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Reflection spectra of various ITO-coated glass substrates (the “Geomatec” sample used a thicker substrate).
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Solar thermal collectors are radiative heat exchangers. Their efficacy is dictated predominantly by their absorption of short wavelength solar radiation and, importantly, by their emission of long wavelength thermal radiation. In conventional collector designs, the receiver is coated with a selectively absorbing surface (Black Chrome, TiNO x , etc....
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Citations
... A comprehensive comparison of selective transmitters was reported in [25], concluding that ZnS/Ag/Zns and ITO perform best at low cost for the latter. ITO was also reported as an antireflection coating for silicon solar cells [26], while filters with appropriate cut-off at 1200 nm, suitable for silicon solar cells, were reported in [27]. ...
Combined concentrated solar power with photovoltaics can provide electricity and heat at the same system while maximizing the power output with reduced losses. Spectral splitting is required in such systems to separate the infrared part of the solar spectrum towards the thermal system, while the visible and near-infrared radiation can be converted by the photovoltaic solar cell. The performance of concentrated solar power plants comprising reflective beam splitters for combined generation of electricity and heat is presented in this work. A 50 MW power plant is considered in this work as a case of study in Southern Crete, Greece. The solar power plant consists of parabolic trough collectors and utilizes beam splitters with varying reflectivity. The dynamic performance of the power plant is modeled, and the annual energy yield can be calculated. Up to 350 MWt of thermal power can be delivered to the photovoltaic system utilizing a 50% reflecting splitter. The penalty to the high-reflectivity system is limited to 16.9% and the annual energy yield is calculated as 53.32 GWh. During summer months, a higher energy yield by up to 84.8 MWh/month is produced at 80% reflectivity compared to 90% as a result of the number of parabolic troughs. The reported energy yields with reflectivity by dynamic modeling can highlight discrete points for improvement of the performance in concentrated solar power photovoltaics.
... The outer surface of the inner glass tube is coated with a copper-aluminum alloy coating that absorbs solar radiation. This material has gained widespread usage due to its low cost and high heat absorption efficiency [43]. To determine if there were significant differences in the cooling of each vacuum tube, three T-shaped thermocouples (−200~350 • C) were installed on the outer surface of the vacuum tubes facing the ground and sky, respectively, along the length of the vacuum tube array at the 1st, 6th, 11th, and 15th vacuum tubes from the left. ...
... Energies 2023, 16, x FOR PEER REVIEW 11 of 2 meant that it had little obstruction to the outward heat radiation during cooling at nigh The outer surface of the inner glass tube is coated with a copper-aluminum alloy coatin that absorbs solar radiation. This material has gained widespread usage due to its low cos and high heat absorption efficiency [43]. To determine if there were significant difference in the cooling of each vacuum tube, three T-shaped thermocouples (−200~350 °C) wer installed on the outer surface of the vacuum tubes facing the ground and sky, respectively along the length of the vacuum tube array at the 1st, 6th, 11th, and 15th vacuum tube from the left. ...
The issue of freezing often occurs when using all-glass vacuum tube solar water heaters during cold winter seasons, leading to problems such as pipe ruptures and tank leakage. In order to further study the nocturnal heat dissipation and freezing characteristics of these heaters, a three-dimensional transient numerical model of their nocturnal heat dissipation was established. The model simulated the nocturnal heat dissipation process, and experimental validations were conducted through nocturnal temperature drops of the collector and temperature drops of individual tubes without a storage tank. Experimental and simulation results revealed that in clear weather conditions during cold winters in Luoyang, the all-glass vacuum tube solar water heaters experienced freezing issues during the night, with freezing predominantly starting from the bottom surface of the vacuum tubes. The frozen length along the tube wall and the thickness of ice at the bottom section reached up to 1180 mm and 5 mm, respectively. In the absence of a storage tank, the freezing situation was severe, with approximately 4/5 of the individual tubes completely frozen. Under specified operating conditions, different storage tank volumes exhibited varying degrees of freezing in the all-glass vacuum tube solar water heaters. When the volume was increased to 15 L, the temperature drop in the storage tank and the vacuum tubes decreased by 12.1% and 7.6%, respectively. Larger storage tank volumes resulted in reduced freezing risks in all-glass vacuum tube solar collectors. This study provides valuable guidance for the design and application of solar collectors and serves as a reference for the development and application of solar energy utilization technologies.
