WO2011004057A1

WO2011004057A1 - Method and arrangement for improved controllability of parallel coupled fuel cells

Info

Publication number: WO2011004057A1
Application number: PCT/FI2010/050439
Authority: WO
Inventors: Kim ÅSTRÖM
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2009-07-08
Filing date: 2010-05-31
Publication date: 2011-01-13
Anticipated expiration: 2012-01-08
Also published as: FI20095776A0; FI123172B; FI20095776A7

Abstract

The object of the invention is an arrangement for controlling current values in a fuel cell system for producing electricity with fuel cells, each fuel cell in the fuel cell system comprising an anode side 100, a cathode side 102 and an electrolyte 104 between the anode side and the cathode side, and the fuel cell system comprises at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, each said stack 103 comprising at least one fuel cell. The arrangement for controlling current values in the fuel cell system comprises means for detecting current sharing between said at least two electrically parallel connected individual fuel cell stacks 103 or groups of fuel cell stacks 103. The arrangement also comprises active control means 112 for controlling said detected current sharing comprising compensational electrical sources 112 being capable of sourcing or sinking a small voltage, being integrated in serial connection to all, or to all except one, at least two electrically parallel connected individual fuel cell stacks 103 or to all, or to all except one, at least two electrically parallel connected groups of fuel cell stacks 103, so that power levels to be converted in said active control means 112 are substantially small compared to total power in said parallel connections.

Description

Method and arrangement for improved controllability of parallel coupled fuel cells

The field of the invention

Especially because of the environmental problems, new energy sources, that are environmentally friendly and having good efficiency, have been developed. Fuel cell devices are promising future energy conversion devices by means of which fuel, for example bio gas, is directly transformed to electricity via a chemical reaction in an environmentally friendly process.

The state of the art

Fuel cell, as presented in fig 1, comprises an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with the used fuel producing water and also typically carbon dioxide (CO₂). Between the anode 100 and the cathode 102 is an external electric circuit 111 comprising a load 110 for the fuel cell.

In figure 2 is presented a SOFC device as an example of a high temperature fuel cell device. SOFC devices can utilize for example natural gas, bio gas, methanol or other compounds containing hydrocarbon mixtures as fuel. SOFC device system in figure 2 comprises of more than one, typically multiple fuel cells in one or more stack formation 103 (SOFC stack(s)). A larger SOFC device system comprises many fuel cells in several stacks 103. Each fuel cell comprises of anode 100 and cathode 102 structures as presented in figure 1. Part of the used fuel may be recirculated in feedback arrangement 109. SOFC device in fig 2 also comprises a fuel heat exchanger 105 and a reformer 107. Heat exchangers are used for controlling thermal conditions in the fuel cell process and there can be more than one of them in different locations of a SOFC device. The extra thermal energy in circulating gas is recovered in one or more heat exchangers 105 to be utilized in the SOFC device or externally. Reformer 107 is a device that converts the fuel such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing all or at least some of the following: hydrogen, methane, carbon dioxide, carbon monoxide, inert gases and water. Anyway in each SOFC device it is though not necessary to have a reformer.

By using measurement means 115 (such as fuel flow meter, current meter and temperature meter) necessary measurements for the operation of the SOFC device are carried out. Only part of the gas used at the anodes 100 is recirculated in the feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100.

Fuel cells are electrochemical devices converting the chemical energy of reactants directly to electricity and heat. Fuel cell systems have the potential to significantly exceed the electrical and CHP (Combined production of Heat and Power) efficiency of traditional energy production technologies of comparable size. Fuel cell systems are widely appreciated as a key future energy production technology.

In order to maximize the performance and lifetime of fuel cell systems accurate control of the fuel cell operating conditions is required.

