In the framework of energy harvesting for microgenerators ,this Work investigates the capabilities of pyroelectric energy harvesting using ferroelectric materials. The energy source in this work is a time varying temperature. First, the energy harvesting using a linear pyroelectric material (PVDF film) was studied using Synchronized Switch Harvesting on Inductor (SSHI) nonlinear technique. Efficiencies of these techniques were compared with Carnot cycle. In order to improve the energy conversion, it was investigated phase transitions at which the pyroelectric and electrocaloric activity are maximum. For this purpose, phase transitions of relaxor PZN-4.5PT single crystals were used associated with the thermodynamic Ericsson cycle. The efficiency of the Ericsson cycle using the ferroelectric-ferroelectric (FE-FE) transition is much higher than techniques using linear properties of pyroelectric materials.