What is combined heat and power (CHP)?
Combined heat and power (CHP) maximizes the usable energy from a fuel source by simultaneously generating thermal and electric outputs. CHP can achieve operating efficiencies of up to 80%, compared to the 45% efficiency typically achieved by conventional energy production. CHP is not a specific technology; rather, CHP applications are customized, site-specific energy systems that may consist of reciprocating engines, combustion or steam turbines, microturbines, generators, and heat-recovery systems.Â This flexibility allows project engineers to choose the appropriate fuel and products that meet their needs.
R.M. Clayton Wastewater Reclamation Center City of Atlanta
The R.M. Clayton Wastewater Treatment Plant in Atlanta, Georgia, previously flared excess methane gas produced by the anaerobic digesters wasting a high-energy fuel but the city’s Office of Sustainability and the Department of Watershed Management collaborated to solve this problem. A new plant is equipped with a combustion engine that can convert waste biogas into nearly 13 million kilowatt-hours of useful energy annually. The system will also capture more than 39,000 million Btu of waste heat and use it as process energy for the anaerobic digesters.
The project stemmed from a city goal to produce 5% of municipal energy from renewable sources by 2015. The system is designed to achieve 88% of that goal and will help the city avoid an estimated 12,700 metric tons of greenhouse gas emissions each year.
While this kind of sustainability performance is important to the city, favorable economics are also a key requirement for city projects. In this case, the R.M. Clayton system will generate approximately $1 million in annual savings after a six-year payback period.
RM Clayton CHP City of Atlanta
The $7.1 million project is financed by a loan from the federal Clean Water State Revolving Fund at an interest rate of 3%, and a $1.5 million grant. All of the energy produced will be used internally at the plant, offsetting approximately 25% of purchased electricity.
The city also controlled upfront costs by approaching the project as design-build,Â meaning that the system was designed to meet the budget on a strict design and construction schedule with a unified contractor-engineer team.
Keys to Success
The project achieves multiple goals for the City of Atlanta including green house gases (GHG) reductions and financial savings. Created through the Clean Water State Revolving Fund, the project is a partnership between the Office of Sustainability and the Department of Watershed Management. The system design avoids standby rates and interconnection.
Design Element Highlights
- 1.6 MW Caterpillar Engine Generator
- .2 MMBTUH of Recoverable Heat
- Iron Sponge H2S Removal System
- Multi-Stage Blowers and Dryer System
- Siloxane Removal System
- Compressor System for Excess Gas Storage
Additional waste-to-energy projects being evaluated by the city include conversion of organic municipal solid waste to methane, and generating electricity or biofuels from yard trimmings.