Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
Concentrating Solar Power (CSP) is a promising technology aiming to capture and concentrate the sun’s energy to reach high temperatures, which is essential for thermal power generation. Among the available CSP technologies, parabolic trough is the most mature and commercially developed technology with the longest operational experience of utility-size plants whereas linear Fresnel technology is only beginning to be deployed in pilot projects with significant potential to reduce capital cost and improve performance (IRENA, 2012).
Linear Fresnel Collectors (LFCs) use a series of long flat or slightly curved mirrors inclined at different angles to concentrate the sunlight on a fixed receiver located several meters above the mirror field. In conventional LFCs each line of mirrors is equipped with a single-axis altitude angle tracking system. Using the flat mirror as a standard mass-produced commodity reduces the capital costs significantly. However, lower concentration ratio and consequently, lower operational temperature are penalties of using LFC.
Further cost saving for line-focus solar collectors is attainable through transition to direct steam generation (DSG) in which the need for an intermediate heat transfer fluid and heat exchangers are removed. Furthermore, an improvement of heat transfer is expected in DSG systems due to the two-phase heat transfer inside absorber tubes. Moreover, the operational temperature and consequently, overall heat loss from the solar field can be reduced in the absence of the intermediate heat exchangers which is beneficial for increasing efficiency and cost effectiveness. Nevertheless, probable flow instability due to two-phase flow in the collector, pressure drop and highly sensitivity to radiation changes are the main challenges still ahead for DSG systems.
This thesis research was defined in CSEM-UAE (Centre Suisse d’Electronique et de Microtechnique in United Arab Emirates) company as a part of an R&D program for CSEM concept so-called Solar Island. The aim of this project is to design a low cost/high-performance DSG-LFR (Linear Fresnel Receiver) for Solar Island which is a large circular platform with rotational mechanism capable of tracking the sun’s azimuth angle. Owing to the utilization of this novel tracking technique, it is possible to use mirrors with fixed inclination angles. As a result, further reduction in initial cost is achievable as all conventional one-axis tracking compartments are replaced with a single rotational disk. On this basis, the initial design of the LFC prototype for Solar Island has been done previously in CSEM-UAE.
The project is tackled, firstly, through developing a two-phase steady state thermal model in the Engineering Equation Solver (EES) program for both non-coaxial and coaxial configurations. In addition, adaptability of the commercially developed products on the market such as Schott Solar and Siemens receivers is studied through performing optical analysis with TracePro. As the next step, a cavity receiver is designed through comparing different alternatives in the case of geometry, material and configuration of tubes.
Heat loss from a cavity receiver has a complex mechanism and cannot be analyzed by conventional methods. Therefore, CFD modeling method is used to determine the heat loss rate from the receiver at a certain temperature of the absorber, wind speed and cavity inner pressure along with estimation of the stagnation temperature under no-flow condition. Heat flux on the absorber is obtained from optical analysis based on actual optical properties of the materials. The overall heat loss correlation is extracted as a function of absorber temperature which in turn can be used in EES modeling for further thermal analysis.
2013. , 110 p.