Fuel cells produce DC current whereas in higher power systems typically AC output is desired and thus a power conversion from DC to AC is required. To allow for practical interfacing and current collection from the fuel cells and subsequent power conversion, the fuel cells are manufactured as stacks containing several series connected individual cells. In fuel cell systems comprising of several stacks, the electrical

interconnection topology of the stacks is a key design parameter. Series connection of several stacks provides for lower cabling and power conversion losses as well as lower cost for components. Electrical isolation limitations as well as the preferred operating voltage level of the fuel cell load typically, however, limit the feasible amount of stacks to be serially connected. Hence, if higher power levels are required than what can be achieved with a single string of serial connected stacks, some sort of parallel connection of stacks or groups of stacks becomes a necessity.

When electrical sources such as fuel cells are connected in parallel, uneven load sharing may occur if there are deviations in the electrical characteristics of the individual sources. With fuel cells, this is a significant issue since uneven load sharing may reduce the efficiency, due to reduced fuel utilization, and/or significantly deteriorate those fuel cells operating above the average current. Due to inherent variances in series resistance between stacks as well as variations due to age, temperature etc, uneven load sharing to some extent is typically to be expected if stacks are connected directly in parallel. Electrical parallel connection of stacks is particularly problematic in high temperature fuel cell systems due to intrinsic negative temperature coefficient of their internal resistance. This characteristic gives rise to a positive feedback behaviour in the load sharing balance between parallel connected stacks ,i.e. a stack with higher current heats up, which tends to increase the current further due to decreased internal resistance. To avoid the current sharing issues, separate converters for each stack or series of stacks are often used, bringing a considerable higher cost to the system.

In Finnish patent publication FI118553 Bl is presented a biocatalytic fuel cell arrangement where fuel cells are connected in parallel or in series. This arrangement comprises controllable switches that are controlled by using a control circuit so that said switches change cyclically to and from conducting state as an object to increase the output voltage of said biocatalytic fuel cell arrangement. Anyway neither FI118553 Bl presents a solution to the described problem of parallel connected fuel cell stacks, because in FIl 18553 Bl is not presented an embodiment where each, or almost each, parallel connection can be individually, and with a substantially small power effect, controlled in relation to other at least one parallel connection.

Short description of the invention

The object of the invention is to accomplish a fuel cell system where many fuel cells can be serially connected so that the loading of fuel cell stacks in a fuel cell system can be actively optimized and a lifetime for each fuel cell can be made longer and thus the lifetime for the whole fuel cell system can be made even substantially longer. This is achieved by an arrangement for controlling current values in a fuel cell system for producing electricity with fuel cells, each fuel cell in the fuel cell system comprising an anode side, a cathode side and an electrolyte between the anode side and the cathode side, and the fuel cell system comprises at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, each said stack comprising at least one fuel cell. The arrangement for controlling current values in the fuel cell system comprises means for detecting current sharing between said at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, and active control means for controlling said detected current sharing comprising compensational electrical sources being capable of sourcing or sinking a small voltage, being integrated in serial connection to all, or to all except one, at least two electrically parallel connected individual fuel cell stacks or to all, or to all except one, at least two electrically parallel connected groups of fuel cell stacks, so that power levels to be converted in said active control means are substantially small compared to total power in said parallel connections.

The focus of the invention is also a method for controlling current values in a fuel cell system for producing electricity with fuel cells, each fuel cell in the fuel cell system comprising an anode side, a cathode side and an electrolyte between the anode side and the cathode side, and the fuel cell system comprises at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, each said stack comprising at least one fuel cell. In the method current values are controlled in the fuel cell system by detecting current sharing between said at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, and by controlling said detected current sharing in active control means by utilizing compensational electrical sources being capable of sourcing or sinking a small voltage, being integrated in serial connection to all, or to all except one, at least two electrically parallel connected individual fuel cell stacks or to all, or to all except one, at least two electrically parallel connected groups of fuel cell stacks, and by converting in the active control means power levels that are substantially small compared to total power in said parallel connections.

