Tubular SOFC, Solid Oxide Fuel Cell Technology
How Acumentrics Fuel Cell Systems Work
A solid oxide fuel cell is an electrochemical device that converts fueled hydrogen directly into electricity in the presence of heat. The process is driven by the flow of oxygen ions from a cathode to an anode through an electrolyte. When these ions combine with hydrogen from the fuel, electrons are released to an external circuit. This process is replicated many times in the fuel cell, in arrays or stacks.
In contrast to planar or membrane designs, the patented Acumentrics tubular solid oxide fuel cell (T-SOFC) design employs many discreet electrolytic tubes in parallel. The anodes are on the inside of each tube and the cathodes are on the outside, in an “anode supported” configuration, as shown in the diagram.
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Using the exclusive Acumentrics “fuel-in-the-tube” technology, fuel is introduced directly inside the tubes in a high-temperature environment, at approximately 750 C. Ambient air (the oxygen source) circulates around the outside of the tubes. The oxygen ions are conducted through the tube, and an electrical potential is generated between the inside and outside of the tube. This potential difference is tapped as electrical energy. The electrochemical process happens in multiple tubes in a stack, producing power systems that can supply a few hundred watts to 10 kilowatts or more.
The net result is a highly efficient system that produces clean electricity, with the byproducts of water, heat, and acceptable levels of carbon dioxide. Efficiency is further enhanced when the high-grade waste heat is recovered for heating, hot water and processing purposes. This results in total fuel efficiency (LHV) levels exceeding 80 percent for residential and industrial cogeneration applications.
Because no internal combustion engines or turbines are used, fuel cells have nearly undetectable levels of SOx and NOx, near-silent operation, and none of the maintenance associated with legacy engine- or turbine-based gensets.
Where Is The Fuel Reformer?
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Many fuel cell designs require a separate reforming process (with associated components, maintenance and costs) to reform the fuel and extract the hydrogen required by the fuel cell generation process. The reformers add bulk, component costs, and maintenance costs.
The Acumentrics system requires no separate fuel reformer. Our system natively reforms fuels from the hydrogen-rich family (i.e., methane, propane, butane, ethane, with the general structure CnH2n+2). This internal reformation bypasses the need for extra equipment and keeps our footprint smaller.
In the Acumentrics design, the reformation process occurs inside the tube, a process that is nearly co-mingled with the generation process itself. With the introduction of a small amount of air with the fuel, the inherent high temperature of the process reforms the fuel, producing the needed hydrogen as well as CO. During the direct electrical generation process the hydrogen is oxidized to produce water, and the CO combines with the process O ions to produce carbon dioxide.
Small Tubes – the Acumentrics Edge
Our small tube fuel cell design avoids one of the biggest problems in many fuel cell concepts – catastrophic damage due to temperature gradients. Gradients occur during thermal cycling in normal start-up and shut down, and are repeated over the lifetime of a unit. The small-radius geometry in our system minimizes gradients. It tolerates many thermal cycles, and faster cycles. An Acumentrics power system can power up within 45 minutes, as opposed to 12 - 24 hours for other high-temperature fuel cell designs.
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Specialized Ceramics Are the Key
The operating principle of solid oxide fuel cell (SOFC) technology is founded on electroceramics: advanced materials that exhibit unusual electrical properties. These ceramic materials are fast ion conductors but poor electron conductors.
A special ceramic electrolyte layer that promotes ion and charge transfer, and separates the anode and the cathode, also insulates the anode and the cathode. This creates an electrical potential between the anode and a cathode. As a result, the electrons can be released to an external circuit. This direct generation process is analogous to the operation of a battery, however the battery contains the necessary reactants internally.
The fuel cell process, in contrast, uses externally supplied components to react with the electroceramic material. This results in a continuous battery-like electrical generation process that can run indefinitely as long as the fuel (propane, natural gas, ethanol, and others) and oxidant (ambient air) are externally supplied. (Battery reactants have a finite life, which is why batteries must be replaced or recharged.)
While electroceramic materials are readily available and not considered exotic, their material and fabrication quality can vary widely. For this reason, Acumentrics makes its own anode-supported tubular oxide fuel cells at its facility in Westwood, Massachusetts. In-house fabrication allows for tighter control over our critical materials and processes.