In the Danish town of Vojens, a 70,000 m2 solar heating plant is being established. The heat is absorbed to heat water, which is then transported to a 200 million litre pit, for storage. Here, the water is saved for district heating consumers to use during the autumn and winter season. Vojens District Heating and Ramboll have designed and constructed the record breaking project.

A floating cover and the world’s largest water bed

Vojens is already famous for world records in speedway, and the town in South-Jutland is now also recognized for two world records in energy. Vojens’ consumer-owned district heating company is establishing a 200,000 m3 heat store in an old gravel pit. The project is driven by an extension of the existing solar heating plant from 17,500 m2 to 70,000 m2, which makes both the storage and the solar heating plant the world’s largest. It is hardly a secret that the sun does not always shine in Denmark.

Therefore, the construction of a seasonal thermal store, where the energy can be deposited, is valuable and convenient. The 13 metre deep storage pit in Vojens has a circumference of 610 metres. It takes about five months to fill the pit to its maximum of 200 million litres with a pumping capacity of 50,000 litres of water per hour. A few metres of surrounding soil are protected against rain water, and therefore the soil will form a several metre thick insulation to the storage pit.

One challenge was to construct an insulating floating lid to cover the entire surface of the pit. The floating cover makes it possible to store the hot water for the Danish winter season when consumers turn on the radiator. The double-sealed bottom plastic liner is supplemented by a floating top liner covered by a 60-cm layer of insulating expanded clay, which again is covered by a top-liner. So besides an insulating floating cover, we have also created the world’s largest water bed, says Flemming Ulbjerg, Senior Consultant and solar heating expert at Ramboll’s Energy division.

Supplying 2,000 citizens with district heating

The storage pit together with the extension of the solar heating plant will be finished and ready for operation in May 2015. It will supply 2,000 district heating consumers in Vojens with solar heating, which corresponds to 45 pct. of the annual district heat consumption.

The large-scale investment will provide consumers with savings of 10-15 pct. on the annual heating bill, and the plant will save the environment from 6,000 tonnes of CO2 per year.

Other Danish cities will follow suit and store the sun’s heat

Besides Vojens, three other cities in South-Jutland will convert heat production to solar heating in the coming years. Gram District heating, not far from Vojens, is establishing a 125,000 m2 storage pit and extending the 10,000 m2 solar heating plant to 44,000 m2. Furthermore, Toftlund District Heating is planning a 70,000 m3 storage pit for storing solar heat and waste heat from industry. Loegumkloster is looking to expand as well, and each individual solar heating plant will be bigger than any other solar heating plant in the world. Ramboll is involved and provides consultancy on all four projects.

The reason why we can implement large-scale solar heating facilities in Denmark, which is not exactly known for its sunny weather, is that we have many district heating plants which can use the heat. This is a unique thing for Denmark and it will probably take a while before we see solar heating plants this big in other countries, says Flemming Ulbjerg and continues: We expect that this new approach to long-term storage of summer heated water for the cold season will be booming in the coming years. Since the cost of solar water heating from large plants is four to six times lower than that of heat from small plants, these large plants are now competitive with some biomass boilers and many electrical heat pump solutions, he says.

Solar heating has world-wide potential

Solar heating is one of the most well-known and environmentally sustainable alternatives to traditional energy sources. Since the early 1990s Ramboll has been involved in more than 20 different solar heating projects in ten countries around the globe, including the establishment of 15 of the largest solar heating plants in Scandinavia. The life span for these facilities is expected to be 30 years.

According to Flemming Ulbjerg, the demand for large-scale solar heating plants is expected to grow in countries with warm climates, a centralised heat supply and a progressive climate change policy. Besides Denmark, a developing progress within solar heating is seen in Germany and the U.K.

South-Jutland Stores the Sun’s Heat in the World's Largest Pit Heat Storage