Abstract
• Radiative cooling with semitransparent radiative coating and participating substrate. • Iterative method to calculate effective radiative properties from spectrally variable parameters. • Overall estimation error of less than 0.2%. • Beneficial for applications requiring lumped solar and thermal radiation related parameters. Radiative cooling is a promising solid-state, non-vapor-compression technology for passive refrigeration and air conditioning. Although this phenomenon occurs naturally, achieving a significant amount of cooling to make it a technically and economically viable technology requires highly engineered, spectrally selective radiative surfaces. These characteristics make radiative cooling difficult to estimate, particularly when it is integrated with other systems such as photovoltaic panels or building envelopes. The complexity further increases when the substrate also participates in the radiative cooling (along with the radiative coating). Energy estimation is becoming increasingly critical because of the recent focus on the semitransparent radiative coatings that transmit a variety of colors to enhance the aesthetic appeal of the system. Here, we propose an iterative method to calculate the effective radiative properties, which provides the same net radiative cooling that would be observed using the spectral properties at both the coating and substrate surfaces. Compared to traditional methods that rely on either computationally expensive full spectral analysis or methods for averaging each radiative surface parameter locally, our proposed method focuses on calculating effective properties that provide the same the net cooling effect as a full spectral analysis by accounting the emissivity, absorptivity, and transmissivity collectively, thereby providing an overall estimation error of less than 0.2%. We believe that this study will be beneficial to the engineering communities that employ complex simulation codes and require lumped solar and thermal radiation related parameters.
Highlights
Radiative sky cooling is gaining tremendous interest as an effective cooling technique and a promising alternative for traditional vaporcompression air conditioners that typically consume large amount of electricity and use refrigerants with high global warning potential.[1]
The cooling capacity and operating time can be substantially improved by combining nighttime radiation cooling with daytime radiation cooling that requires materials with extremely high reflectivity in the highly intensive solar spectrum wavelength (0.3– 2.5 μm) while having high emissivity in the atmospheric window (8–13 μm)
Compared to the traditional method, which relies on averaging each of the radiation-related parameters separately, we focus on the net radiation heat flux comprising absorptance, emittance, and transmittance collectively, thereby minimizing the error in the net radiation cooling calculations
Summary
Radiative sky cooling (aka radiative terrestrial cooling or radiation cooling) is gaining tremendous interest as an effective cooling technique and a promising alternative for traditional vaporcompression air conditioners that typically consume large amount of electricity and use refrigerants with high global warning potential.[1] Radiative cooling is an important natural mechanism that enables the earth to maintain its temperature by balancing heat gain from the sun and heat loss to space. Because it is a natural process, it does not require any external input energy for operation. This study will be beneficial for engineering simulations like EnergyPlus that employ complex numerical codes and require lumped solar and thermal radiation related parameters
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