“Exciting developments in the battery sector”

Expert Interview – May 22, 2023
Dr. Johannes Weniger

The combination of photovoltaics (PV) and battery storage systems is being hailed as an attractive solution on European markets at the moment. High electricity prices and overloaded grids are behind this development. We were curious about how the market and technology for PV storage solutions are evolving – also a hot topic at the 38th PV-Symposium, which brought together PV industry experts from Germany, Austria and Switzerland in Bad Staffelstein, Germany, from February 27 to March 2.

While we were there, we spoke to Dr. Johannes Weniger, a scientist working in the solar storage system research group at the University of Applied Sciences (HTW) in Berlin, about the latest developments in solar storage.

Dr. Weniger, what would you say are the most important technological developments relating to photovoltaic battery storage systems right now?

We’re seeing exciting developments in the battery sector. We’re coming from a market that has been heavily dominated by NCM batteries, which are based on lithium nickel manganese cobalt oxides. But now lithium-iron-phosphate batteries, or LFP batteries, are gaining ground on the market. In 2018, half of all residential storage systems in Germany still featured NCM batteries. Last year, however, LFP batteries were already being used in around two thirds of all residential storage systems.

Things are changing in the balance of systems too. Hybrid inverters are becoming more prevalent, connecting solar installations and battery storage systems. Three quarters of all newly installed PV storage systems in Germany are now DC-coupled solutions with hybrid inverters.

Just four years ago, the split between AC-coupled and DC-coupled systems on the market was still 50/50. Now that more manufacturers are making hybrid inverters, though, this system configuration is becoming more common. Resource efficiency is another key factor in this shift. When it comes to these concepts, I’m in a much better position if I have just one housing and one inverter platform.

Not to mention that hybrid inverters increasingly have an output range of between 15 and 30 kW. This is a considerable improvement from the previous range of 5 to 10 kW. With hybrid inverters getting bigger, we’re now in a position to introduce DC-coupled system solutions to small business properties and multi-family dwellings. We’re seeing more and more systems exceeding the 10 kW mark these days. Up until the start of 2021, we had a market barrier here in Germany because the EEG levy applied to self-consumption of solar power. Now that this levy has been removed, we have seen real progress in the small-scale system segment, with a quarter of all systems in Germany having an output of between 10 and 20 kW last year.

How is the gradual increase in system size affecting the technology used?

When I charge and discharge a hybrid inverter at 5 percent capacity, the efficiency is much lower than if I were to operate it at maximum capacity. The challenge is that electricity consumption usually drops to 300 watts or even less overnight. This means that I would then be operating a 20-kW hybrid inverter at a lower efficiency level than if I were using a 10-kW hybrid inverter.

The trend of using silicon carbide power semiconductors makes a lot of sense to me since switching frequencies can be higher and storage chokes can be smaller. This ultimately results in higher efficiency levels compared to conventional inverter topologies and the silicon-based power semiconductors used previously.

What progress is being made in terms of conversion efficiency and standby consumption for photovoltaic battery systems?

Let’s start by talking about the relevance of efficiency levels, which are often just considered as a technical parameter. What does it actually mean when inverter efficiency only hits 85 percent when discharging? If I have a 10 kWh battery storage system, I can only use 8.5 kWh on average. In other words, the inverter efficiency has a direct impact on what happens within my building’s network.

The best inverters have efficiency levels of 97 or 98 percent. If I discharge the storage system when I’m at that level of efficiency on average, I can take up to 9.8 kWh from my battery storage system and use it to replace grid-supplied power. As you can see, the inverter efficiency largely determines how beneficial a storage system is in a building, with the ultimate aim of avoiding the use of electricity from the grid.

In terms of product development, this trend is being reinforced by the fact that there are more high-voltage batteries on offer in the residential storage segment with a voltage of between 100 and 500 volts, allowing inverters to operate more efficiently. With a smaller deviation between the battery voltage and the inverter internal voltage, the inverter can be operated more efficiently.

In the case of high-voltage battery systems, the inverter efficiency is boosted further by the fact that manufacturers have improved efficiency levels even at the lower end of the partial output range. The conversion efficiency is generally better as a result.

As for consumption in standby mode, a storage system isn’t operating for a full 8760 hours in a year. Instead, it will be completely discharged for 2000 to 3000 hours a year, depending on the size of the system. The question we need to ask here is: How much energy is the storage system using during this time to stay in standby mode? There’s a device that only uses two watts when it’s in standby mode. That’s rather impressive when you consider that a standard WIFI router sometimes uses more than that. But there are also devices out there that use 50 or even 100 watts when they’re in standby mode. In those cases, the benefit can be reduced by 10 to 20 percent when considering the actual purpose of the storage system to reduce the amount of electricity being taken from the grid.

What developments are in the pipeline for photovoltaic battery systems?

One really exciting area is long-term extendability. There are some inverters with more than one battery connection. In fact, it is possible to connect up to three batteries with DC decoupling, providing much more flexibility for retrofitting.

This also applies to the PV side. A standard solar inverter used to have two separate inputs but they are now being developed with three or even four inputs. There’s no doubt that this development offering more flexibility will start to become a trend reflected in more and more products.

The deployment of renewable energy in the grid is making storage systems increasingly important. And yet a shortage of staff is having an impact on projects being rolled out. How can we meet the huge demand given these circumstances?

The most important step we can take is removing as much of the bureaucracy as possible to take the pressure off skilled workers who are already working in the industry and boost the efficiency of each individual. Requirements for grid connection and registration could be condensed and automated. Processes should have the option of being made automatic, with the possibility of interface connection being opened up so that companies can digitally link their own systems.

As far as the grid is concerned, the regulatory requirements also need to be loosened. Why do inverter installers have to submit all the certificates in this day and age? Why can we not just have a central database where manufacturers can simply upload the relevant certification?

We do also need a lot more skilled workers in the industry too. The solution here is to poach people working in similar areas. I also think it’s crucial that we professionalize the training and make the industry more attractive on the whole.

What’s the situation with procuring materials?

We have serious problems with the availability of power semiconductors. It’s putting European manufacturers in a difficult position. But we’re seeing investments being made in new chip factories in Europe as a result. If the prices for individual raw materials skyrocket, there’s obviously a greater incentive to develop new products and technologies.

Looking at what battery manufacturers are doing, there’s a trend toward reducing the amount of critical raw materials. For example, manufacturers relying on cobalt batteries are reducing the amount of cobalt. We’re certainly in a creative industry.

Dr. Johannes Weniger spoke to Sarah Hommel de Mendonça.

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