Abstract
The conversion of solar energy to heat can be performed in direct absorption solar collectors, where the radiation from the sun is absorbed by a fluid. There are various types of fluids that can be used, and recently, nanofluids (i.e., liquids with immersed nanoparticles) have been investigated by researchers. Nevertheless, nanofluids have inherent drawbacks such as cost, toxicity, and clogging. This paper considers the use of fluids that are inexpensive and neutral to the environment, namely, coffee colloids. These types of fluids have already been tested for solar energy applications, but they have not yet been compared with nanofluids. In this research, we conducted a series of simple experiments where both coffee colloids and carbon black nanofluids were analyzed under the same conditions. According to our results, the thermal efficiency of coffee colloid and the nanofluid systems is, respectively, 12% and 16% greater than that of pure water. In addition to the experiments, we developed a mathematical model that is based on the Beer–Lambert law and a heat balance equation. Despite its simplicity, the model predicts the results relatively well.
Highlights
There are usually two techniques for utilizing solar energy: photovoltaic and solar thermal
We focused only on the coffee colloids because the studied carbon black nanofluids had very high values of the extinction coefficient that they became non-transparent when using our setup
This figure is shown as a demonstration of how the process develops over time
Summary
There are usually two techniques for utilizing solar energy: photovoltaic and solar thermal. The present paper is devoted to solar thermal technology. This type of technology uses a collector, where solar irradiation is directly converted to heat. There are usually two types of solar collectors: a surface collector or a direct absorption collector (see, e.g., the study by Gorji and Ranjbar). The first one comprises a solid surface that is exposed to solar irradiation and is the warmest part of the design. This is necessary for heating fluids, but it results in heat loss to the surroundings
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