Heat exchanger for heating of fuel cell combustion air

CROSS-REFERENCE TO RELATED APPLICATIONS

0001This application is a continuation of application Ser. No. 10/375,834, which was filed on Feb. 25, 2003.

TECHNICAL FIELD

0002The present invention relates to solid-oxide fuel cells (SOFCs); more particularly, to heat exchangers for heating incoming combustion air in an SOFC assembly; and most particularly, to an improved heat exchanger for increasing heat exchange efficiency and reducing heat exchanger manufacturing cost and complexity.

BACKGROUND OF THE INVENTION

0003Fuel cells combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).

0004In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic gasoline oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 100° C.

0005Air enters an SOFC fuel cell at ambient temperature and desirably is preheated before being sent to the fuel cell stacks. A convenient and economical way to heat the air is by abstracting heat via a heat exchanger from the fuel cell exhaust which exits the fuel cell combustor at about 950° C. In the prior art, a typical heat exchanger employed for this purpose is of a well known plate-and-frame design wherein a plurality of heat-exchange modules is assembled as a stack. A plurality of alternating hot and cold gas flow spaces are separated by heat transfer plates. A typical prior art heat exchanger for use in an SOFC may comprise more than 100 individual plates and frames and can require more than 200 feet of brazing to seal the edges of all the modules, and is thus complicated and expensive to fabricate.

0006What is needed is an efficient heat exchanger for an SOFC wherein the number of components and fabrication costs are significantly reduced.

0007It is a principal object of the present invention to reduce the cost and complexity of an SOFC heat exchanger.

SUMMARY OF THE INVENTION

0008Briefly described, a heat exchanger for a solid-oxide fuel cell assembly includes a plurality of parallel tubes for conveying a first gas, preferably a hot gas, from a first manifold means to a second manifold means. The only brazing required is to attach each tube to each manifold. Preferably, the tubes are highly corrugated in bellows-like form to increase the wall area and decrease the wall thickness. The tubes are disposed in a jacket through which is passed a second gas, preferably a cool gas. The tubes may be linear between two manifolds, or they may be curved such that the first and second manifold functions are accommodated within a single component.

BRIEF DESCRIPTION OF THE DRAWINGS

0009These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:

0010<figref idref="DRAWINGS">FIG. 1</figref> is an isometric view from the front, partially exploded, of a prior art plate-and-frame heat exchanger;

0011<figref idref="DRAWINGS">FIG. 2</figref> is an exploded isometric view from the front of a first embodiment of a heat exchanger in accordance with the invention;

0012<figref idref="DRAWINGS">FIG. 3</figref> is an exploded isometric view from the front of a second embodiment of a heat exchanger in accordance with the invention; and

0013<figref idref="DRAWINGS">FIG. 4</figref> is an exploded isometric view from the front of a third embodiment of a heat exchanger in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0014Referring to <figref idref="DRAWINGS">FIG. 1</figref>, a prior art heat exchanger <b>10</b> comprises a plurality of alternating hot and cold fluid flow spaces formed by alternating rectangular plates <b>12</b> and frames <b>14</b>. In an SOFC assembly, the hot fluid is hot exhaust gas from the fuel cell stack and the cold fluid is combustion air entering the assembly. Each plate and frame has perforated extensions <b>16</b> at all four edges such that, when assembled into a solid stack <b>18</b>, the perforations define inlet and exhaust manifolds <b>20</b>,<b>22</b> for a first fluid <b>23</b> flowing across the plates in a first direction <b>24</b>, and inlet and exhaust manifolds <b>26</b>,<b>28</b> for a second fluid <b>29</b> flowing across opposite sides of the plates in a second and orthogonal direction <b>30</b>. The extensions of first frames <b>14</b><i>a </i>are open on their inner edges to permit access of first fluid <b>23</b> from manifold <b>20</b> to first sides of plates <b>12</b>, and the extensions of second frames <b>14</b><i>b </i>are open on their inner edges to permit access of second fluid <b>29</b> to the opposite sides of plates <b>12</b>. It will be seen that the sequence of plates and frames <b>12</b>-<b>14</b><i>a</i>-<b>12</b>-<b>14</b><i>b </i>represents a modular repeat, and that the full stack is simply a plurality of such modular repeats, the number of modules being as desired for a particular heat exchange requirement. In prior art SOFC heat exchanger <b>10</b>, the number of modules is typically about 25, requiring 100 or more components. After the entire stack of plates and frames is assembled, the edges of all plates and frames are sealed as by brazing to prevent fluid leakage from the heat exchanger.

