Sondex® heat exchanger technology and VLT® drives from Danfoss ensure world-class efficiency in pump control and heat transfer at the world’s largest solar hot water system. The plant harnesses energy to heat the homes and workplaces of 40,000 citizens, supplying 18-20% of the annual heat consumption in the city of Silkeborg, Denmark. The plant reduces CO2 emissions by 15,700 t annually, helping Silkeborg to reach its ambitious target of CO2 neutrality in heat production by the year 2030.
The Silkeborg plant is designed to produce 80,000 MWh of heat annually. Solar water heating was chosen since it allows storage of solar energy harvested in daylight for use at night or at a different time of year. This extends the added value of the sun and makes solar solutions even more profitable.
The Silkeborg solar water heating plant contains 22 km of piping which links together 12,436 solar heating panels, installed over an area of 50 ha. The solar field is built in four independent sections, to ensure maximum operating reliability. If an operating problem arises in one field, the operators isolate it and then run on the other three.
The plant is designed for a lifetime of 25 years. It is a highly efficient plant, which is 4-6 times more effective than residential solar water heating systems installed typically on rooftops of private homes.
The solar hot water system runs on Sondex® heat exchangers and VLT® drives from Danfoss, which have powered a 30% cost reduction in its first year of operation, compared with traditional drive systems.
Reduced pump energy consumption
Four large pumps run continually in parallel to distribute the hot water to consumers. In addition, four more pumps are available on standby as a backup, should one of the pumps in operation need to be replaced. All eight water pumps are controlled by VLT® AQUA drives to maintain their energy consumption at an absolute minimum.
Successful transfer of energy
A total of four heat exchangers delivered by Sondex® are connected to the solar heating plant. The model is named S221 and has between 884 and 936 plates. At the utility in Silkeborg, the buildings are adapted to the size of the heat exchangers, specifically designed for this application due to the height differences of the landscape.
Silkeborg could have selected a smaller heat exchanger size, but then they would not have achieved the same close temperature on the primary and secondary sides as in the four major ones, which were chosen by the solar panel supplier Arcon Sunmark.
Silkeborg District Heating Utility decided to create a PN10 system, and consequently, the heat exchangers were calculated according to the pressure drop in the solar panels. The Sondex® S221 exchanger is currently the tallest model with connection size DN200 from Sondex.
By having a high temperature differential, it is possible to operate at a lower flow which means that it is not necessary to invest in larger pumps. At the same time, a small LMTD (Logarithmic Mean Temperature Difference) can maintain the temperature on the district heating side as close to the temperature of the solar heat side thereby enabling the transfer of as much energy as possible.