Thermal solar collector with VO2 absorber coating and V1-xWxO2 thermochromic glazing – Temperature matching and triggering (original) (raw)
Related papers
Predicting the thermal performance of thermochromic flat plate solar collectors
Journal of Physics: Conference Series, 2019
A thermochromic vanadium dioxide (VO2) based absorber coating is proposed. The optical response of the multilayered absorber is first simulated, then, the spectral reflectance of the deposited absorber is measured. The determined solar absorptance and thermal emittance values are used to predict the thermal performance of the thermochromic flat plate solar collector. The corresponding stagnation temperature is estimated to be ∼25°C lower than that of a standard collector. This is mostly due to the increase in emissivity over the thermochromic phase transition and, partially, to a lower solar absorptance than in standard absorbers. However, a concurrent increase of the solar absorptance over the phase transition limits the overheating protection abilities of the thermochromic collector. Alternative scenarios based on thermochromic absorbers with assumed, constant solar absorptances and the same temperature dependent thermal emittance as determined for the measured sample, are conside...
Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors
Energy Procedia, 2016
Highly efficient solar thermal systems generally undergo stagnation conditions with temperature inside the solar collectors as high as 190-200°C, as soon as the domestic hot water demand is poor or when the system is off while the collectors are still submitted to a strong solar radiation (> 950 W/m²). These stagnation conditions are known to be one of the major problem of thermal solar systems and often lead to vaporization and glycol degradation, loss of performances, and the need for regular maintenance with associated costs for the end user. Thanks to a novel smart selective coating, characterized by a strong increase of its infrared emissivity (thermochromic effect) at a critical temperature, stagnation temperatures can be reduced to 150°C for solar radiation and ambient temperature of 1000 W/m² and 35°C, respectively. As the novel smart selective coating presents a high solar absorption coefficient (>94%) and a low emissivity (6%) at low temperature, and because the thermochromic effect starts at a temperature around 70°C, the high performance of the new thermochromic thermal solar systems is guaranteed for domestic hot water heating. The properties of this new generation of selective coatings, based on a mixture of vanadium and aluminum oxides (VO 2 /V n O 2n±1 /Al 2 O 3 /SiO 2), are presented and discussed with regard to composition, structure and optical properties analysis. FTIR spectroscopy and infrared camera pictures clearly show the strong increase of emissivity for temperature higher than 70°C. Aging performances (high temperature, humidity, thermal cycling) are also presented in order to guarantee a minimum life time of 25 years for the new generation of thermochromic solar collectors. Finally, stagnation temperatures recorded under the same natural sun radiation on scale one (2.3m²) standard and thermochromic collectors are compared.
Optical Modeling and Measurements for Solar Energy Systems III, 2009
Overheating is a common problem both with the use of active and passive solar energy in thermal solar energy systems and in highly glazed buildings. In solar thermal collectors, the elevated temperatures occurring during stagnation result in reduced lifetime of the collector materials. Highly glazed building facades provide high solar gains in winter, but imply in most cases high energy needs for air conditioning in summer. A solution to such problems might be provided by "smart" thermochromic coatings. A durable inorganic thermochromic material is vanadium dioxide. At 68°C, VO 2 undergoes a reversible crystal structural phase transition accompanied by a strong variation in optical properties. By doping the material with tungsten, it is possible to lower the transition temperature making it suitable as a window coating. In order to simulate the optical behaviour of multilayered solar coatings, precise knowledge on the optical material properties is necessary. Experimental data reported in the literature are rare and controversial. We determined the complex dielectric function for VO 2 :W by spectroscopic UV-VIS-NIR ellipsometry above and below the transition temperature and subsequent point-bypoint analysis of the ellipsometric psi/delta data. For a validation, the solar reflectance, absorptance and transmittance were measured by spectrophotometry in the visible range and in the near infrared range up to 2500 nm. The experimental reflectance spectra have been compared with the computer simulations based on the determined optical material properties. Finally, we collected optical data in a more extended wavelength range by digital infrared imaging to detect the switch in thermal emissivity of VO 2 :W at around 45°C.
Towards coloured glazed thermal solar collectors
Multilayers of TiO 2 and SiO 2 dielectric coatings deposited by reactive sputtering have been characterized by X -ray photoelectron spectroscopy (XPS), in-stitu real-time laser reflectometry, spectroscopic ellipsometry and spectrophotometry. Measurements have been performed on individual dielectric layers of TiO 2 , SiO 2 and multilayers. The experiments show a good agreement between the different techniques. Optical properties can be modelled properly using a Cauchy dispersion model. The optical constants of an individual layer are confirmed by the optical measurements on more complex multilayer structures. Reflectivity of the three-layer films fulfils the requirements for a first step towards coloured glazed thermal solar collectors.
