Inasmuch as insolation varies over time, in most cases a specific load cannot be directly allocated to PV electricity, which must instead be processed or stored in a suitable form. Direct use of PV electricity via stand-alone installations without storage systems is conceivable: for example, for facilities such as irrigation systems (see Figure 1.6), fan operation (e. g. to cool down cars during the summer) or in settings where it is of lesser importance for electricity to be available at all times. Figure 5.1 displays a breakdown of various types of PV systems according to their basic structure. Figure 5.2 displays the typical power ranges of today’s PV systems.
A stand-alone system is an electricity installation that supplies one or more appliances with energy completely independently of a power grid. The possible output of such a system ranges from milliwatts to watts for power supplies for portable devices (e. g. watches, calculators, small radio devices and so on) to tens of kilowatts (for transport infrastructure facilities, large buildings, radio transmitters and so on). Appliances that depend on a steady supply of electricity need an accumulator, usually a battery or battery bank – although such components appreciably drive up electricity costs, in most cases by around €0.66 per kWh.
In the interest of avoiding electricity storage costs, relatively large PV systems with outputs of 1 kW or more are hooked up to the electricity grid. In areas with readily available grids such as Central Europe and the USA, an accumulator can be dispensed with in most cases. During periods when grid-connected PV systems produce more energy than needed, the surplus energy is fed into the grid and vice versa, i. e. energy is drawn from the grid at times when the PV system produces too little energy, namely in inclement weather during daylight hours, and after dark. Hence the electricity grid is in a sense the storage ‘device’ for a grid-connected PV system.
In the sections that follow, in view of the overriding practical importance of stand-alone installations with storage systems, as well as grid-connected installations without storage systems, these two types of PV installations will be discussed.