Solar Energy
Summary
The use of energy derived from fossil combustible is the main polluting
source of our planet and its availability is in depletion. The utilization
of renewable energies can represent the path to continue to develop our
civilization without creating any damage to the environment, thus ensuring
ourselves inexhaustible energy.
Renewable energy sources (R.E.S.)
Solar energy is the source of primary energy per excellence. Every year
the sun irradiates the earth with 19.000 billion EOTs (Equivalent Oil
Tons) , while the annual world energy need is about 8 billion EOTs. In
Italy the annual demand is around 167 million EOTs. All energy forms on
earth derive from solar energy, the only exceptions being nuclear, geothermal
and tide energy. Direct solar energy is the one associated with the light
and the warmth which we perceive during daylight. The energy derived from
water and wind is considered to be a type of indirect solar energy. Even
the biomass, that is to say any kind of organic material of vegetable
origin (from plants to agricultural and urban waste) is a sort of accumulation
of solar energy, through the processes of chlorophyll photosynthesis.
Direct solar energy and its indirect forms are indicated as renewable
energy sources, because they regenerate, after their utilization, on a
human time-based scale. Fossil combustible are not – on the other hand
– a renewable energy source, because their reservoirs are finished and
destined to be depleted. Direct solar energy is the one we feel when we
get warm in the sun’s heat. It is the renewable source per excellence,
because it immediately renews itself or after some hours (such as bodies
which accumulate heat during the day and release it at night time).
- Wind energy origins from a non-homogeneous and variable warming up
of the earth’s surface by the sun.
- Water energy, that is the energy possessed by water in presence of
a difference in height, is a phenomenon that is made possible only thanks
to the sun, the heat of which makes the water cycle run.
- The biomass energy is nothing else than solar energy - stored thanks
to photosynthesis – in the plants’ green (vegetable) tissues.
Fossil combustible energy derives from solar energy too, because it is
stored in the biomass and – in the course of million of years – undergoes
chemical and geological processes. For this reason it cannot be considered
renewable.
Solar radiation
The solar radiation that – passing through the earth’s atmosphere - reaches
its surface, derives from the sum of three components: the direct, the
reflex and the diffuse one. In conditions of clear sky and with the sun
at the Zenit (that is to say on the perpendicular of the place) the power
that reaches the surface is roundabout 1000 Watt/sqm (one thousand watts
per square meter) (Ir)adiation takes place in the form of electromagnetic
waves, whose components, generally called thermal or infrared radiations,
correspond to a wave length interval that ranges from about 1 µm up to
100 µm. Warmth propagates even in the void and in fact solar thermal radiation
crosses interplanetary spaces to reach the earth. When a material body
is run over by a radiating energy, a fraction of it is reflexed, another
one is absorbed and the rest transmitted. The absorption - by the matter
- of this radiating energy, depends on the surface of the body, and in
particular on its being rough ,smooth (glossy), white or black. In fact
black surfaces are those which – in absolute terms – absorbe the most
quantity of energy
Besides these parameters both insolation and radiating depend on the
exposition (of the body to them) , on the surface inclination and on the
terrestrial latitude and longitude.
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Solar radiation in Mediterranean areas [kWh/mq]
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Technologies
For solar technology is generally meant any kind of tool that allows
the transformation of direct solar energy or its indirect forms (wind,
water and biomass) into forms of energy that are useful to man: heating,
electricity, combustible.
Solar thermal plants convert direct solar energy into thermal energy,
that is to say into heat to produce warm water for central heating, or
for the production of electricity: thermo-solar heating, pipe vacuum tube
collectors, plate collectors, monobloc accumulation collectors, thermo-solar
collectors, thermo-solar power generators, solar towers, thermo- photovoltaic
panel systems, parabolic chase concentrators, linear parabolic mirrors,
parabolic concentrator systems, solar pond systems etc.
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A solar tower |
Parabolic concentrator
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Thermo-solar collector |
Wind generators are nothing else than the modern version of wind mills. They are used to convert the energy of the wind into mechanic energy or more often into electric energy.
Mini hydro-electric plants are little plants that allow the utilization of small water falls to produce electric energy.
Biomass plants convert wood, agricultural residues or urban waste into heat and electricity, through processes of combustion and gasification.
Photovoltaic plants allow, instead, the instant conversion of direct solar energy into electric energy.
The working system of photovoltaic devices is essentially based on the capacity of some semiconductor materials, opportunely treated, to convert the energy of the solar radiation into electric energy (direct current) without using any moving mechanical part.
The semiconductor material that nowadays is almost world-wide used to this purpose is silicon.
The basic component of a Photovoltaic plant is the photvoltaic cell. A structure of more assembled and inter-connected cells form the photovoltaic module.
The transfer of energy from the photovoltaic system to the customer takes place through further appliances, that are necessary to transform the direct current produced by the modules and adapt it to the needs of the final user.
References
Caforio-Ferilli, Corso di fisica sperimentale
Link
www.enea.it
www.isesitalia.it
www.trenitalia.it
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