Prof. Bruno Burger, PhD: "We are working on improving efficiency levels for partial load operation". Professor Bruno Burger, PhD, of the Fraunhofer Institute for Solar Energy Systems responds to seven questions on the current stage of development of battery inverters.
Battery and hybrid inverters are used to incorporate electrical storage systems into in-home distribution networks, stand-alone systems or transmission grids. How would you describe the current state of this technology?
Here we are talking about a new market model. It used to be that people would simply feed solar power into the grid and earn money from the feed-in tariff. Today, the feed-in tariff is only ten to twelve euro cents per kilowatt hour, while the electricity purchase price is 30 euro cents. It is becoming more economical to install batteries at home. This way, cheap, self-generated electricity can be stored and consumed later when solar power is not available – in the evening or at night. Inverter technology is evolving with this trend. There used to only be feed-in inverters. Now, we have inverters which can be connected to the photovoltaic installation and the batteries.
What distinguishes these devices technologically?
In the beginning, it was generally the case that there were dedicated PV inverters and battery inverters – two separate devices. Now, we are witnessing a trend towards merging the two technologies in one device, to which the photovoltaic installation and the battery are then connected.
What developments have there been recently?
We are working on improving partial load efficiency levels. We are currently developing devices which are specially optimized for partial load operation. PV inverters also operate in partial load mode, in the morning and in the evening, but not as frequently as battery inverters. These also have to work overnight in partial load mode. We are trying to take that into consideration in our development work.
What aspects of the devices have to then be adapted?
In partial load operation, it is the inverter’s constant losses that determine the level of efficiency – not the power-dependent losses. Some of the constant losses come from the control and regulation systems. Their DSP controllers consume electricity. Additionally, there is a display in the inverter as well as outward interfaces. We also have a power supply unit which supplies the DSP controller. The power electronics themselves are also responsible for some of the constant losses. In order to be able to reduce these losses, we require low-consumption DSP controllers. We need to optimize the onboard power supply units so that they run at a higher degree of efficiency. And we need to use better transistors for the power electronics, for example transistors made of silicon carbide.
What distinguishes devices for off-grid applications from those for on-grid applications?
Historically, normal PV inverters were used for grid-connected systems and battery inverters for off-grid systems. As I said before, a first step involved employing both devices for home use. Now, the technology is converging in devices with two connections – one for the photovoltaic generator and one for the battery. This development is beneficial for off-grid systems because double the demand means larger production runs are needed, leading in turn to more affordable devices. On-grid means, of course, that a grid is always available. But even in these cases, the inverter must be equipped with an uninterruptible power supply function for the event of grid failure. Inverters equipped with such a function do not differ so much from off-grid inverters.
What are the advantages and disadvantages of AC and DC coupling respectively?
With AC coupling, you’re practically back to having two separate devices. Its advantage is that the system is modular and can thus be expanded. However, partial load operation is more difficult – with separate devices, each one has its own controller board, DSP controller and display. They might even have their own housings. If you want to make set-up more affordable and efficient, both devices should be connected, which then automatically gives you DC coupling in the device itself.
How has the price of power electronics changed in the past years and what has led to the lower prices we’re seeing?
Solar inverters have gone through various phases of development similarly to solar modules. The fall in prices is due to new generations of transistors which enable higher switching frequencies and lower rates of loss. Additionally, better circuitry has been developed which enables greater efficiency. In newer devices, multi-level switching is also frequently used in combination with more semiconductors. This reduces the need for passive components, choke coils and capacitors, which are more expensive.