Dr. Mueller’s work suggests that through simulation, transient understanding, and control development, integrated control strategies can be developed and implemented to enable rapid SOFC system transient load following and improve disturbance rejection capabilities. Dr. McLarty put tremendous effort into significantly improving the architecture and computational accuracy of the NFCRC dynamic fuel cell modeling tools. His work improved upon previous fuel cell dynamic modeling studies, where a novel model was developed to spatially and temporally resolve the transient temperature, pressure, and species distributions fora simulated fuel cell stack in a computationally efficient manner. The model accounts for internal manifolding of fuel and oxidant streams and predicts two-dimensional fields associated with the dynamic operation of a single high temperature fuel cell. The readily calibrated novel model can accurately capture the dynamic performance of both planar solid oxide fuel cell and molten carbonate fuel cell systems of various sizes and flow configurations with a significant range of spatial resolution. Higher spatial resolutions in modeling efforts is likely lead to more accurate simulations that identify the precise locations of the most severe thermal gradients for any particular flow geometry considered. Figure 15 illustrates the spatial resolution of the electrolyte and its temperature gradient across the cell. Simulation of any spatial resolution is readily accomplished by specifying the number of rows and columns in the model initialization file. In addition, various air and fuel flow directions can be specified that include co-flow, counter-flow,cannabis grow equipment and cross-flow patterns. Previous dynamic SOFC and MCFC models have developed controls for basic load following operation, but have not captured the spatial resolution or internal heat transfer characteristics necessary for accurate spatial temperature gradients.
The spatial resolution methodology is based upon the quasi dimensional discretization of a single fuel cell node into five distinct control volumes in the through-cell direction. The five distinct control volumes consist of the oxidant separator plate, the cathode gas stream, the electrodes and appropriate electrolyte , the anode gas stream, and the fuel separator plate. Within each volume, the temperature, species concentrations, pressure, voltage, and current density are locally evaluated with dynamic conservation of mass, energy, and momentum equations. Models of additional fuel cell system components such as heat exchangers, external reformers, and thermal oxidizers are typically integrated with these spatially resolved fuel cell models to form complex models of integrated energy systems that incorporate the resolution of individual system component physics, chemistry and electrochemistry. Although the model analyzes the operation of a single fuel cell, the results are representative of the fuel cell stack. In other words, the model takes the two-dimensional operation of a single fuel cell and applies it to the three-dimensional stack by multiplying the model results by the number of cells included in the stack. An illustration of this is shown in Figure 16. Dr. McLarty identified the positive electrode-electrolyte-negative electrode temperature gradient as an important control parameter that is heavily influenced by the power density, inlet temperature, and air flow rate. In addition, transient responses emphasize the drastically different time scales associated with electrochemical performance and cell thermal dynamics. Novel control strategies have the potential to make use of the intermediate time scales for rapid transient response of a fuel cell system with minimal cell thermal fluctuations. Therefore, detailed physical models must be employed to study system level transient responses and determine the delicate balance between performance and longevity under dynamic operating conditions.
Dr. Zhao’s most recent research is particularly relevant to the research discussed in this thesis. Microsoft has chosen to work with the NFCRC to investigate the feasibility of a new data center concept – recall the aforementioned ‘stark’ design – that includes the detailed integration of fuel cell technology into server racks. The stark concept eliminates the need for backup power systems and can introduce significant emissions reductions and energy savings while enhancing data center availability and reliability. Figure 17 illustrates the potential energy savings at each step along the energy supply chain by utilizing in-rack fuel cell technology beginning with the fuel resource and ending at the server level. Note the significant energy savings in the initial step. This conveys that significant reductions in harmful criteria and greenhouse gas emissions associated with the conversion of fuel to useable energy is possible by implementing fuel cells in place of conventional power plants. The technical challenge for using fuel cells for in-rack server power generation is rapid load following because fuel cells are typically designed for relatively steady loads. However, Figure 18 illustrates the most severe transient loads that are characteristic of AC or DC powered servers and that a fuel cell system must be able to handle without failure. The dramatic rise or fall of a server’s power demand is expected to be quite challenging for fuel cell systems to react instantaneously. Although processes inside the fuel cell such as electrochemical reactions and charge transfer processes occur on time scales on the order of milliseconds, load following issues arise when the fuel cell system cannot meet both external system and balance of plant power demands. Limitations could result from conservative control techniques or inherently slow response of subsystem components, such as flow or chemical reaction delays associated with fuel and/or air processing equipment. Dr. Zhao’s analysis and experiments verify that it is possible to achieve low cost, low greenhouse gas emissions, high reliability , and high efficiency by using mid-sized FCs at the rack level, directly supplying DC power to the servers.
