In April 2020, a new lid solution was installed on a 70,000 m3 Pit Thermal Energy Storage (PTES) at Marstal district heating plant. The lid is of a new design, which prevents moisture accumulation and enables easier dewatering of the surface ensuring lower heat loss and contributing to higher efficiency and reliability of the complete energy system.


Client:      Marstal Fjernvarme
End user/Owner:        Marstal Fjernvarme
Location:   Marstal (Ærø), Denmark
Status:   Operational

Minimizing heat loss and boosting efficiency with unique lid design

The project in Marstal included design and installation of a 10,000 m2 turnkey lid solution for the district heating plant, Marstal Fjernvarme’s existing 70,000 m3 PTES. The floating insulating lid retains the heat inside the storage and keeps the storage tight from rainwater.

The original lid on the storage in Marstal was damaged due to moisture accumulation and was no longer able to retain heat resulting in a significant heat loss and decrease in efficiency. The new lid is of a different and optimized design developed with the aim of addressing and solving some of the challenges experienced during previous deployment of the PTES technology. This includes challenges such as accumulation of moisture within the insulation, dewatering of surface water and accumulation of air pockets under the lid.

“At Marstal Fjernvarme our goal is to have 55% of the annual heat supply covered by solar heat. With the old lid, the maximum solar energy share was 45%, but with the new lid, we are approaching 55%. The new lid was installed six months ago, and we can already see that it has a much better warm-up as well as a significantly lower heat loss, and it does not have the same issues with rainwater” – Lasse K. Larsen, Operation Manager, Marstal Fjernvarme.

Design prevents moisture accumulation and enables dewatering

The turnkey project in Marstal included design and installation of the complete lid solution including design of diffuser, liner work, installation of control system, electrical connections as well as installation of a rainwater pump system.

The new 10,000 m2 PTES lid in Marstal is the first large-scale lid solution applying the new patented design. This design has undergone several optimization practices and improvements have been made based on experience from previous pit storages.

The unique design features of the new patented lid solution include a diffusion-open top cover construction, which allows for vapour to diffuse out through the upper liner above the insulation rather than accumulating inside the construction. Moreover, the lid is divided into sections for improved rainfall handling. Each section has an inward fall towards the middle and towards a pumping well, which is able to lead away rainwater. The inward fall on each section prevents minor air pockets, that would otherwise develop underneath the lid construction, from accumulating into larger pockets underneath the lid. Instead of accumulating, the air pockets are guided towards the edges of the sections, where they are led out through venting valves.

The 10,000 m2 lid in Marstal consists of 12 sections and thereby 12 pumping wells. The original lid at Marstal district heating plant had only two pumps placed in the middle of the lid and therefore not enough capacity to pump away all the water in case of rainfall.

These unique design features ensure lower heat loss in the PTES contributing to higher efficiency and reliability of the complete energy system.

The project was carried out by Arcon-Sunmark, but Aalborg CSP has since then acquired all know-how and intellectual properties related to the PTES technology including patents and rights to the new lid design from Arcon-Sunmark.


Client:  Marstal Fjernvarme

Capacity of the plant:

Solar peak production: 24MWth
PTES capacity: 7,000 MWh
PTES load/discharge capacity: 10.5 MWth

Annual energy production:13,400 MWh heat/year
No. of customers:1,6000
Location:Marstal, Ærø, Denmark

Scope of supply:

Design, delivery and installation of a 10,000 m2 lid
for a 70,000 m3 Pit Thermal Energy Storage (PTES).

Expected system lifetime:25 years


Project updates

Click through the pictures to see how the construction progresses in Marstal, Denmark. (The pictures are organized in chronological order, latest project updates appear first.)