Parabolk güneş toplayıcılarında ısınım ve ışınım transferi bazlı enerji/ekserji analizleri

The concept of exergy is used to determine the maximum energy that can be extracted from a system. It is based on both the first and the second laws of thermodynamics and allows us to determine the irreversibilities throughout a process and the losses from the system. In this dissertation, the fundamentals of spectral radiative exergy are developed and applied to determine the maximum conversion of solar energy in concentrated solar power (CSP) systems. There are five primary objectives of this study. First, a new formulation is developed for the maximum efficiency of the solar radiation conversion by considering the radiative energy transfer between two surfaces at different temperatures for a constant volume system. Exergy of spectral radiative transfer is determined, and the formulation for the exergy efficiency maximization is presented in a direct and practical manner. For the calculation of maximum efficiency, the mean temperature of the environment and the sink temperature are used. Second, a new methodology is presented for spectral radiative energy and radiative exergy calculations to evaluate the performances of CSP systems. Spectral radiative properties and the operating temperature of selective surfaces, along with the temperature of the environment, are considered in these analyses. The fundamental quantities needed for the spectral radiative energy and radiative exergy formulations are introduced, and then the spectral performances of five selective coatings are assessed. The spectral analysis is performed in the wavelength range of 250 nm to 20,000 nm, while thermal analysis is carried out for the temperature range of 325 K to 800 K. The third objective is to introduce a new approach for estimating the exergy value of the monthly average daily horizontal global radiation, including several parameters, such as the monthly average daily value of the horizontal extraterrestrial radiation, the number of sunny hours, the day length, the mean temperature and the mean wind velocity. Seven statistical parameters are used to validate the accuracy of all models. The concept is applied to four locations in Iraq and Turkey, to help predicting the maximum available solar radiation based on different weather parameters. The fourth objective is to outline a comprehensive energy analysis for a parabolic trough collector (PTC) system. The analysis considers all heat transfer modes, optical components, and the details of spectral absorption and reflection of solar radiation on the glass envelope. The energy performance of the PTC system is investigated using five gases in an annular space, five selective coatings of the absorber surface, and four common heat transfer fluids following a two-dimensional approach. A model is built using Engineering Equation Solver (EES). The results obtained are compared against the available results from experimental tests and analytical models. This analysis shows the effects of the properties of the absorbing gas, the selective coating and the working fluid on the energy performance of PTC as the key parameters of energy for various operating conditions. The fifth objective of the study is to establish a methodology to analyze PTC systems using the principles of spectral radiative exergy. The fundamental relations for spectral exergy analyses are derived starting from the first and second law of thermodynamics, and the key performance parameters, including exergy losses, destructions, consumption and efficiency are determined using the same parameters mentioned above in the fourth objective. It is noted that the exergy destruction is directly related to irreversibility throughout processes while the exergy losses are due to the thermal and optical losses. Based on these findings, an improvement of PTC design parameters are discussed.