75,000 m3 of softened water for solar heating system

SUNSTORE 4 is an extended solar district heating network placed at the town Marstal at the Danish island of Aeroe. The solar heating system uses a huge water reservoir of 75,000 m3 for storing thermal energy, when the sun is not shining. Water supply for the pit heat storage came from three nearby groundwater wells. A rental water treatment plant from EUROWATER handled the filtration and softening of the groundwater within the 70-day timeframe; a challenge in more than one sense!

Solar energy

The sun undoubtedly plays a major role in the energy supply of the future. Rising energy prices, increased focus on the environment, as well as technologies that are constantly improving, have made the production of heated water and electricity with solar heating a profitable and CO2-friendly option for energy supplies. A sustainable choice that at the same time has a low total cost of ownership (TCO).

Marstal District Heating has been testing the principles with a smaller solar heating system for several years. In 2011, it was decided to significantly expand the previously successful plant. Now the sun should supply half of Marstal's residents with heated water – all year!

The solar heating panels are installed on the outskirts of Marstal. (Photo: Erik Christensen,, CC type BY-SA 3.0)

Renewable energy production with heat storage

Solar heating is an efficient source of energy, but of course only as long as the sun is shining. So, what do you do when it's cloudy, or in winter, when there are fewer hours of sunshine? This is where the heat storage comes in.

To provide heat energy throughout the year, it is necessary to have some kind of buffer that can retain heat, and that is used a pit heat storage.

A pit heat storage is in all its simplicity just a huge water reservoir, with water heated to 75-85°C by a solar heating system. The temperature is calculated based on a trade-off between efficiency and capacity, and an analysis of the surrounding environmental factors.

Water supply and water quality

A consultancy firm carried out the initial soil studies and at the same time examined where the many m3 of water was to be supplied from.

First consideration was given to whether treated wastewater from a nearby treatment plant could be used. However, it was quickly clarified that placing such a large amount of wastewater on top of a groundwater reservoir could cause great concern. In addition uncontrollable and contaminant substances could pose a problem for technical equipment. And it would involve a 1.5 km long pipeline across roads and through the habitation on the island.

It was also examined whether the utilities of Aeroe could supply the necessary amount of drinking water, but it was not possible within the timeframe.

Therefore Marstal District Heating contacted EUROWATER to find a future-proof solution. After a profound analysis it was clear, that the water supply should come directly from three nearby groundwater wells. The water would be filtered by pressure filters followed by softening to attain the right water quality.

The many litres of water were only to be supplied once. A rental water treatment facility was therefore a perfect choice for this temporary assignment. At the same time, the large amount of water had to be delivered relatively quickly to initiate the solar heating system at the desired time. 

A very large hole in the ground

75,000 m3 of water takes up a lot of space. More specifically a lake of 18,000 m2 and 16 meters deep at the deepest point. The excavation took place for a long period of time, and constituted an impressive sight.

After excavation, the hole was lined inside with thick, welded "plastic bag" for insulation and to ensure no water escape. Shortly after installation, the water treatment plant in container was put into operation, and in less than 70 days the heat storage was filled with water of the necessary, softened quality. Finally, an insulating lid of thick foam mats was placed on top of the pit.

Excavation of the heat storage pit. On the left is the 16 m high nozzle. (Photo: Leo Holm)

Water treatment

In a solar heating system, a circulating liquid in the solar panels is warmed up by the sun. With the help of a heat exchanger and heat pump, the heat is transferred and used for heating district heating water. 

At SUNSTORE 4, the solar panels heat both district heating water and the heat storage pit at the same time, as the plant has a calculated overproduction during the active months.

The water from the heat storage pit (like the district heating water) circulates in pipes, heat exchangers and heat pumps, and this places high demands on water quality to avoid corrosion and deposits in the machinery.

The raw water supplied was untreated groundwater and could therefore not be supplied directly into the heat storage pit. It had to be softened first - a process that removes calcium and magnesium. Softening can be carried out chemically free with ion exchange if the water available complies with the quality of drinking water. It was therefore necessary to treat the water for iron and manganese through pressure filters before the softening process.

Two pressure filters were placed in parallel operation, as well as three ion exchange plants with continuous production. By hourly backwash of the pressure filters and regeneration of the ion exchange plants, it was possible to establish a stable supply of softened water to the heat storage pit.

Delivery and installation of the temporary water treatment plant. (Photo: Svend Flensborg)

Permanent installation

Even though the heat storage pit is a closed system, evaporation will occur over time, as well as the risk of small leaks. Therefore, Marstal District Heating has installed a smaller, permanent SM82-F softening plant from EUROWATER to replenish the heat storage pit when necessary.

The heat storage pit is almost filled with softened water and only the lid is missing. In the middle of the lake is the top of the nozzle. (Photo: Leo Holm)

More information

Marstal District Heating's solar heating project:
SUNSTORE 4: Summary technical description


Technical information 

  • Solar heating area: 1,034 solar heating modules totalling 15,000 m2
  • Surface construction: 43.500 m2
  • 4 MW Biomass boiler
  • 750 KW ORC (Organic Rankine Cycle)
  • 1.5 MW heat pump
  • Dam heat storage: 75,000 m2 softened water
  • Replenishment time: 70 days
  • Hardness softened water: < 0.5 °dH
  • ROI: approx. 9 years

Water Treatment Equipment 

  • Pressure filter TF25
  • Pressure filter TF35
  • 3 x Softening plants type SFH 1802-F
  • Pumping station with 2 Grundfos pumps

More references from EUROWATER

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