Cogeneration and where it suits

Cogeneration (combined heat and power, CHP) is the simultaneous production of electricity and useful heat (steam or hot water) from a single fuel source. The heat that a normal power plant rejects to the condenser or exhaust is used for an industrial process or heating.

Principle

          +-----------+   electricity
 Fuel --> | Prime     |---------------> load / grid
          | mover +   |
          | generator |   exhaust / extracted steam
          +-----------+---------------> process heat
                                        (drying, cooking,
                                         heating)

• A conventional thermal plant converts only about 30–40% of fuel energy to electricity; the rest is lost.
• In cogeneration, overall fuel utilisation rises to about 70–85%, because the low-grade heat is also used.

Types

Where is cogeneration appropriate?

Cogeneration suits places that need both electricity and heat at the same site and at the same time:

1. Sugar mills – bagasse fired boiler; steam for juice boiling and power for the mill, surplus to grid (sugar mills in Nepal's Terai).
2. Paper and pulp, textile, chemical and food processing industries – large steam demand.
3. Cement plants – waste-heat recovery from kilns.
4. Rice mills, breweries, dairies – rice husk or biomass fired units.
5. Hospitals, hotels, campuses – electricity plus hot water / space heating.
6. District heating in cold climates.
7. Remote or high-altitude areas where both heat and power are scarce.

Benefits

• Lower fuel cost per unit of useful energy.
• Less CO₂ and pollution for the same output.
• Captive, reliable supply; less transmission loss.

Cogeneration is not appropriate where heat demand is small, far from the plant, or does not match the timing of electricity demand.