Fuel cell devices are electrochemical devices, which can enable production of electricity with high duty ratio in an environmentally friendly process. Fuel cell technology is considered to be one of the most promising future energy production methods.
An exemplary fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. The reactants fed to the fuel cell devices undergo a process in which electrical energy and heat are produced as a result of an exothermal reaction.
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 the used fuel 108 producing water and also, for example, carbondioxide (CO2). Between the anode and cathode is an external electric circuit 111 for transferring electrons e− to the cathode. An external electric circuit can include a load 110.
FIG. 2 shows a SOFC device, which can utilize as fuel for example, natural gas, bio gas, methanol or other compounds comprising (e.g., containing) hydrocarbons. The SOFC device in FIG. 2 can include planar-like fuel cells in stack formation 103 (SOFC stack). Each fuel cell can include an anode 100 and a cathode 102 structure as presented in FIG. 1. Part of the used fuel can be recirculated in feedback arrangement 109 through each anode.
The SOFC device in FIG. 2 can include a fuel heat exchanger 105 and a reformer 107. Heat exchangers can be used for controlling thermal conditions in a fuel cell process and there can be more than one of them located in different locations of SOFC device. The extra thermal energy in a circulating gas can be recovered in the heat exchanger 105 to be utilized in the SOFC device or outside in a heat recovering unit. The heat recovering heat exchanger can thus be located in different locations as shown in FIG. 2. A reformer is a device that converts the fuel, such as for example natural gas, to a composition suitable for fuel cells such as, for example, a composition comprising (e.g., containing) half hydrogen and half methane, carbondioxide and inert gases. The reformer is not, however, necessary in all fuel cell implementations, but untreated fuel may also be fed directly to the fuel cells 103.
By using a measurement device 115 (such as fuel flow meter, current meter and temperature meter) measurements can be performed for the operation of the SOFC device from the through anode recirculating gas. Only part of the gas used at anodes 100 (FIG. 1) of the fuel cells 103 is recirculated through anodes in the feedback arrangement 109 and thus FIG. 2 diagrammatically shows another part of the gas is exhausted 114 from the anodes 100.
A fuel cell device can produce electrical energy in the form of direct current of a low voltage level. The voltage level can be raised by combining several fuel cells or combinations of fuel cells to form a serial connection such as, for example, a stacked formation. Current-voltage characteristics of the fuel cells depend on, for example, reactant compositions, mass flow, temperature and pressure. Electrochemical reactions in the fuel cell can react quickly to fluctuations in the fuel cell load. However, the response capacity of reactants input can be much slower, meaning response times of seconds or even minutes. When trying to obtain more efficiency out of fuel cells than the prevailing input of reactants allows, a weakening of fuel cell voltages can occur, and even an irreversible deterioration of fuel cells is possible. In addition, load changes can cause rapid temperature changes in the fuel cell, which especially in high temperature fuel cells cause harmful thermomechanical stress, resulting in significant reduction of performance and life time of fuel cells. Thus, fuel cell systems are designed so that the load of each fuel cell is kept as constant as possible and a possible change in the load is carried out in as controllable a fashion as possible.
When the fuel cells are used to obtain independent variable AC loads, or to supply power to a distribution network, a DC-AC converter is used to convert DC power to AC power. There may also be a desire for DC-DC converters to raise DC voltage obtained from the fuel cells to a level which is suitable for DC-AC converter. However, due to the highly limited compatibility and capacity of the fuel cells to respond to changes in load, known fuel cell implementations, especially high temperature fuel cell implementations, can be bad power sources for feeding independent variable AC loads or to feed variable power to the distribution network. A well-known way to try to address this issue is the use of an energy buffer, which includes, for example use of lead acid batteries. The function of energy buffer is to feed or consume power in rapidly changing conditions so that the load variation of fuel cell would be controlled. Especially in large fuel cell systems, disadvantages of such known implementations become more serious due to high cost, large size and heavy weight and limited effectiveness. In electrical network coupled applications, an alternative known implementation to maintain a constant fuel cell load is to use a current controlled transform in feeding power to the network. The control based on current controlled transform is not suitable in network independent operation, and thus can not be used as an emergency power source for AC loads inside or outside the fuel cell system.
High temperature fuel cell systems can involve a major heat energy amount for heating systems up to operating temperatures. From this follows that start-up times can be up to tens of hours in length. Wide temperature alternations in shut down and start up sequences can expose the fuel cells and related system components to even excessive thermomechanical stress. Thus, the high-temperature fuel cell systems should be designed to operate continuously for as long time periods as possible, for even thousands of hours, without any shut downs. To achieve this, the system should be designed to fulfill high reliability as well as to minimize such external factors, which might shut down the system or might drive the system to harmful operation conditions. Current controlled converters in fuel cell applications are unable to protect the fuel cells from sudden changes in load, arising from different network disruptions such as power failures, voltage dips or transients.