Solar Radiation Basics

Solar radiation is a general term for the electromagnetic radiation emitted
by the sun. We can capture and convert solar radiation into useful forms of
energy, such as heat and electricity, using a variety of technologies. The
technical feasibility and economical operation of these technologies at a
specific location depends on the available solar radiation or solar
resource.

Basic Principles

Every location on Earth receives sunlight at least part of the year. The
amount of solar radiation that reaches any one "spot" on the Earth's surface
varies according to these factors:

• Geographic location
  
• Time of day
  
• Season
  
• Local landscape
  
• Local weather.

Because the Earth is round, the sun strikes the surface at different angles
ranging from 0º (just above the horizon) to 90º (directly overhead). When the
sun's rays are vertical, the Earth's surface gets all the energy possible. The
more slanted the sun's rays are, the longer they travel through the atmosphere,
becoming more scattered and diffuse. Because the Earth is round, the frigid
polar regions never get a high sun, and because of the tilted axis of rotation,
these areas receive no sun at all during part of the year.

The Earth revolves around the sun in an elliptical orbit and is closer to the
sun during part of the year. When the sun is nearer the Earth, the Earth's
surface receives a little more solar energy. The Earth is nearer the sun when
it's summer in the southern hemisphere and winter in the northern hemisphere.
However the presence of vast oceans moderates the hotter summers and colder
winters one would expect to see in the southern hemisphere as a result of this
difference.

The 23.5º tilt in the Earth's axis of rotation is a more significant factor
in determining the amount of sunlight striking the Earth at a particular
location. Tilting results in longer days in the northern hemisphere from the
spring (vernal) equinox to the fall (autumnal) equinox and longer days in the
southern hemisphere during the other six months. Days and nights are both
exactly 12 hours long on the equinoxes, which occur each year on or around March
23 and September 22.

Countries like the United States, which lie in the middle latitudes, receive
more solar energy in the summer not only because days are longer, but also
because the sun is nearly overhead. The sun's rays are far more slanted during
the shorter days of the winter months. Cities like Denver, Colorado, (near 40º
latitude) receive nearly three times more solar energy in June than they do in
December.

The rotation of the Earth is responsible for hourly variations in sunlight.
In the early morning and late afternoon, the sun is low in the sky. Its rays
travel further through the atmosphere than at noon when the sun is at its
highest point. On a clear day, the greatest amount of solar energy reaches a
solar collector around solar noon.

Diffuse and Direct Solar Radiation

As sunlight passes through the atmosphere, some of it is absorbed, scattered,
and reflected by the following:

• Air molecules
  
• Water vapor
  
• Clouds
  
• Dust
  
• Pollutants
  
• Forest fires
  
• Volcanoes.

This is called diffuse solar radiation. The solar radiation that reaches the
Earth's surface without being diffused is called direct beam solar radiation.
The sum of the diffuse and direct solar radiation is called global solar
radiation. Atmospheric conditions can reduce direct beam radiation by 10% on
clear, dry days and by 100% during thick, cloudy days.

Measurement

Scientists measure the amount of sunlight falling on specific locations at
different times of the year. They then estimate the amount of sunlight falling
on regions at the same latitude with similar climates. Measurements of solar
energy are typically expressed as total radiation on a horizontal surface, or as
total radiation on a surface tracking the sun.

RADIANT ENERGY FROM THE SUN

CONSTANT OUTPUT OF ENERGY

CONSTANT INPUT OF ENERGY

NOT ALWAYS CONSTANT AT THE SURFACE

Document(s) 12 of 15

Introduction

Drawing inspiration from water treatment practices developed in India as long
ago as 2000 B.C., researchers at the American University of Beirut in Lebanon
developed, with IDRC support, a
low-cost, practical means to provide safe drinking water in rural and urban
areas in developing countries. The key to this method lies in the ability of
direct sunlight to destroy bacteria. The treated water is suitable for drinking,
and can also be used to prepare Oral Re-hydration Therapy (ORT) solutions to
treat dehydration suffered by children with diarrhea. Solar radiation is a form
of renewable energy that is abundant and accessible in most southern countries.
The research focused on small quantities of drinking water, sufficient to
satisfy the daily requirements of a small family. 

Sunlight with wavelengths of 315-400 nanometers (nm) on the ultraviolet (UV)
range of the electromagnetic spectrum is most effective at destroying bacteria.
Since colourless glass or plastic can transmit light in this near ultraviolet
range, they are the best materials for disinfection. Visible light (400-750 nm)
next in terms of efficiency, with the visible band of violet and blue light
(400-490 nm) is the most useful within this range. As a result, violet, blue,
and very light green-tinted glass follow colourless glass or plastic in order of
suitability. 

More recent research carried out at Montreal's Brace Research Institute
(BRI), in collaboration with international colleagues, indicates that
transparent plastic bags are the best material for solar water disinfection.
Researchers report that six litres of St. Lawrence River water can be
disinfected in five hours during the summer. Research attention in this regard
has been focused on finding or developing plastics capable of transmitting
sunlight wavelengths below 400 nm. 

Impact

• 
  

  Development of a simple, effective and economical method of water
  disinfection using solar radiation - Earlier work by researchers in Lebanon
  on developing suitable methods to disinfect water in developing countries is
  being carried on at the Brace Research Institute. Identifying simple,
  inexpensive plastic bags for use in disinfection has become a suitable method to
  use in southern contexts. BRI researchers actively promote their findings
  through a variety of networks, including the Global
  Applied Research Network (GARNET).

Prerequisites