The objective of the study is to provide an alternate phenomenological modelling-based theoretical approach to study thermal properties such as caloric effect and pyroelectric energy density of ferroelectrics. Thus, the research introduces a novel theoretical approach based on a thermodynamic framework to obtain pyroelectric characteristics, such as, electrocaloric and elastocaloric properties in terms of adiabatic temperature change. During the course of the study, two different constitutive models have also been developed to address different characteristics of ferroelectrics. A one-dimensional constitutive model with an idea of infinite yield surfaces has been introduced to capture the minor ferroelectric hysteresis responses for low electric field operations. Further, the research work has proposed a three dimensional electro-mechanical constitutive model to predict the ferroelectric and ferroelastic responses of 1-3 piezocomposites of different fibre volume fractions. The proposed non-linear constitutive models have been incorporated into a non-iterative algorithmic setup to improve computational efficiency. The model parameters have been identified based on the experimental results of PbZrxTi(1-x)O3 (PZT) and 1-3 piezocomposites. Using these parameters, simulations are performed to compute the change in polarization and strain in respect of temperature at different values of applied electric fields and stresses, based on the proposed theoretical approaches. Thus, the relation obtained between the polarization temperature and the strain temperature has been used to calculate adiabatic temperature change (ΔT). The dependency of ΔT on temperature, electrical loading, mechanical loading and coupled loading has also been predicted using this theoretical measurement method. The same models have been used to simulate the pyroelectric energy harvesting cycle (Olsen cycle). The aforesaid models have been utilized to estimate the harvested energy density per cycle (ND) of the PZT composites of different fibre volume fractions from waste heat source for low power systems.