Self-adaptive radiative thermostat to surrounding temperature variation

Abstract

Human beings use tremendous energy to keep cool in summer–current effective cooling, e.g., vapor compression and fluid cooled system consumes around 10% of worldwide energy [1]. This high level of energy consumption accompanies with various problems such as ozone depletion and greenhouse effect. To reduce the environmental burden on Earth, passive radiative cooling method is in the limelight in its eco-friend way of lowering the temperature. However, it is unconscious and sustainable method lasts in winter and causes unwanted cooling. Although few prior studies reported efficient thermoregulation techniques [2,3], most of them uses additional energy or stimulation to change heating and cooling states. Here, we suggest surrounding temperature variant radiative thermostat (STVRT) to realize completely passive way for winter heating and summer cooling. The STVRT changes its cooling and heating states depends on designated temperature points. Our design consists of two core parts: partial solar absorber and thermal radiator within long-wavelength infrared region (LWIR) atmospheric window (i.e., 8-13 μm). The appropriate solar absorbing portion is strongly needed to achieve winter heating. Based on thermal equilibrium equation, we extort the combination of ~ 14% of solar absorption layer (0.25-2.5 μm-wavelength) with ~ 85% of heat radiation layer (8-13 μm-wavelength). The proposed STVRT can reach +9.6 ℃ heating and –7.4 ℃ of cooling when surrounding temperatures are 0 and 30 ℃ in ideal case (i.e., non-radiative heat exchange coefficient, hc = 0 W/m2/K), respectively. For considering practical case, where hc = 10 W/m2/K, the archived heating temperature is -2.2 ℃ and cooling temperature is +2 ℃ when surrounding temperature is 30 and 0 ℃, respectively. With these intuitive design, we experimentally demonstrate STVRT by coating using porous poly(methyl-methacrylate) (PMMA) (i.e., radiative layer) on Cu film (i.e., absorbing layer). The coating thickness of porous PMMA controls the solar transparency. Also, the material selection can be expanded as long as each layer takes engineered portion of absorption/emission. The design basic shows generalized solution for temperature homeostasis.