LOW-TEMPERATURE DISTRICT HEATING FOR OLD AND NEW BUILDINGS

Albertslund Utility, Denmark, has cracked the code to lower the district heating temperature all over the town. With shunt valves and temperature zones, the utility can lower the temperature in the 60-year-old grid without changing the existing pipes and upgrading the 6-bar net to a 10-bar net. With shunt valves, both the temperatures and the pressure in the grid can be lowered simultaneously. And the flexibility gained by temperature zones is highly valued by the utility.

By Niels Hansen, Senior Consultant, Albertslund Municipality

Published in Hot Cool, edition no. 6/2024 | ISSN 0904 9681 |

Low-temperature district heating (LTDH) has been in operation for several years in Denmark. Also, in Albertslund, newly built and energy-refurbished housing areas have been supplied with LTDH for the last 15 years. LTDH significantly reduces heat loss and improves the opportunities for exploiting local renewable energy sources, such as industrial surplus heat or geothermal energy.

Albertslund is a town located 15 km west of Copenhagen with around 30,000 inhabitants. Most of the town was built in the 1960s and 1970s. However, in recent years, urban development has been happening in three major areas of Albertslund. The goal is 45,000 inhabitants by 2045. Despite all the new buildings being built, the main concern for the utility has been how to lower the DH temperature for the existing buildings.

When new housing areas in Albertslund were built or when housing areas were energy refurbished, the DH temperature was lowered. It happened with a shunt valve, supplying LTDH to the specific area while higher temperatures were still supplied to the rest of the town. The number of LTDH areas grew.

”In 2016, the municipality of Albertslund agreed on a strategy for 100% LTDH in Albertslund by January 2026. The social housing areas, covering more than 50% of the housing stock, were thoroughly energy refurbished with support from the National Building Fund of Denmark. 

In 10 years’ time, half of the homes of Albertslund would be low-energy buildings with no need for high DH temperatures. Should we continue with two grids – one for high temperatures and one for LTDH? Or should we try to go for 100% LTDH? We chose LTDH, and we informed all building owners two times by mail: Be ready for lower temperatures by 2026”, explains Catarina Nørgaard Marcus-Møller, head of the district heating utility of Albertslund.

A long-term strategy for lowering the temperatures

How to get started? Albertslund Utility created an action plan to fulfill the DH strategy. Initially, there was a lot of focus on energy consulting and guiding the end users. Energy refurbishment concepts were created for privately owned terraced houses. However, it took a lot of effort, and the results were limited. Things simply went too slow.

The next step was changing all the heat meters to intelligent meters, which provided the utility with a lot of data to analyze. Then, a new service was started, where the end users could rent a new DH unit from the utility. To participate in the service, house owners need sufficient radiator capacity and adjustable valves on the radiators.

Catarina Nørgaard Marcus-Møller, Head of the District Heating utility of Albertslund, explains, “The question was how to move a step further towards LTDH in Albertslund. We came up with the idea, and with the help of shunt valves, to divide the grid into smaller islands or temperature zones. Thereby we can target our work towards smaller limited housing areas. 

We can lower the temperature stepwise in one single area without other areas having to wait for lower temperatures. With this approach, we had the opportunity to test, or demonstrate, the REWARDHeat project, funded by the EU Horizon 2020-programme.”

An EU project showed the way

In 2020, with the REWARDHeat project, Albertslund Utility installed a Grundfos iGrid-shunt at the end of a typical Danish street with one-family houses from the 1960s. The DH temperature was lowered step by step, with the return temperature from all 104 houses being monitored. Troubles were expected but never really occurred. 

In February 2021, the whole street was supplied with 60 degrees LTDH, even though the outside temperature was below zero. Five houses had needed consulting. In three houses, smaller adjustments were made without changing any components. In one house, a smaller component was changed. And in the fifth house, the heating unit was changed.

We had the opportunity to test this approach or demonstrate it in the REWARDHeat project, funded by the EU Horizon 2020 program.

The learnings from REWARDHeat and the street named Porsager strengthened the belief in fulfilling the strategy of lowering the temperature in all of Albertslund. Lowering the temperature by 20-30 degrees on a cold winter day in an ordinary Danish housing area was much easier than expected. Due to the REWARDHeat experience, the last two housing areas of Albertslund, heated by natural gas, were converted directly to LTDH areas in 2023.

