|Title of Invention||
A FUEL CELL BASED UNINTERRUPTED POWER SUPPLY SYSTEM
|Abstract||A fuel cell based uninterrupted power supply system comprising a fuel cell stack as the primary DC power source a power controller including a DC/DC converter an inverter, a DC input regulator, an automatic "fast change-over" switching arrangement and a "trickle charge" circuit lor changing the battery bank using power from the fuel cell stack itself, said power controller accepting DC power input either from the fuel coll Stack or the auxiliary battery bank and a master controller for effecting smooth and automatic change-over of the load to the fuel cell stack in the event of mains power failure, for managing the gas flows to the fuel cell stack as a function of the electric load on the fuel cell stack, for managing the temperature of the fuel cell Stack through a closed-loop coolant circulation arrangement and for controlling start up and shut down with the aid of the auxiliary battery bank, without mains power supply.|
|Full Text||This invention relates to a fuel cell based uninterrupted power supply (UPS) system , more particularly, though not exclusively, a UPS system based on a proton exchange membrane or polymer electrolyte membrane fuel cell.
The PEMFC-based UPS system comprising of a fuel cell stack as the primary DC power source. A power controller that converts the variable voltage DC power into a constant voltage AC power, which can accept DC power input either from the fuel cell stack or an auxiliary battery bank, consisting of a DC/DC converter, an inverter, a DC input regulator, an automatic 'fast changeover' switching set up, and a 'trickle charge' circuit for charging the battery bank using power from the fuel cell stack itself. A master controller that effects smooth and automatic changeover of the load to the fuel cell in the event of mains power failure, manages the gas flows to the fuel cell stack as a function of the electric load on the fuel cell stack, manages the temperature of the fuel cell stack through a closed-loop coolant circulation set-up, and controls start-up and shut-down even without mains power supply with the aid of an auxiliary battery bank.
An uninterrupted power supply (UPS) system consists of a back-up power source that supplies power (usually AC power) to an electric load in the case of mains, i.e., grid power failure without interruption of supply to the electrical load. UPS systems are generally designed in such a way that the normal electric load is met by the grid or mains power supply, and during power outages, an automatic "changeover" is effected so that the load is now met by the UPS system. If this changeover has to be fast so that no interruption in power is experienced by critical loads such as electronic instruments and computers (hence the name 'uninterrupted power supply), the UPS system requires a sensing mechanism, usually in the form of a sensing circuit comprising-fast-acting relays, as well as a standby power source that can respond instantaneously to sudden load requirements. The most common power source used in UPS systems is made of 'lead-acid' batteries that can respond instantaneously to sudden loads and load changes. Since AC power is more commonly required, the DC power from the batteries is converted to AC power by a device called the 'inverter'. Some of the drawbacks associated with a UPS system using lead-acid batteries as the primary power source are:
(1) These batteries have to be first charged by using an external DC power source. This is usually obtained from the mains or grid power itself, which is AC power of 220V or 110V, which is then converted to the required DC voltage using a battery charger circuit consisting of a voltage transformer and an AC-to-DC rectifier circuit.
(2) The duration for which the battery-based UPS system can be used is entirely decided by the 'capacity' of the batteries, which is expressed in watt-hours or ampere-hours.
(3) Moreover, the batteries cannot be discharged to less than about 40% of their capacity - this is monitored by the voltage of the batteries which gradually drops as the batteries are discharged - otherwise the life of the batteries is reduced.
(4) Extra power is consumed froln the mains supply for charging the batteries. As mentioned above, the AC power has to be converted to DC for charging of the batteries, and in the case of mains power failure, the DC power from the batteries has to be converted back to AC power, thus resulting in a loss in the overall efficiency" during this conversion from AC to DC and again from DC to AC.
Fuel cells are power generation devices that produce an electric current through an electrochemical reaction of a fuel and an oxidant at efficiencies substantially higher than those of conventional power generation technologies used for the grid power supply. This invention uses a fuel cell as the primary DC power source in a UPS system, with a low-capacity battery-bank used for start-up and shut-down of the UPS system as well as during sudden load increases.
