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Comparison of Selective Transmitters for Solar Thermal Applications

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Comparison of Selective Transmitters for Solar Thermal Applications Comparison of Selective Transmitters for Solar Thermal Applications ROBERT A. TAYLOR,1,2,* YASITHA HEWAKURUPPU,1 DREW DEJARNETTE,3 TODD P. OTANICAR3 1School of Mechanical and Manufacturing Engineering—The University of New South Wales, Gate 14, Barker St., Sydney, Australia, 2052 2School of Photovoltaics and Renewable Energy Engineering—The University of New South Wales, Gate 14, Barker St., Sydney, Australia, 2052 3Department of Mechanical Engineering, The University of Tulsa, 800 S. Tucker Dr., Tulsa, USA, 74104 *Corresponding author: [email protected] Received 10 March 2016; revised 13 April, 2016; accepted XX Month XXXX; posted X Month XXXX; published XX Month XXXX OCIS codes: (310.6860) Thin films, optical properties, (310.7005) Transparent conductive coatings, (350.6050) Solar Energy. ABSTRACT 1. INTRODUCTION Solar thermal collectors are radiative heat exchangers. Their efficacy is dictated predominantly by their absorption of short- An ideal selective solar component perfectly absorbs (or wavelength solar radiation and, importantly, by their emission of transmits) short wavelength radiation - e.g. for wavelengths long-wavelength thermal radiation. In conventional collector cut-off – while perfectly reflecting long designs, the receiver is coated with a selectively absorbing wavelength radiation – e.g. for wavelengths cut-off. This cut- surface (Black Chrome, TiNOx, etc.) which serves both of these betweenoff wavelength 0 and depends λ on operating temperature and the solar aims. As the leading commercial absorber, TiNOx consists of concentration ratio. ‘Selectivity’ is made possible> λ by the fact several thin, vapor deposited layers (of metals and ceramics) on a that terrestrial solar energy does not have a true blackbody metal substrate. In this technology the solar absorption to spectrum after passing through the atmosphere (e.g. after H2O thermal emission ratio can exceed 20. If a solar system requires and CO2 absorption). Thus, incident solar radiation has very an analogous transparent component – one which transmits the little infrared energy, enabling an energy balance ‘win’ with a full AM1.5 solar spectrum, but reflects long wavelength thermal cut-off absorber (or filter). Modern solar thermal receivers use emission – the technology is much less developed. Bespoke ‘heat thin, selectively absorbing/emitting coatings on the outer mirrors’ are available from optics suppliers at high cost, but the surface of an opaque absorber. Selective surfaces are most closest mass-produced, commercial technology is low-e glass. advantageous when the operation temperature is high, but not Low-e glasses are designed for visible light transmission and, as too high (e.g. 100-600oC). They are also best applied when the such, they reflect up to 50% of available solar energy. To address hot surface is encased in a vacuum package to minimize this technical gap, this study investigated selected combinations convective heat transfer loss and chemical oxidization/ of thin films which could be deposited to serve as transparent, degradation. When these conditions are met, the solar and the selective solar covers. A comparative numerical analysis of blackbody spectrums have little overlap and, importantly, feasible materials and configurations was investigated using a radiation heat loss is dominant. This is the case in evacuated non-dimensional metric, termed the ‘Efficiency Factor for tube collectors – a technology which represents the dominant Selectivity’ (EFS). This metric is dependent on the operation solar thermal collector on the market with approximately 350 temperature and solar concentration ratio of the system, so our million square meters installed in China alone [1]. This analysis covered the practical range for these parameters. It was technology operates well in the intermediate temperature found that thin films of indium tin oxide (ITO) and ZnS-Ag-ZnS range (with and without solar concentrating optics), and is provided the highest EFS. Of these, ITO represents the more well-suited to provide heat for industrial processes or to drive commercially viable solution for large-scale development. Based organic Rankine cycles. Since there is an immense demand for on these optimized designs, proof-of-concept ITO depositions 100-600oC heat (some 25-30% of the total energy usage in were fabricated and compared to commercial depositions. developed countries [2]), it is expected that this type of solar Overall, this study presents a systematic guide for creating a new thermal energy technology will increasingly displace class of selective, transparent optics for solar thermal collectors. conventional fossil fuels [3,4]. At