A saline concretion that produces electricity
Graspinghand with Alexander Xanthakis, Yannis Yaxas and Spyros Michas / 2014
Located at the edge of the city, in the old industrial zone and at the estuary of the sea stands the Stensalt. It pumps water during the night and releases it during daytime when the electricity demand is at its peak. On its top, the salt is extracted from seawater. It flows then through pipes to run down along its surface to create layers of saline concretions.
Denmark has over the past decades setting the pace on green and sustainable energy. Currently, with over 30% of renewable energy in the electricity system, it targets to have power entirely by alternative resources by 2050. Moreover, the city of Copenhagen is to become the first carbon neutral capital in the world by 2025.
In this renewable energy mix, wind plays a major role today and will represent an even greater in the future with the envisioned expansion of offshore wind. Yet is has a drawback of being an intermittent source and under certain circumstances the derived production does not match the actual energy needed. A suggestion is to temporarily store energy and deploy it when required. And this is the principle of the Stensalt that calls in the most widely used form of grid energy storage and well proven technology: pumped storage.
The Stensalt consists of a cylindrical tank standing at 30 meters high with a diameter of 30 meters and a height of 50 meters (total height : 80 meters). It has a storage capacity of 40,000 m3 of water. During the early hours of the day (02:00-06:00), electricity to pump the water is used either from the neighboring wind turbines in Refshaleøen or from the grid where the electricity price is low. In the evening time (18:00-22:00) the water is released to generate electricity and meet the peak energy needed in the Danish households. The Stensalt is estimated to release a total daily energy of 5 MWh. This represents the extra energy needed during the evening peak power for 2200 households that is to say approximately 2% of the needs of the city of Copenhagen. The Stensalt annual energy production is expected to be 1800 MWh.
Process of concretion
A source of inspiration for this project was the big salt dome located in North Jutland.
The only material that the site offered plenty of was salt, as a result it became a key element for the project.
The paradox is that the Baltic Sea being the youngest sea on Earth, its level of salinity is relatively low compared to other seas (around 7g/liters, maybe a little more due to the proximity of the straits). But if 10m3 are treated per day it means 70 kilos of salt, and 70 kilos / day means 25 tons/ Year.
Salt is extracted by electrodialysis. Pipes are distributing the salty solution on the surface of the Stensalt by vaporizations and spilling, those pipes being specially treated to avoid blocked suction. Their locations will be frequently changed, to create as much as possible a homogeneous coverage of the surface, the process of concretion being permanent due to the erosion of the salt.
A second skin made of stainless steel mesh is created around the water tank to host concretions. The benefit of stainless steel mesh being that: it is not affected by corrosion, unlike most of metal and it allows creating a soft irregular surface suitable to concretion. The density of wires per m2 should be high, (200 per 200 minimum) in order to get a surface that looks like a creased and transparent textile.
The stainless mesh is maintained by small poles fixed on the water tank at a distance varying from 0.5 meters to 4 meters depending on whether it’s hosting a function or not.
The stainless mesh is maintained with the help of a basket moved by a crane located on the top platform of the structure.
The surface of the stainless skin is of 4700m2. If the quantity of salt extracted per day is 70 kilos, and if 2 kilos of salt are needed by m2 it means that in approximately 200 days it could be fully covered.
Only boats offering adequate protection should be allowed to navigate under the Stensalt because of the risk of falling salt, so a security zone is delimited by buoys.
The stature of the Stensalt, its introduction in the urban landscape of Copenhagen, its situation in the estuary, its ongoing permanent process of concretion and the possibility for visitors to access its 80 meters high platforms are elements that could make of it a major attraction for local people and tourists.
To access the top platform of the Stensalt, visitors can take a cable car or use the stairs. Those stairs are hidden, located between the water tank and the stainless steel wire mesh surface of the Stensalt. Those stairs and platforms are creating deformations on the surface of skin. Several openings in this skin (zones preserved from salinization) allow visitors to enjoy unique views on the city, transforming the ascension into something in between an expedition caving inside a gigantic artificial concretion and an architectural promenade.
On top of the Stensalt visitors can enjoy the views. Machinery necessary to the desalinization of the water will be visible.
All technologies used in the project are clean, from pumps to generators and desalinization machines. They are powered by electricity coming from the grid (if not directly from Lynette’s wind turbines) and produce no waste.
The reason why the Stensalt is placed on the sea is due to the risk of falling of salt. To avoid risk of contamination of the ground and the flora, the most obvious solution was to place it on the sea, where the salt will go back to where it comes from.
The only environmental impact that needs to be study is the fall of salt crystals into the sea, even if the risk seems minimal. They should normally quickly dissolve. But the impact should be precisely studied, to be sure that it won’t affect the ecosystem of the estuary.