Stand-alone PV Systems

Since in most settings a continuous supply of electricity is needed, PV electricity needs to be stored foruse during periods when there is little or no Sun, i. e. during inclement weather or at night. Nowadays, mainly storage batteries (mostly lead batteries, and nickel-cadmium batteries to a lesser extent) are used, although other storage methods are available. For example, short-term storage can be realized using large – capacitance supercapacitors or very high-RPM flywheel gears constructed of extremely robust materials. Long-term storage of PV energy can be realized using storable hydrogen produced via electrolysis that can be converted back into electricity using fuel cells.

Figure 5.3 displays the layout of a stand-alone PV system that integrates a battery bank for energy storage purposes. The 1-У characteristic curves of solar generators (1) are very suitable for battery

Photovoltaics: System Design and Practice. Heinrich Haberlin.

© 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

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charging owing to their power source characteristics. The charge controller (2) keeps the battery from being overcharged, since otherwise the aqueous electrolyte (diluted sulphuric acid) would break down into hydrogen and oxygen, resulting in battery gassing. This process (which forms a detonating gas): (a) to some extent creates a safety hazard, which can be kept under control with good ventilation; and (b) reduces the battery’s life span owing to water loss, which means that the distilled water in the battery needs to replenished manually from time to time. With relatively large installations, in lieu of a charge controller,

Type of plant

Peak PV power of plant in Wp

10-3 1 0-2 10-1 100 101 102 1 03 1 04 1 05 1 06 1 07 1 08

Stand-alone plant Plants with storage

Single devices Mobile equipment Telecommunication equipment Leisure residences, mountain huts Single houses Infrastructure installations

Plants without storage

Irrigation plants Ventilation plants

Grid-connected plants Plants with storage Plants without storage

Small plants (SFH) (230V, 1ph) Large plants (400V, 3ph) Power plants (5kV… 50kV, 3ph) Plants at DC grids (600 V)

Figure 5.2 Typical PV system power ranges

a device called a solar tracker can be used, which ensures that the solar generator is operated at its maximum power point (MPP) at all times and prevents overcharging.

The actual energy storage process occurs in the battery bank (3), whose nominal voltage in a stand­alone system is the same as the installation’s system voltage. The other possible system voltages are 12 V for small installations, and 24 V, 48 V or even higher for larger installations, with the highest voltages being used in installations in the kilowatt range. System voltage determines the operating voltage of directly connected DC appliances (7a). The 12 V systems, which are used for auto accessories, camping equipment and portable devices, provide the highest operating voltages, followed by 24 V systems, which provide considerably less, and by upwards of 24 V systems, which provide virtually none. Apart from system voltage, battery capacity (storable electrical charge as expressed in ampere-hours (Ah)) is also an important factor as it is the key determinant of battery life during inclement weather.

Inasmuch as battery life span is affected by both overcharging and deep discharging, the discharge controller or deep-discharge protection device (4) shuts down the connected appliances if a (possibly current-dependent) minimum battery voltage is undercut. In large installations, the connected appliances can be broken down into groups that are assigned various priorities. Less important groups are shut down first so that electricity is available for longer for critical appliances.

It is often necessary to hook up appliances using different operating voltages (7b), particularly in cases where system voltages exceeding 12 V come into play. DC converters (5) carry out the requisite voltage conversion, which usually takes the form of a voltage reduction. As many appliances run on 230 V AC (7c), a stand-alone PV system must integrate an inverter (6). These devices simplify the appliances-side installation process, but they also add to the system cost and lower overall installation efficiency and exhibit a certain amount of open-circuit loss. Hence inverters should only be activated when an AC appliance is in use. For more on the use of inverters in stand-alone PV systems, see Section 5.1.3.

AC devices that use DC internally (mainly consumer electronics products) can often be converted into DC devices via a minor modification, which often greatly reduces inverter energy consumption by eliminating transformer open-circuit loss. In such cases PV DC is more useful than AC.

In small PV installations without a solar tracker, a charge controller (2) and discharge controller (4) are often combined into a single device known as a charge controller, system controller or battery controller. Such devices are available in various sizes and for various system voltages, feature connectors for the solar generator, battery and appliances, and in some cases integrate a DC distribution board. Figure 5.4 displays a holiday-home stand-alone PV system that integrates a charge controller. Inverters with integrated charge controllers and deep-discharge protection devices are now available that allow for the simple realization of stand-alone systems for 230 V AC after connecting a battery and a solar generator.

Updated: August 8, 2015 — 4:04 pm