At Sink Float Solutions, we have developed and patented an energy storage technology that values two free and unlimited elements: gravity and depth of the seas.
Our technology brings a simultaneous response to 3 principal challenges energy storage is facing today.
It is easy to implement, scalable, and very economical.
Imagine concrete masses attached to floats below the sea surface. Besides, a barge equipped with a winch connected to the electric network by an underwater cable.
When the network displays an excess of electricity of wind or solar origin, the winch will transform it into potential energy by raising the masses of concrete one by one from the bottom of the sea to the surface, further hanging them to the floats.
Then, during periods with no wind or sun, this energy can be transformed again into electricity in order to supply consumers. For this, the winch will not operate as a motor but in generator mode (kind of dynamo) by letting the masses down one by one.
Depending on the distance to the coasts, the depth of the sea, and the necessary storage time (MWh/MW), our system requires 5 to 20 times less investment than competing storage solutions.
Overall, over a complete cycle, the energy efficiency is above 80%.
The potential energy formula that most of us have learned on school benches (Epot = MxGxH) allows us to verify that, in a cubic meter of concrete, across a 3,800-meter elevation difference (average depth of seas), it is possible to store 15 kWh.
1 cubic meter of concrete = 2.5 tons (density 2.5) – 1 ton of Archimedean thrust = 1.5 tons in relative weight.
Energy = 1,500 kg x 9.81 m / s 2 x 3,800 m = 55,917,000 joules.
Knowing that 3,600,000 joules is equivalent to 1 kWh, this represents a storage capacity of 55,917,000 / 3,600,000 = 15 kWh.
With a cost of 100 €/cubic meter, our concrete storage unit costs 100 € / 15 kWh, or 6.6 €/kWh, which is about 85 times cheaper than the Tesla “PowerWall” battery (in 2019: 7,740 €/13.5 kWh = 570 €/kWh, source www.tesla.com)
However, a few additional components are needed: a steel float for each weight, a winch, anchor cables, and an underwater electrical connection. These different components being standard, it is easy to check their costs.
Overall, and depending on implementation conditions (distance from the coast, depth and scale effects), our solution achieves a total cost of storage (Investment + O&M) 5 to 20 times lower than the best competing solutions.
In addition, if we are taking into account a longer lifetime than batteries (10 years) and (non-standard) optimized components, we estimate we can achieve a total cost 10 to 40 times lower than current batteries.
Integrated into a mainly renewable energy mix, our storage system will generate an extra cost of LCOES (actualized cost of electricity with storage) of 5 to 30 €/MWh (+ 10% to + 50% compared to the cost of renewable energy production). An electricity mix that is mainly renewable and able to satisfy the customer permanently then becomes more economical than most conventional polluting technologies (coal, gas, or combined wind/solar + gas, wind/solar + coal, nuclear + gas, etc).
This actualized cost (LCOES) can be calculated easily, case by case, using the following assumptions:
CAPEX power (barge, electric winch, anchor cables): 250 to 500 €/kW
CAPEX energy (masses, floats): 15 to 60 €/kWh
Underwater power line (HVDC):
– below 50 MW: 90 k€/km + 3 k€/MW/km
– above 50 MW: 1 k€/MW/km
– Masses: 30 years,
– Floats: 20 years,
– Mechanical, electrical components: 15 years,
– Underwater electrical cable (HVDC): 60 years
Ratio MWh / MW: solar = 6h to 16h, wind = 12h to 72h. A case-by-case simulation must be carried out in order to optimize the economic compromise and the penetration rate of renewable energies compared to a backup with a low load factor (diesel or agro-fuel) and overproduction (in particular to manage seasonality).
Do you want to store energy for 6 hours? 12h? 24? 3 days? No problem. Our technology will always offer you an optimal economic compromise, meaning, without generating unnecessary costs.
You can write an optimized specification chart according to your needs:
Indeed, the “power” components (how many MW) are independent of the “energy” components (how many MWh). The winch, its support and the submarine power line (storage power) are totally independent of the number of concrete masses and their associated floating capacity (storage energy). It is even possible to change the ratio MWh/MW (number of hours storage) after the first investment by adding or reselling masses that can easily be towed with their floating capacity, from the port to the implementation site.
The storage system can, therefore, be permanently adapted according to the evolution of the energy mix (rate of renewable energy on the grid) and the evolution of the hourly and annual consumption profile, which is set to change with the evolution of the importance of certain consumer uses such as electric cars, and electric heating
It is also possible to optimize the compromise between the energy efficiency (about 80%) and the cost of the winch according to the needs of each customer. By reducing the vertical speed of the winch, energy losses can be reduced. To maintain a given power with a lower speed, then it is necessary to use heavier and less numerous masses, which increases the cost of the winch
Despite some fixed costs (operations) and other scale-related effects (hydrodynamics and cost of the underwater connection), our technology is economically viable for powers of more than 10 MW, which corresponds to a power 50 times lower than that of a conventional cycle gas plant.
