Konvektive Verluste an offenen volumetrischen Solarstrahlungsempfängern

Abstract

The open volumetric air receiver is a promising technology for solar power towers. Researchers have investigated the behavior of the porous Absorber structures and the operational performance under fluctuating incident solar radiation. All of the research work carried out is based on assumptions on the flow conditions at the receiver front, which so far were not investigated. The flow conditions at the front determine to what extent the receiver cycle is driven as a closed loop, which is quantified by the air return ratio. The energy losses due to the partial exchange of hot cycled air by ambient air are referred to as convective losses. In the present thesis, the influence of the air return ratio on the overall System efficiency was analyzed. More specifically, the interaction between receiver cycle and power cycle was investigated to determine optimized design Parameters for future solar power towers. The simulations demonstrated the significant impact of the air return ratio on the overall system efficiency. This result provided motivation for a thorough examination of the flow conditions at the receiver front. The approach was to develop a measurement setup which provides velocity fields in front of a modular section of the receiver, under real operating conditions. The experimental setup was assembled in the high flux solar simulator in Cologne, and using the installed Particle Image Velocimetry measurement system, a quantitative examination of the flow conditions was feasible for the first time. The measurement results showed the turbulent nature of the gas flow and highlighted the phenomena which affect the convective losses. A numerical model of the system was also developed using the open source software OpenFOAM. Velocity fields and integral temperature values were compared with the experimental data in order to validate the simulations. Good agreement between experiment and simulation allowed for an in depth analysis based on the numerical model. Hence, characteristic operating points for the open volumetric air receiver were determined. A weak dependency of the air return ratio on the overall mass flow was found. Furthermore, an intentional partially open loop for the receiver cycle was identified as an operating mode which can increase the performance of solar power towers in the future. Findings in this work lead to a better understanding of the convective losses and the flow conditions at the receiver front. Further investigations are required in order to transfer this knowledge from the laboratory scale to the power plant scale.