In 2017, Næstved Fjernvarme entered into a partnership agreement with the project owner Næstved Municipality in the southern part of Denmark. The purpose of the partnership agreement was to improve energy efficiency in buildings with district heating, minimise energy loss in the district heating system and ensure greater efficiency from the heat pump at AffaldPlus, an inter-municipal waste management company located in the south of Zealand, Denmark.

District heating systems set the pressure and temperature based on the needs of the customer in the worst position to ensure that all buildings receive the necessary heat/energy. It is important for energy efficiency that the return temperature from a building is as low as possible, and thus the district heating water is cooled as much as possible. This means that the energy content of the water is supplied to the building and used in it.

Improving cooling provides several benefits, such as less energy loss in the distribution system and at the individual customer, increased capacity in district heating pipes/system and greater energy efficiency in the heat pump installation.

All district heating customers at Næstved Fjernvarme have heat metres from Kamstrup, submitting data every hour to the central database. The use of data has meant that customers with poor energy utilisation were selected via a metering system on a daily basis. On the basis of this data, customers with the poorest energy utilisation/highest return temperatures were contacted to arrange a visit. After each visit, the customers were given a report with a description of defects and deficiencies in the customer’s district heating system.

A total of 1,221 visits were made during the project period, and some customers received more than one visit. The defects and deficiencies observed were:

• Defective valves/controls
• Limescaled/soiled hot-water tanks/domestic water exchangers
• Incorrectly set or defective installations
• Summer valve (operation error)
• Under-dimensioned radiators/underfloor heating

The visits showed that the usual cause of poor cooling is a defective district heating system. Among other things, Næstved Fjernvarme has introduced an incentive tariff to give customers a financial incentive to repair defective systems. The new tariff increases the variable tariff by 1 percent per degree Celsius when the return temperature exceeds 50 degrees Celsius.

The results of the project are illustrated by the temperature settings for 2017 (reference year prior to project start) and 2020. The average supply temperature at the inlet to the distribution system has been lowered from 85.3 degrees Celsius in the reference year to 73.7 degrees Celsius in 2020,which is a total reduction of 11.6 degrees Celsius corresponding to 13.6 percent. The average return temperature at the outlet from the distribution system in the same period fell from 48 degrees Celsius to 43.9 degrees Celsius, corresponding to 8.4 percent. The same trend is seen for temperatures at customers, as illustrated in the graph below.
More than 300 new district heating units were installed during the project period. This has led to energy savings, and lowering the return temperature has also improved the efficiency of the heat pump installed at Næstved Fjernvarme’s waste incineration plant. This means that it is now possible to produce approx. 800 MWh (2.880 GJ) more a year.

Total energy savings are thus 4.936 MWh (17.769 GJ), and with a heating price of DKK 518/MWh including VAT, this means that total annual savings for customers are DKK 2.6 million. Furthermore, customers have saved DKK 1.4 million including VAT in incentive tariffs. Overall, customers in Næstved have saved around DKK 4 million including VAT – every year.

Næstved Fjernvarme emits 0.127 tonnes of CO2 per MWh, and therefore total saved CO2 emissions amount to 627 tonnes. Laying out a district heating distribution system in a new area would cost approx. DKK 110,000 per building. Total energy savings in the project were calculated as standard consumption for 227 buildings. This means that 227 buildings can be connected without upgrading the existing distribution system. With some caution, it can therefore be assumed that DKK 25 million were saved by not investing in a new distribution system.