Abstract:
A method for humidifying and controlling the temperature of a process gas stream comprising the steps of super-saturating and heating the process gas stream with steam until it reaches a first pre-set temperature; cooling the process gas stream until it reaches a second pre-set temperature; removing excess condensed water from the process gas stream; and heating the process gas stream until it reaches a third pre-set temperature. An apparatus for implementing this method is also disclosed.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to a method and apparatus for humidifying and controlling the temperature of incoming fuel cell process gas. More particularly, the present invention relates to a humidification system capable of providing rapid, accurate and precise control of both the relative humidity and the temperature of the incoming fuel cell process gas. 
   BACKGROUND OF THE INVENTION 
   Fuel cell systems are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy into electrical energy. Proton exchange membrane fuel cells comprise an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane and are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product. 
   Proton exchange membranes require a wet surface to facilitate the conduction of protons from the anode to the cathode, and otherwise to maintain the membranes electrically conductive. It has been suggested that each proton that moves through the membrane drags at least two or three water molecules with it (U.S. Pat. No. 5,996,976). U.S. Pat. No. 5,786,104 describes in more qualitative terms a mechanism termed “water pumping”, which results in the transport of cations (protons) with water molecules through the membrane. As the current density increases, the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point the membrane begins to dry out, at least on the anode side, and its internal resistance increases. It will be appreciated that this mechanism drives water to the cathode side, and additionally the water created by reaction is formed at the cathode side. Nonetheless, it is possible for the flow of gas across the cathode side to be sufficient to remove this water, resulting in drying out on the cathode side as well. Accordingly, the surface of the membrane must remain moist at all times. Therefore, to ensure adequate efficiency, the process gases must have, on entering the fuel cell, a predetermined or set relative humidity and a predetermined or set temperature which are based on the system requirements. 
   A further consideration is that there is an increasing interest in using fuel cells in transport and like applications, e.g. as the basic power source for cars, buses and even larger vehicles. As compared to some stationary applications, this presents some unique requirements. More particularly, it is necessary that the power delivered by a fuel cell be capable of rapid change between different power levels, and these power levels can be quite different. Thus, in urban driving, it is common for fuel cells to be required to frequently switch between minimum, or even zero power, to a maximum power level and back again. Maintaining appropriate humidity levels under such severe operating conditions is not easy. Additionally, a fuel cell must be capable of providing this functionality under a wide range of ambient air conditions. 
   Accordingly, in this art one can find numerous proposals for maintaining humidity in fuel cell systems. One conventional way to humidify a gas stream is to pass a gas as a stream of fine bubbles through water. As long as the process gas has sufficient contact time with the water, controlling the temperature of the water controls the amount of water in the gas stream. However, these bubble column type humidifiers are generally not suitable for fuel cells. The humidifiers tend to be large and costly. Moreover, the humidifiers are unable to react fast enough to meet the load following requirements of the fuel cell system. As a result, at high gas flow rates the system becomes unstable, unreliable and unresponsive. In addition, this humidification system never reaches 100% relative humidity in practice and this limits the flexibility or adaptability of the system. 
   In some prior art fuel cells, incoming process gases are humidified by flowing each gas on one side of a water vapor exchange membrane and by flowing deionized water on the opposite side of the membrane. In this way, water is osmotically transferred across the membrane to the fuel and oxidant gases. However, these systems have process parameter restraints that cause problems and inefficiencies when used in conjunction with fuel cells. Since the membrane is at the same temperature as the fuel stack, there is no independent control of the relative humidity or temperature of the process gases and thus the system is limited in its ability to adjust to different situations. 
   Other humidification methods include exposing the incoming process gas to a source of steam or metering in a quantity of fine water droplets into the gas supply line (U.S. Pat. No. 5,432,020). However, in the past, these systems tended to be large, complex, slow acting, and possessed inadequate dynamic controllability. 
   There remains a need for a humidifier that can offer rapid dynamic control, as well as precise and accurate temperatures and relative humidities for incoming fuel cell process gases. More particularly, such a humidifier should enable relative humidity and temperature to be controlled independently over a wide variety of flow rates, for both the oxidant and fuel systems. 
