Direct Solar Radiation

The sun's radiation is at wavelengths varying from ultra-violet through visible to infra-red. In outer space before it hits the earth's atmosphere its intensity varies around 1400 W/m2, depending on the earth's distance from the sun. At noon on a clear day at ground level, direct radiation is about 900 W/m2 on a surface perpendicular to its rays. Direct radiation is directional.

Diffuse Solar Radiation

In passing through the atmosphere, some of the sun's rays are scattered, and absorbed by dust, gas molecules, ozone and water vapour.  The portion of the solar radiation which is scattered (up to 25% of the direct radiation) is called diffuse radiation and is non-directional. It can reach 150 W/m² on a horizontal surface, and should not be ignored in climatic design; a significant proportion of unwanted summer heat gains can come from windows even those shaded from direct solar radiation.

Reflected Solar Radiation

A substantial amount of solar radiation can be reflected from sunlit surfaces such as roofs and walls, paved areas or bodies of water. This can cause problems, but may also be utilised in climatic design. Well-designed landscaping and planting can reduce the effect of diffuse / reflected radiation on unwanted solar gains in Summer, and can help to warm interiors in Winter. Reflective surfaces can be incorporated in buildings themselves, and reflective panels adjusted appropriately in front of the collectors of solar hot water heaters can increase the input radiation to the collectors and thus the efficiency and effectiveness of the system.

Night Sky Radiation

All matter exchanges heat by Radiation with its surroundings due to its heat content. (Even the coldest object on earth contains heat; a total absence of heat would mean a temperature of Absolute Zero. Heat transfer between two bodies or surfaces is proportional to (t1 - t2)4, the fourth power of their temperature difference.

But the temperature of outer space is only just above Absolute Zero Kelvin (^oK). Therefore surfaces facing the clear night sky, only minimally protected by the earth's atmosphere from direct exposure to the temperature of outer space, lose heat extremely rapidly, and their temperature drops well below ambient air temperature.

This is the explanation for frosts, which occur on horizontal and near-horizontal external surfaces even though the air temperature does not drop below zero, and extreme winter discomfort in houses with uninsulated roofs, which face the night sky and become very cold, absorbing heat from the rooms - and people - below. This is particularly marked at high altitudes, where the air is less dense than at sea level, and thus a less effective insulator. This is why when camping we prefer to sleep in a tent.

But those frosty cloudless winter nights, with high levels of radiant heat loss from building roofs to the night sky, have advantages - during the day those same clear skies allow the sun to heat Passive Solar buildings. On the other hand cloudy nights, the cloud cover serves as a 'blanket' to retain the earth's heat, some recompense for the cloud cover which blocked the sun's rays during the day. Poorly designed houses, with badly oriented windows and inadequate or no Insulation, draw little benefit from clear skies during the day.

Therefore solar hot water heaters should be fitted with a valve to stop reverse flow at night; otherwise the water will circulate back from the tank to the collector, radiating heat to the night sky, and draining heat from the water in the tank. Installing a hinged insulating panel to cover the collector at night will conserve even more energy, and give warm water in the morning.

'Black Body' Radiation

A 'Black Body' is a theoretical body with properties such that it will absorb all radiation falling on it, reflecting and transmitting none. However it does emit radiation, at a rate depending on its absolute temperature: -

q = sAT4

where:-

q = heat

A = surface area

s = Stefan-Boltzmann constant

T = temperature (^oK)

Absorptivity, emissivity and reflectivity of real materials can be defined in terms of the proportion of the respective properties of a "black body" at the same temperature (defined as 1.00). They vary with the wavelength of the incident radiation. For a given wavelength, emissivity (e) + reflectivity (r) = 1.

Emissivity: the surfaces of most building materials such as wood, bricks, tiles and unpainted concrete have an emissivity of 0.90.  Polished aluminium has an emissivity of 0.05 at temperatures usual at the surfaces of building materials.

Reflectivity: the surfaces of most building materials have a reflectivity of about 0.10 - 0.40 depending on wavelength, while white painted surfaces may have a reflectivity of about 0.75 at the wavelengths of solar radiation, and aluminium foil has the high reflectivity of 0.95 at all wavelengths (ie it reflects 95% of the radiation falling on it).  So: - 

Dark paint is a poor reflector at all wavelengths;

White paint is a good reflector of solar radiation but a poor reflector at ambient temperatures, therefore painting the outside surface of the external walls of an uninsulated house white will improve summer performance;

Polished aluminium is a good reflector at all wavelengths, so it is used for the reflective surface in reflective insulation.