Articles & Case Studies

VLT® AQUA Drive optimising district heating pumps – climate-friendly waste incineration plant saves power

Posted: Wednesday 17th August 2011

Aarhus Municipality’s goal of achieving a CO2-neutral heat supply is one step closer to becoming a reality. Two new energy efficient frequency converters from Danfoss have significantly increased the efficiency of the two district heating pumps located in the nearby town of Skejby, which send heat from the waste incineration plant in the town of Lisbjerg to the district heating net in Aarhus. As a result, Aarhus is getting closer to reaching its ambitious goal of becoming a CO2-neutral municipality.

The new frequency converters also run the plant’s existing motors at a much higher efficiency than previously and at the same time create greater stability, as the new converters work better with the motors than the old ones. The savings amount to 50,000 KWhrs per year. Danfoss has delivered a comprehensive solution which, in addition to the frequency converters, includes an extra cabinet for the connection of existing cables and a filter device that protects the motors.

Pumping climate-friendly heat

Ever since the Skejby pumping station was built in 1993, it has pumped cheap and climate-friendly heat from the Lisbjerg Incineration Plant to the district heating net for consumers in the Aarhus area. Residual heat is created as a result of the high temperature incineration of residential and commercial waste, which would otherwise be disposed of in a less sustainable fashion. Instead of allowing this heat to simply escape into the air, it is used to heat private residences, although the pumping station was in need of an overhaul. It had become too expensive to maintain the 15-year-old control system and the old frequency converters needed to be replaced. The two large 500 kW primary pumps had previously been speed-regulated by using the old less efficient frequency converters, which have now been replaced by two high-efficiency Danfoss VLT®frequency converters. The motor flux control mechanism of the new VLT® drives also runs the existing motors at higher efficiency than before, giving a two-fold benefit.

One pump is in constant operation while the other remains in stand-by so that in the event of interruption, it is up and running in just a few seconds. “Both district heating pumps are in good condition due to regular maintenance work. It would not make energy-saving sense to replace them. On the other hand, we’ve cut back significantly on energy consumption by optimising them”, says operating assistant Martin Jensen, who is responsible for AffaldVarmes Århus’ motor control systems and the Skejby Pumpestation.

Great benefits from EnergyBox2

The heat exchangers for the transmission net have also been removed in the optimisation process, so the pipe system from the incineration plant is now connected directly to the city’s district heating net. This results in additional energy savings and has increased the output transfer by 8 MW. A new PLC control with process image display and the installation of improved control regulation programmes have also contributed to the energy optimisation of the incineration plant. The SRO plant is connected to a Profibus network and all operating data is remotely monitored 24 hours a day.

Drive electronics have been greatly improved over the last 15 years. The new frequency converters have a 98% efficiency rate and provide significantly lower motor heat loss than the original converters. This helps lower operating costs. “With the new frequency converters and using the operating performance history which is automatically saved in the frequency converter, we have used Danfoss’ EnergyBox2 calculation programme to calculate savings of approximately 50,000 kWh per year”, Martin Jensen explains. The new converters were delivered with motor filters and features that allowed them to be installed on the existing base and also allowed all existing cables to be reused.

Back channel cooling

The converters are cooled using direct exhaust of cooling air from a separate back channel cooling system, which ensures that no impurities are sucked into the electronics. “The optimisation is a clear improvement of the plant and makes it easier for us to ensure a stable district heating supply for our customers”, comments Martin Jensen.

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