Abstract:
Uninterruptible power supplies (UPSs) are generally discussed herein with particular discussions extended to fuel-cell-based UPSs used in conjunction with DC power supplies for improved operating efficiencies. With a wide voltage DC power supply, a DC-AC inverter may be omitted from the UPS and power from a back up power source, such as a battery or a fuel cell, may be applied directly to the DC power supply without performing two power conversions. The end result is a more efficient system capable of longer operating time.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is an ordinary application of Ser. No. 60/540,676 filed Jan. 30, 2004, the contents of which are expressly incorporated herein by reference. 

   Uninterruptible power supplies (UPSs) are generally discussed herein with particular discussions extended to fuel-cell-based UPSs used in conjunction with DC power supplies for improved operating efficiencies. 
   BACKGROUND 
   Fuel cells are rapidly becoming a significant source of power in our society, and their use in a variety of applications is inevitable. One such application is the use of a fuel cell as a power source in an uninterruptible power supply (UPS) for use with an electronic device or digital equipment, such as a personal computer (PC). 
   One advantage of using a fuel cell instead of a battery as the power source in a UPS is the fuel cell&#39;s high energy density, and therefore, the ability to operate a system for very long periods of time while off the utility grid. However, a fuel cell based UPS is not without limitations. Although it can operate for very long periods of time, it is still limited by the amount of fuel (usually hydrogen) available. 
   Fuel cell based UPSs that have emerged typically operate in a parallel backup configuration to the utility grid and rely on status monitoring and control in combination with a transfer switch or the like to operate when the line power is interrupted. An exemplary prior art fuel cell based UPS is disclosed in U.S. Pat. No. 6,602,627 to Liu et al., the contents of which are expressly incorporated herein by referenced. Typical prior art fuel cell based UPSs generally require appropriate consideration for the hold time of the equipment to be powered and the transfer time of the UPS in switching over to fuel cell operation. 
   In a few fuel cell based UPSs that has emerged, a DC-AC inverter is typically incorporated for converting the fuel cell&#39;s DC power to AC power for use by the equipment to be powered. When use as power backup for a PC, for example, there may be a minimum of two power conversions that take place between the fuel cell and the computer&#39;s actual components. They include a DC-AC conversion in the UPS and an AC-DC conversion in the computer&#39;s power supply. These conversions waste a considerable amount of hydrogen in feeding the fuel cell and allowing the power produced by the fuel cell to dissipate as heat. Additionally, if the inverter in the power supply is incapable of accepting a wide input voltage range that a typical fuel cell provides, an additional DC-DC converter must be used to bring the input voltage to within the inverter&#39;s tolerance, which results in three inefficiencies. 
   Accordingly, there is a need for a fuel cell based true online UPS that has improved operating efficiency. 
   SUMMARY 
   The present invention may be implemented by providing an uninterruptible power supply (UPS) for powering an electronic device comprising a charging unit connected to a rechargeable battery comprising a battery output, a fuel cell stack comprising a fuel cell output connected in parallel with the rechargeable battery with a blocking diode located between the battery output and the fuel cell output, and wherein a DC-AC inverter is absence from the UPS. 
   The present invention may also be practiced by providing an uninterruptible power supply (UPS) for powering an electronic device comprising a charging unit connected to a rechargeable battery comprising a battery output, a fuel cell stack comprising a fuel cell output connected in parallel with the rechargeable battery with a blocking diode located between the battery output and the fuel cell output, a control circuit for switching from battery operation to fuel cell operation when power supplied to the charging unit is under normal voltage. 
   The present invention may yet be practiced by a method for using an uninterruptible power supply (UPS) with an electronic device comprising: connecting AC voltage to an input terminal of a UPS housing; connecting DC voltage output from the UPS housing to the electronic device; wherein the UPS comprises a charging unit connected to a rechargeable battery comprising a battery output, a fuel cell stack comprising a fuel cell output connected in parallel with the rechargeable battery with a blocking diode located between the battery output and the fuel cell output, a control circuit for switching from battery operation to fuel cell operation when AC voltage supplied to the input terminal is under normal voltage. 
   Other aspects and advantages of the present invention are described in the Detailed Description set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will become appreciated as the same becomes better understood with reference to the specification, claims and appended drawings wherein: 
       FIG. 1  is schematic diagram depicting a fuel cell based UPS provided in accordance with aspects of the present invention connected to a power grid and a personal computer; 
       FIG. 2  is a semi-schematic diagram of a DC power supply provided in accordance with aspects of the present invention; 
       FIG. 3  is a semi-schematic diagram of a DC power supply power configuration. 
   

