Aaswath Raman
Radiative Cooling Generator

Transformative? New Device Harvests Energy in Darkness
It doesn’t generate much power, but it works during the one time of day that solar cells can’t : night.
By Rebecca Boyle
Sept. 12, 2019

Aaswath Raman was driving through a village in Sierra Leone in 2013 when an idea came to him as suddenly as, perhaps, a light bulb switching on.

The village was not equipped with electricity, and Dr. Raman, an electrical engineer at the University of California, Los Angeles, was unaware he was in a village until he heard the voices of shadowed human figures.

“It took us about five minutes to realize we were passing through a town, because it was completely dark,” Dr. Raman said. “There wasn’t a single light on.”

Dr. Raman wondered whether he could use all that darkness to make something to light it up, not unlike the way that solar panels generate electricity from the sun’s heat and light.

He did. In new research published on Thursday in the journal Joule, Dr. Raman demonstrated a way to harness a dark night sky to power a light bulb.

His prototype device employs radiative cooling, the phenomenon that makes buildings and parks feel cooler than the surrounding air after sunset. As Dr. Raman’s device releases heat, it does so unevenly, the top side cooling more than the bottom. It then converts the difference in heat into electricity. In the paper, Dr. Raman described how the device, when connected to a voltage converter, was able to power a white LED.

“The core enabling feature of this device is that it can cool down,” Dr. Raman said.

Jeffrey C. Grossman, a materials scientist at the Massachusetts Institute of Technology who studies passive cooling and solar technology, said the work was “quite exciting” and showed promise for the development of low-power applications at night.

“They have suggested reasonable paths for increasing the performance of their device,” Dr. Grossman said. “But there is definitely a long way to go if they want to use it as an alternative to adding battery storage for solar cells.”

Everything emits heat, according to the laws of thermodynamics. At night, when one side of Earth turns away from the sun, its buildings, streets and jacket-less people cool off. If no clouds are present to trap warmth, objects on the Earth can lose so much heat that they reach a lower temperature than the air surrounding them. This is why blades of grass may be glazed in frost on clear fall mornings, even when the air temperature is above freezing. The cloudless atmosphere becomes a porthole to the void, through which warmth flows like air through a porch screen.

Humans have taken advantage of this effect for millenniums. Six thousand years ago, people in what are now Iran and Afghanistan constructed enormous beehive-shaped structures called yakhchal, which used this passive cooling effect to create and store ice in the desert.

Modern scientists have studied how to harness energy from Earth’s day-night swings in temperature, but that work has mostly remained theoretical. In 2014, researchers led by Federico Capasso, an electrical engineering professor at Harvard, calculated that at best only about 4 watts of energy can be extracted from a square meter of cold space. By contrast, a photovoltaic panel, the most common type of solar panel, generates about 200 watts per square meter in direct sunlight.

Nonetheless, a device that could produce any amount of electricity at night would be valuable; after the sun sets, solar cells don’t work and winds often die down, even as demand for lighting peaks.

Shanhui Fan, an electrical engineer at Stanford and an author on Dr. Raman’s study, has been at the vanguard of this research. Last fall, Dr. Fan’s team described a device that can generate electricity with solar panels during the day, then use the passive cooling effect to chill a building at night. Earlier this year, they also tested an infrared photodiode, similar to the technology used in most solar cells but which uses warmth, not sunlight, to generate wisps of electricity in the darkness.

The prototype built by Dr. Raman resembles a hockey puck set inside a chafing dish. The puck is a polystyrene disk coated in black paint and covered with a wind shield. At its heart is an off-the shelf gadget called a thermoelectric generator, which uses the difference in temperature between opposite sides of the device to generate a current. A similar device powers NASA’s Curiosity rover on Mars; its thermoelectric generator derives heat from plutonium radiation.

Usually, the temperature difference in these generators is stark, and they are carefully engineered to separate hot and cold. Dr. Raman’s device instead uses the atmosphere’s ambient temperature as the heat source. The shift from warm to cool is very slight, meaning the device can’t produce much power.

His puck-in-a-dish is elevated on aluminum legs, enabling air to flow around it. As the dark puck loses warmth to the night sky, the side facing the stars grows colder than the side facing the air-warmed tabletop. This slight difference in temperature generates a flow of electricity.

When paired with a voltage converter, the prototype produced 25 milliwatts of power per square meter. That is about three orders of magnitude lower than what a typical solar panel produces, and well short of even the roughly 4-watt maximum efficiency for such devices. Still, several experts said the prototype was an important contribution to a new and relatively unusual space in the renewable energy sector.

“This is a neat combination of radiative cooling — a technique where Raman has pioneered real working devices — with thermoelectric materials that generate electricity if one side is hotter than the other side,” said Ellen D. Williams, a physics professor at the University of Maryland and a former director of the Department of Energy’s Advanced Research Projects Agency-Energy. “Both technologies are proven and practical, but I haven’t seen them combined like this. They did this with inexpensive materials, suggesting it could be made into useful products for the developing world.”

One challenge will be improving the device’s efficiency without raising its costs, said Lance Wheeler, a materials scientist at the National Renewable Energy Laboratory in Golden, Colo. Although thermoelectric devices are less efficient and more expensive than photovoltaic cells, they can be more durable.

“You could call this a long play,” he said. “It is just a piece of metal with spray paint on it. It could last for a super long time, and its rivals, photovoltaic cells and batteries, don’t. It can enhance any thermoelectric device as long as it’s outside facing the stars.”

Conceivably, Dr. Raman said, thermoelectric devices could complement solar-powered lights in areas where changing batteries is a challenge, like on street lamps or in remote areas far from electrical grids.

