METHODANDAPPARATUSFORMODIFYINGTHEPOWER

OUTPUTOFFUELCELLS

[0001] The present invention relates to a method as described in the preamble of claim 1 and an apparatus as described in the preamble of claim 8 for modifying the power output of fuel cells.

[0002] Most of the energy of the world is produced by means of oil, coal, natural gas or nuclear power. All these production methods have their specific problems as far as, for example, availability and friendliness to environment are concerned. As far as the en¬ vironment is concerned, especially oil and coal cause pollution when they are combusted. The problem with nuclear power is, accordingly, storage of used fuel.

[0003] Because of these problems, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed. One future energy source is the fuel cell by means of which fuel is directly transformed to electricity via a chemical reaction. For example, natural gas, hydrogen, suitable mixture of hydrogen, methane or methanol can be used as fuel. Oxygen is also a reagent in the reaction, the oxygen being introduced into the fuel in the form of air, for example. The advantages of fuel cells include good efficiency, silence and very small need of moving parts. Another advantage is that being only water or water vapour, the emissions are environmentally friendly and clean. Fuel cells can be used in a number of applications, from power outputs of milliwatts to power outputs of hundreds of kilowatts and even larger than that. Fuel cells can be used, among others, as power sources for vehicles as well as separate power stations and emergency power sources as well as for example for production of heat and electricity for a single house or a block of flats.

[0004] In practice one problem of the fuel cells is, however, the DC voltage produced by them that is not suitable for all applications as such. Further, the problem with current solutions is that the voltage produced by the fuel cell is highly dependent on the load so that as load increases the voltage decreases.

[0005] There is an attempt to fix these disadvantages in, among others, US patent ap¬ plication 2003/0206424, disclosing a solution for converting the DC voltage produced by a fuel cell into a single-phase AC voltage. Being a single-phase solution, it is meant for small systems, whereby the problem is, as far as larger systems are concerned, that there is no voltage stabilization in the solution and the said load dependency has not been removed effectively enough. In an attempt to remove the load dependency the apparatus is connected to an additional battery maintaining the voltage high enough as

the load increases, until the fuel cell can respond to the increase of load. This solution does not work in larger systems. Further, the solution according to the US application has no zero potential output, by the solution does not support 5-wire system and therefore it can not be used in applications requiring 5-wire system.

[0006] The aim of this invention is to eliminate the above-mentioned disadvantages and to produce as versatile a method and apparatus as possible for modifying the power of fuel cells so that the DC voltage produced by the fuel cells is converted into three- phase AC voltage. Additionally, an aim of the invention is to produce a method and apparatus for stabilizing the said AC voltage and facilitating the 5-wire system. Yet another aim of the invention is to eliminate the load dependency of voltage occurring in fuel cell solutions. It is also an aim of the invention to produce an independent power station based on fuel cells that can be used as a stand-alone power station for producing three-phase electric energy in 5-wire system while the power station operates either as a main power source or an emergency power station, whereby the power station can produce electricity for small houses, large buildings and facilities, electrically operated machines and apparatuses, such as elevators and cranes and various vehicles. The method according to the invention is characterized by what is disclosed in the characterizing part of claim 1. The apparatus according to the invention is accordingly characterized by what is disclosed in the characterizing part of claim 8. Other embodiments of the invention are characterized by what is disclosed in other claims.

[0007] The solution according to the invention comprises a method and an apparatus for modifying the power output of fuel cells, in which method the voltage of DC electricity from a fuel cell unit is converted into AC electricity by means of a frequency converter. According to the invention, the DC electricity produced by the fuel cell unit is directed from the fuel cell unit to a three-phase frequency converter, in which the DC electricity is converted into three-phase AC electricity. A positive output and a negative output as well as a neutral point output having a potential between these are arranged in the fuel cell unit, whereby DC electricity from the fuel cell is directed as positive and negative voltage in relation to the neutral point output.

[0008] According to a preferable embodiment of the invention the AC voltage produced by the three-phase frequency converter is adjusted by means of a voltage regulator.

[0009] According to another preferable embodiment of the invention the voltage of the DC electricity from the fuel cell unit is adapted to suit the requirements by varying the number of the fuel cell stacks or internal voltage or connections of the fuel cell stacks.

[0010] According to yet another preferable embodiment of the invention the voltage of the DC electricity from the fuel cell unit is adapted to suit the requirements by means of a DC/DC -converter.

[0011] According to yet another preferable embodiment of the invention the neutral point is grounded, if necessary.

[0012] According to yet another preferable embodiment of the invention capacitances are connected between the both the positive output and the negative output as well as the neutral point output of the fuel cell unit for stabilizing the current from the fuel cell unit.

