Daytime Passive Radiative Cooling

Passive radiative cooling, an emerging technology, provides sustainable, energy-saving solutions. Taking advantage of outer space as a cold sink, a well optical-designed material can passively block incoming heat flux while simultaneously providing cooling effect by emitting thermal radiation. Sub-ambient cooling can be achieved, even under direct solar irradiation. Compared with conventional radiative cooling strategy, passive radiative material leads the way to an eco-friendly cooling without ozone depletion and the greenhouse effect.

Here is a short video that I have made during my Master's degree that briefly explaines about concept of daytime passive radiative cooling and also my research works. Enjoy!

Bio-inspired daytime passive radiative cooling

Saharan silver ants can maintain their body temperature below ambient air due to unique triangular shaped hairs that enhance solar reflection and thermal emission through a transparent window that lies in the atmosphere. Applying this thermoregulatory prismatic structure to polydimethylsiloxane (PDMS), highly emsissive in the 8–13 μm spectrum, we present a geometrically modified polymer-based daytime passive radiative cooler. The selective thermal emitter was fabricated based on the optimized prismatic structure from Finite Difference Time Domain (FDTD) simulations. The average emissivity within the 8–13 μm spectrum was enhanced to 0.98 by the gradient refractive index effect, while the average solar reflectivity in the visible and near-infrared spectrum was measured to be 0.95. The net radiative cooling power is estimated to reach 144 W/m², exceeding records of previously reported radiative coolers. Last, in Hong Kong's hot and humid climate, a field test successfully demonstrated cooling by 6.2 °C below the temperature of ambient air corresponding to a net cooling power of 19.7 W/m² in a non-vacuum setup during the peak daytime with shading. This is the largest temperature reduction observed in a tropical region for daytime passive radiative cooling. Our work presents an alternative method to enhance passive thermal emission and may facilitate its world wide application in eco-friendly space cooling.

Design and optimization method

We propose the bio-inspired passive daytime radiative cooler with a design of a top MIR emissive layer of PDMS and SiO₂ that strongly radiates in the 8–13 μm spectrum and a bottom solar reflective layer composed of Ag which can provide solar reflection above 95%. Optimization was mainly focused on achieving the highest emissivity within the targeted MIR range (8–13 μm) due to the major contribution in net radiative cooling performance. PDMS and SiO₂ are already well known for being selectively emissive in 8–13 μm wavelength range due to a high value of imaginary permittivity. Furthermore, the proposed cooler possesses an advantage of easy fabrication due to widely available SiO₂ substrate wafer (400–500 μm) and flexible material property of PDMS which can be easily shape to desired geometries on the surface by a nano-imprinting process. The great freedom to shape PDMS allows us to study the improvement of optical properties especially emissivity in different configurations. The geometrical structure on the surface of the PDMS emitter was optimized by utilizing FDTD simulations considering different geometries and sizes. After optimization, a comparison study regarding MIR emission between our optimized bio-inspired cooler and conventional flat surface cooler was performed. Prior to the fabrication and field experiment, FDTD optical computational analysis which directly solves Maxwell equations was utilized to achieve the highest net cooling power by optimizing geometrical structures of the PDMS emitter surface and the overall design of the cooler.

(c) FDTD simulation result for averaged MIR emissivity (8–13 μm) of three different geometrical configurations (triangular prism arrays (black solid line), circular rod arrays (red solid line), and uniform flat layer (blue solid line)) along with characteristic length of 2–15 μm (d) FDTD simulation result for MIR emission spectrum (8–13 μm) of emitters with triangular prism arrays (black solid line), circular rod arrays (red solid line), and uniform flat layer (blue solid line) at a constant characteristic length of 8 μm.

Micro-fabrication of the radiative coolers

Fabrication process flow of the bio-inspired passive radiative cooler.

(a) Preparation of a 525 µm thick P-type 4-inch silicon wafer cleaned by Piranha and hydrofluoric acid solution.

(b) Thermal oxidation of the silicon wafer for 60 nm.

(d) AOE process to etch the thermal oxide layer for 30 s.

(e) Photo resist layer strip off process.

(f) Silicon wet etching process with TMAH solution

(g) BOE process to remove the remaining thermal oxide layer.

(h) Sputter 160 nm thickness chromium on the surface of the mold.

(i) Sputter 160 nm Silver at the back of the silicon dioxide substrate.

