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Corrosion is among the most observed phenomena that threaten the profitability of concentrated solar thermal power plants and constitutes a real risk factor for metallic materials. Solar reflectors are between the impacted materials by corrosion because they are composed of metallic reflective layers that can easily be oxidized in air. To avoid this, paint systems could be applied in the back of the reflective layers as a barrier, that prevents the diffusion of corrosive elements to the metal layer. Metallic structures that support those reflectors can also be easily corroded and are replaced with composite structures that are not affected by corrosion. However, the durability of the paint protection systems and the composite structures must be inspected to ensure their effectiveness in protecting solar mirrors against corrosion, since they are found in sites that are often characterized by arid conditions, which affects corrosion protection solutions. This work is devoted to study the corrosion aspects and the parameters that affect the resistance of the protection systems of glass solar mirrors and solar mirrors fixed on composite structures by the use of a real outdoor exposure facility on a coastal site. The results show the important role of the paint coating layers number on the protection against corrosion. Also, the role of edge protection and shape were highlighted. Solar mirrors fixed on composite structures show a weak resistance because of the degradation of the fixing glue which creates areas of retention of salts and moisture that cause an aggressive corrosion of the metal layers.

The objective of this research work is to understand the impact of soiling on the thermal performance, electrical production and Levelized Cost of Electricity of possible Concentrating Solar Power plants in both Morocco and Portugal. For this matter, a joint soiling measurement campaign, following the same protocol, has been conducted in the two countries to assess and evaluate the impact of dust on the optical properties of soiled solar mirrors. After that, and using high-quality irradiation data measured at ground level, simulation of a 3.6 MWth molten-salt test facility has been conducted with and without taking soiling into consideration for both sites. Results show that soiling in Morocco is three times higher than in Portugal. The daily Soiling rate average is around 1.6% and 0.06% for both sites, respectively. In addition, results show that both sites are favorable for CSP implementation. The simulated electrical output for a clean field is ∼1.4 GWeh and ∼1.3GWeh with a Levelized Cost of Electricity of ∼0.194€/kWh and ∼0.205€/kWh for Morocco and Portugal, respectively. Nevertheless, when soiling is taken into consideration both sites produce similar amount of electricity of ∼1.25GWeh with a similar Levelized Cost of Electricity of ∼0.231€/kWh.

Soiling severely reduces solar mirror performance, requiring dispendious water consumption for cleaning operations and causing an increase in the levelized cost of energy (LCOE). An emerging technology for facing this problem consists of developing transparent self-cleaning coatings, able to be washed with a small amount of water by virtue of the modulation of surficial wetting properties. Nevertheless, the beneficial effects of coatings decrease in the first year, and coated mirrors show even higher soiling than non-coated ones. Moreover, it is important that coating production processes are economically convenient, consistent with the intended reduction of overall costs. The aim of this work is the research and development of a cheap and scalable solution, compatible with mirror fabrication steps and, in such a sense, economically advantageous. It involves the substitution of the alumina last layer of solar mirrors with more hydrophobic, potentially auxetic aluminum compounds, such as nitrides. In particular, 2D inorganic aluminum nitride thin films doped with metals (such as aluminum and silver) and non-metals have been fabricated by means of reactive sputtering deposition and characterized for the purpose of studying their self-cleaning behavior, finding a trade-off between wetting properties, optical clarity, and stability.

This Paper investigates the concept of solar mirrors in an Earth orbit to provide large-scale terrestrial equatorial solar farms with additional solar power during the hours of darkness. A flower constellation of mirrors is considered in highly eccentric orbits (semimajor axis=20,270.4 km) in order to increase the time of visibility over the solar farms, and through this architecture, only two mirrors are needed to provide complete night coverage over three equatorial locations. Selecting the proper value for the orbit eccentricity, solar radiation pressure and Earth’s oblateness perturbations act on the mirrors so that the apsidal motion of the orbit due to these perturbations is synchronized with the apparent motion of the sun. Therefore, it can be guaranteed that the perigee always points toward the sun and that the mirrors orbit mostly above the night side of the Earth. With respect to geostationary orbit, the family of orbits considered in this Paper allows a passive means to overcome issues related to orbital perturbations. Moreover, because of the large slant range from geostationary orbits, a larger mirror is required to deliver the same energy that could be delivered from a lower orbit with a smaller mirror. As a result, a single antiheliotropic flower constellation composed of two mirrors of 50 km2 would be able to deliver energy in the range of 4.60–5.20 GW·h per day to 1000 km3 solar farms on the equator. Finally, it is estimated that, deploying 90 of these constellations, the price of electricity could be reduced from 9.1 cents to 6 cents per kW⋅h.

Soiling of optical surfaces due to sand and dust accumulation is the main cause for decreased efficiency of concentrating solar power and photovoltaic installations in desert areas. Nanostructured coatings with tailored surface roughness can reduce the rate of soil accumulation and maintain the high optical performance of the solar mirrors and cover glass. Here, we investigate the correlation between the size and structure of the surface features of the coating and its anti-soiling and optical properties. To control the morphology of the surface features we developed two types of coating: (1) based on small size (30–50 nm) silica particles with nanostructured surface and (2) based on unstructured nanoparticles with bimodal size distribution (80 and 35 nm). We tailored the surface features of the coatings to achieve synergistic improvements over different length-scales and thus decrease the adhesion force between the soil particles and the surface. Adhesion force measurements were performed using atomic force microscopy. The adhesion force and energy required to separate a silica particle from the surface of coated solar glass was significantly lower than the respective values from the surface of uncoated solar glass. A falling sand abrasion test, modeled after the procedure in ASTM D968, was performed. The optical properties of coated and uncoated solar glass were measured before and after the soiling test. Coated solar mirror samples were tested in the field. The results of the field test provided evidence that the anti-soiling coating is effective at reducing soiling and improving the specular reflectances of the coated mirrors. The use of the developed anti-soiling coating can be extended to other power-grid applications where reducing the soil accumulation is valuable.

