Efficiency Of Solar Thermal Systems Research Articles

Thermal and dynamic improvement of a double pass solar air heater for drying applications

The low efficiency of solar thermal systems is a drawback that hinders the effective utilization of renewable energy. In this context, a feasibility study was conducted on a double-pass solar air heater designed for solar drying applications. Given that low efficiency often results from inefficient thermal conversion, an optimization of the V-shape absorber was performed by varying the corrugation angle. The CFD method, validated by experimental results, was employed in the current study. The meteorological station located at Mohamed First University in Morocco provided real and unstable weather conditions that were inserted into the software. The results obtained were analyzed and discussed regarding the air domain temperature, collector outlet temperature, collector efficiency, and average drying tray temperature together with its distribution. The findings show that the average air temperature and outlet temperature reach a maximum of 345.53 k and 365.5 k, respectively. For absorber corrugation angles of Φ=100°, a maximum collector efficiency of 86% was achieved. In addition, different mass flow rates (MFRs) ranging from 0.003 to 0.068 kg/s were evaluated in this study. From the results, it was noticed that the optimal MFR varies depending on the products to be dried, with values of 0.028, 0.035, 0.049, and 0.068 kg/s for sweet potato, fig, cauliflower, and orange, respectively.

Efficiency Improvement and Comparative Studies of Solar Organic Rankine Systems using Nanofluids

Solar power generation has emerged as one of the most rapidly growing sources of renewable energy. The solar thermal system with a Rankine cycle used to harness solar energy and generate electricity from a low temperature heat source is an emerging technology. The major drawback of solar thermal power generation is its poor efficiency, which is around 10% to 15%. Although prior attempts to improve the efficiency of the solar thermal system and use of Nano fluids in heat transfer applications have been carried out, very little work has been carried out using Nano fluids in Rankine cycle systems. Thus, the objective of the research undertaken was to primarily improve the efficiency of a small-scale solar thermal system by selecting working fluids, optimizing the system parameters and using Nano fluids with improved heat transfer properties for capturing heat. Thermodynamic simulation tool Aspen Hysys was used to carry out simulations of the Rankine and Regenerative Rankine systems. The system was simulated with combinations of Dowtherm-Cu O/Ag/Al2O3/TiO2 as the heat transfer fluid, and n-butane, n-pentane, n-hexane, or R-134a as the working fluid. System parameters such as mass flow, temperature, and pressure were optimized to obtain maximum power output and efficiency, keeping the system constraints and practicality in mind. The use of Nano fluids improved heat transfer to the working fluid in the heat exchanger by a maximum of 50%. The efficiency of the basic Rankine cycle was determined 16.03% for n-pentane, 14.90% for n-hexane, 13.83% for n-butane, and 9.82% for R-134a as the working fluid. Further, the use of regeneration improved the efficiency of the system by 6%, 3 %, 7% and 2% respectively. Highest efficiency of 27.96% was obtained when 6% volume concentration of Al2O3 was used in the heat transfer fluid, and n-pentane was used as the working fluid in the Regenerative Rankine cycle.

Dual Fresnel lens and segmented mirrors based efficient solar concentration system without tracking sun for solar thermal energy generation

We propose and experimentally demonstrate a combination of two large Fresnel lenses and six segmented mirrors for concentrating sunlight efficiently within a common area throughout the day without mechanical tracking the sun. The proposed system was used for efficient solar thermal power generation, i.e., efficient heating of water. The angular position of two lenses is designed and mounted in such a way that the lenses are in direct line-of-sight with the sun for efficient sunlight collection during early morning, mid-day and for evening hours. Further, the height gap between Fresnel lenses and receiver unit is sufficient enough such that there is no shadow effect on the receiver unit hence the receiver unit is also exposed to direct sunlight rays throughout the day. Due to concentration of sunlight throughout the day the efficiency of solar thermal system is significantly enhanced. This is due to the fact that with the help of the proposed system, apart from concentrated sunlight made incident on to the solar thermal receiver it is also exposed to direct sunlight. The thermal receiver unit is placed in the common area and fluid is heated very fast. The design of system using ray optics in ZEMAX and experimental study of the direct and collected incident sunlight radiations using the proposed concentrating system on the receiver unit is analyzed. The thermal performance of the proposed system is compared with combination of single large Fresnel lens and segmented mirrors. The instantaneous thermal efficiency of the proposed system is around 20% or more in comparison to single lens concentrating system. The result shows that this type of system is highly efficient, and very much useful for countries where sunlight is available in plenty. Therefore, the proposed system is very much helpful for sustainable development of country.

