Some industries, like chemical, paper, and food processing, require energy input in the form of heat, called process heat. This section will introduce cogeneration power plant, which produce process heat and electric power from the same power plant.

Cogeneration of Electricity and Heat

Some industries, like chemical, paper, and food processing, require input process heat. Process heat in these industries is usually supplied by steam at 5 to 7 atm and 150 to 200^oC. So it is possible to have a power plant that produces electricity while meeting the process heat requirements of their industrial processes. In this way, more of the energy transferred to the fluid in the boiler is utilized for a useful purpose. In general, cogeneration is the production of more than one useful form of energy from the same energy source. Thus, this kind of power plant is called a cogeneration power plant.

Either a Rankine cycle or a Brayton cycle or their combination cycle can be used as the power cycle in a cogeneration power plant. The fraction of energy that is used for either process heat or power generation is called the utilization factor of the cogeneration power plant.

Schematic of an Ideal
Cogeneration Power Plant

For unit mass of steam flowing through the boiler, heat and work interactions and process heat supply by a steam-turbine cogeneration power plant are:

      Heat Input: q_in = h₄ - h₃
      Heat Output: q_out = (1 - y - z)(h₁ - h₇)
      Work Output:
             W_turb,out   = (1 - y)(h₄ - h₆) + (1 - y - z)(h₆ - h₇)
      Work input:
             W_pump,in = (1 - y - z)(h₂ - h₁) + y(h₃ - h₈)

One common cogeneration arrangement involves district heating. In this application a power plants is integrated into a community so that it provides electricity for industrial, commercial, and domestic use together with steam or hot water for process heat requirements.