There are many ways to convert freely available ambient energy into usable electrical energy. One of them, which so far has led a shadowy existence, could possibly still make it big: the capacitive transducer. So far, it has practically only been constructed on a microscopic scale with outputs of a few microwatts. Larger models have so far seemed utopian, but that is about to change now. The latest developments are intended to provide output powers of up to 100 W.
The core element is a capacitor with a variable capacitance - for example with plates that are move with respect to each other. It is charged to a certain voltage and then isolated from the outside world.
If the plate spacing is increased, the capacity is reduced. However, because the charge remains constant, the voltage rises accordingly. At the same time, the stored energy increases in proportion to this, because force must be used to pull the attracting plates further apart. This makes it very easy to convert mechanical energy into electrical energy.
If this process runs periodically, it can be used to generate continuous power. In a state of low capacitance, i.e., high voltage, the charge is discharged via a switch to a second capacitor of fixed size. After charge equalization, the switch opens again and the capacity is increased by approaching the plates. The capacitor is reconnected to the input voltage, recharged, and the whole thing starts again.
As with a charge pump, the second capacitor gradually charges up to a saturation value. With regular repetition - stretching, charging, relaxing, discharging - a continuous current can be drawn. For the very first start, the system needs an externally supplied voltage, after which it can supply itself.
The generated power depends on the switching frequency, the input power, the maximum and minimum capacitance, and the inserted mechanical power. The converters of this type built up so far are far behind other types of converters in terms of power. However, their advantage is that they can be constructed in a micro-miniaturized form, i.e., in single-crystalline silicon using standard MEMS technology - with electronic functional elements on the same chip if required.
Silicone Film as Capacitor
If you want to build capacitive transducers in a large version for high power, the first question is how to find suitable capacitors. They must have both a high basic capacity and a large capacity variation ratio. Researchers at the Fraunhofer Institute for Silicate Research in Würzburg in the CeSMa (Center for Smart Materials) department had a creative idea.
Silicones - highly elastic plastics with some extraordinary properties - are a major field of research at CeSMa. Thin films made of silicones are ideal for this purpose. They can be stretched by several hundred percent without tearing and are also very stable over the long term and highly resistant to electric shocks.
Such a foil is coated with graphite on both sides, followed by an insulating layer. The Würzburg team led by Dr. Bernhard Brunner has succeeded in applying the coating in such a way that it does not crack when stretched by up to 100 %, but remains conductive throughout. The area increases and the thickness decreases; the capacity increases. Extensive tests have shown that the films are very well suited as variable capacitors. In thicknesses of a few 100 µm they can withstand voltages up to the kV range.
The mechanical force acting from the outside should be a force that costs nothing, i.e., already exists anyway. The current of waters is abundantly available. How is it possible to stretch and release films periodically with the help of the flow?
The solution found at CeSMa: The water flows through a pipe that is narrowed in a limited area. Due to the "venturi effect", an air vacuum is created in a laterally attached riser tube, through which a film stretched in a circular pattern bulges inwards (Fig. 1). The foil used in the test arrangement has a diameter of 16 cm.
The capacity increases as a result of the stretching; in this state it is charged. Once the maximum deflection is reached, a ventilation valve opens automatically and the film is straightened again. The capacity decreases and the voltage rises accordingly. The procedure already described then takes place. The frequencies of the test system are between 0.1 and 0.5 Hz. At a charging voltage of 4,000 V and a discharge voltage of 8,000 V, an electrical energy of the order of 0.1 J is generated with each cycle.
The connected power electronics convert the high voltage into a lower, conveniently usable voltage. The mechanics and electronics used are standard technology that do not require expensive or hard-to-procure raw materials. The test system was set up as part of the research project DEGREEN ("Dielectric Elastomer Generators for REgenerative ENergies"), which is part of the Bavarian concept for research and technology development in the energy sector.
Unused potentials of renewable energy generation - for example in flowing small waters - should become usable in an economically viable fashion and without harming the environment. The Bavarian Ministry of Economic Affairs is funding the work coordinated by the Fraunhofer ISC with 8 million euros.