The field of the invention

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 environment is concerned, especially oil and coal cause pollution when they are combusted. The problem with nuclear power is, at least, storage of used fuel.

Especially because of the environmental problems, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed.

Fuel cell's, by means of which energy of fuel, for example biogas, is directly converted to electricity via a chemical reaction in an environmentally friendly process, are promising future energy conversion devices. 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 106 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 fuel 108 producing water and also typically carbon dioxide (C02). Between anode 100 and 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 device can utilize as fuel for example natural gas, bio gas, methanol or other compounds containing hydrocarbons. SOFC device in figure 2 comprises more than one, typically plural of fuel cells in stack formation 103 (SOFC stack). Each fuel cell comprises anode 100 and cathode 102 structure as presented in figure 1. Part of the used fuel can be recirculated in feedback arrangement 109 through each anode. SOFC device in fig 2 also comprises a fuel heat exchanger 105 and a reformer 107.

Typically several heat exchangers are used for controlling thermal conditions at different locations in a fuel cell process. 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 hydrogen and methane, carbon dioxide, carbon monoxide and inert gases. Anyway in each SOFC device it is though not necessary to have a reformer.

For example inert gases are purge gases or part of purge gas compounds used in fuel cell technology. For example nitrogen is a typical inert gas used as purge gas in fuel cell technology. Purge gases are not necessarily elemental and they can be also compound gases.

By using measurement means 115 (such as fuel flow meter, current meter and temperature meter) necessary measurements are carried out for the operation of the SOFC device. Part of the gas used at anodes 100 may be recirculated through anodes in 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 fuel. Advantages of SOFC device include high efficiencies, long term stability, low emissions, and cost. The main disadvantage is the high operating temperature which results in long start up times and both mechanical and chemical compatibility issues.

The anode electrode of solid oxide fuel cell (SOFC) typically contains significant amounts of nickel that is vulnerable to form nickel oxide if the atmosphere is not reducing. If nickel oxide formation is severe, the morphology of electrode is changed irreversibly causing significant loss of electrochemical activity or even break down of cells. Hence, SOFC systems require purge gas, i.e. safety gas, containing reductive agents (such as hydrogen diluted with inert such as nitrogen) during the start-up and shutdown in order to prevent the fuel cell's anode electrodes from oxidation. In practical systems the amount of purge gas has to be minimized because an extensive amount of, e.g. pressurized gas containing hydrogen, are expensive and problematic as space-requiring components.

There is simultaneous need for minimization of the purge gases and heating of the system in the start-up situation and also simultaneous need for minimization of the purge gases and cooling of the system in the shut-down situation. According to prior art applications the amount of purge gases during normal start-up or shut-down is minimized by anode recirculation, i.e. circulating the non-used purge gases back to the loop. However, in emergency shut-down (ESD) that may be caused e.g. by gas alarm or blackout, and there won't be active recirculation available increasing the amount of needed purge gas. In addition, the cathode air flow is not cooling the system during the ESD, because the air blower has to be shut down, and hence the amount of needed purge gas is even more increased as the time to cool the system down to temperatures where nickel oxidation does not happen is even three-fold compared to active shut-down situation. As described, current SOFC stacks require reducing purge gas to protect the anode from oxidation during abnormal situations, like emergency shutdowns. However, still the amount of purge gas is considerable for real field application, especially with larger unit sizes. Stacks are vulnerable towards detrimental nickel oxidation above a certain critical temperature, which lies typically somewhere between 300-400 degrees Celsius. Below this

temperature, nickel oxidation reaction is so slow that there is no more need for reductive atmosphere on the anode. In passive emergency shutdown (ESD) situations, the cooling of the unit is extremely slow (even up to 10 hours or more) due to non-existing air flow through the system, high heat capacitance of the components, and good thermal insulation of the system. Even if active air cooling could be utilized, the cooling is slow because of high efficiency recuperator bringing most of the heat back to the system.