“I figured the amount of electricity we could get would be pretty small, and it was,” he said. “But walking around in Sierra Leone, I realized lighting remains a big problem, so it’s an opportunity as well.”

Generating Light from Darkness
Aaswath P. Raman, Wei Li, Shanhui Fan
A thermoelectric generator is built whose cold side radiates heat to the sky
Night-time power generation of 25 mW/m 2 is demonstrated, sufficient for a LED
Pathways to performance > 0.5 W/m 2 using existing commodity components exist
This approach is immediately practical for lighting and off-grid sensors

A large fraction of the world’s population still lacks access to electricity, particularly at night when photovoltaic systems no longer operate. The ability to generate electricity at night could be a fundamentally enabling capability for a wide range of applications, including lighting and low-power sensors. Here, we demonstrate a low-cost strategy to harness the cold of space through radiative cooling to generate electricity with an off-the-shelf thermoelectric generator. Unlike traditional thermoelectric generators, our device couples the cold side of the thermoelectric module to a sky-facing surface that radiates heat to the cold of space and has its warm side heated by the surrounding air, enabling electricity generation at night. We experimentally demonstrate 25 mW/m 2 of power generation and validate a model that accurately captures the device’s performance. Further, we show that the device can directly power a light emitting diode, thereby generating light from the darkness of space itself.
Cell, Volume 3, ISSUE 1, P101-110, January 16, 2019
November 08, 2018
Simultaneously and Synergistically Harvest Energy from the Sun and Outer Space
Zhen Chen, et al.
Theoretical limit of energy harvesting from the sun and outer space simultaneously
Experimental demonstration of heating and cooling using the same physical area
The absorber (cooler) reaches 24°C above (29°C below) the ambient temperature

The sun and outer space are the two most important fundamental thermodynamics resources for human beings on Earth. The capability for harvesting solar energy has been of central importance throughout the history of human civilization. Harvesting the coldness of outer space using radiative cooling technology also has a long history and has received renewed interest recently. However, simultaneously and synergistically harvesting energy from these two thermodynamics resources has never been realized. Here we report the first experimental demonstration of such simultaneous energy harvesting using a configuration where a solar absorber that is transparent in mid-infrared is placed above a radiative cooler. The solar absorber is heated to 24°C above the ambient temperature and provides a shading mechanism that enables the radiative cooler to reach 29°C below the ambient temperature. This work points to a new avenue for harvesting of renewable energy resources.
Applied Physics Letters > Volume 114, Issue 16 > 10.1063/1.5089783
23 April 2019
Experimental demonstration of energy harvesting from the sky using the negative illumination effect of a semiconductor photodiode featured
Masashi Ono, Parthiban Santhanam, Wei Li, Bo Zhao, and Shanhui Fan
We experimentally demonstrate electric power generation from the coldness of the universe directly, using the negative illumination effect when an infrared semiconductor diode faces the sky. Our theoretical model, accounting for the experimental results, indicates that the performance of such a power generation scheme is strongly influenced by the degree of matching between the responsivity spectrum and the atmospheric transparency window, as well as the quantum efficiency of the diode. A Shockley-Queisser analysis of an ideal optimized diode, taking into consideration the realistic transmissivity spectrum of the atmosphere, indicates the theoretical maximum power density of 3.99 W/m2 with the diode temperature at 293 K. The results here point to a pathway towards night-time power generation.

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Various aspects as described herein are directed to a radiative cooling apparatuses and methods for cooling an object. As consistent with one or more embodiments, a radiative cooling apparatus includes an arrangement of a plurality of different material located at different depths along a depth dimension relative to the object. The plurality of different material includes a solar spectrum reflecting portion configured and arranged to suppress light modes, thereby inhibiting coupling of the incoming electromagnetic radiation, of at least some wavelengths in the solar spectrum, to the object at a range of angles of incidence relative to the depth dimension. Further, the plurality of material includes a thermally-emissive arrangement configured and arranged to facilitate, simultaneously with the inhibiting coupling of the incoming electromagnetic radiation, the thermally-generated electromagnetic emissions from the object at the range of angles of incidence and in mid-IR wavelengths.

US2018023866 -- Ultrahigh-Performance Radiative Cooler
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A radiative cooler is provided having a thermally insulated vacuum chamber housing that is configured to support a vacuum level of at least 10-5 Torr, an infared-transparent window that is sealably disposed on top of the thermally insulated vacuum chamber and is transparet in the range of 8-13 μm, a selective emitter disposed inside the chamber, a mirror cone on the infared-transparent window, a selective emitter inside the chamber and is configured to passively dissipate heat from the earth into outer space through the infared-transparent window and is thermally decoupled from ambient air and solar irradiation but coupled to outer space, a heat exchanger with inlet and outlet pipes disposed below the selective emitter to cool water flowing through the pipe, a sun shade disposed vertically outside the chamber to minimize direct solar irradiation, and a mirror cone to minimize downward atmospheric radiation.

[ PDF ]

Various aspects as described herein are directed to a radiative cooling device and method for cooling an object. As consistent with one or more embodiments, a radiative cooling device includes a solar spectrum reflecting structure configured and arranged to suppress light modes, and a thermally-emissive structure configured and arranged to facilitate thermally-generated electromagnetic emissions from the object and in mid-infrared (IR) wavelengths.

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Aspects of the present disclosure are directed to providing and/or controlling electromagnetic radiation. As may be implemented in accordance with one or more embodiments, an apparatus includes a first structure that contains an object, and a second structure that is transparent at solar wavelengths and emissive in the atmospheric electromagnetic radiation transparency window. The second structure operates with the first structure to pass light into the first structure for illuminating the object, and to radiatively cool the object while preserving the object's color.