Heat Transfer of Nanomaterial over an Infinite Disk with Marangoni Convection: A Modified Fourier’s Heat Flux Model for Solar Thermal System Applications

The demand for energy due to the population boom, together with the harmful consequences of fossil fuels, makes it essential to explore renewable thermal energy. Solar Thermal Systems (STS’s) are important alternatives to conventional fossil fuels, owing to their ability to convert solar thermal energy into heat and electricity. However, improving the efficiency of solar thermal systems is the biggest challenge for researchers. Nanomaterial is an effective technique for improving the efficiency of STS’s by using nanomaterials as working fluids. Therefore, the present theoretical study aims to explore the thermal energy characteristics of the flow of nanomaterials generated by the surface gradient (Marangoni convection) on a disk surface subjected to two different thermal energy modulations. Instead of the conventional Fourier heat flux law to examine heat transfer characteristics, the Cattaneo–Christov heat flux (Fourier’s heat flux model) law is accounted for. The inhomogeneous nanomaterial model is used in mathematical modeling. The exponential form of thermal energy modulations is incorporated. The finite-difference technique along with Richardson extrapolation is used to treat the governing problem. The effects of the key parameters on flow distributions were analyzed in detail. Numerical calculations were performed to obtain correlations giving the reduced Nusselt number and the reduced Sherwood number in terms of relevant key parameters. The heat transfer rate of solar collectors increases due to the Marangoni convection. The thermophoresis phenomenon and chaotic movement of nanoparticles in a working fluid of solar collectors enhance the temperature distribution of the system. Furthermore, the thermal field is enhanced due to the thermal energy modulations. The results find applications in solar thermal exchanger manufacturing processes.

Review of the Photothermal Energy Conversion Performance of Nanofluids, Their Applications, and Recent Advances

Nanoparticles have been thoroughly investigated in the last few decades because they have many beneficial and functional qualities. Their capability to enhance and manipulate light absorption, thermal conductivity, and heat transfer efficiency has attracted significant research attention. This systematic and comprehensive work is a critical review of research on the photothermal energy conversion performance of various nanofluids as well as the recent advances in several engineering applications. Different nanofluids used in the photothermal energy conversion process were compared to identify the suitable applications of each nanofluid in thermal systems. An analysis of the previous investigations based on experimental and numerical studies has established that nanomaterials have the potential to increase the efficiency of solar thermal systems.

Design, fabrication and optical characterizations of large-area lithography-free ultrathin multilayer selective solar coatings with excellent thermal stability in air

A sub-micron-thick selective multilayer solar thermal absorber made of tungsten, SiO2 and Si3N4 multilayer thin films was theoretically designed and experimentally demonstrated in this work. The optical performance was optimized by the particle swarm optimization algorithm for this multilayer absorber, whose spectral selectivity is associated with the Fabry-Perot resonance and anti-reflection effects. The designed multilayer absorber was deposited by sputtering and chemical vapor deposition techniques on the wafer scale. Its spectral absorptance characterized by a Fourier Transform Infrared spectrometer (FTIR) was demonstrated to be greater than 0.95 in the solar spectrum and less than 0.1 emittance in the mid-infrared with angular insensitivity. Temperature dependent FTIR measurements with an optical fiber setup revealed stable optical performance up to 600°C in ambient, while thermal cycle testing showed its long-term thermal stability at 400°C. Theoretical analysis of solar to power efficiency for a Carnot heat engine driven by the Solar heat was performed, which clearly shows that the proposed ultrathin selective multilayer absorber with spectral selectivity, angular insensitivity as well as high temperature stability could significantly boost the conversion efficiency of solar thermal systems at mid to high temperatures.

INFORMATION SUPPLY FOR SOLAR THERMAL SYSTEMS MATHEMATICAL MODELING

Solar thermal system are its constituent elements with their connection between each other, thermal processes within them and also input/output data. The conjunction of external and internal factors determines the efficiency of solar thermal system. No excess heat as well as its deficiency displays us high level efficiency of system. The initial data for modeling of solar thermal systems functioning are dissimilar. Parameters of system’s equipment are constant. Solar radiation amount and water consumption are variable data. The more close initial data to reality, the more definite simulated result is. The main problem is in unpredictability of water consumption by the reason of daily regime and requirement of each user. In this way user is the most instable element of the system. In this study the input data for mathematical modeling of solar thermal systems was analyzed. The climatic databases and standard specifications of hot water demand were also analyzed. The operability estimation method for solar thermal systems with variable input data was offered. The extent of suitability of any solar thermal system can be defined by certain characteristic. The value of this characteristic displays energy accumulation process.