High temperature fuel cell systems, especially SOFC systems, typically contain several fuel cell stacks within one or more insulation enclosures. With respect to explosion safety the fuel cell stacks themselves shall be regarded as potential sources of fuel leakage to their surroundings as well as any flanges and sealing surfaces used within the fuel supply lines. In case of high temperature fuel cells the temperature of the surroundings is typically above the self ignition temperatures of gaseous fuels that are used. Therefore, the traditional approach for dealing with the leakages has been to place all potential sources of leakage within an air or exhaust stream taking care that air is always available in excess amounts to combust any leaking fuel. This approach requires a gas tight enclosure suitable for the pressure level of the air/exhaust stream and large enough to contain all fuel manifolding with non- welded connections (i.e. source of leakage).

The above described method for dealing with leakages is convenient particularly if leakages are relatively high. In some process and layout configurations, however, the requirement to fit all fuel connectors within a gas tight space may negatively impact system compactness, maintainability and cost. Particularly in the case of open-air manifolded stacks, i.e. stacks are to be sealed against the perimeter of an air feed duct, fitting in all fuel connectors within such a duct may be impractical.

Short description of the invention

The object of the present invention is to accomplish a fuel cell system, which comprises an advanced ventilation arrangement to meet explosion safety requirements. This is achieved by a ventilation arrangement for high temperature fuel cell system, 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, the fuel cell system comprising the fuel cells in fuel cell stacks, air feed-in piping for feeding air to the fuel cell stacks, fuel feed piping for feeding fuel to the fuel cell stacks and the fuel cell system comprising potential sources of leakage of explosive gas. The ventilation arrangement comprises means for arranging an insulation space containing the fuel cell stacks and at least part of the fuel feed piping, means for arranging at least outward ventilation flow of the insulation space, means for generating a negative pressure with respect to the pressure inside the insulation space to facilitate a forced ventilation flow, piping arrangement for providing a defined flow route for at least the outward portion of said ventilation flow and the ventilation arrangement comprises flow monitoring means for monitoring ventilation flow and gas composition to form essential leakage information and means to alter said defined flow route to

significantly increase the amount of heat removal from the insulation space caused by said ventilation flow. The focus of the invention is also a ventilation method for high temperature fuel cell system, in which fuel cells are arranged in fuel cell stacks, air is fed to the fuel cell stacks through an air feed-in piping, fuel is fed to the fuel cell stacks through a fuel feed piping and in the fuel cell system exists potential sources of leakage of explosive gas. In the method is arranged an insulation space containing the fuel cell stacks and at least part of the fuel feed piping, is arranged at least outward ventilation flow of the insulation space, is generated a negative pressure with respect to the pressure inside the insulation space to facilitate a forced ventilation flow, is provided a defined flow route for at least the outward portion of said ventilation flow and in the method is monitored ventilation flow and gas composition to form essential leakage information and is altered said defined flow route to significantly increase the amount of heat removal from the insulation space caused by said ventilation flow.

The invention is based on the utilization of an insulation structure, which is formed substantially near the fuel cell stacks. By the formation of the insulation structure an insulation space is being formed containing the fuel cell stacks and at least part of the air feed-in piping. The practical idea is to locate all potential leakage sources outside the insulation space on the other side of the insulation structure than the stacks and the air feed-in piping in the insulation space. According to the invention is arranged and defined at least outward ventilation flow of the insulation space. At least the outward ventilation flow and gas composition are monitored to form essential leakage information and the defined ventilation flow route is altered to significantly increase the amount of heat removal from the insulation space caused by said ventilation flow.

The benefit of the invention is that fuel feed piping can be arranged outside a space, which is purged by reactant flows (e.g. air). Significant advantages can be achieved both in terms of lower system cost and more compact system size.

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 first preferred embodiment according to the present invention.

Figure 4 presents a second preferred embodiment according to the

present invention. Figure 5 presents a third preferred embodiment according to the present invention.

Detailed description of the invention

Solid oxide fuel cells (SOFCs) can have multiple geometries. The planar geometry (Fig 1) is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte 104 is sandwiched in between the electrodes, anode 100 and cathode 102. SOFCs can also be made in tubular geometries where for example either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. This can be also arranged so that the gas used as fuel is passed through the inside of the tube and air is passed along the outside of the tube. Other geometries of SOFCs include modified planar cells (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are promising, because they share the advantages of both planar cells (low resistance) and tubular cells.