[0013] An advantage of the solution according to the invention is that the environmentally friendly DC electricity produced by the fuel cells can be converted into three-phase AC electricity with a very good efficiency of about 97 % so that the electricity can be used in a number of applications. Another advantage is that the produced AC electricity is not load-dependent and due to the voltage regulation solution the AC voltage from the apparatus is stable. A further advantage is that by means of the solution according to the invention the disturbing distortion can be eliminated from the voltage. Still another advantage is that in a solution according to the invention there is no need for a transformer that causes interference as well as adds costs and weight of the apparatus and there is no need for a DC/DC-converter, either, that would also cause additional costs and weight. Another advantage is that by means of the apparatus returning power can be used and overload situations can be controlled. A further advantage is that the solution according to the invention makes it possible to use environmentally friendly fuel cells as power sources producing electric energy for a number of purposes, whereby the apparatus according to the invention acts, for example, as a stand-alone power station.

[0014] In the following the invention is disclosed by means of an exemplary embodiment and by reference to the appended drawings, in which

[0015] figure 1 illustrates schematically and in a simplified way a wiring diagram of a power modifier for fuel cell according to an embodiment of the invention,

[0016] figure 2 illustrates as a simplified schematic drawing the mutual electrical connections of the fuel cell stacks of figure 1,

[0017] figure 3 illustrates as a simplified schematic drawing a detail of a connection making it possible to process return power,

[0018] figure 4 illustrates schematically and in a simplified way a wiring diagram of a power modifier for fuel cell according to another embodiment of the invention, and

[0019] figure 5 illustrates as a simplified schematic drawing the mutual electrical connections of the fuel cell stacks of figure 4.

[0020] In this context the apparatus solution according to the invention is also called a power modifier of a fuel cell unit, the modifier comprising at least a power electronic part connected to the fuel cell unit 1 and an electronics controller 2, 2' of the power electronic part. The main components of the power electronic part according to the

invention are a three-phase frequency converter 3 and a three-phase filter part 4. The electronics controller 2 is arranged to control the three-phase frequency converter 3.

[0021] In a solution according to the exemplary embodiment fuel cell stacks Ia forming the fuel cell unit 1 are mutually connected so that one fuel cell group is formed by twelve fuel cell stacks Ia connected in series, as is shown in Figure 2. Each fuel cell stack produces a chosen DC voltage Ul. Two groups of twelve fuel cell stacks are further connected mutually in parallel, whereby the total voltage potential between the positive output 5 and the negative output 6 of the fuel cell unit 1 is 12xUl. Neutral point output 7, a so-called neutral wire, is taken symmetrically from the middle of the fuel cell unit 1. Thus, there is a voltage +6xUl between the neutral point output 7 and the positive output 5 of the fuel cell unit 1 and a voltage of -6xUl between the neutral point output 7 and the negative output 5 of the fuel cell unit 1. Further, operating voltage for running the electronics controller 2 is taken from a suitable point of the fuel cell unit 1, the voltage being in this case +Ul. The operating voltage of the electronics controller 2 can, if necessary, be taken from another place than that described above. The DC voltage from the fuel cell unit 1 is adapted to suit a number of various purposes at a certain voltage level by changing the amount of fuel cell stacks Ia, internal voltage or connections. In the mutual connection of fuel cell stacks Ia shown in Figure 2 the in¬ termediate wires 13 connecting both fuel cell groups can, depending on the application, be in use or disabled. The advantage of the use of the intermediate wires 13 is dividing the load among the operating fuel cell stacks in case one fuel cell stack is broken.

[0022] Subsequent to having chosen the number, connection and internal voltage level of the fuel cell stacks Ia for a certain purpose, the level of DC voltage produced by the fuel cell unit 1 is already of the correct magnitude for each purpose, so there is no need for increasing or decreasing the DC voltage prior to feeding it into the three-phase frequency converter 3.

[0023] However, as the DC current produced by the fuel cell unit varies according to the load, the load of the fuel cell unit 1 is stabilized by filtering the ripple content of the current by means of capacitances C4 and C5, connected prior to the three-phase frequency converter 3 so that a capacitance C4 is located between the neutral point output 7 and the positive output 5 of the fuel cell unit 1 and a capacitance C5 is corre¬ spondingly located between the neutral point output 7 and the negative output 6 of the fuel cell unit 1. Three-phase frequency converter 3 is connected to the DC output of the fuel cell unit 1, the frequency converter comprising at least six power switches B1-B6, the power switches being power semiconductors according to prior art and also known as IGBT switches. The DC electricity produced by the fuel cell unit 1 is converted into AC electricity in the three-phase frequency converter 3 with pulse width modulation principle so that the output voltages of the three-phase frequency converter are sine-

formed AC voltage.

