Ultrasonic vibrations could bring
energy efficient clothes drying

Homes owners who have grappled with the nuisance of vibrating clothes washers and dryers may be surprised to learn that coming generations of appliances could use vibrations as a way to boost energy efficiency.

So say researchers at the at Oak Ridge National Laboratory (ORNL) in Tennessee. They have come up with a way to use ultrasonic vibrations, rather than heat, to dry clothes. This new technology is expected to dry clothes in half the time and use 70% less energy than conventional dryers.

Ultrasound waves have been used for years to dry a variety of media ranging from cardboard to food. The usual drying mechanism is the creation of small cavitation bubbles of liquid that result in a release of moisture. In contrast, the ORNL technique essentially forces moisture off fabric by shaking. The ultrasonic driving force must overcome the inertia of water and a resistive force caused by capillary action in the fabric.

ORNL researchers say ultrasonic clothes dryers can be more energy efficient because conventional evaporation-based dryers must overcome the latent heat of evaporation in clothing and thus ideally can dry about 3 to 3.7 lb. of clothing per kilowatt-hour of energy (2,453 to 2,257 kJ/kg of water). Considering losses, existing dryers perform at 54–66% of their theoretical maximum efficiency, ORNL researchers say. For the same amount of kilowatt-hours the new dryer is three to five times more efficient, claims chief ORNL researcher Ayyoub M. Momen.

ORNL and GE Appliances are in the process of scaling up this technology. The approach uses piezoelectric transducers vibrating at ultrasonic frequencies to turn water into a cool mist as it’s removed from the fabric. ORNL’s Momen says the prototype could dry a piece of fabric in just 14 seconds. It would take several minutes to dry the same piece of fabric in a conventional dryer, he says.

Momen says researchers are now designing the final form factor of the dryer and finalizing the piezoelectric transducers. The first prototypes had the look of a clothes pressing machine with a plate containing the piezoactuators put up against the cloth to be dried. But machines designed for commercial use are more likely to resemble conventional tumble driers but with piezoactuators embedded in the chamber walls.

Another factor undergoing development work is the piezoactuator drive electronics. Conventional piezoactuator drives can be relatively simple. The piezoactuators themselves are highly capacitive and generally operate in a tank circuit that generates the vibration frequency. But piezoactuators for drying clothes must work in what ORNL calls burst mode, in a partial duty cycle. It turns out that accelerating fabric in short bursts dries it more efficiently than vibrating it continuously. So ORNL researchers are trying to optimize the length of the bursts and the shape of the piezoactuator driving signals.

However, the toughest technological problem is the design of the piezoelectric transducers, says ORNL’s Momen. The prototype uses a mesh of 24 piezoactuators which have been optimized for vibrating fabric. Momen says past that, he can’t elaborate because much of the IP is proprietary. Most of the technical work takes place at ORNL, though GE provides equipment and gives development guidelines. The two entities will collaborate on a final design. Momen says it is tough to predict when an ultrasonic dryer might reach the market but he thinks five years could be a possibility.