Nevertheless, like conventional power plants, larger unit sizes also reduce the overall cost of the storage device.
In theory, there is no power limit. Currently, offshore cranes with the largest load capacity can lift masses of 4000 tons, which, for a vertical speed of 20 km/h, would correspond to a power of 200 MW. Several winches on the same site can be used in parallel.
All these elements already exist and have been used on an industrial scale, for decades, in the offshore sector. We just need to assemble those differently.
Our storage technology can, therefore, be industrialized in less than a year while outsourcing component manufacturing to many existing shipyards.
The installation can be done at a lower cost because the storage units are, by definition, equipped with an integrated lifting device, which will allow a “self-installation” of hanging points, masses, and cables.
Before assembly, the masses, hooked to their respective floats can be towed to their place of use, with conventional marine shipping transports.
Storm resistance, control of masses movement, hooking/unhitching operations, control of maintenance, and anchoring costs.
Our technology is available in many variants in order to reduce costs while facilitating its operation. Those variants are grouped into several patent families.
Here is a non-exhaustive list of variants aimed at reducing costs, increasing the life of the system and facilitating lifting and masses hooking operations:
 Positioning floats several tens of meters below the surface:
i) Floats are the largest part of the system. By stabilizing them permanently below the surface, they will not be subjected to wind and waves forces, even during exceptional storms. Stabilized with anchoring cables, one of the variants is planned to fill in/empty the floats with compressed air in order to permanently adapt their floating power to the number of masses that are attached to them. This makes it possible to subtract these volumes from weather hazards while considerably reducing, even eliminating, the cost of anchoring cables for these components. The energy consumption required to compress air at such a depth impacts the total energy efficiency of the system by less than 2%.
ii) Other solutions also patented are planned to achieve these same objectives, including the use of floats accompanying the mass during the descent. In that case, the float will see its volume drop exponentially with the pressure/depth, whether it is rigid structures gradually filling with water or flexible envelopes.
 Positioning the winch on a stabilized support several tens of meters below the surface. This component can be secured or separated from the float group; it can be positioned on the surface or underwater according to a principle similar to that described above.
 Closed loop cable hoisting system with driving pulleys and pulleys at the bottom to prevent gyratory movements to facilitate the control of the horizontal position of the phase of deposit / seizure of the masses on the seabed, to have a ratio torque/power constant (own weight of the cable), to use a set of driving pulley rather than a drum of winch (simpler and less expensive),…
 Attachment system guiding/masses unhooking: by mini winches, thrusters, other,…
 Hanging / unhooking system: from the simplest (conventional synthetic slings, ROV) to the most sophisticated,…
 Taking into account the elongation of the cable over a charge cycle,…
 Taking into account the weight of the hoisting cable(s),…
 Optimized design to limit hydrodynamic friction losses on masses and cables (total efficiency > 80% over one complete cycle).
 Taking into account dead times linked to the hooking unhitching phases: multiple barges, auxiliary storage systems, other,…
 Choice of materials for masses: concrete, construction waste, steel,…
The materials entering into this technology are available in inexhaustible quantities and are ecologically clean.
Located far away from the coasts (10 to 200 km depending on the cases), the storage system is not visible and, in its complete underwater version, is not hindering maritime traffic.
Despite a size that can exceed several hundred meters long, in its complete underwater version, the storage system becomes undetectable and, therefore, impossible to sabotage or destroy with conventional means.
Surely, concrete manufacture consumes energy and emits CO2.
However, with 4,000 meters of unevenness, the amount of concrete required to store each kWh is very small, equivalent to a concrete cube of 0.06 m3 (40 cm).
Most of the other components (cables, floats) are made of steel, an infinitely recyclable material.
The masses can also consist of construction waste or any other material denser than water and thus constitute a new outlet for recycling streams.
Furthermore, unlike almost all conventional production, storage or transmission infrastructure (nuclear-, coal-, gas power plants, high-voltage overhead lines, hydroelectric dams, pumped hydroelectric energy storage or battery stations) that can be a target for any hostile group or state, our energy storage system could be significantly threatened only by a few entities equipped with an underwater attack force.
In addition, by offering the possibility to manage intermittency economically, the growing share of wind turbines and photovoltaic parks will make the energy mix much more difficult and costly to destroy, as it consists of several tens of thousands of independent intertwined production units operating without fuel supply.