Energy-neutral living with underground heat storage from HoCoSto | Brabant Brand Box

Energy-neutral homes thanks to HoCoSto's underground heat storage system

HoCoSto's underground buffer stores heat in the summer that you can use in the winter. So you can heat your home without gas. An interview with HoCoSto founder René Geerts, who's always full of ideas.

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René Geerts is full of ideas. He has always been very inventive. Retirement was just not an option for the man behind HoCoSto - one of the winners of the Brabant Start-up Awards in 2018. “If I don't do something productive, all those ideas will drive me crazy.” So, after selling his successful LED technology company, he finally decided to commercialise an idea that had tantalised him for more than ten years: underground heat storage.

Creative freedom

Creative and innovative minds need space. Rigid frameworks, restrictive regulations: nothing could be more frustrating for someone like René Geerts. He founded Rena Elektronica in 1985, long before LED lighting was widely recognised as a sustainable technology. However, the increasing popularity of LED lighting also led to restrictive rules and standards. And René faced increasing competition from major players in the lighting industry. That made the activity a lot less fun for him. In addition, Rena had grown into a company employing about 120 employees. As the company's director/owner, he hardly had time for the technology, which was the area that fascinated him most. So when he received an attractive offer for Rena from a potential buyer, he jumped at the opportunity: “Time to quit. And enjoy my retirement.”

Energy-neutral living with underground heat storage from HoCoSto | Brabant Brand Box
Photo: Peter van Trijen


His ‘retirement’ did not last long; after just three days he already felt like he was in a rut. That was when his idea for creating a heat storage system bubbled to the surface again. But he was still undecided about the details. The penny dropped at Shanghai airport when he was waiting for a flight to Amsterdam. As he studied the ingenious roof structure, he asked himself in a flash of sudden inspiration: “Why don't I do exactly the same thing underground”?

Home-built prototype

René discussed his idea with several design consultancies, which all recommended using concrete. “But”, he says, "I had already done the costings and concrete was far too expensive. So after talking to five design consultancies, I realised I was wasting my time and decided to build a prototype myself.” Together with his wife Gerda, he designed and developed a strong structure made from aluminium piping, which they assembled themselves. They excavated a pit with sloping walls in their garden in Achtmaal (in the Municipality of Zundert), covered the sides with strong rubber sheet and glass fibre-reinforced fabric, placed their aluminium-tube structure in the pit, filled the space between the rubber and the fabric with insulation material and then opened the tap to let 120,000 litres of water flow into the resulting reservoir. Next, they laid six pipes to connect this reservoir to the vacuum tubes in the eight solar collectors on the roof of their house and finally covered it over with insulating material, glass fibre fabric and soil. They discovered that the collectors were capable of heating up the water in the reservoir to between 75 and 80 degrees in the summer. This is the perfect temperature for heating René and Gerda's home in the spring and autumn. In the winter, their heat pump kicks in to offer a helping hand, so they no longer use any gas at all.

Mum’s the word

This all sounds quite simple, but obviously they had to overcome quite a few challenges, says René with a laugh. “We had to do all the hands-on design work ourselves: how do you make sure the hole is exactly big enough, how do you get the structure into the ground, how do you insulate the reservoir? Things go wrong occasionally, and you have to stop and rethink your strategy. Fortunately, the answers come to me in my sleep. When I am preoccupied with a problem, the thought processes carry on at night and I wake up the next morning with a solution.”

A further complicating factor was that René and Gerda had to keep their plans secret. Having learned from previous mistakes at Rena, they immediately decided to apply for a patent for their Seasonal Thermal Energy Storage (STES) system. And that meant: mum's the word. As chance would have it, The Evening Four Day Marches event, which passes through Achtmaal, coincided with their excavation and assembly activities. René: “Hundreds of walkers marched past our construction site and asked what we were doing. Rather than lying to everybody, we put up a letterbox with a sign next to it: ‘Guess what this is and win a sightseeing flight for the right answer’. The answers were surprising and intriguing and ranged from a helicopter platform to a support structure for a diving board. We didn’t have to arrange a sightseeing flight in the end though.”

The underground buffer from HoCoSto in Achtmaal stores heat in the summer that you can use in the winter | Brabant Brand Box
Photo: Peter van Trijen

“Answers come to me in my sleep. When I am preoccupied with a problem, the thought processes carry on at night.”

Market launch

After tests proved that the STES does what it is supposed to do and some minor improvements had been made, René and Gerda decided to launch their invention on the market. They choose HoCoSto, an abbreviation of Hot-Cold-Storage, as the name for their new venture. René calls the first few months “an interesting time”. The market for their product turned out to be very different from what they expected beforehand. “We initially approached people who were not connected to the gas mains and consequently had to use expensive propane from a gas tank in the garden. Our system would save them a great deal of money. However, when we exhibited at the Energiebeurs in 's-Hertogenbosch, we discovered that people were mainly interested in our system as an alternative to heat cold storage because you can’t drill boreholes everywhere.” HoCoSto has now installed several STES units in homes and gardens, particularly in areas where drinking water is produced from groundwater. The heat storage system is also used by schools and businesses to provide combined sustainable heating and cooling.

‘Turning off the gas’ in sports facilities

In 2018, HoCoSto hit the headlines when the Ministry of Health, Welfare and Sport opened a competition for “start-ups offering progressive and innovative energy saving and energy generation methods to make sports facilities energy-neutral”. The company from Achtmaal was one of five start-ups that were each awarded 100,000 euros to fund a pilot project. The site that benefited from HoCoSto's STES system is Wernhout football club in Zundert, which is scheduled to ‘turn off the gas’ in 2019. There are more than 24,000 sports facilities in the Netherlands. They have signed a covenant that commits them to significantly reducing their energy consumption by 2020 and - like other public buildings in the Netherlands - achieving energy neutrality by 2040. René Geerts: “A wealth of opportunities for us.”

Administrative challenges

The world of energy-neutral homes and offices also puts René’s creativity to the test. Because no legislation on underground heat storage exists as yet, he spends a lot of his time arguing about permits and explaining his system to municipalities. “They are not quite sure how to approach this technology at the moment”, he says. “One municipality tells you that you need a building permit, another believes that you need an excavation permit. In spite of these setbacks, all our storage systems were installed under the soil without a permit; in the end, the municipalities discovered that no permits were required. But you still have to spend months arguing your case.”

Living labs

HoCoSto participates in “Nagele in Balans”, a living lab project that is right up René’s street. Nagele, in the Municipality of Noordoostpolder, is one of 27 neighbourhoods selected for the Natural Gas-Free Neighbourhoods Programme, which is part of the government's set of initiatives for achieving the Paris Agreement climate targets. The aim of the programme is to build knowledge and refine techniques for making existing neighbourhoods more sustainable in a feasible and affordable manner. This project is all about learning and experimenting, so René has all the freedom he needs. “Because you can’t solve the energy problem if your hands are tied by rules and regulations.”

Upgrading neighbourhoods one at a time and making them more sustainable: René Geerts believes that this is the best way of implementing sustainability in the Netherlands. It is easy to manage and affordable. That is where the future of HoCoSto lies in his view. Although he doesn’t really find the challenges of tomorrow all that fascinating. “My focus lies on the more distant future.”

“You can't solve the energy problem if your hands are tied by rules and regulations.”

René Geerts is the founder of HoCoSto in Achtmaal |Brabant-Brand-Box.jpg
Photo: Peter van Trijen

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Article last updated on: June 11th, 2019. 

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