How to build your electrical grid using renewable energy

Electrical current, anytime, anywhere – something we all take for granted. More than two billion people, however, live in another reality: in so called “remote areas”, far away from power plants and electrical grids. And for this reason, they must do without many of the conveniences of our civilization.

Photovoltaics offers solutions here like no other technology: PV systems are robust, durable, easy to install and maintain. For years SMA has been motivated to develop a self-contained energy supply system based on solar power. As the only manufacturer in the world, SMA has followed not only the concept of DC coupling, but the AC coupling as well, which distinguishes itself with a host of advantages.

With AC coupling, all consumers and generators are connected using an AC voltage bus, whereas with DC coupling, the interconnection takes place in a DC voltage grid. For residential as well as industrial power generation, AC coupling has been used throughout the world, while DC coupling systems have been employed mostly in small applications of up to 100 watts as well as for telecommunications purposes.

Different Types of coupling

For the assembly of solar energy supply systems, which save the generated energy in batteries, there are three different concepts: pure DC coupling, mixed systems, and pure AC coupling. In the beginning, the first developments offered a pure DC coupling variation: both the solar modules and the batteries were DC voltage sources, so that a conversion of the power with the then still expensive inverter technology could be avoided. With some 100 watts of power, the performance of these systems was quite low and many of the appliances consuming the energy, such as lamps, radios or televisions, were also available in a DC voltage version.

Variants of the DC coupling

Powerful systems pose a difficult problem for pure DC coupling: since the voltage is generally limited to 48 volts for reasons of personal safety, there is a huge current strength. Sockets, cables and fuses with around 80 amps had to be resilient in order to carry the around 3,700 watts of power put out by a common household socket. The costs for an installation of this type are significantly greater, in addition there is very few 48 V DC compatible consumers in the world.

For this reason, mixed systems are used, at least with regard to larger solar energy supply systems: inverters convert the lower DC voltage of the batteries into grid conforming AC voltage of 230 volts. Only the consumers were connected to this AC voltage, while the PV modules and batteries were still connected with DC cables. If their distance is limited, you can make up for it with a charge controller, which prevents battery overload.

The SMA Solution for DC coupling

Since the voltage of the PV modules varies with the battery voltage, they almost never reach their optimal operating point (MPP) in such systems. In this way, as much as 30 percent of the valuable solar energy is lost. In order to avoid this, one needs an MPP charge controller like the Sunny Island Charger. It increases or decreases the voltage coming from the battery to the current MPP voltage of the solar module. The somewhat higher costs can be recovered very quickly: The solar yield increases by 20 to 30 percent. The solar system can be designed to be relatively smaller, which more than makes up for the additional costs of the charge controller.

Critical Power Supplies - How to build your electrical grid using renewable energy image 1
Figure 1. DC coupling: better with the Sunny Island Charger

Flexibility with AC coupling

Should the solar system and the batteries lie some distance apart, the optimal solution can be found in a complete AC coupling. Instead of building a costly DC grid on the generator side, a solar inverter converts the solar power into AC current and feeds it into the already existing AC island grid. This creates a number of advantages:

- Lower installation costs due to the lower current strength and the AC voltage technology.
- Simpler and more flexible planning, since large distances between solar generator and battery are unproblematic. Feeding energy into any point of the AC grid is now a de facto possibility.
- Better usage of solar power especially during high-use periods of the day.
- Utilization of technological advantages from the grid connection, such as optimized MPP tracking processes, high efficiencies, OptiCool, etc.
- Lower overall costs for solar systems with more than four kilowatts of power.

Critical Power Supplies - How to build your electrical grid using renewable energy image 2
Figure 2. AC coupling: the basis for a highly flexible stand-alone electricity supply

Solutions for all stand-alone systems

SMA is the only manufacturer in the world to use all advantages of AC coupling for self-sufficient energy supply systems. Both the multiple award winning Sunny Boy and the Sunny Mini Central solar inverters are put into use. Smaller systems can also be built as DC-coupled solar systems with the Sunny Island Charger. No compromising in terms of design, cost or installation: SMA offers optimum solutions for all off-grid systems ranging from two to 100 kW.

This information was produced by SMA Germany and all trademarks / copyright are respected.