To date, the topic of storage has played a rather minor role in the energy transition in Germany. From when on, will this have to and will it change?
It made sense for us to focus on expanding renewable energies first. Unlike with renewables, however, we should not build as much storage as possible, but rather what is currently required. Every storage facility incurs additional costs, and its use causes efficiency losses. In various studies, we have been able to show quite well that, in fact, up to a share of about 50 percent renewable energies, there is no real need for storage at all from the point of view of the power grid. And long-term storage is only needed beyond a share of 80 percent renewables.
Now, however, we are exactly at the point where storage facilities are necessary in order to get away from fossil power plants. Now, there are increasingly periods when conventional power plants are not in fact needed. However, in order to actually be able to shut them down during these phases, we need storage facilities for the instantaneous, primary and secondary reserve. This is necessary to balance out fluctuations and ensure grid stability.
Another important point for me is that we always talk about storage. Viewed from the perspective of the power grid, however, we don't necessarily need storage facilities, but flexibility. In other words, if there is a lot of renewable energy, we need something to utilize the surplus power for a useful purpose. Furthermore, if there is too little electricity, we have to release power. This can also mean switching off loads. From a technical point of view, a storage facility is the ideal element for this, however, the task of balancing supply and demand can also be achieved in other ways.
In the political discussion, people also like to refer to future innovations that are necessary. Do the storage technologies for the energy transition still have to be invented?
No, the technologies are already available to us today. We have solutions for short-term storage with lithium-ion batteries and also with classic lead batteries. Long-term storage in the electrical sector is gas, i.e. hydrogen or methane produced from green electricity. Between these technologies we don't need that much more. When it comes to innovations, the question can only be: can we make the technologies more affordable in terms of lifecycle costs? Storage systems are still expensive, of course. Battery storage systems have significant investment costs that have to be amortized over their life cycle. Research must focus on finding alternatives for limited raw materials on the one hand, and on making the technologies more cost-effective in terms of lifecycle costs on the other. Investment costs, efficiency and service life, but also recycling, play an important role here.
Then let´s get specific: which storage technologies will be used for future supply security as well as grid stability?
Anything that needs to be balanced on a 24-hour basis is the domain of battery technology. Charging and discharging takes place every day, and that is where efficiency plays a major role. Therefore, you would not choose gas for a daily cycle, as the conversion from electricity to gas and back to electricity has an efficiency of only about 40 percent in the case of hydrogen as a gas. That means for every kilowatt-hour that will be returned, there will be a loss of one and a half kilowatt-hours. In comparison, a lithium-ion battery including converter achieves an efficiency of 85 to 90 percent, which is quite a different story.
On the other hand, we need a buffer for the so-called dark and wind scarce periods of up to three weeks. The energy system must, therefore, be able to supply itself from storage facilities for up to three weeks when fossil fuels are no longer available. From what we can see today, this task can only be performed by gas. In this case, the relatively low efficiency no longer plays such a decisive role. Instead, it is almost exclusively the investment costs that matter.
This can be calculated quite easily: if we use a very inexpensive stationary battery storage system, the installation costs amount to around 100 euros per kilowatt hour. If we assume a 20-year service life and need the storage unit on average once a year, then each kilowatt hour sold would have to earn an average of five euros. This does not yet include loss compensation, maintenance, repair or capital costs. The long-term average price of electricity on the stock exchange is five cents, just one percent of that. If we compare that with a gas storage facility in a salt cavern in Germany, we are worlds apart. Only around 50 cents need to be invested for one kilowatt hour of storage capacity in the cavern. Calculated over the same 20 years, each with one cycle, this results in 2.5 cents per kilowatt hour. In addition, there is the depreciation for the electrolyzer and gas turbine, but these are rather subordinate. That is perfectly okay for a long-term storage facility, which is a form of insurance cover. In this sample calculation you could also add five to ten cents per kilowatt hour for the losses that result from the poor efficiency of a hydrogen storage system, without that being a problem. Of course maintenance, repair or capital costs must also be taken into account here.
For this reason it so important to make a clear distinction between the forms of storage: short-term storage must have high efficiency above all else, and long-term storage must have low investment costs as it is so rarely used.
In your view, are there any storage technologies that are currently under the radar but have greater potential?
I would say it is more the other way around. There are still many technologies that play a role in public opinion. However, it is quite clear that these technologies will never attain economic viability.
Unfortunately, it is very brutal: For storage in the stationary sector, it is virtually only about lifecycle costs. In the mobility sector, weight still plays a major role, as does performance in terms of super-fast charging. In contrast, the requirements we have for stationary storage are all harmless. The performances are manageable, which means we need maybe half an hour, maximum four hours of electricity from the storage. From a storage perspective, the maximum currents are small. Weight and volume play a very minor role. Therefore, in the storage sector, the qualification for stationary applications is almost exclusively based on the cost factor.
Redox flow batteries are discussed from time to time as possible alternatives. However, the problem is that lithium-ion batteries have a 30-year head start on the market. In 1991, the price was perhaps $3,000 per kilowatt hour; today, the price paid by vehicle manufacturers is perhaps $100 per kilowatt hour. In other words, a factor of 30 has been gained here, largely through economies of scale in production. It is virtually impossible to catch up.
There are always discussions about technologies such as flywheels, double-layer capacitors or superconducting coils for even shorter hold-up times. However, we find that they are not economically competitive – and that is always the essential factor. In other words, technically the systems all work, but over operating times of 15 minutes or longer, these systems are simply not economical. For this reason we really only see them in special applications and less for the system integration of renewables. An example would be a container crane that lifts a container every minute and puts it down again and has a high peak power demand for this or a regenerative power demand through recuperation. A similar application exists, for example, in the substations of subways or tramways. In all cases, peak load capping is the motivation and not the integration of renewable energies.
In your view, will electric cars also play a noticeable role in balancing out fluctuations in demand within a day?
If I just take the smallest wallbox with three kilowatts as the charging station and assume 40 million cars, I get 120 gigawatts of power. If you compare that with the six gigawatts we have in pumped storage power plants or the 80 gigawatts of peak power, you can immediately see where the potential is. And it's absolutely clear: vehicle batteries don't get broken from driving, but from standing around. If we don't succeed in utilizing this huge amount of investment in the field of electric cars, it would be extremely counterproductive. It would mean that we would have to build additional storage facilities, which would cost additional money and make electricity and energy more expensive for all of us.
I not only see this as a great opportunity, but even as a necessity. On average, a vehicle runs for one and a half hours a day. Therefore, there are huge capacities available most of the time.