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Solar Terms

Solar terms are the words used in any discussion about solar energy. Here we will take a look at some of these solar terms and how they apply to solar energy and solar power.

Solar Constant : The Solar Constant is the amount of incoming solar electromagnetic radiation reaching a square foot of material. This is assuming the material is perpendicular to the sun's rays, at the outer limits of the earth's atmosphere, for one hour. The Solar Constant would then be about 429.2 BTU's. Only about 35-40 percent of this solar radiation actually makes it to the earth. Much of the solar energy is reflected back into the space while some of it is absorbed in water vapors and atmospheric gases.

The point to remember is that the more atmosphere the solar radiation has to go through, the less of it reaches the earth. With this in mind it becomes easy to understand why the rotation of the earth, and it's seasonal tilt, play a big part in the field of solar energy. These two events set the limit of solar energy that can reach the earth at any location on any day of the year.

Energy Density : As we learned above, Solar Constant requires the surface to be perpendicular to the sun. Since this is not usually the case, the angles created by the sun's rays not being perpendicular are reflected in the Energy Density. The more angle, the less dense are the ray's at any one spot. This goes back to the strong effects the earth rotation has on solar power.

Energy density is a solar term used for the amount of energy stored in a given space per unit volume or per unit mass. The per unit mass version is better know as specific energy.

Angle of Incidence : The angle of the sun's rays are measured in a solar term know as the angle of incidence. This angle is created by incoming radiation and a line perpendicular to an intercepting surface. Put another way, the angle of incidence is a measure of deviation of something from straight on.

Radiation and Surfaces : Radiation is the process in which energy is emitted by one body and travels through a medium, or through space, to ultimately be absorbed by another body. Another important consideration in the field of solar power is the type of surface the sun is hitting. The sun striking a surface is either reflected, transmitted or absorbed, in any combination. The way the ray's are affected depend on the texture, color and clarity of the surface. A rough surface will general reflect sunlight in a scattered fashion. A smooth surface, however, will uniformly reflect the rays at an angle equal to the angle of incidence.

Color plays a role also in affecting which of the solar ray's are absorbed and which are reflected. A White glossy surface will reflect all the wavelengths in the visible spectrum equally and opposite to the angle of incidence. A red surface will reflect wavelengths in the red band and absorb the rest. A black surface will absorb all the visible wavelengths and a transparent surface will allow nearly all the radiation to pass through. It should be noted, however, that very few materials are excellent absorber, transmitters or reflectors of the sun's radiation.

Heat Absorption : Sunlight transforms from solar energy to heat energy when impacting a material by absorption. The color of the material plays a role in the temperature of the heat energy. Black surfaces absorbs more heat and is therefore hotter than a white surface under the same radiation. Heat is a result of molecular movement and is measured in temperature.

Conduction : As a material absorbs heat, and molecular movement continues to accelerate, the heat energy is redistributed through the material due to the natural phenomenon of maintaining equilibrium. As the molecules are stimulated, and vibrating faster, they bump into molecules vibrating at a slower rate. This action causes the heat to be conducted away from the heat source as the material seeks to distribute this energy equally across it's surface. The density of the material determines the rate at which the energy flows, or is conducted. Metals are fairly good conductors while gases are of poor quality.

Heat Transfer : When heat is passed from a solid material to a fluid medium it is called a heat transfer. The bumping molecules in the solid material bump the transfer fluid molecules creating heat. When the fluid is moving across the solid surface the molecules are heated and then move away from the heat source, being replaced by new, unheated molecules. This process of fluid movement is called natural convection. This means the fluid is not being moved by machinery. Forced convection is when this process is helped by use of a pump or a fan.

Radiant energy : This is the energy of electromagnetic waves. It has to do with the amount of infrared radiation emitted from a material at low temperatures. The energy emitted is determined by the temperature of the material and the nature of the surface. Radiant energy can be generated electrically by infrared lamps, or can be absorbed from sunlight. This heat energy can then be emitted by warm elements such as floors, walls or overhead panels.

Specific Heat : Sometimes called specific heat capacity, it is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a unit of temperature. The symbol for specific heat is c. More heat energy is required to increase the temperature of a material with high specific heat capacity than one with low specific heat capacity.