   DETAILED DESCRIPTION 
   The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments and methods for making fuel cell based UPSs provided in accordance with aspects of the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features and the steps for constructing and using the fuel cell based UPSs of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like or similar elements or features. 
   Referring now to  FIG. 1 , an exemplary schematic diagram depicting a fuel cell based uninterruptible power supply (UPS) unit  10  connected to a personal computer (PC)  12  and to the utility power grid  14  is shown. In one exemplary embodiment, the UPS unit or UPS  10  comprises a housing  16  (represented by dot-dashed lines), which houses at least one of the following components: a system monitor and control circuit  18 , a charging unit  20 , at least one rechargeable battery  22 , and a fuel cell stack  24 . The fuel cell stack  24  can be any number of fuel cells including Polymer Electrolyte Membrane (PEM), Direct Methanol, Solid Oxide, Alkaline, Phosphoric Acid, Regenerative, and Molten Carbonate. Preferably, the fuel stack  24  is of the PEM type and receives its fuel from a fuel source  26 , which is preferably mounted external of the housing  16  for maintenance and for refueling purposes. In an exemplary embodiment, the fuel source is a hydrogen tank. In an exemplary embodiment, the fuel cell stack  24  is part of a prior art fuel cell system comprising a cooling system, shut-off valve, pressure regulator, etc., connected to the battery  22  in parallel configuration. 
   Broadly speaking, the UPS  10  is an online type UPS or true UPS and when powered by the utility grid  14 , operates like prior art online UPSs. When the UPS  10  is plugged into the utility grid, power from the grid travels to the charging unit  20  to charge the battery  22 . In an exemplary embodiment, the charging unit comprises an AC-DC power supply providing a DC voltage suitable for the battery being charged. More preferably, the charging unit  20  comprises battery monitoring circuitry, and current or voltage is controlled by the circuitry to the battery for maintaining a safe charge rate for the battery. In one exemplary embodiment, the battery comprises a sealed lead acid gel type preferably capable of providing a voltage compatible with the fuel cell being used. For example, if the fuel cell has an output voltage range of 26V to 40V, a battery of 24V would work well, but 12V would be less preferred. Preferably, the fuel cell (based on the number of cells) should be designed to provide a suitable voltage output based on the battery&#39;s output. In an exemplary embodiment, the fuel cell output to the battery output should be between about 1.1 to 3 fuel cell output to about 1 battery output. 
   In an alternative embodiment, rather than incorporating a wide input voltage computer PSU, a single wide input DC-DC converter that would convert the fuel cell&#39;s wide output voltage to a single regulated voltage just slightly higher than the battery&#39;s fully charged voltage could be incorporated. While this alternative embodiment would be somewhat less efficient than a direct fuel cell-to-computer&#39;s PSU connection, it is simpler since a standard DC power supply may be used rather than a wide input voltage power supply unit. 
   During normal operation, the charging switch  28  is activated by, for example, a relay  30 , and charges the battery  22 . The battery then feeds the load, which in the present embodiment is a PC  12  comprising a monitor. During normal operation, the fuel cell switch  31  opens and the fuel cell is isolated from the load. In a preferred embodiment, the fuel cell stack is turned off during normal operation and no fuel is supplied to the fuel cell stack. Fuel cell operation in an existing fuel cell system is well known in the art. 
   Power supplied by the battery  22  to the PC  12  is by way of the power supply unit (PSU)  32 . In an exemplary embodiment, the PSU  32  is a wide voltage DC power supply unit of an ATX form factor,  FIG. 2 . The PSU  32  is similar to prior art ATX form factor power supplies in that it provides different DC voltages to different computer components inside the PC  12 , has printed circuit boards and electrical components for providing standby power and communications between the mother board and the PSU using a plurality of connectors  34 , provides cooling through one or more fans  36 , has an input voltage selector  38 , and has a power plug receptacle  40 . However, the PSU preferably does not incorporate a DC-AC inverter. Among other things, one would not be needed as power supplied to the PSU  32  from the UPS  10  is DC type voltage.  FIG. 3  is a schematic diagram of an exemplary 4 to 1 DC power supply unit  52  provided in accordance with aspects of the present invention. In one exemplary embodiment, the DC power supply unit  52  comprises a DC input terminal  54  for receiving DC power from the fuel cell  24  or the battery  22 . As an example, DC input can range from between about 18 VDC to about 75 VDC depending on whether the UPS is operating in battery mode or fuel cell mode. DC power from the input terminal  54  then feeds a circuit board (not shown) comprising circuitries for communicating and powering the PC  12  in a manner similar to an ATX form factor power supply unit. In one exemplary embodiment, the circuitries include one or more DC-DC converters for stepping up or stepping down the input voltage. For example, the one or more DC-DC converters may step down input voltage to produce +3.3 V, +5 V, −5 V, +5 V standby, +12 V, and −12 V output voltages. However, the input to output voltage ratio may vary depending on the needs of the electronic device to be powered by the UPS provided in accordance with aspects of the present invention. For example, rather a 4 to 1 input voltage range, the UPS may be configured for a range of about 2-4 input to 1 output voltage range. 
