Concentrated Solar Power (CSP) also known as concentrated solar thermal is the systematic act of using mirrors and lenses to concentrate direct sunlight to a specific focal point so that solar radiation can be converted into useful electrical energy. The genesis of CSP technology dates back from the 1800s, when August Mouchout used a Parabolic Trough Collector (PTC) to produce steam. CSPs are known to have different types of collector technology which include the enclosed trough, solar power tower, fresnel reflectors, dish stirling, and parabolic trough collector.
It has been established that a Parabolic Trough collector (PTC) is the most developed form of technology that a concentrated solar power plant can utilize to harness the energy of the sun. PTCs are commonly used by large scale plants to collect a large amount of solar radiation and incorporate it into their many functions. PTCs are energy reactors which enables heat exchange between solar energy and a transport fluid medium. They are ‘parabolic’ in shape, consist of an absorption tube located at the focal concentrating point, a bearing structure, and a shiny reflector surface. PTCs can either have a one or two-axis tracking system. PTCs with a two-axis tracking system are more efficient because of their zero-incidence angle, however they are generally more costly to maintain and have higher thermal losses involved. It is highly imperative that the PTC reflector surface has constant good reflectance as this is where the Direct Normal Irradiance (DNI) is absorbed and emitted to the focal line of the PTC. Strict design requirements such as high UV reflectance, corrosion resistance and weather resistance are necessary. Past research has proven that aluminium is a good choice of material because of its low cost and high reflective properties.
A PTC system can be applied in different areas according temperature output requirements. For lower temperature requirement (100°C- 250°C) they are used for domestic heating, heat driven refrigeration systems and air conditioning units. For higher temperature requirement (300°C- 400°C), they are used in CSP plants arranged in array form with multiple PTC units connected together ultimately forming a PTC plant. Generally, all CSP plants are located in dessert arid regions where the exposure to sunlight is hardly hindered. Maximum exposure to sunlight is necessary for a CSP plant as the solar energy that reaches the earth is about 170 trillion kWH and the major aim of all CSP plants is to harness as much as possible effectively. In this regard, the reflectivity of mirror facets of a collector unit needs to be kept clean and free of any substance that reduces reflectivity. Due to CSP plant location, dessert storms and sandstorms occur frequently causing sand particles to be deposited on the surface of mirrors. Mirror soling is defined as the deposition of dust particles on mirror facets resulting from particle movement from one region to the mirror. Dust particles can absorb and deflect solar rays that hit the mirror facets limiting reflectivity and limiting the performance output of a CSP plant. Mirror soiling is phenomena that cannot be easily prevented 100% as there are different sizes to particles and one would have to stop the weather and climate altogether.
PTC plants as well as other CSP plants experience mirror soiling on a daily basis of their operation. In dealing with this problem, plants have employed cleaning methods that commonly utilize a large volume of water which gives favourable results in trying to maintain high reflectivity. However, for a location that is dry and arid, it is not economical to carry on using water where it is a considerable finite resource. To minimize water usage in handling this problem to a significant number or nil, researchers have tried automated novel methods, water saving methods and dust prevention methods. A dust prevention method that has proven to reduce mirror soiling to a significant number is the installation of a wind barrier. It has been numerically proven and validated that a wind barrier, placed in the prevailing wind direction, can deflect dust particles away from a defined mirror location.
The presented thesis and research aim to re-introduce porous barriers and non-porous barriers as a simple economical practical approach that can minimize mirror soiling and present it as an alternative solution that lessens the volume of water used to clean collector facets. The thesis is purely simulation-based and incorporates particle mechanics and computational fluid dynamic (CFD) to show results and performance of wind barriers ultimately deriving an optimum candidate that can be manufactured and used in CSP plants. The study used ANSYS 2019 packages as the simulation tool to perform simulations and optimization procedures. Results showed that an optimum porous barrier has the capability to increase a CSP plant efficiency by a significant percentage.
Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2021.