Image: One heat pump at the location of Marienhütte.
A steel and rolling mill used for increased environmental friendliness. That might sound strange, but is actually happening in Graz, Austria, where the mill is connected to the district heating grid. One part of the success story is two large and highly efficient heat pumps installed at the mill. Thanks to them, excess heat of temperatures as low as 30 °C can be utilized for district heating.
This excess heat from the steel production is then used by the energy service provider and district heating network operator Energie Graz GmbH & Co KG (“Energie Graz”). The steel and rolling mill, Marienhütte GmbH, needs high temperatures for the steel production, and these high temperatures creates a need for cooling to avoid overheating. Instead of letting this thermal energy go to waste, it has been introduced into the district heating system.
The cooperation between Marienhütte and Energie Graz started already in 1992. Waste heat of temperatures up to 100 °C was then directly introduced into the system. During the years, the size of the cooperation has expanded, and since 2011 the annual delivery is some 60 GWh, making up for approximately 5% of the total heat supply through the district heating grid.
Through the introduction of heat pumps in 2016 this number has now grown even larger, as waste heat with temperatures as low as 30-35 °C is used. The installation consists of two large R1234ze heat pumps. They each have two turbo compressors which can be operated in parallel or serially, with the higher heating capacity related to parallel operation. Together, the heat pumps have a total heating capacity of up to 11.5 MW. The estimated annual input to the district heating network is 40,000 MWh. As the heat pumps are supplied with renewable electricity, their introduction into the system is estimated to lead to an annual decrease of CO2 emissions with up to 11,700 tons.
The next step for cooperation is already planned. A new low-temperature district heating network will be built in Graz during the upcoming years. This will be decoupled from the existing grid, and support about 12,000 inhabitants in the new district Reininghaus. The plan is to provide this new grid mainly with heat from the newly installed heat pumps. In this system, the needed supply temperature is lower, 70 °C as compared to 95 °C in the existing network. This leads to an increase in COP from 3.3 to 4.5, which is an improvement of 36%.
Alexander Arnitz and René Rieberer, Austria (Institute of Thermal Engineering, Graz University of Technology) Veronika Wilk, Austria (Austrian Institute of Technology GmbH) Helmut Unger and Peter Schlemmer, Austria (Energie Graz GmbH & Co KG Austria)