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.
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.
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.
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.
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.
New form of optical material based on developed graphene
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