PV surplus self-consumption maximisation logic with hot water buffer

Step by step.

Large or high-yield PV surfaces enable a high surplus of PV energy. In periods of PV energy production, the current energy demand is thus completely covered by the sun. Through energy management, variable consumers such as car charging stations, washing machines, dryers and the like can be activated during periods of energy production.

As soon as all possibilities of optimised energy self-consumption have been exhausted, there is a surplus of generated energy. In addition to the possibility of feeding it into the supply grid, this surplus can also be discharged into a heat storage tank (for hot water supply or as a heat storage tank for the heat pump). In this case, however, only the energy surplus should be used, without drawing additional energy from the supply grid.

In our example a heating element is operated with the surplus energywhich is controlled precisely to the wattage and dynamically via a dimming object. The dynamic, stepless control of the heating rod takes place in the LOGIKEDITOR of the EIBPORTs.

Structure of the control system

To set up this control, a 0-10V dimmer is used, which drives a thyristor controller. The thyristor controller passes on the corresponding power to the heating rod with wattage accuracy (where 0V = 0W, 5V = 50% of maximum power and 10V = maximum power of 3000W). In the example, a single-phase heating rod is used, but with an appropriate thyristor controller, multi-phase heating rods can also be used. The measured values of the PV system are transmitted via the Modbus (Smart Home App in the APPMODULE) is transmitted to the KNX and the consumption of the heating rod is realised via an actuator with current measurement.

The aforementioned tasks are realised via a logic group in the LOGIKEDITOR and is briefly explained below.

About the function of the logic group

A percentage value for the dimmer control of the heating rod is calculated with the input value "Current reference power" (feed). As soon as the heating rod is switched on, the measured power at the input "Current reference power" is reduced by the energy consumption of the heating rod. This consumption is recorded at the actuator that switches the heating rod, calculated as consumption and transferred to the "Heating rod boiler consumption" input (current consumption power in kW). This value is added to the input value "Current consumption power" to know the real value of the supply. The "Heating rod setpoint in %" is calculated from this value. In the comparator element, it is checked whether the percentage value is less than 0. If so, a 0 is sent to the output to prevent the heating rod from being fed with mains supply.
The fixed value "Value in W (buffer surplus)" serves as a buffer when larger consumers are switched on (e.g. oven) in order to plan for fluctuations in consumption. The cyclic transmitter is used to synchronise the processing of input values in the logic elements.


Input values

  • Current reference Power

The value is read out from the inverter via the APPMODULE.
(EIS9, DPT 14.* 4 byte float)


  • Boiler heating rod consumption

The value is measured via the KNX at the actuator (current measurement) of the heating rod.
(EIS9, DPT 14.* 4 Byte Floate)


  • Value in W (buffer surplus)

Fixed values to be defined, depending on the unit types and their load behaviour.



Output values

  • Heater set point in %

Value for controlling the heating rod; a thyristor is controlled via a 0-10V interface.
(EIS 6, DPT 5.*, 1 byte)