[0024] The modulation frequency of the pulse width modulation, which is about 5 kHz with high-voltage semiconductors, must be filtered out so that it does not cause in¬ terference in the system. The filtering is carried out by means of a three-phase filter 4 connected to the voltage output of the three-phase frequency converter 3, the filter having at least resonance circuits formed by the inductances L1-L3 and capacitances C1-C3. A suitable frequency, higher than 50 Hz, is chosen as the frequency for the resonance circuit, such as 500 Hz. Thus, at a modulation frequency of about 5 kHz the dampening is about 40 dB.

[0025] In the solution according to the invention the DC electricity produced by the fuel cell unit 1 is converted by means of a three-phase frequency converter 3 and a three- phase filter 4 into a three-phase AC electricity, the outputs of which in the apparatus are designated by letter A, B and C in the Figures 1 and 4. The apparatus further has a neutral point output 7 and if necessary, a ground 7a may be connected thereto.

[0026] The electronics controller 2 of the three-phase frequency converter 3 in Figures 1 and 4 is provided with at least a power source 8 for the electronics, a three-phase generator 9, a dedicated voltage regulator 10 for each phase, an AC pulse width modulator 11 and an IGBT controller 12 connected to each other. The power source 8, 8a of the electronics gets its energy from the fuel cell unit 1, with the output located such that the magnitude of the voltage is +Ul. The power source 8 of the electronics feeds current to the components of the electronics controller 2.

[0027] The output of the three-phase generator 9 is connected to the voltage regulators 10, by means of which the output voltage of all phases of the three-phase frequency converter is separately stabilized in relation to the neutral point output 7. The voltage regulators 10 allow the removal of the load dependency acting on the three-phase frequency converter. The load dependency is caused in the three-phase frequency converter mainly by the heat losses of the resistors and power switches Bl -B 6 of the inductances L1-L3. The load dependency caused by the inductances L1-L3 is indirectly removed by suitably increasing the reference value of the three-phase voltage as a function of the load current. The amount of the load current can be measured by means of the current measurement unit VMl. Additionally, the phase voltage controllers 10 correct in pulse width modulation the voltage distortion caused by the delay of the power switches B1-B6. The pulse width modulator produces a necessary pulse width reference which is fed via an IGBT-controller 12 connected to the output of the pulse width modulator 11 to the power switches B 1-B6 of the three- phase frequency converter 3.

[0028] An important feature in power station applications is the possibility of utilizing and processing return power. Certain kinds of loads, such as elevators, escalators and the

like, can in some conditions return power to the electrical network. In elevator use, for example, such a situation occurs when the counterweight of the elevator pulls an empty or lightly-loaded elevator cab upwards. The apparatus according to the invention can be connected as shown in Figure 3 so that a power resistor Rl and a power switch B9, connected mutually in series are connected prior to the three-phase frequency converter 3, between the positive 5 and the negative 6 output, by means of which the returning power is converted to heat. This connection further includes a cooling fan of the resistors, the fan not being shown in the Figure.

[0029] In Figure 4 there is shown an embodiment the main components of which are two DC/DC-converters 20, a three phase frequency converter 3 and a filter part 4. An electronics controller 2' has been arranged to control the DC/DC-converters 20 and re¬ spectively an electronics controller 2 has been arranged to control the three-phase frequency converter 3. In this embodiment fuel cell stacks Ia forming the fuel cell unit 1 are mutually connected so that one fuel cell group is formed by six fuel cell stacks Ia connected in parallel, as is shown in Figure 5. Each fuel cell stack Ia produces a chosen DC voltage Ul. Two groups of six fuel cell stacks Ia are further connected mutually in series, whereby. Respectively there are twelve fuel cell stacks Ia connected in same manner symmetrically with each other between the negative output 6 and the neutral point output 7, whereby also the voltage potential of 2xUl is generated between the negative output 6 and the neutral point.

[0030] The negative output 7 has been provided symmetrically in the middle of the fuel cell unit 1, whereby the power of both fuel cell groups at the positive side of the neutral point output 7 and the negative side of the neutral point is symmetrically equal.

[0031] According to embodiment shown in Figure 4 there are two DC/DC-converters 20 connected in the arrangement, which converters adapt the DC voltage provided by the fuel cell unit 1 to a required level. Because in this embodiment the available DC- voltage 2xUl with respect to zero potential is lower than is needed a consumption device, the voltage must be increased in most of the applications with said DC/ DC-converter to suitable level. DC/DC-converters 20 are connected to the fuel cell unit 1 so that the first DC/DC-converter is connected to the positive output 5 and the other DC/DC-converter is connected to the negative output 6 of the same fuel cell unit 1. The internal construction of the DC/DC-converter is known as such.

