Solar Thermal Energy: 3D Compound Parabolic Concentrator with Heat Storage: Design, Construction and Testing of a 3D Compound Parabolic Concentrator with Heat Storage
Doctoral thesis
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https://hdl.handle.net/11250/3172703Utgivelsesdato
2025Metadata
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Sammendrag
The key objective of this thesis is the design, construction and testing of a 3D Compound Parabolic Concentrator (CPC) with heat storage. The 3D CPC with a reflecting surface may be difficult to construct, therefore, the surface has been simplified by using two sets of 4 flat mirrors in a squared channel arrangement. The system should be low cost, robust, safe, and technically simple to construct and operate as a small-scale solution for storing heat for cooking. The CPC was designed such that no solar tracking is required. However, tilting of a concentrator at intermediate times during a day can be accepted.
As the CPC can be built using mirror tiles or as a smooth surface, a sensitivity analysis regarding the dimensions of mirror tiles on the CPC surface was performed. Ray tracing analysis showed that uniform and square tile sizes up to 15 cm gave quite similar solar interception ratios (less than 10 % changes) compared with the smooth surface. For the strongly simplified case of only 8 flat mirrors, the height separating the 4 lower mirrors from the 4 upper mirrors was optimized using an in-house ray tracer. The optimal dividing position of the mirrors was found to be 25 % of the height of the CPC.
A comparison of the 3D CPC with a normal Parabolic Reflector (PR) for similar concentration ratios was also performed. The comparison was made using ray tracing, by scanning sun angles over the acceptance angles and comparing the interceptions on a cylindrical absorber between the two systems. The results showed that for systems with low concentration ratios, the performances of the two reflectors were similar, with around 10 % improvement in interception ratios with the CPC compared to the PR.
Tests on the simplified CPC system with a heat absorber were performed both during sunshine periods and in the laboratory. For the case of CPC tests during sunshine periods, the discretized CPC consisted of 8 mirrors, with a 0.2 m diameter and 0.3 m length cylindrical absorber placed inside the CPC. The maximum temperature reached on the cylindrical absorber was about 140 ℃. The experimental work in the laboratory included testing of the insulated iron cylinder for storing heat for cooking. The system was constructed and demonstrated for the case of boiling water and cooking beans. Cooking beans is a particular important case because the cooking time is long and charcoal is currently the most common energy source for cooking beans in Tanzania. The iron cylinder was heated by grid-powered heating elements to about 220 ℃. After heating the cylinder, a cooking pot was placed on top of the cylinder and cooking of beans was successfully demonstrated. A computational analysis also indicated that cooking can also be made at the lower temperatures experienced in the CPC heating case. The present study forms the basis for further improvement of the concept and considerations of the designed and simplified CPC for storing heat for cooking.
Further work could be on field tests of similar systems in the more favorable solar conditions in Tanzania. The idea of the system is that a light weight CPC can be placed over a heat storage system for charging during sunshine hours. The CPC can then be replaced with an insulating cover over the heat storage for heat retention. The heat storage can then be moved for indoor cooking at times when needed.
Består av
Paper 1: Lwiwa, Casiana Blasius; Nydal, Ole Jørgen. 3D Raytracing for optimizing the size of surface tiles on a Compound Parabolic Concentrator. Southern African Sustainable Energy Conference, South Africa, 2021Paper 2: Lwiwa, Casiana Blasius; Nydal, Ole Jørgen. 3D Ray tracing for optimizing a mirror tiled Compound Parabolic Concentrator 16th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT) 2022
Paper 3: Lwiwa, Casiana Blasius; Nydal, Ole Jørgen. A Ray Tracer for optimizing solar concentrating systems: Thecase of discretized Compound Parabolic Concentrator. I: Proceedings of the 63rd International Conference of Scandinavian Simulation Society, SIMS 2022 https://doi.org/10.3384/ecp192003 This is an open access article under the CC BY license
Paper 4: Lwiwa, Casiana Blasius; Nydal, Ole Jørgen. Interception on Solar Absorbers: Ray Tracing for Comparison between a Parabolic Reflector and a Compound Parabolic Concentrator. ISES and IEA SHC International Conference on Solar Energy for Buildings and Industry, EuroSun2022 Proceedings
Paper 5: Lwiwa, Casiana Blasius; Nydal, Ole Jørgen. Sensible heat bean cooker. Tanzania Journal of Engineering and Technology (TJET) 2023 ;Volum 43.(1) s. 79-84 https://doi.org/10.52339/tjet.v42i1.890 This is an open access article under the CC BY - ND license
Paper 6: Lwiwa, Casiana Blasius; Nydal, Ole Jørgen. Experiments on a discretized 3D compound parabolic concentrator with a sensible heat storage. Next Energy 2024 ;Volum 7. https://doi.org/10.1016/j.nxener.2024.100224 This is an open access article under the CC BY-NC license