Electricity from the Ocean Waves
WACKER is the first silicone manufacturer to successfully produce ultrathin precision silicone films as rollstock. When these silicone films are laminated with electrodes, they act as dielectric elastomers. They can be used for developing innovative technologies, such as power plants that derive energy from ocean waves.
Ocean waves could theoretically produce up to 29,500 terawatt hours of electricity per year – more than the planet’s entire annual energy demand.
When waves strike the quay wall sending spray high into the air, even land lubbers can appreciate the massive energy being released. According to United Nations projections, the world’s wave energy potential is 29,500 terawatt hours – more than the entire planet’s annual energy demand. Up to now, however, no systems have been available that are technologically mature enough to offer a reliable, cost-effective method of harvesting the latent power of the sea. While prototypes of hydraulic wave power plants have been developed, these are repeatedly destroyed by the energy they are actually meant to harvest – by the waves themselves, in other words, mostly as a consequence of winter storms.
Principle of a wave park using electroactive polymers
Silicone elastomers repel water but are permeable to certain gases. ELASTOSIL® Film is no different in this respect. The silicone films hold back water, but grant free passage to water vapor and certain gases. This gas permeability is highly selective: carbon dioxide, oxygen and water vapor pass through the silicone layer much faster than nitrogen. ELASTOSIL® Film could therefore serve as a membrane for removing a specific gas, such as carbon dioxide.
A new, environmentally friendly approach to harvesting electricity from ocean waves is now available, however, thanks to the industrial research project Silicone-Based Electroactive Polymers for Energy Generation, sponsored by the German Federal Ministry of Education and Research (BMBF). Wacker Chemie AG is providing the base material for this project – an ultrathin film based on silicone that can be employed as a dielectric elastomer. Dielectric elastomers belong to the family of electroactive polymers (EAPs), which change their shape when electricity is applied, thereby converting electrical energy to mechanical energy. Because their mechanism of action is similar to that of natural muscles, EAPs are frequently referred to as “artificial muscles.”
“Silicone films withstand over ten million compressive cycles without the slightest sign of material fatigue.”
Dr. Andreas Köllnberger
Global Product Development Manager, Engineering Silicones
EPoSiL Project Manager Dr. Istvan Denes tests the demo model at Darmstadt Technical University.
Up to now, however, mass-producing parts based on dielectric elastomers has proved difficult. “What we needed most were elastic polymers with properties that had been optimized for these kinds of applications and that can be easily processed,” explains Dr. Andreas Köllnberger, the global product development manager for Engineering Silicones at WACKER. With the launch of ELASTOSIL® Film, WACKER has introduced an elastic, extremely thin film on the market that is suitable for large-scale production as rollstock, allowing manufacturers to begin mass producing EAP components. Precision silicone films are also suitable for applications beyond EAP technology, however, as they can be used as functional membranes or for dressing wounds.
Manufactured using a patent-pending process that obviates the need for solvents, these continuous films are made from addition-curing silicone rubber compounds and are commercially available in thicknesses less than 100 microns – they can even be obtained in thicknesses as low as 20 microns.
Thinner than a human hair: ELASTOSIL® Film and SILPURAN® Film – precision silicone films – are suitable for applications such as functional packaging films, wound dressings, sensors, actuators and energy-saving electrical relays.
The manufacturing process yields homogeneous, flawless films that are characterized by their extremely uniform thickness, which varies by no more than 5 percent across the entire width and length of the film web. And to ensure that no particles protrude from the film surface, the rubber blends are produced under cleanroom conditions.
Silicone films possess all the key properties for which silicones are noted, namely heat resistance, low-temperature flexibility, chemical inertness and biocompatibility. Their surface is highly hydrophobic, i.e. water-repellent, and they exhibit silicones’ typical high resistance to many different physical and chemical influences. Their favorable elastic properties also help prevent them from fatiguing under years of mechanical stress.
With SILPURAN® Film, WACKER has developed a high-purity version of its ultrathin, ultra-high precision silicone rubber films, which are particularly targeted at the needs of the health-care and medical industries. The silicone rubber used in these films has passed selected tests for biocompatibility according to ISO 10993 and US Pharmacopeia Class VI. SILPURAN® silicones are manufactured and packaged under cleanroom conditions in accordance with WACKER’s own standard for medical materials (WACKER CLEAN OPERATIONS) to eliminate any impurities. For example, breathable adhesive plasters that help boost the healing process can be made from SILPURAN® Film. SILPURAN® Film also offers new possibilities in medical equipment and prosthetics.
ELASTOSIL® film can be used, e.g., to produce deformable capacitors. This is done by coating the upper and lower surfaces of the films with a flexible, electrically conductive material. When a DC voltage is applied, the electrodes are attracted to each other due to their opposite electrostatic charges, and compress the soft film material. The layer of elastomer material becomes thinner, and spreads out in the plane. As a result, the capacitor becomes flatter and wider overall. When the capacitor is discharged, the elasticity of the film causes it to return to its original shape.
