Photothermal Conversion Characteristics Of Carbon Nanotubes In Microchannels

Solar energy is abundant but energy density is low and collection is difficult.This paper combined excellent photothermal materials with microchannel structures to provide two methods for efficiently collecting solar energy-nanofluids and nanomaterial coatings.Due to the addition of nanoparticles,nanofluids could exhibit far better light absorption properties than base fluids,and the temperature rise process could be quickly completed in the microchannels.The coating made of nano materials avoided the problem of poor stability of nanofluids while taking into account the high efficiency of photothermal conversion,and because of the large specific surface area of the microchannels,compared with the conventional indirect absorption solar collectors,loading the photothermal material on the inner wall of the microchannels greatly enhanced the heat exchange process between the coating and the working mediums,which was beneficial to improve the efficiency of the collector.In this paper,a microchannel collector was designed,and the photothermal conversion characteristics of nanofluid and nanomaterial coatings were studied at the laboratory level.Firstly,CNT nanofluids with good stability was prepared.Under the range of 0~0.02 wt% of CNT,the photothermal conversion efficiency of the microchannel collector system increased with the increase of mass fraction,but the growth rate gradually getting slower.The increase of the flow rate could also improve the photothermal conversion efficiency to some extent.When the flow rate was 1.6 cm/s,the photothermal conversion efficiency of the 0.02 wt% CNT nanofluid reached 91.2%.On the basis of CNT nanofluids,CNT/CuO binary nanofluids were prepared.The addition of nanometer copper oxide particles further improved the photothermal conversion performance of 0.0025 wt% CNT nanofluids in microchannel collectors.In the nanotube nanofluids,the addition of 0.025~0.2 wt% of nano-copper oxide has an effect on the photothermal performance,but the increase with respect to the mass fraction after the 0.1 wt% is not obvious.The photothermal properties of each mass fraction of CNT nanofluids were further improved by using nano-copper oxide and CNT nanofluid doping.The CNT/CuO mixed nanofluid with a mass fraction of 0.02%/0.1% could achieve a photothermal conversion efficiency of 95% at a flow rate of 1.6 cm/s.In view of the instability of nano-photothermal materials in fluids,a preparation method of CNT/PVA coating on microchannel inner wall was proposed,including wall pretreatment and coating material loading,due to the advantages of microchannel large specific surface area.The immobilized photothermal material could still achieve good photothermal conversion effect.The microchannel-loaded coating prepared from the mixture with a CNT content of 0.2 wt% could achieve 86.9% photothermal conversion efficiency at a working fluid flow rate of 1.6 cm/s.