The ceramics used in SOFCs do not become ionically active until they reach a very high temperature and as a consequence of this the stacks have to be heated at temperatures ranging from 600 to 1,000 °C. Reduction of oxygen 106 (Fig. 1) into oxygen ions occurs at the cathode 102. These ions can then be transferred through the solid oxide electrolyte 104 to the anode 100 where they can electrochemically oxidize the gas used as fuel 108. In this reaction, water and carbon dioxide byproducts are given off as well as two electrons. These electrons then flow through an external circuit 111 where they can be utilized. The cycle then repeats as those electrons enter the cathode material 102 again. In large solid oxide fuel cell systems typical fuels are natural gas (mainly methane), different biogases (mainly nitrogen and/or carbon dioxide diluted methane), and other higher hydrocarbon containing fuels, including alcohols. Methane and higher hydrocarbons need to be reformed either in the reformer 107 (Fig 2) before entering the fuel cell stacks 103 or (partially) internally within the stacks 103. The reforming reactions require certain amount of water, and additional water is also needed to prevent possible carbon formation (coking) caused by higher hydrocarbons. This water can be provided internally by circulating the anode gas exhaust flow, because water is produced in excess amounts in fuel cell reactions, and/or said water can be provided with an auxiliary water feed (e.g. direct fresh water feed or circulation of exhaust condensate). By anode recirculation arrangement also part of the unused fuel and dilutants in anode gas are fed back to the process, whereas in auxiliary water feed arrangement only additive to the process is water. Because anode electrode of a solid oxide fuel cell typically consists of a porous, nickel matrix ceramic-metallic structure which morphology is critical for cell performance, oxidation of nickel may change the fuel cells performance irreversibly. This is why SOFC systems require purge gas containing reductive agents, such as hydrogen diluted with inert such as nitrogen, in order to prevent anode electrodes of the fuel cell system from oxidation. In practical fuel cell systems it is uneconomical to maintain an excessive purge gas storage, i.e. the amount of safety gas should be minimized. Also a pressurization arrangement, which is needed for the use of purge gas, has a significant effect on the physical size of the fuel cell system.

In the presented approach, fuel connectors are at least partially placed outside an air feed duct, within an insulation space at essentially atmospheric pressure. Forced ventilation is applied to the space within the insulation such that explosion safety codes and regulations are met. In practice this means a ventilation rate of not less than 12-30 (depending on which regulations are used) air changes per hour or a flow dimensioned according to the expected "normal" leakage rate within the area. Since the stacks are still within the air ducting, the expected leakage within the insulation space is limited to that of fuel connectors. This leakage rate can by proper choice of components be low whereby the previously stated ventilation rate of 12-30 air changes per hour (together with leakage monitoring means) can be regarded sufficient. This range of flow is small compared to the cathode air flow, presumably significantly less than a tenth of the mass flow for foreseeable volumes of insulated spaces.

In ventilation arrangements according to the present invention, the fuel cell system comprises at least one heat exchanger 105 for controlling thermal conditions in the fuel cell system, air feed-in piping 130 for feeding air to the fuel cell stacks 103 and fuel feed piping 132 for feeding fuel to the fuel cell stacks 103. The ventilation arrangement comprises means 138 for arranging an insulation space 136 containing the fuel cell stacks 103 and at least part of the air feed-in piping 130. Means 138 for arranging an insulation space are an insulation structure 138, which is formed for example from micro porous insulation material with for example metallic or ceramic supporting structure. The insulation structure 138 is formed substantially near the fuel cell stacks 103, an inside of the boundary structure 138 being the insulation space 136. Potential leakage sources 131 of the fuel feed piping 132 such as for example fuel connector(s) are placed at least mostly outside a gas tight enclosure 139, i.e. outside an air feed duct, within the insulation space 136 at essentially atmospheric pressure. The fuel feed piping 132 comprises at least one fuel connector. Preferably every fuel connector 131 locates outside the air feed duct, because it is a potential leakage source 131, as described. An essentially atmospheric pressure is maintained inside the insulation space 136. The ventilation arrangement comprises means 152 for arranging forced ventilation of the insulation space 136. In one embodiment a suction blower 152 on an exhaust side is used as the means 152 to generate a small negative pressure inside the insulation space to prevent leaking fuel from leaking out through the insulation structure 138. In another embodiment the ventilation arrangement comprises an ejector 152 as the means 152 to generate suction through the insulation space 136. Embodiments according to the invention comprise a piping arrangement 133 for providing a defined flow route for at least the outward portion of the ventilation flow of the insulation space 136. Flow monitoring means 140 monitor feed-in flows into the insulation space 136 and flows out from the insulation space to form leakage information to be utilized in the ventilation of the insulation space 136. The ventilation arrangement preferably comprises the flow monitoring means 140 in the inlet side of the ventilation. For example, a single gas detector preferably locates in a ventilation suction line 146 for forming leakage information from the fuel cell system space 148 outside the insulation space 136. In one embodiment of the invention the ventilation arrangement may comprise a passive cooling source 135 available in emergency shutdown or utility blackout situations to keep the ventilation of the insulation space 136 running in said situations.

