Such tests of complete SOFC systems are both expensive and complex; therefore, novel modeling practices were developed to avoid costly testing. The balance of plant typically includes thermal insulation, pipework, pumps, heat exchangers, heat utilization plant, fuel processors, control system, start-up heater and power conditioning. Arguably, the BOP is the dominant part of the system. The Engen-2500 system includes a host of BOP components that are commonly used to sustain the SOFC operation. More specifically, these BOP components consist of an external reformer, an oxidizer, a heat exchanger, a condenser with a water drain, a desulfurizer, an evaporator, a pump, two blowers, and three valves. For a visual illustration of the Engen-2500 system setup, refer to Figure 22, “Reproduced design schematic for the Engen-2500 system,” in section 4, Experimental Setup and Results. It is important to note that the Engen-2500 was originally designed for CHP applications and therefore the system is currently designed to produce high-quality waste heat so that the end-user can recycle the heat for additional purposes like building heating. Furthermore, only the external reformer, thermal oxidizer, heat exchangers, and blower components were included in the Engen-2500 system model. The additional BoP components such as the valves, pump, desulfurizer, evaporator, condenser, and water drain were not included in the system model in order to significantly reduce computational time and because these components do not have a significant impact on the overall thermodynamics and performance of the system. Its primary purpose is to chemically convert a readily available fuel such as a hydrocarbon fuel into a hydrogen-rich fluid that can be oxidized at the fuel cell’s anode. It also serves to convert fuel or oxidant not consumed at the fuel cell’s anode and cathode into useful energy. A fuel processing subsystem consists of a series of catalytic chemical reactors that convert hydrocarbon fuel into a low impurity, high-hydrogen content gas.
Some of these chemical processes release heat , while others require heat to be supplied . For high-temperature fuel cells,drying weed the required heat may be supplied by the fuel cell itself. The size and complexity of an external fuel processor depend on the type of fuel reformed, whether impurities or poisons need to be removed, and how much reformate needs to be produced. SolidPower’s Engen-2500 system uses an annular external reformer located in the HotBox section of the system and encompasses the oxidizer component. Placing the external reformer in contact with the oxidizer suggests that the heat produced from the combustion of fuel and air within the oxidizer is transferred via conduction across the walls of the oxidizer and to the reformer. The heat transferred from the oxidizer to the reformer functions as the heat input required for the endothermic steam-methane reformation reaction to occur. Reforming hydrocarbon fuel to hydrogen-rich fuel in the Engen-2500 system is accomplished by two subsequent reactions: steam-methane reformation and water-gas shift . Realistically, while fuel cells stacks cannot use all of the fuel in the anode compartment, fuel cell systems are unable to make use of all the fuel provided. Therefore, the remaining fuel that leaves the anode is sent to the thermal oxidizer – where it is usually mixed with cathode off gas – and undergoes combustion to produce heat and clean the anode off-gas emissions. The oxidizer releases the remaining fuel energy of the anode off-gas as heat and reduces the composition of methane and carbon monoxide to satisfactory levels. The Engen-2500 system has three gas streams that enter the oxidizer: extra natural gas, anode off-gas, and ambient air. A valve controls the addition of natural gas to the oxidizer when additional heating is necessary. As mentioned previously, the external reformer encompasses the thermal oxidizer and therefore the heat produced from the thermal oxidizer is used for the endothermic reformation process occurring within the external reformer component. Therefore, the valve controller controlling the input of extra natural gas to the burner must consider the instantaneous degree of heat being provided to the external reformer so that the system maintains a desired external to internal reforming ratio. The anode off-gas has very little heating value because the majority of its composition is steam and carbon dioxide.
Mixing of these three streams is determined by solving the conservation of mass equations to find the molar flow rate and mixed species concentrations. In any fuel cell system, there are process streams that must be heated. The challenge imposed on the system designer is to use the heat available from one stream to heat another in the most efficient way. The gas or liquid to be heated passes through pipework that is heated by the gas or liquid to be cooled. The pipework is generally referred to as a heat exchanger and is a mechanical device that conveys thermal energy or heat from a hot-fluid stream on one side of a barrier to a cold-fluid stream on the other side without allowing the fluids to directly mix. There are many types of heat exchangers such as shell and tube, plate, fin, tubular, regenerative, phase change, and printed circuit exchangers that all use different geometries and physical means of energy transfer. The key features to consider for selecting a heat exchanger are the surface area available for heat transfer, the types of fluids/solids exchanging heat, inlet temperatures and flow rates, and geometric configuration. The heat exchanger design implemented by SolidPower for their Engen-2500 system is assumed to be a typical cross-flow plate and fin heat exchanger. Entering on the hot side is the thermal oxidizer exhaust stream mixed with the cathode outlet exhaust. The combination of these streams carry high quality heat that preheats the ambient air before it enters the cathode. The ambient air is blown through the cold side of the heat exchanger via a blower. The high operating temperatures of SOFC systems melt most common metals, therefore, high temperature heat exchangers are typically manufactured from high temperature stainless steel or ceramic materials, for which the planar configuration is most common. Incorporating a heat exchanger into the model involved discretizing each plate into a specified number of control volumes, referred to as nodes, along the length of the air flow. This gives detailed temperature profiles, and avoids pinch-point limitations of a bulk counter-flow heat exchanger model. Heat exchanger size varies with the number of plates, allowing dimensional constants to remain fixed. This modeling technique gives an approximation of the surface area required to meet the needs of each particular design and scale.