The invention is based on the detection of current sharing between said at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, and on the utilizing of active control means for controlling said detected current sharing. Said active control means comprises compensational electrical sources being capable of sourcing or sinking a small voltage, being integrated in serial connection to all, or to all except one, at least two electrically parallel connected individual fuel cell stacks or to all, or to all except one, at least two electrically parallel connected groups of fuel cell stacks and this is accomplished so that power levels to be converted in said active control means are substantially small compared to total power in said parallel connections.

The benefit of the invention is that by using a considerably small amount of compensating power is achieved a needed control capacity for even current sharing between parallel connections with significant savings in economical cost, physical size and operation power losses. Short description of figures

Figure 1 presents a single fuel cell structure.

Figure 2 presents an example of a SOFC device.

Figure 3 presents a preferred embodiment according to the present

invention.

Figure 4 presents a first preferred implementation for compensational electrical sources 112.

Detailed description of the invention

According to the invention can be provided an arrangement in which fuel cell stacks or series of fuel cell stacks can be connected in parallel with active control means for maintaining even current sharing without the need for driving all of the power from the members in the parallel connection through separate converters. This active load sharing control according to the invention is accomplished by an introduction of active control means comprising small compensational electrical sources in series with every stack string or all stack strings except one. The compensational electrical source maintain the desired current in the stack string by sourcing or sinking a small voltage proportional to the deviation of the stack string compared to the other parallel connected stack strings.

Since the expectable variance between different stack strings in the parallel connection can be expected to be relatively small, i.e. a few percent and even less if the string comprises of several stacks, the compensational electrical sources only need to source or sink a remarkably small voltage compared to the full voltage of the stack string. Hence, the power rating of these compensational electrical sources is only a small fraction of the total power of the corresponding stack string whilst still maintaining the ability to avoid uneven current sharing. Correspondingly, the price, size and

conversion losses of the compensational electrical sources are only a small fraction of that of a full-power converter, e.g. buck or boost converter, for each stack string.

In figure 3 is presented a preferred arrangement according to the present invention. In this fuel cell device several fuel cell stacks 103 are serially connected and the groups of serially connected fuel cell stacks 103 are parallel connected. Although the example figure 3 shows only three fuel cell stacks 103 in serial connection, there can be many more fuel cell stacks in serial connection and this group of serially connected stacks 103 can be in parallel connection to one or more other group(s) of serially connected stack(s). In figure 3 active control means 112 are integrated to each group of serial connected stacks 103 but it is also possible that among the group of serial connected stacks of fuel cell stacks 103 is one group, or even more groups, where active control means 112 do not exist. The integration of active control means 112 is accomplished so that power levels to be converted in said active control means 112 are substantially small compared to total power in said parallel connections.

As referred to figure 3 active control means 112 comprise measurement means for sensing voltage differences between said parallel connections. Active control means 112 comprise compensational electrical sources in said parallel connections to maintain a substantially even current sharing between said parallel connections. Active control means 112 also comprise a system 119 for controlling said compensational electrical sources in said parallel connections to source or sink a small voltage proportional to said serial connected stacks 103, i.e. stack strings, on the basis of said sensed voltage differences. The system control 119 can be located separately or as integrated into the same unit with other active control means 112. Said system control is preferably processor based and can it be implemented as a software and/or a hardware based topology. The arrangement in figure 3 also comprises a DC-AC converter 121. An arrow 117 describes an AC output power from the DC-AC converter 121. Also as a converter can be a dc-dc converter and then as said output power is dc power.

In a detailed example (not figured as such) of a preferred embodiment according to the invention, the arrangement comprises 40 stacks 103, each stack operating nominally at voltage of 65V and at current of 5OA. The power is being fed to a DC/AC converter 121, operating at 600-800V DC-link voltage. According to the prior art, to avoid current sharing issues the stacks would not be connected directly to the DC-link, but they would be equipped with separate converters to control the load of each series connected stack string. In this example 13OkW of DC/DC conversion is carried out, giving rise, in the prior art situation, to losses typically in the range of 1-4% of the transferred power (i.e. 1.5-5kW). According to the invention, the stacks could be arranged in four paralleled series of 10 stacks, each series being equipped with a small compensational current source 112. Assuming that the voltage of individual stacks 103 varies in the range of ±5% from the average then statistically the variance between the four strings becomes

