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Photonic Radiative Cooling, Dr. Aaswath Raman

Research Associate, Ginzton Laboratory, Dept. of Electrical Engineering, Stanford University; aaswath@stanford.edu

Dr. Aaswath Raman, Prof. Shanhui Fan, Dept. of Electrical Engineering, Stanford University

3 relevant patent applications through Stanford's OTL

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Photonic radiative cooling exploits a renewable resource that has to-date not been exploited by our energy systems: the cold darkness of the universe. We have demonstrated at prototype-scale that this resource can indeed be meaningfully used by specifically designed sky-facing surfaces that reside on the roof of a building, even when the sun is shining strongly, to enable cooling for buildings that requires no electricity or water input. By using state-of-the-art advances in nanophotonics and metamaterials, we have tailored the thermal emissivity and absorptivity of a sky-facing surface by photonic multilayer design to selectively emit its thermal heat at wavelengths where the atmosphere is transparent while simultaneously being strongly reflective of sunlight. Together this enables the surface to remain at least 5 degrees C below air temperature at all hours of the day completely passively, and enables a heat rejection capacity of 40 W/m2 when cooling a fluid such as water below air temperature. Modeled system performance in hot, arid climates is 100+ W/m2 of thermal heat rejection. Funded by ARPA-E we are building on this core breakthrough (published in Nature in 2014) to both scale these sky-facing surfaces and build cooling systems around them. Specifically we are current building 1-2 m2 (10-20 square feet) cooling modules that use these photonic radiative cooling surfaces to chill water 3-5 degrees C below the ambient air temperature. These modules would have a similar form factor to solar thermal water heating modules, except they generate chilled water by using the cold of outer space at all hours of the day. These systems will work particularly well in hot, dry climates, where we expect to reduce cooling costs by 10-20% for a typical system.

During the NSF I-Corps program we identified high-end (class A) office buildings as a potential market fit given the heat loads, desire for LEED certification and cooling-driven electricity use that peaked when the grid's loads peak (middle of the day). In particular we believe hot, dry regions with large building cooling loads and water scarcity are a good fit for our technology's benefits.

The core photonic radiative coolers are multilayer films that are deposited via evaporation, and are not dissimilar to other coatings (window coatings) that are done at very large scale. Many manufacturers exist that already do this at scale. We are currently working to ensure our designs can be reliably deposited by their large-area machines and evaluating the cost.

Unlike existing chillers and air-conditioners, our system requires no water or direct electricity input to cool a fluid (water, air) or solid (phase-change material, thermal mass) below the air temperature. In particular, it works well even when the sun is shining and air temperature is high, including late-afternoons when PV generates less electricity but A/C demand is still high.

Some barriers are similar to those faced by solar thermal and building PV: meeting short payback period expectations of building owners, soft installation costs and permitting. Integrating with chillers is challenging given conservative approach to new technology adoption in HVAC and buildings sector. Low energy-density per unit area.

They want to know the potential payback period and total installed cost vs. lifetime performance of the system. Typical commercial building retrofits are looking for less than 5 year paybacks, but some owners and institutions will consider longer paybacks. No water losses and dealing with peak cooling loads are of interest, but combining with thermal storage gets mixed feedback.

Core Technology: Photonic Radiative Cooling

Radically Improving Building Cooling Efficiency & Water Use

radCool Stanford University !

Cooling buildings is a major use of electricity globally and a key driver of peak demand in the middle of the day. Our team of electrical and mechanical engineers at Stanford University has devised and demonstrated a passive, water-free way of achieving below-air-temperature cooling at all hours of the day. Funded by ARPA-E, we are now demonstrating cooling systems based on this technology.

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Our core technology (published in Nature in 2014) is driven by engineered, sky-facing surfaces that can simultaneously reflect nearly all incoming sunlight and radiate their heat as thermal radiation into space

Daytime Radiative Cooling Surface!

cold outer space! (3 K)!

sun!

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These photonic radiative coolers remain below air temperature even under sunlight. Here we demonstrate 9°F (5°C) below air temperature and a 40 W/m2 cooling capacity:

through a combination of material properties and interference effects.SiO2 has a strong peak in its absorptivity near 9 mm due to its phonon–polariton resonance. HfO2 also presents non-zero absorption and henceemission in the 8–13mm wavelength range28. The combination of all theselayers results in a macroscopically planar and integrated structure thatcollectively achieves high solar reflectance and strong thermal emission.