(j) Spin-coat 20 µm PDMS on the silicon dioxide substrate.

(k) Nano-imprint process

(l) Removal of the mold after baking.

Myself in the clean room facility in Hong Kong University of Science and Technology

(a) A fabricated 4-inch wafer size 8 μm characteristic length triangular PDMS-SiO₂-Ag daytime passive radiative cooler under direct sunlight. (b) SEM image of the fabricated silicon mold with 8 μm characteristic length triangular arrays with a scale bar at 100 μm. (c) SEM image of the fabricated silicon mold with 8 μm characteristic length triangular arrays with a scale bar at 10 μm.

Field investigation of daytime passive radiative coolers

To validate the numerically estimated cooling performance and study the actual cooling behavior in non-ideal atmospheric conditions, 24-h field investigations were conducted under the sky condition of Hong Kong. During the daytime, even under the clear sky, it is clear that both coolers without shading were unable to produce a cooling effect. At the peak daytime, both the triangular PDMS cooler and the uniform PDMS cooler without shading was measured with a temperature higher than the ambient air temperature by 5.1 °C and 6.0 °C, respectively. Due to the characteristics of the tropical climate, both coolers, without shading, were unable to deliver cooling performance under the peak solar irradiation as incident solar energy exceeded the radiated thermal energy from the coolers to the atmospheric transparency window. With the solar shades partially blocking the incoming sunlight, at the peak daytime, daytime cooling was achieved with the maximum temperature reduction (average temperature difference between 12:00–13:00 (Date: 31-Sep-2018)) of 6.2 °C and 5.1 °C below the ambient for the triangular and uniform, respectively.

Field investigation of daytime passive radiative coolers using shades and without them on the roof of academic building

(a) Temperature profile of the unshaded uniform 100 μm thick PDMS-SiO₂-Ag (grey solid line), the unshaded 8 μm triangle PDMS-SiO₂-Ag (green solid line), and the temperature of ambient (blue solid line), the shaded uniform 100 μm thick PDMS-SiO₂-Ag (yellow solid line), the shaded 8 μm PDMS-SiO₂-Ag (orange), through a 48 h cycle (Date: 30-Sep-2018 to 31-Sep-2018). The average daytime relative humidity was 87% and 0–1 oktas sky condition was observed. The average global solar intensity was measured to be 1010 W/m² at peak daytime (11:00 a.m. to 13:00 p.m.). (b) Zoom-in of the temperature profile (Date: 31-Sep-2018) with the device under direct solar irradiation. The shaded 8 μm triangle PDMS-SiO₂-Ag cooler (orange solid line) achieved a temperature 6.2 °C below the temperature of ambient. (c) Net cooling power (square box) and temperature (line) profiles of the shaded 100 μm uniform PDMS-SiO₂-Ag cooler (yellow) and the shaded 8 μm triangular patterned PDMS-SiO₂-Ag cooler (orange) at the ambient air temperature (blue) during a 24-hr cycle (Date: 08-June-2019). The average relative humidity during the daytime and nighttime was 85% and 65%, respectively. 0–1 oktas sky condition was observed. The average global solar intensity was measured at 1020 W/m² during the peak daytime (from 12:00 p.m. to 2:00 p.m.).

Passive radiative coolers for building indoor cooling

This study describes the development of an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Some micro-channels are fabricated on the back side of the passive radiative cooler, allowing fluid to flow in an isolated loop such that the coolant can be chilled and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. Considering the steady state energy balance within the radiative cooling panel integrated HVAC systems, the cooling performance and indoor air temperature are evaluated by numerical analysis. A 100 m2 passive radiative cooling panel could chill water for the cooling of air, reducing indoor air temperature by 10 °C, equivalent to a net cooling power of 1600 W. This study suggests that the proposed passive radiative cooling system should be used to pre-cool the ambient hot air such that the overall energy consumption of a traditional air-conditioning system can be reduced. The findings promise the application of passive daytime radiative cooling in building HVAC systems.

Schematic diagram of the radiative cooling panel integrated HVAC system

(a) Cooling capacity with respect to the supplied air temperature and the parsitic heat transfer coefficient at the airflow rate of 200 m3 /h. (b) Cooling capacity with respect to the supplied air temperature and the parsitic heat transfer coefficient at the airflow rate of 4000 m3 /h