Stanford Continuous Absorption Type of Cooling Unit
Passive Radiative Cooling
Amir Kader
November 3, 2021
Submitted as coursework for PH240, Stanford University, Fall 2021
Introduction
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| Fig. 1: Building with air conditioning units and a high need for cooling. (Source: Wikimedia Commons). |
Air conditioning has transformed the way that humans integrate themselves into their environments, as shown in Fig. 1. It has made places with torrid climates more inhabitable and improves human efficiency. Electricity use for cooling by all sectors globally in 2016 was about 2000 Terawatt hours (7.2 × 1018 J), which constituted about 10% of total electricity consumption globally that year. [1] With growing efforts worldwide to mitigate climate change, reducing energy consumption for cooling and refrigeration systems has the potential to drastically reduce emissions. Radiative cooling systems that consist of thin film materials adhered to rooftop panels are capable of reflecting sunlight and emitting infrared heat which keeps the panels and the refrigerant fluid flowing under them cool. With zero electricity input, these panels are capable of keeping the refrigerant in a cooling system 3-5°C below ambient temperature, which can improve cooling efficiency by up to 50% when the panels are integrated as a simple add-on in ideal conditions. [2]
How Radiative Cooling Works
All objects emit heat in the form of infrared radiation, a form of electromagnetic radiation that encompasses invisible light. One major challenge of utilizing this form of radiation in useful ways comes from the fact that heat from the sun offsets any of its potential cooling effects. At SkyCool Systems, a company founded based off of research done at Stanford, scientists developed a material capable of radiating infrared light in the range that escapes the atmosphere as well as reflects 97% of sunlight. [3] At 3°C above absolute zero, outer space serves as an excellent heat sink and SkyCools radiative material can be used to emit heat during the day and night, even when it is directly under sunlight. [4] The optical properties that a material would need to overcome the sun's heat include that it must (i) reflect the solar spectrum in wavelengths from 200 nanometers to 2.5 micrometers even more effectively than white paint, which is already up to 94% reflective, and (ii) absorb and emit as close as possible to 100% of the wavelengths in the crucial 8-13 micrometer range, of which earth's atmosphere is transparent to. [5] The wavelengths to which the atmosphere is transparent are shown in Fig. 2. Research and testing performed over 3 days suggests that enough heat can be emitted from the material to consistently stay 3°C-5°C cooler than ambient temperature, which translates to about 40.1 watts per square meter. [3] Computer models based on collected data demonstrate that in hot and dry climates, the panels can stay up to 10°C cooler than the surrounding air. [5] Pipes carrying water or any other refrigerant are embedded under the panels and the temperature difference between the two materials cools the water without any electricity input, reducing the load on the rest of the refrigeration system. Manufacturing the panels is possible with conventional manufacturing methods and results in a sheet that has a total thickness of less than 2 microns. The sheet is composed of 7 different layers of either silicon dioxide or hafnium dioxide at specified thicknesses and is backed by a layer of silver which acts as a mirror. The arrangement and thicknesses of the layers were optimized on a computer to manipulate the energy levels of the incoming light in order to preferentially reflect or emit certain wavelengths. [4]
Benefits of Passive Cooling Technology
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| Fig. 1: Radiative cooling materials must be extremely reflective (even more than white paint). They also absorb wavelengths between 8 and 13 micrometers, and emit them into space. (Source: A. Kader.) |
Cooling systems utilized 10% of all electricity generated and produced 12% of all greenhouse gas emissions globally in 2016. According to the International Energy Agency, 1130 million tonnes (1.12 × 1012 kg) of CO2 were emitted in 2016 from space cooling. [1] With the increasing use of air conditioning in developing nations, rise in electric vehicle usage, and proliferation of data centers, the demand for air conditioning is expected to triple by 2050. [6] The implementation of passive cooling technologies could result in significant energy savings for applications such as air conditioning, refrigeration, data center cooling, cooling towers, economizers, and electric vehicles. Assuming that all cooling applications adopt passive cooling technologies by 2050 and that the systems reduce energy use by a conservative estimate of 10%, passive cooling could reduce CO2 emissions in 2050 by 1 gigaton (1012 kg). A company called Radi-Cool is aiming to commercialize a radiative cooling, glass-embedded plastic and estimates that integrating their material into commercial buildings with water chillers could reduce electricity consumption for cooling in the summer by 32-45% in cities like Phoenix, Miami, and Houston. [5] The researchers from SkyCool Systems simulated the addition of their panels to a two-story commercial building in Las Vegas that uses a vapor-compression refrigeration system and found that in the summer, the panels could reduce electricity use by an average of 21%. [5]
