Solar Energy and Radiative Balance

Radiation balance. Source: Wikimedia Commons

Solar energy is defined as a form of heat energy from the sun which reaches the earth's surface by radiative and conventional processes. The solar energy that reaches the earth's surface covers a broad range of electromagnetic spectrum. This spectrum includes short gamma rays, X-rays and radio waves etc. The quantity of radiant energy that is emitted and received vary greatly from time to time and with the direction of emissions or observations. Similarly, the flux density of the solar radiations received on a given surface depends on;

• The solar attitude.
• The intensity of the radiations.
• The turbulence of the air.
• The type and nature of the cloud.
• The nature of the receiving surfaces.
• The presence of the atmospheric gases.

Usually, the maximum flux density of the radiation does not exceed 1.23kw/m2 hence solar radiation is called short wave radiation. Radiant energy travelling in the form of electromagnetic waves has the dimensions of energy and is measured in joules, erg, calorie or kilowatt.

The solar energy travelling to the earth's surface is intercepted and absorbed by the layers of these gases like Ozone(O3),Carbon dioxide (CO2), Water vapour (H2O), Hydrogen sulphide (H2S), Carbon(C), Carbon monoxide (CO) etc. Some these gases are absorbed and reflected by both the clouds and the earth's surface. It has been observed that about 40% of solar energy or radiations was reflected back into space or the atmosphere. At the earth's surface, the radiant energy is converted into heat energy, thereby increasing the temperature of the air, the soil and the entire environment.

The radiant energy tends to increase the duration of sunlight hours of any given place. The rise in the environment temperature is as a result of the imbalances between the incident and reflected radiations. If the incoming radiations or incident radiations is known as (R1) and the outgoing or reflected radiations as (Ro), then R1=Ro.

It is noteworthy that;

• Whenever the incoming radiation is greater than the outgoing radiation, the environmental temperature tends to increase and more heat is generated.

• Whenever the incoming radiation is lower than the outgoing radiation, the environmental temperature will reduce and less heat will be produced. Consequently, the quantity of radiations reaching the earth's surface at any time depends on the the following factors:

1. The elevation of the sun at the place.
2. The zenith angle of the sun (Z) which is the angle of inclination of the sun with reference to the position of the place.
3. The latitude of the place.
4. The time of the year.
5. The time of the day.

The factors that influence the quantity of radiations received by a place at any given time are as follows.

1. Because the earth is spherical.
2. The sun is far from the earth.
3. The rays of the sun are parallel.
4. The earth is always rotating on its axis.

Locations and Climatic Impacts (A and B)

The location of a place on a world map determines the intensity and quantity of radiations it may receive from the sun. The quantity of the radiations will also play a role in the environmental temperatures as well as the physical development of the crops growing in the particular place. Here is a rough illustration of direction of rays at position A and B.

The environment at A would recieve maximum direct rays with higher radiative intensity. This is associated with an increase in heat production. The climatic influence at A will be:

• High rainfall events.
• High soil temperatures.
• Lower relative humidity.
• Higher evapotranspiration rate etc.

While the environment around B would receive partial rays transverse a greater distance before hitting the surface hence the solar energy or heat produced is low. The climatic influence at B will be:

• Moderate to low temperature.
• Low rainfall depths.
• High temperatures.
• Low evapotranspiration rate etc.

Radiative Balance

Radiative balance is the average energy reaching a horizontal surface from the upper limit of the atmosphere. It can be said to be energy per unit area which provides about 99.97% of heat energy required for the physical processes taking place on the earth's atmospheric systems. The sun emits different quantities of radiation every year through the incoming radiations and the receives a varying quantities of radiations every year. Radiative balance is therefore the relationship existing between the incoming and outgoing radiations which an environment receives within a period of time. A balance radiation can only be obtained when the quantity of radiation or heat received by the earth's surface is equal to the amount of radiation or heat emitted by the sun.

Radiative energy must be transferred meridionally from lower to higher latitudes. This implies that heat must be transferred from earth's surface (heat source) to the atmosphere (heat sink) and from the equator to the poles. This process will ensure that no part of the earth gets warmer or stays cold noticeably over a given period of time.

Types of Radiations

There are six types of radiations that exist between the sun and the earth radiative systems. They are :

• Global radiation .

• Atmospheric radiation.

• Terrestrial radiation .

• Oceanic radiation .

• Polar radiation .

• Equatorial radiation.

• Global radiation: This is the sum of radiation emitted by the sun and the sky over a stated period. An environment usually receive a given quantity of radiations during a given period.

• Atmospheric radiation : This is the sum of the radiation absorbed and reflected by the earth and those scattered by the particles in the air. Carbon (C) and Carbon dioxide (CO2) are some of the gasses that keep the earth's atmosphere warm by trapping the infra-red radiations of heat produced when solar radiation strike the earth surface.

• Terrestrial radiation : This is the quantity of radiations that reach the earth's surface from the sun. It is either absorbed or reflected back to the atmosphere.

• Oceanic radiation : This is quantity of radiations absorbed by the surface water over a given period. The unequal heating of the land (Terrestrial radiation) and sea (Oceanic radiation) tend to result to the popular natural phenomenon called land and sea breezes.

• Polar radiation : This is the quantity of radiant energy absorbed and reflected by those areas surrounding the North and South poles. Usually, the quantity of radiation received at the poles are lower than that received at the equator.

• Equatorial radiation : This is the quantity of radiations received by the equatorial regions of the world during a given period.

Instruments Used to Measure Radiation

• Pyreliometers: used to measure solar intensity
• Pyranometers: used for total short wave radiation from incident on a horizontal surface of the ground.
• Pyrgeometers: It is used for infra-red radiations.
• Pyranadiometers: Used for both infra-red and solar radiations measurement.
• Net radiometers: used to measure net radiation or radiation balance.
• Gunn bellenic radiation integration.
• Camp bell Stokes sunshine recorder.

Uses of Solar Energy

Solar Energy. Source : wikimedia commons

• It is used in green house effects.
• It provides the energy required for surface and plants water evaporation.
• It provides the energy required for the photosynthesis of plants.
• It is can be converted to heat energy for the heating of buildings and homes particularly at the polar regions.
• It can be converted into electrical energy for homes, farms and industries.
• It creates unequal heating of lands and seas which helps to provide land and sea breezes.

Reference

[1] Solar Energy, Wikipedia
[2] What is Solar Energy?
[3] Solar Energy -simple English, Wikipedia
[4] Tribal Energy and Environmental Information
[5] Radiative Balance
[6] Radiation and Energy Balance of the Earth System
[7] Energy Balance
[8] Outgoing longwave radiations, Wikipedia
[9] Shortwave radiation
[10] Pyrgeometer

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