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Combined Heat and Power: Pros and Cons

RP Siegel | Monday April 30th, 2012 | 3 Comments

There is no perfect energy source. Each and every one has its own advantages and compromises. This series will explore the pros and cons of various energy sources.  Learn about other forms of energy generation here.

Combined heat & power (CHP) or cogeneration, is really not an energy source itself, but rather more of an energy multiplier, squeezing more usable energy out of each unit of fuel most everywhere it is applied.

According to the EPA, CHP is not a single technology, but rather, an integrated energy system that provides electricity and heat, usually in the form of hot water or steam. Heat is an inevitable byproduct of any power produced by gas or steam turbines, which would include all gas, coal, oil or nuclear power plants in use today. In these turbines, pressurized hot gas or steam is expanded across a turbine, which spins the blades that ultimately drive the generator. The hot gas needs to be cooled immediately after leaving the turbine for the system to work. This was traditionally done with a condenser or cooling tower, but using the heat to keep a building comfortable or drive a production process, with steam or hot water, will also do the job with little loss in efficiency. However, in order for the heating application to replace conventional cooling systems, the demand for heat must be continuous. Typical CHP system will reclaim upwards of 80 percent of the heat that would otherwise be wasted.

A conventional fossil fuel plant achieves a thermal efficiency of approximately 33 percent.

When integrated into a CHP system, the same power plant can achieve efficiencies between 60 and 80 percent. It is estimated that CHP systems can reduce carbon emissions by up to 30 percent.

Without CHP, fuel is used to provide electricity and then additional fuel is used for heat, which in many cases is a missed opportunity.

Cogeneration systems are generally installed on-site for large facilities, such as factories, institutions, commercial buildings, multi-unit residential buildings and district energy systems. It is particularly attractive for facilities with a high heating demand. These facilities then have their own source of electricity as well as a source of heated water or steam. The CHP needs to be located close to where the heat will be used so it won’t cool down. It is therefore an inherently distributed energy source. CHP is seen to be cost-effective in areas where the cost of electricity is seven cents per kWh or higher.

CHP is generally based on fossil fuel  technology, though some companies are now marketing solar cogeneration products: Naked Energy in the UK, and Cogenra in the US.

The number of building with CHP potential in this country has been estimated at well over a million, with energy usage close to 80,000MW. Cutting that by 30 percent would be equivalent to shutting down twelve 1000MW coal plants.

The systems can be installed by any of a number of contractors, many of whom are members of the EPA’s CHP Partnership.

Pros

  • Increased efficiency. CHP systems act as energy multiplier which:
    • saves energy
    • saves money
    • reduces carbon emissions by up to 30 percent
  • Increased reliability. System is independent of the grid and therefore immune to grid-level blackouts.
  • The technology is available and in use today.

Cons

  • Not an actual energy source, only a means of extending energy
  • Could end up preempting more sustainable options
  • Only suitable where there is a need for both electricity and hot water on site
  • Heating and electricity demand must remain fairly consistent
  • Capital intensive
  • Not long term sustainable when based on fossil fuel technology
  • Heating demand must be continuous
  • Efficiency claims are sometimes overstated since heat energy and electricity are not equivalent

When used with a fossil fuel input source, CHP cannot be considered an ultimately sustainable solution for the long term. However, it can help slow the rate of carbon emissions with substantial energy savings in situations where more sustainable options are not available or affordable. On the other hand, when used with renewable energy sources, such as the recently announced solar cogeneration, or certain biomass applications, CHP can act to enhance the efficiency of sustainable energy systems.

***

What about other energy sources?

Image credit: Courtesy of US EPA (with permission)]

RP Siegel, PE, is the President of Rain Mountain LLC. He is also the co-author of the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water in an exciting and entertaining format. Now available on Kindle.

Follow RP Siegel on Twitter.


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  • http://twitter.com/Districtenergy District Energy

    CHP plants do not have to burn fossil fuels. For example, Eastern Illinois University’s CHP plant consumes biomass (waste wood) to produce district heating & cooling and 2-cents-per-kilowatt electricity. See: http://www.districtenergy.org/eastern-illinois-university-renewable-energy-center/ and http://www.districtenergy.org/assets/pdfs/2012-Campus-Arlington/Presentations/Wednesday-A/5A1BONDANKREEDEVERSEIU-Renewable-Energy.pdf.
    The potential for utilizing purpose-grown energy crops on marginal agricultural land, such as fast-growing willow or poplar varieties developed respectively at SUNY-ESF and the University of Minnesota, as fuel changes the game.

  • RP Siegel

     Good point. I stand corrected. So noted.in the revised closing paragraph.

  • M. Goodell

    Our group designed and built the world’s first “solar trigeneration” energy system in 2002-2003. Now we are developing a 200 home subdivision in SoCal, which will be OFF the grid. All 200 homes will be “Net Zero Energy” using our Solar Trigeneration energy system.

    Synthesis Gas, from organic waste can be used to fuel cogeneration and trigeneration energy systems.  Synthesis gas, or Syngas, can be produced via “Biomass Gasification.”

    Similarly, “Biomethane,” which is generated from “Anaerobic Digesters” located at wastewater treatment plants and dairy farms, can also be used as a green fuel to run cogeneration & trigeneration energy systems.