The regulations and significant advances in technology over the decades have resulted in the production of modern engines, power plants, and furnaces with dramatically reduced emissions. This has yielded significant improvements in air quality in locations where strict regulatory enforcement occurs. Figure 8 compares the United States’ changes in energy conversion with criteria pollutant and greenhouse gas emissions to the population, vehicle miles traveled and gross domestic product over the past few decades since the enactment of the Clean Air Act of 1970. Noticeably the Clean Air Act has been beneficial to the nation’s emissions as there have been significant reductions of aggregate pollutant emissions despite only minor, yet meaningful, reductions in greenhouse gas emissions and energy conversion. Further developments for reducing greenhouse gas and criteria pollutant emissions can be made by implementing advanced alternative methods of power production that are proven to be more environmentally friendly and reliable replacements to traditional combustion-based power methods. Fuel cells are a great alternative to traditional combustion-based power methods and can convert fossil or renewable fuels into electricity more efficiently and with lower emissions. Typically, combustion processes mix and burn fuel and oxidant to produce heat that is subsequently converted to a useful form of energy through a heat engine. Conversely,pots for cannabis plants fuel cells directly convert the fuel’s chemical energy to electricity through electrochemical reactions that keep fuel and oxidant separate and form a complete circuit.
These fundamental differences can lead to much higher electrical efficiencies in fuel cells, reaching greater than 50% in some simple-cycle systems and greater than 70% in some hybrid cycle systems. The basic operation of a fuel cell is extremely simple. Scientist Cristian Friedrich Schönbein and lawyer William Grove accomplished the first demonstration of the concept in 1839 with an experimental setup consisting of two separate platinum electrodes submersed in a dilute acid electrolyte with a wire connected to both electrodes. Grove noticed that water could be separated into hydrogen and oxygen by passing an electric current through it – a process known as electrolysis and would become a popular method for producing hydrogen gas. Conversely, he noticed that the reverse reaction would occur naturally as the hydrogen and oxygen would favorably recombine to form liquid water whilst producing an electric current and releasing heat. The latter case has become the primary model for fuel cell energy conversion. Therefore, fuel cell designs have advanced significantly since Grove’s discovery. Today, the electrolyte and electrodes are made very thin, flat, and highly porous to maximize the surface area in contact between the gas, electrode, and electrolyte. The electrodes consist of one porous positive electrode with a catalyst layer – the cathode – and one porous negative electrode with a catalyst layer – the anode – sandwiched around an electrolyte that only allows for the flow of ions and not electrons. The typical voltage produced by an individual fuel cell is approximately 0.7 volts [V]. To achieve higher voltages, one can simply stack multiple cells together in series to create a fuel cell stack of suitable power and voltage output. Fuel cells can be stacked numerous times until an appropriate power rating is achieved for the specific application; anything from watt to megawatt systems can be created as shown in Figure 10.
In addition to the modularity, fuel cells offer a number of additional benefits that allow for progressive means of power production beyond the status quo. Fuel cells can be all solid state and mechanically ideal – meaning no moving parts – contributing to their silent operation, and potential for highly reliable, continuous power while undesirable aggregate pollutant and greenhouse gas emissions are virtually zero. Even more beneficial is the higher efficiencies associated with directly utilizing the chemical and heat energy from the fuel’s chemical reaction instead of the additional losses incurred by converting the heat energy to produce electricity. Despite the numerous advantages, several major disadvantages hinder the rapid succession of fuel cell power production; the first and most prohibitive of which is the cost of implementation followed closely by limitations in high-temperature capable materials. As with all new technology, significant advances need to be made in the manufacturing and availability of fueling infrastructure to reduce the cost of these systems. Power density, which defines how much power is produced per unit volume , is another limitation that challenges the efficacy of preferring fuel cells to internal combustion engines or batteries in portable or automotive applications. Lastly, for widespread acceptance in even the most remote regions of the world, the issue of fuel availability and storage must be resolved. Fuel cells operate best on hydrogen gas, a fuel that is not widely available, has low volumetric energy density, and is difficult to store. Although specific types of fuel cells – namely high temperature fuel cells – are fuel flexible and have the ability to operate using more common alternative fuels , these fuels are still challenging to use directly and typically require reforming.
