Annals of West University of Timisoara
Series Chemistry 17 (2) (2008) 25-30
TRENDS IN THE USE OF SOLAR ENERGY
Co r n e lia A ng h el a , C odr u ta - Oa na Ha ma t b
a
University EFTIMIE MURGU of Reşiţa, Faculty of Engineering, Department of
Computer Science, Traian Vuia Square, 320085 Reşiţa, ROMANIA
b
University EFTIMIE MURGU of Reşiţa, Faculty of Engineering, Department of
Mechanical Engineering, Traian Vuia Square, 320085 Reşiţa, ROMANIA
SUMMARY
The paper presents the tendencies in solar energy and the present researches in
this comprising domain. Some applications are described and we consider that the future
belongs to thermo-solar energy.
Keywords: solar energy, thermo-solar
INTRODUCTION
A huge quantity of solar energy reaches the surface of the Earth every day. This
energy can be captured and used under the form of heat in thermo-solar applications, or can
be directly turned into electricity with the help of photo - voltaic cells (PVC).
In order to understand the way PVCs and thermo-solar systems capture solar
energy, it is important to understand the latter’s course from the Sun to the Earth and the
way this flux periodically changes.
How does the Sun produce energy?
The Sun is a sphere with the diameter of around 1.4 million km, made of gases
with very high temperatures (the Sun’s interior temperature is of around 15 million degrees
Kelvin). This immense temperature combined with a pressure 70 billion times higher than
that of the Earth’s atmosphere, creates the ideal conditions for the fusion reactions.
The fusion reactions in the Sun take place between hydrogen atoms, which
combine and form atoms of helium. (Figure 1):
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A N G H E L C. , H AMAT C. O.
Figure 1. The fusion reaction
As a result of this process, energy is released under the form of high-energy
radiations, especially gamma rays. While these radiations migrate from the centre to the
exterior of the solar sphere, they react with different elements from the interior of the Sun
and turn into small-energy radiations. The Sun has produced energy in that manner for about
5 billion years, and will continue to do so for another 4 -5 billion years.
How is energy transported to the Earth?
The Earth turns around the Sun at a distance of approximately 150 million km/s.
The radiations extend at the speed of 300,000 km/sec, the speed of light. The time necessary
to reach the Earth is about 8 min. (Figure 2.)
Figure 2. The radiations released by the Sun while
it moves along its orbit
The quantity of solar energy reaching a certain place on the surface of the Earth at
a given moment is called the solar constant, and its value depends on several factors. If the
sun is at noon and the sky is clear, the radiation on a horizontal surface is of around 1000 W
per square meter. We witness the decrease of the solar constant when the surface is not
oriented perpendicularly to the Sun’s rays.
Figure 3. Plotting of the axis and inclination angle
of the Earth’s rotation.
The Earth spins around its axis in a day and turns around the Sun, along an elliptic
orbit, in a year. The axis around which the Earth spins has an inclination of about 23.5
degrees from the vertical. This inclination gave birth to seasons: when the axis of the Earth
is inclined towards the Sun, the northern hemisphere receives more solar radiations (in
summer). Six months later, when the axis is not inclined towards the Sun, summer comes to
the southern hemisphere, and thus the quantity of solar radiations reaching the Earth is
higher. (Figure 3).
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TRENDS IN THE USE OF SOLAR ENERGY
MATERIALS AND METHODS
The Thermo-Solar energy
The “thermo-solar” technologies use the heat of the sunrays to produce hot water
and electric power and to heat dwellings. The thermo-solar applications range from mere
residential water-heating system to very large stations for electric power generation.
The Thermo-Solar energy
The “thermo-solar” technologies use the heat of the sunrays to produce hot water
and electric power and to heat dwellings. The thermo-solar applications range from mere
residential water-heating system to very large stations for electric power generation.
The thermo-solar electric power is obtained with the technologies using solar
radiations in order to obtain steam. This steam feeds turbines, which generate electricity.
