'Green Machine' Turns Engine Heat into Electricity
The site produces biogas (methane) through anaerobic digestion to fire a 500kWe GE Jenbacher 312, which generates electricity to sell to the grid and uses the waste heat to dry crops during harvest. Reno, Nev.-based ElectraTherm says this is the first known application where an organic Rankine cycle (ORC) is incorporated into a biogas facility to supplement efficient use of renewable energy.
The Electratherm unit generates distributed power from waste heat. Hot water enters the unit at 190°F (88°C) where it heats a working fluid into a pressurized vapor using ORC technology. The high pressure vapor expands through a twin-screw power block, spinning an electric generator and creating up to 65kWe. For the size engine and available waste heat at the Austrian site, the power generated averages 32kWe. Since its commissioning in September 2011, the machine has operated approximately 3,000 run-time hours.
The production process requires heating of the anaerobic digester tanks, which keeps the bacteria at the appropriate temperature for effective biomass decomposition to create methane. The system recovers as much waste heat as possible to sustain the anaerobic digestion process as well as produce both fuel- and emission-free electricity.
The Jenbacher 312 engine has an exhaust-gas heat exchanger to combine into a single stream both the heat available in the exhaust and the heat available in the jacket water. The engine’s waste heat then heats the anaerobic digestion process and dries maize after harvest. The excess heat not used for anaerobic digestion or seasonal grain drying is used to produce fossil fuel-free electricity 24 hours a day, seven days a week, as needed.
The power generated by the green machine meets the utility’s European feed-in tariff requirements, paying up to approximately 24 cents per kWe, says Karl Leisch, project developer.