SOLAR ENERGY

On this page the topic of solar energy will be discussed. There will be links provided to other websites because there is not enough room here for a full discussion of the topic. We will provide nominal information regarding availability, collection and storage of solar energy because those are the variables or factors that influence the applications and flexibility of this resource.

AVAILABILITY

Generally, the amount of energy coming from the sun is related to one constant and several variables. The constant is the amount of energy that the sun itself emanates. The variables are time of day, time of year or seasonal change, location north or south of equator, and perspective. The temperature at the sun’s surface is said to be 10,340F due to the distance the rays travel to the earth’s surface and that they spread out as the travel here the amount of energy that reaches the earth’s outermost atmosphere is 1367 W/m² as the rays travel through our atmosphere some of that energy is dissipated to the water and elements there. The net energy that reaches the earth’s surface is said to be 1000 W/m² at the equator at noon on a cloudless day during the summer. The previous condition is regarded by most as “perfect” which means that the collector surface-the earth is perpendicular to the sun’s ray and gets direct sunlight. As the earth turns, relative to the location of a specific spot or collector if the collector does not change its orientation the amount of solar radiation that strikes the collector gets less because the angular intensity of the rays gets less. For example, at night there is no direct radiation on a spot or collector. At sunrise, the angle of the sun’s rays relative to noon is largest and therefore the sun’s rays are of least quality. As the earth rotates on its axis this increases until noon and then decreases until sunset. At which point the sun disappears for a couple of hours and then reappears. In addition to its rotation on its axis or spinning which gives us day and night, the earth also travels around the sun in a path that takes 365 days. The seasonal changes which we experience are a result of this path where the sun’s rays emanate from a central point to a moving sphere that has a tilted axis as the sphere moves the axis in reality does not change but with relation to the central point the axis gets closer and then further from the central point or the sun. This change of the earth relative to the sun gives us our seasons and therefore locations around the equator get nearly constant solar radiation all year round while the north and south hemispheres get alternate and reverse seasons year after year. In line with this is another variable---location. The further north or south of the equator since the angle of solar radiation changes, its quality decreases which is another factor affecting the amount of solar energy. Another variable is weather, mostly cloud cover, which prevents the sun’s ray from directly hitting the earths surface. This is a variable that has many influences. Some being the rotation of the earth around the sun—seasonal variations, some being environmental—man caused.

COLLECTION

There are two types of solar collection devices—photovoltaics and thermal. Photovoltaic collectors use some sort of semiconductor to convert light into an electric current. These devices prefer direct solar radiation but can use diffuse solar radiation to generate to a lesser degree. They are fairly expensive and are 15-20% efficient.

Thermal collectors are of two basic types: high and low temperature. The low temperature devices are usually of flat plate design. Basically, a flat box painted black with copper piping snaked in to it with a flow of water through it. The systems are usually used for domestic hot water systems. The high temperature systems use some sort of concentrating device to focus the sun’s rays on to a focal point pipe which is configured in some way to heat water or a thermal fluid or oil to high temperatures then the fluid is pumped to a central heat exchanger immersed in water. Most concentrating devices are approximately 60% efficient. The hot fluid gives its heat to the water creating steam as in a normal power generating system. Other devices use mirrors to concentrate the sun onto a Stirling engine. These systems are limited by the same inefficiencies that have been discussed in the power generation section, which means there are additional losses to be considered.

ENERGY STORAGE

Due to the limited availability of solar energy discussed above and our increasing demand for power 24/7 there is a need for storage of solar energy.

Photovoltaic collectors have in the past used batteries to store electricity for times when solar energy is unavailable either due to weather or our schedules. The systems are very expensive and complicated because you need at least 3times the area as you need power and of course there is inefficiency of the collectors to be considered.

Thermal storage—there have been attempts to do this but most have failed because these systems are expensive and they cannot hold the amount of heat needed---the lose heat over time.