There are five major types of fuel cells that are differentiated from one another by their electrolyte materials: phosphoric acid fuel cell , polymer electrolyte membrane fuel cell , alkaline fuel cell , molten carbonate fuel cell , and solid oxide fuel cell . While all these fuel cell types are based upon the same underlying electrochemical principle as discussed with the PEMFC example, their individual operation differs significantly leading to differences with the specific ion that is conducted through the electrolyte, the operating temperature, cell components and performance characteristics. Figure 11 illustrates the electrochemical operation of the major types of fuel cells and their associated operating temperatures. Refer to Appendix A at the end of this thesis for characteristic differences between the electrolyte, operating temperatures, typical stack sizes, applications, and advantages and disadvantages for each fuel cell type. It is important to note that the operating temperatures of MCFCs and SOFCs differ greatly from that of PEMFCs, AFCs and PAFCs, therefore MCFCs and SOFCs are labeled high-temperature fuel cells. The high temperature operation is an important and uniquely exploitable characteristic that is explored throughout this thesis with specific focus on SOFC systems. As mentioned previously, the power output is directly proportional to the cell area; therefore, two plausible designs have been pursued: a stack of flat plates or an array of parallel tubes. Planar cells have a few advantages over tubular cells that make them better for centralized power generation: being readily manufactured by standard processes like screen printing, stacked together with narrow channels to achieve high power densities, and can provide short current pathways through the interconnect. To build a single cell, five basic components are needed: an electrolyte, anode, cathode, and two interconnect wires all combined in the configuration shown in Figure 12 [38]. The component that typically distinguishes a solid oxide fuel cell among the other fuel cell types is the material composition of the electrolyte. SOFCs are completely solid-state devices that use an oxide ion-conducting ceramic material as the electrolyte. The solid oxide electrolyte is made from a ceramic of zirconia doped with 8 to 10 mole % yttria to form yttra stabilized zirconia . YSZ is the most effective electrolyte for high-temperature SOFCs and is highly stable in both reducing and oxidizing environments that are found at the anode and cathode of the fuel cell, respectively. The cathode of SOFCs consists of a porous structure that allows rapid mass transport of reactant and is where reduction occurs. Typically, the ideal SOFC cathodes are ceramic materials that exhibit mixed ion-conducting and electronically conducting behavior. The most commonly used material is strontium-doped lanthanum manganite or commonly abbreviated as LSM. Materials like LSM are ideal because they have good oxidation resistance and high catalytic activity in the cathode environment.High-temperature fuel cells are considered perfect replacements for combustion-based power generation because the high quality heat involved can be used for internal reformation of hydrocarbon fuels and/or for combined heating and power purposes. For these reasons,indoor cannabis grow system the data center industry has chosen to study the on-site centralized application of fuel cells by: installing high-temperature fuel cells to power an entire data center, utilizing combined heat and power technology within the data center for better efficiency, and performing economic, reliability and energy efficiency assessments. As is characteristic of high temperature fuel cells, SOFCs are quite fuel flexible; methane, carbon monoxide or hydrogen can be used as fuels.
Despite this appealing trait, to achieve the highest efficiencies possible, hydrogen is the preferred choice of fuel for fuel cell operation. Current methods of producing hydrogen economically require reformation of natural gas , which obviously fails to eliminate greenhouse gas emissions and fossil fuel use. Although reformation of hydrocarbon fuels leads to some carbon dioxide emissions, highly efficient fuel cell systems operating on hydrocarbon fuels produce significantly lower emissions than burning the same fuels in conventional power plants. Figure 14 compares the differences in emissions for centralized coal-fired power plants and fuel cell power production of the same scale. The ideal alternative to achieve sustainability is hydrogen production via electrolysis – passing electricity through water between two electrodes, breaking it down into its two components, hydrogen and oxygen. Electrolysis of water has received major recognition as an extremely clean and emissions free way to store excess wind or solar power that would otherwise have been curtailed. In addition hydrogen can also be extracted from renewable bio-gases, such as landfill gas produced during natural bacterial decomposition of organic material; anaerobic digest gas generated at wastewater treatment plants, breweries, and agricultural processing facilities; or from biomass resources such as agricultural lumber waste. The hydrogen gas produced from any of these methods can then be injected into the existing natural gaspipeline infrastructure, where it can be widely transported, or stored and used at a later time to generate electricity in a stationary fuel cell or even used to fuel up fuel cell electric vehicles. The first fuel cell model developed at the NFCRC has been significantly improved upon through the years as numerous graduate student researchers have developed sophisticated and novel dynamic fuel cell system models for their research activities. The research work from previous graduate students identified applications where fuel cell systems would thrive and other applications fuel cell systems experienced limitations requiring a novel control strategy to be implemented for successful fuel cell operation. The most notable and relevant fuel cell simulation studies that pertain to my research work are discussed in this section. Throughout Dr. Mueller’s tenure as an NFCRC graduate student researcher, he worked with four fully integrated system dynamic models, which consisted of a 25 kW Siemens integrated SOFC simple-cycle system, a 220 kW Siemens SOFC gas turbine hybrid system, a 60 kW Capstone recuperated gas turbine, and a 5 kW Plug Power stationary PEM fuel cell system. With these systems, physical based dynamic models of integrated SOFC systems were developed for transient analyses, controls development, and evaluation of SOFC system transient and disturbance rejection capabilities. The challenge for SOFC systems is maintaining the system within recommended operating requirements and minimizing system transients during transient load demands and disturbances. To begin his work, Dr. Mueller developed a two-dimensional dynamic model of a Siemens Westinghouse tubular 25 kW SOFC system and included all simulation modules for the balance of plant system components required for continuous SOFC operation. The model results were shown to predict the actual steady state SOFC system characteristics within 3% margin of error in power and 5% margin of error in temperature values. Furthermore, the model was used to analyze the load-following capability of the system when subjected to transients. It was determined that high fuel utilizations were found to be problematic when fuel flows were not changed during power transients. Therefore, Dr. Mueller developed a current-based fuel flow controller that was shown to minimize transients in utilization and stabilize the response of the fuel cell. In this control strategy, fuel actuator response time was shown to be a major limitation to fuel cell transient capability. Additional simulation studies with solid oxide fuel cell – gas turbine hybrid systems led to the development of control concepts that prevent fuel depletion, avoid gas turbine surge, as well as reduce fuel cell and combustor temperature transients.