   SUMMARY OF THE INVENTION 
   A method of humidifying a process gas stream for delivery to a fuel cell, the method comprising:
         (a) introducing steam into the process gas stream, so as to humidify the process gas stream at a first temperature and so as to provide the process gas stream with excess humidity;   (b) cooling the process gas stream in a first heat exchanger at a second temperature, lower than the first temperature, to cause condensation of moisture;   (c) removing condensed moisture from the process gas stream;   (d) passing the process gas through a second heat exchanger to give the process gas stream a third temperature, and delivering the process gas stream at the third temperature, whereby the absolute humidity level in the process gas stream is determined from the maximum relative humidity at the second temperature;   (e) supplying the humidified process gas stream at the third temperature to the fuel cell; and   (f) providing a common coolant supply and removing excess heat from the first and second heat exchangers with the common coolant supply;   wherein step (b) includes passing a first heat transfer fluid through the first heat exchanger to cool the process gas stream to the second temperature, step (d) comprises passing a second heat transfer fluid through the second heat exchanger to heat the process gas stream to the third temperature; and   wherein the method further includes passing the first heat transfer fluid through a first temperature control circuit, including a first heater and a third heat exchanger, for controlling the temperature of the first heat transfer fluid, passing the second heat transfer fluid through a second temperature control circuit, including a second heater and a fourth heat exchanger, for controlling the temperature of the second heat transfer fluid, and passing coolant from the common coolant supply through the third and fourth heat exchangers.       

   In one embodiment, the method further comprises the step of maintaining the third pre-set temperature of the process gas stream from step (d) until it reaches an inlet of a fuel cell. 
   In accordance with another embodiment of the present invention, there is provided an apparatus for humidifying a process gas stream for an operating fuel cell, the apparatus comprising:
         (a) a means for super-saturating and heating the process gas stream with steam until it reaches a first pre-set temperature;   (b) a means for cooling the process gas stream until it reaches a second pre set temperature;   (c) a means for removing excess condensed water from the process gas stream; and   (d) a means for heating the process gas stream until it reaches a third pre-set temperature.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which show a preferred embodiment of the present invention and in which: 
       FIG. 1  illustrates a schematic flow diagram of one embodiment of a humidification system for a fuel cell; 
       FIG. 2  shows a perspective view of a second embodiment of the humidification system for a fuel cell; 
       FIG. 3  shows a detail of  FIG. 2  in a corresponding perspective view on a larger scale; 
       FIG. 4  shows a perspective view of part D of  FIG. 3  in a corresponding perspective view on an enlarged scale; 
       FIG. 5  shows details of elements of a steam line; 
       FIG. 6  shows a part of  FIG. 5  on an enlarged scale; and 
       FIG. 7  shows a schematic view of the humidification system of the second embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , this shows a schematic flow diagram of a process gas stream for one process gas. It is to be understood that the invention is applicable to both gases, i.e. to both of the fuel and the oxidant, and for this purpose the flow diagram of  FIG. 1  would be duplicated for the two process gas lines. The embodiments of  FIGS. 2–6  shows the invention as applied to both process gas lines. 
   Referring to  FIG. 1 , and in that process gas stream  12  and a steam line  14  are both connected to a saturator  16 , for increasing the humidity of the gas stream. Steam is supplied from a steam supply indicated schematically at  18 . The saturator  16  includes an injector for injecting steam into the process gas stream, so as to both heat and humidify the process gas stream. 
   A line  20  exits from the saturator  16  and contains super-saturated process gas. The line  20  enters a first heat exchanger  22 . The first heat exchanger  22  can be a plate heat exchanger or other suitable heat exchanger, and has an inlet  26  and an outlet  28  for a water stream. It is to be understood that the stream could be comprised of at least one fluid, including but not limited to water, oil, and/or ethylene glycol. While a variety of heat exchange fluids could be used for the specific embodiments described here, all the heat exchange fluids are water in the described embodiments. The inlet and outlet  26 ,  28  are part of a first temperature control circuit, also referred to as a dewpoint cooling section, including a pump  30 , an inlet  32  for make-up water and a third heat exchanger  34 . Additionally a first heater  36  is provided. Thus, in the first temperature control circuit, the make up water enables the level of fluid to be maintained, and this stream can be either cooled with the third heat exchanger  34  or heated with the first heater  36  to a desired temperature. 
   The first heat exchanger  22  has an outlet for cooled gas connected by a line  24  to a separator  38 . The separator  38  is for separating water condensed out of the gas in the first heat exchanger  22 , and has a discharge or outlet  40  for condensed water. An outlet of the separator  38  is connected by a further line  42  to a second heat exchanger  44 . 
   The second heat exchanger  44  is intended to reheat the gas, and has an outlet connected to line  46  for the reheated gas. Like the first heat exchanger  22 , the second heat exchanger  44  has an inlet  48  and an outlet  50  for a water heating stream. The inlet and outlet  48 ,  50  are part of a second temperature control circuit, also referred to as a reheating section, including a pump  52 , a make-up inlet  54 , a fourth heat exchanger  55 , and a second heater  56 . Thus, like the arrangement of the first heat exchanger  22 , the pump  52  circulates the make up water, and this stream can be either cooled with the fourth heat exchanger  55  or heated with the second heater  56  to a desired temperature. 
