Tubular SOFC, Solid Oxide Fuel Cell Technology

How Acumentrics Fuel Cell Systems Work

 A solid oxide fuel cell (SOFC) is an electrochemical device that converts hydrogen and carbon monoxide from hydrocarbon fuels into electricity. The process is driven by the flow of oxygen ions from a cathode to an anode through an electrolyte that is comprised of solid zirconia. When these ions combine with hydrogen and carbon monoxide from the fuel, electrons are released to an external circuit. This process is replicated many times in the fuel cell, in arrays or stacks, and it results in highly efficient power generation with virtually no greenhouse gas emissions.

In contrast to planar or membrane designs, patented Acumentrics tubular solid oxide fuel cells are constructed from 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.

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.

The net result is a highly efficient system that produces clean electricity, with water, heat, and low levels of carbon dioxide as the only by-products. 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 operation is below the high temperature of internal combustion engines or turbines, fuel cells have nearly undetectable levels of nitrous oxides (NOx). Sulfur oxides (SOx) are reduced due to the high efficiency of our fuel cells that also provide near-silent operation and none of the maintenance associated with legacy engine- or turbine-based gensets.

Where Is The Fuel Reformer?

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 reforms the fuel inside the fuel cell stack of tubes from hydrogen-rich fuels such as methane, propane, butane, ethane with the general structure CnH2n+2. This internal reformation bypasses the need for extra equipment, resulting in a smaller footprint.

With the introduction of a small amount of air with the fuel, the inherent temperature of the process reforms the fuel, producing the needed hydrogen as well as carbon. During the direct electrical generation process, the hydrogen is oxidized to produce water and the carbon combines with oxygen to produce carbon dioxide – each of these processes liberate electrons that flow from a system as electrical current.

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 thermal cycles, and faster cycles. An Acumentrics power system can power up within 30 minutes, as opposed to 12 - 24 hours for other high-temperature fuel cell designs. Additionally, internal reforming of hydrocarbons would be destructive to planar fuel cell systems due to the resulting temperature gradients resulting from this process. Our tubular structure tolerates the stresses from this process and, therefore, eliminates the large, expensive, external reformers that are common in planar fuel cell systems.

Specialized Ceramics Are the Key

The operating principle of solid oxide fuel cell (SOFC) technology is founded on electro-ceramics, which are 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 cathode. As a result, the electrons are released to the external circuit. This direct generation process is analogous to the operation of a battery. With a battery however, the necessary reactants are contained within the battery.

The fuel cell process uses externally supplied components to react with the electro-ceramic material. This results in a continuous battery-like electrical generation process that can run indefinitely as long as the fuel and air are externally supplied. (Battery reactants have a finite life, which is why batteries must be replaced or recharged.)

While electro-ceramic materials are readily available, 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 critical materials and processes.

Ceramic fuel cells can run at high heat, and are fuel-flexible. They are able to reach efficiencies almost double that of proton exchange membranes (PEMs), and are not poisoned by carbon monoxide. Several other advantages are:

  • Due to their high operating temperatures, cogeneration cycles can be effectively employed, with the resulting efficiencies of these cogeneration power plants approaching 90%.
  • The raw materials for these ceramics are relatively inexpensive.
  • Given the high operating temperatures of SOFCs, the need for expensive precious metal catalysts is negligible.
  • SOFCs can operate on hydrocarbon fuels such as natural gas, diesel fuel, jet fuel, and biofuels with minimal fuel processing. Hydrogen-rich gases containing carbon monoxide do not need to undergo additional “gas clean-up” steps.