Cycle-proof flywheel storage instead of battery storage?

The basic idea isn't new, but established

It is actually surprising that someone is only just now starting to use this technology for charging stations. Of course, it's been around for decades. Some still remember the flywheel-driven Porsche prototype and even buses. Instead of burning the energy via the brakes when braking, they spin up the flywheel and then use up the power again when accelerating.

In the car itself, however, the system had only very moderate success. Above all, one issue is that the flywheel also gives the car poor driving characteristics. Flywheels are used in ships to make them more stable. So they drive smother in the waves and changes directions less. Anyone who has lifted the front wheel of their bike in the air and then turned it quickly knows the phenomenon: It is becoming increasingly difficult to turn the handlebars because the rotating wheel no longer wants to leave its position.

This phenomenon is perhaps not quite as relevant for a bus, but a passenger car or even a sports car loses the ability to drive properly through curves. In addition, such a flywheel must of course also have a lot of weight in order to be able to absorb and deliver power. Which also does not improve the handling of the vehicle at all.

On the other hand, something like this is quite common in the stationary sector. For example, ABB supplies flywheels for data centers. These are kept at constant speed and in case of a power outage, they can step in immediately and supply the servers with power until they have saved everything and have been shut down safely. Small-capacity power storages can deliver particularly high performance for a short time. Something that rechargeable batteries can only do directly with a lot of effort and costs.

The technology of the ZOOZTER 100 power storages

The ZOOZTER 100 has a total of 8 flywheels. Together they have a usable capacity of 25 kWh and an output of 100 kW. In addition, each flywheel weighs around 450 kg and is in its own housing. The whole thing is rather heavy, because of course it also has to be stable enough to be able to hold the flywheels themselves. The flywheel itself is more or less stacked metal plates. According to ZOOZ, this ensures that the storage itself is significantly more environmentally friendly and recyclable than rechargeable batteries.

The flywheels are mounted with corresponding heavy-duty bearings in their enclosure and in order to minimize losses, there is a vacuum in every enclose as well. This is maintained permanently by means of a vacuum pump. There is also oil-based droplet lubrication and a corresponding oil circuit with filter.

The ZOOZTER 100 is fed in the classic way with 400 V three-phase current and is charged with up to 100 kW. To do this, the electric motors begin to accelerate the flywheels to up to 17,000 revolutions per minute. When they are running, you can hear them quietly on the outside and when you put your hand on it vibrates noticeably, but less than what a cell phone can do. However, since they vibrate, all the flywheel housings are each mounted individually via three points in the ZOOZTER and fixed with another three points in such a way that they would also be held if the mounting of the first three points failed. It is "discharged" with the same motors that accelerated the flywheels. As with the electric car with the brakes, you brake the flywheels down again via the electric motor. The electric motor then acts as a generator.

Although they are three-phase motors, they have a different voltage and frequency than our public power grid. Therefor the ZOOZTER 100 also has a handful of SolarEdge inverters installed. There is also a dedicated 24 V DC network for sensors and the like.

As Jan Waldert from ZOOZ Power Germany told to me, the whole system has to be nicely balanced when it is set up. Maintenance, on the other hand, is quite simple. The storage modules from Israel would only have to be subjected to a visual inspection every year. In addition, the oil and the filters are replaced if necessary. But it also needs every 3-5 years an expert maintenance where the flywheels are taken out and opened. Then obviously things like the heavy-duty ball bearings of the flywheels will also be replaced.

Where is the field of application of such power storages?

Batteries have caught up a lot in the last 10 years, so the system has to measure itself against them. ZOOZ promises to be significantly more cycle-resistant than rechargeable batteries. They would guarantee 200,000 cycles, provided the system is maintained according to their specifications. On the other hand, there are battery storage systems, which according to the ZOOZ Power Deutschland CEO only manage up to 8,000 cycles and then have to be replaced at great expense.

With a first calculation example, the first questions come to me:

200,000 cycles multiplied by 25 kWh result in 5 GWh (5,000,000 kWh), which the system should experience in its lifetime. On the other hand, there is the ChargePost from ADS tech energy with just over 200 kWh. If you assume 8,000 cycles here, you have a turnover of 1.6 GWh (1,600,000 kWh). Due to the ZOOZTER's low capacity, its cycle stability is by far not as much in comparison as it appears to be. It's only about 3x more cycle resistant. Is it worth it with the much more intensive maintenance? The acquisition costs of a ZOOZTER are also significantly higher than those of a battery solution and this would only pay off after 20-30 years.

But the system is great, especially for locations with many cycles, they promise. That would be on the freeway and other locations with heavy traffic. But this is exactly where you have large charging parks. But due to the low capacity you would have to set up one ZOOZTER per charging station. However, these are very large and cannot be placed arbitrarily close together. Otherwise, expert maintenance is no longer possible, since additional flaps must be opened on the right and left.

This is where it starts getting strange. The storage systems are intended for locations with high cycles, but cannot keep up due to the low capacity. Increasing the capacity would mean building it even larger, since you simply need more flywheel mass. Increasing the density of flywheels is not really possible, and with around 80% efficiency, the system can not double the capacity in the same volume. Even if you could get the full 20%, it would still only be 35 kWh. The average fast charging process already needs slightly more energy today and that will not decrease in the future.

A suggestion from Mr. Dappen, the head of Germany, was to combine ZOOZTER with rechargeable batteries. Instead of such a hybrid system, I would prefer to have only rechargeable batteries. These have a higher energy density and also require significantly less maintenance. What these are possibly inferior to the ZOOZTER in terms of cycle stability, you have to put back into the ZOOZTER in the form of maintenance.

Another point is that it is mainly short-term storage. They lose their stored energy much faster than rechargeable batteries. ZOOZ Power Germany itself states the losses at 1-1.5% per hour, which corresponds to a over 300 watts the system needs even in idle. So after 2-3 days the entire stored energy is used up. If the two air conditioners to cool the system start up, it should be much faster.

My conclusion

It is a little strange that ZOOZ Power is very cautious about the price of the system. In addition, they cannot really show how they intend to expand in the next few years and where they see the use case of the ZOOZTER 100. The system is also technically suitable for other applications than just charging stations. Because it has a normal 400 V three-phase input and output. Nevertheless, it seems that the focus is only on e-mobility. It seems to me like a free rider of the e-mobility hype. I will go into more detail on this topic in this post.

The idea itself is really exciting and it was also nice to see how something like this looks and works in reality. But it stands or falls with the capacity of this power storages, that this is very low and does not make up for it with the presumably better cycle stability. No matter how I think about it, as a project manager for charging infrastructure, I don't really know where I should use such a system.

Even if the stories about flywheel power storage sound wonderful and seems logical at first. As long as the batteries in the car can withstand the charging process, they will being integrated in a charging station.