Capstone Microturbine Generators: Turning Wastewater into Clean Energy May 5, 2005 Disclaimer The Pennsylvania Department of Environmental Protection does not endorse or recommend any of the technologies described herein. The technical articles are provided for informational purposes only. Persons seeking additional information about the described technologies should contact the parties listed in the article. Overview The Capstone Microturbine Generator, a relatively new technology, first debuted in 1998 as a product of the Capstone Microturbine Corporation. It is designed to operate on a variety of liquid and gaseous fuels, so it can be used in many different applications. In particular, in wastewater treatment the microturbine is used by facilities that employ anaerobic digesters as the anaerobic digesters provide the fuel for the microturbine. Each unit is about the size of a refrigerator and can produce up to 30 kW or 60 kW (depending on model) of electricity and the units can be linked together into large arrays if necessary. They also come with the option of utilizing the exhaust heat for increased efficiency. How It Works The Capstone Microturbine Generator uses the methane gas that is provided by the anaerobic digester as fuel to produce electricity. Just like conventional turbines the Capstone unit uses the expansion of high-pressure gas to turn the generator to produce electricity, but what makes the Capstone Microturbine different is that the fuel is mixed with air, compressed, combusted and expanded using only 1 moving part, a single shaft rotating on air bearings. The unit is designed to be capable of spinning up to 96,000 RPM under full load. It is also air-cooled and the air bearings require no liquid lubrication, which means that there are no liquids or oils to change. The Capstone Microturbine has a recommended maintenance schedule of 8,000 hours with the first maintenance consisting of changing the air filter. The capstone units produce electricity, which is used to power equipment in the facility, very efficiently but they also produce heat. The heat exits the unit through the exhaust and can be utilized in a few different ways. Since the microturbines run on a lean air mixture when burning their fuel, their exhaust is oxygen rich and can be used to heat directly. The exhaust can be run through a heat exchanger to heat water or steam. It can also be used with an absorption chiller to create cold water. Combining the efficient combustion/generation process with thermal energy utilization results in very high-energy efficiency in the range of 70% or greater. Reported Advantages Over Conventional Technology • Uses waste gas to produce valuable heat and electricity • Uses air bearings for high reliability and low maintenance • Very low emissions without the use of after combustions emissions controls Potential Disadvantages/Concerns • Only useful in plants that employ anaerobic digestion Technology Verification and Usage In May 2004 the city of Albert Lea, Minnesota installed 4 Capstone Microturbines in its wastewater treatment facility. The facility anaerobic digesters produce a gas comprised mostly of methane, which is used to fuel the microturbines. The system was set up to operate efficiently on any combination of the 4 microturbines in order to promote optimal use of the fuel supply, which can vary greatly. The 4 microturbines can produce up to 2500kWh of electricity and 28 million BTUs of thermal energy per day. The electricity produced is used to power the plant’s equipment while the heat is used to keep the digesters at optimal operating temperature. Installing this upgrade has cut the energy costs of the Albert Lea wastewater treatment plant significantly. Another wastewater treatment site which has capstone equipment installed is San Elijo Joint Powers Authority Water Reclamation Facility located in Cardiff-by-the-Sea, CA. This 3 MGD wastewater treatment plant employs 3 Capstone Microturbines that produce up to 80kW of electricity, approximately 15% of the plant’s peak power usage. The plant has experienced a drop of $4,000 - $4,500 in monthly electric costs and, combined with approximately $100,000 in grants they received, expects a 3 to 4 year payback period on the project. Appendix A A typical single microturbine setup courtesy of www.capstoneturbine.com. Sources of Additional Information about Capstone Microturbines are available by writing Capstone Turbine Corporation, 21211 Nordhoff Street, Chatsworth, CA 91311, by calling 818-734-5300 or 866-4-CAPSTONE, or at www.capstoneturbine.com.
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