Application of compact flow channel geometries to pressurised solar receiversa numerical and experimental analysis

Abstract

Resumen Concentrated solar thermal energy systems have an immense potential to renewably and sustainably address the growing global energy demand. The application scope of these technologies is broad and ranges from power generation to industrial process heating which in itself is wide ranging. The ability of concentrated solar thermal technologies to produce heat, via a working fluid, at high temperatures is what allows for this broad range of application. Operating temperatures in excess of 1000 °C can well be attained through concentrated solar thermal. A critical component of any concentrated solar thermal system is its receiver which is the subsystem that absorbs the concentrated solar radiation incident on it and transfers it to a heat transfer fluid that passes through it. The efficiency and effectiveness with which the solar receiver is able to transfer the incident radiation to the heat transfer fluid largely decides the overall performance of any concentrated solar thermal system. In this thesis, a novel type of solar receiver, is proposed and explored with the objective of developing receivers with performances rivalling or bettering those of the state of the art. The proposed receivers are based on compact heat exchanger concepts in so far that the flow channels of the receivers imitate those typically used in compact heat exchangers. The motivation behind this are the well understood and demonstrated performance enhancements achieved in compact heat exchangers, especially when the working fluid is a gas or a supercritical fluid. This improved performance owes itself to the compactness of the flow channels which boosts the heat transfer to the fluid though at the expense of an increased pressure drop. Smaller sized receivers, which is an inherent feature and advantage of compact structures, results in savings in material costs. There are several compact flow channel geometries, commonly used in compact heat exchangers, which may be employed in solar receivers. In order to evaluate the performance of each of these flow channel geometries, a numerical model of solar receivers using a pressurised fluid has been developed. The numerical model has been programmed in such a way as to easily facilitate the inclusion of different flow channel geometries and vary their respective geometrical configurations. Applying the developed numerical model to a central solar pressurised air receiver power plant coupled to a supercritical carbon dioxide Brayton cycle, a steady state parametric and optimisation analysis was performed on six different receiver flow channel geometries. The six geometries selected were the plain rectangular, plain triangular, wavy, offset strip, perforate and louvred fin flow channels. Four geometrical parameters, common to all flow channel geometries, were identified and varied in the parametric study. These are the channel height, channel breadth, channel wall thickness and number of vertical channels. Performance indicators for receiver evaluation were studied and it was determined that exergy efficiency, which accounts for both heat transfer to the fluid and pressure drop in it besides the incident solar radiation, is a useful tool for optimisation and comparison. The parametric study revealed that perforated fin receivers, followed by plain rectangular and wavy fin receivers, exhibited the highest exergy efficiencies with taller and narrower channels with thicker walls and fewer vertical channels improving this efficiency. The methodology used in this analysis, besides the receiver operating conditions and system modelling assumptions, greatly affects the results and relative performances of the receiver configurations. A validation of the model and some of its underlying assumptions was conducted by comparing it to a previous study and a more complex three-dimensional computational fluid dynamics model. To substantiate the findings of the numerical model, an experimental campaign on receivers of differing flow channel geometric configurations was proposed. The high flux solar simulator of the IMDEA Energy institute, namely KIRAN-42, was employed as the radiation heat source for the experiments. A calorimetric testbed was designed, assembled and commissioned for the purpose of experimentation on pressurised gas receivers. Procedures for the operation and control of the pressurised receiver testbed were established after a series of preliminary test runs. Four variants of the plain rectangular fin receiver were designed and fabricated using additive manufacturing. The geometrical variations in the receivers were increased height, increased breadth, and reduced channel wall thickness respectively. The receivers were manufactured in stainless steel and Inconel 718 though only the stainless-steel receivers were experimented on. An experiment plan was drawn out specifying the experimental characterisation to be performed on each receiver. This was performed varying the mass flow rate of air, receiver inlet pressure and incident radiation peak flux. The experimental campaign confirmed important findings and predictions of the pressurised receiver numerical model. These include the maximum thermal efficiency and pressure loss occurring at the smallest channel size and also the positive effect of taller and narrower channels. The maximum thermal efficiency observed was 94.7% at an inlet pressure of 12 bar, a mass flow rate of 2 g s-1 and a peak incident flux of 400 kW m-2 with the corresponding pressure drop measured at below 1% of the inlet pressure. This performance, in terms of thermal efficiency and relative pressure drops, is on par and even surpasses the state-of-the-art receivers of its type. Such high thermal efficiencies (above 90%) and low relative pressure drops (below 1%) were observed for other operating conditions and receiver geometries as well. The numerical model of the receiver was modified to better represent experimental realities such as the flow channel surface roughness, non-uniform incident radiation, uneven receiver surface absorptance and receiver inlet/outlet section pressure losses. While the pressurised receiver numerical model generally corresponded well with the experiments, within the bounds of experimental error, a sensitivity analysis was performed to evaluate the influence of operational parameters that had significant associated uncertainties. These included the mass flow rate, incident radiation flux, inlet pressure, air composition and receiver surface absorptance. The performance indicators evaluated in this sensitivity analysis were the receiver outlet temperature, pressure drop, thermal and energy efficiencies. In conclusion, the use of compact flow channels in pressurised gas receivers has been numerically and experimentally demonstrated to produce high performance receivers. When optimised for geometry, these receivers can effectively transfer incident solar radiation to the heat transfer fluid at thermal efficiency and pressure drop combinations that rival and excel the state of the art in pressurised gas receivers.