Human beings, animals, and plants alike are dependent on some gases, nutrients, and solids that are available rather abundantly and almost freely in nature for their survival. Among these the most precious ones are the air in the atmosphere that living organisms breathe and the water that is available in the hydrosphere either in the troposphere as a vapor (humidity) or in the lithosphere as a liquid (rainfall, runoff, groundwater, seas, lakes) or a solid (glaciers, snow, ice, hail). The atmosphere has evolved over geological time and the development of life on the earth has been closely related to the composition of the atmosphere. From the geological records, it seems that about 1.5 billion years ago free oxygen first appeared in the atmosphere in appreciable quantities (Harvey 1982). The appearance of life was very dependent on the availability of oxygen but once a sufficient amount had accumulated for green plants to develop, photosynthesis was able to liberate more into the atmosphere. During all these developments solar radiation provided the sole energy source.
In general, there are six different heat and mass exchanges within the atmosphere. These exchanges play the main role in the energy distribution throughout the whole system. The major energy source is solar radiation between the atmosphere and space. This energy source initiates the movement of heat and mass energy from the oceans (seas) into the air and over the land surfaces. The next important heat energy transfer occurs between the free surface bodies (oceans, seas, rivers, reservoirs) and the atmosphere. Thus water vapor, as a result of evaporation, is carried at heights toward the land by the kinetic energy of the wind. Such a rise gives the water vapor potential energy. After condensation by cooling, the water vapor appears in the form of precipitation and falls at high elevations forming the surface runoff which due to gravity flows toward the seas. During its travel toward the earth's surface, a raindrop loses its potential energy while its kinetic energy increases. Water vapor is the inter-mediator in such a dynamic system. Finally, the water is returned to the seas via streams and rivers, because gravity ultimately takes over the movement of masses. The natural energy cycle appears as an integral part of the hydrological
Zekai Sen, Solar Energy Fundamentals and Modeling Techniques DOI: 10.1007/978-1-84800-134-3, ©Springer 2008
cycle (§en 1995). During this cycle, no extra energy is produced within the atmosphere. Such movements result from the fine balance that has existed for so long between the output of radiation from the sun and the overall effects of the earth's gravitation.
Groundwater and surface water bodies become acidified due to trans-boundary air pollution causing harm to human health, and tree and forest loss. Unfortunately, there is not enough data for assessment of these dangers in the developing countries. One of the greatest and most famous scientist all over the world Ibn Sina (Avicenna, 958-1037) recommended some 1000 years ago seven points for a human being to sustain a healthy life in this world (Sen 2005). These are:
1. Spiritual healthiness
2. Choice of food and drinking water quality
3. Getting rid of extra weight to feel fit
4. Healthiness of the body
5. Comfortable dressing
6. Cleanliness of the inhaled air
Two of these points, namely, choice of water and air clarity will constitute the main topic of this chapter related to renewable energy sources in general but to solar energy in particular.
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