Optimisation results for a day in May are shown in Fig. 3. The left diagrams illustrate the thermal balance and on the right the electric balance in the grid is plotted. Because of high irradiation for the selected day, around noon the PV plants provide more than the electrical power needed in the grid. In the KWK-G-scenario the CHP operates thermally driven always for short time periods. So only a small part of the storage is used.

In the scenarios with variable tariffs (VDE and LOCAL) the operation is shifted to high price times and the storage is maximally charged to use the heat in low price times. In the VDE-scenario the highest return can be achieved during noon. Consequently most CHP operation is shifted in this time. But at the same time the PV plant feeds in maximal and already exceeds the local grid load. The additional CHP power can not be used locally and must be transported to the medium voltage grid.

In the LOCAL-scenario the highest return can be achieved, if the grid load is high and additional the PV input is small. These times are in the early morning and evening. Because at these times no PV power is available almost all CHP power can be used in the grid and as a result a bigger part of the electric load profile is covered by local generation.

Fig. 3. Optimisation result for a day in May: Electric and thermal balance of the local grid.

In all three scenarios the CHP produced 225 kWh of electricity at the chosen day. With a gas price cfuel = 0.05 €/kWh and an electric efficiency of nel = 30 % this results in fuel costs of 37.50 €. The return for the sold electricity was lowest in the KWK-G scenario with 19 €. In the scenarios the operation was shifted to high price times and results in significant higher returns: VDE: 31.85 € and LOCAL: 32.56 €.