A Study Of A Spectral-selective Surface For Diurnal Solar Heating-Nocturnal Radiative Sky Cooling

The flat-plate solar collector and the nocturnal radiative sky cooling device are highly similar in structure and complementary in working hours.Moreover,both of them are renewable energy technologies.If the two are comprehensively utilized,not only the multifunctional characteristics of the device can be realized,but also more efficient energy utilization can be realized.Based on this idea,this paper proposes a spectral selective surface for the comprehensive utilization of diurnal photothermic-radiative sky cooling(PT-RSC),and based on this,the following research work is carried out:1)In this paper,a selective composite surface of diurnal photothermic-nocturnal radiative sky cooling is prepared by wet coating method combined with TiNOx selective absorption coating and polydimethylsiloxane(PDMS)thin film.The preparation process is simple and easy to operate with low cost and suitable for industrial production.2)Full spectral transmittance,reflectance and absorptivity/emissivity are characterized for the prepared spectral selectively composite surfaces.Spectral tests show that this surface has a high emissivity(absorptivity)within the solar radiation band(i.e.,0,3～3 μm)and atmospheric window band(i.e.,8～13 μm),with values at 0.94 and 0.86,respectively.A low emissivity(absorptivity)within the left and right mid-infrared band(i.e.,3～8μm and 13～25 μm)is also found,with values at 0.38 and 0.37,respectively.The composite surface absorptivity(emissivity)shows a"high-low-high-low" trend,with good spectral selection characteristics.3)In order to further test the solar heating and radiative sky cooling of the spectral selective composite surface,an outdoor testing device is designed and built in this paper and preliminary experimental tests are carried out.The results show that the surface temperature can reach 98.9℃ during the daytime,which is 71.6℃ higher than the ambient temperature,and it reached 10.4℃ during the nighttime,which is 9.3℃ lower than the ambient temperature.From the experimental point of view,it is verified that the composite surface has excellent diurnal solar heating-nocturnal radiative sky cooling performance.