In the first preferred embodiment presented in figure 3, forced ventilation of the insulation space 136 is arranged by the suction blower 152, which preferably arranges said forced ventilation through a ventilation heat exchanger 125. The suction blower is placed on the exhaust side in the flow path to generate a small negative pressure inside the insulation space 136 to prevent any leaking fuel from leaking out through the insulation. Also as any leakage through the insulation boundary 138 is inward, local overheating issues at the outer boundary are efficiently removed. A negative pressure inside the ventilated insulation space 136 has the additional benefit of preventing leak-out of burn-in fumes from the insulation materials during first start-ups. Burn-in fumes can thus be discharged to a safe location from the ventilation outlet 146 pipe connection. In the example embodiment according to the figure 3 the ventilation air is circulated first through other parts of the fuel cell system and then through the insulation space 136.

Therefore the whole fuel cell system will be at slightly negative pressure with respect to the surroundings whereby burn in fumes from the other parts of the fuel cell system will also be captured to the ventilation stream(s).

Flow monitoring is required to assure that the ventilation is functional. In the example embodiment according to the figure 3 flow monitoring means 140 have been placed on the inlet side of the ventilation. In addition to monitor the flow, a measurement placed in the inlet stream also safeguards that outward leakage from the gas tight enclosure 139 does not exceed the ventilation flow thereby reversing the direction of leakage through the insulation structure 138. The amount of leakage can be determined by measuring both inward and outward flow. Alternatively, the proportions of the flows can be calculated from the inlet and outlet temperatures of the heat exchanger 105 streams. A third method of monitoring the inward leakage is to compare the inlet flow information to a known characteristic of the heat exchanger or blower.

Still referring to the first preferred embodiment of figure 3, a cooling source 135 may be required in the suction line upstream of the suction blower 152 to keep the blower operation temperature sufficiently low, particularly if leakage to the insulation space 136 is significant. Since the overall cooling duty is relatively low, the cooling source 135 could be e.g. a passive convection cooler placed e.g. on the roof of the fuel cell system. A

controllable bypass or three-way valve may in this case be needed to avoid condensation in cold ambient conditions. If the cooling source 135 is a passive cooling source, then cooling is available also in emergency shutdown or utility blackout situations whereby the ventilation could be kept running with an UPS-backupped EX-approved blower 152. In such a configuration, the optional bypass valve 137 could be opened to facilitate rapid cooling of the insulation space 136 by feeding in cold air, possibly also at a higher flow rate than in nominal operation. This could significantly reduce the amount of purge gas (i.e. safety gas) required for anode protection during an

emergency shutdown situation. As the ventilation does flow into the gas tight enclosure 139, stacks will be protected to a degree from excessive thermal stress due to cold air feed.

Moreover, as the ventilation arrangement according to the present invention allows for reducing the size of the gas tight enclosure 139, i.e. cathode 102 flow duct, surrounding the stack 103, possible shutdown purging of the cathode side can be accomplished more efficiently and with less amount of purge gas. In shutdown situations, the pressure inside the cathode flow duct will approach pressure of the fuel cell system air intake i.e. pressure of the surroundings. Hence, the pressure in the insulation space 136 outside the duct is negative compared to the pressure inside the duct whereby air will not leak into the purged duct.