Soiling effects influence the output of solar thermal plants, significantly causing unwanted transmittance, reflectance, and absorbance losses. Research is needed to identify what type of reflective surfaces are best suited for semitropical climates, such as the southeastern United States. This paper initially presents a review of several concentrating solar power (CSP) reflector testbeds used to analyze the soiling effects of various reflective materials. A soiling testbed is developed for this study that comprised six sets of reflective surfaces mounted at a fixed tilt of 30 degrees: three sets of thin-film surfaces and three sets of glass types. Two generations of 3M solar mirror film (SMF), 3M SMF 1100 and 3M SMF 2020, were used along with Konica Minolta SMF, silvered Corning Willow Glass, a dichroic cold mirror, and a standard mirror. Results show that the 3M SMF 2020 and Konica Minolta SMF performed the best during regular cleaning intervals, whereas the silvered Corning Willow Glass gave the best performance if only cleaned naturally. The other glass types showed the least average loss due to soiling throughout this study but gave the lowest performance for specular reflection.

The SolarPACES Reflectance Guideline provides an essential tool to obtain comparable reflectance measurements, but because of the lack of adequate commercial instrumentation, till now the exhaustive characterization of reflectance behaviour versus incidence θi and (half) acceptance angle φ is unachieved. An expert group in Task III has been working to outline some practicable solutions. The Equivalent Model Algorithm (EMA) was found quite promising: with few input data, EMA allows to predict any reflectance feature by computation. The recent availability of reliable reflectance measurements at oblique incidence made possible the refinement of EMA by analysing a representative set of commercial solar mirrors. This paper describes the new EMA for solar mirror, named EMA4SM, and reports its validation1.

International audience; Durability tests of solar materials are crucial to guarantee the profitability of the Concentrated Solar Power (CSP) plants and to ensure a proper efficiency during their lifetime. As solar mirrors and absorbers, optical performances of receiver glasses are affected by soiling phenomenon and surface degradations, especially in desert and coastal environments. Indeed, the deposition of dust particles, salts, organic materials or any other contaminants present in the solar field or around the CSP installation sites can generate an important optical loss. Some coatings have been developed by a manufacturer to avoid the deposition of a soiling layer on the glass surface and to keep good optical performances. Their formulations are confidential but this paper presents the results and the comparison of the four different anti-soiling coatings, deposited on glass substrates and subjected to several accelerated aging tests. The conclusion of this work will help the manufacturer to make a choice about the most durable coating for industrialization.

Concentrated power on the receiver and homogeneity of irradiance distribution are optimised in the optical design of solar mirrors. However, the manufactured mirror may contain some fabrication defects, which alter the irradiance distribution on the receiver. Surface defects introduce aberrations that cause an enlarged spot on the receiver and the resulting energetic losses depend on the collection features of each solar plant. To minimise these losses, it is essential to control the surface curvature, especially on large solar reflectors like heliostats. Facility of application, low-cost equipment, and elaboration simplicity are the key advantages of the proposed optical control. Exploiting techniques of image analysis, it assesses the mirror surface quality comparing an acquired image with a reference image. The application requires only a photographic camera and a screen with a calibrated pattern. The surface defects of the reflecting surface appear as deformations in the image of the pattern. This optical measurement could be adapted to examine mirrors with various shapes and sizes. It is an optical control that can be applied during manufacturing of the mirrors or on the produced samples.

The dust accumulation on solar mirrors is a complex and site-specific phenomenon. It strongly depends on the environment parameters such as wind, precipitation, ambient temperature and relative humidity. Therefore, finding the relationship between these parameters we can predict the impact of this accumulation on the mirrors optical efficiency. Currently the Artificial Neural Network (ANN) is one of the best solutions that can be used for a performant prediction of such problematic. In this paper, a new approach using ANN and different meteorological parameters is used to predict the soiling level for a solar mirror with a maximum accuracy. As first results we reached an accuracy value of 95% using only 7 months of daily measurements of the environment data in Green Energy Park (GEP) research facility.

Beginning in 2018, the National Renewable Energy Laboratory restarted exposure campaigns on new and archived samples as part of a multiyear project, some with outdoor exposure for more than a decade. By resuming exposure and collecting and analyzing data on thousands of samples going back decades, several goals can be advanced that can be difficult to determine within the timeline of most projects: 1) correlating an accelerated exposure campaign to outdoor aging, specifically with xenon arc lamp exposure chambers; 2) drawing conclusions between specific corrosion mechanisms and weather patterns; and 3) finding novel relationships between mirror composition and performance. In addition to building and mining a database, we will experiment with new characterization techniques, primarily focused on macroscopic and microscopic imaging. In introducing these techniques more broadly, it may be possible to reveal a more direct line between optical performance and exposure campaigns by better understanding the degradation mechanisms occurring.

