Carbon dioxide (CO2) is a colourless and odourless gas; it is 0.035 percent by volume of surface air. Steam, carbon dioxide, and other minor gases absorb the heat radiated by the Earth, thereby causing the "greenhouse effect".
CO2 plays a central role in today's discussions on possible climate changes caused by mankind.Since pre-industrial times, the CO2 content in the atmosphere has risen by almost 30 °/o, primarily as a result of burning coal, oil and gas.
At least 50% of the greenhouse effect attributable to human activity has been caused by increased CO2 emissions. The remaining 50% comes from minor gases reacting with the atmosphere, especially chlorofluorocarbons (CFCs), methane, ozone, and di-nitrogen.
Researchers today have largely agreed that further increases in heat-absorbing gases - especially CO2 - in the atmosphere will lead to a rise in the Earth's temperature.
Far-reaching ecological changes would then be expected. Operators of power stations are taking discussions of possible climate changes seriously and are making an effort to reduce the CO2 impact of their plants despite the uncertainty contained in the findings available until now.
District heating can be generated together with electricity in CUP plants. Buildings can then be supplied with ready-to-use heat for space and water-heating via pipelines in which high temperature water can circulate.
Joint generation of electricity and heat in CHP plants, also known as cogeneration, makes particularly efficient use of fuels. (See the Figure below, in which separate electricity and heat generation in individual heating systems and a condensation power station is compared to joint electricity and heat generation in a CHP plant. For this, all losses have been taken into account up to the respective heating units and power outlets.)By extracting heat from the electricity-generation process, fuel input can be reduced considerably compared to individual heating systems.
Under favourable conditions, up to 2/3 of the fuel input for conventional individual heating systems can be saved.
Greater utilisation of this effect can only be successful in conjunction with building up and expanding district heating networks.
Saving energy during the generation process has many advantages:Depending on the fuel used (in CHP plants, it is primarily coal and gas) harmful emissions (SO2, NOx, particulate, etc.) can be lowered in accordance with the plant technology employed. In any case, CO2 emissions can be reduced considerably, as whatever does not have to be burned due to energy-saving measures cannot produce any CO2. Natural gas generates less CO2 than coal because with gas (CH4) the relative carbon content is low and hydrogen (H2) is also burned. Specific CO2 emissions kg/kWH of useful heat
With joint generation of electricity and district heating, "common" emissions occur. The proportion of CO2 emissions attributable to district heating can be easily determined by comparing the CO2 emissions of a CHP plant with the CO2 emissions of a condensing boiler plant with the same electricity output (it was assumed that electricity was generated with 80% coal and 20% nuclear power).
The chart above shows the results for typical coal-fired CHP plants in comparison to various individual heating systems.
With gas-fired CHP plants (not shown in chart), there can be very low levels of CO2 emissions which in some cases can even lead to emissions credits. The choice of fuels for CHP plants, however, cannot be made with regard to environmental protection alone.
In addition to the environmental-protection aspects, general considerations on energy policy will also have to play a role. Heat service should not only be environmentally sustainable, but should also be reliable and economical on a long-term basis.
Coal can only be utilised in an economical and environmentally sustainable way in larger power stations. Natural gas can make it possible to make use of cogeneration in smaller plants. Wherever technically and economically feasible, sources of waste heat - and later on renewable energy sources - will be used. The heat required for large district heating networks is usually supplied from several sources (usually from CHP plants or heat-only plants).This will also increase the reliability of uninterrupted service.
In times of unfavourable price developments, district heating networks can make it possible to switch to the most economical fuel for generating heat. However, the district heating network remains unaffected. District heating from CHP plants is a classic example of replacing energy with capital.
Energy savings on the one hand are offset by additional investments for transporting and distributing heat to consumers.
Building up a district heating system can only be done gradually, as the investments for transporting and distributing heat must be financed by the respective utility in advance up to maximum utilisation of network capacity.
By being connected to a district heating network, buildings can be supplied conveniently and economically.
Every connection to district heating
- especially in light of the CO2 aspect -
is a further step toward maintaining a viable environment.