The key is to use direct current. By passing a direct current in a coil, a static magnetic field is generated lossless. A static magnetic field does not in itself generate currents in the metal to be heated, but by rotating the metal, currents are induced and the associated resistive losses generate heat. The rotation makes the metal, in this case a massive cylinder (a billet), “experience” a time-varying magnetic field, which is required to induce currents. A motor rotates the billet and thereby mechanical
energy from the motor is transferred into heat in the billet. Electrical motors in the 500 kW range have energy efficiencies above 90% and the overall efficiency of the system becomes around 90%. This number is to be compared to 55-60%, which conventional state-of-the-art induction heaters operate at.
With the basis in the new idea and some preliminary studies, SINTEF Energy Research has initiated and established a consortium to develop the technology towards an industrial product. The consortium, which consists of selected European universities, research institutes and companies (see the box), has received support from the European Commission for a three-year project. The aim of the project is to design and build a prototype to be tested at a Polish extrusion plant.
There are several challenges in the project, especially when it comes to the mechanical gripping device that should hold the rotating billet, and the superconducting coil system. A two decimetre in diameter and six decimetre long billet is rotated with up to 3000 rpm in a strong magnetic field. When rotating the billet, a large torque is acting on the grippers and on the coil system. The superconductor to be used for the coils is magnesium diboride, discovered as recent as in 2001. In addition, and this may be the most important issue, the entire system is to be designed for a tough industrial environment.
Innovative researchers, Niklas Magnusson (left) and Johan Skjølberg, Photo: Mette K. Høiseth
Designing superconducting coils
The superconducting coil system will be designed and constructed by SINTEF Energy Research. The coil design involves sophisticated three-dimensional numerical modelling. The coil system provides the means of controlling the magnetic field and the temperature at different positions in the billet. Magnetic field strength, rotational speed and coil geometry are all closely linked together.
The coils need to sustain major electromagnetic forces, as well as cooling to low temperatures. The low temperatures cause thermal contraction and pose special demands on the materials to be used. A measurement system has been developed to test different coil configurations for temperatures down to 20 K (-253 °C), and initial experiments with the new magnesium diboride superconductor are under way.
The lossless conduction of electric current has been the driving force behind comprehensive speculations of the use of superconductors in the future power system. However, two properties of the new materials have to a large extent limited the progress.
The first is the operating temperature of the superconductor, and the second is the behaviour of the superconductor under AC operation. Superconductors have to be cooled to very low temperatures to become superconducting. Typical operating temperatures are -196 °C (the boiling point of liquid nitrogen) or lower. Maintaining a material at such low temperatures does not present a major problem. However, a superconductor is only truly superconducting under DC conditions. Under AC conditions, which prevail in our power system, losses will always occur. Cooling of these losses is expensive and has so far prevented a wide use of superconductors in power systems.
There are two ways of handle these problems:
- One is to reduce the AC losses in the superconducting material to an acceptable level
- The other possibility is to simply use DC.
Induction heating of aluminium
Induction heating is used to heat metals and other electrically conducting materials by applying a time-varying magnetic field which induces currents in the material to be heated. The magnetic field is normally generated by an AC current in a copper coil. At aluminium extrusion plants billets are pre-heated by induction heaters before being extruded to profiles. When heating aluminium the energy efficiency is poor, due to large losses in the copper coils.
Induction heaters have power ratings of up to and above 1 MW. As nearly half the supplied power is lost in the copper coils, the losses correspond to an annual cost of almost 1 MNOK for a standard induction heater.