Session: 05-03 Solar Receiver Design 1

Paper Number: 108736

108736 - Experimental Investigation of Heat Transfer Mechanisms in a Beam Down Molten Salt-Based Volumetric Solar Thermal Receiver 

Molten salt concentrated solar power systems with tubular surface absorbers are known to be limited by low capture efficiencies. A lower capture efficiency leads to higher levelized costs of electricity. On the other hand, volumetric receivers can act as promising alternatives to surface absorbers. These receivers consist of semi-transparent media (e.g., molten nitrate salts) that are directly irradiated and absorb solar radiation volumetrically. Potentially, they can sustain higher solar fluxes as compared to the surface absorbers and eliminate temperature differences between the absorber and the heat transfer fluid. This phenomenon can in turn help in reducing heat losses. Nevertheless, no studies have experimentally quantified the limits of the performance of volumetric receivers. This has happened mainly due to complex interactions between radiation, natural convection, and volumetric heating which are involved in analyzing these systems. Recently, we reported a mechanistic model of liquid-based volumetric receivers for solar thermal energy conversion. The model was employed to understand the theoretical limits of the capture efficiency and temperature uniformity of a volumetric receiver for a range of design parameters. Utilizing the model, three characteristic regimes single-mixing-layer, triple-layer, and conduction-dominant were identified. Moreover, a dimensionless volumetric receiver number capable of predicting the regime transitions for a wide range of operating parameters was reported. However, the mechanistic model was based on certain assumptions and there was a disagreement (< 1.7 K difference for bulk temperatures and < 9 K for boundary temperatures) between computational fluid dynamics models (used for benchmarking) and the mechanistic model. One of the reasons for this disagreement was the lack of studies done on such systems and the unavailability of experimentally determined Nusselt number correlations. In order to use the mechanistic model in real-life applications and gain more confidence in its reliability, an experimental validation is required. In this study, we are utilizing a lab-scaled high solar flux (6.5 kW) facility to validate the findings of the mechanistic model. Basically, a powerful xenon lamp (spectral response very close to that of sun) is used to replicate the input solar flux conditions. The lamp is equipped with an elliptical reflector to concentrate the energy onto the receiver top. By moving the lamp vertically up and down, the flux diameter and flux concentration level can be changed. The receiver is nothing but an open top stainless-steel container (6 in tall, 4 in diameter) filled with molten nitrate salt (NaNO₃:KNO₃, 60:40). The salt is first melted (melting point 493 K) using the solar lamp energy and then the increase in salt temperature is recorded using K-type thermocouples inserted into the salt vertically at different locations. Moreover, an infra-red camera is employed to estimate the exposed liquid surface temperature. Thus, a vertical temperature profile can be obtained by noting the temperatures at each time interval and compared with the mechanistic model’s findings. More details of the experimental design and parameters will be discussed in the ES 2023 talk.

Presenting Author: Muhammad Taha Manzoor McGill University

Presenting Author Biography: Taha is a PhD Candidate at McGill University. He did his undergrad from NUST, Pakistan and masters from KAIST, South Korea. Currently, he is investigating heat transfer mechanisms in volumetrically absorbing solar thermal receivers from a design optimization perspective. He also developed low-cost concrete based molten salt storage tanks during his PhD.