Investigation on Stability and Optical Properties of Titanium Dioxide and Aluminum Oxide Water-Based Nanofluids

Water is regarded as a poor absorber of solar energy. This affects the efficiency of solar thermal systems. The addition of nanoparticles to heat transfer fluids used in solar thermal systems can enhance their optical properties. These new-generation heat transfer fluids are known as nanofluids. The present study investigates the stability and optical properties of three nanofluids, including aluminum oxide (13 nm and <50 nm) and titanium dioxide (21 nm) nanofluids. The stability of the nanofluids was observed through a photo-capturing method and zeta potential measurements. Ultraviolet–visible (UV–Vis) spectrophotometer was used to measure the absorbance and transmittance of the prepared nanofluids. The effect of factors such as type of particle, type of surfactant, and pH of the solution on the optical properties of the nanofluids was also investigated. We found that the titanium dioxide nanofluid had better optical properties but lower stability compared to aluminum oxide nanofluids.

Integrated analysis on the volumetric absorption characteristics and optical performance for a porous media receiver

The absorption of concentrated solar radiation within volumetric receiver is the key step that governs the global efficiency of solar thermal systems. An integrated radiative transfer model combined the dish concentrator, solar window and porous absorber is developed. The distribution of absorbed radiation energy in the porous absorber and the optical performance of the entire system are investigated by a Monte Carlo ray tracing simulation. The detailed radiation transport within the solar window and porous absorber is fully considered. The effects of porous structure parameters, slope error of concentrator and the alignment error of receiver are analyzed. The results show that the optical losses due to the back scattering from porous absorber and the absorption and reflection of solar window have a significant influence on the optical performance of system. The porous structure parameters greatly affect the distribution of solar radiative source within the porous absorber. In addition, the optical efficiency decreases noticeably when the slope error and alignment error exist.

Ultra-refractory Diboride Ceramics for Solar Plant Receivers

Concentrating Solar Power (CSP) is considered to be one of the most promising and sustainable technologies for electricity production in the future, and as efficiency of solar thermal systems rapidly increases with increasing working temperature, the big challenge for future is to develop novel solutions for solar receivers. In this framework, Ultra-High Temperature Ceramics (UHTCs) are mainly studied as thermal protection materials for aerospace and military applications, but their peculiar properties (very high melting points and good thermo-mechanical properties at high temperatures) can be advantageously exploited to increase the operating temperature of thermodynamic solar plants in concentrating solar power systems. This work is devoted to the study and characterization of the spectral reflectance of hafnium and zirconium diborides containing MoSi2 as secondary phase in order to evaluate their potential as novel solar absorbers. To assess the spectral selectivity properties, room-temperature hemispherical reflectance spectra were measured from the UV wavelength region to the mid-infrared, considering different levels of porosity for each system, in order to understand how porosity affect spectral reflectance. Moreover, for zirconium diboride and hafnium diboride composites containing 10vol% of MoSi2 sintering aid, the thermal emittance was measured in the 1100–1400K temperature range in PROMES-CNRS solar furnace. Data obtained were compared with spectral characteristics and high temperature emittance of a monolithic silicon carbide.

Transparent thermal insulation materials and systems: state of the art and potential for the future

Transparent or translucent insulation materials (TIM) represent a class of materials with a high potential for increasing the efficiency of solar thermal systems. A large number of materials and material combinations have been subjected to theoretical and experimental investigation. An overview of generic types, characterisation methods, simulation models, commercial materials, and actual systems is given, and the future potential of transparent insulation is shown as well as current market penetration. Results of selected demonstration projects with active and passive systems are presented. The most prominent application of transparent insulation was in the self-sufficient solar house in Freiburg. A detailed energy balance of this house is given. In addition the use of TIM in retrofitting as well as in low energy apartment blocks is discussed.

TRANSPARENT INSULATION MATERIALS AND THEIR APPLICATION IN ACTIVE AND PASSIVE SYSTEMS

Transparent or translucent insulation materials (TIM) represent a new class of materials with a high potential for increasing the efficiency of solar thermal systems. A large number of materials and material combinations has been subjected to theoretical and experimental investigation. An overview of generic types, characterisation methods, real existing materials available on the market, and the future potential of transparently insulated systems will be given as well as first results of applications in active and passive systems.