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

SYNOPSIS Sky radiation affects numerous systems but is often under estimated. This paper is especially oriented towards its determination in hot countries for cooling purposes. It includes expressions for clear weather conditions and shows some properties and the power of the flux according to the climates. Observations in Kuwait and in the Ivory Coast support the corrections established concerning hazed skies. Correction for cloudiness is treated theoretically. The limited interest of anti-convective selective coatings is finally pointed out to warn the reader against unfeasible performances.

Expressions for computing the radiative cooling flux from the sky in different climates

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.

C.N. Awanou

Papers

Clear sky radiation as a function of altitude

Expressions for computing the radiative cooling flux from the sky in different climates

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