Bogus High Efficiency of Combined Heat and Power Plants
People still ask whether combined heat and power (CHP) achieves efficiencies in the 80% and 90% range, which compares with 32% for traditional coal-fired plants, 45% for a gas turbine, and 60% for a natural gas combined cycle (NGCC) power plant. (These are approximate efficiencies with variations occurring among different plants.)
The answer is no – CHP can achieve slightly higher efficiencies, but not as high as those reported by Greenpeace and others.
The mistake arises when people assign the same value to the heat extracted as steam from a turbine, with the electricity produced by the power plant. The steam has low heat content and therefore has less value than the electricity produced by the power plant.
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 Power Magazine: An automobile’s engine using gasoline has considerable horse power 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.
The 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.
The opposite was true in Europe, where CHP is still used extensively in many European countries.
In Europe, the steam and heat from turbines is used to heat buildings in the area around the power plant. The steam is conveyed by large pipelines, covered with insulation, to the buildings using the steam for heating. The close proximity of buildings in Europe, rather than homes spread out in suburbs, as in the United States, makes CHP convenient in Europe, but impractical in most instances in the United States. Once again, the history of how the United States developed, capitalizing on abundant and low cost land, compared to Europe where people reside primarily in cities, played a role in Europe’s adopting CHP.
An exception in The United States was where Consolidated Edison in New York City used steam to heat buildings, and did so for nearly one hundred years.
With closure of power plants in or around cities, the use of CHP in the United States becomes less and less appropriate.
Greenpeace in its energy plan, the Energy [R]evolution, says, “The lack of district heating networks [in the United States] is a severe structural barrier to the large-scale utilization of geothermal and solar thermal energy [and CHP].” This of course, reflects another Greenpeace objective which is to have Americans live in mixed-use communities, preferably near mass transit which would also eliminate the use of automobiles.
Another factor to consider when evaluating efficiency, is where the steam is extracted from a turbine. If the steam is extracted before it passes through the last few stages of the steam turbine, it can actually reduce the thermal efficiency of the power plant. Extracting steam in this manner prevents the steam from passing through the last rows of buckets and doing useful work, generating electricity.
It’s the second law of thermo dynamics that establishes that the value of low temperature steam is lower than the value of electricity. The value of electricity is higher because it has a greater ability to do useful work.
Care should be taken when touting the high efficiencies of CHP – they are not 80% or 90% efficient when compared with NGCC systems. There is a place for CHP, but there must be a need for the low temperature steam, which rarely exists in the United States.
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