Abstract
Solar energy has become an important renewable energy source for reducing the use of fossil fuels and to mitigate global warming, for which solar collectors constitute a technology that is to be promoted. The use of nanofluids can increase the efficiency of solar into thermal energy conversion in solar collectors. Experimental values for the specific heat, thermal conductivity and viscosity of alumina/water nanofluids are needed to evaluate the influence of the solid content (from 0.25 to 5 v%) and the flow rate on the Reynolds, Nusselt and the heat transfer coefficient. In the laminar flow regime, thermal conductivity enhancement over specific heat decrement is key parameter, and a 2.34% increase in the heat transfer coefficient is theoretically obtained for 1 v% alumina nanofluid. To corroborate the results, experimental tests were run in a flat plate solar collector. A reduction in efficiency from 47% to 41.5% and a decrease in the heat removal factor were obtained using the nanofluid due to the formation of a nanoparticle deposition layer adding an addition thermal resistance to heat transfer. Nanofluids are recommended only if the nanoparticle concentration is high enough to enhance thermal conductivity, but no so high so as to avoid wall deposition.
Highlights
Increase in the energy global demands and the use of non-renewable energy sources like fossil fuels have reduced the availability of these sources and have produced strong negative environmental effects, such as air pollution and global warming
The thermal conductivity, specific heat and viscosity of the alumina nanofluid were measured within a wide range of temperature and solid content and results were modeled
It was concluded that the mixture rule could be used to predict the nanofluid properties, as previously done in the literature by other authors
Summary
Increase in the energy global demands and the use of non-renewable energy sources like fossil fuels have reduced the availability of these sources and have produced strong negative environmental effects, such as air pollution and global warming. In order to mitigate these inconveniences, research works have focused on improving the efficiency of technologies using renewable energy sources like solar energy [1,2]. Solar collectors are used to convert solar energy into thermal energy using a heat exchanging fluid. The collector absorbs solar radiation by an absorber plate and transfers heat to the absorber fluid by, increasing its internal energy, which can be used for further applications. Flat plate solar collectors (FPSC) are used within the 40–100 ̊C range, with no optical concentration. Their simplicity, easy maintenance and low operating costs make them suitable for domestic applications. The working fluids used as absorbers are mainly water and mixtures of water and ethylene glycol, but the main drawback of these conventional fluids is their poor thermal properties as they confer the conversion process poor thermal efficiency
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