0015Referring to <figref idref="DRAWINGS">FIG. 2</figref>, a first embodiment <b>110</b> of an SOFC heat exchanger in accordance with the invention includes a plurality of parallel metal tubes <b>112</b> having ends <b>113</b> (one set of ends not visible). Lower end plate <b>114</b> is provided with a plurality of openings <b>116</b> arranged in a pattern, each opening being surrounded by a lip <b>118</b> for receiving a first end (not visible) of a tube <b>112</b>. Upper end plate <b>120</b> is similarly provided with openings <b>122</b> and lips (not visible). The exemplary pattern of openings and tubes is four staggered rows of five tubes each. Obviously, other patterns are possible within the scope of the invention. The tubes are attached to the end plate lips as by brazing.

0016Metal tubes <b>112</b> preferably are axially corrugated as by hydro-forming into bellows form such that the surface area of each tube is substantially greater than the surface area of a non-corrugated tube having equal length and diameter. Preferably, the surface area is at least doubled. In addition, the bellows-forming process, which is well known in the art, causes thinning of the tube wall. As a result, the thermal conductance of heat exchanger <b>110</b> can be as much as 200% greater than that of prior art heat exchanger <b>10</b> of comparable size.

0017Preferably, tubes <b>112</b> are formed of a nickel-based high temperature alloy, for example, Inconel 625.

0018A base plate <b>124</b> has a planar upper surface <b>126</b> for mating against a planar lower surface <b>128</b> of lower end plate <b>114</b>. Surface <b>126</b> is relieved in three areas. One is a central well <b>130</b> defining an intermediate manifold for mating with the central two rows of ten openings <b>116</b>; the other two are lateral wells <b>132</b><i>a</i>,<b>132</b><i>b</i>, each of which defines an intake and exhaust manifold, respectively, which mates with a respective lateral row of five openings <b>116</b>. Well <b>132</b><i>a </i>is provided with slots <b>134</b> extending through plate <b>124</b> for mating with a supply such as an intake manifold (not shown) of a first fluid <b>23</b>, preferably the hot exhaust gas from the fuel cell stack. Well <b>132</b><i>b </i>is provided with similar slots <b>136</b> for mating with a return pathway through an exhaust manifold (not shown) for first fluid <b>23</b>.

0019A cover plate <b>138</b> has a planar lower surface <b>140</b> for mating against a planar upper surface <b>142</b> of upper end plate <b>120</b>. Surface <b>142</b> is relieved in two wells <b>144</b><i>a</i>,<b>144</b><i>b</i>, each of which defines a first and second crossover manifold, respectively. Each well contains two respective lateral rows of five openings <b>122</b>. Wells <b>144</b><i>a</i>,<b>144</b><i>b </i>are separated by a median <b>146</b>.

0020The result of this arrangement is an “M” shaped path for gas through five parallel tube assemblies. A first gas (fuel cell exhaust gas) at a first starting temperature enters through slots <b>134</b>, passes through openings <b>116</b> into the first staggered row of five tubes <b>112</b>, passes upwards through openings <b>122</b> into crossover manifold <b>144</b><i>a</i>, passes downwards through openings <b>122</b> into the second staggered row of five tubes <b>112</b>, passes through openings <b>116</b> into central well <b>130</b>, passes upward through openings <b>116</b> into the third staggered row of five tubes <b>112</b>, passes upward through openings <b>112</b> into second crossover manifold <b>144</b><i>b</i>, passes downward through openings <b>112</b> into the fourth staggered row of five tubes <b>112</b>, passes downward through openings <b>116</b> into lateral well or manifold <b>132</b><i>b</i>, and passes out of heat exchanger <b>110</b> via slots <b>136</b>.

