Carbon Nanotube Coatings for Spectrally Selective Solar Thermal Absorbers

Solar thermal heating is one of the promising candidates for widely applications of renewable energies in Norway and worldwide. Research and development of low cost and high efficient absorber layers, the most important part of the solar thermal heating system, is essential. This project aims to enhance the efficiency of the solar thermal absorbers combining the latest findings within nanotechnology. A new type of the absorber material based on Carbon nanotubes is developed and investigated to increase the efficiency of the energy absorbents, without compromising the low cost perspective. The ideal absorber should absorb as much as possible visible light, i.e. absorptance is close to 1, and its emittance is close to 0 in infrared light regions. In the project, both computational simulation and experimental work are used to synthetize, characterize and optimize the new absorber layer. Firstly, optical simulations of spectrally selective solar absorbers using carbonaceous materials graphite, soot and single-walled carbon nanotubes (SWCNTs) were carried out to define the optimized thickness of the absorber layer. The simulation results have shown that nano structured carbonaceous materials are highly potential candidates for spectrally selective absorbers. For SWCNTs, the optimal thickness is about 430 nm and the achieved solar absorptance and thermal emittance are 0.88 and 0.07. Even with such a single layer of carbonaceous material, the solar absorptance and thermal emittance of soot and SWCNT absorbers are already commercially interesting. Their performance can be furthered improved by adding an anti-reflection layer on top of absorber surfaces. The simulation pointed out that SWCNT absorbers with silica layer can achieve absorptance/emittance values of 0.91/0.07, respectively. In our experiments, functionalized multi-walled carbon nanotubes (MWCNT) from different suppliers were deposited on aluminum substrates by electrophoresis. Two types of CNT absorbers using N-CNT and P-CNT exhibit good spectral selectivity. The optimal spectral selectivity achieved for the two types of CNT abosrbers were 0.83 and 0.84 respectively, which is only little lower than that of commercial products of solar absorbers (>0.85). However, the coating process of CNT absorbers employed in this work has several advantages: 1) Electrophoretic deposition of CNT coating takes only a few seconds; 2) It does not require any advanced coating production equipment; 3) It uses aqueous suspension and consumes little chemicals, i.e. is environment-friendly; 4) The CNT absorbers only needs to be heat treated in air, but no need of inert atmosphere. Our novel CNTs spectrally selective absorbers coated with protective silica films exhibit an excellent thermal stability and they are qualified in accelerated ageing tests. The project is carried out in collaborations with experts in electrophoretic deposition at Erlangen-Nuremberg University and in solar thermal heating at University Tromsø. Beside fundamental research, the project is also industry oriented by collaboration with Norwegian companies operating in the solar energy sector. The results achieved so far were presented in two scientific conferences in 2014: Renewable Energy Research Conference (RERC), held in Oslo, June 2014 and International Conference on Solar Energy and Buildings EuroSun, held in France, September 2014. Some of the project results were recently published in Journal Energy Procedia. In 2015, based on the new results of the project, we have submitted 1 scientific paper to the Journal Solar Energy Material and Solar Cells, 1 conference paper to the World Renewable Energy Congress and other publications are under writing. The PhD thesis will be defended in 2016.

Solar thermal heating is per today the most energy efficient way to provide heating for households. The most efficient solar thermal collectors for hot water production use a spectrally selective absorber that absorbs and converts solar radiation into hea t. There are already high-performing selective absorbers but there are several difficulties with all of them, such as long-term durability, moisture resistance, adhesion, scratch resistance and costly technically advanced production techniques. In order t o make thermal solar collectors more accepted and widespread, the price per unit has to be reduced. The most costly component of a solar thermal collector is the spectrally selective absorbing surface. The aim of this project is to investigate whether m ultiwalled carbon nanotubes (MWCNTs) poses the appropriate optical/IR properties combined with easy processability and durability to simplify the manufacturing process of the spectrally selective absorbing surface.