Aluminium, first surface, plastic mirrors have been developed to be used as heliostats in concentrated solar power stations. The Al reflective layer, the adhesion of the reflective layer to the polycarbonate (PC) substrate and the protective over-coat have all been optimized in order to achieve a highly reflective, robust and durable light-weight mirror. The Al layers were deposited by means of magnetron sputtering and the change in reflectance was investigated as a result of variations in the sputtering conditions. The influence of the substrate cleaning process and the addition of an interfacial seed layer on the adhesion of the reflective layer were also systematically studied. A thick sputtered SiO2 film and a dip-coated transparent hard-coat resin were examined as protective coatings against weathering tests. All prepared samples were characterised using, adhesion and abrasion tests, spectrophotometry, nano-scratch, salt-spray tests and both accelerated and outdoor weathering.The optimal three-layer stack was found to be 15 nm of SiO2 as an adhesive layer, 80 nm of Al as the reflector and 4.5 μm dip-coated, hard-coat resin as the protective layer. The three-layer stack plastic mirror gave an overall total solar weighted specular reflectance of ≈88% which was maintained even after aggressive abrasion tests such as Bayer and Steel Wool, 40 days within a salt-spray chamber and after 5 months under accelerated weathering. Refereed/Peer-reviewed

Soiling impacts energy harvesting of solar thermal power plants and operation and maintenance cost. Our research showed that reflectance of soiled glass reflectors diminishes significantly with increasing angles of incidence. The incidence angle dependent attenuation due to dust on the reflector surface can be modelled based on the Lambert-Beer law. In this study we show the effect of incidence angle dependent reflectance on the expected yield of exemplary power plants by integrating the model into ray tracing. Results illustrate the additional reduction of the solar yield in dependence of the mean incidence angles on the mirrors of a heliostat field, a parabolic trough field and a linear Fresnel collector array, depending on the power plants site and seasonal effect. Results show mean hourly incidence angles and optical performance for every hour of the year. In conclusion, not considering angle dependent reflectance can lead to on overestimation of the annual plant performance of up to 2%. For line focusing collectors the effect is dominant in winter, in the northern hemisphere, underlining the importance of cleaning during this season.

This paper presents an indirect method to estimate the near-normal sun-conic reflectance of solar mirrors, which is considered to be the relevant parameter to describe the reflectance characteristics of reflectors employed for CSP systems. The indirect method has been validated by direct measurements with the Spectral Specular Reflectometer measurement device reported in previous conferences. Good agreement between indirect and direct measurements was found for new and aged mirrors but not for soiled mirrors, where different phenomena seem to occur. The indirect method is thus only applicable for clean mirror surfaces in new and aged condition. The benefit of the indirect method is that it is simple and applicable with commercial equipment, providing a useful tool to determine near-normal sun-conic reflectance for all laboratories which do not own advanced reflectometers.

Concentrating Solar Power (CSP) plants are often being built in regions with high dust loads such as the Middle East and North Africa. Current soiling measurement methods in solar fields are labor and cost intensive and they only cover small measurement spots. Even a high number of manual measurements can only cover a minimal portion of the total solar field mirror surface. Airborne measurement solutions such as QFly have been developed in recent years to measure the geometry and tracking accuracy of the large CSP mirror areas. Soiling on solar mirrors is qualitatively visible in raw QFly measurement images. In this study we present a method to automatically derive the mirror cleanliness from aerial images taken with QFly. We present scattering investigations that correct for the directional particle scattering, illumination and camera viewing angles. A promising correlation was found between the thus corrected camera RGB signals and reference reflectance measurements that shows an RMSD of 3% for the investigated dust.

The objective of this study is to evaluate the erosion resistance of coated solar glass mirrors compared to a solar mirror with a blank surface. The evaluated coatings are a hard and an antisoiling coating. The study is conducted to understand the limits and the advantages of modifying surfaces by coating adding on the initial reflectance of the glass mirrors and also on their resistance towards degradation under harsh conditions. The experimental erosion simulations are conducted on a horizontal test bench propelling a rate of sand particles on the target surfaces with different test conditions (air velocity and impact angle). The obtained results have showed a good resistance of the hard coatings samples towards erosion compared to the blank surface and the antisoiling coating. The resistance of the hard coated samples is reflected by a minor degradation of optical properties (specular and hemispherical reflectance) in the extreme test conditions and by a surface degradation less important compared to the tested materials, namely, roughness modification and impacts properties.

Solar mirrors samples are exposed in a Moroccan natural aging site where the climatic stresses could create different degradationsand failures on samples materials that consist mainly on metal corrosion, detachment of protective layers and surface erosion. The climatic parameters that cause stress are temperature, wind velocity, rain, solar irradiation and sandstorms frequency. The useful lifetime of reflectors is conditioned by the effect of these parameters on the optical performance deterioration. During the exposure period, it has been found that weather conditions had generated surface erosion as well as physical and chemical degradation of glass surfaces of tested mirrors. The intensity of both phenomena depends on the mirrors surface nature, climatic conditions and samples exposure orientations. Optical performance of mirrors decreases considerably under extreme climatic conditions and erosive environment.

A passive daytime radiative cooler is made of a sky facing surface which can preserve the indoor air temperature below ambient without energy consumption by simultaneously reflecting solar radiation and emitting thermal radiation to the universe through the atmospheric window located between 8–13 μm of the electromagnetic spectrum. After the first demonstration of radiative cooling under direct sunlight, a solar mirror coated with a mid-infrared (MIR) emissive thin film has become the standard device architecture. This study firstly reviews recent developments in daytime passive radiative cooling, followed by describing the development of an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Some micro-channels are fabricated on the back side of the passive radiative cooler, allowing fluid to flow in an isolated loop such that the coolant can be chilled and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. Considering the steady state energy balance within the radiative cooling panel integrated HVAC systems, the cooling performance and indoor air temperature are evaluated by numerical analysis. A 100 m2 passive radiative cooling panel could chill water for the cooling of air, reducing indoor air temperature by 10 °C, equivalent to a net cooling power of 1600 W. This study suggests that the proposed passive radiative cooling system should be used to pre-cool the ambient hot air such that the overall energy consumption of a traditional air-conditioning system can be reduced. The findings promise the application of passive daytime radiative cooling in building HVAC systems.