Durable innovative solar optical materials - the international challenge
Optical Coatings for Energy Efficiency and Solar Applications, 1982
A variety of optical coat.inJa are dfacmsed ln the context of .olar energy utlllr.atlon. Will kncMl eo11t1nga such as hut mirrors, •lectlve abmrt.rs, and refl-=tive films are covered briefly. Bnphasis is placed on the materials limltaUons and desi9n choices for varlom lesser known optical coatings and materials. llbysical and optical properties are detailed for s;rotectlve• mtireflection films, fluorescent concentrator 111ter1a!s, bolograpiic films, cold mirrors, redlatlvo cooling surfaces, and optical switching films lncludlng electrochrC1Dlc, themmchrClllic, photochrcmic, and liCJJid crystal types. !br many of these materials rwarch la only now being considered, Grxl varicus desi.CJn and dnmhility iUIJe:S mmt tie addressed.
Effect of Chromium Trioxide Coating on the Thermal Performance of Solar Thermal Collector
Karbala International Journal of Modern Science
This paper compared the thermal performance of two solar collectors. The first collector is coated by a dark black absorbent solar collector, and the second one coated by the chromium trioxide. The results of the second collector show a better selectivity in terms of greater radiation absorption, and less emission compared to the first collector. At the minimum solar irradiance time, the absorbed energy is increased from 908.28J to 1221.5J, and the thermal efficiency is improved from 37.3% to 50.1% when the chromium trioxide coating is used. Besides, at the maximum solar irradiance time, the absorbed energy is increased from 1340.5J to 1528.4J, and the thermal efficiency is improved from 63.9% to 78.9%. After three months exposing the collectors to outdoor conditions, the chromium trioxide coated collector show superior thermal performance over dark black coated collector. At the minimum solar irradiance time, the absorbed energy is increased by 877J and the thermal efficiency is improved by about 34% when the chromium trioxide coating is used. In addition, the absorbed energy and thermal efficiency are also increased from 501.1J and 20% to 1440.7J and 62.2% respectively when the solar irradiance is at its maximum value
Thermochromic VO 2 -based multilayer films with enhanced luminous transmittance and solar modulation
physica status solidi (a), 2009
Vanadium dioxide (VO 2 ) shows an abrupt and reversible change in optical and electrical properties when the temperature is raised beyond a critical point of ∼68 • C. Films made from this material have a potential to be used in energy efficient "smart" windows with temperature-dependent throughput of solar radiation. Two of the drawbacks of this material have been its low luminous transmittance and limited solar modulation of transmittance during switching. In this work we report calculations and experiments on multilayers of VO 2 and TiO 2 , produced by reactive DC magnetron sputtering, that significantly improve the luminous transmittance and solar modulation of the films during switching. We also explore the angular-dependent transmittance of five-layer TiO 2 /VO 2 /TiO 2 /VO 2 /TiO 2 films and demonstrate that the modulation of luminous and solar transmittance can be enhanced at non-normal angles of incidence.
Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings
Buildings
Today's advances in materials science and technology can lead to better buildings with improved energy efficiency and indoor conditions. Particular attention should be directed towards windows and glass facades-jointly known as "glazings"-since current practices often lead to huge energy expenditures related to excessive inflow or outflow of energy which need to be balanced by energy-intensive cooling or heating. This review article outlines recent progress in thermochromics, i.e., it deals with materials whose optical properties are strongly dependent on temperature. In particular, we discuss oxide-based thin surface coatings (thin films) and nanoparticle composites which can be deposited onto glass and are able to regulate the throughput of solar energy while the luminous (visible) properties remain more or less unaltered. Another implementation embodies lamination materials incorporating thermochromic (TC) nanoparticles. The thin films and nanocomposites are based on vanadium dioxide (VO 2), which is able to change its properties within a narrow temperature range in the vicinity of room temperature and either reflects or absorbs infrared light at elevated temperatures, whereas the reflectance or absorptance is much smaller at lower temperatures. The review outlines the state of the art for these thin films and nanocomposites with particular attention to recent developments that have taken place in laboratories worldwide. Specifically, we first set the scene by discussing environmental challenges and their relationship with TC glazings. Then enters VO 2 and we present its key properties in thin-film form and as nanoparticles. The next part of the article gives perspectives on the manufacturing of these films and particles. We point out that the properties of pure VO 2 may not be fully adequate for buildings and we elaborate how additives, antireflection layers, nanostructuring and protective over-coatings can be employed to yield improved performance and durability that make TC glazings of considerable interest for building-related applications. Finally, we briefly describe recent developments towards TC light scattering and draw some final conclusions.