... By stacking combinations of these quarter wavelength designs together, the resulting dichroic mirror can be designed to create a simple edge-pass/edge-stop filter or a band-stop/band-pass optical element (such as the one used in Green et al. [421]). Several companies offer stock and custom designs of these materials [422][423][424], but they are typically orders of magnitude too expensive (e.g., 1 × 10 5 to 1 × 10 6 USD/m 2 ) for PV-T collectors [425]. Ignoring their cost, nearly ideal optical properties can be achieved through careful design. ...
... With these so-called 'low-emissivity' windows, modern buildings can reduce their solar heat gain (and indoor heat loss) without reducing the amount of natural daylight they receive. This feature is enabled by spectral splitting dichroic coatings deposited on the inner pane of a double or triple-glazed frame (which reflects all solar wavelengths except the visible light) [425]. Given their ubiquitous use in high-efficiency buildings, dichroic-coated glass might also be mass-produced for PV-T products in the future. ...
... Given their ubiquitous use in high-efficiency buildings, dichroic-coated glass might also be mass-produced for PV-T products in the future. This would require two fundamental changes from today's architectural glazing manufacturing processes: (a) broadening of the transmitted spectrum (perhaps requiring different materials and layer thicknesses) to shift the transmission window from about 450 to 750 nm to, say, 700-1100 nm band for Silicon cells [425]; and (b) use of low-iron or 'solar glass' substrates (rather than typical window glass), which is less absorbing of these longer wavelengths. ...
In this paper, we provide a comprehensive overview of the state-of-the-art in hybrid PV-T collectors and the wider systems within which they can be implemented,
and assess the worldwide energy and carbon mitigation potential of these systems. We cover both experimental and computational studies, identify opportunities for
performance enhancement, pathways for collector innovation, and implications of their wider deployment at the solar-generation system level. First, we classify and
review the main types of PV-T collectors, including air-based, liquid-based, dual air–water, heat-pipe, building integrated and concentrated PV-T collectors. This is
followed by a presentation of performance enhancement opportunities and pathways for collector innovation. Here, we address state-of-the-art design modifications,
next-generation PV cell technologies, selective coatings, spectral splitting and nanofluids. Beyond this, we address wider PV-T systems and their applications,
comprising a thorough review of solar combined heat and power (S–CHP), solar cooling, solar combined cooling, heat and power (S–CCHP), solar desalination, solar
drying and solar for hydrogen production systems. This includes a specific review of potential performance and cost improvements and opportunities at the solargeneration
system level in thermal energy storage, control and demand-side management. Subsequently, a set of the most promising PV-T systems is assessed to
analyse their carbon mitigation potential and how this technology might fit within pathways for global decarbonization. It is estimated that the REmap baseline
emission curve can be reduced by more than 16% in 2030 if the uptake of solar PV-T technologies can be promoted. Finally, the review turns to a critical examination
of key challenges for the adoption of PV-T technology and recommendations.
... In recent years, thin films have attracted great attention for various applications such as solar cells [1], supercapacitor, counter electrode, thermal sensors [2], catalysts, solar selective coatings [3], diluted magnetic materials [4] and anode materials. Because of these semiconductor materials showed unique physical properties [5], optical [6] and electrical behaviors. ...
