Role of Combined Heat Power
Environmental organizations repeatedly attempt to deride the efficiency of existing power plants and promote the use of combined heat and power (CHP).
Some radical environmental organizations, such as Greenpeace, claim that CHP has en efficiency of over 90%. Greenpeace makes this claim in its plan, the Energy [r]evolution, which is riddled with hype and misinformation.
To make the 90% claim appear more awesome, it is compared with traditional coal-fired power plants that have a thermal efficiency of 33%.Even today, the American Council for Energy-Efficient Economy (ACEEE) claims that CHP has an efficiency of 80%.
Amazingly, an editor from a leading industry magazine, who is a lawyer, also said CHP can reach 90% efficiency. He has probably accepted at face value the claims of some manufacturers, some of whom make the claim, but always seem to miss the obvious truth about waste-heat energy.
These claims are misleading at best, and are probably better described as bogus.
Proponents of CHP arrive at these thermal efficiency numbers by comparing apples with oranges.
The mistake arises when people assign the same value to the heat, extracted in exhaust steam from a turbine, with the electricity produced by a power plant.
The exhaust steam has low heat content and therefore less value than the electricity produced by the power plant.
Under the second law of thermodynamics, the exhaust steam can do less work.
The best analogy is one suggested by the former editor of Power magazine:
“An automobile’s engine using gasoline has considerable horsepower and also heats water in the engine’s cooling system. The hot water is then used to heat passengers during the winter. While this takes advantage of the heat in the water, the water doesn’t have the power to drive the automobile. Gasoline has high energy density, while hot water has a low energy density. Using the hot water for heating the car does not increase the engine’s efficiency.”
Organizations promoting CHP have become imbued with the idea that CO2 is causing climate change and insist that America change its ways.
While there are legitimate applications for CHP, such as in chemical plants that need the steam for processes, the only people pressuring for the adoption of CHP are environmental organizations and their allies in government.
At a recent meeting in Washington, DC, the keynote speaker was from the White House. She was the “federal environmental executive”, a position established by executive order. She noted that CHP was a way to cut CO2 emissions, and that the President had issued an executive order to double federal CHP systems, with a target of adding 40,000 MW of CHP generation by 2020.
States are likely to adopt CHP as part of their plans to comply with the EPA’s proposed regulations for cutting CO2 emissions 30%, under the mistaken idea that CHP significantly improves energy efficiency.
Once again, the fear of CO2 induced climate change is affecting energy policy.
In practice, CHP systems have a thermal efficiency of around 60%, not 80% or 90%.
They are no more efficient, and possibly less efficient, than natural gas combined cycle (NGCC) power plants that can have a thermal efficiency of 63%.
At the core of promoting CHP is the use of steam to heat buildings.
District heating is widespread in Europe, largely because so many people live in cities.
It’s ludicrous to attempt to apply CHP heating to America’s suburbs. (Suburbs with free standing residential buildings are an anathema to city planners and to those promoting CO2 induced climate change, so CHP fits a strategy that promotes mixed use development.)
An examination of Europe’s district heating infrastructure and its adoption of regulations promoting CHP is beyond the scope of this article. Basically, it’s predicated on the need to cut CO2 emissions.
The allure of CHP is to use exhaust steam for area heating. This reduces the need for individual heating units in buildings. Fewer heating units in buildings probably reduces CO2 emissions, as emissions can be more easily controlled by a single central CHP plant.
In addition, the European CHP plants promoted in Power magazine use fuels such as biomass and syngas rather than coal or natural gas.
In countries other than in Europe, CHP is used where there is a need for steam for industrial or chemical processes.
Finally, as is so often the case when discussing energy issues, the proponents of CHP switch horses and obfuscate the discussion.
CHP has always, until now, been associated with centralized installations for power generation, using steam and gas turbines, or diesels and natural gas engines.
The switch is to include fuel cells for CHP.
- Fuel cells are a separate technology, and are more closely related to batteries than power generation installations. They can generate electricity and be used for distributed installations, including heat for local use, say in a building.
- Fuel cell CHP is less efficient, as measured by thermal efficiency, than NGCC power plants. They also cost more. Fuel cell costs in 2010, according to DOE, were around $5,000 / KW, compared with $1,100 / KW for NGCC power plants.
- Fuel cells for CHP are currently a distraction, and do not achieve 90% efficiency, with the possible exception of units using hydrogen. Hydrogen isn’t readily available, and besides, the losses incurred to produce hydrogen would negate any efficiency advantage Fuel Cells might have.
Higher cost CHP is being promoted by the federal government to reduce CO2 emissions and promote mixed use development, using the excuse that CHP is more efficient than existing power plants, which is not true.
Natural gas combined cycle power plants produce electricity at the lowest cost, with the possible exception of large scale hydro.
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