Mar
28
A New Cooling Panel That Works in the Sun
March 28, 2013 | 1 Comment
A Stanford team has designed an entirely new form of cooling panel that works even when the sun is shining. The team’s new structure could vastly improve the daylight cooling of buildings, cars and other structures by reflecting sunlight back into the chilly vacuum of space.
The paper describing the device was published March 5th in Nano Letters.
The press release asserts homes and buildings could be chilled without air conditioners. Car interior designs are possible that don’t heat up in the summer sun. “Tapping the frigid expanses of outer space to cool the planet. Science fiction, you say?” sayeth the author.
Prof. Shanhui Fan with Grad Students Aaswath Raman (L) and Eden Rephaeli. Click image for the largest view.
Shanhui Fan, professor of electrical engineering and the paper’s senior author explains, “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. 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 human made structures even in the day time.”
The engineering technology is twofold. 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 purpose of cooling. The second challenge is that the structure must efficiently radiate the heat back out into space. Thus, the structure must emit thermal radiation very efficiently within a specific wavelength range at which the atmosphere is nearly transparent. Operate outside this range and the Earth’s atmosphere simply reflects the light back down again. Many people are familiar with this phenomenon. It’s better known as the greenhouse effect – the supposed cause of global climate change.
Stanford’s Solar Absorption/Emission Graphic for the New Cooling Panel. Click image for the largest view.
Stanford’s new structure accomplishes both goals. It’s an effective a broadband mirror for solar light, reflecting most of the sunlight. It also emits thermal radiation very efficiently within the crucial wavelength range needed to escape Earth’s atmosphere.
The new panel answers one difficult to handle problem. Peak demand for cooling occurs in the daytime thus moving radiation to after dark has been studied extensively as a mitigation strategy for climate change.
Eden Rephaeli, a doctoral candidate in Fan’s lab and a co-first-author of the paper reflects on the issue, “No one had yet been able to surmount the challenges of daytime radiative cooling – of cooling when the sun is shining. It’s a big hurdle.”
The Stanford team has succeeded using nanostructured photonic materials. These materials can be engineered to enhance or suppress light reflection in certain wavelengths.
Aaswath Raman, a doctoral candidate in Fan’s lab and a co-first-author of the paper explains, “We’ve taken a very different approach compared to previous efforts in this field. We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant. In particular, we’re very excited because this design makes viable both industrial-scale and off-grid applications.”
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 results are impressive, 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% efficient solar panels generate the 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.
The “at home metaphor” is a typical one-story, single-family house with just 10% of its roof covered by radiative cooling panels could offset 35% its entire air conditioning needs during the hottest hours of the summer.
Perhaps even more significantly the comparison is one of active power generation, conversion and transforming to grid power – an expensive undertaking vs. a passive unit.
Radiative cooling has a profound advantage over all other cooling strategies such as air-conditioning – it’s a passive technology. It requires no energy. It has no moving parts. It is easy to maintain. You put it on the roof or the sides of buildings and it starts working immediately.
This technology has a big future. Beyond the commercial implications, Fan and his collaborators foresee a broad potential social impact. Much of the human population on Earth lives in sun-drenched regions close up to the equator. Electrical demand to drive air conditioners is skyrocketing in these places, presenting energy, economic and environmental challenges. These areas tend to be poor and the power necessary to drive cooling usually means fossil-fuel power plants or even more risky nuclear fission demands.
Fan points out, “In addition to these regions, we can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time. There are large numbers of people who could benefit from such systems.”
It concept looks good and should work straight away where the humidity of the air is already low. But where the humidity is high some method has to be employed to pull out the water vapor. Then the question comes up on cycling out heat from the building. Is the team suggesting a forced air design or other heat-moving device? There remain a lot of practical questions.
Yet the idea is a groundbreaker for a huge market of perspiring folks who would dearly love a more comfortable existence, work environment and home life.
It’s a truly grand idea with much larger prospects than the team may realize. Let the commercial scale folks take off with it. Priced right, they’ll be measuring production in square kilometers.
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The article seems to suggest that the device is removing other sources of cooling load from the home that are being removed by the standard air conditioning unit. These would be solar gain through the windows conductive heat through the shaded walls. It makes no mention of the internally generated loads like light bulbs, etc. If the device cannot remove these heat gains, and only prevents the load from the roof (usually less than 20% of the total), then it is not so dramatic, and may not be economical if its cost is too high relative to the alternative.