This thesis describes a study to evaluate the energy recovery potential and challenges associated with the application of Organic Rankine Cycle (ORC)s. Application of ORCs for both waste heat streams and deep geothermal sources are considered. A model which calculates the thermodynamic performance of ORCs for any source heat or sink stream and cycle configuration was developed. Simulations for three waste heat case studies showed a potential thermodynamic benefit from using zeotropic working fluids. An experimental rig was built to explore the reported discrepancy in performance between theory and experimental observations for zeotropic mixtures. Experiments were carried out with a near azeotropic working fluid, R410a, and a zeotropic mixture R407c. Results show that the global heat transfer coefficient of the zeotropic mixture was lower than for the azeotrope. The availability of theoretical models to accurately calculate heat transfer for zeotropic mixtures was explored. Appropriate models are available in the literature. However, to incorporate these into the ORC model is a significant bit of work beyond this project. The thermodynamic performance, footprint and cost of an ORC plant are key parameters that will determine the feasibility or otherwise of an ORC plant. These factors are considered together and the interdependence of them is discussed. Three deep geothermal heat sources are considered, within the context of these three factors. The reasons for the feasibility or otherwise of fuelling an ORC with each of these heat sources is discussed. Ultimately, while simulations show there is potential improvement in thermodynamic performance, by using zeotropic working fluid, experimental work shows there may be a penalty to pay in terms of the size of the system. The analysis of the deep geothermal case studies shows that finance and social factors also have a huge influence on whether a project to recover low enthalpy heat will evaluate or not.