   Referring again to  FIG. 1  and assuming that a power grid  14  failure, such as an under normal voltage condition, is experienced, the battery  22  will feed the PC  12  using its stored power. At the same time, the system monitor and control circuit  18  will sense a power drop in the input power line  42 . In an exemplary embodiment, an appropriate time delay is incorporated before the control circuit  18  activates the fuel cell  24  subsequent to sensing the drop in power. During this time delay, the control circuit  18  verifies that the sensed power condition is not a momentary power dip. If a power failure is confirmed, the control circuit  18  closes the fuel cell switch  31  to turn on the fuel cell  24  to power the PC using power supplied by the fuel cell  24 . In a preferred embodiment, the control circuit  18  will continue to recheck the line voltage periodically for normal line power and will switch back to battery mode when normal line power is detected. 
   In one exemplary embodiment, when the fuel cell  24  is activated, the battery  22  should be isolated as the voltage of the battery will dictate the voltage of the system and render the fuel cell inefficient when the battery and the fuel cell are connected in parallel configuration. Accordingly, a transfer switch may be incorporated between the output of the fuel cell  24  and the output of the battery  22 . However, incorporating a transfer switch will present hold time and transfer time issues, which can be overcome with proper planning and component selections, but more complicated than necessary. Thus, in a preferred embodiment, a blocking diode  48  is incorporated. With the blocking diode  48 , the fuel cell  24 , which provides a higher voltage than the battery  22 , will pick up the load automatically as soon as it is applied to the load. In an exemplary embodiment, a standard commercially available rectifier diode of sufficient voltage and current capability for the load is used. 
   An optional DC-DC charger  50  may be incorporated to charge the battery  22  using power from the fuel cell  24 . If incorporated, the charger  50  is connected from between the fuel cell and the battery. The charge controller  50  should incorporate a blocking diode similar to the blocking diode  48  between the battery  22  and the fuel cell  24  to only allow current to flow to the battery, and not feed back to the fuel cell. 
   As is well known in the art, fuel cells can dehydrate and experience a drop in power as well as take a short time to come up to full power upon start up. Thus, in a preferred embodiment, a three-way switch, also known as a Single Pole Double Throw (SPDT) switch,  44  and a resistive load  46  of about 10-20 ohms resistant are incorporated. Before running the load on the fuel cell  24  or when the fuel cell  24  is dehydrated following a prolonged period of non-use operation, the three-way SPDT switch  44  is toggle to the resistive load  46  to rehydrate or to come up to full voltage power. In an alternative embodiment, a fuel cell hydration system may be used rather than running the fuel cell output to resistive load. Rehydration occurs automatically as water is produced by the reverse electrolysis process occurring in the fuel cell. In an exemplary embodiment, a timer may be incorporated for operating the fuel cell under a resistive load before switching the fuel cell over to power the PC. More preferably, a voltage sensor is incorporated in the monitor and control circuit  18  for sensing the fuel cell voltage output. If an appropriate voltage is detected by the control circuit  18 , the three-way SPDT switch  44  will be switched over to power the PC. 
   In one exemplary embodiment, during fuel cell operation, the battery switch  28  is opened to isolate the charging unit  20  from the battery  22 . This step is incorporated as input current to the charging unit can spike during under voltage conditions. Input current will increase due to a constant output power and a decrease in input voltage. Isolating the charging unit  20  will prevent it from overheating. 
   In a preferred embodiment, the control circuit  18  is powered by the grid power during normal operation. However, during power backup operation, the control circuit  18  may be powered by either the fuel cell  24  or the battery  22 , which can be arranged to provide a redundant power source for the control circuit. 
   In an experiment conducted using a prior art fuel cell based UPS and a personal computer having a standard power supply unit, power consumed by the system was found to be:
 
3.09 A×33.7 V=104.1 W.
 
   In a second experiment using a fuel cell based UPS provided in accordance with aspects of the present invention and a PC comprising a DC power supply, power consumed by the second system was found to be:
 
1.89 A×34.3 V=64.8 W.
 
   The modified system had a power consumption reduction of:
 
(104.1−64.8)/104.1×100=37.8%.
 
   The difference is attributable to the elimination of a DC-AC inverter in the UPS and a AC-DC inverter in the power supply. The operating time for the system using a standard fuel cell powered by UPS with a K/UK hydrogen cylinder was approximately 120 hours. Using the same cylinder at the improved efficiency would result in:
 
120 hours×1.606=192.7 hours.
 
   Although limited preferred embodiments and methods for making fuel cell based UPSs and their components have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. For example, various switching and monitoring may be accomplished using different electronic or software scheme. Accordingly, it is to be understood that the fuel cell based UPSs constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims.