Passive Daytime Radiative Cooling.

The most effective form of renewable, low-carbon energy is energy not used at all. Passive daytime radiative cooling (PDRC) is a method proposed to ameliorate global heating, by enhancing the radiation of heat to outer space using thermally-emissive surfaces placed around the Earth. There is no energy consumed in running this technology, and hence no associated greenhouse gas emissions. Since all natural materials absorb more heat during the day than at night, PDRC surfaces are designed with a high solar reflectance (to minimize heat gain) and strong thermal (heat) radiation transfer through the atmosphere's infrared window (in the region, 8–13 µm), so that temperatures are reduced during the daytime. PDRC offers the advantage over solar radiation management that it increases the emission of radiative heat, rather than merely reflecting solar radiation back into space before it is absorbed by the environment and heat thus generated from it.

It has been estimated that if PDRC were installed over 1–2% of the Earth's surface area, a brake would be applied to relentless global heating, and temperature increases reined in to survivable levels. The cooling potentials are greater for desert and temperate regions than for tropical climates, since both humidity and cloud cover inhibit the efficiency of the devices. Cheap materials have been developed for PDRC that can be mass produced, including coatings, thin films, aerogels, and metafabrics, to reduce the need for air conditioning, attenuate the urban heat island effect, and cool human bodies in conditions of extreme temperature.

Scientists at MIT have invented a PDRC device that can cool things down by more than 13 degrees Celsius, and significantly below the ambient air temperature, in full sunlight on a cloudless day. The critical component for this device is a polyethylene foam insulating material called an aerogel. The foam is extremely lightweight (just 1/50th of the density of water), looks and feels somewhat like a marshmallow, and both blocks and reflects the visible rays of sunlight, thus preventing them from passing through it. It is also transparent to the infrared radiation wavelengths that transport heat, so they can escape and be radiated out and away.

Hot objects cool down as a result of radiative heat loss, emitting midrange infrared radiation. Since air is effectively transparent to these wavelengths, the heat energy is lost into space.

The basic concept was demonstrated a year ago, using a narrow strip of metal, as a physical barrier to shade the device from direct sunlight, and prevent it from heating up. However, its cooling power was less than one half that the new system, with its highly efficient insulating layer, without which the heat from the surrounding air raises the temperature of the device.

The use of air conditioners and electric fans already accounts for about 20% of the total electricity consumed in buildings around the world, which amounts to around 10% of current total global electricity consumption. It is expected that demand for air conditioning will increase from the current 1.6 billion to 5.6 billion AC units by 2050, becoming one of the top drivers of global electricity demand, despite negative consequences in terms of increased energy use, costs, and global warming, described as a "vicious cycle.”

This situation may nonetheless be mitigated, since PDRCs are most often applied to building envelopes, which can significantly lower the temperatures within. When a  reflective white roof was combined with a PDRC, a doubling of the energy saved for cooling could be obtained. Elsewhere, it is quoted that PDRC coatings directly covering a roof reflect a large proportion of solar radiation and achieve a lower roof temperature, which can reduce cooling loads by 18%–93%. By covering 10% of a building's roof with a multilayer PDRC surface, some 35% of air conditioning used during the hottest hours of the day can be avoided. Hence, PDRCs can act to replace, or reduce the energy demand of, air conditioning, and also help to ease the pressure on energy grids during periods of peak demand.

It has been reported that, in suburban residential areas in the United States, PDRCs can result in a 26%–46% reduction in energy use and an average lowering of temperatures by 5.1 °C.

With the addition of "cold storage to utilize the excess cooling energy of water generated during off-peak hours, the cooling effects for indoor air during the peak-cooling-load times can be significantly enhanced" and air temperatures may be reduced by 6.6–12.7 °C. 

As global temperatures increase, such PDRC cooling devices may find widespread applications, with the advantage that they use no energy, incur no greenhouse gas emissions, and hence do not add to the burden of global heating, unlike conventional refrigeration and air conditioning systems which need electricity to run them.

Integrated, hybrid systems, that combine thermal insulation, evaporative cooling and radiative cooling, can also be used to perhaps double the time that fruit and vegetables can be kept fresh, and in remote regions where refrigeration is not viable due a lack of a reliable electricity supply.

However, there are a number of challenges attendant to a wide scale commercialisation of PDRC, that must be considered: for example, the cost and availability of the materials employed to fabricate particular devices, along with their durability (lifetime) and performance under prevailing environmental conditions, which will vary appreciably according to location.