Case study: university campus CHP
Cogeneration systems—also known as combined heat and power (CHP) systems—generate both electricity and usable thermal energy. These systems typically are used on campuses that have high heat load requirements.
In 2007, a major East Coast research university’s climate change task force introduced climate commitments for the university, including a 51% reduction in carbon emissions by 2025. Subsequent investigation of opportunities for emissions reduction from different sources determined that the largest opportunity would be the installation of a CHP facility at the central utility plant serving the university’s 5-million-sq-ft main academic campus. By generating electricity and capturing waste heat to heat buildings in winter and operate central cooling equipment in summer, the CHP system represented an opportunity to reduce energy consumption and operating costs, and reduce the campus carbon footprint.
To select an appropriately sized unit for the university’s current and future needs, duration curves, monthly electric usage data, and campus demand totals were assembled. The total load factor for electricity use was developed incorporating commodity cost. The local grid and regional transmission grid verified a real electrical cost (it had doubled over the previous decade). After including consideration of the steam-load duration curve, an initial recommendation for a 3,500-kW CHP unit was made.
With interest in future campus growth, further analysis was completed to weigh any potential negative impacts in selecting a larger unit that could provide excess steam load to be used in the summer. Ultimately, the owner favored a system consisting of a nominal 4.6-MW CT and a heat recovery steam generator (see Figure 3). This CHP is operated as a base-load unit generating 4.6 MW of electrical power at 13.2 kV and 25,000 lb/hr of steam at 125 psi. The steam output is connected to the existing campus steam distribution system to supplement existing boiler steam generation and serve the campus heating loads (see Figure 4). The electrical output operates in parallel to the incoming electric utility to reduce overall energy demand from the campus’ electrical distribution system. To fully use the CHP’s electric and steam output, modifications were made to the campus’ 13.2-kV electrical distribution system to shift campus electric loads to the CHP system. To fully use steam loads during summer, a large electric-driven condenser water pump used for campus cooling was changed over to a steam-turbine-driven pump.
To make the transition as simple as possible for operators new to cogeneration, the CHP system was base-loaded and natural gas and electric contracts were obtained through the payback period. No back-end pollution controls were required, nor was additional plant operations staff needed.
Completed in June 2011 at a construction cost of $7.4 million, the project has resulted in operational cost savings of approximately $1.5 million/year and is reducing greenhouse gas emissions by 9,500 metric tons of carbon dioxide per year (equivalent to eliminating 1,750 automobiles, or planting 2,200 acres of forest), as well as reducing nitrogen oxide—associated with ground-level ozone, a severe nonattainment area concern—by 45 tons/year.
About the authors
Jerry Schuett is a principal and leader of the energy and utilities market at Affiliated Engineers Inc. with more than 35 years of experience designing and managing energy and utility projects.
David Cunningham is a project manager at Affiliated Engineers Inc. with more than 15 years of experience designing major utility projects—including heat and power—across the country.