Power Plant Engineering

 9.5 COGENERATION
Decentralized combined heat and power production-cogeneration is a very flexible and efficient
way of utilizing fuels. Cogeneration based on biomass is environmentally friendly, and all kinds of
biomass resources can be used.
The role combined heat and power production plays in Danish energy supply originates from the
decision in 1978 to establish a national natural gas grid. At present the natural gas system is one factor
blocking the utilization of biomass and natural gas in decentralized cogeneration plants, because a great
part of the heat market is lost for decentralized cogeneration due to the individual gas supply.
In June 1986 it was decided that 450 mW decentralized heat and power plants should be estab-
lished. These are very efficient and environmentally compatible, if they are based on natural gas or
biomass. The interest in biomass as basis for combined heat and power production is caused partly by
environmental considerations, and partly by the desire in agriculture and forestry to get rid of an increas-
ing surplus of residue products, typically straw and wood chips.
But exceeding the problem with an insufficient heat market, the energy policy has caused that
until now there has been no sufficiently purposeful and ambitious aiming at the cogeneration technolo-
gies, that first of all shall lead to an increased use of biomass in heat and power supply.
9.5.1 COGENERATION — WHY
There is a large political interest in changing the local heat supply to combined heat and power
supply—this means cogeneration of heat and power.
It is a fundamental physical condition that not all-latent energy of a fuel can be converted into
tractive power, e.g. to run a car. The main part of the energy is necessarily transformed to waste heat,
which in the car example disappears by motor cooling and with the exhaust.
Cogeneration plants can be used in all situations where a given heat demands exists. This in-
cludes all together an extremely large number of district heating plants, institutions, co-operative build-
ing societies, industries, etc.
For the cogeneration technologies, the primary interest is due to, that a very large percentage of
the fuel’s energy content is utilized, typically 85-95%. This must be compared to the relatively low
energy efficiency of centralized thermal power plants, the annual mean efficiency is about 55% in the
ELSAM area (Jutland, Funen).
Another important reason for the interest in decentralized cogeneration is the possibility to uti-
lize renewable bio fuels straw, wood, manure, etc. There are furthermore a few circumstances which are
not that much noticed in the political debate.

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POWER PLANT ENGINEERING
First of all a large number of cogeneration plants increase the security of power supply. It is not
usual that the large power units break down, but it happens. It is obvious that the consequences of
missing a large unit are much more significant, than if it is one of the much smaller cogeneration plants.
Second there is a considerable energy loss from the power grid. In the ELSAM area it is good 7%
in average. But this figure covers very large variations through the day, and furthermore depends very
much on the voltage level. Thus the energy loss from the low-voltage grid is much larger than from the
high-voltage grid. All in this entire means that e.g. on a winter day at 5 pm there is a large energy loss
from the low-voltage grid.
Exactly because many of the cogeneration plants are coupled on the low-voltage grid, they also
reduce the grid loss, which influence the overall energy efficiency.