**2. The sun, origin of solar energy**

4 Solar Radiation

barrels per day. This would represent about 38.5% of demand. This indicates the

As the world population increases and the economic standard of third world countries improves, there is an expectation of an unprecedented rise in the global energy demands. To allow the traditional energy sources, that is, fossil, nuclear, or hydro fuel to meet these increasing energy demands now and for too long in the future will be unwise and suicidal.

 There is a strong international consensus on the threat of dangerous climate change due to pollutants emitted from fossil fuels powered engines. This threat is heightened by the rapidly increasing demand for fossil fuels, which in recent years propelled the price of crude oil above US\$ 60 per barrel for the first time. This has demonstrated that production of "cheap" fossil fuels, which we may deplete by the middle of this century, can no longer cope with the demand. We therefore have to pay more to quickly bring about dangerous climate change and, if we survive that, wait for the highly probable

The ecological impact of turning every river into a dam for hydroelectric power if

It has also been recognized that the heavy reliance on fossil fuel has had an adverse impact on the environment. For example, gasoline engines and steam-turbine power plants that burn coal or natural gas send substantial amount of sulphurdioxide (SO2) and nitrogen oxides (NO2) into the atmosphere. When these gases combine with atmospheric water vapor, they form sulphuric and nitric acids, giving rise to highly acidic precipitations which are very dangerous to plants and human beings. Further more, the combustion of fossil fuels also releases carbon dioxide into the atmosphere; the amount of this gas in the atmosphere has been observed to have steadily risen since the mid 1800, largely as a result of the growing consumption of coal, oil and natural gas. More and more scientists believe that the atmospheric built up of carbon dioxide (along with that of other industrial gases such as methane and chlorofluorocarbon) may induce a green house effect, causing the rising of the surface temperature of the earth by increasing the amount of heat trapped in the lower atmosphere. This condition could bring about climate changes with serious repercussions

Similarly, nuclear power generation as a source of alternative energy faces lots of social objections due to the possible radiation hazard that it may cause during production. Scientists cannot estimate the extent and gravity of destruction, both immediate and long term, that nuclear radiation hazard can cause when nuclear power reactor accident occurs such as the case of the Russian's Chernobyl nuclear power plant accident in 1987, and the recent nuclear energy plants accident(tsunamis) in Japan, which gravity and extent of damage to life and properties cannot now be estimated and for how long the damaging radiation will be absolutely controlled. By this many countries are signing off nuclear

Moreover the nuclear power material if inappropriately stored could end up in wrong hands and get turned into weapon of mass destruction that will make terrorism assume a

incapability of oil to continue to meet the energy demand of the world.

The reasons for this strong opinion being:

possible, is scary and hard to imagine.

for natural and agricultural ecosystems.

energy utilization.

much more dangerous dimension.

energy crisis.

Here we will not bother ourselves with detailed specifications of the Sun, but give us just some relevant data of it.

The Sun is one of the many billion of stars in the Milky Way Galaxy, the galaxy of our solar system in the universe. It is the closest star to our planet earth; its effect and importance to us on the earth results from its closeness.

The sun is learnt to be formed about 5000 million years ago(Okeke and Soon 2004,). It is a great ball of hot gases with diameter of about 1.4x 106km, which is about 109 times that of the earth, and it is about 1.5 x 108km distant from the earth. It is the most important celestial object to us because it is the source which supplies the energy that allows life to flourish on earth.

The energy of the Sun is derived from a process similar to that of nuclear fusion in which hydrogen nuclei are believed to combine to form helium nucleus. The excess mass in the process is converted to energy in accordance with Einstein's theory i.e., **E= mc2**.

Thus, the Sun produces a vast amount of energy but only a tiny part of it reaches the earth. The energy comes from the nuclear fusion occurring at the core of the sun. The sun is a stable star, it thus promises to remain at the same magnitude of its properties and surface temperature for a long time. It is interesting to note that the Sun is not one of the hot stars, but one of the cooler stars. Cooler stars are yellow in colour and the Sun is yellow in colour. Yet its heat from 93million miles away is very effective in keeping us warm and sustains lives on our planet earth.

The Sun radiates about 3.86 x 1026 Joules of energy every second, a value which is more than the total energy man has ever used since creation. Although some of this energy is lost in the atmosphere, the amount reaching the earth's surface every second, if properly harnessed, is still probably enough energy to meet the world's energy demand (Maniel, 1974). Today it is a common knowledge that the Sun is the primary source of energy for all the processes taking place in the earth-atmosphere system. All lives on earth depend upon its radiant energy directly or indirectly to survive.

The Sun, therefore, is one of the popular emerging feasible sources of energy being looked into and sought by the world today for long–term, possible source of renewable and reliable energy. The Sun is available free for all land and mankind. It is free of politics. It only needs suitable devices to capture its rays and translate it into useful heat or work.

The amount of solar energy available for any land depends only on its location with respect to the Sun. If we examine the following expression for the solar energy available at the top of the atmosphere of any location from the sun,

$$\mathbf{H}\_o = \mathbf{24} \text{/n} \text{ I}\_{\text{sc}} \text{Cosq} \text{Cos0} \text{(Sino}\_{\text{s}} \text{ - n/180)} \text{o}\_{\text{s}} \text{Cos0} \text{s} \tag{1}$$

two angles in this expression are related to the location of a site on the earth's surface with respect to the sun:

Φ, the latitude, and **δ**, the declination angle of the Sun.

The amount of solar energy received per unit area per second at the outer edge of the earth's atmosphere above a site is known as Extraterrestrial radiation, and is about 3.0 x 1026 Joules. The extraterrestrial radiation being received at the normal incidence (i.e. Sun – earth average distance) at the outer edge of the atmosphere of a site is known as the solar constant Isc which is about 4921kJm-2h-1.

If the Sun emits energy as said above, in form of electromagnetic radiation given by

$$\mathbf{E} \equiv \mathbf{m} \mathbf{c}^2 \tag{2}$$

where **m** is mass and **c** is velocity of light, the energy therefore, radiated by the sun, is equivalent to a mass loss by the sun every second and can be evaluated to be:

#### **m=3x1026/c2 =3.3x109 kgs-1**

If the Sun thus loses mass at this rate, it can be estimated that the Sun may extinct in about 2x104 b years. Hence the energy of the Sun can be said to be in-exhaustible by the earth, i.e., the Sun is with us for some time to come.

However the amount of the energy reaching the earth's surface is about 1.00 x 103Wm-2 at noontime at the equator. The depletion of the Sun's energy as it passes through the atmosphere to the earth's surface, coupled with the seasonal, night and weather interruptions, constitutes the major impediment to the full realization of solar energy utilization. This notwithstanding, solar energy is proving by far the most attractive alternative source of energy for mankind.

Solar energy is pollution free, communitarian, conservational, decentralizable, adaptable, and the related devices to utilize it require very little or no maintenance, safe and cost effective. Solar energy utilization has come to stay as the possible future long–term energy resource. It can be argued that it is the only recurrent source, large enough to meet mankind demands of energy supply if properly harnessed. All other renewable energy sources depend directly or indirectly on the Sun for their existence.
