Anyone looking out from the canteen at bell pepper grower Duijnisveld in De Kwakel on Friday afternoon would have seen clear blue skies and bright sunshine. Ideal weather for solar panels, excellent for plant growth—and perfect conditions for a thermal battery. Inspired by the centuries-old concept of the fruit wall, the company Thermeleon has developed a system capable of both heating and cooling.
"Right now, growers are simply venting away free solar energy," noted Peter de Munck, a grower since January 1 who is well acquainted with Thermeleon's technology. "And when you vent, you also lose CO₂—a resource that's becoming increasingly valuable for plant growth. Sunlight alone isn't enough. In fact, too much sun can actually be counterproductive."
A few years ago, Peter considered investing in the system developed by the Rotterdam-based scale-up but ultimately decided against it. Koppert Cress, however, chose to proceed and has had the system in operation for a year and a half. On the afternoon of Friday, April 11, during a knowledge café at Greenport Aalsmeer, Thermeleon founder and director Jildou de Jong, along with Peter, shared the first results of the system's performance.
Passive technology for smarter greenhouses
"Raise your hands," says Peter, as five key reasons to invest in a thermal battery appear on the presentation screen. Just like in a small survey conducted by Thermeleon in 2022, most growers raise their hands for the same top priority: a quick return on investment. Other major reasons include improving crop quality and the environmental benefits of reducing fossil fuel and electricity use.
Thermeleon's thermal battery is a passive technology. It works without electricity or moving parts, using special Phase Change Materials (PCM) placed inside the greenhouse. These materials take advantage of the natural heat buildup caused by the greenhouse effect. They store heat when temperatures are high and release it slowly when it cools down. No power cables are needed, and no harmful chemicals are used, making it a safe and sustainable solution—very different from traditional batteries, as founder Jildou de Jong points out.
Heating and cooling in one system
The basic idea is simple: stored heat can be reused to warm the greenhouse. Thermeleon's PCM stores 13 times more heat than the old-fashioned brick fruit walls. This stored heat is then gradually released, helping to maintain the optimal climate that growers can set in collaboration with Thermeleon.
Peter, sharing early results from a full hectare of greenhouse space, highlights the cooling power of the system. At Koppert Cress, they compared two greenhouses—one with a thermal battery, one without. The one using the battery stayed noticeably cooler during the summer. Even a few degrees of difference can make a big impact, Peter says. And in the fall, heating can be postponed, saving even more energy.
Smarter Energy Use
It's a common misconception that installing a thermal battery means the boiler can be removed. In reality, like many new energy technologies, Thermeleon's thermal battery is designed to complement existing systems—not replace them. It becomes part of a broader energy mix, helping growers cover their base energy load more efficiently.
By combining multiple technologies, growers gain flexibility and reduce energy demand during peak times. This flexibility is increasingly valuable, especially with ongoing changes in government energy policies. With new regulations constantly emerging from The Hague, long-term planning has become more difficult.
After the main presentation, around thirty growers and other visitors gathered with Jildou, Peter, and engineer Wouter Bentvelsen for a deeper discussion. One key question: Can the thermal battery charge in winter? The answer, based on data from Koppert Cress, is yes—there are still sunny days in winter that provide enough energy, and the system is designed to retain heat well, even in colder periods.
Another common question is how to best integrate the system into existing greenhouses. "That depends on the type of crop," explains Wouter. "The system can be installed in different parts of the greenhouse to help maintain an even climate." The technology aligns well with the principles of Next Generation Growing—a more closed growing strategy that reduces energy loss. For growers aiming to limit venting, especially during hot periods, the cooling effect of the thermal battery is a major advantage.
Jildou in conversation with a table full of interested people.
Minimum heating temperature
Peter encountered many of the same questions from growers that came up during the session on Friday. But seeing the system in action only strengthened his confidence. "There are benefits to be gained for every crop type," he said. "The biggest advantages appear in spring and summer, but the impact during autumn and winter should not be overlooked."
Thermeleon sees the most potential in crops that require a heating temperature around 17°C. "The system is also a great fit for growers who intentionally follow low-energy cultivation strategies," adds Jildou. "In those cases, retaining heat becomes even more important."
When subsidies are applied, the system can pay for itself within just a few years. The exact payback period depends on the type of crop and how the system is integrated into the greenhouse, Jildou notes. "Since the system is passive, there are no ongoing operating costs, and growers save on energy each year after installation. Added benefits include improved CO₂ retention and a more stable greenhouse climate—key factors for long-term plant health and productivity."
For more information:
Thermeleon
Van Nelleweg 1
3044 BC Rotterdam
[email protected]
www.thermeleon.nl