The use of shunt valves became permanent when the benefits became clear

The work with shunt valves has changed in Albertslund. Initially, shunt valves were regarded as a temporary component in the grid, awaiting the lowering of the temperatures in the DH grid. Today, shunt valves have become a permanent part of the DH grid. With no grid division, the DH plant’s temperature should be much higher than 60 degrees to provide the last customer with 60 degrees DH. With this approach, many of the end users would never reach LTDH.

Instead, Albertslund Utility will supply higher temperatures in the main pipes and then lower the temperatures with shunt valves to the end users. Even if the temperatures in the main pipes exceed 60 degrees, the temperature will be much lower than today. Albertslund Utility expects to have around 35 shunt valves in operation in a few years.

The heat loss is bigger in the small pipes

With the use of shunt valves, more end users will be supplied with LTDH – compared to a central lowering of the temperature. Thereby, the temperature is lowered in a greater part of the entire net. The shunt valves increase the overall efficiency and limit the overall heat loss in the grid. 

The heat loss in the main pipes is smaller compared to the heat loss of the smaller pipes supplying the end user with DH. In Albertslund, the heat meters are placed indoors by the end users. Therefore, all the heat loss in the grid is covered by the utility – or included in the heating price.

Shunt valves improve the flexibility

Probably the most important thing learned from the work with shunt valves is the flexibility obtained in the grid—flexibility concerning temperatures and flexibility regarding the pressure in the pipes. For example, if one housing area needs higher temperatures on a cold day, Albertslund Utility can increase the temperature in this area while keeping the temperature lower in other areas.

The DH grid of Albertslund has been in operation for 60 years and is dimensioned as a 6-bar grid. When lowering the temperature, a given need for energy will result in a larger flow of water. It can be solved with larger pipes and/or with higher pressure.

Traditionally, the DH pressure is created from the DH plant or with strategically placed booster pumps in the grid. However, according to the local circumstances, shunt valves can increase the pressure locally for each individual DH area. Therefore, Albertslund Utility can lower the temperature in the existing 6-bar grid without needing to upgrade to a 10-bar grid. It would have been costly for the utility and for the end users who were still directly supplied with DH without a heating unit.

There are also downsides to working with shunt valves: They take up space. They need service. They cost money. The division of the greater old grid into smaller islands means a limited number of buildings will never have LTDH. However, the temperature will be lowered for all end users.

The transition to LTDH is one of Albertslund Utility’s contributions to the green transition of Albertslund Municipality. By lowering the temperature, the DH supply was made more efficient with less heat loss and improved the possibility of exploiting local renewable energy resources. Overall, Albertslund’s DH is prepared for efficient interaction with the electricity market.

Albertslund Utility believes that if one day a geothermal plant is considered, the heat from the underground can be exploited as DH with no need to increase the temperature. This improves the business case significantly. Smaller quantities of surplus heat will also be easier to exploit. Since 2015, surplus heat from a data center in Albertslund has been exploited. Albertslund expects a greater part of the DH base load will be covered by surplus heat in the near future.

Involvement of end users

The DH Utility of Albertslund has, since the beginning in 1964, been operated by the municipality. Since 1980, the User Council, with representatives from all housing areas, has been asked on all new initiatives, before they were approved by the city council. Direct citizen involvement has played a major role in the development of Albertslund Utility.

The REWARDHeat project, which established a shunt valve on the street named Porsager, was carried out in close cooperation with the homeowners’ organization. Meetings were held with the homeowners, and they had a direct phone number for an energy consultant during the process.

”Finally, the service of renting out heating units has proven to be a major success. Most end users come to us when their heating unit needs replacement. More and more, we deliver comfort, rather than ‘just’ DH, to our customers”, Catarina Nørgaard Marcus-Møller says.

Three good advice

• Find out what temperatures are really needed. There is a lot of focus on keeping the temperature on the coldest day; however, how often is it really that cold? If we cannot supply our end users with LTDH all year, then LTDH 350 days/year is far better than high temperatures all year.
• Communication and open dialogue with the end users. Be reachable. Pick up the phone. Show up if invited. Be present.
• With the service of renting out heating units, it is easier for the end users to obtain a well-functioning, high-quality heating unit. There is no money upfront; it’s a pure subscription. The service brings the utility closer to the end users.

4 things we have learned

• The buildings perform much better with LTDH than expected
• Improved flexibility with shunt valves and the division of the grid into smaller islands
• Minimal need for upgrading the grid when lowering the temperatures
• DH end users are generally positive towards the transition into LTDH

For further information, please contact: Niels Hansen, nej@albertslund.dk