## 2 Related Work  
In recent decades different systems have been designed for animal tracking. Some of them make use of satellites which locate the animal's position [1]. These systems allow the determination of the position of animals which have been equipped with a satellite emitting system. They have been widely used in turtle [2], duck [3] or whale [4] tracking. However, its use is extremely expensive and requires all the satellite transmitters in the animals to be updated in the satellite database. Moreover, satellites are not able to take more than some tens of measurements per day. Other implementations are based on GPS devices which allow a larger data rate update [5]. However, commercially available tracking systems lack the data storage capacity needed to collect animal location data frequently over long-term deployment periods. Some commercial systems have remote data-download capabilities, reducing the need to recapture tagged animals for data retrieval. These systems download data via satellite, mobile communications (GSM) or radio communications. However, satellite systems are excessively expensive and GSM coverage is extremely limited in some areas within the Globe. Therefore, only radio communications or manual download are feasible for most of standard systems. Nevertheless, these battery-powered systems need to be replaced once exhausted and suffer from the disadvantages already mentioned in Section 1. Different researchers are working on energy harvesting systems which allow animal tracking systems to take advantage of renewable energy, avoid battery replacement, or reduce its use to a minimum. When energy harvesting is applied to mobile sensor networks, the energy from external power sources can be used to power the nodes and increase their lifetime. Energy harvesting can be divided into two categories: _i)_ energy to use and _ii)_ energy to store. In the former, the system directly powers the sensor node and it continues operating until the harvester stops supplying energy. In the latter, a storage system (typically a battery or super-capacitor) is present and the energy produced by the system is stored in the battery for its later use. Systems which generate energy for its direct use are the ones that must be actively pursued for an ecological environmental operation. However, most of the systems found in the literature make use of storage systems. Most of these strategies make use of solar energy as the environmental energy source [6; 7]. In [8], for example, a single-storage energy harvesting system based on solar energy is built on a Mica2 platform [9]. The energy obtained is used to recharge a AA-size Ni-MH battery of 1,800 mAh capacity. In [10], the authors presented a system based on a double storage method. The primary system is recharged using a solar cell. If there is an excess of energy in the primary system, it recharges the secondary system. When there is not enough energy in the primary system, the node switches to the secondary system for its supply until the primary system gets recharged again. In [11], a non-battery-powered node is presented. It uses a super-capacitor which is recharged by means of solar energy. The authors claim that the system is able to operate for 20 years at 50% duty cycle. In [12], the authors also make use of a super-capacitor which is recharged when some of the photodiodes are enabled because of the light detection. Some of these implementations have been used for animal tracking, as in the _zebranet_ project [13] or the _Turtlenet_ project [14]. However, solar energy is severely limited for latitudes where daily sunlight may be short in some seasons or the irradiance is not enough to power the system. To deal with solar energy restrictions, some other techniques have been studied. In [15], the authors describe a system which takes advantage of human motion and obtains enough energy for transmitting information. The system is based on a piezo-electric film (_PolyVinylidene Fluoride_) which generates energy when the film is bent.