International audience; During the life-time of Concentrated Solar Power (CSP) plants, optical performance of solar mirrors are affected by soiling phenomenon and surface degradations, especially in desert and oceanic environments such as those prevailing in North Africa. This optical loss results from the deposition of dust particles, salts, organic materials or any other contaminants present in the solar field or around the CSP installation sites. This phenomenon depends on the different exposure sites with every location having its own meteorological and geological characteristics. Moroccan CSP sites are planned in many locations with different environmental conditions: arid, semi-arid and Saharan, with or without oceanic influence. These environmental factors can have an aggressive impact on the CSP mirrors. Indeed, large particles will tend to degrade the mirror by scratching or breaking the glass surface, while small particles have more chance to deposit on solar mirrors and thus create a soiling layer. These particles are generated from many sources (sand storm, pollution, vehicular movements, etc.) and are transported through the air. This mode of transport is determined as a function of mineral, size, shape and hardness. In this paper, we present a simple methodology for analyzing the chemical and physical characteristics of the sand particles, characterization techniques, and their appropriate laboratory equipment. All the factors previously mentioned could be critical for the CSP mirrors. That is why analyzing these data may be a key point for the industry to understand the effects of soiling and degradation on the CSP mirrors in order to increase the global economic profitability of their solar plants.

The solar photovoltaic water pumping system (SPVWPS) is increasingly popular in non-electrified population in fulfilling the irrigation and drinking water demand. It can be a potential option than conventional pumps to draw down water in developing countries like India. However, to gain wider acceptance, scientific information and results are needed with the system installation and sizing. In this study, the performance of SPVWPS is investigated with helical rotor pump. The effect of solar radiation and total head on water output of SPVWPS has been analyzed with the optimized PV array configuration. Four pumping head have been analyzed (4 bar, 6 bar, 8 bar and 10 bar). The experiments have been performed for various heads on clear sunny daylight hours in an artificial well at VNIT campus Nagpur (India). The effect of variation of radiation on the performance of pump has also been studied. The best system efficiency is founded for the total head of 10 bar which is suggested for helical rotor submersible pump. Also, for Nagpur (India), the system should be sized as per the irradiance range 400–800 W/m2, as more than 60% of the total solar energy falls in this range.

The performance of a novel solar energy concentrating system consisting of a cylindrical mirror placed in a stationary position and a receiver tracking system is investigated. Via a computational tool based on the ray-tracing method, the sensitivity of the geometric concentration ratio, rim angle, shadow angle, incidence angle and hourly insolation level to the optical performance of the concentrator are evaluated. The simulation includes a model of solar concentrating system based on the approximation of the optical behavior of a cylindrical reflector. In fact, the optical efficiency of the solar concentrator system is examined. The results prove that the maximum of the optical efficiency is achieved for 20 × geometric concentration ratio and 80°-rim angle. Moreover, the concentrated solar density and the Local Concentration Ratio (LCR) distribution along the tracked receiver tube are analyzed and compared to those obtained by the classical one-axis tracked parabolic trough concentrator system.

One significant maintenance problem and cost associated with solar energy conversion systems is the soiling due to the accumulation of dust and other pollutants. In this work, we describe a scalable approach for applying antisoiling coatings based on superhydrophobic (SH) silica particles using a spray coating process. A large water contact angle (WCA) is one of the characteristics of excellent SH surfaces and because of the low surface energy and low adhesion forces the soiling rate is reduced. Our findings indicate that the WCA depends strongly on the ratio of the polymer binder and the nanoparticles. The nanoparticle surface coverage of the spray coated samples was substantially improved after rinsing with solvent. This process tended to remove large aggregates and excess polymer binder and further increased the WCA by allowing exposure of the functionalized nanoparticles. The durability of the SH coatings was enhanced when the substrate was pretreated with polymer binder and an optimal curing time between 30 and 60 min. The abrasion tests of the SH coatings we report in this study showed that the WCA decreased from ~ 166° to ~ 157° after exposure to 2.6 g of sand. Such coatings will help reduce costs of periodic cleaning of solar energy conversion systems (photovoltaic panels and concentrated solar mirrors).

This paper addresses the design and specification of solar collector networks for the capture of solar radiation and its transformation into thermal energy for use in low energy intensity processes. All glass evacuated tube solar collectors are the type of technology considered in this work. Target temperature and heat load are the design objectives in the design of solar collector networks and they are achieved by specification of two design variables, namely: a) number of collectors in series in a row and, b) number of rows in parallel. The variability of ambient conditions is accounted for by means of the specification of the critical point conditions for the design of the network. From the thermal point of view, the number of solar collectors or heat transfer surface area required to achieve the targets depends on the ambient conditions chosen for the design. The targeting approach for the specification of the network structure is based on the use of a thermal model. The various design options available to the designer to specify the number of solar collectors in a row, are presented in a graphical way as a function of mass flow rate, inlet temperature, solar radiation intensity and target temperature.

Aluminum sheets-based mirrors are finding applicability in high-temperature solar concentrating technologies because they are cost-effective, lightweight and have high mechanical properties. Nonetheless, the reflectance percentages obtained by electropolishing are not close to the reflectance values of the currently used evaporated films. Therefore, controlling key factors affecting electropolishing processes became essential in order to achieve highly reflective aluminum surfaces. This study investigated the effect of both the electropolishing process and previous heat treatment on the total reflectance of the AA 1100 aluminum alloy. An acid electrolyte and a modified Brytal process were evaluated. Total reflectance was measured by means of UV–Vis spectrophotometry. Reflectance values higher than 80% at 600 nm were achieved for both electrolytes. Optical microscopy and scanning electron microscopy images showed uneven dissolution for the acid electropolished samples causing a reflectance drop in the 200-450 nm region. The influence of heat treatment, previously to electropolishing, was tested at two different temperatures and various holding times. It was found that reflectance increases around 15% for the heat-treated and electropolished samples versus the non-heat-treated ones. A heat treatment at low temperature combined with a short holding time was enough to improve the sample total reflectance.