An alternative to an active or passive cooling source for keeping the operation temperature of the suction blower 152 within its operation temperature limits is to mix in cold air to the possibly hot stream upstream of the blower 152. This kind of cold air supplement could be a part of the fuel cell system ventilation air supplement whereby the cooling function is accomplished without adding fuel cell system cost or energy consumption of the fuel cell system.

In figure 4 is presented a second preferred embodiment, where the ejector 152 is used to generate the suction through the insulation space 136. Also this embodiment preferably comprises a ventilation heat exchanger 125, through which said suction, i.e. forced ventilation is arranged. In this kind of embodiment, fuel cell system ventilation stream can serve as primary flow in the ejector 152 whereby a low entrainment ratio, high efficiency subsonic ejector can be used. In either of the preferred embodiments presented in figure 3 and 4, leakage monitoring of the cold unit compartments can be accomplished with a single gas detector 140 in the unit ventilation suction line. The suction can be drawn from multiple locations within the fuel cell system to maximize the detection probability of leakage or to provide local cooling to critical components. In embodiments according to the invention oxygen content level inside the insulation space 136 can be determined in flow

measurements made by an oxygen sensor 141. Preferably for the leakage monitoring in the insulation space 136, an oxygen partial pressure sensor 141 can be used if operating temperatures are higher than gas self ignition temperatures. A catalyst may be included in the exhaust line to reduce e.g. CO-slip originating from normal flange leakages 131, which are examples of sources 131 of leakage. A source of ignition could also be included to allow for using the oxygen partial pressure monitoring for gas safety also when the air in the insulation space 136 is below fuel self ignition temperatures e.g. during start-up conditions. In embodiments according to the invention is made possible to rinse leakages from the insulation space 136. By this way is achieved the advantage to locate potential sources 131 of leakage, such as for example fuel connectors and flange joints 131, to the rinsed insulation space 136. The presented approaches according to the invention for leakage handling in insulation spaces offer potential for significant advantages in terms of system cost and compactness, gas safety, commissioning procedures (routing of burn-in fumes) and/or emergency cool down speed. In the embodiment where the emergency cool down speed is faster, a substantial amount of energy production time can be spared because of the faster cooling process.

In figure 5 is presented a third preferred embodiment according to the present invention. In this embodiment the piping arrangement 133 comprises a fuel cell system space 148 ventilation, which is arranged by an active common ventilation device for both the insulation space 136 and the fuel cell system space 148 outside the insulation space 136. This embodiment also comprises bypass valves 143 to increase the outward ventilation flow especially in emergency shutdown situations. In this third preferred embodiment the flow needed to ventilate the insulation space is small enough not to cause too large heat loss even without a ventilation heat exchanger. This provides the most simple implementation where inward flow of air to the ventilation space can take place at arbitrary locations through the insulation structure or alternatively through a dedicated route. Flow monitoring means 140 are in this case placed in the outward flow piping preferably in relation to flow restriction means 149 used to achieve a suitable flow when a common suction blower 152 is used to ventilate both the fuel cell system space 148 and the insulation space 136.

A gas detector for monitoring presence of explosive gases can be shared between the insulation space 136 and the fuel cell system space 148. By placing a gas detector in a pipe branching from the system ventilation suction line and connecting to the insulation space ventilation line 146 through a controllable 2- or 3-way valve 147 the gas content can be sampled either from the insulation space or from the fuel cell system space depending on valve position. Such an arrangement can be satisfactory from a safety point of view in the case that the insulation space is normally above self ignition temperature for all explosive gas components whereby traditional gas monitoring is needed only in special situations (start-up and shut-down) of short duration. During normal operation explosion safety is based on the flow monitoring and on the residual oxygen content monitoring. In shutdown situations the bypass valves 143 to bypass the flow restriction and possibly enlarge the inward flow routes can be opened to radically increase the ventilation flow inside the insulation space 136.

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.