Global energy requirements have displayed a significant increase over the last few decades due to increased world population and effects of technology in the daily lives of humans. This brought about the necessity to invent state-of-the-art solutions to world energy problems. However, most of the proposed systems and devices were not ecological, which led to global warming issues. With the current advancements in nanotechnology, it is now possible to design environmentally friendly devices with enhanced efficiency. Scientists have proposed new materials, thermal management options, and coatings to tune the devices to application-specific requirements. At our lab, we investigate each aspect individually to combine the learnings into final, ecological energy harvesting devices. We employ phase change phenomena to utilize the latent heat in order to attain optimal thermal management solutions. Specifically, we fundamentally study the evaporation and condensation of heat transfer liquids to gain a deeper understanding of the phase change dynamics on recently developed micro/nanostructured surfaces. Furthermore, we investigate the possibility of atmospheric water harvesting through coatings inducing radiative cooling to tackle water scarcity issues. We further study the utilization of the harvested water to enhance the device efficiency by inducing a pseudo evaporative cooling effect and display significant improvement in the overall thermal efficiency with room for further improvements. In order to utilize the knowledge that we gathered from thermal management studies into the creation of high-efficiency energy harvesting devices, we also examined energy conversion materials. We investigate radiative transport through energy conversion materials to tune their optical and thermal properties for a plethora of applications including solar-thermal energy harvesting and thermophotovoltaics. Additionally, we examine optically transparent – thermally insulating (OTTI) silica aerogels to create an artificial greenhouse effect on solar absorbing surfaces, effectively increasing their temperature and thus the device efficiency. We obtained silica aerogels of 88% transmission to solar rays and 1% transmission to emitted rays from the absorbing surfacefor a nominal thickness of 1 cm. Combinations of green energy generation devices with structured phase-change inducing surfaces would enablethe fabrication of high-efficiency standalone devices, and could bring about significant changes into the renewable energy agenda. Our work sheds light on the study of thermal management coatings, energy conversion materials, and phase change fundamentals and offers design priorities for the next-generation green thermal devices.