A thermal cloak could help conserve EV batteries from temperature fluctuations

How the thermal cloak works

The cloak works by isolating the vehicle from its surrounding environment. It has two components, an outer layer that reflects sunlight and an inner layer that traps the heat inside. Since the cloak has dual functionality, the researchers refer to it as a Janus cloak, after the two-faced Roman God.

Much like how the Earth cools itself using radiative cooling, the outer layer of the cloak emits the energy it absorbs in a long-wave infrared spectrum. These waves escape the atmosphere and therefore not only help the car stay cool but the planet too.

When researchers tested the cloak on an EV parked outside in Shanghai, the cloak reduced the cabin temperature by 72 - 82 Fahrenheit (22.8°C—27.7°C) as compared to an uncovered car, whose temperature reached a toasty 123 Fahrenheit (50.5°C).

While this feature is helpful in summer months, it could have a chilling effect when the temperatures drop in the winter or at night. To counter this, the researchers used an effect called 'photon recycling' that ensures that energy trapped between the cloak and car stays there and does not escape.

This helped the car maintain a cabin temperature nearly 44 Fahrenheit (7^oC) higher than the ambiance.

The passive cloak can be scaled

The cloak's design is highly effective since it does not actively need any energy to either heat or cool the car down. However, the researchers were also conscious that they used materials that could be manufactured easily so that the technology could be scaled.

The outer layer of the cloak is made from thin fibers of silica, which were then coated using boron nitride flakes, a material similar to graphite that improves the reflection of sunlight. The fibers were then braided and woven into a fabric and stuck to the inner layer, made from aluminum alloy.

Making the outer layers with thinner silica fibers could have further improved the cooling of the car, but such fibers cannot be made at scale with techniques that are readily available.

By using materials like boron and aluminum, the team has also ensured that the weight of the cloak is low and so are its costs. The cloak is also fire retardant and extremely durable, making it ideal for regular use.

The research findings were published today in the journal Device.

Abstract:

Global climate change and high energy costs pose urgent demands in the development of low-cost, passive thermal-regulation technologies. Despite intensive efforts invested, high-efficacy, all-season temperature regulation on real-world objects has not yet been achieved through truly passive, reliable structures. We propose and experimentally realize a Janus thermal cloak (JTC) composed of an all-ceramic, radiative-cooling phononic metafabric facing the sky and a photon-recycling foil facing indoor space. The phononic metafabric, which supports broadband hyperbolic phonon polaritons (HPhPs) and strong scattering of phonontransport, shows outstanding resilience under harsh thermal, mechanical, and corrosive environments. Field tests on electric vehicles (EVs) show that the JTC realizes daytime sub-ambient cooling by 8.0C (by 27.7C compared with non-covered EVs) in summer and nighttime supra-ambient warming by 6.8C in winter. The JTC does not involve dynamic phase change or moving parts and thus is promising toward practical applications in buildings, vehicles, and extraterrestrial environments.