The photonic radiative cooler’s absorptivity/emissivity spectrum isexperimentally characterized and shown in Fig. 2. The cooler shows min-imal absorption when integrated from 300 nm to 4 mm, where the solarspectrum is present, in Fig. 2a, reflecting 97% of incident solar powerat near-normal incidence. In Fig. 2b we observe that the cooler has strongand remarkably selective emissivity in the atmospheric window between8mm and 13mm. Moreover, the photonic radiative cooler’s thermal emis-sivity persists to large angles (see Extended Data Fig. 1), a useful featureto maximize radiated power Prad, a hemispherically integrated quantity—see equation (2)—and reminiscent of the behaviour of hyperbolic meta-materials29. Photonic design fundamentally enables these spectral prop-erties, which in turn are essential to achieving below-ambient radiativecooling. This spectral behaviour, and below-ambient cooling, is notachievable using these materials individually with conventional metal-lic reflectors.

We demonstrate the performance of the photonic radiative cooler ona clear winter day in Stanford, California, by exposing it to the sky on abuilding roof during daylight hours and comparing its steady-state tem-perature to the ambient air temperature. As shown in the temperaturedata of Fig. 3a, immediately after the sample is exposed to the environ-ment (shortly before 10:00 local time in Fig. 3a), its temperature dropsto approximately 4u to 5u Celsius below the ambient air temperature,

even though significant solar irradiance is already incident on the sam-ple. This is a key signature of radiative cooling, and a counterintuitiveresult during the day: we typically think of surfaces increasing their tem-perature when removed from the shade and exposed to the Sun duringthe day. We observe the photonic radiative cooler’s temperature for overfive hours under direct sunlight. Over 800 W m22 of solar power is inci-dent on the sample for three of the five hours. The cooler maintains asteady-state temperature substantially below the air temperature overthe entire day, and is 4.9uC 6 0.15uC below the air temperature bet-ween 13:00 and 14:00 (local time) when the solar irradiance is in therange 800–870 W m22. To illustrate the significance of this result, wecompare in Fig. 3b the photonic radiative cooler’s performance against200-mm wafers in identical apparatuses coated with conventional ma-terials: carbon black paint and aluminium. The black paint reaches near80uC, which is more than 60 uC above the ambient air temperature,while the aluminium reaches nearly 40uC, which is 20uC above the am-bient air temperature. Typical roofing material has strong solar absorp-tion and hence significantly heats up under direct sunlight, as emulatedby the black paint result here. Also, one still sees very strong heatingwith an aluminium film, even though it provides relatively strong solarreflection.

We next characterize the photonic radiative cooler’s cooling power.We allow its temperature to reach the previously achieved steady-statevalue under peak sunlight conditions of nearly 900 W m22. We then in-put heat to the cooler in steps over the course of one hour and observed

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Figure 2 | Emissivity/absorptivity of the photonic radiative cooler from theultraviolet to the mid-infrared. a, Measured emissivity/absorptivity at 5uangle of incidence of the photonic radiative cooler over optical and near-infrared wavelengths using an unpolarized light source, with the AM1.5 solarspectrum plotted for reference. The cooler reflects 97% of incident solarradiation. b, Measured emissivity/absorptivity of the cooler at 5u angle ofincidence over mid-infrared wavelengths using an unpolarized light source,with a realistic atmospheric transmittance model plotted for reference26.The photonic cooler achieves strong selective emission within theatmospheric window.

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Figure 3 | Steady-state temperature of photonic radiative cooler. a, Rooftopmeasurement of the photonic radiative cooler’s performance (blue) againstambient air temperature (black) on a clear winter day in Stanford, California.The photonic radiative cooler immediately drops below ambient onceexposed to the sky, and achieves a steady-state temperature Ts of4.9 uC 6 0.15 uC below ambient for over one hour where the solar irradianceincident on it (green) ranges from 800 W m22 to 870 W m22. b, Comparingthe photonic radiative cooler’s performance against two reference roofingmaterials: black paint and aluminium. The paint reaches a temperature up to80 uC, or 60 uC above ambient, while the aluminium reaches nearly 40 uC,or 20 uC above ambient. Only the photonic cooler stays well below ambientunder direct solar irradiance.

RESEARCH LETTER

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Macmillan Publishers Limited. All rights reserved©2014

We are developing systems based on this core technology that can enable cooling at all hours of the day. See to right for modeled performance in an arid, hot climate (Phoenix, AZ) throughout the year (panel temperature below ambient, top) and in June specifically (cooling power, bottom)

Key benefits include: •  No water or electricity input •  Reduces peak cooling loads

(especially commercial office buildings)

•  Improves existing chiller efficiency and capacity

•  Excellent fit for LEED and Netzero buildings (both new and retrofit)

