What to think of that award-winning Dutch Ocean Battery?

Ocean Grazer

At the consumer electronics fair CES2022, the Dutch startup Ocean Grazer won an innovation award this January. Ocean Grazer is developing the Ocean Battery, an offshore energy storage system for wind farms.

A spin on pumped hydro storage

Since winning the CES award, the Ocean Battery received a lot of positive press coverage. Recently the BBC published an extensive article on the concept. I’m afraid it’s time to temper expectations a bit.


Due to international attention for Ocean Battery, this article has been translated. The original article, as well as the rest of WattisDuurzaam.nl, is written in Dutch. Some links in this article also refer to Dutch articles. If desired so, Google Translate may help.


Based on a proven concept: Pumped Hydro Storage

The Ocean Battery concept is a twist on Pumped Hydro Storage (PHS), a form of energy storage that has been contributing to a reliable electricity supply in mountainous areas for decades. A conventional pumped-storage power station consists of a water basin high on a mountain and a basin at the foot of that mountain. When electricity is widely available, electric turbines pump water from the lower basin upwards.

When demand for electricity rises, the high basin is emptied via the turbines and the pumped-storage power station supplies power back to the grid. A well-known pumped-storage power station is Coo-Trois-Ponts in Belgium. This power station consists of two high basins of together 8.5 million cubic meters and a single low basin of 8.5 million cubic meters.

Coo generates a maximum of 1,160 megawatts (MW) from a difference of up to 275 meters in elevation. The storage capacity is 5,500 megawatt hours (MWh); the plant can generate power for almost 5 hours at full capacity. The round trip efficiency is about 75%. About 1,800 MWh is lost to friction with each full charge and discharge.


The storage capacity is determined by the depth of the sea

Ocean Grazer has converted the operating principle of the pumped-storage power station into a concept that works on the high seas. Four concrete tanks dug into the seabed serve as the low basin. The high basin consists of 4 flexible rubber bags on the seabed with the same volume. When the wind blows but there is no need for electricity, the Ocean Battery pumps water from the concrete tanks into the bags.

A building with 300 apartments

When there is no wind and the demand for electricity is high, the bags are emptied back into the concrete tanks via turbines. Ocean Grazer mentions a storage capacity of 10 MWh per system of 4 tanks and 4 bags.

An important difference with conventional pump storage is that the water column above the rubber bags adds to the height difference. The water in the bags effectively lifts all seawater above the bags a bit. The storage capacity therefore also depends on the local depth of the sea. The deeper, the more capacity.

Source: Ocean Grazer (PDF)

In a brochure (PDF) Ocean Grazer mentions a volume of 20,000 cubic meters for each of the concrete tanks. Each system therefore pumps 80,000 cubic meters between the 4 tanks and the 4 rubber bags. For a storage capacity of 10 MWh, a height difference of 45 meters is required. That is feasible in the Dutch part of the North Sea, certainly including a little digging in the seabed. With 80,000 cubic meters, the Ocean Battery is 100 times smaller than the pumped-storage power station at Coo, but that primarily shows how large Coo is.

For those who love converting random volumes into Olympic-sized swimming pools: 80,000 cubic meters will fill 32 pools of the official size. For those who are more familiar with undersea tunnels: 80,000 cubic meters fill almost 2 kilometers of a Channel railway tunnel between France and Great Britain. For those who prefer to think in apartments with a size of 75 square meters: 80,000 cubic meters fill more than 300 such homes. In other words, in order to store 10 MWh of wind power, Ocean Grazer will have to bury a 300-home appartement building under the seabed.


‘Low maintenance, no critical raw materials required and ecofriendly’

We could stop here. A modern offshore wind turbine delivers 10 MWh in an hour. Digging concrete shafts the size of an apartment building under the seabed to store one hour’s production from one wind turbine is not going to happen. But while we’re at it, let’s take a closer look at Ocean Grazer’s other unique selling points:

  • Long life, limited maintenance. In the brochure, Ocean Grazer states that the system will last at least 20 years without the need for major maintenance. The rubber bags contribute to that. In principle* the concept would also work directly on seawater and thus without the bags. In that case, however, the turbines are exposed to saltwater, sand and bio fouling. Ocean Grazer therefore fills the concrete tanks with 32 Olympic pools of clean fresh water. That alone is quite the operation on the high seas, but maintenance with divers on the seabed is complex and expensive so it might be worth it;
  • A million cycles. In the interview with the BBC, CEO Frits Bliek states that the system can handle a million charge and discharge cycles. Over 100 times more than equivalent lithium ion batteries. Assuming 30 years of operation, that means 90 cycles per day. Assuming a pump power equal to a wind turbine, charging and discharging takes 2 hours. More than 12 cycles a day is therefore not an option, and in practice it will probably be 0 to 2 cycles daily. A million cycles is pointless;
  • No critical raw materials. Ocean Grazer claims to be independent of critical raw materials such as lithium, cobalt and rare earth metals. The system consists of concrete, steel and rubber, materials that are readily available worldwide. However, the Ocean Battery works thanks to generators that inevitably contain copper and most likely rare earth magnets. If the pump power is equal to the power of a wind turbine next to it, the use of critical materials will be similar;
  • Ecofriendly. Ocean Grazer states that the concept is environmentally friendly, referring to the safe and widely available raw materials. However, the production of steel, concrete and rubber is far from environmentally friendly, and burying a block of flats in the seabed is also not good for the seabed habitat and local wildlife;

All in all, I find it hard to grasp that Ocean Grazer has been selected to represent the Netherlands at CES2022, hard to grasp that the jury at this event was impressed by the startup and hard to grasp that the national and international press is so wildly enthusiastic about this concept. Extrapolating this trend, a merger with a SPAC is a probable next step.


*Copycat. A simpler version of the Ocean Battery was tested 5 years ago by the German Fraunhofer Institute. At the bottom of a 300 meter deep lake in the Alps, Fraunhofer tested a concrete sphere with a diameter of 3 meters. This sphere was not buried and it exchanged (fresh) water with the lake, so without filling rubber bags. Apart from that, the concept is identical.


Sources: Ocean Grazer, BBC / Imagecredit: Ocean Grazer

Dit vind je misschien ook leuk...