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A nationally unique district heat storage developed in Varkaus

District heating company Varkauden Aluelämpö is planning a district heat storage investment that is unique at the national level. The idea is to store district heat in rock using heat pump technology. Sweco is responsible for the challenging preliminary design of this timely project.

In many ways, Varkauden Aluelämpö is a pioneer in the energy sector. The company has succeeded in reducing the carbon emissions of its district heat production to a fraction of what they were a decade ago. Next, the company strives to get rid of light fuel oil. Varkauden Aluelämpö is planning an innovative district heat storage that optimises the operations of its district heating network.

“Our number one goal is to promote the practical use of energy produced by the Riikinvoima waste-to-energy plant and balance the production and demand of energy even better,” says Mikko Onkalo, CEO of Varkauden Aluelämpö. Winters are no longer as long and cold as they used to be due to the great variation in heat loads caused by climate change. “Heat is wasted even during the heating season. The recovery of waste heat must become more efficient.”

District heat is stored in rock

The company’s long-term development has already paid off in Varkaus and the energy loss of the district heating network has been reduced. The new heat storage will help Varkauden Aluelämpö achieve the full potential of the network. The most important thing is to optimise the temperature of district heating return water at all times.

“This boosts the flue gas scrubbers of the heating plant, which means more efficient recovery of particulate emissions and waste heat,” explains Tuomas Salminen from Sweco. In practice, a larger quantity of energy is recovered from the plant for practical use.

The heat storage will utilise several technologies and processes. “A geothermal heat pump of this size has never before been connected to district heating while using heat pump technology simultaneously,” says Salminen. The new heat storage will be the first storage of this scale in Finland. “There are only a couple of storages of this size in the whole world.”

The idea is to store extra district heat in rock whenever possible and utilise it using heat pumps whenever necessary. “This solution is an answer to the common issue at CHP and waste incineration plants where the need for heat and heat production is often unbalanced,” says Riku Maidell, Energy Solution Specialist at Sweco. The challenge of co-producing electricity and heat is that electricity must be produced and waste must be incinerated throughout the whole of the year. “This generates waste heat. The purpose of this project is to recover this heat and store it for the cold season.”

Investment decision will be made in late 2021

As the preliminary planners of the heat storage investment, Sweco’s experts will be collecting information for procurement surveys and the investment decision to be made in late 2021. “We will design a working system, scale it and prepare a cost estimate,” says Maidell. “All the technical solutions are optimised to ensure the maximised efficiency of district heating production.”

A duplicable energy storage solution

The total cost of the investment will be moderate compared to the cost of digging a rock cave or building an underground water supply, for example. According to a preliminary estimate, the scale of the investment will be MEUR 3–5. “As a whole, this is a very sensible and profitable solution,” Maidell estimates. “The quantity of waste energy generated is so large that the investment will pay for itself in a short period of time.”

Maidell emphasises that this energy recovery solution has also national significance. “A system optimised to meet the needs of Varkauden Aluelämpö can be duplicated and adapted to other energy companies’ and regions’ needs. If realised, the investment will indirectly support Finland’s carbon neutrality goal.”

Image: Riikinvoima