±5%/sqrt(10) « ±1.6%. Hence, the current sources 112 can be dimensioned for sourcing/sinking e.g. ±2% of the total voltage/power whereby the required DC/DC conversion volume is 2%*130kW = 2.6kW. Since the extent of power is now only a small fraction of the total power, low-price topologies with moderate efficiencies may be used without violating the over all efficiency. Even with a very moderate conversion efficiency of 85% in the compensational current sources, the total worst case loss in the example is only 1.6%*130kW*15% « 300W i.e. approximately a decade less than in the prior art arrangement.

In figure 4 is presented a first preferred implementation for compensational current sources 112 as compensational electrical sources, that are designed to have one pole in common, whereby the individual current sources can be implemented with a cost-effective non-isolated topology having one common isolated power source 123 feeding all compensational current sources 112. A standard isolated 24 V power supply 123 can be used to supply for compensational current sources 112 having for example a common + pole (e.g. buck DC/DC converters). This allows for dimensioning the common isolated supply 123 for the average current of all compensational sources 112 rather than the maximum current. The compensational current source is in serial connection to each string of fuel cell stacks. In the example of figure 4 there are three strings of fuel cell stacks in parallel connections, but there are no limitation for this amount so there can be from two up to very many strings in parallel connections.

The compensational current sources 112 in figure 4 comprise coils L, switches S, diodes D, and capacitors C. Switches S in parallel connections to diodes D are not necessary. In addition to said compensational current sources, active control means 112 in this preferred embodiment comprises a system control 119, that is for example a programmable logic controller (PLC). Active control means 112 also comprise current measuring means I integrated to compensational current sources to sense current differences between said strings of fuel cell stacks for producing control signals. The system control 119 utilizes said control signals in controlling switches S to open or closed positions to accomplish a substantially even current sharing between said strings of fuel cell stacks. Switches S are preferably bipolar- or FET-transistor or some other transistor switches. This arrangement in figure 4 also comprises a load 121 that is e.g. a DC-AC converter. An arrow 117 describes an AC output power from the DC-AC converter 121.

As an example of other implementations for compensational electrical sources 112 is mentioned an embodiment where compensational current sources are implemented as e.g. individually isolated converters to be placed for example arbitrarily in each serially connected string. This embodiment can be referred to figure 3.

The system control 119 can comprise different kind of analogical and/or digital electronics implementations that are for example programmable processor based. The active control means 112 can also comprise different kind of measuring arrangements for sensing voltage and/or current differences in said at least one parallel connection. As well as described with SOFCs the present invention can also be utilized with MCFCs (Molten Carbonate Fuel Cells) and other fuel cells. MCFCs are high temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic matrix. Also the fuel cell system, where this invention is utilized, does not need to have a feedback arrangement though the feedback arrangement is described in SOFC device example in figures 2.

Although the invention has been presented in reference to the attached figures and specification, the invention is by no means limited to those, as the invention is subject to variations within the scope allowed for by the claims.

Claims

1. An arrangement for controlling current values in a fuel cell system for producing electricity with fuel cells, each fuel cell in the fuel cell system comprising an anode side (100), a cathode side (102) and an electrolyte (104) between the anode side and the cathode side, and the fuel cell system comprises at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, each said stack (103) comprising at least one fuel cell, characterized by, that the arrangement for controlling current values in the fuel cell system comprises:

- means for detecting current sharing between said at least two electrically parallel connected individual fuel cell stacks (103) or groups of fuel cell stacks (103), and

- active control means (112) for controlling said detected current sharing comprising compensational electrical sources (112) being capable of sourcing or sinking a small voltage, being integrated in serial connection to all, or to all except one, at least two electrically parallel connected individual fuel cell stacks (103) or to all, or to all except one, at least two electrically parallel connected groups of fuel cell stacks (103), so that power levels to be converted in said active control means (112) are substantially small compared to total power in said parallel connections.

