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 (CO ₂ ). 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 from the anode gas. 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.

A solid oxide fuel cell (SOFC) device is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Advantages of SOFC device include high efficiencies, long term stability, low emissions, fuel versatily and cost. The main disadvantage is the high operating temperature which results in long start up times and both mechanical and chemical compatibility issues.

In large fuel cell systems, such as in SOFC systems, there are several fuel cell stacks each providing part of the required power. In addition to cost, from system and manufacturing complexity point of view the stacks cannot be separately actively controlled flow-wise. This means that there has to be manifolding that provides the inlet fuel to all stacks in a uniform manner in order to enable high fuel utilization (FU) rate in all stacks, and hence also high system efficiency. Anyway, as stack manufacturing is not trivial and mature technology, the degradation rate between stacks may be inherently different, and hence some stacks may be in their end-of-life performance significantly sooner than others in a system. Different end-of-life performance may also be due to possible inhomogeneities in stack operating environment, and also due to simple statistical flaws e.g. in cell materials. Different performance levels in due course, and different end-of-life performances means that the system operation cannot be continued before poor stacks are replaced and after that long start-up times has to be again introduced. Another alternative is that the system has to be operated with significantly lowered efficiency as part of the fuel is flowing through stacks that practically do not contribute to power generation.

Short description of the invention

The object of the invention is to accomplish a high temperature fuel cell system that continues successfully to produce electricity by blocking out fuel cells which are in their end-of-life performance and thus avoiding long start- up times in producing electricity with the fuel cell system. This is achieved by an arrangement for controlling high temperature fuel cell system efficiency for producing electricity with fuel cells, each fuel cell in the fuel cell system comprising an anode side, a cathode side, an electrolyte between the anode side and the cathode side, and the fuel cell system comprising a fuel delivery channel for feeding fuel flow to the anode sides. The arrangement for controlling high temperature fuel cell system efficiency comprises means for grouping fuel cells into groups, each said group comprising at least one fuel cell, means for monitoring the performance of said groups and noticing if one or more of said groups performs below a determined performance level, and means for derating the power level of the fuel cell system by switching off one or more of said groups performing below said performance level and by preventing the fuel flow to the anode sides of said one or more switched off groups.

The focus of the invention is also a method for controlling high temperature fuel cell system efficiency for producing electricity with fuel cells, in which method the fuel is fed to anode sides of the fuel cells. In the method fuel cells are grouped into groups so that each said group comprising at least one fuel cell, the performance of said groups is monitored by noticing if one or more of said groups performs below a determined performance level, and the power level of the fuel cell system is derated by switching off one or more of said groups performing below said performance level and by preventing the fuel flow to the anode sides of said one or more switched off groups.

The invention is based on that fuel cells are physically grouped into groups and the performance of said groups is monitored by noticing if one or more of said groups performs below a determined performance level. If one or more of the fuel cell groups performs below said level, these groups are switched off and the fuel flow to the anode sides of these groups is prevented by thus derating the power level of the high temperature fuel cell system.

The benefit of the invention is that a long term fuel cell system operation in producing electricity can be achieved, that is very important for example when the fuel cell system is used for back up power operation.

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 launching arrangement according to the preferred embodiment of the invention. Detailed description of the invention

In the embodiments according to the invention the stacks are grouped in a way that there are several stack "bunches" consisting e.g. from 2 or 4 stacks that have separate current control, can be shut off from receiving fuel to the anodes. When poor performing stacks can be electrically switched off and the fuel flow to these prevented, the system can be driven with a de-rated power level, but still with high efficiency as the high fuel utilization rate can be maintained, meaning that the system can continue producing power with high efficiency without the need of expensive and time consuming stack replacing procedure until some specified number of stacks need to be changed. The invention also increases the availability of a fuel cell system, as its operation can be continued longer with derated power level even if some stacks are partially or even totally malfunctioning. Increase in availability is important especially in back-up power applications, which are considered one of the key market segments for the fuel cell systems.

