Stand-alone systems are composed of one or more electric generators, typically PV- modules that use the energy from the sun or wind generators in hybrid systems. Through a charge controller, the current is charged into the batteries, where it is stored until it’s needed. To consume the energy it is removed from the battery and converted from direct current (DC) to alternating current (AC) by an inverter. In this way it is possible to use conventional home appliances, as if the power came from the public grid. (Click on the image to enlarge)
There are also systems which do not have an inverter. Only devices built for use with direct current can be operated in this system, usually 12 or 24 V. This is the simplest system, suitable for small installations.
It is surely the photovoltaic energy in the form of PV modules, in other words, the energy from the sun. Portugal is one of the best countries in Europe for solar energy thanks to its high insulation rate and radiation intensity. In terms of wind energy, Portugal is a poor country excepting some coastal regions. In addition, there are strong fluctuations in wind energy, which is therefore unsuitable for the charging batteries without any additional source. In conclusion, this means that the focus should be set primarily on photovoltaic energy, which can be supported in so-called hybrid systems with an additional wind generator.
Unfortunately not. On the one hand, there are always losses in the different system components, which occur most strongly in the case of the batteries. The proportion of the energy produced which can be consumed varies greatly from case to case. On the other hand, the stand-alone systems are always designed for the winter months, since the system’s energy yield will double during the summer months due to the high radiation intensity. Thus, in the summer months too much energy is produced, which cannot be consumed.
A stand-alone system should be installed only in places where the connection to public grid is impossible. Compared to grid-connected systems, stand-alone systems must have two times more installed power to produce the energy for the total annual consumption. In short, the cost of a stand-alone system is at least two times higher than the cost of a system connected to the public grid.
Yes. At a time when the energy yield is not sufficient, additional generators such as gasoline or diesel generators can be used to charge the battery. For this purpose, it is necessary to have an inverter that works simultaneously as a battery charger, or a conventional battery charger.
The planning of a stand-alone system is much more complex than the calculation of a grid-connected system, because precise and detailed information about the energy consumption of the customer is needed. In addition, for the estimation of solar radiation, it is absolutely necessary to know the location of installation. In addition, the desired system autonomy is requested.
Given the climatic conditions in Portugal, we recommend a battery autonomy of 3 to 5 days, at a battery"s depth of discharge (DoD) of 50%.
In some cases. One example is the case that you have an inverter of the Studer Xtender series, where the system "Optimal solar backup" can be installed. This system favors the renewable energy sources in your system but is, however, available to receive a portion of the required energy from the public power grid in the case that you’re not producing enough energy to cover all your needs. This system does not inject any power to the public grid.