Computational investigation of swirling jet impingement in a concentrated solar tower receiver

Abstract

With growing concern of climate change and environmental pollution the need for better renewable technologies is a necessity. Solar energy shows the most promise in meeting global energy needs and competing with fossil fuels economically. Currently solar power is generated with photovoltaic (PV) panels and stored in batteries. The disadvantages of PV are expensive batteries, limitations on panel efficiencies and electrical grid considerations to balance electricity generation. Concentrated solar power (CSP) is an alternative that addresses PV limitations and shows potential in a hybrid power generation mix, especially because of its thermal storage capabilities and ability to provide process heat directly. CSP consists of a variety of systems. Of all available CSP technologies, solar power towers (SPT) show potential to reach high temperatures and effectively store thermal energy. For SPT the central receiver shows promise for improvement in effectively capturing heat. Of the many methods available to improve heat transfer, jet impingement with swirl can improve heat transfer for the receiver fluid. Jet impingement heat transfer is well known to enhance local heat transfer because of the local increase in the heat transfer coefficient and Nusselt number. Swirling flows have also shown to enhance heat transfer for internal pipe flow arrangements and other heat transfer applications. The effect of swirl and jet impingement are not often considered cumulatively as in the current study. For a proposed solar receiver design, a swirling impinging jet is proposed to enhance heat transfer. The flow behaviour is investigated numerically using computational fluid dynamics (CFD). Ansys Fluent is used to model the flow behaviour and to validate the model with available experimental results. From the validation study the Transition Shear-Stress-Transport turbulence model is shown to predict jet impingement the best. A 2D axisymmetric assumption is however shown to not predict the heat transfer well while a costly full 3D transient Large Eddy simulation does. As LES is too expensive for use in a parametric investigation, both 2D and 3D RANS simulations were used as an engineering tool to improve and optimise heat transfer, keeping in mind their shortcomings. Swirling jet impingement is further investigated for a curved impingement surface. This is the first investigation of its kind where swirl, jet impingement and a curved impingement surface are considered. From the validation study, a CFD model is used to investigate how curvature affects heat transfer. The parameters show that surface curvature has a large effect on heat transfer and it is shown that a potential optimal curvature exists for the unique flow arrangement. A surrogate optimisation model is used from the numerical results to improve the design. To provide a realistic heat source on the solar receiver, Monte Carlo ray tracing (MCRT) is used to model the heliostat field. The MCRT model can better predict the solar flux distribution on the receiver absorbing surface. The solar flux distribution is an important consideration for the receiver design. The CFD model of the receiver showed that while swirling jet impingement did not increase the outlet temperature of the heat transfer fluid, it did however show potential to reduce the receiver’s maximum surface temperature and as well as radiation losses. The thermal enhancements made do however come at the cost of an increased pressure drop.