Shutting the fuel flow is problematic to accomplish with the aid of motor- driven valves, as from practical point of view the flow distribution has to be made in hot compartment, where temperature of the fuel going into stacks is approximately between 600-700 Celsius degrees. Valves operating at such high temperatures are expensive and/or non-reliable. Hence in the invention a "passive" method for flow shut off is suggested. One possible approach is to utilize a physical blockage in the feed-in channel that can be launched e.g. with purge gas already existing in the system. Anyway such a blockage does not need to be absolutely gas tight.

In figure 3 is presented a preferred embodiment according to the present invention. This embodiment comprises means 120 for grouping fuel cells physically into groups so that each group comprises at least one fuel cell stack. Further this embodiment comprises means 122 for arranging a separate current control between each said group. These means 122 are for example switching means for arranging a switching arrangement where current control is switched separately between each said group. Another alternative is for example to build an own, physically discrete current control for each group.

The monitoring of performance of fuel cell groups and noticing, if one or more of said groups performs below a determined performance level, is carried out by means 124 that comprises measurement means for measuring current and/or voltage of fuel cell groups, and a control processor 124 to notice when some measurement value is below said determined performance level. This level can be calculated beforehand in the control processor or in a separate processor and be stored to the control processor. Said performance level can also be calculated simultaneously during fuel cell process in the control processor 124 or in the separate processor. One or more groups noticed to perform below said performance level are switched off from electricity production by using, for example, switches controlled by said control processor 124.

The fuel flow is prevented to the anode sides of said one or more switched off fuel cell groups by using a launching arrangement according to the preferred embodiment of the invention. This launching arrangement utilizes gravity force in its operation to achieve the objects of the invention. This arrangement is presented in figure 4 comprising a fuel delivery channel 130 wherefrom the fuel is fed to the anode sides of fuel cell groups through a separate fuel feed-in channel 128 for each said group and the arrangement comprising a launchable blockage 126 to be launched into the fuel feed-in channel 128 when a fuel cell group is noticed to perform below the performance level. In this exemplary preferred embodiment the launching order is given by the control processor 124 and the launchable blockage 126 is launched for example with purge gas through a impulse line 134. This launching, by a preferably instant high pressure effect, can of course be accomplished with other gases and other kind of arrangements but purge gas usually already exists in this kind of high temperature fuel cell systems. When a high pressure, for example 10 bar, purge gas hits a spring sleeve 136, it moves (to left in figure 4), so that a dropping piece 138 falls off through a hole 140 in the spring sleeve 136. By utilizing gravity force this makes the launchable blockage 126 to drop and hit corresponding surfaces 132 and settle down against them thus blocking the fuel feed to this fuel cell group. The sealing ability of the blockage does not need to be 100%, but it needs to be substantially sufficient to block the fuel flow to the stack, or group of stacks, that are disconnected from the system.

The preferred launching arrangement presented in figure 4 further comprises insulating materials 142 in several locations to prevent fuel leakage. Soft parts 148 can be inserted to prevent vibration for example when utilizing this launching arrangement in a ship environment. The spring leeve 136 can be coated with a ceramic material to prevent it from welding against other arrangement part in this hot environment. The wire 146 may be partly in spring formation. When the operation of the fuel cell system is shut down for example for service purposes, dropped blockages 126 have to be lifted up. The blockage 126 can be lifted for example by means of a wire attachment to the dropping piece 138, which wire attachment has the enough free movement capacity to allow the dropping procedure to happen when the blockage 126 is at the upper position during fuel cell system operation according to the invention. It is also to be noticed that, instead of utilizing gravity force, same kind of arrangement as described with figures 3 and 4 can be arranged by utilizing for example spring force.

Other alternatives to the embodiments according to the invention, instead of using the above described launching arrangement, are for example to use a hot environment, 400 ⁰ C - 700 ⁰ C (or even warmer), tolerating valve arrangement, or to use a mass part that is shot to close the feed-in channel, the mass part also tolerating the said hot environment well enough to be long lasting. As well as described with SOFCs the present invention can also be utilized with MCFCs (Molten Carbonate Fuel Cells) and other high temperature fuel cells that operate at 400 ⁰ C and higher temperatures. 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 high temperature fuel cell system, SOFC or MCFC, where this invention is utilized, does not need to have a feedback arrangement though the feedback arrangement is described in SOFC device examples of figures 2 and 3.

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.