Although we already harvest energy from many sources, new sources are always welcome. Ignoring the up-front costs, harvesting energy is often free, convenient and eases problems of several types of practical installation and replacement. Energy harvesting requires three fundamental stages: a reliable source of energy, the harvesting electronics or the converter and a load where the energy can be gainfully employed.
Apart from finding a reliable source of energy, there is one other factor before the energy goes into the converter and load. This factor is the transducer, which will allow the energy to be gathered with reasonable efficiency and reliability. Suitable transducers examples are blades for airflow, piezo-based components for vibrations or a water-driven vehicle using temperature gradient for propulsion.
Currently we use several potential sources of energy for harvesting. Prominent among them are solar radiation, temperature difference, sound, vibration, airflow and motion. One of the most common sources that have been very hard and baffling to capture is static electricity generated from friction (also referred to as Electro Static Discharge or ESD). Generally famous for the damage it causes, triboelectricity it may finally have been tamed by the work done at the Georgia Institute of Technology.
Although a precursor to generating and collecting static electricity does exist in the form of the Van De Graaff generator, the team from GIT has developed polymer materials, which are inexpensive and flexible. In addition, these materials are very good at developing a charge through rubbing and holding it until it is extracted as current flow. Static electricity is easy to generate – simply walking on carpet is good enough – and hold – that is, until you touch the doorknob – but very tricky to extract.
The team generates power from triboelectricity by sliding two materials together and then separating them to create a gap in between. Although this may be interesting, the next question could be a tricky one – how much harvestable energy does this mechanism offer?
The GIT press release cites power outputs of the order of 300W, which is of course, significant. However, there is another side to energy harvesting. This concerns power delivery to the load. Although energy collection may happen in dribbles on the generation side, when releasing energy it to the load, it has to be done in the form of power, which is the rate at which energy is expended.
The reason for this is not hard to fathom. All loads require a minimum threshold or power to function. The 300W generated may translate into barely meaningful power levels at the load. When energy levels to be harvested are on the low side, it may actually be difficult to collect enough power after accounting for acceptable losses.
The team from Georgia Tech has done something impressive and intriguing. They claim that the materials they have developed have a volume power density of more than 400KW per cubic meter and the efficiency figure is more than 50%. In addition, the team says that their material is suitable for generating energy from contact with flowing water. That is an interesting proposition, opening up new opportunities where sinks and faucets could become the hidden sources for charging mobiles and or lighting up the kitchen.
