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Synthesis of coffee-based colloids. a) Moka coffee maker used for brewing coffee (top left); particle size distributions of suspended coffee (top right); Scanning electron microscope (SEM) images of coffee particles (bottom). (b) Colloids with different concentrations of G30 (from right to left): pure G30 fluid (56.17 g / l of suspended particles); G30w10 fluid (10% dilution); G30w1 fluid (1% dilution in water); pure water. Credit: Scientific reports, doi: 10.1038 / s41598-019-39032-5
Solar energy is one of the most promising resources to help reduce fossil fuel consumption and reduce greenhouse gas emissions to ensure a sustainable future. The devices currently used to convert solar energy into thermal energy rely primarily on the indirect absorption of sunlight, whose efficiency is generally limited due to significant heat loss by convection in the surrounding environment. A promising alternative is the direct absorption of sunlight, where a fluid can serve as both a solar energy absorber and a coolant. The advantage of this technique lies in the reduction of convective and radiation heat losses, as the temperature peak moves from the absorbing surface (indirect absorption) to the main region of the carrier fluid (direct absorption). In a recent study, Matteo Alberghini and colleagues from the Departments of Energy, Applied Science and Technology and the National Optics Institute of Italy studied a durable, stable and cost-effective solution-based colloid. of coffee to implement direct solar absorption. The results of their work are now published on Scientific reports.
In the work proposed by Alberghini et al. the colloid consisted of distilled water, Arabica coffee, glycerol and copper sulfate to optimize the properties and biocompatibility of the fluid. Scientists analyzed the photothermal performance of the proposed direct solar absorption fluid and compared its performance to that of conventional flat plate collectors. They showed that collectors could be customized and made with 3D printing for experimental testing.
Existing carbon-based nanocolloids have disadvantages, despite the promising thermo-physical properties suitable for direct solar absorption, due to cytotoxicity and adverse effects on the environment. In the course of pioneering experimental work, the researchers used a black fluid containing Indian ink in water (3.0 g / l) for the direct absorption of solar thermal energy. They observed an encouraging performance that led to the use of nanocolloids, also called nanofluids, to allow direct absorption of the sun. The fluids are generally characterized by a suspension phase capable of imparting improved photothermal properties at the base of the fluid. If they are designed appropriately, these nanocolloids will have promising potential for solar-thermal conversion.
Optical properties of coffee – based colloids (dilutions at 1%, 10% and 100% in water). (a) Comparison of spectral extinction coefficient of coffee-based colloids at different dilutions and suspension at 0.05 g / l of carbon nanohorns in water27. Preparation G30 (100% dilution) is coffee with 2 ppm copper sulfate and 30% by weight. glycerol; G30w1, G30w10 are respectively 1% and 10% volume fractions of G30 in distilled water. (b) fraction of stored energy (EF) versus path length for the three coffee-based colloids considered. The solid lines correspond to the fraction of energy obtained with the Planck blackbody distribution, while the dashed lines are obtained with the AM1.5 standard spectrum. By way of comparison, the curves for a suspension of 0.05 g / l of carbon nanohorns in water are also presented. Credit: Scientific reports, doi: 10.1038 / s41598-019-39032-5
In the present work, Alberghini et al. first optical characterization of proposed coffee-based colloids. Because coffee is a complex substance, scientists used Arabica coffee prepared in an aluminum coffee maker known as "mocha" for cooks, for consistency. They followed a protocol to prepare the "student's coffee" allowing for an increased suspension of caffeine particles in the water and performed a scanning electron microscope (SEM) to evaluate the particle size distribution in the solution. resultant. They then introduced glycerol into the preparation to lower its freezing temperature for outdoor use in cold or frosty weather. Finally, the scientists added copper sulphate (CuSO4) to reduce the risk of algae formation or mold in the liquid. They examined five variants of the proposed colloid for the experiments, which were stable throughout the six-month period. The five variants were the primary colloidal solution containing glycerol (30% w / v) and CuSO4 (2 ppm), which scientists have named G30, followed by fractions of 1%, 10%, 20% and 50% of G30 in distilled water called; G30w1, G30w10, G30w20 and G30w50 in the study.
The scientists conducted characterization studies of the optical properties of the proposed colloids with respect to the extinction coefficient and calculated the fraction of stored energy of the fluids. They calculated the extinction coefficient in the study by summing the absorption and scattering coefficients for a given wavelength. Scientists recorded an extremely intense optical coefficient for the G30 fluid, which they attributed to the contents of the coffee. The height of recorded peaks decreased with the increase in dilution in the water. Subsequently, Alberghini et al. calculated the fraction of stored energy of the solutions as a function of the incident solar radiation and the penetration distance in the fluid, called the path length. The G30 fluid had the largest stored energy, which gradually decreased with the increase in water dilution.
Set up for solar absorption tests. (a) Flowchart for the design and manufacture of solar collectors: from the CAD model to the 3D printed sensor, through the final assembly. During the field tests, the performance of the direct solar absorber is compared to that of the traditional flat plate sensor. (b) Schematic of the experimental device used to test the effectiveness of coffee-based colloids for the direct absorption of solar thermal energy. The solid lines represent the hydraulic hoses for the colloidal flow; dotted lines of electrical wires for data acquisition. Credit: Scientific reports, doi: 10.1038 / s41598-019-39032-5.
