Inverter and PV System Technology
Industry Guide 2013

Stand-alone systems are the original preserve of photovoltaics. The simplest installations consist of a PV module and a device that consumes DC, such as a water pump. Photovoltaic systems are straightforward to install and benefit from low operating costs. In remote regions, located at great distances from the power grid, they are therefore often unrivalled on price – particularly when the only alternatives are diesel generators that consume expensive fuel. Continually falling module prices together with rising prices for fossil fuels are also clearing a path for such systems in other markets.

If small, stand-alone installations require a 24-hour power supply, the PV plant is combined with a battery system, as is the case in weather stations, navigational aids and transmitter masts. Here, electronic charge controllers are employed to ensure that the power supplied by (mostly individual) PV modules is stored in the batteries as efficiently as possible. DC power consuming equipment (such as lamps and refrigerators) is connected to the charge controller and is thus supplied either by the solar power generated at a given moment or by power stored in the batteries. This principle is that of the “solar home system”.

Backup systems (e.g. diesel or vegetable oil generators) improve supply security. This has led to the creation of hybrid systems that can be used, for example, in hunting cabins and refuges, and even on large yachts.

With the development of PV technology, stand-alone systems have grown into autonomous grids and have become more diverse. If intended to supply power to schools, hospitals, entire villages or even small islands, the PV systems are usually supplemented by small wind turbine generator systems as well as batteries and diesel or vegetable oil generators. Biogas plants can also be integrated into these autonomous grids.

As such grids increase in size, cheap devices that consume AC (refrigerators, TVs and other household appliances) are used in addition to those that consume DC, meaning that inverters become necessary alongside charge controllers.

It is not only possible to structure hybrid systems as either pure DC systems or mixed AC/DC systems, pure AC configurations are also available that are flexible and can be expanded. Special inverters are needed for such systems. A stand-alone inverter primarily has two tasks: It charges the batteries to store any solar power not used immediately and creates a stable AC network.

Additional PV systems and the fuel driven generators feed into the AC network, coupled with a wind turbine generator system (preferably also with a special inverter) or biogas plant when large amounts of power are needed. The better the levels of insolation and wind complement one another over the course of a day, week or year, the less frequently the back-up generator is used.

Power storage systems are needed to stabilize the grid. In addition to ensuring that electricity is available around the clock as far as possible, they also decrease short-term power fluctuations which arise as a result of clouds passing overhead, for example, and cause PV output to fall by up to 80%.

If a hybrid system supplies an entire village, a micro grid is created. Several of these stand-alone systems can then be combined to form a mini grid. Stand-alone systems gradually grow together into ever larger grid units, thus representing a major contribution to rural electrification in developing countries.

Bridging bottlenecks

Between PV plants that supply stand-alone systems and those that feed into the public grid come installations that generate power in parallel to the grid (grid-compatible parallel operation).

Grid-parallel operation is necessary anywhere where a public grid exists but the power supply is unreliable. Moreover, such systems are also practical in situations where a large power consumer (such as a factory) is connected to a weak grid spur and the power demand regularly exceeds the capacity of the grid connection. In both cases, photovoltaics assists in stabilizing the power grid and bridging bottlenecks in supply.

To date, this task has been performed by diesel generators. But in view of rising oil prices and PV generation costs that continue to fall, it makes far more practical sense to install PV systems. This is especially true in regions with high levels of insolation. In many cases, photovoltaics is already capable of generating power for profit in these regions, as illustrated by this simple case study: A factory in India that operates around the clock, but is plagued by frequent power failures, relies on a diesel generator to supply power during the power cuts. This generator can produce power at all times of the day for 20 euro cents/kWh. Thanks to the high levels of insolation there, a PV installation is able to generate electricity throughout the day for 10 euro cents/kWh. Both systems operate in parallel to the grid. During the day, the PV installation has priority, while at night the diesel generator is responsible for securing the power supply. Surplus solar power produced during the day can also be stored using a battery system, increasing the availability of that power even further. This increases the availability of the battery and makes it possible to use solar power at nighttime.

A progression of this system is based on a situation where the PV installation produces distinctly more power than the factory requires and consists of two PV generators. The larger of these supplies the factory with electricity throughout the day and feeds any surplus power into the battery system. The smaller PV generator is tasked solely with ensuring that the battery is fully charged, so that enough power is available during the night. Wind energy installations can also be connected to this system and supply power to the factory. The diesel generator is currently still needed as a backup power source, but provided the latest radical developments in battery technology continue, it will foreseeably become redundant in the medium term.