   The reheated gas with the known moisture content is then passed through line  46  to the fuel cell stack indicated schematically at  60 . 
   The arrangement of  FIG. 1  is intended to provide controlled humidification of the gas stream supplied to the fuel cell stack  60 , and to enable both the temperature and humidity to be precisely controlled. This is explained further, by the detailed description of the mode of operation of the apparatus of  FIG. 1 . Thus, dry incoming process gas is supplied to the saturator  16 , and gas is super-saturated with steam in the saturator  16 , to a humidity level greater then that ultimately desired for the gas. Both the flow of the gas through line  12  and steam through line  14  are controlled and metered. The effect of injecting steam into the gas is also to heat the gas to a first pre-set temperature. Typically, on leaving the saturator  16 , the gas is supersaturated at the first pre-set temperature of around 90° C., although the gas may be supersaturated at any temperature in the range of 10° C. to 120° C. 
   In the first heat exchanger  22 , the gas is cooled down to a second pre-set temperature of, for example, 80° C. Again, for example, this temperature could be in the range 10° C. to 120° C. The effect of this is to remove excess moisture from the gas stream, and to give a humidity level equal to 100% relative humidity at the temperature to which the gas is cooled in the first heat exchanger  22 . 
   The reason for first super-saturating and then cooling the gas to remove excess moisture is to ensure that the absolute humidity level is accurately achieved. Achieving a reliable humidity level in the saturator  16  is not practical. Thus, the approach has been taken of adding excess moisture and then removing excess moisture by condensation, since the absolute humidity level is known, when the gas is saturated and the dewpoint temperature of the gas is known. 
   The excess moisture in the gas will form droplets, and the gas with the condensed droplets is passed to the separator  38 , where the condensed droplets are collected or separated from the gas flow and drained out through the outlet or discharge line  40 . 
   The saturated gas is then passed through line  42  to the second heat exchanger  44 . Here, the gas is reheated to a third pre-set temperature of, for example, 85° C. More generally, the gas can be reheated to a temperature of 10° C. to 120° C. Heating the gas will reduce the relative humidity level but the absolute humidity level will remain constant. 
   Thus, reheated process gas from the second heat exchanger  44 , passes through line  46 , and it will then have a known, third pre-set temperature and a known humidity level. As indicated schematically at  58 , a heater is provided to maintain the line  46  at a constant temperature, to ensure that the gas does not cool or otherwise change in temperature during passage to the fuel cell stack  60 . Practically, it has been found that heat traces, comprising electrical heating wires, wound around the line  46  provides a necessary heating function. This ensures a gas delivered to fuel cell stack  60  is at the desired temperature and with the desired humidity. 
   As mentioned above, changing demands on the fuel cell stack are accomplished by changing the flow rate for the gas passing through the line  12 . If it is desired to change the temperature and/or the humidity of the gas flow then this is achieved by control of the operating conditions of the first and second heat exchangers  22 ,  44 . 
   Thus, the temperature of the cooling fluid in the first temperature control loop or circuit of the first heat exchanger  22 , passing through the inlet and outlet  26 ,  28 , is controlled, so as to control the temperature of gas exiting from the heat exchanger  22 , and hence the absolute humidity level of this gas. 
   Commonly, it is expected that gas will be cooled in the first heat exchanger  22 , and the second heat exchanger  44  will ensure that the cooling water passing through the cooling loop will be at a desired temperature. The third heat exchanger  34  enables the temperature in the cooling loop to be lowered quickly if desired. 
   Where the operational requirement is to increase the temperature of the gas exiting from the first heat exchanger  22 , then the water in the cooling loop needs to be heated. For this purpose, the first heater  36  is provided to enable the water to be rapidly heated. It has been found, in practice, that response times of less than a minute can be provided for a 12 kW fuel cell. 
   Correspondingly, in the second heat exchanger  44 , the second heater  56  is adjusted to heat water in a cooling loop passing through the inlet and outlet  48  and  50  to the desired temperature. The fourth heat exchanger  55  enables the temperature in the cooling loop to be lowered quickly if desired. 
   Reference will now be made to  FIGS. 2–7 , which show an implementation of the present invention. 
   Referring to  FIG. 7 , there is shown a schematic view of a humidification circuit according to a second embodiment. Here, a steam inlet  70  is connected to a steam supply and is provided with a pressure sensor  72 , connected to a pressure switch (not shown) for tripping the fuel cell system if the steam supply pressure is too low. The line  70  then passes through a main shut off valve  74  and a trap  76  is provided for draining off any condensation which may have formed. The steam line then passes through a pressure regulator  78  and includes a pressure gauge  88 . 
