The electric and thermal loads at one or more sites (building, industrial unit, etc.) are usually covered by purchasing electricity from the local electricity network and by generating useful heat by burning a fuel in a boiler or a furnace located at the site. However, the production of electricity in a power plant is accompanied by production of heat, which results in a huge waste of energy in case the heat is rejected to the environment via the exhaust gases and the cooling circuits of the plant. Most of this heat can be recovered and used to cover thermal loads, thus converting the power plant to a cogeneration system, which increases the efficiency of fuel use from 40%-50% to 80%-90%. Many definitions of cogeneration (called also Combined Heat and Power) have appeared in legislature and in the general literature. The following one is proposed here: Cogeneration is the simultaneous generation of work and useful heat from the same primary energy source.