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Polysun also provides for the simulation of cogenerators. Cogenerators work on the cogeneration principle to simultaneously generate both heat and electricity. Cogenerators are mainly used in decentralised energy supply systems for single or multi-family houses as well as for industrial plants.

Power Modulation

A great number of cogenerators are non-modulating, on/off devices. As soon as a request is received from the controller, the cogenerator increases its output to a specific operating point, converting the fuel energy input into heat and electricity at a fixed ratio. In addition to these, cogenerator models also exist that can be operated within a given range by modulating their power output. Output modulation makes it possible to positively affect cycle behaviour and service life which, in turn, can have a positive impact on key factors such as maintenance intervals and economic viability.

In the catalog, the power range of the respective cogenerator may be defined by means of two reference points, the maximum and minimum operating point based on the fuel power input. The fuel power input, the thermal efficiency value as well as the electrical efficiency value are required for each reference point. Should it not be possible for the cogenerator to be operated in modulating mode, matching minimum and maximum values should be entered.

Depending on the selected modulation, the model performs its calculations based on the fixed power data or interpolates between the two reference points as needed.


Basically, a cogenerator can be operated in two distinct modes: the heat-driven operation mode and the power driven operation mode. When operated in the heat-driven operation mode, the cogenerator strictly operates to meet the specified heating energy demand. The electrical energy output is either used to cover self-consumption needs or fed into the public grid. In the power-driven operation mode, the device is operated based on electricity demand. It must, however, be ensured that the heat output may be directly or indirectly decreased at any time by means of appropriate tank modules. Should this not be the case, the surplus heat must be dissipated by means of appropriate emergency cooling devices. This, however, negatively affects the system’s overall efficiency and should, therefore, be avoided as far as possible. Thus, an interesting alternative is provided by combined operation, whereby priority is given to meeting the specified heating demand while simultaneously attempting to supply the most possible amount of electrical energy  so that no excess heat is dissipated unused to the environment.

In Polysun, the cogenerator’s operation mode may be defined by selecting the appropriate control mode. The programmable controller makes it possible to operate the cogenerator both according to heat and electricity demand or based on any combination of these.


The component “cogenerator” may be operated both through the heat generator controller as well as by means of the programmable controller. It should be noted that for the heat generator controller only (and directly) the heat-driven operation mode is available as a default setting. Alternatively, only the status or additionally the cogenerator’s controlled output may be selected for the corresponding modulating operation. It should also be noted that a cogenerator may only be operated in controlled mode if the relevant reference points have been appropriately parameterized in the database; otherwise, despite the controlled operation having been selected, the cogenerator will not operate in modulating mode.

Next to the control mode, an additional output is available for the programmable controller that may alternatively be set to “1” (heat-driven operation mode) or “2” (power-driven operation mode). As in the preceding case, both the status and the controlled power output must be selected. The control mode allows the user to define whether the requested output is a thermal (1) or electrical (2) output. Accordingly, the model performs its calculations with thermal or electrical values from the database. The third available control mode is the “0” mode. If this is active, the cogenerator will operate in non-modulating mode, even if essentially it could also operate in modulating mode. This makes it possible to study the influence of modulation.

Table: Cogenerator settings in the programmable controller

DescriptionControl valueAdditional controller outputsFunction description
Fixed maximum power0The cogenerator runs at the maximum power level 
Heat-driven operation1“Controlled power heat generator“The cogenerator delivers, as far as possible, the desired thermal power set via the “Controlled power heat generator“ controller value. (default mode)
Electrically-driven operation2“Controlled power heat generator“The cogenerator supplies, as far as possible, the desired electrical power set via the “Controlled power heat generator“ controller value.