2. An arrangement in accordance with claim 1, characterized by, that said compensational electrical sources (112) are compensational current sources (112).

3. An arrangement in accordance with claim 2, characterized by, that said compensating current sources (112) are implemented as individually isolated converters.

4. An arrangement in accordance with claim 2, characterized by, that said compensational electrical sources (112) are implemented as a nonisolated current source topology comprising one common isolated power source (123) feeding said compensational electrical sources (112), that are designed to comprise one pole in common.

5. An arrangement in accordance with claim 1, characterized by, that the active control means (112) comprises said compensational electrical sources (112) being capable of sourcing or sinking a small voltage on the basis of sensed current differences so that power levels to be converted in said active control means (112) are substantially small compared to total power in said parallel connections.

6. An arrangement in accordance with claim 1, characterized by, that the active control means (112) comprises said compensational electrical sources (112) for accomplishing a substantially even current sharing between said at least two electrically parallel connected individual fuel cell stacks or parallel connected groups of serial connected fuel cell stacks.

7. An arrangement in accordance with claim 1, characterized by, that the active control means (112) comprise a system control (119) for controlling said compensating electrical sources (112) in said parallel connections to source or sink a small voltage proportional to said individual fuel cell stacks or groups of serial connected fuel cell stacks on the basis of sensed voltage and/or current differences for maintaining a substantially even current sharing between said at least two electrically parallel connected individual fuel cell stacks or parallel connected groups of serial connected fuel cell stacks.

8. A method for controlling current values in a fuel cell system for producing electricity with fuel cells, each fuel cell in the fuel cell system comprising an anode side (100), a cathode side (102) and an electrolyte (104) between the anode side and the cathode side, and the fuel cell system comprises at least two electrically parallel connected individual fuel cell stacks or groups of fuel cell stacks, each said stack (103) comprising at least one fuel cell,

characterized by, that in the method current values are controlled in the fuel cell system by detecting current sharing between said at least two electrically parallel connected individual fuel cell stacks (103) or groups of fuel cell stacks (103), and by controlling said detected current sharing in active control means (112) by utilizing compensational electrical sources (112) being capable of sourcing or sinking a small voltage, being integrated in serial connection to all, or to all except one, at least two electrically parallel connected individual fuel cell stacks (103) or to all, or to all except one, at least two electrically parallel connected groups of fuel cell stacks (103), and by converting in the active control means (112) power levels that are substantially small compared to total power in said parallel connections.

9. A method in accordance with claim 8, characterized by, that in the method said compensational electrical sources (112) are compensational current sources (112).

10. A method in accordance with claim 9, characterized by, that in the method said compensating current sources (112) are implemented as individually isolated converters.

11. A method in accordance with claim 9, characterized by, that in method said compensational electrical sources (112) are implemented as a nonisolated current source topology comprising one common isolated power source (123) feeding said compensational electrical sources (112), that are designed to comprise one pole in common.

12. A method in accordance with claim 8, characterized by, that said compensational electrical sources (112) are capable of sourcing or sinking a small voltage on the basis of sensed current differences so that power levels that are converted in said active control means (112) are substantially small compared to total power in said parallel connections.

13. A method in accordance with claim 8, characterized by, that in the method a substantially even current sharing between said at least two electrically parallel connected individual fuel cell stacks (103) or parallel connected groups of serial connected fuel cell stacks (103) is accomplished by utilizing said compensational electrical sources (112).

14. A method in accordance with claim 8, characterized by, that in the method a substantially even current sharing is maintained between said at least two electrically parallel connected individual fuel cell stacks (103) or parallel connected groups of serial connected fuel cell stacks (103) by controlling compensating electrical sources (112) in said parallel connections to source or sink a small voltage proportional to said individual fuel cell stacks or groups of serial connected fuel cell stacks on the basis of sensed voltage and/or current differences.