   A reference inlet, e.g. for air, is provided at  86 . This inlet  86  is connected through a pressure regulator  84  and a three way valve  80  (with a temperature controller  82 ) to the pressure regulator  78 . 
   The steam line  70  then passes through a T-connection to two separate lines  90 ,  92  for supplying steam to the separate gas lines for the fuel and oxidant gases. Many elements of these two lines  90 ,  92  are common, and for simplicity, a description is given just of these elements in the line  90 . The corresponding elements in the line  92  are given the same reference numeral but with a suffix “a”, it being understood that they have essentially the same function. 
   Thus, the line  90  includes a steam regulator or shut off valve  94  connected to a further regulating valve  96  Valve  96  is a metering valve which controls the flow of steam into the gas lines. The valve  96  is connected to a temperature controller  98  and a back pressure regulator  102 . 
   A fuel gas is supplied through a line  112 . Steam is injected into the fuel gas at an injection port  114 . Steam is supplied to injection port  114  through a non-return valve  116 . Correspondingly, on the oxidant side, there is a supply line  112   a , for example for air, and a steam injection port  114   a . A temperature sensor is provided at  100 . 
   The fuel gas containing steam, which is then in a supersaturated condition passes through a first heat exchanger  118 , which is cooled, so as to promote condensation of excess moisture. 
   The cooled fuel gas then passes to a separator  120  with a trap and drain arrangement  122 , for separating out also droplets. The fuel gas with 100% relative humidity then flows through a line  124 , that is insulated to maintain the temperature and humidity level of the fuel gas to a second heat exchanger  126 . A temperature sensor  128  is provided downstream from a separator  120 , for detecting the temperature of the fuel gas stream. Knowing that the fuel gas stream will be at a 100% relative humidity, the absolute humidity of the gas stream can be determined. 
   From the second heat exchanger  126 , the fuel gas flows to the fuel cell stack indicated at  130 . Again, standard sensors can be provided as indicated at  131 , immediately before the inlet to the fuel cell. 
   Each of the first and second heat exchangers  118 ,  126  has its respective temperature control circuit, and these are now described separately. 
   Referring first to the first heat exchanger  118 , a temperature control circuit indicated at generally  132  includes a first secondary heat exchanger  134 , a pump  136  and a heater  138 . 
   Auxiliary elements of the circuit include a connection  140  for make up water and a pressure relief valve  142 . Chilled cooling water is supplied to the secondary heat exchanger  134  through supply and return lines  144  and  146  with a control valve being provided at  148 . A temperature sensor  150  is provided in the cooling circuit, to enable the temperature in the first heat exchanger  118  to be set as desired. Other standard control elements would be provided as required. For example, a temperature controller  152  is connected to the temperature sensor  150  and to the heater  138 , and also to the control valve  148 . Thus, the temperature controller  152  can open the valve  148  to increase the flow of cooling water to cool down the temperature in the circuit, or alternatively actuate the heater  138  to increase the temperature in the circuit, as required. 
   The temperature control circuit for the second heat exchanger  126  generally corresponds. Thus, the circuit is indicated at  162 , and includes a second secondary heat exchanger  164 , a pump  166  and a heater  168 . A make up inlet  170  is provided, together with a pressure release valve  172 . 
   The chilled water supply and return lines  144 ,  146  are connected through the second secondary heat exchanger  164 , through a control valve  178 . 
   A temperature sensor  180  is connected to the second temperature control circuit  162 , and a temperature controller  182  is connected to a temperature sensor  180 , control valve  178  and the heater  168 , for control as for the first temperature control circuit  132 . 
   The present invention has many advantages over the prior art. The combination of the dewpoint cooling section and the reheating section allows rapid changes in operating conditions, with typical response times which are less than one minute. Furthermore, the system can be dynamically controlled to provide precise and accurate inlet fuel process gas stream temperatures and relative humidities, which are both essential for the efficient operation of a proton exchange membrane fuel cell over a wide range of current densities. 
   While the above description constitutes the preferred embodiments, it will be appreciated that the present invention is susceptible to modification and change without departing from the fair meaning of the proper scope of the accompanying claims. For example, the present invention might have applicability in other types of fuel cells, which include but are not limited to, solid oxide, alkaline, molton-carbonate, and phosphoric acid. In particular, the present invention may be applied to fuel cells which operate at much higher temperatures. As will be appreciated by those skilled in the art, the requirement for humidification is very dependent on the electrolyte used and also the temperature and pressure of operation of the fuel cell. Accordingly, it will be understood that the present invention may not be applicable to all other types of fuel cells.