The anti-soiling (AS) performance of solar mirrors coated with a highly transparent, superhydrophobic nanoparticle-textured coating has been characterized. The AS coatings were created on the mirror surface by depositing nano-textured silica nanoparticle layers of ∼250 nm thickness using a draw-down coating process, followed by fluorination of the nanoparticles in a molecular vapor deposition process. Highly uniform surface features of the AS-coated mirrors (20 × 30 cm2, no measurable loss in specular reflectance, and water contact angle >165°) provided an outstanding AS performance. A 4× reduction in the rate of dust accumulation as determined by gravimetric measurement of the accumulated dust on coated versus uncoated mirrors was observed. Additional evidence of a significant reduction in soiling rate was determined during measurements of specular reflectance in an outdoor environment test. The adhesion force between a model sand particle and nano-textured coatings in the hydrophobic to superhydrophobic range was also studied. A dramatic decrease in adhesive force acting on the particle was observed with increasing surface hydrophobicity. The results align well with the observed dust accumulation on the AS-coated mirrors. The AS-coated mirror maintains a high reflectivity by shedding dust and resisting dust accumulation, providing a potential benefit when applied to mirrors in the solar field of a concentrated solar power generation plant.

The use of solar energy concentration systems for achieving performance enhancements in the Photovoltaic/thermal hybrid solar systems and reduction of initial costs is an idea that has been studied for years. In this article a new structure for parabolic trough photovoltaic/thermal collector is proposed and its thermal and electrical performances are experimentally investigated. The receiver of this concentrator contains a triangular channel with an outer surface covered with photovoltaic cells and thermoelectric modules with a specific arrangement so that in addition to absorbing heat, a larger portion of the solar radiation is directly converted to electricity. Hence, the performance of the system is enhanced. Performance evaluation of this combined heat and power system that is equipped with one-axis solar tracker in polar mechanism shows that daily average electrical and thermal efficiencies can reach 4.83% and 46.16%, respectively. In addition, the electricity generation capacity of the photovoltaic module applied to this system has become fourfold in comparison with its standard working conditions. As a result, besides simultaneous generation of heat and electricity, the proposed model has a satisfactory performance.

Harvesting solar energy for vapor generation has attracted large amount of attention due to its promise for applications in water purification, desalination, power generation, and so on. Many structures based on the two-layered design or the plasmonic enhanced evaporation have been reported to promote the efficiency of solar vapor generation. Inspired by the transpiration phenomenon in plant, we report that a capillary-driven pump can be used for highly efficient solar vapor generation. The pump is mainly consisted of a porous hydrophilic modified NiO (M-NiO) disc and a one-dimension water supply channel. The M-NiO with a three-layered structured TiAlON-based nanocomposite absorber deposited on its surface can efficiently capture solar radiation (absorptance of 0.97). Driven by the capillary force in the porous M-NiO, the pump can continuously wick water via the one-dimension channel to the solar absorber for evaporation, achieving solar-to-vapor efficiency of 73% at 1 sun and 90% at 4 suns. The high conversion efficiency can be attributed to the high absorption ability of the M-NiO disc and the one-dimension water supply design to limit the thermal loss. This new design with advantage of easy scale-up provides an efficient approach to harvest sunlight for solar vapor generation in low solar concentrations.

This paper shows the relationship between the incident solar radiation and the optimum outlet temperature of a solar field to produce the highest amount of electrical power. Various simulations were made for different values of incident solar radiation, calculating for each one the optimum temperature which produces the maximum electrical power and demonstrating that to operate the field at the highest allowable temperature is not the optimal operating point from certain values of solar radiation; a situation which takes special relevance during cloudy days. These simulations were carried out using two connected models, one for the solar field and another one for the power cycle.

The solar chimney power plant (SCPP) is one method of solar thermal utilization with high/super-high chimney. There are few methods to decrease the solar chimney height. One possible method is the sloped solar chimney power plant (SSCPP), which depends on the special geographical condition. In the present paper, another system is proposed to overcome this problem, which is named solar double-chimney power plant (SDCPP). Mathematical models of the SDCPP are established and its performances are analyzed. It is found that, for an SDCPP with 5 MW configuration size, the average temperature rises of the horizontal and tilted solar collectors are 5.64 K and 7.87 K respectively. The highest wind speeds in the inner chimney and in the interlayer of the inner and outer chimney are 15.28 m/s and 19.41 m/s respectively. The annually average SDCPP power productivity and power efficiency are 4.72 MW and 1.2% respectively. The power productivity of the SDCPP is 1.59 times larger than that of the CSCPP and 2.77 times larger than that of the SSCPP. Through comparing with the CSCPPs in the literature, the SDCPPs can increase their power productivities by 21%–55%. The SDCPP is a promising method for non-concentrating solar thermal power generation.

