Energy Harvesting for Electronic Systems

Energy harvesting enables products to become more self-sufficient of power and offers the possibility to avoid battery exchange and power cables. The performance and applicability of the most promising or popular small scale energy harvesting technologies are investigated in this thesis. The focus lies on the amount of potential energy that can be harvested and how limited the technology is to a specific environment. Different power management and energy storage methods are also examined. Two prototypes demonstrating different energy harvesting phenomena are presented. The first prototype is able to harvest an average of 152 μW at 302-346 lux using 4 AM-1417 photovoltaic cells with an area of 19.46 cm2. The 302-346 lux span is measured in indoor fluorescent office lighting. The second prototype uses a DC motor to harvest the energy from the motion of opening a door. It is possible to harvest more than 470 μJ from a single door opening with the prototype. This requires a minimum average revolving speed of 7.5 rpm during the door opening. The thesis concludes that photovoltaic cells are relatively easy to use in practice and that an environment with sufficient energy is not difficult to find. Piezoelectricity and vibration based electromagnetism has a low applicability due to the fact that the transducers only gain a useful power output at their resonance frequency. However, electromagnetic pushbuttons or rotation generators are proven to be useful. Thermoelectric elements are highly dependent on specific thermal conditions, making them applicable in some cases but hard to use as an all-round energy harvesting device. Radiofrequency energy harvesting is also investigated but it is concluded that this technology is not yet developed for practical use.With the ever decreasing power consumption of low level electronics new ways of powering products are on the rise. It is now not only possible to power small electronic devices through batteries and cables but also through energy found in everyday environments. There are products on the market today with such a low power requirement that they can be powered by their own environment. This technology is called energy harvesting. The devices powered by energy harvesting essentially works as tiny power plants meaning they provide a small relief to the power grid. This offers the possibility to avoid battery exchange and power cables. Avoiding this can be very beneficial in places hard to reach for maintenance or where mobility is of significance. An example of low power products possible to power with energy harvesting are wireless sensor nodes. Wireless sensor nodes are self-powered sensors communicating with each other over a network. These can for example be combined with temperature, occupation or tracking sensors. This thesis evaluates the most promising or popular energy harvesting technologies. The focus lies on the amount of energy that can be harvested and how limited the technology is to a specific environment. The technologies evaluated are photovoltaic, electromagnetic, piezoelectric, thermoelectric and radiofrequency generators. Photovoltaic cells are relatively easy to use and it is not difficult to find an environment with a sufficient amount of energy for a practical purpose. However, there are different types of photovoltaic cells suitable for different environments. Amorphous cells are best suited for indoor lighting and monocrystalline cells are best in outdoor sunlight. One of the prototypes is based on amorphous cells and manages to harvest an average of 152 µW in indoor office lighting. This prototype proves that a sufficient amount of energy can be harvested in indoor office lighting for a practical purpose. Electromagnetic harvesters utilizes electromagnetism to convert movement into electricity. The movement can be both vibrational and rotational. Vibration generators work best when something is vibrating at a certain frequency. It is difficult to make a small generator that works with vibrations found in everyday environments. Rotation generators can be made out of electromagnetic motors. In the thesis one of these are mounted on an office door. There is enough energy harvested to use it with, for example, an Internet of Things application. Thermoelectric generators can be very useful where a continuous temperature difference can be found, for example in combustion engines in cars. This continuous temperature difference is not easy to find in many everyday environments. Piezoelectric harvesters are vibration dependent, making them difficult to use in many environments. The energy available for radiofrequency harvesters is too low for a useful energy harvesting application. Figure 1. Energy harvesting technologies. Source: maxim integrated. To conclude, photovoltaic cells and electromagnetic harvesters are the most promising harvesting techniques. The other methods were not found to be as useful as the photovoltaic cells or electromagnetic harvesters, since they require very specific environments to generate a usable amount of energy