This paper looks into the possible methods of harvesting energy for applications involving wireless sensors from renewable or sustainable sources. An overview of the fundamentals of energy harvesting models using renewable sources and the advancements in the components used for this purpose is given. It also provides an analysis of the issues that are restricting the current harvesting models and indicates possible advances in technologies that may soon solve those problems. There are two main issues that plague these harvesting models. Firstly, the technology involved in the actual harvesting of renewable energy sources is not well developed and offer poor efficiency. In addition to that, most of the models need a permanent storage device, such as a battery or capacitor in order to operate. This is a serious drawback in applications involving wireless sensors as these devices have limited capacity, lifetime, and require a large area to work efficiently. This is most often not favorable as the attractions of using wireless sensors are in its portability and form. This paper looks into the studies dedicated to solving these problems. The latter part of the paper contains a discussion regarding the technologies that are best suited for applications in wireless sensors. The two most readily available and feasible options of a sustainable energy for these applications are found out to be RF (Radio Frequency) signal and solar radiation. Therefore, the final part of the paper concentrates on reviewing a model that may potentially be able to serve as a basis for a singular model for any kind of energy harvesting from renewable sources.