Thin films have been synthesized using different deposition methods such as chemical bath deposition, vacuum evaporation, thermal evaporation, sputtering, hydrothermal, dip coating method and pulsed laser deposition. In this work, UV-visible spectrophotometer technique was employed to study the optical properties and band gap of the obtained thin films. UV-visible spectrophotometer technique has many advantages such as rapidly analysis ability, cost effective, versatile and simple to use. This technique measures the absorption or transmission of light that passing through the samples as a function of wavelength. The band gap was about 1.5 eV, matched the solar spectrum.
... The glass tube outer surface is also covered with anti-reflective coating, to facilitate the transmittance of the incident solar radiation. A wide comparative study [5] of 32,000 dielectric-metal-dielectric (DMD) and transparent conductive oxide (TCO) coatings concluded to the ZnS-Ag-ZnS and indium tin oxide as the most efficient coatings. The comparison utilized a selectivity factor metric that considers the temperature, the solar concentration ratio and the radiative efficiency of the receiver. ...
In this paper, the technological advances in concentrating solar power are reviewed. A comprehensive system approach within this scope is attempted to include advances of highly specialized developments in all aspects of the technology. Advances in geometric optics for enhancement in solar concentration and temperature are reviewed along with receiver configurations for efficient heat transfer. Advances in sensible and latent heat storage materials, as well as development in thermochemical processes, are also reviewed in conjunction with efficient system integration as well as alternative energy generation technologies. This comprehensive approach aims in highlighting promising concentrating solar power components for further development and wider solar energy utilization.
... The downside of using nanofluid-based DASCs should be considered. Firstly, as indicated by Hewakuruppu et al. [18] and Taylor et al. [19], rather than using a low-emissivity selective surface, thermal emission emanates the basefluids of DASCs, so: Q radiation loss ¼ A surface s ε basefluid T 4 basefluid , where ε basefluid $ 1. This can become a major source of energy loss as T basefluid increases much above ambient temperatures. ...
... This can become a major source of energy loss as T basefluid increases much above ambient temperatures. This means most of the DASC proposed in the literature would be inefficient and uncompetitive at temperatures >100 C. To address this issue, transparent solar selective surfaces (SSS) have been proposed as a cover for DASCs [19]. Another issue for nanofluid-based DASCs is that nanofluids intrinsically increase the pumping power (over pure fluids) [20] because they usually exhibit higher viscosity than their basefluids. ...
... As mentioned earlier, a transparent substrate coated with a layer of ITO was selected as the best available SSS for solar thermal application. As this ITO was custom-made, the modeled refractive indices were kindly provided by a fellow researcher [19]. These constants were plugged into OpenFilters [26], then transmission and reflection spectra were given to an ITO manufacturer and a sample with~250 nm thick ITO (corresponding to the optimised thickness for 400 C in Ref. [19]) on quartz was fabricated via magnetron sputtering by Geomatec, Japan (see Fig. 1h). ...
Solar thermal receivers are designed to absorb as much sunlight as possible (e.g., to increase energy inputs) while minimising heat losses and parasitic pumping energy requirements (e.g., to reduce energy losses). The field of direct absorption solar collectors has devised numerous elegant solutions for increasing energy inputs. Mitigating the energy losses, however, represents the other—generally overlooked—factor. To address this research gap, this study investigates the impact of nanofluids, superhydrophobic walls, an anti-reflective coating, and a transparent selective on the thermal efficiency of a microchannel-based solar receiver. The proposed modifications were ranked by their impact on the overall collector efficiency. It was found that compared to a pure water reference case, using a nanofluid working fluid provides the biggest reduction in optical losses (validating the literature's focus on this aspect). However, a micro-patterned surface achieved nearly the same improvement in optical efficiency. Adding a nanofluid, an anti-reflective cover, and a superhydrophobic surface (in concert) can boost the efficiency by ∼7% (compared to the reference case). Overall, this study provides a systematic approach to investigating features to mitigate energy loss mechanisms in direct absorption solar collectors via detailed component-level experimental testing and collector-level theoretical analysis.
