Transparent Conductive Oxides as Selective Transmitters for solar thermal applications


Transparent Conductive Oxides as Selective Transmitters for solar thermal applications

Lungwitz, F.; Schumann, E.; Janke, D.; Escobar-Galindo, R.; Gemming, S.; Krause, M.

Abstract

In solar-thermal power plants the receiver tubes are one of the key components for increasing the efficiency of concentrated solar power (CSP) technology. Absorber materials of those tubes have to exhibit high-temperature and air stability, high optical absorption in the solar region and low thermal emittance. Temperatures of up to 400 °C and up to 550 °C are reached in currently operated parabolic trough and central receiver plants, respectively. The CSP efficiency could be increased by 15 to 20% applying operation temperatures of around 800 °C. In state of the art central tower plants black paints are used as absorber material. Due to limited stability they have to be periodically replaced. Furthermore the high emissivity of those paints leads to high radiative energy losses. Most of the R&D approaches for high-temperature solar receiver materials are focused on complex multilayer coatings.
Here, an alternative concept for high-temperature stable solar-selective coatings is presented. It consists of a transparent conductive oxide (TCO) deposited as solar-selective transmitter on a black body absorber. The latter is responsible for high absorption in the solar spectral range (300 nm – 2000 nm), the former ensures low emissivity in the infrared range (> 2000 nm) and oxidation resistance. The concept is easily implementable and combines significant improvements of CSP technology performance and cost competiveness.
This study is focused on the solar-selective transmitter component of the concept. For this purpose, SnO2:Ta thin films are reactively magnetron co-sputtered from tantalum doped and undoped tin targets at high temperatures on fused quartz, silicon and carbon substrates. Their optical properties are tailored to meet the specific requirements of a solar-selective transmitter coating. The correlation between structural, optical, and electrical properties is analyzed by Raman spectroscopy, spectroscopic ellipsometry (SE) and Rutherford backscattering spectrometry (RBS). All the techniques are applied in situ at high- temperatures in a cluster tool. In order to simulate real operating conditions, cyclic heating tests and heating in reactive atmospheres are conducted. Additionally, X-ray Diffraction (XRD), UV-VIS spectrometry, and Hall Effect measurements are performed. It is shown that structural parameters like e.g. grain sizes and dopant concentration result in different electrical properties and as a result determine the optical behavior like spectral selectivity of the TCO.
Financial support by the EU, grant No. 645725, project FRIENDS2, and the HGF via the W3 program (S.G.) is gratefully acknowledged.

Keywords: TCO; solar-thermal electricity (STE); concentrated solar power (CSP); solar selectivity

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Verknüpfte Publikationen

  • Vortrag (Konferenzbeitrag)
    MRS Spring Meetings & Exhibit 2017, 17.-21.04.2017, Phoenix, Arizona, USA

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