The fuel cell based uninterrupted power supply system* according to this invention, comprises a fuel cell stack as the primary DC power source; a power controller incorporating a DC/DC converter, an inverter, a DC input regulator, an automatic fast changeover switching arrangement and a trickle charge circuit for charging the auxiliary battery bank using power from the fuel cell stack itself, said power controller accepting DC power input either from the fuel cell stack or the auxiliary battery bank; and a master controller for effecting smooth and automatic changeover of the load to the fuel cell stack in the event of mains power failure.
This invention will now be described with reference to accompanying drawings which illustrate, by way of example, and not by the way of limitation, in Fig. 1 a block circuit diagram of an embodiment of this invention and Fig.2 the assembly of the said embodiment.
Fuel cell stack
The fuel cell stack contains several PEM fuel cells assembled in series, each cell comprising ^rf^an anode, a cathode and a polymer membrane electrolyte sandwiched between the anode and the cathode, forming a 'membrane-electrode assembly' or MEA.
The electrochemical reaction taking place in a hydrogen-oxygen fuel cell can be written as
The hydrogen ions or protons produced at the anode migrate through the electrolyte towards the cathode, where they combine with oxygen ions and electrons to produce water and heat, which is the heat of reaction. It can be seen that the cathode reaction requires electrons, which are released at the anode - these are routed to the cathode through an external 'load' where the electromotive force or e.m.f. of the electrons is tapped as electric power. Since the theoretical voltage obtained from a hydrogen/oxygen fuel cell is only 1.23 volts per cell, it is common practice to assemble several cells in series to form a fuel cell stack to obtain higher voltages for practical applications. Electrical resistance due to the presence of several components assembled in series results in the generation of additional heat, which is generally proportional to the square of the current drawn from the fuel cell stack. The voltage of all fuel cells gradually drop as more and more current is drawn, while simultaneously producing more heat as the cell voltage drops.
In a PEM fuel cell stack, each MEA is assembled between pairs of 'bipolar' plates which are made of a current-conducting material such as graphite. These bipolar plates serve the
functions of supplying the fuel gas, containing hydrogen, to the anode and oxidant gas, containing oxygen, to the cathode and also act as a current collector. Additionally, the bipolar plates provide structural support to the MEA's and also contain a cooling arrangement comprising ^'coolant plates' which are inserted at suitable intervals between the MEA's. A coolant such as water is circulated through the PEMFC stack to remove the heat generated during operation of the PEMFC stack and to maintain the stack at the desired operating temperature, which can be in the range of 50 to 85 C.
As mentioned above, the PEM fuel cell stack needs to be maintained at a desired temperature in the range of 50 to 85 C. If the heat generated in the stack during operation, that is, when an electric load is applied, is not removed, the temperature of the stack would go on increasing to a point where there would be a drop in the stack performance, followed by even irreversible damage to the cells. The desired temperature is maintained by circulating a coolant such as water through 'coolant plates' that are inserted at suitable intervals between the bipolar plates housing the fuel cells. The flow rate of the coolant is so adjusted that a controlled amount of heat is carried away by the coolant so as to maintain the desired temperature. This is done by sensing the temperature of the fuel cell stack at few points along the stack using temperature sensors such as thermocouples or RTDs that are connected to an automatic temperature controller. The temperature controller, in turn, switches a coolant circulation pump 'ON' and 'OFF' to regulate the stack temperature at the desired level. Alternatively, the temperature controller can control a variable-flow coolant pump by providing a control signal that varies with the amount of heat to be removed at any instant so as to maintain the desired temperature. This 'variable' signal would also depend on the 'inlet temperature' of the coolant into the stack.
