Two Energy World Records in Vojens



















The town Vojens is known all over the world for Vojens Speedway Center, starting with Ole Olsen, world champion several times.

Today Vojens is also known to be the solar city number one. The local consumer-owned district  heating company Vojens Fjernvarme is in 2014/2015 in the process of establishing the world largest, 70,000 m2, solar heating plant and the world largest, 200,000 m3, underground thermal seasonal heat storage in an old sand pit.

Both records is a factor two larger than the next largest. However, the record will not last long, as several similar projects combining solar and seasonal storage are in the pipe line. Soon the this storage technology may also store surplus useful heat from industries and waste-to-energy plants, which else would be wasted.

The huge storage will be operated as an interseasonal heat storage allowing the solar heating plant to deliver more than 50% of the annual heat production to the network. The rest of the heat will be produced by 3 gas engines, a 10 MW electric boiler, an absorption heat pump and gas boilers.

The storage is excavated in an old sand pit. The 200,000 m3 water volume will be seperated from the district heating water by a heat exchanger. A huge “plastic bag”, formed by a special welded plastic liner, will ensure that the water does not dissapear into the sand and remain clean. The surface of the water will also be covered by the liner and moreover an insulated cover and draining system to remove rain water.

The maximal temperature is in principle 95 dgr. C, but it is planned not to be much higher than 80 dgr. C in order to prolong the life-time of the liner.

On the picture we see that the most of the liner is installed and that the pipe construction for warm inlet on top and colder inlet in the bottom is under construction.

Ramboll has in the design introduced several improvements based on experience from the previous smaller pit storages.

Special attention is paid to:

  • protection of steel pipes against corrosion
  • keeping the water clean without organic material
  • damaging the insulated cover
  • installation of the liner
  • production of process water

The cost of heat in winter from the solar heating combined with the interseasonal heat storage is competitive against the heat from gas boilers, due to economy of scale.

The plant is purely commercial without any subsidies except the indirect subsidy in terms of energy tax on gas.

The solar plant is almost completed, and the pit almost filled with treated water. We hope that the storage pit will be ready when the sun starts to shine and warm up the panels this summer.

For more information, contact Flemming Ulbjerg




The Hidden Backbone of the Liveable City

Ramboll Hidden back bone in the liveable city


Imagine you are standing on a green roof top. You can-not see, hear or smell any sign indicating how the energy is generated and what happens to waste water and waste. Only in the distance you notice some activity in an industrial area. In the farm land there is no landfills, no poluted rivers and no power plants. Instead the farmers produce surplus straw and wood to be processed at the large energy plants in the outskirts of the city.

What is the secret ? Very few know it and it is difficult to explain. It is my experience that one simple picture can explain more than a big report, and thousand numbers. Therefore I got the idea to this picture, showing a little but important corner of our “liveable city” and how it interacts with the farm land and the rest of the world through markets for electricity, gas and other fuels delivered by ship.

It is a challenge to create liveable cities. Cities, which are not only sustainable, but also worth to live in. In such a city we take it for granted that all energy and environmental services are carefully planned and implemented to be efficient, environmental friendly and not least cost effective. The more cost effective the more money we have available for our national and private budgets and thereby increasing our welfare.

In our vision of a liveable city, the energy infrastructure is a hidden back bone of the city. Thanks to this infrastructure for electricity, district heating, district cooling and natural gas, hidden below the ground, you cant see, hear or smell any sign of energy production. The urban area reserved for human activity.


  • The power grid supplies all buildings, except remote islands and transmits mainly renewable electricity from wind turbines and CHP plants to the buildings.
  • The district heating grid delivers surplus heat and renewable heat to all buildings in the city, whereas buildings outside, which are too expensive to supply with district heating are supplied with individual heat pumps and building level plants.
  • The district cooling grid supplies efficient cooling to buildings in the city center, to commercial buildings and to institutions, also limited by the criteria of cost effectiveness.
  • The gas grid supplies the power plants and industries with process energy.


Thus the energy grids have the same importance as water and waste water in modern cities.

Thanks to the grids for energy and environment the plants, which produce our energy and solve our environmental problems can be allocated in certain areas dedicated for this activity and benefitting for the synergies among them.


  • The waste water treatment plant delivers biogas to the gas grid and the sludge incineration plant heat to the grid.
  • The waste incineration plant use all waste, which can-not be recycled, to generate heat and electricity to the grids.
  • The CHP plant generates electricity and heat to the grids based on gas, coal and an increasing share of biomass.
  • The district cooling plant generates cooling and heat to the grids based on electricity or it generates cooling based on surplus heat in the district heating grid.
  • The hot water storage optimizes the heat production day to day or even longer
  • The cold water storage optimizes the cooling production and reduces the need for installed capacity
  • The cooling plant could include an aquifer storages for cold and warm water and there could be access to sea water cooling
  • The district heating could have access to deep geothermal heat.
  • The industries, which have process energy consumption, could use all the energy services or they could deliver surplus capacity and energy to either district heating or district cooling.

Our buildings have of course to be energy optimized with a good in-door climate and be able to use low temperature district heating and high temperature district cooling.


As a result we can use our green top floors and enjoy a view to other green and read roofs without any chimneys and other appliances. Only in the distance we can observe some tall stacks with white clean smoke and rotating wind turbines indicating where our energy services are generated.