IDistributedPV analysis and comparison showed that the technical recommendations are not country specific. The main findings and recommendations are based on the conversion chain of PV battery systems, in the section battery and inverter, the grid integration and the control issues.
Battery systems show a big influence on system losses, which is the most crucial factor for overall performance. Data sheet values state a one-way efficiency of about 95%. But the conversion is applied twice – for charging and discharging. Thus, a round trip efficiency of 0.95 * 0.95 = 90% can be achieved for charging and discharging in the best case. Batteries, as the project’s simulation results showed, are still not beneficial from the economic point of view at the moment. However, they are necessary for the grid, for example in the case of the controllable load solution, where batteries can buffer the effect of demand peaks in the grid. Therefore, the battery should be as efficient as possible. The average battery energy efficiency in private PV battery systems is 75-95 %.
iDistributedPV simulations showed for all regarded countries (Spain, Germany, Poland, Lithuania, Greece) that a battery system installed in addition to a PV system reduces the impact on the grid even if it is controlled in the default control and the voltage band is stabilised by the battery systems. Additional control targets could provide further benefits for the grid and lead to the reduction of the feed-in peak. If the peak is reduced by the decentralized control, voltages are closer to nominal voltage and more hosting capacity for renewables in the grid can be achieved. For further increase of grid friendliness and grid support additional information of the grid operator are necessary. A common communication standard for control is necessary, since there many different grid operators with different needs which should collaborate.
Lastly, the onsite control needs to combine the external input with the requests of the users. This can be very different depending on the system. The homeowner and the controllable load solutions indicate that batteries should be technically improved to be able to act grid-friendly. This includes the possibility to act according to the electricity demand and PV generation prognosis. The virtual power plant solution showed that batteries should be able to act also as primary control reserve for the grid, as this is cost-effective and beneficial for the grid. In multifamily houses, an automatic measurement system measuring the load depending on the time of day, regardless if participation or not, should be installed at low cost in each flat.