The Beam-Down Solar Thermal Concentrator: Experimental Characterization and Modeling

  • Marwan Basem Mokhtar

Student thesis: Master's Thesis

Abstract

Solar thermal technologies have emerged as a strong candidate for large scale power generation. Unlike other renewable energy technologies, Concentrated Solar Power (CSP) offers the capability of supplying reliable dispatchable power. Among CSP technologies point-focus power tower or central receiver plants promise higher thermodynamic conversion efficiencies compared to line focus technologies due to their higher achievable temperatures. The Beam-Down solar thermal concentrator is a variation of central receiver plants with Cassegrainian optics. It was constructed in Masdar City in 2009 in order to investigate the potential of the unconventional plant design. The Beam-Down optics and the heliostat construction are considered to be the two main modifications on regular central receiver plant designs. In March 2010 our research team at LENS (the Laboratory for Energy and Nano-Science) performed a comprehensive test on the Beam-Down heliostat field using the existing flux measurement system consisting of a CCD camera and heat flux sensors. It was concluded that two main issues require further investigation and became the focus of this thesis: • The reliability of the flux measurement set up used to characterize the performance of the plant, • The concentration quality of the individual heliostats which directly affects the performance of the plant. Based on that, and in order to investigate and analyze the flux concentration quality of the unique heliostats, a model was considered necessary. A geometrical optics model is presented. The model is developed to investigate the causes of optical aberrations of the heliostats. It is also adapted to assess the effect of varying optical errors and sunshapes on concentration quality and radiation spillage. The optical model uses geometry and vector algebra to maintain the computation speed high and to simplify implementation compared with stochastic methods. Efforts were also directed on enhancing the accuracy of the heat flux sensors which are used as a reference for the optical flux measurement system consisting of a Charge-Coupled Device (CCD) camera. A regression model for correcting the heat flux sensor measurement is proposed. It is founded on a convective and radiative heat transfer model of the flux sensor. As will be shown, the proposed model for calibration reduces the measurement error significantly, especially at low radiation fluxes. In addition, since the model is based on easily measurable variables it can be applied conveniently on existing measurement systems without major modifications. An analysis of the overall Beam-Down performance is presented. The analysis is based on a theoretical receiver design. Performance curves of plant under varying operating temperatures, ambient conditions and concentration levels are presented. This gives an overview of the plant performance as a whole. The analysis is based on full concentration test data obtained experimentally at the plant in March 2011 using the retrofitted measurement system. A part of this thesis is dedicated to discussing some topics of practical importance in solar concentrating system with an optical lay out similar to our Beam-Down concentrator. Optical alignment procedures of the central reflector mirrors and the tracking sensors of the heliostats are discussed. In addition the canting procedure of individual heliostat facets and their effect on amount of intercepted radiation is also addressed. 
Date of Award2011
Original languageAmerican English
SupervisorMatteo Chiesa (Supervisor)

Keywords

  • Solar Thermal Energy

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