High-flux solar simulators consist of lamps that mimic concentrated sunlight from a field of heliostats or parabolic dish. These installations are used to test promising solar-thermal technologies for technical viability. Ideally, the conditions in a given high-flux solar simulator closely approximate those in solar furnaces, experimental facilities that are driven by actual sunlight. During the characterization of a new high-flux solar simulator at the University of Colorado an artifact predicted in high-flux solar simulators, but not solar furnaces, was observed experimentally. Specifically, power measurements from the 18 lamp, 45kWelectric, device increased by 11.1% depending on the wider optical environment, which was either specularly reflective or diffusely absorbing. This calorimetry result was confirmed by a flux gauge, which showed that optical conditions could inflate incident flux by more than 20% at the radiation target. An “observer effect” in high-flux solar simulators was first suggested by Monte Carlo ray traces, and refined ray traces recapitulated the experimental results. Thus, solar-thermal designs may behave differently when evaluated in solar furnaces, versus high-flux solar simulators. Artifacts can be minimized by insuring that solar targets, including radiation measurement instrumentation, present diffusely absorbing surfaces to high-flux solar simulators. Under these conditions the new high-flux solar simulator featured a peak flux of 12.50 MW/m2 and delivered 9.076 ± 0.190 kW onto a ∅10 cm target for a mean flux of 1.155 MW/m2.

The solar flux distribution on the receiver of a solar power tower is usually not uniform, which can cause a number of problems for the energy efficiency and system safety, particularly, the local hot spot and the thereby caused thermal stress and thermal deformation. Therefore, homogenization of the solar flux distribution is critical and important. The objective of the present study is to homogenize the solar flux distribution while keeping the optical loss (reflection loss) as low as possible through optimization of the distribution of the solar absorptive coating. An integrated approach coupling the Monte-Carlo ray tracing method and the Gebhart method is applied to simulate the process of the solar radiation transfer in the solar power system. The multi-objective optimization of the distribution of solar absorptivity is performed by using the non-dominated sorting genetic algorithm. The following conclusions are drawn from the study. (1) The improvement of the uniformity of the distribution of solar flux can lead to more reflection loss due to the fact that more solar energy is distributed on the position with greater view factor to the aperture; (2) The Pareto optimal front obtained from the multi-objective optimization provides the trade-off between the non-uniformity of the solar flux distribution and the reflection loss. (3) The optimal solar absorptivity distribution provided by the Pareto optimal front can significantly flatten the solar flux distribution at a minimum cost of optical loss. (4) The optimal distribution of the solar absorptivity is approximately opposite to the distribution of solar flux projected onto the active surfaces from the heliostat fields.

Silver is a strategic material for renewable energy. Among other applications related to PV, it is used as reflective layer in solar mirrors for concentrated solar power (CSP), thanks to its high reflectance in the whole solar range. CSP plants require large mirror areas, typically in the 0.1 to 1 km2. However the estimated reserves of silver do not exceed 20 years. To sustain the deployment of CSP as a valuable input in the future energy mix, silver must be replaced with a less critical material, ideally low cost. In this theoretical study we propose to replace silver by aluminum, adding a dielectric multilayer coating on top, to reach similar optical performance. This design is inspired by Bragg mirrors but differs in that the stack parameters (layer thicknesses) are individually optimized, so as to maximize solar reflectance. A thin aluminum layer with MgF2/TiO2 aperiodic stack on top can theoretically reach solar reflectance of 96.1%, higher than silver (95.5%). Other materials are also found suitable: SiO2, ZnO, ZrO2. A reasonable number of layers (〈20) not only enhances the optical performance of metallic reflectors, but also allows tuning said performance, which is not possible with classical mirrors relying on a single metal reflective layer. For instance, one can optimize the stack to reach a better performance than silver at typical mean incidence angles of solar light in CSP plants, for different locations. A solar mirror without rare or precious materials and more effective than traditional silver mirrors is thus obtained.

Concentrated solar thermal technologies play an essential role in the energetic transition which is currently facing our society. The energy generation in this technology vastly depends on the optical behaviour of the reflector materials of the solar field. Corrosion of solar reflectors might be an issue in locations with high corrosive environments because an excessive corrosion of the solar mirror could be catastrophic for the profitability of the concentrated solar thermal plant. This research is focusing on modelling the durability of four different solar reflector materials exposed outdoors by accelerated aging tests. For this purpose, ten locations suitable for concentrating solar thermal applications were classified depending on their corrosive aggressiveness. Commercial, free-lead and low-cost reflectors samples were exposed in all the sites to determine the influence of the corrosion in its durability. Corrosion defects appeared in the solar reflectors during outdoor exposure were properly reproduced by CASS test. Novel lifetime prediction models were developed for all the solar reflectors depending on the corrosive aggressiveness of the place. Number and thickness of the paint coatings employed in the solar mirrors were identified as one of the most important parameters to improve the energy generation of a CSP plant in corrosive environments. A reduction of the capital invested in the solar mirror purchase is expected for sites with low corrosivity.

In this article, we propose a new solar concentrator based on spectral splitting of sunlight. Spectral splitting has the objective to collect different spectra onto spectrally adapted solar cells for a more efficient use of the Sun’s spectrum. Its combination with solar concentration makes an alternative to classical technologies. The proposed concentrator is composed of a diffractive/refractive optical element that spectrally splits and focuses the light onto a waveguide. The light is then conducted by total internal reflection towards the two specific solar cells. The optical concept and optimization of each element is presented in this paper. An adaptation for dye sensitized solar cells is performed. A geometrical factor around 5 × is reached. Finally, theoretical optical efficiency, the manufacturing process and experimental testing with a collimated Sun simulator are presented.

Solar collectors are the main components of a solar heating system. The collectors collect the sun’s energy, transform this radiation into heat, and then transfer this heat into a fluid, water or air, which has many household or industrial applications. This paper introduces a new technology to improve the performance of the solar thermal collectors. The solar reflector used here with the solar collector to increase the reflectivity of the collector. Thus, the reflector concentrates both direct and diffuse radiation of the sun toward the collector. To maximize the intensity of incident radiation, the reflector was allowed to change its angle with daytime. The radiations coming from the sun’s energy were converted into heat, and then this heat was transferred to the collector fluid, water. A prototype of a solar water heating system was constructed and obtained the improvement of the collector efficiency around 10% by using the reflector. Thus, the present solar water heating systems having the best thermal performance compared to the available systems.