0021Referring still to <figref idref="DRAWINGS">FIG. 2</figref>, tubes <b>112</b> and upper plate <b>120</b> are surrounded by a jacket <b>150</b> defining a jacketed space <b>152</b> between jacket <b>150</b> and the walls of tubes <b>112</b>. Jacket <b>150</b> is sealed to cover plate <b>138</b> and to lower end plate <b>114</b>. Lower end plate <b>114</b> is attached to base plate <b>124</b> as by bolts (not shown) through bores <b>154</b>. Lower end plate <b>114</b> and base plate <b>124</b> are provided with slots <b>158</b>,<b>160</b>. A second gas at a second starting temperature (air to be heated) enters through inlet slots <b>158</b>, passes into jacketed space <b>152</b>, passes around corrugated tubes <b>112</b> abstracting heat therefrom, and exits through exhaust slots <b>160</b>.

0022Referring to <figref idref="DRAWINGS">FIG. 3</figref>, a second embodiment <b>210</b> is identical in gas flow path to embodiment <b>110</b> but is substantially simplified in construction. The “M” shaped flow path is clearly visible in five staggered M-tubes <b>212</b> having ends <b>213</b>. Each M-tube <b>212</b> preferably is corrugated along its four linear portions as shown. Upper plate <b>120</b> and cover <b>138</b> are eliminated, their functions being cast into a closed jacket <b>250</b> conformable with M-tubes <b>212</b>. Further, lower end plate <b>214</b> is simplified to have only ten openings <b>216</b> rather than twenty openings <b>116</b> as in embodiment <b>110</b>. The total brazing required between tubes and plates is reduced from forty joints to ten. When tubes <b>212</b> are one-half inch in diameter, the total length of brazing required is about 15 inches, as compared to 200 inches required for prior art exchanger <b>10</b> or 60 inches for first embodiment <b>110</b>. Preferably, bottom plate <b>214</b> is provided with a plurality of attached fins <b>280</b> disposed adjacent M-tubes <b>212</b> for improving air flow around the tubes. Base plate <b>224</b> is simplified to eliminate central well <b>130</b> from embodiment <b>110</b>.

0023The “M” flow path indicated in first and second embodiments <b>110</b>,<b>210</b> can give rise to undesirably high back pressures because of the relatively long flow path. Referring to <figref idref="DRAWINGS">FIG. 4</figref>, a third embodiment <b>310</b> reduces the flow path by half, albeit at a cost of sixty joints as in first embodiment <b>110</b>. The basic flow arrangement provided by the twenty corrugated tubes <b>312</b> (shown for clarity via a cutout in jacket <b>350</b>), having tubular inserts <b>313</b>, is ten parallel U-shaped flow paths instead of five M-shaped flow paths. Lower end plate <b>314</b> is substantially identical with plate <b>114</b> in <figref idref="DRAWINGS">FIG. 2</figref>. Base plate <b>324</b> is configured as essentially a frame having two openings <b>332</b><i>a</i>,<b>332</b><i>b </i>which become inlet and exit chambers when plate <b>324</b> is disposed between a mounting manifold (not shown) and plate <b>314</b>. Upper end plate <b>320</b> is welded to tubes <b>312</b> as in <figref idref="DRAWINGS">FIG. 2</figref>, and a single upper manifold space <b>380</b> is provided by a cutout in a new spacer frame element <b>382</b> into which tubes <b>312</b> debouch. Cover plate <b>338</b> is similar to cover plate <b>138</b>. The flow path then is simply from inlet opening <b>332</b><i>a </i>upwards through the forward ten tubes <b>312</b> into manifold space <b>380</b>, then downwards through the rear ten tubes <b>312</b> into exhaust opening <b>332</b><i>b</i>. Jacket <b>350</b> may be substantially identical with jacket <b>150</b>.

0024A potential drawback of flowing a gas through corrugated tubing is stagnation of gas within the recesses of the corrugations. Referring still to <figref idref="DRAWINGS">FIG. 4</figref>, each tube <b>312</b> preferably is provided with an internal spiral turbulator <b>355</b> which is installed into the tube prior to brazing. (For purposes of clarity, each turbulator is shown partially removed from the respective tubes). Turbulator <b>355</b> is formed from sheet metal, preferably a high-temperature alloy, and twisted through an axial angle such as 180° about its axis. The turbulator induces a swirling flow of gas through the tube, promoting flushing of gas from the corrugation recesses.

0025While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.