Sustainable By Design: Double Duty: Combined Heat And Power

By Anne Cosgrove

Published in the October 2005 issue of Today’s Facility Manager

Energy efficiency is a concept facility managers must embrace in order to meet the challenge of rising energy prices as well as ever present budgetary constraints. Increased efficiency also helps to reduce the demand on natural resources. While alternative energy technologies—such as wind and solar—are gaining momentum, the fact remains that the lion’s share of energy consumed in today’s society is derived from fossil fuels.

So, until the day when facilities derive all their energy from the sun (or other renewable resources), what can facility managers do to stem the tide of consumption? Implementing combined heat and power (CHP) systems to generate a portion of a facility’s energy needs is one approach.

CHP (or cogeneration) systems produce two forms of useful energy from one fuel source. In most of these applications, energy from a fuel source is converted to both mechanical and thermal energy. The mechanical energy generates electricity, while the thermal energy (or heat) is recovered by the system and expended for another application, such as water heating, space heating, or steam production.

According to the U.S. Department of Energy, in conventional conversion of fuel to electricity, more than two-thirds of the energy input is discarded as heat. By recycling waste heat, CHP systems achieve efficiencies of 60% to 80% , as compared to the average 33% efficiency of conventional fossil fueled power plants.

Because of its higher energy efficiency, a CHP system consumes nearly 40% less fuel than conventional systems, according to the Northeast CHP Application Center, University of Massachusetts, Amherst. In turn, the lower fuel consumption can significantly reduce energy costs.

Microturbines represent one technology through which combined heat and power can be introduced to a facility. Approximately the size of a refrigerator, microturbines are small combustion turbines with power output ranging from 25 kilowatts (kW) to 500 kW. These can be located on-site and offer a number of potential advantages for small scale power generation, including the ability to recover waste heat (when outfitted with a heat recovery unit) to create a CHP system.

A microturbine can be designed to run on a one type of fuel or varying fuel types. Fossil fuel sources are an option. Or, a green conscious specifier could choose renewable fuel sources such as biogas.

The basic process of a microturbine consists of mixing compressed air with fuel, which is burned in the microturbine combustor under constant pressure. The hot gas produced then travels through the turbine to create power. When the microturbine is used for a CHP application, it is outfitted with a heat recovery unit through which the heat normally wasted during power generation is captured for a secondary use such as heating water.

In Minnetonka, MN, the Hennepin County Home School recently installed a 60 MW natural gas fueled microturbine coupled with a heat recovery unit. This CHP system is expected to provide 5% of the electricity used at the 170,000 square foot facility. The heat recovered from the electricity generation process is used for preheating boiler water and for space heating. “The heat provided is 7% to 8 % of the total [demand],” says Roy Earl, senior mechanical engineer in the design and construction division of the county property services department.

Earl explains that the CHP system is part of a pilot program that also includes a fuel cell installation. “The county wanted to have the pilot projects to show how distributed generation can be part of the solution to the [projected] electrical shortfall,” he says. “[This installation] has the benefit of localized power generation without the transmission losses with heat recovery.” Xcel Energy, the local electric provider, was instrumental in the installation.

CHP systems also reduce the amount of chemical emissions released into the atmosphere. By capturing the heat exhaust for a secondary use, the system does not have to generate as much electricity as a conventional system. This helps to reduce the amount of nitrous oxide (NOx), sulfur dioxide (SOx), and other chemicals that are released into the environment during electricity generation.

“Microturbines produce energy far cleaner than utility power plants,” says Keith Field, director of communications at Capstone Turbine Corp, a microturbine manufacturer located in Chatsworth, CA. “An independent test posted on the U.S. EPA Web site confirms that a Capstone C60 system, at full output, emits 0.15 pounds of nitrogen oxides NOx per MWh. According to the EPA , the average U.S. power plant is 20 times higher: three pounds of NOx per MWh.

“With less fuel, less emissions, and less cost than utility power rates in California, New York, and other high rate states, on-site microturbine CHP is a no-brainer,” says Field. “As fuel costs have risen, microturbine CHP solutions mitigate those costs by serving thermal loads without [creating the need for] additional fuel. There is also the generation of a more valuable commodity—electricity—that slashes monthly power bills.

“These systems pay for themselves,” adds Field. “And the new 10% federal microturbine tax credit, more favorable distributed generation natural gas rates, and state rebate programs for clean, efficient on-site energy supplementation make that return on investment even faster.”

Optimizing the materials used to build the equipment as well as facilitating the integration of CHP equipment for operation in a facility are prime issues. With economic and environmental concerns dominating the energy market, it is timely for facility managers to watch the developments.

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