New experiments at the Matrimandir Lake

“All the earth from the digging of the Test Lake was deposited on that hill over there,” says Rohan, pointing at a now 14 metres high hill behind the Matrimandir workshops. “The depth of the Test Lake is 10 metres, of which about 4.2 metres has been filled up by the monsoon rains. With a level difference of about 20 metres the idea came up to generate electricity by pumped hydro by creating a reservoir on the hill.” 

Pumped hydropower is generated when, under the influence of gravity, water moves down from a higher to a lower reservoir passing through a turbine. The hill is well-suited to host the upper reservoir, while the Lake serves as the lower one. Energy produced by the Matrimandir Solar Power Plant will be used to pump water from the Lake back into the upper reservoir. “Pumped hydro-power storage is considered to be ‘state-of-the-art’ energy-storage,” says Michael Bonke, who came up with the idea.

Is pumped hydro necessary given that the Matrimandir already has its own solar power plant? “The problem with solar energy is its storage,” explains Michael. “Solar energy is generated in excess during the day, while in the mornings and evenings we must draw from the TNEB grid, unless we can store the surplus energy from the lunch-hours. For many solar systems the excess energy is stored in batteries, which is not ideal. As compared to hydro storage, batteries are environmentally unfriendly (the production of lithium-batteries is a “dirty” business), inefficient (only around 60% as compared to 85% for hydro-power storage) and expensive (collective hydro-power storage costs only a quarter compared to individual battery storage).”

The Matrimandir Solar Power Plant generates more electricity than it can consume at certain times of the day, e.g., during lunchtime and on Sundays. The excess electricity is exported to the ‘Auroville High Tension Zone’ where it can be used by other Auroville consumers. But there are other solar-energy producers inside that high-tension zone, like on the roof of the Town Hall. This results in this zone producing excess energy at certain times, which is then exported into the TNEB grid, for which Auroville receives no compensation. 

The new hydro storage could allow for the storage of some of the energy produced by the solar plant. “It is an experiment,” concedes Rohan. “The digging work for the upper reservoir (35 x 20 x 4 metres) and its lining with 2 mm thick HDPE foil is finished and the reservoir was filled for the first time with water from the Test Lake on May 31st. The 25 cm diameter pipeline between the upper reservoir and the Lake has been installed and a small turbine will be installed soon. Then we’ll see how it works. We hope the hydro storage will be able to generate about 100kWh of electricity a day.” This will contribute towards the energy being consumed by Matrimandir, and in fact Matrimandir can then function in the evenings and mornings with its own hydro-stored electricity.

The expansion of the Lake – starting section two

It has been decided and will remain decided that the Matrimandir will be surrounded with water. However, water is not available just now and will be available only later, so it is decided to build the Matrimandir now and surround it with water later, perhaps in a few years' time...

- From a letter written by André, on The Mother’s behalf, to Huta, 14 February 1969

Roger Anger, Matrimandir’s architect, had approved a basic design proposal for the Lake of about 90 metres width and a central depth of 10 metres. The Matrimandir Executives have announced that they would like to complete the entire Lake by Mother’s 150th birth anniversary, February 21, 2028. It’s a huge challenge.

The Lake’s second section is envisaged to be about 250 metres long (more than twice the length of the Test Lake) and will be excavated on the south side of the Matrimandir as the existing workshops block lake construction on the east side. For this, the current public Viewing Point has to move. Work has started; trees at the Viewing Point are being pruned in preparation for their transplantation; the boulders from the old ‘Rock Garden’ area are being  removed; and a new, higher Viewing Point, just behind the current one, outside the boundary of the Lake section two, is being made. Once all that is ready, the removal of the existing public Viewing Point and the full excavation of section two will start. The Executives hope that this section will be functional, (e.g., lined with geotextile and HDPE foil, covered with granite gravel, and the end wall of ‘Lego’ blocks installed) before the winter monsoon of 2024.

At the same time, work on the first section of the Lake continues. The reinforcement of its two end walls and their height increase from 5.3 to 8.5 metres will start soon, so that they will be high enough to contain the rainwater from the 2023 winter monsoon.