The following energy balance equations are applied to each control volume. The most general function of a condenser is to change the physical state of gas to its liquid state by cooling it. The condenser can be used to capture the latent heat of condensation. In a fuel cell system, a condenser is important for both recapturing heat and recovering liquid water to achieve neutral system water balance. Neutral water balance is achieved when all of the water that is consumed by the system components is produced by other components internal to the system. In other words, no additional water needs to be suppled from an external source. The Engen-2500 system utilizes a condenser at the tail-end of the SOFC system cycle after the mixing of the cathode outlet exhaust and thermal oxidizer exhaust streams flow across the hot side of the heat exchanger. The hot side exhaust is comprised of mostly carbon dioxide and steam, with extremely small concentrations of methane, carbon monoxide, and hydrogen. The high concentration of steam present in the hot side exhaust is mixed with additional ambient tap water via a valve to help reduce the temperature and condense the high steam concentration to liquid water in the condenser. A water drain is included to capture all the liquid water that leaves the condenser. The remaining gases pass through and are emitted into the atmosphere as exhaust. Two blowers are included in the Engen-2500 system to take ambient air and direct it into the system components. One blower provides ambient air to the oxidizer for combustion of the anode off-gas and extra natural gas to produce enough heat for the endothermic external reformer. A second blower provides ambient air to the heat exchanger so that the air can be preheated before entering the cathode of the SOFC stack. Blowers operate much like a compressor, but with a much lower pressure ratio. Both blowers have their own controllers to provide the necessary pressure rise to its specific component. For the blower that directs air to the oxidizer component,vertical growing systems this blower’s controller attempts to regulate the heat produced by the oxidizer such that it maintains a desired external to internal reforming ratio designated by SolidPower. Considering the blower that directs air to the heat exchanger, this blower’s controller simply regulates the air flow required by the SOFC stack to maintain a constant cathode outlet temperature. A simplified thermodynamic analysis was used to analyze the blowers operation. The dynamics of the blowers are assumed sufficiently quick to minimally affect the remainder of the system due to low rotational inertia of the blowers. The blowers are treated as compressors with an isentropic efficiency factor and are provided enough power to generate the necessary pressure gain. The calculation of the necessary power for the blower is performed by applying an energy conservation analysis to the control volume with the following expression accounting for energy lost to inefficiencies and cooling. Sulfur in any form is harmful to the SOFC stack. It poisons the nickel-containing anode and reduces the stack performance. The Sulphur level must generally be below 0.1 ppm to avoid a performance loss. Natural gas and petroleum liquids naturally contain organic sulfur compounds that normally must be removed before any further fuel processing can be carried out. Even if sulfur levels in fuels are extremely low, 0.2 ppm, some deactivation of steam reforming catalysts can occur. The most common desulfurization techniques use conventional sorbents such as activated carbon, alumina and/or zeolites at low capacity.
Another common technique is to use zinc oxide and/or copper oxide at high capacity with hydrogen and an operating temperature >200°C. The latter reactively adsorbs sulfur and is regenerable. Although the specific type of desulfurizer is not explicitly known, it is acceptable to assume the desulfurizer is removing enough sulfur for continuous, unhindered operation of the SOFC system. Fluid systems generally involve a method of increasing the pressure, velocity, and/or elevation of a fluid. This can be accomplished by supplying mechanical energy to the fluid via a pump, fan, or a compressor. Pumps require mechanical work in the form of shaft work produced by an electric motor, which is transferred to the fluid as mechanical energy. There exists only one pump in the Engen-2500 system, the purpose of which is to pump recycled liquid water from the water drain to the evaporator before it enters the external reformer. Throttling valves are any kind of flow-restricting devices that cause a significant pressure drop in the fluid without involving any work. There are three valves included in the Engen-2500 system, the purposes of which are all the same: vary the pressure change of the fluid to increase or decrease thereby influencing the mass flow rate. Two separate valves vary the mass flow rate of methane going to the external reformer and the thermal oxidizer, independently. A SOFC system controller controls one valve to vary the mass flow rate of methane entering the external reformer such that the system maintains a steam-to-carbon ratio greater than two after taking into account the mass flow rate of steam. Fuel flow delay to the fuel cell can be a primary fuel cell transient limitation. In fuel cell systems, the most significant fuel flow delay is due to fuel flow control valves and pressure transients in the fuel processing system. Another system controller controls a second valve that adjusts the mass flow rate of methane entering the thermal oxidizer such that enough methane is being supplied along with air and anode off-gas to produce enough heat through combustion that maintains a designated external to internal reforming ratio. A third system controller simply controls a third valve to adjust the addition of tap water entering the SOFC system in order to maintain a neutral system water balance. To begin the verification process, the most influential piece of information is verifying that the model performance curve matches that of the experimental results.