6. A method as in claim 3 further comprising adding a dilutant to said fuel supply conduit downstream of said fuel reformer to vary one of the heat and electric output of said plurality of fuel cell stack assemblies.

7. A method as in claim 6 wherein said dilutant comprises one of H2O and N2.

8. A method as in claim 2 wherein said oxidizing agent supply conduit is a first oxidizing agent supply conduit and said heater further comprises a second oxidizing agent supply conduit for supplying said oxidizing agent to said plurality of fuel cells of said plurality of fuel cell stack assemblies, said method further comprising:
supplying said oxidizing agent only to said first oxidizing agent supply conduit to achieve a first heat and electric output of said plurality of fuel cell stack assemblies;
supplying said oxidizing agent only to said second oxidizing agent supply conduit to achieve a second heat and electric output of said plurality of fuel cell stack assemblies which is different from said first heat and electrical output; and
supplying said oxidizing agent to both said first oxidizing agent supply conduit and said second oxidizing agent supply conduit to achieve a third heat and electric output of said plurality of fuel cell stack assemblies which is different from said first heat and electrical output and said second heat and electrical output.

9. A method as in claim 1 wherein said heater is disposed within a bore hole of an oil containing geological formation.