Classification of Energy Harvesting devices

Classification of Energy Harvesting devices
We may classify the different energy harvesting devices in two ways: considering who or what provides the energy for conversion, and what type of energy is converted. Table 2 relates the two classification schemes. In the first classification scheme we can distinguish between two kinds of devices. First, devices that use part of the energy of the user of the electronic appliance. It will usually be a human, but it could be also an animal , for example for a remote monitoring device. We call this first kind of devices
The second kind of energy harvesting device gets its energy from the environment, and thus we call them Environment Energy devices. This classification takes into account that, following the first principle of thermodynamics, a greater amount of energy must be spent to obtain a certain amount of electrical energy. In the case of Human Power it is the user that in some way or other provides this energy and, though the energy levels are very small, the effect may prove noticeable when several devices depend on the activity of a single use.

In order to evaluate the burden of energy harvesting on user activities, it is possible to use a simple biomechanical model to calculate the energy involved in a human step,11 obtaining around 40 J. In comparison, the energy of a short RF transmission can be evaluated in the order of 100 μW. This means that the extra energy demanded to obtain enough energy is very small for some applications, and therefore it makes sense to consider human beings as a possible energy source
The second classification scheme may consider three types of energy: kinetic, electromagnetic radiation (including light and RF), and thermal. For Human Energy devices only kinetic and thermal energy are available. In the case of kinetic Human energy, one may distinguish between those actions made specifically to generate energy and casual movements made during normal behavior. These two cases are called by the Human Power research group of the Delft University of Technology12 Active and Passive Human Energy respectively. Following this definition, thermal Human Energy is always passive. Environment Energy sources include kinetic energy in the form of vibrations, radiation as solar energy or RF radiation, and thermal energy. The energy harvesting devices may pick up vibrations when located on machines, building elements or other places near vibrating sources. Radiation may come from natural or artificial sources. Thermal energy depends on the existence of a temperature gradient. While the transducing methods may be similar to the Human Energy devices, the excitation magnitudes, frequency spectra and periodicity are very different, and therefore each case must be studied separately. This will also have consequences in the electrical power conditioning circuit.