... Solar selective surfaces are well-developed and they have been commonly realized using cermets, metal oxides, metals coated with thin-films comprising dielectric/metal/dielectric sandwich structures or semiconductors, and metal surfaces textured on the micron scale [2][3][4][5][6][7]. While a wide range of selective surfaces that absorb solar energy are available for different applications, their transparent counterparts are underdeveloped [8]. Transparent selective surfaces, or "transparent heat mirrors", are technologically desirable for the advancement of concentrated solar technologies that employ volumetric receivers [9][10][11][12][13]. ...
Numerical calculations are performed to determine the potential of using one-dimensional transparent photonic crystal heat mirrors (TPCHMs) as transparent coatings for solar receivers. At relatively low operating temperatures of 500 K, the TPCHMs investigated herein do not provide a significant advantage over conventional transparent heat mirrors that are made using transparent conducting oxide films. However, the results show that TPCHMs can enhance the performance of transparent solar receiver covers at higher operating temperatures. At 1000 K, the amount of radiation reflected by a transparent cover back to the receiver can be increased from 40.4% to 60.0%, without compromising the transmittance of solar radiation through the cover, by using a TPCHM in the place of a conventional transparent mirror with a In2O3:Sn film. For a receiver operating temperature of 1500 K, the amount of radiation reflected back to the receiver can be increased from 25.7% for a cover that is coated with a In2O3:Sn film to 57.6% for a cover with a TPCHM. The TPCHM that is presented in this work might be useful for high-temperature applications where high-performance is required over a relatively small area, such as the cover for evacuated receivers or volumetric receivers in Sterling engines.
... Nanofluids-water, oils, gases containing nanoparticles-have been proven to provide heat transfer enhancements [2][3][4] and control over the optical properties [5][6][7] of their base fluid. However, these benefits (e.g., increased convective heat coefficients) are often largely cancelled out by the increased viscosity resulting from the addition of nanoparticles [8]. ...
Solar energy can be converted into useful energy via photovoltaic cells or with a photothermal absorber. While these technologies are well-developed and commercially viable, significant benefits can be realised by pulling these two technologies together in photovoltaic/thermal (PV/T) systems which can provide both heat and electricity from a single collector. Emerging configurations in the PV/T field aim to incorporate micro and/or nanotechnology to boost total solar utilisation even further. One example of this is the nanofluid-based PV/T collector. This type of solar collector utilises nanofluids—suspensions of nanoparticles in traditional heat transfer fluids—as both an optical filter and as a thermal absorber. This concept seeks to harvest the whole solar spectrum at its highest thermodynamic potential through specially engineered nanofluids which transmit the portion of solar spectrum corresponding to the PV response curve while absorbing the rest as heat. Depending on the nanoparticle concentration, employing nanofluids in a flowing system may come with a price—an efficiency penalty in the form of increased pumping power (due to increased viscosity). Similarly, microchannel-based heat exchangers have been shown to increase heat transfer, but they may also pay the price of high pumping power due to additional wall-shear-related pressure drop (i.e., more no-slip boundary area). To develop a novel PV/T configuration which pulls together the advantages of these micro and nanotechnologies with minimal pumping power requirements, the present study experimentally investigated the use of nanofluids in patterned hydrophobic microchannels. It was found that slip with the walls reduced the impact of the increased viscosity of nanofluids by reducing the pressure drop on average 17% relative to a smooth channel. In addition, flowing a selective Ag/SiO2 core–shell nanofluid over a silicon surface (simulating a PV cell underneath the fluid) provided a 20% increase in solar thermal conversion efficiency and ~3% higher stagnation temperature than using pure water. This demonstrates the potential of this proposed system for extracting more useful energy from the same incident flux. Although no electrical energy was extracted from the underlying patterned silicon, this study highlights potential a new development path for micro and nanotechnology to be integrated into next-generation PV/T solar collectors.