Scientists then experimentally investigated the photothermal performance of coffee-based colloids versus a selective absorber with specifically designed solar collectors. They used similar geometries in the experiments to study the direct and indirect absorption of sunlight. Scientists first designed solar thermal collectors with the aid of computer-aided design (CAD) software prior to their manufacture.
During direct absorption, colloids flowing into the channel directly absorbed sunlight. For indirect absorption, Alberghini et al. mounted a selective surface absorber on the collector so that water can flow into the underlying channels. With the aid of a peristaltic pump, they ensured a constant flow of fluid in the channels and controlled the temperature of entry of the fluid with the aid of a thermostatic bath. To compare the efficiency between the two sensors, they calculated thermal losses and optical efficiency through the conservation of energy in the system. They also tested colloids at three different flow rates and reported the corresponding average optical efficiency of fluids at flow rates.
<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2019/4-coffeebasedc.jpg" title = "Thermal Performance Modeling. (a) Decomposition and Analysis power components (1D model) for different configurations (direct absorbed and surface-selective flow) at flow rates of 0.276 ml / s (upper histogram) and 0.414 ml / s (lower histogram) Higher fluid velocity reduces heat losses to the environment The absorption of the irradiance is not influenced by the different mass flow rates, which allows the fluid to capture the highest possible degree of irradiance, namely the fluid G30w50 (b) Fluid temperature profiles at the outlet (inlet temperature is constant) obtained with the 2D model: the colloids have a surface temperature lower than that of the surface receiver and the maximum thermal losses are lower From Lower fluid concentrations result in reduced surface temperature and less sharp profiles. Scientific reports, doi: 10.1038 / s41598-019-39032-5. ">
Thermal performance modeling. (a) Decomposition and analysis of power components (Model 1D) for different configurations (direct and selective surface absorption) at flow rates of 0.276 ml / s (upper histogram) and 0.414 ml / s (lower histogram). Higher fluid velocity reduces thermal losses to the environment due to lower operating temperatures. The irradiance absorption is not influenced by the different mass flow rates and the design therefore favors the fluid capable of capturing the highest irradiance, namely the G30w50 fluid. (b) Fluid temperature profiles at the outlet (the inlet temperature is constant) obtained with the 2D model. The colloids have a surface temperature lower than that of the surface receptor and the maximum thermal losses are lower. Lower fluid concentrations result in reduced surface temperature and less sharp profiles. Credit: Scientific reports, doi: 10.1038 / s41598-019-39032-5.
In addition, Alberghini et al. numerical models developed and validated against experimental data. For this, they used two models; 1) a one-dimensional model based on an electrical analogy and 2) a 2D fluidic computation model (CFD). They indicated that the optical losses did not depend on the flow rate, but on the optical properties of the flowing fluids and the composition of the collectors. Scientists have maintained the sensor's efficiency by establishing a balance between heat absorption and reflection for optimal thermal performance.
<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2019/3-coffeebasedc.jpg" title = "Photothermal Performance. (a) Results obtained for the # The optical efficiency of the proposed coffee-based colloids at different dilutions (G30 volume fraction of 10%, 20% and 50% in water) and the selective surface absorber. the steady state (sampling frequency of 5 minutes) for three different flow rates (0.138, 0.276 and 0.414 ml / s) are reported.The error bars were obtained by quantizing the uncertainty about experimental data and model parameters (b) Temporal evolution of the experimental optical yield of fluid G30w50 (black), selective surface (blue), and irradiance (red) for l & # 39; Experimental test at a flow rate of 0.138 ml / sec. Scientific reports, doi: 10.1038 / s41598-019-39032-5. ">
Photothermal performance. (a) Results obtained for the optical efficiency of the proposed coffee – based colloids at different dilutions (G30 volume fraction in water of 10%, 20% and 50%) and selective absorber of surface. The average value obtained in the equilibrium state (sampling frequency of 5 minutes) for three different flow rates (0.138, 0.276 and 0.414 ml / s) is indicated. The error bars were obtained by quantifying the uncertainty of the experimental data and the model parameters. (b) Evolution over time of experimental optical efficiency of fluid G30w50 (black), selective surface (blue) and irradiance (red) for experimental testing at a flow rate 0.138 ml / s. Credit: Scientific reports, doi: 10.1038 / s41598-019-39032-5.
In this way, Alberghini et al. have shown that the proposed coffee-based colloids have competitive optical and thermal properties for direct solar absorption. The experimental results are consistent with the numerical models and validate that these fluids work in a similar way to the traditional indirect absorption technique. Scientists discovered that during operation, optimal dilution guaranteed the best energy storage capacity. The results will pave the way for the development of an unconventional family of biocompatible, environmentally friendly and inexpensive colloids for solar applications. Scientists propose using this technique in other solar-powered applications such as:
- Solar evaporation
- Desalination of sea water
- Domestic water heating, and
- Sustainable solar cooling.
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More information:
Boyle, G. Renewable Energy: Energy for a Sustainable Future. (Oxford OUP, 2012). global.oup.com/academic/produc … 59751? cc = us & lang = en &
Matteo Alberghini et al. Coffee-based colloids for direct sun absorption, Scientific reports (2019). DOI: 10.1038 / s41598-019-39032-5
Peng Tao et al. Solar interfacial evaporation, Nature Energy (2018). DOI: 10.1038 / s41560-018-0260-7
Andrej Lenert et al. Optimization of nanofluid volumetric receivers for the conversion of solar thermal energy, Solar energy (2011). DOI: 10.1016 / j.solener.2011.09.029
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