The effective use of solar energy has become significantly important due to unnatural weather changes and fossil fuel exhaustion. Concentrating Solar Power (CSP) technology is a promising approach to harvest solar energy in the form of heat using solar selective absorber coating. These coatings are expected to absorb maximum incoming solar radiation (α ≥ 0.95) and prevent loss of the absorbed energy as infrared radiation (e ≤ 0.05). Efficiency of the absorber coating can be evaluated by a metric called “Solar selectivity (α/e)”. In recent years, a number of attempts have been made to achieve remarkable selective property and high temperature stability of the absorber coating using the concept of Surface Plasma Polaritons (SPPs). The SPPs have the capability to capture solar energy by confining electromagnetic field at the metal-dielectric interface. Solar absorption, can be maximized by tailoring the optical constants of the metal and dielectric. In this review, we have described different types of solar absorber coatings with particular emphasis on dielectric-metal-dielectric (DMD) -based absorber coatings. We have presented a brief theoretical overview to comprehend physics of DMD coatings. This review additionally highlights some of the case studies based on the DMD -based absorber coatings with the high temperature stability and their importance in the context of CSP technologies.

The preheating of the feedwater in a Regenerative Rankine Cycle power plant with solar thermal energy, termed Solar Aided Power Generation, is an efficient method to use low to medium temperature solar thermal energy. Here, we compared the use of medium temperature (200–300 °C) energy from concentrating solar collectors (e.g. parabolic trough collectors) to displace the extraction steam to high temperature/pressure feedwater heaters with that from low temperature (100–200 °C) non-concentrating solar collectors (e.g. evacuated tube collectors) to displace the extraction steam to low temperature/pressure feedwater heaters of the power plant. In this paper, the in terms of net land based solar to power efficiency and annual solar power output per collector capital cost of a Solar Aided Power Generation using concentrating and non-concentrating solar collectors has been comparted using the annual hourly solar radiation data in three locations (Singapore; Multan, Pakistan and St. Petersburg, Russia). It was found that such a power system using non-concentrating solar collectors is superior to concentrating collectors in terms of net land based solar to power efficiency. In some low latitude locations e.g. Singapore, using non-concentrating solar collectors even have advantages of lower solar power output per collector capital cost over using the concentrating solar collectors in an SAPG plant.

We propose a procedure for calculating geometrical parameters of a flat mirror concentrator of solar energy. The procedure describes construction of the path of rays between the mirror and the collector in the form of a system of degree coordinates α . We obtained analytical dependences for the course of illumination of air collector by the sum of direct and reflected flows Е β ( τ ) and a flow gain coefficient k. This makes it possible to estimate efficiency of a flat mirror concentrator for different azimuthal angles of orientation of parameters of the horizontal receiving surface of air collector S and the system of degree coordinates α . The rational values for a dihedral angle (focline) α between the mirror and the collector are established, which are 90...120 o. It was found that a gain coefficient of the flow of solar energy k is implemented with the least expenditures from 1.8 to 1.2. We report results of experimental research into energy characteristics of work of the mirror concentrator in a combination with an air collector. It was established the application of a mirror concentrator in the solar dryer makes it possible, in the morning and in the evening, to improve annual average capacity of incoming solar radiation on the receiving surface of air collector by an order of 3.41 kW/m 2 during daylight. This enables receiving, amplifying, and directing to the absorber of AC from 1.5 to 2.3 kW·h of energy per day from 1 m 2 of FMC. The results obtained could be used for design and improvement of technical means of drying, in order to enhance technological and energy efficiency of the process.

The paper investigated a hybrid experimental space heating system that comprises the novel mini-channel solar thermal and PV/T panels, as well as a heat pump. The major advantage of the system lies in the multiple functions of providing energy for space heating in winter and domestic hot water in other seasons, and generating electricity annually to balance the electricity demand of the system. Compared to traditional flat-plate solar collectors, the mini-channel solar thermal and PV/T panels have the smaller cross-sectional area, higher fluid flow velocity and thus higher heat transfer rate between the heat-carrying fluid and panel walls, thus leading to enhanced solar thermal and electrical efficiencies. It is found that during the typical-day testing, the mini-channel thermal panels had average solar efficiency of 50.8%; while the mini-channel PV/T panels had average solar efficiency of 45.0% and the whole system had average Coefficient of Performance (COP) of 4.9. The system made use of solar and gas sources with the fractional solar consumption of 44%, while its operation was under the pre-set time schedule, i.e., solar panels on from 9:00 to 6:00, heat pump on from 18:00 and 20:00, and gas heater on from 22:00 to 9:00. Throughout the testing period, the room temperature was maintained at above 18 °C, thus enabling achieving the required thermal comfort. Concerning the complexity of such a system, a simplified system comprising the mini-channel thermal and PV/T panels and a heat pump was suggested. In brief, the research has produced fundamental data for the mini-channel-based solar thermal and PV/T panels and established a foundation for designing a simple and low-cost mini-channel based solar space heating system for rural houses in cold climatic regions of China, which will subsequently contribute to significant fossil fuel saving and carbon emission reduction in the global extent.