In our/design, the heat generated in the fuel cell stack is removed using water, whose flow rate is controlled so as to maintain the stack at the desired temperature. If the coolant water exiting the stack is not recirculated, a large amount of water would be required during long durations of operation. Therefore a 'closed-loop' coolant subsystem is used so that the coolant can be recirculated. This is achieved by passing the coolant exiting the stack in hot condition through a heat exchanger where heat is lost by the coolant to a second cool fluid such as air. The coolant exiting this heat exchanger is fed to a 'coolant reservoir' from where the coolant is pumped into the fuel cell stack. In order to minimize the outer dimensions of this heat exchanger, a 'finned tube' design is adopted. Further, in order to augment the heat
dissipation from the coolant in the heat exchanger, a fan is used to blow air on to the finned tubes, thus achieving forced convection. Switching 'ON' and 'OFF' of this fan is controlled by a temperature controller by sensing the temperature of the coolant water exiting the heat exchanger. Alternatively, the speed of the fan can be regulated by this temperature controller. The dimensions of the heat exchanger and the reservoir are so chosen that a minimum amount of electric power is spent to run the coolant circulation pump and the fan, and to allow continuous operation at maximum electrical load for several hours before fresh or make-up coolant would be required.
PEM fuel cells using polymeric membranes such as Nation™ made by Dupont require that the proton conducting membranes be kept in wet condition in order to sustain ion transport from the anode to the cathode, since there is generally a net water transport from the anode to the cathode, leading to a progressive drying up of the membrane starting on the anode side. This is offset by supplying moisture to the membrane by one of several means. One of the means of supplying this moisture is by pre-humidifying the hydrogen-containing fuel gas before feeding into the anode. One of the means of pre-humidifying the hydrogen-containing gas is by passing the fuel gas along one side of a water-permeable, but gas-impermeable membrane. Hot water is passed on the other side of the membrane. The gas picks up moisture, which is then fed to the fuel cell anode. A portion of the water which is used as the coolant in the fuel cell stack in our design and which exits the stack at a temperature substantially higher than ambient temperature is passed through the 'humidification' subsystem where the fuel gas picks up moisture, thus saving on the energy needed to heat the water for humidification. Otherwise, the heat needed to raise the temperature of the water used for humidification would have to be provided using external energy input, such as by using electric resistance heaters. An additional benefit of the membrane humidification is that it also simplifies the overall system. The humidification subsystem is designed and closely integrated with the fuel cell stack in such a way that the fuel gas is sufficiently pre-humidified at all ranges of the fuel cell stack operation, since the flow rates of the fuel gas as well as the coolant water vary roughly proportional to the electric power drawn from the fuel cell system.
Gas flow controller
The required flow rates of the fuel gas and the oxidant gas vary roughly proportional to the electric power drawn from the fuel cell system . The electric power drawn can be described as the product of the current and the voltage of the fuel cell stack. Since the UPS system is meant to be left unattended during operation, an automatic gas flow controller is employed to regulate the flow rates of the gases so that at any time during operation, only the required amounts of gases are fed to the fuel cell stack to avoid wastage of excess gases as well as to avoid 'starving' of the cells. This is done by sensing the current drawn from the stack. This 'current signal' is used by the automatic gas flow controller consisting of a microprocessor, among other components, to compute the required flow rates of fuel and oxidant gases. The required flow rates are pre-programmed into the gas flow controller based on theoretical as well practical considerations. Separate 'control signals' or 'set point signals' are then generated by the gas flow controller for the fuel and oxidant gases, which are fed to the respective gas mass flow controllers. The valves of these mass flow controllers open or close in a manner proportional to the control signal applied, thus regulating the gas flow rates.
