Environmental Engineering Reference
In-Depth Information
4.1.4 Combined Heat and Power - Using Fuel Twice
When it comes to increasing effi ciency, combined heat and power (CHP) is often
promoted as a promising candidate. Using electricity generated by conventional
steam turbine power plants that burn brown and hard coal, the level of effi ciency is
between 35 and 45%. Modern gas and steam turbine power plants that use natural
gas achieve up to 60%. However, this means that at least 40% of the primary energy
fi zzles out unused through the cooling tower of the power plant.
CHP, or cogeneration, plants use the heat from electricity generation effectively and
are able to exploit up to 90% of the fuel. As a result, in an optimal case less carbon
dioxide is produced than when electricity and heat are generated separately.
Many comparisons of cogeneration power plants that generate heat and electricity
separately using fossil fuels show possible carbon dioxide savings of up to 50%.
However, these comparisons usually pit CHP plants against antiquated electricity
plants. If the comparison is made on the basis of optimal functioning plants on both
sides, the carbon dioxide savings are reduced to a meagre 15 to 20% (Figure 4.4)
- too little to save the climate. Furthermore, these savings are only possible with
optimal plant operation. For example, in summer when a cogeneration plant is only
supposed to generate electricity but not heat, it will have great diffi culty in even
coming close to the dream effi ciency level of 90%. A cogeneration plant can some-
times end up producing even more carbon dioxide than a straightforward electricity
power plant.
Figure 4.4 Comparison of primary energy requirements and carbon dioxide emissions
between power-heat coupling and separately generated heat and electricity in modern power
plants.
If, on the other hand, a suffi cient requirement for heat exists over the entire year,
cogeneration plants can help to reduce carbon dioxide. With cogeneration plants
that use fossil fuels the savings for effective climate protection are too low. Those
that use renewable energy sources, such as biomass and geothermal energy, are
totally carbon-free and can continue to accelerate the switch to carbon-free energy
supply.
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