A Janus Passive Cooler for Migrating the High Temperature of the Automobile

Abstract

As car is one the most important transportation for each individual, cooling private vehicles has highly demanded. In particular, the extreme temperature can develop in stationary automobile under the greenhouse effect, when the windows are transparent for incoming solar radiation but opaque for outgoing long-wave thermal radiation. The temperature inside the cabin tremendously increased to approach 60 °C, which is causing human discomfort and car aging problem. Current cooling methods mostly rely on vapor compression and fluid-cooled systems. However, 28.4% of the total vehicle power is consumed for space cooling and place enormous stress on fuels consumption and CO2 emission. Passive cooling strategy, which is sustainable alternatives to active cooling technologies, can release heat without energy consumption or pollutant emission. Recently reported passive radiative coolers have shown outstanding sub-ambient cooling during the daytime. Those coolers are attached on exterior materials, roof, or human skin to draw heat from the periphery through conduction and convection. However, this attractive solution has explicit limitation in thermally sealed space (e.g., stationary automobiles). Since most of conventional radiative coolers only consider emissivity of the side exposed to the sky, they cannot thermally access to inside of the enclosure. Emissivity of the inner side open the path to inside of the enclosure by increasing the inner radiation flux and could allow the trapped heat to escape. Accordingly, strategic design of emission spectra on both sides of the cooler is vital for releasing the trapped heat. Here, we propose a Janus cooler that acts as a selective emitter on the top side and a broadband emitter on the bottom side. This design effectively draws heat because the bottom side can absorb thermal input in a broad spectral range while the top side emits heat to space without disturbing ambient radiation. We first design a Janus cooler comprising of an Ag-polydimethylsiloxane layer on micro-patterned quartz substrate. The induced spoof surface plasmon polariton helps overcome inherent emissivity loss of the polymer, and creates near-ideal selective and broadband emission on the separate sides. Next, we demonstrated several modification methods to improve adaptability as a car roof, such as mechanical strength and scalability, while maintain its optical properties. For example, we replace quartz to PMMA and PDMS substrate enable large-scaled Janus cooler with affordability. The hardness as well as elastic modulus and scratch resistance can be improved by using harder material such as h-PDMS. Last, we theoretically and experimentally demonstrate the remarkable cooling performances of Janus cooler for enclosures. We numerically solved the steady-state heat transfer model of enclosed space to estimate the impact of emissivity of each layer on cooling. The Janus cooler was more effective in drawing heat away as compared to materials with no emissivity and one-directional emissivity. Using Janus bi-directional emission characteristics, the Janus cooler lowered the temperature of a radiative object inside an enclosure by ~4 °C compared to the Conventional radiative cooler in experiments simulating a stationary automobile environment. The significance of the Janus cooler for migrating the high temperature of the automobile is illuminating the importance of emissivity of the substrate itself meanwhile designing the selective emitter in an interpretive way. With the superior ability to passively cool the enclosure, this advanced design leads to comfortability and cost effectiveness in unconscious way, which are critical demands for situation where heat entrance is present and exit is absent (e.g., stationary automobiles).