The small systems of water heating use flat-tray collectors in order to capture the
heat of the Sun, while the electric plants fed by thermo-solar energy use more complex
procedures for capturing radiations.
The flat-tray collectors transfer the Sun’s heat to water, either directly, or by means
of other liquids or a heat—changing system. (Figure 4)
Figure 4. The parts of a flat tray
The collector is covered in glass or another transparent material in order to keep
the solar heat within. The posterior part of the collector is covered with an insulating
material, in order to prevent heat from releasing. Between the transparent material and the
insulator there is a heat-absorbing material.
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A N G H E L C. , H AMAT C. O.
Figure 5. The three types of heating systems
There are 3 types of thermo-solar water-heating systems using the flat-tray
collector, i.e.:
A: The system is made of a pump, a collector and a storage tank. The pump
circulates the water through the collector, the latter heats the water, which is then stored in
the tank.
B: The system is made of a collector representing at the same time the storage tank
C: The system is made of a collector and a water-storage tank
The transformation of the thermo-solar energy into electric power
The thermo-solar power plants produce electricity using a turbine fed with the
steam produced by means of boiling a liquid with the help of the Sun’s radiations.
RESULTS
APPLICATIONS:
Hot water can be produced at a small scale for domestic uses or at a large scale for
the feeding of thermo-solar power plants. The small-scale applications generally use flattray collectors, whereas the electric power plants use systems of concentration of solar
radiations.
1. Hot water for domestic use. Installing a system using solar energy for heating is
economical and can satisfy 60-80% of the total necessary hot water. SRCC or Solar Rating
& Certification Corporation is a non-profit organisation dealing in the assurance of the
quality of domestic thermo-solar systems.
2. Pool heating
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TRENDS IN THE USE OF SOLAR ENERGY
3. Commercial and domestic uses
4. Thermo-solar power plants. Using runs, parables or central receivers, the
thermo-solar power plants concentrate the Sun’s rays to collectors, which reach very high
temperatures (up to 600 degrees Celsius sometimes). Nowadays there are many active
commercial plants, and some bigger ones are about to be built.
PARTICULAR CASES:
1. The St. Rose Hospital in San Antonio, Texas uses a thermo-solar heating system
for 90% of the total necessary hot water. The system has a 30000-litre tank and a 5000sq.m. collector; this manner of obtaining hot water helps the hospital save 17,000$ per year.
2. The tenants of a block of flats in Honolulu, Hawaii opted back in 1984 for the
use of thermo-solar energy to heat water, because of the high prices of oil. The system uses
around 50 flat-tray collectors of 48 square meters, and a 13500–litre tank in order to provide
70% of the total necessary hot water.
3. In Southern California (U.S.A.) there are 9 plants with run systems, also called
SEGS (Solar Electric Generating Systems) totally generating 354 de megawatts. These
systems are the most reliable and economical among the thermo-solar systems.
DISCUSSION
In order to increase the efficiency of the systems we should use lighter materials in
the construction of thermo-solar collectors. Scientists within the American Energy
Department have found that melted salt absorbs and stores solar heat very efficiently.
Moreover, experiments have been conducted at the University of Chicago in order to
develop a concentration system increasing the sun intensity 60,000 times. The American
government has planned for the following year the building of 100-MW power plants using
collecting systems with central receiver.
CONCLUSION
The advantages of the thermo-solar electric energy are the following:
• Obtaining electric power and hot water at the same time
• The power plants can be adapted to the applications for which they are
used
• The pollution is very reduced or null
• The building of thermo-solar power plants is done more rapidly than in
the case of conventional power plants.
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A N G H E L C. , H AMAT C. O.
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Bădileanu, M., “Electric power prices and analysis of value”, Economic Publishing House,
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Dumitrache C., “Solar protuberances”, Cartea Universitară Publishing House, Bucharest, 2006
Paulescu M., “Algorithms for the assessment of solar energy”, Matrixrom Publishing House,
Bucharest, 2007