Energy harvesting – The second generation

Harvesting energy from the environment sounds like a dream, but is already reality. By utilising solar energy, pressure and vibration as well as heat, energy harvesting systems are already in use today in building automation, consumer goods and the automotive sector. Now there is a new energy harvesting generation, which uses not just one natural energy source, but combines two.

Sunlight, wind power, heat and movement are not constantly available. However, when you have two sources of energy to choose from, it is more likely that at least one of them will function. Thus hybrid systems offer higher availability and can produce more energy when both energy sources are active at the same time.

Solar energy is by far the most popular of the energy sources used in normal and hybrid energy harvesting systems, since it produces the most energy. In direct sunlight a photovoltaic cell can supply 100 mW per square centimetre, while in artificial light performance falls to 100 µW. Solar systems can be combined with electrodynamic energy harvesting to good effect. At about 700 µW per square centimetre, the power output is relatively high even when energy is obtained through temperature differences. Here thermogenerators exploit the Peltier effect or inversely the Seebeck effect, in which Peltier elements are used. In Peltier elements, energy is released in the form of heat by the flow of current, or by a change in the energy level of electrons. Conversely, if a temperature difference prevails, a current flow or voltage can be generated from it, which energy harvesting systems make use of.

Application scenarios for hybrid harvesters utilising solar and heat energy include supplying power to sensors used in building, plant and process automation. Both thermostats and light sensors can be operated in this way, as can gas and pressure valves. Due to the low efficiency of energy harvesting systems, the energy saving as compared to a conventional power supply is still relatively small. But already today savings on installation costs make energy harvesting very attractive. Costs mount up when cables have to be laid for the installation of conventional sensors in buildings and industrial plants. Wireless sensors, which generate their own energy, cost far less by comparison. They are also to be recommended for facilities and parts that are hard to access, for example in wind farms or on oil rigs. Standalone hybrid harvesters not only offer higher availability and greater efficiency, but also an additional safety factor. If a system fails, a redundant second system is available, so that the risk of a total failure is minimised.

Hybrid systems can offer a further benefit for wireless sensors. Thanks to their higher efficiency, they are better able to generate more power. Wireless sensors frequently have to transfer data every millisecond. Conventional energy harvesting systems, which only use one energy source, are often unable to provide sufficient energy. With two sources, however, the chances to do so increase.

Falling component prices, smaller form factors and higher levels of capacity mean that, in future, new areas of application will open up for energy harvesting systems.

Arrow Electronics Launches Energy-harvesting Evaluation Kit