Designing Cogeneration for the Real-world: A Comprehensive Approach

Digester gas is an asset utilities’ can use to maximize beneficial reuse of their resources and reduce their climate impact. In Fairfield, CA, the Fairfield-Suisun Sewer District (District) plant is already an industry leader by employing cogeneration, solar photovoltaics, and wind turbine sources to generate renewable energy and produce the majority of its electricity demands onsite. When looking to replace systems and further advance their energy program, the District engaged Brown and Caldwell to complete a digester gas utilization study. Due to increasing complexity of electric utility rate schedules, the existing varied energy resources at the facility, net metering/export limitations, and potential gas/energy storage, the typical annual average evaluation approach was no longer sufficient to reach realistic conclusions. New tools and methods were required that incorporated time-domain analysis and plant-level energy balance. Simulating minute-resolution digester gas production, real time energy production, and gas storage, allowed for a more accurate evaluation for the District, while using annual averages caused an over estimation of the benefits of the various scenarios evaluated. The selected design was one 1,100 kW engine with natural gas blending to accommodate variability in gas production, take advantage of spark spread, and reduce flaring.