Key Terms Used in Calculating CHP Efficiency

CHP system. The CHP system includes the prime mover (e.g., combustion turbine, engine, microturbine), the electric generator, and the heat recovery unit that transforms otherwise wasted heat to useful thermal energy.

Total fuel energy input (QFUEL). The heating value of the total fuel input. Total fuel input is the sum of all the fuel used by the CHP system. The total fuel energy input is often determined by multiplying the quantity of fuel consumed by the heating value of the fuel.

Commonly accepted heating values for natural gas, coal, and diesel fuel are: 

  • 1020 Btu per cubic foot of natural gas
  • 10,157 Btu per pound of coal
  • 138,000 Btu per gallon of diesel fuel

Net useful electric output (WE). The gross electric output of the generator minus any parasitic electric losses. In other words, the net useful electric output is the total electric output from the CHP system that is put to a useful purpose. 

Gross electric output is the total electric output of the generator. Parasitic electric losses are the electrical power consumed by the CHP system; for example, the electricity used to compress natural gas before it is used as fuel in a combustion turbine.

Net useful thermal output (∑QTH). The gross thermal output of the CHP system minus any thermal output that is not put to a useful purpose. In other words, the net useful thermal output is the total thermal output from the CHP system that is put to a useful purpose. 

In the case of a CHP system that produces 10,000 pounds of steam per hour, with 90 percent of the steam used for space heating and the remaining 10 percent exhausted in a cooling tower, the energy content of the 9,000 pounds of steam per hour is the net useful thermal output.

Gross thermal output is the total thermal output of the CHP system.