EHS Management

Combined Heat and Power 101

Conventional power generation is a notoriously wasteful operation in the United States where the average efficiency of fossil-fueled power plants is just 33 percent, the same as it has been since the 1970s. The remaining energy produced is simply released in the form of heat, a curious fact when you consider that so much of the energy produced and purchased is actually used to keep us warm.

According to a 2012 report by the Department of Energy’s (DOE) and the Environmental Protection Agency (EPA), CHP is used in only about 8% of annual U.S. power generation or about 82 gigawatts (GW), but it saves 1.8 Quads (quadrillion Btu) of energy annually and eliminates 240 million metric tons of carbon dioxide (CO2) equivalent to emissions of more than 40 million cars.

These numbers are not surprising because CHP power production is typically 60 to 80 percent more efficient than conventional systems. The EPA cites one Texas oil refinery with a 470 MW CHP system that has actually reached 88 percent operating efficiency, uses 37percent less fuel than conventional systems, and has reduced CO2 emissions by an estimated 2.4 tons per year, the equivalent of the emissions from 397,000 passenger vehicles.

 


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How does CHP achieve these rates? In a typical CHP gas turbine or reciprocating engine power production scenario, the hot exhaust gases that are normally vented to the atmosphere, are routed through a heat recovery unit that converts the heat into thermal energy such as steam or hot water, which can then be used for heating or cooling. CHP can also be used with a boiler system with steam turbines, which normally create electricity as a by-product of steam generation, and are especially suitable for industrial settings where there is a ready supply of solid fuels like waste products, biomass, or coal.

Another option is microturbines that have evolved from uses in cars, trucks, and aircraft to become a great CHP alternative for small-scale power generation. These units are classified by the physical arrangement of their components, and the DOE defines two classifications for CHP uses: simple cycle and recuperated. Simple-cycle turbines cost less, are more reliable, and have more heat available for CHP while recuperated units have a higher thermal-to-electric ratio than simple-cycle turbines and can achieve fuel savings of 30 to 40 percent.

Thermally activated technologies are also part of the mix and are essential to gaining the desired energy savings and return on investment. These technologies are very diverse, according to the DOE, and include absorption chillers that use ammonia and water or lithium bromide to harness waste heat to provide cooling in CHP systems and desiccant systems that use a drying agent or sorbent (such as silica gel, activated alumina, lithium chloride salt, or molecular sieves) to dehumidify the air while at the same time increase system efficiency as much as 80 percent.

 


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Since CHP has actually been around for about 150 years and provides all these economic, energy, and environmental pluses, it is hard to believe it isn’t already the number one way to produce fuel-derived electricity. But things may be about to change. In fact, CHP is a big part of the Obama administration’s clean energy campaign with a goal to achieve 40 GW of new, cost-effective CHP by 2020, doubling the current CHP capacity. According to the 2012 report, meeting this challenge would also:

  • Save energy users $10 billion annually compared to current energy use.
  • Save one Quad of energy or about 1% of total U.S. energy used.
  • Reduce emissions by 150 million metric tons of CO2 annually, equivalent to emissions from more than 25 million cars.
  • Create $40 to $80 billion in new capital investments in manufacturing and other U.S. facilities over the next 10 years.