by Linda Bertelsen
Heliac solar panels for high-temperature district heating

Heliac solar panels generate heat using large lenses that focus sunlight the same way magnifying glasses do. This allows the solution to meet the heat demand even in district heating (DH) networks requiring output temperatures up to 130°C. In fact, the panels may deliver 160°C if needed.

By Jakob Jensen, Commercial Director, Heliac

This article was published in Hot Cool, edition no. 4/2023.

A full-scale, 1.6 MW solar field is in operation in Hørsholm, Denmark. It produces an estimated 1,400 MWh of heat annually for the local DH network operated by the company Norfors. Norfors’ primary heat production is based on waste incineration. Norfors supplies households in five municipalities north of Copenhagen with DH.

Integrated into one of Norfors’ storage tanks, the solar field receives DH return water at 40°C and heats it again to 90°C-110°C before returning it to the network. The specific temperature is adjusted according to the network’s demand which typically runs on higher temperatures in the winter than in the summer.

The solar field heats pressurised water in a closed loop delivering the generated heat to the DH system via a heat exchanger connected to a storage tank installed at a peak load station. Integrating into the network via the storage tank makes it easy to control both the output and adjust the existing heat production to balance demand and supply.

The property of the pressurised water is the same as what is used in the DH pipes, i.e., without the use of glycol. Avoiding glycol eliminates environmental risks and also improves the water’s viscosity, thereby slightly reducing the pumping power needed. 

Instead, to counter the risk of freezing, the solar field circulates the return water in its pipes. This is cheaper than the cost of using glycol and provides an environmentally safer solution.

Being able to provide heat at a stable high temperature makes the solution work well together with heat pumps (HP) for DH: HP efficiency is lower in cold weather than in warm weather. This affects the annual efficiency and, therefore, the entire economy.

Further, in regions where electricity from wind turbines plays a significant role, the cost of the electricity that powers the HP is generally higher in summer than in winter due to less wind. 

Combining HP with heat produced by solar fields addresses the annual variation in the cost of electricity. It makes it possible to increase the yearly efficiency by using stored heat as feed-in to the HP.

Panelprincip by Heliac

Solar fields are built from multiple rows, each with six panels in series, where each row can be individually controlled. This design gives two advantages: First, in case of a component failure (e.g., a leak), it is possible to shut down a row for maintenance without shutting down the entire solar field – resulting in less maintenance downtime.

Second, in contrast to standard flat panel solutions, it permits optimizing the economics of solar fields by designing them with a peak power larger than the maximum offtake. 

Such a design will increase the total number of decarbonised MWh delivered. The optimal design will balance the extra cost of a larger solar field with the value of reduced exposure to carbon emission taxes and the value of the efficiency increase of heat pumps.

Considering the cost of carbon emission permits, distribution and transmission, and boiler losses, solar fields may prove profitable even in regions with limited solar irradiation. 

This is because these costs bring the total cost of fossil-based energy to €80 (natural gas) – €95 (coal) per MWh cost at today’s prices (April 2023). Solar can compete with this cost in most places in Europe.

Other than being able to serve high-temperature DH networks, having higher temperatures delivered enables storing more energy, e.g., in a water tank or pit storage, without significantly increasing the cost of the storage. In the end, the energy density per invested capital is increased with higher temperatures.

For visionary DH operators, the higher temperatures also open an opportunity to serve industrial heat users with decarbonised process heat for steam-driven processes. Adding industrial process not only increase revenue but may further benefit from additional flexibility in the network.


Originally tasked with creating yogurt-repellent structures for a dairy manufacturer so that yogurt won’t stick to the inside of yogurt containers, Heliac’s mother company, Inmold, developed a fast, inexpensive method for making microstructures in plastic. 

Realizing that the methods can also be used to make microstructured Fresnel lenses, which are lens structures invented 200 years ago to direct the light from lighthouses. It thus can be used to focus light as efficiently as any magnifying glass; Heliac was founded to commercialize this opportunity.

Based north of Copenhagen, Denmark, Heliac has 50 employees. The company holds several patents protecting its invention.

For further information please contact: Jakob Jensen, jj@heliac.dk

“Solar heating for high-temperature district heating” was published in Hot Cool, edition no. 4/2023. You can download the article here:
Solar heating for high-temperature district heating

Meet the author

Jakob Jensen
Commercial Director, Heliac