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CHP Creates More Energy Confusion

July 11, 2014

Combined Heat and Power (CHP) is mentioned in the EPA’s proposal to cut CO2 emissions 30%, as a possible approach for achieving improved energy efficiency.

But how does CHP improve energy efficiency, and if so by how much.

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.

Greenpeace, and other radical environmentalists, claim that using exhaust steam from turbines, to heat homes and businesses, will dramatically improve thermal efficiency.

The use of CHP is prevalent in Europe where people live in close proximity to power plants, and the steam can be piped to their homes and businesses. For example, in Denmark, Finland and the Netherlands, CHP accounts for over 30% of total generating capacity. The EU’s 2004 Cogeneration Directive required member states to promote CHP.

Conditions favorable to CHP are seldom present in the United States, though some cities, such as New York, have used CHP.

A claim of 90% efficiency is important, since traditional coal-fired power plants have a thermal efficiency of around 33%, a simple cycle gas turbine around 45%, and a natural gas combined cycle power plants around 65%.

Large steam turbine and generator

Large steam turbine and generator

Achieving a 90% thermal efficiency would, obviously, be highly beneficial.

The claim that CHP achieves an efficiency of over 90% is, however, bogus.

The mistake arises when people assign the same value to the heat, extracted in exhaust steam from a turbine, with the electricity produced by the 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 EPA also made the same mistake on its web site by arriving at an efficiency of 75% for a hypothetical plant, valuing electricity and low temperature steam equally.

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 engines efficiency.

Low temperature steam has some value, but not a value that is equal to electricity.

CHP was in vogue in the United States during the first part of the twentieth century, before the grid supplied low-cost electricity to manufacturing plants. These plants installed CHP to generate electricity for the plant and to supply steam for the plant’s various processes, including heating the plant. After the grid was in place and cheap electricity was available from the grid, these CHP plants fell out of favor.

CHP is still used where there is a need for large amounts of steam, such as in chemical plants and refineries.

The use of centralized power generation in the United States remains the most efficient method for generating and distributing electricity at the lowest cost. Distributed generation, such as with PV solar and CHP, is more costly.

However, Greenpeace and other radical environmental organizations promote CHP.

The Obama administration has established a target of adding 40,000 MW of CHP generation by 2020, and states are likely to adopt CHP as part of their plans to comply with the EPA’s proposed regulations for cutting CO2 30%, under the mistaken idea that it significantly improves energy efficiency.

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2 Comments leave one →
  1. donb permalink
    July 11, 2014 3:54 pm

    The energy DENSITY contained in electricity versus lower-temperature steam is illustrated using the two to heat a living or working space. This occurs because heat flow between the heater and the space to be heated depends on the temperature difference (conduction-convection) or T^4 if by radiation (e.g., a heat lamp). Electrical heating generally involves a quite hot emitter, so it delivers energy at a relatively rapid rate. Low-temperature steam is cooler, so is less efficient at transferring that energy. Plus, used steam has to be returned to the source and reheated or discarded (vented). Electricity does not.
    Chemical energy (e.g. in gasoline combustion) can also be a concentrated and available energy source, but is different form of energy storage than electricity or steam.

    • July 11, 2014 5:45 pm

      Thanks for your comment and the additional analysis.

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