Fuel cells produce DC electric power, with the voltage gradually decreasing with increasing currents. This is true both for a single cells as well as a stack, since the latter is made up of several single cells arranged in series, and the stack voltage is nothing but the sum of the individual cell voltages. The curront-voltage performance,, also known as the polarization ii in M iif ,i ijpii'iil Pin 1 II 11 n Eilmwn in Firim Since most practical applications demand AC electric power of constant voltage and frequency, our UPS system employs a custom-developed power controller that converts the variable voltage DC power into a constant voltage, constant frequency AC power. An additional feature of this power controller is that it can draw DC power from an auxiliary power source such as a battery bank. Though the PEM fuel cell stack is the primary power source for the UPS system, the need for this auxiliary power source is explained as follows:
1) By definition and by design, the system described herein is expected to function as a UPS. Most common UPS systems are designed in such a way that the normal electric load is met by the grid or mains power supply, and during power outages, an automatic changeover is effected so that the load is now met by the UPS system. If this changeover has to be fast so that no interruption in power is experienced by
critical loads such as instruments and computers (hence the name 'uninterrupted power supply), the UPS system requires a sensing mechanism, usually in the form of a sensing circuit comprising of fast-acting relays, as well as a standby power source that can respond instantaneously to sudden load requirements. The most common power source used in UPS systems is made of 'lead-acid' batteries that can respond instantaneously to sudden loads and load changes. The present invention, however, employs a PEM fuel cell stack as the primary power source. While PEM fuel cell stacks also respond instantaneously to sudden loads and load changes, the main difference is that they also require the corresponding amounts of fuel and oxidant gases to be supplied. The gas flow control in the present design is effected by means of gas mass flow controllers which require a finite time for start-up and for gas flow to start, since the mass flow controllers use an electromechanical valve which is controlled by a flow control signal or 'set point' signal and opens in a proportional manner for control of gas flow. This start-up time can range from several milliseconds to several seconds, depending on the manufacturers design of the mass flow controller. When the PEM fuel cell stack is used as part of a UPS system, this would mean that the gases have to be supplied at all times so that the stack is ready to supply power at the time of mains power failure. In order to avoid this wastage of the gases when the mains power supply is present, our system is designed in such a way that at the instant of mains power failure, the electric load is instantaneously absorbed by an auxiliary battery bank of very small capacity until the fuel cell stack is ready to supply electric power.
2) The UPS system includes instruments and controllers which need power supply, though in very small amounts. If, as described above, the gas flow has to be started only after the mains power failure, the power needed to run the gas flow controllers would have to come from another standby source until the fuel cell stack is ready to supply power, that is, after the reactant gases have reached all the cells.
3) If the UPS system has to be started even without mains power supply, the power needed to run the instruments and controllers would have to come from another standby source until the fuel cell stack is ready to supply power.
4) A UPS system is expected to support any sudden changes in the electrical load. While the PEM fuel cells can also respond instantaneously to sudden load changes, as in the case of start-up, a limitation arises due to the time lag involved in supplying the reactant gases corresponding to the electrical load. For example, if the UPS is loaded
from 25% to 75% of its rated capacity, the reactant gas flow rates also have to be increased corresponding to the increase in the load. Since this involves the controller generating control signals for the reactant gas mass flow controllers and the valves opening as per the signals, a finite time lag which can be in the range of several milliseconds is encountered, during which time the stack voltage might tend to drop due to insufficient gas availability - the standby battery bank can 'share' a part of the electrical load during this transient period, that is, during the period when the valves in the gas mass flow controllers open up to supply 'new' flow requirement. Once the new flow rates are established, once again, the entire electrical load is supplied only by the PEM fuel cell stack. 5) It is common practice at the time of shutting down of fuel cell stacks to purge the cells of the reactant gases by using an inert gas such as nitrogen in order to avoid any accidents or gradual degradation of the cell components. The present UPS system consists of a nitrogen-purge sub-system, which, when activated, shuts the hydrogen and oxygen gas flow controllers OFF, opens nitrogen flow through solenoid valves for a certain duration which is programmed in the master controller, after which the nitrogen flow is stopped. This activity also requires a standby power source other than the fuel cell stack, since the reactant gas flows to the fuel cell stack are cut off during the nitrogen purge and therefore the stack cannot supply power.
Because of the requirements described above, the present design includes a standby battery
bank of small capacity which is used during (l)'start-up' without grid supply, (2) at the time
of grid power failure (3) sudden load increases and (4) during safe shut-down of the UPS
system which requires a nitrogen purge of the fuel cell stack.
The power controller consists of a DC/DC converter and an inverter. The DC/DC converter
accepts DC power of variable voltage from the fuel cell stack or from the standby battery
bank and converts it to a fixed voltage DC power at a voltage level that is suitable for
converting into AC power by the inverter.
An input regulator upstream of the DC/DC converter automatically regulates the DC power
input either from the fuel cell stack or from the battery bank, depending on whichever is at a
An automatic 'fast changeover' switching set up senses the presence or sudden failure of grid
power supply and accordingly supplies the load with power from either the grid or from the
UPS system, respectively and effects smooth changeover from one source to the other.
