School of Mechanical Engineering Seminar

Monday, February 21, 2022 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Solar Beam Splitting Collector for Open Field Agrivoltaics

Shahaf Baron

M.Sc. student of Prof. Abraham Kribus^a and Dr. Helena Vitoshkin^b

^aSchool of Mechanical Engineering, Tel Aviv University, Tel Aviv, 69978, Israel

^bInstitute of Agricultural Engineering, Agricultural Research Organization, the Volcani Center, P.O.B. 15159, Rishon LeZion, 7505101, Israel

Installing photovoltaic (PV) collectors above agricultural fields (Agrivoltaics) is an attractive solution to the shortage of available land area for solar power generation. This is an emerging area of research with many experimental projects being installed over greenhouses and open fields, over different crops, and using other PV technologies. However, shading by the collectors blocks sunlight required by the agricultural crop for photosynthesis. This conflict limits the density of collectors (land coverage ratio) and reduces the feasibility of the agrivoltaics solution for most crops. The proposed solution is a spectral splitting collector that transmits the visible light (photosynthetically active radiation, PAR) to the crop, directing the rest of the solar spectrum (infrared and near-infrared) to a PV panel. Such a collector should minimize or even eliminate the reduction in crop yield while allowing the total density of the collector field. The objective of the research is to validate this concept, using an experimental demonstration of a spectrum-splitting collector and measurement of its impact on the electricity production yield. The spectrum-splitting collector includes a spectrally selective ‘hot mirror’, a standard Si-based PV panel, and a single-axis tracking mechanism for daily (East-West) tracking. The experimental work is accompanied by a simulation of the optical and electrical performance under the same conditions for a deeper analysis and validation of the results. An expanded simulation allows to predict the corresponding performance of a future large-scale field covered with collectors.

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School of Mechanical Engineering Seminar
Monday, January 10, 2022 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Ben Uziel

MSC Herman

Generation of smaller microbubbles under inertial regime

In recent years, new techniques of microbubbles generation are being widely investigated. Its intensively studied due to its importance in novel medical applications, pharmacology, material science, and the food industry. There is a growing demand for robust generation techniques with highly controlled microbubbles parameters, such as bubbling frequency, the mean bubble diameter and the polydispersity index (PDI). For instance, there is a tremendous need of stable mass production techniques of monodisperse micro bubbles as an ultrasound imaging contrast agent, besides drugs\ genes and oxygen means of delivery, with highly controlled parameters to insure safe use. The present study examines the establishment of a new monodisperse micro bubble generation technique. The preliminary goal of this work is to develop, design and assemble a flexible experimental micro bubble generation apparatus – a gas feed micro needle in the center of a liquid co-flow-focusing microchannel. As well, to develop the experimental ability of capturing several micro bubbles for subsequent use for analysis.

This study focuses on bubbles generation under an inertial regime at relatively high Reynolds numbers (Re>100), as compared to current methods, without the use of surfactants. The experimental system developed enabled high-speed visualization and comparison to  two phase dynamic simulations.

The high flexibility of the  experimental system allowed the study of bubbles generation using various microneedles diameters and different locations of the needles relatively to the channel entrance. The system could be examined under wide range of Reynolds numbers using derived pressures (separately controlled) of air and ultra-pure water. Three known regimes of microbubbles generation observed, in agreement with the known regimes map exist in literature.

Uniform sizes of microbubbles in wide diameter range of 9-50 um were accepted in high  generation rate of >10⁵ Hz. In contrast, the capture of microbubbles was executed under high inertial effects in which the microbubble breaks down over reverse microneedle into a smaller bubbles of the size of the needle tip. Here, various and less uniform sizes of microbubbles could be reached, but effectively, deacceleration of the microbubbles could be observed, an ability allows the controlled extraction of several microbubbles.

Several  interesting phenomena observed, such as coalescence of microbubbles in relatively low flow rate of the liquid in early stages of bubble formation and coalescence of bubbles downstream the channel, what complete a full research of a previous study of the research group, when the jetting to dripping phenomena could be observed.

Successfully, at the same system, the generation and capture of microbubbles of sizes down to below 10 um was executed while both methods rely on inertial effects although the microscale.

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