[0032] The first DC/DC-converter is provided with at least an inductance L4, IGBT- switch B7, diodes Dl and D2, and a capacitance C7. Respectively the second DC/ DC-converter is provided with at least an inductance L5, IGBT- switch B8, diodes D3 and D4, and a capacitance C6.

[0033] To describe in simplified manner, the DC/DC-converters 20 operate so that the

IGBT-switch B7, B8 is firstly in leading stage and increases current into the inductance

L4, L5. At the moment when the IGBT-switch stops leading, the current continues to flow to the voltage intermediate circuit of the three-phase frequency converter 3 due to inductive slowness. When the energy in the inductance L4, L5 has been used begins the IGBT-switch B7, B8 lead again and inductance K4,L5 is reloaded with energy. The IGBT-switches B7 and B8, as well as the IGBT-switches B1-B6 of the three-phase frequency converter 3 to be described more closely later are power semiconductors according to prior art, and in the following they are called simply as power switches.

[0034] Because the current needed by the DC/DC-converters 20 the loading of the fuel cell unit 1 has been stabilized by filtering the ripple content of the current with ca¬ pacitances C4 and C5, like in the embodiment shown in the Figure 1. These are connected prior to the DC/DC-converters 20 so that a capacitance C4 is located between the neutral point output 7 and the positive output 5 of the fuel cell unit 1 and a capacitance C5 is correspondingly located between the neutral point output 7 and the negative output 6 of the fuel cell unit 1. The three-phase frequency converter 3 has been connected to the output of said DC/DC-converters 20, which frequency converter 3 comprising at least six power switches B1-B6. The DC electricity provided by the DC/DC-converters 20 is converted into AC electricity in the three-phase frequency converter 3 with pulse width modulation principle so that the output voltages of the three-phase frequency converter are sine-formed AC voltage. The modulation frequency of the pulse width modulation, which is about 5 kHz with high- voltage semiconductors, must be filtered out so that it does not cause interference in the system. This is accomplished mainly as is described in connection with the Figure 1.

[0035] In the Figure 4, the electronics controller 2' of the DC/DC-converters 20 is provided with at least a power source 8a for the electronics, a DC- voltage set value input 15 for both of the DC/DC-converters 20, a DC- voltage regulator 16 and a DC- current regulator 17, a pulse width modulator 18 and IGBT- controller 19 connected to each other. The power source 8a of the electronics gets its energy from the fuel cell unit 1 and feeds current to the components of the electronics controller 2' .

[0036] The DC-voltage controller 16 has been arranged to maintain the DC- voltage provided by the fuel cell unit 1 at constant level based on voltage measurement in¬ formation and DC- voltage set value 15 and thus to eliminate the load dependency acting on the DC-voltage. The voltage is measured at location after the DC/ DC-converter 20 of voltage +U2 via measurement wires 16' and 16". The regulation of voltage makes it possible to arrange the previously mentioned neutral point output 7, because with the voltage regulation the relative neutral point may be maintained in the middle between of the maximum positive and negative values.

[0037] The embodiment of Figure 4 the DC- voltage obtained from the fuel cell unit 1 is increased with DC/DC-converters 20 to required value U2, the nominal value of which

is greater than 2xUl. Respectively the DC-current regulator 17 has been provided for stabilizing the DC-electricity provided by the fuel cell unit 1. The DC-current regulator 17 receives measurement information from current measurement units VMl and VM2. Additionally there is a current limiter provided in the DC-current regulator, the limiter operating as overload safeguard for the fuel cells. The current limiter is not shown in the Figure.

[0038] With the pulse width modulator 18 of the electronics controller 2' the control of the power switches B 7 and B 8 is altered so that the power switches B 7 and B 8 are at leading stage or non-leading stage according to the controlled. The modulation frequency is for example 5 kHz. Respectively the output voltage of the DC/ DC-converters 20 is controlled by changing the modulation ratio. The control of the power switches B 7 and B 8 has been realized through IGBT-controller 19.

[0039] It is obvious to one skilled in the art that the invention is not limited to the above- mentioned example, but it can be varied within the following claims. Thus, the structure and connections of the apparatus can differ from that described above. It is also obvious that the number and mutual connection of the fuel cell stacks in the fuel cell unit can vary as needed from that described above. Also, the components used can be different from that described above as long as they are connected so as to achieve the result according to the invention.

[0040] It is also obvious to one skilled in the art that the solution according to the invention can be used as a power station or a power source for other applications than those described above.