present, the global installation rate of solar thermal collectors is about 9% per year [1], yielding a doubling in global installed capacity every 7- 8 years – signifying a large and rapidly growing global market. A. Solar Selectivity cut-offs in the visible spectrum and are typically Fabry-Perot – based depositions which consist of 100s of layers of alternating Although selective absorbing surfaces are well developed high and low refractive index materials [6–8]. As an example, and very successful commercially, little research has gone into the Brightline ‘shortpass filter’ from Semrock has good long the development of analogous selective transparent wavelength cut-off for this application (1326 nm), but comes a components. If developed, these could be useful in similar solar poor shortwave cut-off at 700nm, which actually makes it a thermal applications, but would enable alternative receivers bandpass filter for solar applications [6]. Since these filters are which: a) employ volumetric (or direct) absorption receiver or sold in small quantities (for optics laboratories), their list price b) use simple, low-cost black absorbers with a selective heat is in the range of 0.1-1 Million USD/m2, depending the on mirrors to improve their performance. Thus, this type of supplier/product [6–8]. This is orders of magnitude out of the transparent component could facilitate a new class of low-cost range of solar collection technologies, which need to be << solar receivers that have a similar radiative exchange 1,000 USD/m2 of aperture area [9] [10]. performance with TiNOx. With this as our motivation, the current paper reports on feasible selective coatings for transparent substrates which allow transmission of solar energy, but reflect infrared emission. 1. Selective Absorbers (Commercial) At present, the state-of-the-art commercial selective absorption material is TiNOx, which consists of multiple thin layers on top of a relatively thick metal substrate [1]. A proprietary multi-step method involving advanced vapor deposition techniques is used to produce these coatings. TiNOx is generally used on the inner (metal) tube of an evacuated tube receiver, where it is never exposed to oxygen. It is then sealed inside an outer (glass) tube, which generally stays at temperatures well below 100oC during operation. This outer tube is exposed directly to the ambient meaning that convection (not radiation) heat loss dominates. The outer glass tube can be given either an anti-reflective coating or simply left Figure 1. Measured transmission spectra of commercial uncoated. It should be noted that the metal-to-glass seal of this transparent materials compared with the normalized direct technology is difficult and is the source of many failure modes. normal solar irradiance (DNI) spectrum (from [11]) 2. Selective Transmitters (Commercial) B. Incorporating Transparent Selective Components There is one product on the market which is indeed Due to the fact that the transparent components of designed to be selective in transmitting sunlight and reflecting conventional solar collectors operate at low temperature and infrared light – low-e glass. Low-e glass is mass-produced and usually experience more convective than radiative heat loss, is rapidly replacing ordinary clear glass in buildings and there has been little impetus to develop selective transparent automobiles the world over [5]. Unfortunately, low-e glasses components. While selective solar transmitters are under- seek only to deliver maximum visible light and, in fact, minimize developed, they could be gainfully employed in the following overall solar heat gain (which is clearly not the goal of solar two applications. Note that for both selective surfaces are either thermal collectors). Low-e glasses reflect much of the solar not possible and/or would likely be of higher cost. spectrum which could be converted to useful heat. To demonstrate this, Figure 1 gives experimental measurements 1. Direct Solar Absorption using UV/Vis (from a Shimadzu UV-Vis 2600) and IR (Perkin With the advancement of nano-fabrication techniques, Elmer Spectrum Two) spectrophotometer data for a few direct absorption solar collectors are now possible to create at commercially available low-e glasses as compared to clear relatively low cost through ‘nano-engineering’ [12,13]. Direct glass. It can be seen that low-e glasses, if used as solar absorption (or volumetric absorption) systems need selective collectors, would reject 30-50% of the energy available in the solar transmitters since they will generally have a transparent AM 1.5 direct normal irradiance (DNI) spectrum. Figure 1 also component in which radiative heat losses are large. Aside from shows that normal ‘clear glass’ (window glass) also rejects the optical tunability afforded by these systems, one of the main ~20% of the solar spectrum,
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