Radiative cooling to deep sub-freezing temperatures through a 24-h day–night cycle

Abstract

Radiative cooling technology utilizes the atmospheric transparency window (8–13 μm) to passively dissipate heat from Earth into outer space (3 K). This technology has attracted broad interests from both fundamental sciences and real world applications, ranging from passive building cooling, renewable energy harvesting and passive refrigeration in arid regions. However, the temperature reduction experimentally demonstrated, thus far, has been relatively modest. Here we theoretically show that ultra-large temperature reduction for as much as 60 °C from ambient is achievable by using a selective thermal emitter and by eliminating parasitic thermal load, and experimentally demonstrate a temperature reduction that far exceeds previous works. In a populous area at sea level, we have achieved an average temperature reduction of 37 °C from the ambient air temperature through a 24-h day–night cycle, with a maximal reduction of 42 °C that occurs when the experimental set-up enclosing the emitter is exposed to peak solar irradiance.

Radiative sky cooling: fundamental physics, materials, structures, and applications

“…According to (5), solar absorption and non-radiative heat transfer act as parasitic cooling losses, thereby reducing the overall cooling efficiency and increasing the cooling time to reach thermal equilibrium. Remarkably, it has been experimentally demonstrated that significant temperature reduction up to 40 K below ambient has been reached by radiative cooling within 1 h [47]. For systems with greater heat loads (e.g.…”

Daytime Radiative Cooling Using Near-Black Infrared Emitters

“…25 Achieving such low temperatures requires high vacuum to minimize parasitic conduction and convection. 21 If such parasitic mechanisms are present, expanding the bandwidth of thermal emission is likely to be benecial despite the increase in sky radiation absorption as shown in Figure 4b.…”

“…Thus, expanding the spectral range of high emissivity can be benecial under some circumstances. 21,25 For a cooler temperature of 300 K, Case 1 achieves a cooling power density 158 Wm -2 , 43 Wm -2 higher than Case 2.…”

“…Subsequently, Chen et al was able to demonstrate an average temperature reduction of 37 • C below ambient by combining a selective emitter with an apparatus consisting of a vacuum chamber. 21 Related to these experiments, there have been other recent theoretical works in designing various photonic structures for radiative cooling purposes. 12,2225 Most of these radiative coolers are designed to emit only in the atmospheric transparency window to avoid exchanging radiation with the atmosphere.…”

Tuning Transpiration by Interfacial Solar Absorber‐Leaf Engineering

“…On the basis of global water cycle, moderate global temperature, humidity, and precipitation may be attainable through the tunable plant transpiration. Moreover, photothermal conversion shows the potential as the regulator of the nature, besides the emerging applications, such as radiative cooling,42, 43, 44, 45 thermophotovoltaics,46, 47, 48, 49 water purification,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 seawater desalination,24, 25, 26, 27 and power generation 28, 29…”