Cogeneration is a technique for producing both heat and electricity with a single engine and a single fuel (usually natural gas).     THE ADVANTAGE

With cogeneration, higher overall yield (electricity + heat) can be achieved than with separate heating and electricity systems. The saving on primary energy is about 20%.

COGENERATION SYSTEMS

Boiler + steam turbine Gas engine + heat recuperator Gas turbine + heat recuperator

Cogeneration with a gas turbine is very reliable and requires little maintenance (a major revision every 30,000 to 40,000 hours). To run it on natural gas, the gas must be at a minimum pressure of 15 to 20 bar, therefore requiring connection to the high-pressure network or the use of a compressor. The energy in the flue gases is recovered, either to supply an industrial process directly or to heat a heat transfer fluid such as steam. Cogeneration with a gas engine is usually used to produce hot water and hot air. It can be used to produce cold by compression or absorption, but this use is not widespread in France, where the winter operation period is short. Unlike the turbine, the heat engine does not require high-pressure gas (supply from the 4-bar network is sufficient). It gives a fairly high yield in electricity (37%). Because of its ready availability (95%) and rapid start, the internal combustion engine is one of the solutions best suited to the varied requirements of the manufacturing and services sectors. Internal combustion engines currently account for 53% of cogeneration units in Europe. By way of indication, at end June 2001, France had 700 cogeneration units with an electrical power capacity (installed or in course of installation) of 4.3 MW, equivalent to nearly 4 nuclear units, or 1 ½ times the power capacity of inner Paris. Most of these cogenerators use gas engines. About 25% of the total are gas turbines. Gas turbine technology is mainly used in high power output units, but it is likely to find wider application in future.

OVERALL TECHNICAL CHARACTERISTICS

Gas engine Gas turbine Electrical yield 30 to 35 % 25 to 35 % Thermal yield 45 to 50 % 50 to 55 % Heat recovery Hot water (+ steam) Steam Type of recovery Cooling (3/5) Exhaust (2/5) Exhaust Feed gas pressure 100-300 mbar to 3 bar > 15 bar

MARKET SEGMENTATION

Cogenerators are classified by their electrical power output. In practical terms, the current stock can be broken down as follows.

Type of cogeneration Electrical power output Large Cogeneration 12 MW < P Medium Cogeneration 2.5 MW < P < 12 MW Small Cogeneration 215 kW < P < 2,5 MW Mini Cogeneration 36 kW < P < 215 kW Micro Cogeneration P < 36 kW

THE ADVANTAGE OF NATURAL GAS COGENERATION

At equal power output, natural gas emits less carbon than coal Cogeneration close to the point of consumption avoids losses from distribution network Cogeneration makes use of the heat discharge that is usually lost in conventional thermal generators and combined-cycle generators.

ENVIRONMENTAL IMPACT

Helps to combat the greenhouse effect: savings on primary energy Limits nuisance due to overhead electrical cables Less heating of environmental water No air cooling structures (visual nuisance, risk of creating microclimates).