“People usually see space as a source of heat from the sun, but away from the sun outer space is really a cold, cold place,” said Shanhui Fan, a professor of electrical engineering and the paper’s senior author. “We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools man-made structures even in the daytime.”
From an engineering standpoint, there are two challenges. First, the reflector has to reflect as much of the sunlight as possible. Poor reflectors absorb too much sunlight, heating up in the process and defeating the goal of cooling.
The second challenge is that the device must efficiently radiate heat (from a building, for example) back into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range in which the atmosphere is nearly transparent. Outside this range, the thermal radiation interacts with Earth’s atmosphere. Most people are familiar with this phenomenon. It’s known as the greenhouse effect, which is considered the cause of global climate change.
The new device accomplishes both goals. It is an effective broadband mirror for solar light — it reflects most of the sunlight. It also emits thermal radiation very efficiently within the precise wavelength range needed to escape Earth’s atmosphere.
The Stanford research team has succeeded by turning to nanostructured photonic materials. These materials can be engineered to enhance or suppress light reflection in certain wavelengths.
Using engineered nanophotonic materials, the team was able to strongly suppress how much heat-inducing sunlight the panel absorbs, while it radiates heat very efficiently in the key frequency range necessary to escape Earth's atmosphere. The material is made of quartz and silicon carbide, both very weak absorbers of sunlight.
The new device is capable of achieving a net cooling power in excess of 100 watts per square meter. By comparison, today’s standard 10-percent-efficient solar panels generate about the same amount of power. That means Fan’s radiative cooling panels could theoretically be substituted on rooftops where existing solar panels feed electricity to air conditioning systems needed to cool the building.
To put it a different way, a typical one-story, single-family house with just 10 percent of its roof covered by radiative cooling panels could offset 35 percent of its entire air conditioning needs during the hottest hours of the summer.
The researchers also note that radiative cooling has another substantial advantage. It is a passive technology. It requires no energy. It has no moving parts. It is easy to maintain. It can be installed on the roof or the sides of buildings and it starts working immediately.
Publication date: 5/6/2013
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