Filling the Lake

The 2022 winter monsoon and the healthy 2023 summer rains have filled the Test Lake to a level of 4.20 metres. The high level is due to direct rainfall on the Lake and to the harvesting of runoff rainwater in the 7.5 metres wide rainwater-harvesting channel around the Matrimandir Gardens, which was fed into the Lake. “But this level will soon go down due to evaporation,” says Rohan. “It will take maybe two more monsoons before the 10 metres deep Test Lake will be fully filled up, and even more when the second section of the Lake will be added to the first.”

Is it possible that the level will go down because of leakage, for example due to bad welding of the HDPE foil? In an open letter to the Executives of Matrimandir, Auroville engineer Jan Imhoff mentioned this possibility and warned that leaks might show up once the lake is full, as then the pressure will more than double. Michael doesn’t believe this will be the case. “The technology of waterproofing a water body with HDPE foil is well proven. In Germany, from where our foil is imported, producers of HDPE foil give a guarantee of 100 years on the leakage-free performance of their foil, which includes the welds. This is fixed by law, because for German garbage dumpsites the manufacturers have to guarantee that no toxic chemicals can percolate into the groundwater. There are more than 1,000 artificial water-bodies and garbage dumps in Germany which have proven that HDPE is leak-free.”

The underwater water storage

How is the Lake level to be kept at its maximum? “Roger Anger, the architect of the Matrimandir, had the vision of a filled-up lake with a water level that should not vary more than 80 cm. That’s a big challenge, given the climate change and high levels of evaporation,” says Michael. He came up with the idea of an underwater drinking water storage system. “Once the Auroville desalination plant is up and running [this is expected to manifest in a few years, eds.] we need to find a place to store all the potable water which is not immediately used. The idea came up to store it underwater in large HDPE cushions inside the Matrimandir Lake, and pump it out when needed. The cushions have the additional advantage of pushing the lake level upwards.”

Experiments with different cushion models have meanwhile started: a balloon, a cube, a cuboid and other models. “These models are HDPE foil sheets welded together in inventive manner,” says Rohan. “We don’t yet know how they will respond to repeated underwater compression and refilling. We’ll soon find out, using ordinary drinking water. If they work, we will be able to create a huge underwater drinking water storage reservoir for Auroville.”

Calculations have shown that this system will be cheaper than storing water underground or in water towers. The experimental cushion that will be used will contain 1,000 cubic metres of water, seven times the water volume held by the Elephant Water Tower near Arati community. But the costs of this system will be only a fraction of the costs of building a water tower and an underground tank. “So there is a huge potential in this technology, once it is proven to be functioning well,” says Michael.

The Auroville Water Service is happy with the development. “There is a big demand for more drinking water storage. The new system will probably be used immediately once it has shown to be reliable,” says Michael. In this way the Matrimandir Lake would become part of Auroville’s drinking-water management, a possibility foreseen by The Mother more than 60 years ago.

A future expansion?

Michael has not only been thinking about the lake level and energy requirements of the Matrimandir, but also about the water and energy requirements of Auroville itself. “HDPE foil is allowed for drinking-water storage all over the world, as it is the most durable and indestructible foil in existence. So the combination of HDPE foil, under-water storage of a different type of water, and hydro-power energy production, gives a lot of possibilities which have been unthought-of till now,” he says.

He muses about building huge underwater storage cushions in the open sea and filling them with water from the desalination plant. Such an underwater reservoir could then act as the lower lake of a large hydro-power storage system, while the cushions inside the Matrimandir Lake would function as the upper lake. “Calculations have shown that with the level variation of about 45 metres between these cushions – the Matrimandir is situated on a plateau with an elevation of 50 metres above sea level – Auroville could become independent from outside energy-sources. The energy required for pumping the water upwards would be obtained from solar power. So Auroville could produce all its required power by locally installed solar-energy, and store it battery-free.”

Underwater storage cushions could also benefit the nearby villages, which are suffering from saline water intrusion in the aquifers and which now rely on water tankers for their drinking water. “Once our pilot desalination plant is up and running and we have cushions installed in the sea, Auroville can advise municipalities on this water management system,” says Michael. “For this purpose patents have been applied for, to give Auroville the possibility to generate additional income while propagating these environmentally friendly technologies of water and energy management.”