The terms cogeneration of heat and power plant and district heating central plant stand for the simultaneous production of thermal and electrical energy in a single process. District heating central plant is a compact cogeneration plant.

Cogeneration of electric power and heat optimally uses fuel: cogeneration plants convert between 70 - 90% of the fuel energy into usable energy (electrical current and heat). By comparison, conventional caloric power plants achieve efficiencies of 30 - 55%.

Cogeneration plants can be operated with natural gas, fermentation gas or diesel (fuel oil). In addition, technologies are available for the generation of electricity from biomass. The overall efficiency of a cogeneration plant is substantially higher than with a conventional power station. Thus fuel consumption is reduced significantly for meeting requirements of electric power and heat.

Cogeneration of heat and power makes sense:

• for a simultaneous current and heat requirement of at least 5000 hours per year

• if the heat requirement is about twice that of electric power

• with relatively high electricity rates

• if the fuel unit price is relatively low or could become more favorable with increased use

• if changes in the current or heat supply are necessary

Advantages

Cogeneration can yield clear cost savings for consumers with high simultaneous power and heat requirements. From the ecological point of view, cogeneration is positive if the plant uses alternative fuels (e.g., biomass). A positive side effect is that cogeneration can avoid the drawing of emission-laden current from the net (e.g., current from caloric power stations; the origin is often noted on the electricity bills).

Approaching a cogeneration project

The basis for a cogeneration project is a feasibility study and careful preliminary planning by independent consultants (e.g., an energy service provider) that should cover the following points:

• Exact assessment of heat and power requirements in order to prevent wrong design

• Measures for energy conservation to be accomplished before the design of the cogeneration plant in order to adapt the plant to the reduced requirements

• Analysis of the efficiency of different cogeneration module sizes and technologies (learn about subsidies)

• Environmental impact assessment and effects on the operational situation

Sizing is based on the heat basic load and on daily load profiles for power and heat in order to cover the exact requirements for heat and current. If the heat requirement is too small in the summer months, it might pay to search for further utilization possibilities. Examples include replacing electric heat production units or using waste heat for cooling purposes via absorption coolers (cogeneration of heat, power and refrigeration).

Heat accumulators should be used if possible in order to avoid heat surplus and to reduce peaks. The cogeneration plant needs an area with sufficient ventilation. Noise and vibration must be suppressed by suitable measures. The same applies for the exhaust system, which must fulfill relevant emission and noise regulations. Integration with the heating and control system must guarantee that the cogeneration plant is used as the primary source of heat and power. Integration with existing hydraulics and controls must be planned and implemented carefully.

Source: Austrian Chamber of Commerce