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At the 2013 plastics trade fair in Düsseldorf, Reifenhäuser Extrusion Technology will present a concept for a single-screw extruder that no longer requires water for cooling the intake area, but rather works with thermoelectric generators (TEG). The concept is based on the winning idea of the first web-based Open Innovation Competition in the branch, the Extrusioneers Innovation Contest held by the Reifenhäuser Group in 2011.
The winner, Norman Friedrich, proposed to convert the large temperature difference during plastics processing into electrical current with the aid of thermoelectric generators. The phenomenon known as the Seebeck effect enables the conversion and was initially employed in aeronautics for operating space and measuring probes far away from the sun.
Reifenhäuser Extrusion Technology has carried out series of tests on this idea in the last two years. The results have shown that operating temperatures of over 200°C in the ranges of cylinder, tool and piping tempering combined with the low degree of efficiency of the TEGs limit their possible uses for generating electricity in the extrusion (refer to the info box).
During the series of tests, the engineers further developed the original idea and found a more efficient utilisation of the TEGs. Namely, TEGs can function in the other direction: If current is applied to an element, a temperature difference arises – known as the Peltier effect.
Reifenhäuser Extrusion Technology had successfully installed such Peltier elements for cooling and targeted heating of the grooved bush area of a single-screw extruder. By skilfully combining the elements with ventilation blowers, it is possible to completely dispense with the usual water tempering in grooved bush extruders. This saves resources and frees producers from being dependent on water quality. Temperature control via Peltier elements is additionally very precise and also robust.
Although the concept is already functional today and offers clear technical advantages over the current solution, the practical implementation of waterless cooling is still more of a dream than reality. At this time, the comparatively high costs for Peltier elements inhibits the technology from being economically feasible. In the coming years, however, a significant drop in the costs of Peltier elements and further improvement of the degree of efficiency are expected. This is because Peltier elements are increasingly utilized in automotive engineering and CPU cooling. The engineers at Reifenhäuser are anticipating the elements being economic feasible by the year 2020.
At the K 2013 Reifenhäuser Extrusion Technology will present the finished concept within the scope of an extruder future study including further innovative features. The expo will be held from the 16th to the 23rd of October in Düsseldorf.
Limitaitions and chances for the use of thermoelectric generators (TEG) on the extruder
Optimum operation of TEGs
At optimum operation TEGs generate a power density of 1W/cm². TEGs operating at optimum require a heat flux density of approx. 15 to 16W/cm² (approx. 6% degree of efficiency). From this thermal power, 6% is converted into electrical power. However, 94% has to be dissipated. If this energy dissipation has to be forced, instead of arising from a natural work environment such as in space or by the air flow over a vehicle, it has to be carefully checked whether the costs of forced cooling are larger than the benefits:
Generation of dissipation power at the extruder
It would be advantageous, if the excess dissipation energy could be used for the operation of TEGs in the ranges of discharge and homogenisation of the extruder. For single-screw extruders dependent on screw geometry, screw speed and polymer properties, one finds:
Taking into account the cylinder surface of commercially available extruders, the currently required optimum power density of 15 W/cm² for energy recovery is 3-10 times larger than what is feasible at this time.
Chances for the use of thermoelectric generators (TEG) on the extruder