Clean (and speci cally renewable) energy is steadily improving its global share. However, nite availability of fossil fuels and the growing e ects of climate change make it an urgent priority to convince the industry and governments to incen- tivize investment in the renewable energy eld and to make it more attractive by decreasing the capital cost. Until recently, uncertainties in funding limited renew- able energy development, especially in the US. That limitation has been one of the barriers to progress. Another limitation of many renewable energy systems is the variability in their output, which makes them unsuitable for baseline power production. Therefore, fossil fuels are still the dominant source of energy globally. The estimated US energy consumption in 2015 relied heavily on fossil fuels which generated about 82% of US primary energy. The share of solar energy in 2015 US energy consumption was just 0.43%. This is a disappointingly small share for a zero carbon source of energy. Nuclear energy as another clean energy source has a small share as 8% of the total US energy consumption. Although it is one of the most reliable/stable and low carbon sources of energy, the nuclear power industry is currently facing several challenges. First, nuclear generated electricity is not cost-competitive with other types of generation. Second, there is a diminished availability of cooling water to reject heat from large power plants. Third, the penetration of solar and wind generation systems into the electrical power market is producing signi cant uctuations in the demand for nuclear generation. Open Air-Brayton systems are one of the solutions here since the ultimate heat sink for nuclear supplied power is the atmosphere, so a more direct method of dumping this heat would be useful. An open Air-Brayton system can also provide a great deal of exibility in adjusting power plant electrical output without signi cantly ramping reactor power output. This dissertation develops a common framework for understating and improving the solar and nuclear clean energy system com- ponents which are based on Brayton cycles. For this purpose, experimental and numerical studies of solar and nuclear systems are conducted. The open air Brayton cycle of a solar chimney power plant is studied in this investigation in di erent cases as a solar power cycle. Additionally, the air Brayton cycle of a nuclear power plant is considered for several di erent cases, including a combined nuclear-solar cycle. Air ow is driven by buoyancy in the open air Brayton cycle of a solar chimney power plant system (SCCPS). In SCPPS, the energy of buoyant hot air is converted to electrical energy. SCPPS includes a collector at ground level, covered with a transparent roof that collects the solar radiation, which heats the air inside and the ground underneath. This dissertation proposes and studies new modi cations and optimizations to increase the thermal eciency of the SCCPS, as well as combining SCCPS cycles with other clean sustainable cycles. The nuclear-combined air Brayton cycles are studied with the focus on producing low-carbon energy and combining pressurized water reactor and small modular reactor cycles with another thermal cycle, leading to increased combined eciency. In this manuscript, chapters are organized with respect to the type of their ther- mal cycle. Part I, includes three chapters focusing on simple/single Brayton cycle. Part II contains two chapters regarding combined Brayton cycles. Each chapter in this investigation is based on at least one published or accepted/ready to pub- lish article, and which have undergone peer review. The citation for each original source manuscript is included as a footnote on the bottom of the rst page of each chapter. Therefore, each chapter is in the format of a journal article, including: an introduction, motivation and background, theory, numerical approach, experimen- tal approach, results and dissections, future work and conclusion. The