Off-grid solar power

If you are in a remote location, or you suffer from poor or unreliable power an off-grid system could be your best solution to your energy needs. This could be in the form of a single PV or wind system providing you with clean power or a hybrid system with any combination solar, wind, diesel, battery and grid drawn power.

Hybrid systems can be designed to source and draw power on any priority you set for your system. You may design you system to draw from solar first, followed by stored battery power and finally, in rare case where poor whether yields extended periods of low solar yield, relying on diesel as the final fall-back for your power.

Off-grid systems utilising  AC power opens up new opportunities for providing a stable and powerful energy supply independent of the power distribution grid. AC off-grid systems allow stand-alone system to provide the supply of power at a quality usually associated with power distribution grids.

When designed in a modular structure, off-grid systems can be simple to install and can be expanded freely. Through the integration of renewable energy sources, such as solar and wind, which avoid fuel costs and are steadily becoming more economically viable, such systems are today already more cost effective than conventional systems running on diesel generators.

Off grid systems may be eligible for Federal Government Solar Credits (3 x STC multiplier) on systems up to 20kW.

Contact Greener Energy if you would like to find out more about how off-grid or hybrid systems can keep you powering you business.

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Why Alternating Current?

AC coupling is the basis for modular off-grid power supply of up to 300 kW. In contrast to DC coupling, all energy sources and consumers are connected through an AC power grid, which offers a number of advantages. Standard commercial, and therefore affordable , AC power devices can be used on the consumer side. On the generator side, any number of renewable or conventional energy sources can be incorporated including solar, wind, diesel or batteries. Alternating current grids can be expanded without specialised technical knowledge and using standard components that are available everywhere. Compared to components for direct current grids, these components are considerably cheaper because they normally have to carry higher voltages but also much lower currents. There is also much more planning flexibility involved since even large distances between the generators, batteries and consumers can be implemented. Moreover, the connection of generators and consumers in alternating current grids is possible practically at any random point in the network – thus providing optimum conditions for subsequent expansion.

1. The Grid and Battery Management

The central component of the stand-alone grid is the battery inverter. In its role as grid former and manger, it maintains the stability of the AC grid and ensures that voltage and frequency remain within permissible limits. Surplus energy is stored in the battery. Conversely, when generation is below demand the grid is supplied with battery current. Thus, with a bidirectional battery inverter, often referred to as a combined inverter/battery charger, such devices are particularly cost-effective, since they execute both functions via the same power semiconductor devices. Greener Energy designs grids with quality and performance in mind and recommends the SMA range of Grid Management products.  For example, the Sunny Island is equipped not only with grid management capabilities, but also with a highly developed battery management, including monitoring functions. Thus it is continuously updated on the batteries' exact state of charge and as system manager makes ongoing decisions. At times when the batteries are empty and there is little generation capacity, it can activate a permanently available backup energy source (e.g. a diesel generator) or even switch off certain consumer loads. It also determines the optimum strategy for charging the batteries, and in doing so, increases their lifespan. Apart from all this, the compact device provides further special functions specifically geared to the requirements of stand-alone grids.

Other benefits of the SMA range of grid management products:

Automatic Reactive Power Compensation

With a possible shift of -90 degrees to +90 degrees (power factor cos (f) from 0 to 1 lagging/leading), the Sunny Island can, if necessary, provide its entire nominal power in the form of reactive power. Thus, it is capable of compensating for phase shifts in the stand-alone grid caused by lagging or leading loads (e.g. engines, transformers, cable lines).

Generation Management by Remote Control

When the batteries are full and electricity demand is low, the Sunny Island reduces the electricity produced by all renewable energy generators – whether PV plants, hydropower systems or wind turbine systems. Sole proviso: All inverters are SMA devices and their stand-alone grid mode is activated. If this is the case, the devices will no longer disconnect from the grid with rising frequency, but will continuously reduce their power. As grid manager, the Sunny Island specifies the frequency in the stand-alone grid and can thus limit the power of all generators and maintain the energy balance of the grid – without any further lines of communication.

Extreme Overload Capability

When certain loads are switched on, high start-up currents are frequently encountered which can be well in excess of the normal operating current. In addition, some consumers may require a lot of energy just for a short while, thus creating short peaks on the load profile. For the sizing of off-grid systems, it is extremely important to use battery inverters with a high overload capacity; this will ensure that such load peaks can be handled without a problem. The Sunny Island 5048 can handle 6.5 kW for 30 minutes, 8.4 kW for 1 minute and as much as 12kW for 3 seconds – i.e. approximately two and a half times its nominal power.

1. Three Phases – No Problem

For the transmission of large capacities in alternating current grids, a three-phase configuration has proven effective – this is true both for large power distribution grids and for stand-alone grids. Due to the overlap of the phase-shifted individual capacities, three-phase consumers have practical power at their disposal. The generators in diesel power units or wind turbine systems also supply three-phase power to most cases. It is actually possible to set up single-phase systems up to a battery inverter power of 20 kW (parallel connection of four Sunny Island 5048 to one phase). For larger capacities, however, three-phase stand-alone grids are usually preferred. This is accomplished by always having three Sunny Island inverters connected to a three-phase cluster. A master device sets the frequency for both slave devices, which in turn operate exactly at 120 or 240 degrees phase-shifted from each other. Synchronization is achieved via a special communication connection between the devices. Another advantage of this connection is that the entire cluster can be configured using only the master inverter. If the system is properly wired and configured, a three-phase stand-alone grid can function just as well as the single-phase version. An asymmetric grid load caused by a single-phase consumer can also be compensated for automatically by having the inverter draw more power from the battery based on the phase with the greater load. Even completely unbalanced operation is possible, for example when solar power is fed into the stand-alone grid to phase 1 and there is consumption on phase 3. However, in this case, because the energy must be put through two additional Sunny Island inverters, to maximize efficiency, it is still better to keep the distribution of loads and generators as balanced as possible.