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Combined Heat and Power Partnership

Efficiency Benefits

Learn More About
Calculating Efficiency
  • The Methods for Calculating Efficiency page describes the two most common methods for calculating CHP system efficiency: total system efficiency and effective electric efficiency. Learn how and when to use these methods for your calculations.

Although, CHP systems typically achieve system efficiencies of 60 to 80 percent, higher efficiencies can be achieved. For example, ExxonMobil's Beaumont Refinery in Beaumont, Texas, operates a 470 MW CHP system that achieves an operating efficiency of 88 percent, requiring approximately 37 percent less fuel than typical onsite thermal generation and purchased electricity.

Based on this performance, the CHP system reduces carbon dioxide (CO2) emissions by an estimated 2.4 million tons per year, which is equivalent to 595,000 acres of pine or fir forests storing carbon for one year or the emissions from 397,000 passenger vehicles per year.

photo of Exxon Mobil plant

Combined heat and power (CHP) is an efficient and clean approach to generating power and thermal energy from a single fuel source.

The average efficiency of fossil-fueled power plants in the United States is 33 percent and has remained virtually unchanged for four decades. This means that two-thirds of the energy in the fuel is lost—vented as heat—at most power plants in the United States.

By using waste heat recovery technology to capture a significant proportion of this wasted heat, CHP systems typically achieve total system efficiencies of 60 to 80 percent for producing electricity and thermal energy.

Because CHP is more efficient, less fuel is required to produce a given energy output than with separate heat and power. Higher efficiency translates into:

  • Lower operating costs
  • Reduced emissions of all pollutants
  • Increased reliability and power quality
  • Reduced grid congestion and avoided distribution losses

The illustration below demonstrates the efficiency gains of a 5 megawatt (MW) natural gas-fired combustion turbine CHP system compared to separate heat and power.

Conventional Generation vs. CHP: Overall Efficiency

This graphic compares the efficiency of separate heat and power versus combined heat and power (or CHP). Separate heat and power has an overall efficiency of 49 percent, while the CHP system has an overall efficiency of 75 percent. Both systems generate 30 units of electricity and 45 units of steam.

In this example of a typical CHP system, to produce 75 units of useful energy, the conventional generation or separate heat and power systems use 147 units of energy—91 for electricity production and 56 to produce heat—resulting in an overall efficiency of 51 percent. However, the CHP system needs only 100 units of energy to produce the 75 units of useful energy from a single fuel source, resulting in a total system efficiency of 75 percent.


High-Resolution Image >>

A CHP system's efficiency depends on the technology used to generate the electricity and thermal energy, the system design, and how much of the thermal energy is used by the site. Therefore, every CHP system will have a different, site-specific efficiency once installed. However, the six most commonly installed CHP prime movers tend to offer fairly standard ranges of achievable efficiency as follows:

  • Steam Turbine: 80 percent
  • Diesel Engine: 70-80 percent
  • Natural Gas Engine: 70-80 percent
  • Gas Turbine: 70-75 percent
  • Microturbine: 65-75 percent
  • Fuel Cell: 65-80 percent

More information about these technologies and their applications can be found in the Catalog of CHP Technologies.

Efficiency can be calculated a number of different and legitimate ways; however, the efficiency numbers that EPA cites are referred to as "total system efficiency." Total system efficiency is the total power and useful thermal energy output of the system divided by the fuel used to produce the power and heat.

Total system efficiency and effective electric efficiency are the two most commonly used methodologies for determining the efficiency of a CHP system. Learn about the difference between these methodologies and how to use them for your calculations by visiting Methods for Calculating Efficiency.

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