Island in the sun

Danish Ærø is a relatively small island with the big plans. The island is well on its way to becoming self-sufficient in terms of heating and electricity, in large part thanks to its solar district heating plant – by far the world’s largest.

DATE 2023-11-28 AUTHOR Ulf Wiman

There is something special about islands. The southern Danish island of Ærø is no exception. While most of Denmark’s larger Baltic Sea islands are connected by bridge to the mainland or each other, Ærø is not, which probably accounts for its special atmosphere.

Of Ærø’s approximately 7,000 inhabitants, 2,500 live in Marstal, the largest of the three small towns on the island. With its low, colourful houses lining narrow, winding streets, Marstal is postcard perfect. Its modern-day laid-back charm belies the fact that it once was one of Denmark’s most active maritime centres, second only to Copenhagen in importance.

Part of Ærø’s culture is the islanders’ close relationship with nature. Today this manifests itself in their eco-friendly mindset. In 1997–1998 Ærø competed to be recognized as "The Danish Renewable Energy Island." Although the island didn’t win, the competition had an impact on the residents, who sought to become 80 to 100 percent self-sufficient in terms of electricity and heating by 2008 – in an environmentally friendly way, using only renewable resources.

These initiatives have earned Ærø both international recognition and a number of prestigious awards, both domestic, such as the Solar Town 2000 award, and international, such as the Energy Globe 2001 and a European Union sustainable community award. Given that background, it is fitting that world’s largest solar collector system for heating is on Ærø.

"It was a coincidence that set the ball rolling," explains plant manager Leo Holm as he stands looking at the plant’s 18,365 square metres of solar panels. The rows of panels look surprisingly natural under the big sky, especially with sheep – "our lawn mowers," says Holm – grazing between them.

"In 1993, when the Marstal indoor swimming pool had difficulties paying its heating bill, the idea developed of using the sun to heat the pool water," Holm says. "With help from the Danish Association of Consulting Engineers, we got 75 square metres of solar collectors up on the roof the following year."

The system also served as a test to find out if it were possible to use solar heating on a major scale in connection with the Marstal Fjernvarme (Marstal District Heating), which at the time was run with waste oil.

"Using solar energy would be a lot more eco-friendly and also more cost-efficient," Holm says. "It turned out well, so six months later we erected 8,000 square metres of solar panels for the district heating system."

Benefits for the environment

Denmark is one of the world leaders in district heating. It covers more than 60 percent of the country’s space and water heating. According to the Danish District Heating Association, district heating halves energy consumption, compared with individual oil-fired boilers, and 40 percent of all district heating is generated without any carbon dioxide emission.

Other benefits are that the heat is generated under controlled conditions and that pollution is easier to control. There is also the possibility of recovering waste heat from, for example, industrial processes and refuse incineration.

District heating is often produced in cogeneration plants, where heat and electricity are produced simultaneously, which benefits the environment.

But soon after the initial installation, Marstal Fjernvarme determined that its central solar heating system did not have sufficient capacity. So in 1999 roughly 1,000 square metres of solar panels were added, followed in 2003 by an additional 8,000 square metres.

"The solar heating paradox," Holm says, "is that when the sun shines the most, the heating needs are the least. In 1996 we installed a 2,100-cubic-metre water steel accumulator tank to which we added a 3,500 cubic metres of sand storage in 1998."

With the 2003 solar panel installation, it was again time for the Marstal Fjernvarme to come up with a new storage solution.

Holm explains: "We decided to build a pit heat storage, which in practice is a covered and insulated 6.5-metre-deep pool containing 10,000 cubic metres of water."

Attracting professionals

The Marstal district heating network relies on solar energy from 1 April to 1 October. Solar energy amounts to 30 percent of the annual output of roughly 28,000 MWh. The rest of the year, heat is supplied by six boilers with a total output of 18.3 MWh.

"During this summer," says Per Mortensen, assistant with Marstal Fjernvarme, "we converted the boilers to run on biofuel instead of waste oil, so we’ll soon be as eco-friendly as the other two district heating plants on the island. Since converting to solar energy, we have saved roughly 820,000 to 880,000 litres of oil annually, and that equals annual emissions of approximately 2,200 tonnes of carbon dioxide, 2,600 kilos of sulphur dioxide and 2,900 kilos of nitrogen oxide."

The Marstal district solar heating plant provides heating for 1,450 end users in Marstal. But it draws attention from a lot more people. Being the world’s largest solar heating plant, the site is visited by professionals, journalists and tourists in a constant stream. It attracts more than 2,000 visitors every year.

"They come from all around the world," Holm says. "We’re very pleased with all the international acclaim. We’ve put a lot of effort into this and will continue to do so to reach our sustainability goal. We’ve come a long way since humping those first panels up on the swimming pool roof."

Marstal Fjernvarme and Alfa Laval

When the district solar heating plant in Marstal was started, there were some initial problems getting the system to run according to plan.

"At first we didn’t get the delta T right – that is, the cooling of the water in the district heating network," says Leo Holm, plant manager, Marstal Fjernvarme (Marstal District Heating). "These are problems that are inherent in district solar heating, and Alfa Laval solved them to our satisfaction."

"The problems," says Alfa Laval service engineer Henrik Juul, "are caused by the low temperatures used. There is usually only a two-degree [Celsius] difference between the primary and the secondary sides of the heat exchanger, which puts great demands on the unit. Alfa Laval’s heat exchanger plates feature the high heat-transfer efficiency needed to extract energy from the solar energy."

At the moment Marstal Fjernvarme has installed 10 Alfa Laval plate heat exchangers to transform the solar energy from the solar panels into heating that can be distributed through the district heating network’s 32 kilometres of pipes to the 1,450 end users in the town of Marstal. The heat exchangers also separate the solar system from the district heating system, one important aspect being to keep the solar system glycol from the district heating system. There are three brazed heat exchangers and seven gasketed plate heat exchangers in various sizes.

"We have worked with Alfa Laval since 1995," Holm says, "so there was never any question about choosing them as heat exchanger supplier when we upgraded the plant in 2003. Now we have run the new equipment for four seasons without any trouble."

Plate heat exchangers are the perfect choice in heating applications that must balance high demands for comfort, reliability and safety, such as encountered in district heating networks. Alfa Laval plate heat exchangers feature high heat-transfer efficiency and are able to handle the pressure differences very well.