One main advantage of these radiative cooling panels is that they can be retrofitted to fit into a buildings already existing refrigeration system. Unlike other electricity generation and conversion technologies, which all produce waste heat, passive cooling materials are one of the few technologies that is capable of dumping waste heat back into space. According to SkyCool Systems website, SkyCools panels save 2x - 3x as much energy as solar panels of the same area produce, weigh nearly half the weight, can operate when there is no sunlight, and do not need to face south so they can complement solar panels. To be more specific, SkyCool Systems website claims that a SkyCool panel saves 500 to 600 kWh m-2 y-1 while a solar panel of the same size generates 250-300 kWh m-2 y-1. Developing nations with less reliable electricity grids would also benefit from passive cooling and its energy savings. Depending on the conditions of the climate, passive cooling technology could cut down one of the largest sources of greenhouse-gas emissions by reducing electricity used for cooling by 10-70%. [3]
Barriers to Adoption of Passive Cooling Technology
The idea of passive radiative cooling has existed for centuries. Long before refrigeration existed, In India and Iran where temperatures are often above freezing, ice was made by filling insulated, ceramic pools with water and leaving them outside on clear nights. [6] Implementing the discovery of radiative cooling into a transformational cooling technology has been a major challenge though due to manufacturing, cost, and weather conditions. Theoretical estimates of how much energy passive cooling systems can save have been based on models in dry climates with clear skies, which is where the technology works best. In cloudy and humid environments, water vapor traps the infrared radiation and diminishes the effect that the cooling films produce. Precipitable water vapor (PWV) represents the depth of water that would result if all the vapor in a column of the atmosphere above a certain location were condensed into rain. A locations PWV is an important indicator of humidity, and most successful demonstrations of radiative cooling have been performed in dry areas of North America where the PWV is about 1-10 mm. In such dry environments, the atmosphere is transparent to light with wavelengths 8-13 micrometers and also 16-25 micrometers, which makes efficient thermal radiation possible. In comparison, warm regions in Asia like Japan usually experience a high PWV of more than 20 mm, in which the atmospheric window from 16-25 micrometers almost completely closes. [7]
Furthermore, in urban settings, buildings, people, and other obstacles can block the panels from emitting the infrared radiation to outer space. During the winter when heating is required, passive cooling technologies might even increase heating costs. Despite these constraints, regions with high demands for air conditioning like the southwestern United States or certain areas in the Middle East have arid climates suitable for passive cooling technology. There are also additives that can be added to the panels that would passively eliminate their cooling effects in the winter. [5] Panels can also be positioned strategically or additional reflective materials can be integrated into systems so that the panels always emit radiation to outer space, even in urban environments. The largest obstacle to making the technology widespread is undoubtedly the high manufacturing cost and large-scale production limits. The hafnium dioxide which composes 3 of the panels 7 layers is expensive and will need to be replaced by a cheaper material like titanium dioxide. The reflective layer of silver is also an expensive component of the cooling material which makes the technology uneconomical today. [4]
Conclusion
Rising temperatures, increases in air conditioning use in developing nations, the expansion of data centers, and rising demands in transportation will only increase the demand for cooling and refrigeration. The irony is that an increase in cooling will lead to more emissions and o nly increase temperatures globally. In 2016, 2000 terawatt hours were used worldwide for space cooling, and the International Energy Agency expects this number to triple by 2050. [8] Air conditioning is becoming more popular in regions like India, Indonesia, and the Middle East. In India, 8% of homes are currently equipped with air conditioning and coverage is expected to rise to 50% by 2050. In some parts of the southwestern United States and Middle East, cooling accounts for 70% of electricity use on hot days. [8] Although passive cooling materials have not yet been fully realized due to high manufacturing costs and limits as to which types of environments they can be utilized in, they carry the potential to improve cooling efficiency and reduce greenhouse gas emissions, both of which must be accomplished for the sustainability of humanity and the planet.
© Amir Kader. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
References
[1] "The Future of Cooling," International Energy Agency, 2018.
[2] E. A. Goldstein. A. P. Raman., and S. Fan, "Sub-Ambient Non-Evaporative Fluid Cooling with the Sky," Nat. Energy 2, 17143 (2017).
[3] J. Temple, "A Material That Throws Heat Into Space Could Soon Reinvent Air-Conditioning," MIT Technology Review, 12 Sep 17.
[4] "A Cool Idea," The Economist, 27 Nov 14.
[5] X. Z. Lim, "The Super-Cool Materials That Send Heat to Space," Nature 577, 18 (2020).
[6] S. Kaplan, "Bringing the Chill of the Cosmos to a Warming Planet," The Washington Post, 7 Oct 20.
[7] T. Suichi et al., "Performance Limit of Daytime Radiative Cooling in Warm Humid Environment," AIP Adv. 8, 055124 (2018).
[8] C. Ospina, "Cooling Your Home but Warming The Planet: How We Can Stop Air Conditioning from Worsening Climate Change," Climate Institute, 7 Aug 18.
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Source: http://large.stanford.edu/courses/2021/ph240/kader1/