Enhancing concentration ratio of solar concentrators
Creators: name: Rodriguez Sanchez, David
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401204
subject: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)
classificationCode: 510299
subject: Atomic, molecular and optical physics not elsewhere classified

description: Solar irradiation is approximately 1kW/m2 on the surface of the Earth and utilising only 1 % of global solar resources could be enough to achieve the recommendations of international organisations to achieve a long-term climate stabilisation. Concentration of solar energy is a possible solution to overcome the low density disadvantage and to achieve a more efficient conversion to other types of energy such as electricity.<br><br>Concentration is limited by the Sun-Earth geometry. The maximum limit of concentration for an ideal concentrator is established by the laws of thermodynamics for those concentrators which concentrate the Sun onto a point-like receiver (3D concentrators). In the past, a simplification of this limit was conducted to calculate the limits of those concentrators which concentrate the Sun onto a linear receiver (2D), and it was called the thermodynamics limit of 2D receivers. This limit was not directly calculated as a product of thermodynamics laws but as a simplification of the 3D case and in this work, it will be demonstrated how this limit was underestimated by 20 %.<br><br>Solar concentrators have been designed in the past considering that no rays are to be missed in the receiver after being reflected (or refracted) in the primary concentrator. This assumption makes it possible to analyse concentrators by considering a cross-section along the symmetry plane of the systems. By applying the edge-ray theorem while maintaining the etendue of the solar bundle, the receiver size can be calculated for a given concentrator. Most of solar concentrators are based on parabolic reflectors and the assumption that no rays can miss the receiver further limits the maximum concentration achievable. This limitation depends on whether the parabolic concentrator is a dish (3D) or a trough (2D) and ranges between ¼ and ½ the theoretical limits of concentration.<br><br>In this work, an increase of concentration ratio is explored by sacrificing optical efficiency. If rays are missed in the absorber, concentrations up to the theoretical limits are found for parabolic concentrators by maintaining the simplification of the cross-section analysis. Further research demonstrates how this simplification, though practical for designing 100 % optical efficient concentrators and accurate if no rays are missed, is not accurate if rays are missing the target. Calculations of concentration for parabolic concentrators without cross-section simplifications conducted in this work lead to the conclusion that the thermodynamic limits of concentration are achievable for ideal 3D parabolic concentrators while there can be a concentration surpass of a 20 % the previously stated as limit of concentration for 2D systems. These new limits are reached with a reduction in optical efficiency and they would be not practical for real concentrators. However, it is demonstrated how concentration ratios around 80 % the previous theoretical limit can be achieved for parabolic troughs with cylindrical receivers while maintaining optical efficiencies above 70 % with concentrations close to the previous limit can be achieved with troughs and flat receivers maintaining optical efficiency above 80 %. Ray tracing simulations conducted to validate the theoretical development matched the results obtained in this thesis.<br><br>The missed radiation can be redirected to the receivers by the addition of a second-stage optics. Previous works have addressed complicated geometries for the secondary optics, and concentration and thermal improvements have been achieved. However, due to the complicated geometries required, the large dimensions of the secondary mirrors or to a combination of both, the proposed secondary optics have not become a practical solution. In this thesis, a flat secondary reflector is proposed as an alternative to increase the concentration ratio while maintaining simplicity. A theoretical analysis to calculate the appropriate dimensions of a secondary flat reflector for a given primary mirror is developed in this thesis and ray tracing is conducted to validate the equations obtained. A flat reflector will have a minimal economic impact in the cost of a parabolic trough and it allows larger concentration ratios for identical primary mirror areas compared to a standard parabolic trough. Increases of concentration ratio up to 80 % are observed when a secondary flat reflector is inserted in a commercial system, while the shadow area introduced in the primary mirror is usually less than 15 % of the primary mirror area. The increase in pumping power is offset by the increase in system efficiency.<br><br>The inclusion of the flat secondary reflector changes the trough tolerance to misalignments and the flux distribution along the receiver's surface. Both effects are analysed with ray tracing simulations considering the flat secondary mirrors and the original absorber of representative benchmark parabolic mirrors. In some cases, the required dimensions for the flat secondary reflector would make it a non-realistic solution due to the impossibility of encapsulating it within a glass cover. To overcome this issue a proposal of a shortened version of the secondary mirror is evaluated in the simulations. A shortened secondary flat reflector will decrease the pernicious effects of the shading that the secondary mirror projects over the primary mirror, but will change the tolerance to misalignments of the troughs as well.<br><br>An additional advantage of the inclusion of a secondary optics in the parabolic trough is the increase of uniformity of the flux around the absorber; that will reduce temperature gradients and thermal stress and could have implications on the thermal performance of the troughs. Literature is not completely clear about the effect of the flux distribution on the performance of parabolic trough receivers and to offer a better understanding of them CFD simulations are conducted for an evacuated and a non-evacuated absorber under different flux profile conditions, considering as well the influence of flow rate in the heat transfer fluid and wind velocity on the glass cover. The influence of the flux distribution on the thermal performance of the absorbers is demonstrated, but the decrease of performance caused by irregular fluxes can be compensated with an increase of flow rate which will not cause a noticeable increase in pumping power.<br><br>The increase of the concentration achieved with the secondary flat reflector is significant, but the associated shading will decrease the energy input to the mirror. CFD simulations for two representative benchmark absorbers and their modifications with a secondary flat receiver show a potential increase of performance, especially for an evacuated tube and a shortened flat secondary reflector. A similar performance was obtained when evaluating the receivers with no wind, maximum flow rates, and low emissivities. However, the secondary flat reflector receiver will improve the performance of parabolic trough receivers at maximum flow rates up to 3 % if emissivity and wind velocity increases and the increases are higher if the flow rate is decreased. As power plants are required to work at different flow rates depending on the solar irradiation available, the lower drop of performance when the flow rate is decreased would help to increase the efficiency of the plants.<br><br>In the case of the non-evacuated absorber, the increase of performance does not happen in the range of working temperatures of those collectors, which is lower than the evacuated ones, if low emissivities are considered. However, the emissivity of non-evacuated absorbers is normally higher than the evacuated ones. The inclusion of an insulation layer in the upper part of the receiver or the modification of the secondary reflector to act as a flat plate collector in its upper surface to recover the shadow losses would make the secondary flat reflector receiver offer higher performance than the standard one.<br><br>In any case, it is shown how the secondary flat reflector can improve the thermal performance of the collectors if the output temperature is increased. This would help to increase the efficiency of the Rankine cycle for electricity generation or increase the applications for industrial heat. If alternative fluids such as Molten Salts allow working temperatures near 500 °C the flat secondary receivers could become a promising solution to increase parabolic troughs performance.<br><br>Finally, an experimental comparison was conducted for both a standard and a secondary flat non-evacuated absorbers. Due to limitations in the experimental facility the temperature range of the test was limited to 100 °C. In this case, due to real misalignments and inaccuracies on the primary mirror, the flat secondary reflector offered a higher performance than the standard one, and it is proof that, in real conditions, the optical efficiency of those collectors can be higher than the standard ones due to a better tolerance to misalignments.