Doing so effectively replaces the power distribution system in a data center with a gas distribution network and eliminates reliance on the electrical utility grid. Reducing components in the data center energy supply chain not only cuts costs but also reduce points of maintenance and failure, which improves availability of the data center. In addition, by utilizing the fuel cell DC output, 53% energy efficiency in a single server rack can be achieved . The data obtained from steady state and dynamic response simulations of the PEMFC system to server and system dynamics can be used to determine energy storage requirements and develop optimal control strategies to enhance the dynamic load following capability. Although a PEMFC system has demonstrated its dynamic load-following capabilities, its insatiable thirst for hydrogen was a major drawback for immediate implementation. Due to limitations in the burgeoning hydrogen infrastructure, Microsoft decided to opt for a high temperature SOFC system for its fuel-flexible capability, which would allow for quick installment and implementation in any building with an existing natural gas network. An added bonus to operating on natural gas is the highly reliable nature of the natural gas network, which subsequently contributes to the highly reliable nature of fuel cell systems. A relatively inexpensive and commercially tested SOFC system was needed to begin the research work. Solid Power’s Engen-2500 system was selected because it was commercially available and touted as one of the most efficient SOFC systems to date. Solid Power, formerly known as SOFCpower, began as a spin-off of another Italian company,indoor grow cannabis the Eurocoating SpA Turbocoating Group. Turbocoating is a privately held company focused on developing and manufacturing coatings and special processes for gas turbine and aero engine component manufacturers. As a fledgling Italian company, SolidPower aimed to become a leader in the development and commercialization of stacks and power generation units integrated into SOFC systems. In 2006, SolidPower acquired Swiss-based HTceramix for the industrial production and commercialization of the latter’s integrated solid oxide fuel cell system, HoTbox. HTceramix was a developer of SOFCs with a mission to manufacture and deliver fully integrated SOFC generators to system integrators at competitive prices. At the heart of its development is the SOFConnex based stack, which used a unique approach for stacking ceramic fuel cells. During the period of the acquisition, HTceramix had expanded its facilities to cope with a large increase in orders from the Asia-Pacific region and added multiple test benches for stacks and HoTboxes. Meanwhile, SolidPower was setting up a pilot production line that would be operational by 2007 to begin producing SOFC systems. SolidPower is now an experienced developer of SOFC-based systems, displaying its newly developed Engen-2500 micro-Combined Heat and Power SOFC appliance for home and industry at the recent Hannover Messe trade fair in Germany in 2015. At the trade fair, Guido Gummert, CEO of SolidPower GmbH , explains, “In our development of energy cell technology, we have succeeded in bringing down the operating temperature to around 700°C, which means that we can work with less heat generation for the current we produce. Our objective has been to achieve the highest possible electrical efficiency, but without compromising the total efficiency of the system.
With an electrical efficiency of 50% and a total efficiency of 90% , we are right out in front” . The brand new micro-cogeneration system, Engen-2500, is a good solution for projects ranging from 2.5 kW up to 20 kW of electrical power. It has been granted the coveted A++ classification under the European Energy Related Products directive, certifying a high level of electrical efficiency with maximum micro CHP efficiency. This targets end-users with larger electricity and heat requirements, such as small and medium-sized businesses, or groups of several office units within a building. The device is distinguished by the low percentage of dissipated thermal power and the high life in operation that result in a substantial cost savings. The Engen-2500 system is a floor-standing unit generating a maximum of 2.5 kWe of net AC power and can run solely on natural gas fuel from the grid at normal supply pressure. A connection for tap water supply is required to startup the system. The heat available is recovered with water, exchanged within the Engen-2500 and then transferred to an external water storage tank. When integrated for mCHP purposes, the system is controlled by the heat available at output, meaning the integrated system controller modulates power output following a heat demand command from an external energy manager. As an alternative, the system can also be operated in load-following heat-capped mode where the power can be modulated between 30% and 100% total stack power. To form a better understanding of the design structure for the Engen-2500 SOFC system, it is necessary to know what components are involved and how they interact with one another. Fortunately, SolidPower was willing to provide a design schematic of their Engen-2500 system in addition to some verbal details. Conveniently, the Engen-2500 system can be characterized by its two main segments, which house all the necessary components for operation – the HotBox and ColdBox . Although very few details were provided from SolidPower regarding the position of each component in the Engen-2500 system, it was reasonably speculated that the HotBox contains two SOFC stacks , an external reformer, an oxidizer, and a heat exchanger. These components are surrounded by insulation to mitigate heat losses to the environment . The remaining components necessary for SOFC operation are housed in the ColdBox section and include a desulfurizer, condenser, water drainage tank, pump, three valves, two air blowers, and the electronics associated with control and operation. All components with the exception of the SOFC stack are referred to as the balance of plant . The function of each component in the balance of plant is discussed in detail in the balance of plant section of this thesis. The design schematic provided by SolidPower for the Engen-2500 system was reproduced and illustrated in Figure 22 on the next page. The schematic clearly defines the connections between the balance of plant components and the SOFC stack. It is important to note that SolidPower designed their system to recirculate the anode off-gas into the oxidizer to reduce CO and H2 emissions of the system. In order to keep the oxidizer burning hot enough to supply heat to the endothermic external reformer via heat transfer, additional natural gas fuel and ambient air is mixed with the anode off-gas. Additionally, the cathode exhaust is designed to mix with the oxidizer exhaust where the mixture would be used to provide ample heat to preheat the ambient cold air in a heat exchanger before the air reaches the cathode.