Have been successfully developed the solar drying integrated solar collector and photovoltaic (PV). However, heat waste is transfer from the back side of solar collector to surrounding over operation. A thermoelectric generator (TEG) relies on the "Seebeck effect" to directly convert heat into electricity. TEG is applied to use in specific area such as detector, egg incubator, exhaust car, watch etc. The objective of this research is focused on development a solar drying technology with using solar collector combined with TEG module. The system was constructed and installed at School of Renewable Energy Technology, Naresuan University, Phitsanulok Thailand. Temperature profile the back side of solar collector, power of TEG module, air flow rate of blower were measured and analyzed. As the results, temperature at behind of solar collector was decreased with increasing the produced power of TEG module which corresponded with solar irradiance. Solar collector can be operated and produced hot air temperature about 70-80 °C supported to chamber for drying. TEG can be produced the electricity about 0.0813 V/module. The deference of temperature surface was 1.77 °C and the average temperature of back side 45.4 °C. The ambient temperature was 35.8 °C. In addition, the system can be used blower for controlling air flow rate by using electricity from TEG module. It can be concluded the solar collector can be combined with TEG module for producing the electricity to supply the blower within tunnel drying, including with a guideline for improve the performance of solar drying technology.

Performance investigation of a prototype of solar updraft tower (SUT) power plant is carried out in this study. The diameters of the solar collector and chimney are of the 3.5 and 0.6 m respectively. The objective of this work is to investigate the performance of SUT and to tabulate all the inputs and estimated parameters with the materials of a SUT power plant. All the three main components’ (turbine, solar collector and a chimney) process parameters are estimated and discussed. Appropriate materials are discussed and selected for solar collector, chimney, turbine and heat storage materials. Solar beam, diffuse and global radiation are estimated to analyze the performance of collector cover. Energy losses in solar collector cover and transmissivity estimations are performed to calculate theoretical energy collected in solar collector. Pressure drop inside the chimney is estimated and from that the actual power output of the turbine is calculated. The quantity of heat storage materials needed is evaluated in terms of both mass and volume. Theoretically the maximum velocity of air is achieved at the chimney base and is 2 m/s. The maximum overall efficiency of the plant is estimated as 0.0028%. The maximum theoretical power output of the plant is 0.633 W.

A complex space environment will influence a space solar telescope during its in-orbit operation, and the temperature change of the optical window will affect directly the imaging quality of the optical system behind it. The purpose of the thermal design is to ensure that all the parts of the optical window keep their temperature in a normal range, more importantly, to ensure that the window can rapidly return to its working state as soon as the Earth shadowing is ended, and to complete the operation in a whole period. In order to obtain the temperature distribution and the temperature variation of the window under the space thermal load in the whole period, we have made the steady-state simulation analysis and transient-state simulation analysis of the window with and without heating during the Earth-shadow time. A good thermal control result is obtained by comparing the two kinds of transient state simulation results of the temperature distribution, and by accordingly taking the appropriate thermal control measures on the window.

A solar collector is a device which helps to harnesses solar radiation into useful form of energy. Among various solar collectors, the parabolic trough collector (PTC) is considered to be the best option for medium temperature (150–400 °C) heat requirements. The popularity of solar parabolic trough technology has generated interest in higher efficiency energy recovery potential. The absorber tube, also called the receiver tube or heat collection element, is one of the main functional units of a solar PTC in addition to other elements like parabolic mirrors, metal support structure and tracking unit assembly. Parabolic mirrors reflect to a focal point and concentrate falling sun rays onto an absorber tube, which is made up of a long metallic structure covered by an evacuated glass envelope to reduce convective heat transfer losses. Many techniques were attempted to enhance the heat transfer potential in the receiver tube portion of a solar PTC which includes techniques such as half insulation receiver, cavity receivers, vacuum outer shell, inclusion of inserts, baffles, artificially roughened sinks, selective coatings etc. In some of the research works, nanoparticles were also used to enhance heat transfer properties of the heat transfer fluid. In this paper, detailed review of the experimental and numerical works carried out on heat transfer enhancement techniques which focus on minimization of heat loss, use of turbulators, addition of nanofluid and selective coatings in the receiver tube of a solar PTC are presented. Further the major reasons for heat loss in the receiver tube and comparative study of various heat transfer enhancement techniques are summarised.

As the main electrical power source of spacecraft, solar cells require high electrical performance and light weight, to provide enough electrical power for missions in space and to lower costs for launching. Multi-junction compound semiconductor solar cells generate relatively high power, but their rigid substrates are tens or hundreds of times thicker than the solar cells. In this paper, methods are described to realize flexible space solar cell arrays that can be wrapped on non-planar surfaces, by removing the rigid substrates and integrating the thin solar cells on flexible substrates. Cover glasses were integrated over the solar cells on flexible substrates through an guided-printing method, providing both flexibility and radiation shielding. Experimental results and mechanical analysis demonstrate the mechanical and electrical performances of these flexible space solar cell arrays.

Energy-yield loss caused by soiling of photovoltaic modules and concentrated solar power (CSP) mirrors in utility-scale power plants installed in semiarid lands and deserts would result in unsustainable demands for fresh water needed for cleaning. This paper reviews the progress of the electrodynamic screen (EDS) film technology for frequent water-free cleaning with low-energy requirements. Results presented here, based on laboratory-scale EDS-film-laminated solar panel cleaning, show that the output power can be restored higher than 95% of the initial power under clean conditions. For solar mirrors, the specular reflection efficiency can be maintained over 90% ensuring high efficiency of the CSP plants. Operation of the EDS film for maintaining high optical efficiency of solar collectors requires less than 0.2 Wh/m ${}^2$ /cleaning cycle. Principles, optical modeling, construction, lamination of the EDS films on modules and mirrors, and experimental data showing optical efficiency restoration without water consumption are presented. Current challenges in developing electrodes that would meet optical and conduction properties, low-cost production, and meeting long-term outdoor durability of the EDS films are discussed.