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• Trinary micro-porous 32/4/4 MgO/PVDF/PMMA blend with atmospheric window emissivity and solar reflectivity exceeding 97% is developed. • Compared to the baseline radiative cooling materials, trinary micro-porous 32/4/4 MgO/PVDF/PMMA blend enhanced surface temperature suppression by 1.6 ℃ at daytime. • Functionalized glass-PDMS micro-pillar heat exchanger chills water by 2.5 ℃ at an efficiency of 6.3% at nighttime. • Compared to the saturation limits, reduced temperature reduction and efficiency for low Peclet number low is attributed to axial conductive viscous dissipation. Chilled water harvesting is a fundamental application of passive radiative cooling and promotes energy conservation for space cooling in buildings potentially, which relies on well-designed radiative cooling materials and heat transfer interface. This paper reports a scenario leading to viscous dissipative thermo-photonic energy conversion, which takes place in low Peclet number regime of an order of magnitude of 100, where heat transfer is non-Graetzian. Compared to benchmarked glass-polydimethylsiloxane radiative cooler and barium sulphate coating, a newly developed trinary micro-porous 32/4/4 magnesium-oxide/poly(vinylidene-fluoride)/poly(methyl-methacrylate) radiative cooling blend, featuring high atmospheric window emissivity and solar reflectivity, both exceeding 97%, demonstrated a superior cooling performance with additional temperature reduction of 1.6 °C at daytime. Meanwhile, it chilled water at a flow rate of 6.3 µL/s by 1.3 °C upon coating on a glass-polydimethylsiloxane micro-pillar heat exchanger. Quantitative evaluation on the chilled water capacity was carried out at nighttime when the system ran pseudo-steadily. Cooling power measurement on a radiative cooler of same materials recorded a cooling power of 134 W/m2 which is close to the ideal limit. And measured water temperature reduction and cooling efficiency were 2.5 °C and 6.3% respectively. They were significantly lower than the saturation limit. Degraded thermal and energy conversion performances, attributive to extended Graetzian viscous dissipation, were discussed theoretically. [ABSTRACT FROM AUTHOR]

Passive radiative coolers can preserve the surface temperature below ambient by simultaneously reflecting incoming solar radiation and emitting thermal radiation to the sky. Apart from thermo-optical properties of the materials, radiative cooling performance is affected by various environmental factors which determine the atmospheric transmittance. As such, field investigations lack convergence and completion. And energy balance consideration, which aids in interpreting the field investigative results, is a deterministic cogitation on convective and radiative heat transfer by the radiative cooler that ignores the uncertainties abundant in field study. In this work, we examine the cooling performance of radiative cooling materials under different subtropical weather conditions in Hong Kong and approach the problem based on probabilistic regression modelling as an alternative. At nighttime, the response variable of surface temperature reduction can be correlated with a single predictor variable of sky temperature difference, which is a lumped parameter of ambient temperature, relative humidity, and cloud fraction. At daytime, it should be parametrized with an additional variable regarding solar intensity. The regression analysis reveals that, the higher the thermal emissivity, the larger is the temperature reduction at nighttime, especially obvious for large sky temperature difference. And heavy solar heat load is absorbed by the coolers at daytime even they feature reasonably high solar reflectivity. In this regard, further increment in solar reflectivity poses the top priority in improving daytime radiative cooling performance. [ABSTRACT FROM AUTHOR]

Owing to excellent solar reflectivity and sky window emissivity, disordered heterogenous materials, including filler-abundant matrices, paints, and coatings, as well as foam-like, fiber-stacked and composite porous structures, form a major class for efficient passive radiative cooling. Contrary to well-established empirical understanding, this work offers a generalized analytical overview of their macroscopic thermo-optical properties from the microscopic electromagnetic perspective of Maxwell-Garnett effective medium theory. With the family of micro-porous poly(vinylidene-fluoride)/poly(methyl-methacrylate) blends as a representative example, procedures for tailoring mid-infrared spectral emissivity via effective permittivity are outlined. Theoretical framework and design scheme are validated by finite difference time domain simulation and Fourier transform infrared spectrometry. It is shown that poly(vinylidene-fluoride) and poly(methyl-methacrylate) form a pair of complementary constitutive materials for near unity thermal emission through the atmospheric window. Optimized binary polymeric blend, prepared by spray-coating method, features a window emissivity of 98% and realizes nocturnal radiative cooling with a temperature reduction of 6.8 °C and a cooling power of 94 W/m2 in an outdoor field investigation. It can serve as a promising bifunctional material for simultaneous radiative heat dissipation and capacitive energy storage, which meets the demand for nocturnal, radiative cooling aided thermoelectricity generation and storage potential. • Maxwell-Garnett effective medium theory is employed for disordered heterogeneous radiative cooling materials design. • Thermal-radiative properties of PVDF/PMMA blends are optimized. • Temperature reduction of 6.8 °C and cooling power of 94 W/m2 are realized in nocturnal field investigation. [ABSTRACT FROM AUTHOR]