Aars Golf Club, which is located in the beautiful nature of Jutland, Denmark in Europe, formed the framework for an ELFORSK project, which aimed to investigate the use of return water in the district heating network as a heat source. A heat pump was installed on the return pipeline, and the golf club was connected via a pipeline to the eight kilometre (5 miles) transmission line running between the cities Aars and Hornum. The clubhouse was provided with the re-circulated return water that had already been used for heating the town of Hornum.
The idea was to optimize and improve the efficiency of the utilization of energy in the district heating system. The re-circulated water often runs back to the district heating facility with relative high temperatures, after it has been utilized in central heating systems at small and large customers.
Multiple customers using the same DH network
A heating network has a certain capacity – a maximum amount of energy that can be distributed. This capacity is for traditional district heating determined by supply temperature and the flow itself. When the supply water has delivered its energy, the general assumption is the energy in the network is depleted.
However, there is also energy in the water that returns to the plant. The temperature of the return water is typically between 25°C to 50°C (77°F to 122°F) and especially at the higher end of this range, low grade energy is still available. This provides an opportunity to sell more energy and at the same time improve the efficiency of the district energy system.
For the ELFORSK project, the Aars Golf club installed a 16 kW (54 mBtu/hr) heating pump of the same brand as the ones used in a standard geothermal pump system. For this application, the pump is used to supply the return water from the district heating network instead of geothermal hoses on the cold side of the plant. Since then, the facility has heated the golf club's clubhouse and workshop – just as it has provided the facility with heated domestic water all year round.
The ELFORSK project demonstrates many positive gains for district heating utilities. The Aars district heating system can utilize energy that has already been produced, more efficiently. This can be done without extending their distribution network. Because of the utilization of the return water heat, the Aars district heating utility now has capacity to connect several additional customers to the network without increasing capacity.
Cooler return water is a big advantage
The re-cycling of the return heat means that the heating plant eventually receives return water at a lower temperature than prior to the ELFORSK project. This is another added gain when the return water has to be re-heated before it again is sent out to the district heating customers.
The ELFORSK project has, in the specific example of the Aars district heating system, calculated that if the temperature in the return water of the plant is lowered from 35°C to 25°C degrees (95°F to 77°F), this would have the potential of an efficiency improvement for the plant of 1.35 MW (4,6 mmBTU/hr). This corresponds to a profit of app. 300.000 euro annually (USD $360,000).
The ELFORSK project was the result of a broad collaboration between many players. NIRAS was the lead consultant and project manager conducting all the analysis of the project results. The Aars district heating company delivered the return heat to the plant from the Aars Golf club. The Danish Heat Pump Industry (DVI) supplied the heat pump system and was responsible for optimization of the operation of the heat pump system.