The cell/stack technology was developed on an 8 x 8 inch active area cell (412 cm2). The metallic support consists of commercial grade stainless steel sheet-metal brazed together. The low-cost sheet-metal enables the stack to use internal air and fuel manifolds, greatly simplifying the stack build and hotbox design. The flow channels across the active area are designed in a cross flow configuration. The flow field has been modeled with CFD tools to confirm pressure drops and flow profiles across the active areas which have been verified experimentally.
Some unique advantages of this stack design, which lead to high reliability and reproducibility, include the ability to QC the fuel side pressure drop of each individual cell before stacking. This ensures all cells in a stack are free of any defects that lead to a fuel leak. This eliminates the need for electrochemical testing or conditioning before installation in the system or hotbox. Also, the high thermal conductivity of the metal substrate allows for the operation at relatively high current densities and on a variety of fuel types with acceptable thermal profiles for large scale stacks.
During 2016 the pilot facility at Malta NY produced over 10,000 cells at a yield over 90%. The bulk of the cells were used for system testing, although for quality and process control purposes 5% of cells from the production line were sampled and tested in 10 cell stacks to measure performance at the 1 kW level. In total over 200 stacks were tested in 2016.
For system testing the stacks were built 170 cells high and operated at 100 A on pipeline natural gas with anode recycle. The 50 kW system was designed to operate with a maximum of four stacks connected in a hotbox with parallel air and fuel supplies. Over the course of multiple system tests leading up to the 500 hour run, the deviation in stack voltage was typically less than 2% indicating excellent reproducibility though-out the production process. A second 500 hour test was completed later in the year using two stacks at 25 kW output with similar results.
Although the 8 inch cell has been the standard development platform, GEFC has demonstrated successful scaling of the design, both up and down at the 1 kW level. A 4 x 4 inch (100 cm2) active area cell has been demonstrated in stacks up to 50 cells high, including a 20 cell 4 inch stack operating for 4000 hours without any sign of chromium poisoning from cell cross sectional analysis. Also, the cell design has been successfully scaled up to 9.25 x 9.25 inches without any loss in performance. This larger format has a 30% larger active area (550 cm2) and is designed to operate at larger stack heights to deliver twice the power of the 170 cell 8 inch stack.