The power controller further consists of a battery charging circuit. A special feature of the present UPS system is that the charging of the standby battery bank is done by the fuel cell stack itself during operation, and NOT using the grid power as is the case in conventional UPS systems. Safety features are incorporated to prevent the reverse flow of current from the battery into the fuel cell. It may be repeated here that the capacity of the batteries is so chosen as to absorb the full load of the UPS only for a short duration during which the fuel cell stack is turned ON and takes over the load. Therefore, the charging current required for 'topping-up' the batteries back to their full capacity is also designed to be very small, and flows from the fuel cell stack to the batteries through a 'trickle charge' set up wherein the current and voltage required for charging the batteries are controlled by a battery-charging circuit which takes the variable voltage DC fuel cell power as input and converts it to a fixed voltage DC power for charging of the batteries. During grid power failure:
At the instant of mains power failure, the electric load is instantaneously absorbed by the auxiliary battery bank and simultaneously the master controller turns the gas mass flow / controllers 'ON'. Fuel and oxidant flow control signals are then applied to the respective gas mass flow controllers based on the current sensed by the gas flow controller. As the fuel and oxidant gases flow into the anode and cathode chambers, respectively, of the fuel cell stack, the stack quickly tends to reach the 'open circuit' voltage, which is designed to be substantially higher than that of the auxiliary battery bank. As the voltage of the stack becomes higher than that of the battery bank, the input regulator to the DC/DC converter described above automatically transfers the electrical load on to the fuel cell stack. Further, a small portion of the fuel cell stack's power output is now used to charge the standby battery bank back to its full capacity. Start-up without grid supply:
In this case, the UPS system actually functions as a 'stand-alone' power source. The master controller is first turned ON using power from the standby battery bank, after which electrical load can be applied immediately. Simultaneously, the gas mass flow controllers are also turned ON, and gas flows are then regulated depending on the electrical load on the system. As the voltage of the stack becomes higher than that of the battery bank, the input regulator to the DC/DC converter described above automatically transfers the electrical load on to the fuel cell stack. Further, a small portion of the fuel cell stack's power output is now used to charge the standby battery bank back to its full capacity.
During shut down of the UPS system
In the automatic shut down mode the electrical load is fully disconnected, that is the load on the fuel cell stack is brought to nil which also automatically reduces the gas flows to the fuel stack to a minimum, Subsequently the nitrogen purge sequence is activated during which the fuel and oxidant gas mass flow controllers as well as their respective solenoid valves are turned OFF and simultaneously the nitrogen supply through separate nitrogen solenoid valves connected to the anode and cathode sides are turned ON. Nitrogen which is kept in a ready condition upstream of the nitrogen solenoid valves begins to flow into the fuel cell stack, purging the cell compartments of the fuel and oxidant gases. After a preset duration the nitrogen flow is cut off by closing the solenoid valves and the controller displays N2 purge completed, safe to shut down".
1. A fuel cell based uninterrupted power supply system comprising a fuel cell stack
as the primary DC power source; a power controller incorporating a DC/DC
converter, an inverter, a DC input regulator, an automatic fast changeover
switching arrangement and a trickle charge circuit for charging the auxiliary
battery bank using power from the fuel cell stack itself, said power controller
accepting DC power input either from the fuel cell stack or the auxiliary battery
bank; and a master controller for effecting smooth and automatic changeover of
the load to the fuel cell stack in the event of mains power failure.
2. A fuel cell based uninterrupted power supply system substantially as herein
described with reference to and as illustrated in the accompanying drawings.
|Indian Patent Application Number||409/MAS/2002|
|PG Journal Number||30/2009|
|Date of Filing||28-May-2002|
|Name of Patentee||SPIC SCIENCE FOUNDATION|
|Applicant Address||MOUNT VIEW 111 MOUNT ROAD, GUINDY, CHENNAI 600032|
|PCT International Classification Number||H02J9/00|
|PCT International Application Number||N/A|
|PCT International Filing date|