Exergy and Economic Analysis of Two Different Fuel Cell Systems for Generating Electricity at Waste Water Treatment Plants
Authors: Siefert Nicholas , Ashok Gautam 
Published: July 2013

Generating electricity at wastewater treatment plants is a promising near-term application of fuel cell systems. The scale of most wastewater treatment plants is such that there is a good match with the scale of today’s fuel cell systems. This paper presents an exergy analysis and an economic comparison between two fuel cell systems that generate electricity at a wastewater treatment plant. The first process integrates an anaerobic digester (AD) with a solid oxide fuel cell (SOFC). The SOFC was modeled using publicly-available data from the tests on the Rolls-Royce pressurized SOFC. The second process has the wastewater sent directly to a microbial fuel cell (MFC). An MFC is an electrochemical cell in which bacteria convert acetate, sugars and/or other chemicals into protons, electrons and carbon dioxide at the anode electrode. The MFC was modeled as a PEM fuel cell as used for vehicle applications, but with a few changes: (a) anaerobic bacteria, such as geobacter, grow directly on the surface of the anode electrode, (b) there is no anode gas diffusion layer (GDL), (c) iron pyrophyrin, rather than platinum, is used as the catalyst material on the anode, in addition to the bacteria, and (d) the Nafion electrolyte is replaced with a bipolar membrane in order to minimize the transfer of non-proton cations, such as Na+, from the anode to the cathode. The rest of the equipment in the MFC is the same as those in commercial vehicle PEM fuel cells in order to use recent DOE cost estimates for PEM fuel cell systems. In both cases, we generated V-i curves of SOFC and MFC-PEM systems from data available on a) PEM & SOFC electrolyte conductivity and b) anode and cathode exchange current densities, including the effect of platinum levels on the cathode exchange current density of PEM fuel cells. A full exergy analysis was conducted for both systems modeled. The power per inlet exergy will be presented as a function of the current density and the pressure of the fuel cell. Using various Department of Eneregy (DOE) cost estimates for fuel cell systems, we perform parametric studies for both the MFC and AD-SOFC systems in order to maximize the internal rate of return on investment (IRR). In the MFC case, we varied the platinum loading on the cathode in order to maximize the IRR, and in the AD-SOFC case, we varied the current density of the SOFC in order to maximize the IRR.

Finally, we compare the IRR of the two systems modeled above with the IRR of an anaerobic digester integrated with a piston engine capable of operating on biogas, such as the GE Jenbacher. Using an electricity sale price of $80/MWh, the IRR of the AD-SOFC, the microbial fuel cell and the AD-piston engine were 9%/yr, 10%/yr and 2%/yr, respectively. This economic analysis suggests that further experimental research should be conducted on both the microbial fuel cell and the pressurized SOFC because both systems were able to generate attractive values of IRR at an electricity sale price close to the average industrial price of electricity in the US.