Radiative sky cooling cools an object on the earth by emitting thermal infrared radiation to the cold universe through the atmospheric window (8–13 μm). It consumes no electricity and has great potential to be explored for cooling of buildings, vehicles, solar cells, and even thermal power plants. Radiative sky cooling has been explored in the past few decades but limited to nighttime use only. Very recently, owing to the progress in nanophotonics and metamaterials, daytime radiative sky cooling to achieve subambient temperatures under direct sunlight has been experimentally demonstrated. More excitingly, the manufacturing of the daytime radiative sky cooling material by the roll-to-roll process makes large-scale deployment of the technology possible. This work reviews the fundamental principles of radiative sky cooling as well as the recent advances, from both materials and systems point of view. Potential applications in different scenarios are reviewed with special attention to technology viability and benefits. As the energy situation and environmental issues become more and more severe in the 21st century, radiative sky cooling can be explored for energy saving in buildings and vehicles, mitigating the urban heat island effect, resolving water and environmental issues, achieving more efficient power generation, and even fighting against the global warming problem.

Radiative sky cooling: Fundamental principles, materials, and applications

Over the last 20 years, dew harvesting has evolved to fruition because of a better understanding of its physics, thermodynamics, and the radiative cooling process of condensing substrates. Although resultant yields are relatively small, dew positions itself as a viable water resources supplement because it occurs naturally and frequently in many locations globally, particularly in the absence of precipitation or when more traditional water sources are subject to depletion. Moreover, dew water is generally potable, especially in rural locations, where it is most beneficial. This review summarizes dew harvesting research achievements to date including formation processes, collection in various environments, prediction models, water quality, and applications. The paper concludes with outlining existing gaps and future research needs to improve the understanding and performance of dew harvesting in the context of adaptation to climate change.

Dew as a sustainable non-conventional water resource: a critical review

There has been a drastic increase in the use of air conditioning system for cooling the buildings all around the world. The last two decade has witnessed a severe energy crisis in developing countries especially during summer season primarily due to cooling load requirements of buildings. Increasing consumption of energy has led to environmental pollution resulting in global warming and ozone layer depletion. Passive cooling systems use non-mechanical methods to maintain a comfortable indoor temperature and are a key factor in mitigating the impact of buildings on the environment. Passive cooling techniques can reduce the peak cooling load in buildings, thus reducing the size of the air conditioning equipment and the period for which it is generally required. This paper reviews and critically analyzes various passive cooling techniques and their role in providing thermal comfort and its significance in energy conservation.

An Overview of Passive Cooling Techniques in Buildings: Design Concepts and Architectural Interventions

Estimating dew yield worldwide from a few meteo data

Investigations are reported on infrared (IR) signature studies from internal sources of aircraft and helicopters, e.g. engine heated surfaces and exhaust plume. Due to the increasing sensitivities of IR sensors, irradiances from external sources are investigated here, namely sunshine, skyshine, and earthshine. Their role in determining the IR contrast in the 1.9–2.9, 3–5, and 8–12 µm bands is studied. Estimation of earthshine is emphasized because of its tactical significance in low altitude missions and IR imaging studies. A comparison of IR radiance from aircraft's rear fuselage due to internal and external sources shows different roles in the 3–5 and 8–12 µm bands. The important roles of earthshine and temperature-based radiance of aircraft surfaces are identified in the 8–12 µm band.

https://iopscience.iop.org/article/10.1088/1464-4258/11/4/045703/pdf

Study of sunshine, skyshine, and earthshine for aircraft infrared detection

Clear sky radiation as a function of altitude

Human thermal balance is the result of metabolism and heat and mass exchange. In hot dry climates, the efficient parameters for comfort recovering are evaporation, low radiant temperature and low activity. In such climates the sky radiative temperature may fall 30°C below the air temperature. However the dew point cannot be reached by natural cooling and water is too rare to be consumed for evaporation systems. The diode roof is a heat pipe system constituded of a pebble roof wetted with water and protected with white TiO painted polyethylene bags. This cheap system is efficient to favorize heat losses and to refuse solar gains. The experiments have demonstrated the possibility of a passive cooling 4°C below the outside minimum air temperature.

Thermal comfort in hot dry countries : radiative cooling by “diode” roof

Living in a habitat with comfort is requested by all. Cinder block bricks have poor thermal properties, leading people to use fan heaters and air condition-ers to regain comfort. To overcome this problem of thermal discomfort in buildings, we used lightweight concrete such as foamed concrete which is a material that has improved thermal properties for thermal comfort. In addition, this material was compared with local materials used for the construction of buildings such as BTC, adobe and BLT mixed with binders. The results showed that foamed concrete is a material that has good thermal and mechanical properties compared to local materials mixed with binders. The foamed concrete having acceptable thermo-mechanical properties was that having a density of 930 kg/m 3 . It has a thermal resistance of 0.4 m 2 ∙K/W for a thickness of 20 cm. The foamed concrete having acceptable thermo-mechanical properties was that having a density of 930 kg/m 3 . It has a thermal resistance of 0.4 m 2 ∙K/W for a thickness of 20 cm. For sunshine on a daily cycle equal to 12 hours, the characteristic thickness achieved by this material is 7.29 cm. It also has a shallow depth of heat diffusion having a lower thickness than other materials. This shows that foamed concrete is a promising material for the construction of buildings.

/pdf/comparative-study-of-the-thermal-and-mechanical-properties-6ybysmcr.pdf

Comparative Study of the Thermal and Mechanical Properties of Foamed Concrete with Local Materials

Aims: The energy domain is responsible for the production of many gaseous, liquid and solid pollutants, strongly contributing to climate change. The provision of bioclimatic habitats to households is therefore necessary since it will contribute to the reduction of energy consumption. This work consists in making a study of the thermal behavior of a habitat designed with foamed concrete (FC) in order to evaluate its thermal performance for the improvement of its thermal comfort and to compare its average internal temperature with those of other materials. 
Study Design: A calculation model developed under the COMSOL Multiphysics 5.3a software was used to simulate the thermal behavior of a foamed concrete habitat. The meteorological data used are those of Ouagadougou in the month of April (hottest month in Burkina Faso) on April 15, 2019 (with April 15, the hottest day of the year 2019) 
Methodology: The temperatures of each side of the walls were determined. Moreover, the study of the influence of the thickness of the walls on the average internal temperature made it possible to determine an optimal thickness. Then, the thermal phase shift, the thermal amplitude reduction and the damping factor were carried out. 
Results: The results obtained in the weather conditions of April in Ouagadougou, lead to an average internal temperature of the building of about 304 K, for a wall thickness of 17.5 cm. There is a thermal phase shift of 8 hours, and a reduction in thermal amplitude of 9°C or a damping factor of 8.6%. The maximum average internal temperature of the foamed concrete was compared with those of the cement block, the CEB, the adobe, the CLB which present respectively 311 K; 309.2K; 309K; 308.5K. 
Conclusion: A building constructed with foamed concrete has a low average internal temperature compared to cinderblock, CEB, adobe and CLB. Thus, this material makes it possible to provide more thermal comfort and can be used for the construction of habitats with good energy efficiency.

F. Kieno

Papers

Clear sky radiation as a function of altitude

Thermal comfort in hot dry countries : radiative cooling by “diode” roof

Comparative Study of the Thermal and Mechanical Properties of Foamed Concrete with Local Materials

Evolution of the Average Temperature of the Interior Atmosphere of a Habitable Cell Made of Foamed Concrete in Burkina Faso