Solar thermal collectors and applications
The sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 X 109 m. The solar energy strikes our planet a mere 8 min and 20 s after leaving the giant furnace, the sun which is 1.5 X 1011 m away. The sun has an effective blackbody temperature of 5762 K . The temperature in the central region is much higher and it is estimated at 8 X 106 to 40 X 106 K. In effect the sun is a continuous fusion reactor in which hydrogen is turned into helium. The sun’s total energy output is 3.8 X 1020MW which is equal to 63 MW/m2 of the sun’s surface. This energy radiates outwards in all directions. Only a tiny fraction, 1.7 X 1014kW, of the total radiation emitted is intercepted by the earth . However, even with this small fraction it is estimated that 30 min of solar radiation falling on earth is equal to the world energy demand for one year.
Man realised that a good use of solar energy is in his benefit, from the prehistoric times. The Greek historian Xenophon in his ‘memorabilia’ records some of the teachings of the Greek Philosopher Socrates (470-399 BC) regarding the correct orientation of dwellings in order to have houses which were cool in summer and warm in winter.
Since prehistory, the sun has dried and preserved man’s food. It has also evaporated sea water to yield salt. Since man began to reason, he has recognised the sun as a motive power behind every natural phenomenon. This is why many of the prehistoric tribes considered Sun as ‘God’. Many scripts of ancient Egypt say that the Great Pyramid, one of the man’s greatest engineering achievements, was built as a stairway to the sun .
Basically, all the forms of energy in the world as we know it are solar in origin. Oil, coal, natural gas and woods were originally produced by photosynthetic processes, followed by complex chemical reactions in which decaying vegetation was subjected to very high temperatures and pressures over a long period of time . Even the wind and tide energy have a solar origin since they are caused by differences in temperature in various regions of the earth.
The greatest advantage of solar energy as compared with other forms of energy is that it is clean and can be supplied without any environmental pollution. Over the past century fossil fuels have provided most of our energy because these are much cheaper and more convenient than energy from alternative energy sources, and until recently environmental pollution has been of little concern.
Twelve winter days of 1973 changed the economic relation of fuel and energy when the Egyptian army stormed across the Suez Canal on October the 12th provoking an international crisis and for the first time, involved as part of Arab strategy, the threat of the ‘oil weapon’. Both the price and the political weapon issues quickly came to a head when the six Gulf members of the Organisations of Petroleum Exporting Countries (OPEC), met in Kuwait and quickly abandoned the idea of holding any more price consultations with the oil companies, announcing that they were raising the price of their crude oil by 70%.
The reason for the rapid increase in oil demand occurred mainly because increasing quantities of oil, produced at very low cost, became available during the 50s and 60s from the Middle East and North Africa. For the consuming countries imported oil was cheap compared with indigenously produced energy from solid fuels.
But the main problem is that proved reserves of oil and gas, at current rates of consumption, would be adequate to meet demand for another 40 and 60 years, respectively. The reserves for coal are in better situation as they would be adequate for at least the next 250 years.
If we try to see the implications of these limited reserves we will be faced with a situation in which the price of fuels will be accelerating as the reserves are decreased. Considering that the price of oil has become firmly established as the price leader for all fuel prices then the conclusion is that energy prices will increase over the next decades at something greater than the rate of inflation or even more. In addition to this is also the concern about the environmental pollution caused by the burning of the fossil fuels. This issue is examined in Section 1.1.
In addition to the thousands of ways in which the sun’s energy has been used by both nature and man through time, to grow food or dry clothes, it has also been deliberately harnessed to perform a number of other jobs. Solar energy is used to heat and cool buildings (both active and passive), to heat water for domestic and industrial uses, to heat swimming pools, to power refrigerators, to operate engines and pumps, to desalinate water for drinking purposes, to generate electricity, for chemistry applications, and many more. The objective of this paper is to present the various types of collectors used to harness solar energy, their thermal analysis and performance, and a review of applications.
There are many alternative energy sources which can be used instead of fossil fuels. The decision as to what type of energy source should be utilised must, in each case, be made on the basis of economic, environmental and safety considerations. Because of the desirable environmental and safety aspects it is widely believed that solar energy should be utilised instead of other alternative energy forms, even when the costs involved are slightly higher.