... Another noted improvement is the potential for reduced life cycle energy because of the elimination of the absorber surface [44], but it should also be noted that operating a DASC has some inherent limitations. A seminal work by Hewakuruppu et al. identified and studied the limitations of DASC as it relates to selectivity [45], and based on earlier studies by Taylor et al. [46]. This limitation shows up in considering the parameter a 2 in equation (2), the non-linear loss term associated with radiation (and to a lesser degree in term a 1 ). ...
... Taylor et al. identified an inherent problem in the DASC, especially for temperatures operating above 100 C, in that the base fluids have inherently high emissivity in the infrared [46]. A typical solar thermal absorber operating at high temperatures is designed to be solar selective with a high solar absorptance and low thermal emittance with an ideal cutoff wavelength defined by the operating temperature [47]. ...
Over the last 100 plus years, solar thermal energy has been used for residential heating applications, industrial process heating, electricity generation, and thermochemical reactions. Because of the vast number of applications, numerous designs have been developed to improve the efficiency of converting incoming solar energy into useful heat and to lower the cost. Conventional solar thermal collectors required a solid surface to absorb and convert incoming solar energy to useful thermal energy. Developments in materials science have enabled a new type of absorber—a volumetric absorber—which utilizes nanoparticles suspended in a fluid to absorb sunlight. Since most working fluids only weakly absorb sunlight, well-engineered ‘nanofluids’ are attractive because only a low volume fraction of nanoparticles is needed to obtain a large shift in the optical properties. This review, on the 10-year anniversary of the first appearance of nanofluid-based direct absorption solar thermal collectors, provides a forward-looking perspective on the challenges and opportunities associated with nanofluids as direct absorbers. Through a critical comparison of design considerations, as well as the most recent experimental results of less well explored areas like hybrid photovoltaic/thermal systems and direct steam generation, this review aims to provide discourse on the next steps for development.
... Some of these are already commercialized [19]. Very recent research on multilayer [9,20] or composite SSCs [21] addressed even temperatures in the range up to 1000 K. Textured surfaces [22] and selective transmitters [23] can also provide solar selectivity. The application of transparent conductive oxides (TCOs) for low emissivity glasses was studied intensively as shown in the work of Granqvist [4], but only a few studies using TCOs on an absorbing substrate were published. ...
... The stack was thermally stable at 973 K in vacuum for up to 10 h when prepared with a SiO 2 interlayer between metal substrate and TCO [26]. Taylor et al. studied a wide range of TCOs including F-, Sb-and Cd-doped SnO 2 (FTO, ATO, CTO), Sn doped In 2 O 3 (ITO) and B-, Al-, Ga-and F-doped ZnO (BZO, AZO, GZO, FZO) [23]. In that theoretical study, ITO and FTO showed the best solar selectivity in the temperature range up to 873 K. ...
The transparent conductive oxide (TCO) SnO 2 :Ta is developed as a selectively solar-transmitting coating for concentrated solar power (CSP) absorbers. Upon covering with an antireflective layer, a calculated absorptivity of 95% and an emissivity of 30% are achieved for the model configuration of SnO 2 :Ta on top of a perfect black body (BB). High-temperature stability of the developed TCO up to 1073 K is shown in situ by spectroscopic ellipsometry and Rutherford backscattering spectrometry. The universality of the concept is demonstrated by transforming silicon and glassy carbon from non-selective into solar-selective absorbers by depositing the TCO on top of them. Finally, the energy conversion efficiencies of SnO 2 :Ta on top of a BB and an ideal non-selective BB absorber are extensively compared as a function of solar concentration factor C and absorber temperature T H. Equal CSP efficiencies can be achieved by the TCO on BB configuration with approximately 50% lower solar concentration. This improvement could be used to reduce the number of mirrors in a solar plant, and thus, the levelized costs of electricity for CSP technology.
