In this chapter, we will be focusing on the secondary data used for the study based on books, journal articles, magazines, newspaper, etc. This will be focusing on Piezo electric power generators ,weaknesses of solar power energy generators for the wearable devices and IOT based technology .
• Piezo electric power generators
(L. Mateu, 2006)Piezoelectric material will generate electric energy when it is pressed. For example, two piezoelectric ?lms which were connected in parallel directly put inside a shoe to generate electric energy from foot step Ref 9.
(L.M. Goncalves, J.G. Rocha, P.F. Rocha, M.P. Silva, S. LancerosMendez, 2010) Rocha, Goncalves and Silva realized a prototype based on an electrostatic generator located in the bottom of the shoe. The energy was gathered by the deformation of dielectric elastomer loaded. Researchers in Portugal have created a prototype using two PVDF piezoelectric sheets placed in zones subject to greater change of pressure .it is observed that energy has increased By coupling piezoelectric elements in this study .
Few prototype of smart shoes were developed by MIT media laboratory in Cambridge using different technologies. One of prototype consisted of glued 8 sheets of PVDF piezoelectric material with dominant polarization on the ?exible plastic sheet . The system recovers energy depend on the flexibility of the sole, generating a mean power of 1.1mW. The second prototype has been developed by the pressure exerted by the heel and using a layer of PZT (piezoelectric ceramic material) mounted on a sheet of spring steel. It could be generating a mean power of 1.8 mW. The last prototype adapts with a standard rotatable electromagnetic generator through a back plate located on the side of the shoe and it is activated by the pressing on the foot , It could be generating a mean power of 0.23 W. (Paradiso, 2001), (Kendall, 1998).According to the MIT studies ,It is observed that more electric power can be generated by second prototype which is capable to generate 1.8mW .The last prototype mentioned above is totally fail because less power generates and physical structure of the device is relatively large and it makes user uncomfortable, Actually it cannot be considered as a wearable smart device .
The smart energy harvesting systems incorporating piezo stack ,piezoelectric PVDF, buzzer with the aim of showing several different possible solutions to recover the energy needed to provide GPS module. As the ?rst solution, a PVDF piezoelectric sheet has been positioned at the place parallel to the heel of the sole (Luchetti, 2013). The shoe should have capability to subject considerable higher stresses .Thus, piezoelectric materials those are characterized by brittle behavior have been considered unsuitable for smart shoes and the choice has fallen on the PVDF material as its ?exibility.
In the second solution there are two energy generating systems four buzzers on the rear of the sole and the ?lm of PVDF piezoelectric material placed on the front of the sole . Though this model is most expensive and complex , this is the most ef?cient during the device operation even in lower weight user situations. The dual power system ensures the continue power generating even in case of failure of one power generating system or ensures the adaptation to any style of walking .
In the third solution, an Energy Harvesting system is based on ?-PVDF piezo electrical material which is capable of generating electric current not only walking, but in compression and when it is bent. In order to exploit this feature the piezoelectric sheet material must be kept folded .In this method maximum bent angle is limited and therefore more piezoelectric sheets should be merged to produce enough voltage to consume .
Also in the another solution, Dual Energy Harvesting method have been used. 12 piezo stack positioned in the heel area and a folded sheets of piezoelectric material is positioned in the forefoot area . The piezo stacks in the heel are positioned inside using a 3D printer, which has as its main purpose to provide a rigid base to piezo stacks otherwise it would sink into the rubber plate of the sole reducing large sector of their deformation and thus the power generated.in this method is relatively good because of dual energy harvesting but it is more costly using 3D printer. (Luchetti, 2013)