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In a conventional district heating power plant,
district-heating water is circulated through one or two heat
exchangers as shown in the following figure.
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 [enlarge]
Figure 3. District Heating Power Plant.
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The temperature of district heating water varies
in relation to ambient temperature. For example, typical average
values for incoming water temperatures can be 50-55 oC and
for outgoing temperatures 80-85 oC. Depending on the design
of the network, the maximum outgoing temperatures can during cold
winter days reach 120-150 oC. However, the increase of
water temperature is often accomplished by using hot water boilers
connected in series with the power plant. It has to be noted that the
heat generation in hot water boilers is not considered as CHP Heat.
The higher the temperature of the district heating water after the
turbine heat exchangers, the less electricity can be generated in
relation to heat. This relation of electricity generation to heat
generation in cogeneration power plants is called the Power
to Heat Ratio.
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