Because capacitor deformation can be repeated any number of times and systematically controlled, manufacturers can use EAP films in actuators for converting electrical voltage into movement. The reverse is also possible: EAP films can convert mechanical movement into electrical voltage. This property means that EAP technologies are suitable for producing sensors and novel types of generators. The deformable capacitor is always the base unit of the system.
Since electroactive polymers convert mechanical into electrical energy, silicone films are also suitable for use in sensors.
as thin as 20 micrometers.
To operate such a base unit as a dielectric elastomer actuator, a voltage must be applied. The level of this operating voltage depends on the film thickness – the thinner the elastomer film, the lower the voltage needed. The actuators currently in development typically have film thicknesses between 20 and 60 microns, and are therefore much thinner than a human hair. The voltages involved are in the order of several kilovolts. At these levels, the thickness of the film decreases by 5 to 30 percent, with the exact figure depending on the material. At the same time, the surface area typically increases by 50 percent.
Application of a voltage to a single actuator base unit produces a deformation of just a few microns. That is not enough for industrial use. However, if large numbers of them are stacked together and connected up in parallel, changes in the order of several millimeters and even centimeters are possible. These stacks, which can have any shape, can be used to generate different types of movement.
Relays, Switches and Valves
EAP actuators are lightweight, can be controlled with precision and are extremely efficient. The word has got around, particularly in the electrical sector. Relays, switches and valves based on dielectric elastomer actuators are almost ready for launch, and should reach the market in the next two to five years. EAP actuators are also set to be used in the automotive industry, where they could eventually replace electric servomotors. In contrast to their conventional electromagnetic counterparts, EAP relays only require power when switching. Considering that relays are found in vast numbers of devices and equipment, the energy-savings potential is huge. Another potential beneficiary of EAP technology is the valve industry, where EAP actuators would provide precision control over fluid flow, a task not readily mastered by conventional fluid valves.
With No Material Fatigue
A further important property is the films' exceptional resilience. Their dielectric, mechanical and elastic properties do not vary with the applied voltage or the temperature. Nor do they deteriorate over time, a fact which gives silicone films the edge over dielectric films based on organic polymers. “The property profile of the films is typical of silicones, making EAP parts rugged, durable and low-maintenance,” explains WACKER chemist Dr. Andreas Köllnberger. “Our trials on prototypes have shown silicone films to survive over ten million compressive cycles without the slightest fatigue.”
ELASTOSIL® Film is produced under clean room conditions and is available as rollstock in thicknesses of 20 microns or more.
This durability and resilience are key reasons why silicone films can be used in what is known as energy harvesting. Silicone-Based Electroactive Polymers for Energy Generation is a joint research project in which a consortium of four industrial companies and two universities under the direction of Robert Bosch GmbH is working to develop wave power generators. In this context, researchers at Darmstadt Technical University created a preliminary model of a wave power plant resembling a buoy: whose upper half floats on the surface of the water while its lower half is anchored to the sea floor. The two halves are joined together by a stack of thousands of conductively coated silicone films which change shape at intervals of three to ten seconds in response to the motion of the waves. A positively charged electrode is mounted above the highly insulating silicone, and a negatively charged electrode below it. The action of the waves first compresses the silicone and then relaxes it again. As the water level rises and falls, the two electrodes above and below the silicone elastomer move toward one another and then apart. As soon as the silicone film has relaxed and recovered its thickness, the two electrodes, and therefore their charges, move apart - and the electrical energy in the converter is increased. This produces the desired outcome of converting the mechanical energy from the wave into electrical energy. “The electrical currents in the individual layers are cumulative,” explains Dr. Istvan Denes of Central Research at Bosch, who heads the project.
Silicone elastomers are rubber-elastic materials consisting of inorganic polysiloxanes that crosslink irreversibly to yield a three-dimensional network. Silicone elastomers have a property profile that makes them indispensable in many industrial applications: extraordinary heat resistance, low-temperature flexibility, chemical inertness and biocompatibility. These materials have a strongly hydrophobic, i.e. water-repellent, surface, are selectively permeable to gases, and are very good electrical insulators. A typical characteristic is their high resistance to a large number of physical and chemical influences, which is why, unlike organic rubber compounds, they do not age. Thus, their chemical, physical and technical properties remain virtually constant over the temperature range of roughly -45 to +200 degrees Celsius. Silicone elastomers can cope with constant mechanical and electrical loads as well as continuous exposure to oxygen, ozone and UV radiation.
Multiple converters generate electricity in an array. The efficiency of the pilot plant in Darmstadt was determined to be around 50 percent, which is higher than that of conventional power plants. The first reduced-to-scale model of this kind of wave power generator will now be launched in the wave flume at Hamburg-Harburg Technical University.
Generators based on silicone films do not require hydraulic parts or turbines, making them virtually maintenance free and hence significantly cheaper to operate than conventional wave power plants with their fragile, hydraulic moving parts. This means that, in the not too distant future, we may well see wave power plants anchored off of our coasts, generating electricity from the ceaseless up-and-down motion of the waves.