Future system on a rooftop

Photonic Radiative Cooling Cleantech to Market Application Supplementary Materials Aaswath Raman | aaswath@stanford.edu Technology Summary & History Our team received funding in 2013 from ARPA-E to demonstrate, for the first time, passive, radiative cooling of a surface under direct sunlight during the daytime. In this approach to electricity-free cooling, we design the optical properties of a coating on a sky-facing surface such that it reflects sunlight strongly while simultaneously radiating its heat away thermally in the mid-infrared, to the cold of outer space. This isn’t science fiction! We successfully completed the first phase of our project, using a scalable multilayer photonic approach and demonstrate passive cooling of a sun and sky-facing surface 5-7°C (10-14°F) below air temperature under peak solar irradiance, and with 40 W/m2 of cooling power at air temperature. In hot climates this number can be as high as 120 W/m2. We passionately believe that photonic radiative cooling will be a fundamentally disruptive approach to reduce cooling loads in the hottest places, and during the hottest hours of the day. Beyond being a roofing material, we have modeled that by substituting directly for the interior cooling needs of buildings (through HVAC systems), our approach has the potential to reach payback periods below 10 years while attacking key pain-points surrounding peaking/ electricity use during the day, and water usage in arid regions. We are now building multiple square-meter (10-40 square feet) of product-scale prototypes that demonstrate performance milestones relevant to key markets. In particular we are building a water-cooling system based on these radiative panels. Think of something similar to a solar water-heating system on a roof, except our modules can occupy north-facing parts of a roof and will generated chilled water instead of hot water. Chilled water is what most commercial buildings use to cool interior spaces and these systems thus have the potential to be a ‘plug and play’ solution for both new and existing buildings. Preliminary Market & Customer Assessment

We participated in and completed the NSF’s I-Corps program in Arlington, VA in the summer of 2014. During the program we interviewed over 100 potential customers and partners across a wide range of segments. We believed that key customers for this technology would be operators of commercial or industrial buildings with large, year-round cooling loads. In particular we originally thought data centers would be an attractive early market segment where servers generate heat at all hours of the day. However, data centers turned out to be a poor fit: the heat loads they generated were too large for the ~100 W/m2 heat rejection provided by our radiative cooling panels, and moreover they were moving to higher-temperature operation.

However, in the buildings space, we did find a potential fit with high-end class A office buildings that are seeking to meet California’s Title 24 requirements and/or LEED Gold/Platinum status. Their overall cooling loads match well with available roof space and the heat rejection our panels could provide. Moreover, they were the customer segment most open to

considering new technologies given the stringent requirements being imposed on new construction and significant retrofits.

Our technology meets the need of these customers for a water and electricity-free, passive cooling solution that reduces their cooling load at peak hours of the day when demand surcharges are the highest, and so are temperatures. In particular, we believe there is a real opportunity to provide differentiation by bringing the benefits of water-cooled systems without the water losses. Currently, larger buildings in regions that can afford the water losses use cooling towers to pre-cool water through evaporative cooling. This water is either directly used for chilling, or more often, used as the sink into which the condenser of an air conditioner dumps heat. Thus, water-cooled systems are more efficient than air-cooled systems since in the latter the condenser can only dump heat into the ambient air’s temperature. However, cooling towers are costly and are only justifiable for buildings past a certain size, and moreover have huge water losses which are problematic for arid regions like the U.S. Southwest, Australia and the Middle East.

Our approach enables the benefits of water-cooled systems, providing a lower ambient for the condenser to dump heat into, without the water losses of evaporative cooling or the need for an expensive and complex cooling tower. It could thus enable much more efficient cooling for small-medium buildings that are currently forced to use air-cooled air conditioning units: a new market segmentation. Moreover, combining our system with thermal storage will allow us to complete eliminate chiller use for a few hours by storing the even-colder temperatures we are able to generate using our system at night. Targeting this stored thermal mass for use during peak cooling demand hours could also allow us to take advantage of expensive peak/ time of day electricity rates, and bring utilities on-board as partners.

We are however open to new opportunities and early-entry markets beyond the buildings sector. Buildings are a very hard market for a range of reasons, including the slow product cycle, inherent conservativeness in purchasing decisions and entrenched sales channels and distribution networks. We have considered satellites, military applications and off-grid scenarios more generally. Thus, in addition to buildings, we would be thrilled to be a part of C2M to find new scenarios where the ‘clean’ coldness and chilled water we generate will solve a key customer pain-point and find product-market fit.

References

Raman, A. P., Fan, S., & Rephaeli, E. (2013). “Structures for radiative cooling,” U.S. Patent Application 13/829,997.

Raman, A. P., Anoma, M. A., Zhu, L., Rephaeli, E. & Fan, S. (2014). “Passive radiative cooling below ambient air temperature under direct sunlight,” Nature, 515, 540-544.

Rephaeli, E.; Raman A.; Fan S. (2013). "Ultrabroadband photonic structures to achieve high-performance daytime radiative cooling, " Nano Letters 13, 1457-1461.

Zhu, L.; Raman, A.; Fan, S. (2013). “Color-preserving daytime radiative cooling” Applied Physics Letters 103, 223902.