Transcript ΗΛΙΑΚΗ ΕΝΕΡΓΕΙΑ

Solar power
The sun is the basic source of energy of our planet. The sun is a star of medium size
that because of the big temperatures of its elements which compose it, the
hydrogen, the molecules but also their particles are in a stage of “plasma”.
In these temperatures, certain millions of oC, the very rapidly moved cores of
hydrogen (H) are join together , and create cores of element of helium (He). This
nuclear reaction - fusion of cores is “exothermic” and is going with enormous
quantities of energy or heat or as is used to be said : solar energy, that radiated to
all directions in space.
This happens continuously for 5 billions years roughly, the sun contains enormous
quantities of hydrogen and it is not expected to be a reduction of energy which is
radiated by the sun.
In our country the sunlight lasts more than 2700 hours per year. In Western
Macedonia and the Ipirus it presents smaller prices from 2200 until 2300 hours,
while in Rhodes and southern Crete it exceeds the 3100 hours annually.
Solar power
The solar power
that earth takes
from the sun in
one week is
equal in the
energy of all
fuels in the
planet
The Earth receives
only the one billion of
energy the Sun
radiates ( huge
quantity )
Solar Energy
Solar hot water systems use sunlight to heat water.
Commercial solar water heaters began appearing
in the United States in the 1890s. These systems
saw increasing use until the 1920s but were
gradually replaced by relatively cheap and more
reliable conventional heating fuels. The economic
advantage of conventional heating fuels has varied
over time resulting in periodic interest in solar hot
water; however, solar hot water technologies have
yet to show the sustained momentum they had
until the 1920s. Solar water heating technologies
have high efficiencies relative to other solar
technologies. Performance will depend upon the
site of deployment, but flat-plate and evacuatedtube collectors can be expected to have
efficiencies above 60 percent during normal
operating conditions In addition, solar water
heating is particularly appropriate for lowtemperature (25-70 °C) applications such as
swimming pools, domestic hot water, and space
heating. The most common types of solar water
heaters are batch systems, flat plate collectors and
evacuated tube collectors.
Heating, ventilation, and air conditioning (HVAC) systems of buildings are closely
interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and
reasonable installation, operation, and maintenance costs. Conventional HVAC systems
account for roughly 28 percent of the energy used in the United States and European
Union.[Many solar heating, cooling, and ventilation technologies can be used to offset a
portion of this energy. Thermal mass materials store solar energy during the day and
release this energy during cooler periods. Common thermal mass materials include
stone, cement, and water. The proportion and placement of thermal mass should
consider several factors such as climate, daylighting, and shading conditions. When
properly incorporated, thermal mass can passively maintain comfortable temperatures
while reducing energy consumption. More advanced thermal mass systems can be also
be used for ventilation.
Electricity can be generated from the sun in several ways. Photovoltaics (PV) has been
mainly developed for small and medium-sized applications, from the calculator powered
by a single solar cell to the PV power plant. For large-scale generation, concentrating
solar thermal power plants have been more common but new multi-megawatt PV plants
have been built recently. Other solar electrical generation technologies are still at the
experimental stage.
Active Solar Domestic Water Heating
The active water systems that can be used to heat
domestic hot water are the same as the ones that
provide space heat. A space heat application will require
a larger system and additional connecting hardware to a
space heat distribution system.
There are five major components in active solar water
heating systems:
Collector(s) to capture solar energy. Circulation system
to move a fluid between the collectors to a storage tank
Storage tank
Backup heating system
Control system to regulate the overall system operation
There are two basic categories of active solar water
heating systems - direct or open loop systems and
indirect or closed loop systems.
Direct Systems The water that will be used as
domestic hot water is circulated directly into the
collectors from the storage tank (typically a hot
water heater which will back up the solar heating).
Indirect Systems that use antifreeze fluids need
regular inspection (at least every 2 years) of the
antifreeze solution to verify its viability. Oil or
refrigerant circulating fluids are sealed into the
system and will not require maintenance. A
refrigerant system is generally more costly and
must be handled with care to prevent leaking any
refrigerant.
Passive Solar Energy
Passive solar energy systems require no energy to
operate and are an intrinsic part of the home design.
Passive systems add little additional cost, operate with
almost no supervision and require little or no
maintenance. The basic elements of all passive
systems are south-facing windows and internal thermal
mass. Solar heating is simply sunlight entering the
house that is absorbed and converted into heat energy
which is later released inside the house as it cools. A
passive solar home is one where the design and
construction of the home itself is made to keep the
house naturally warm in the winter using the sun's
energy. The design should also keep the house
naturally cool during the summer .The sun is a very
intense source of energy. When designed properly, a
passive solar home can experience heating costs that
are 80% to 95% lower than for the average home. Air
conditioning costs can also be reduced to a minimal
level.
The basic idea of passive solar home design is to invite
sunlight into the house during the winter, and once it is
inside the home, to hold it in and store it until nighttime.
Conversely, the sun needs to be kept out during the
summer.
Electricity from the Sun
Electricity can be generated from solar energy in two ways. The first is to capture heat
from the sun and use this to power a conventional turbine or generator. The other is to
use the photovoltaic effect, which converts light directly into electricity using materials
called semiconductors.
Solar Thermal Electric Power Plants
The two main types of solar thermal power
plants are Solar Chimneys (where heated
air in a tower rises to drive turbines) and
Concentrating Solar Power (CSP) plants
(which use various types of reflectors to
concentrate sunlight into a heat absorber).
These are both industrial scale applications
which are not suitable for the urban
environment.
Photovoltaic Cells
The word photovoltaic is a marriage of the words ‘photo’, which means light, and ‘voltaic’, which refers to the
production of electricity. Photovoltaic technology generates electricity from light. Electricity is the existence (either
static or flowing) of negatively charged particles called electrons. Certain materials, called semiconductors, can be
adapted to release electrons when they are exposed to light. One of the most common of these materials is silicon
(an element found in, amongst other things, sand), which is the main material in 98% of solar PV cells made
today.
All PV cells have at least two layers of such semiconductors: one that is positively charged and one that is
negatively charged. When light shines on the semiconductor, the electric field across the junction between these
two layers causes electricity to flow - the greater the intensity of the light, the greater the flow of electricity.
Although the photovoltaic effect was known to the Victorians, it was not until humanity launched into the space
race that the unique qualities of solar PV as a power source began to be fully explored. Following this kick-start
the technology has raced along a path to commercialization and the cost of PV generated electricity has
plummeted as manufacturing costs have decreased and cell efficiencies have improved.
Anatomy of a Solar Cell
Before now, our silicon was all electrically neutral. Our extra electrons were
balanced out by the extra protons in the phosphorous. Our missing electrons
(holes) were balanced out by the missing protons in the boron. When the
holes and electrons mix at the junction between N-type and P-type silicon,
however, that neutrality is disrupted. Do all the free electrons fill all the free
holes? No. If they did, then the whole arrangement wouldn't be very useful.
Right at the junction, however, they do mix and form a barrier, making it harder
and harder for electrons on the N side to cross to the P side. Eventually,
equilibrium is reached, and we have an electric field separating the two sides.
This electric field acts as a diode, allowing (and even pushing) electrons to
flow from the P side to the N side, but not the other way around. It's like a hill -electrons can easily go down the hill (to the N side), but can't climb it (to the P
side).
So we've got an electric field acting as a diode in which electrons can only
move in one direction.
When light, in the form of photons, hits our solar cell, its energy frees electronhole pairs.
Each photon with enough energy will normally free exactly one electron, and
result in a free hole as well. If this happens close enough to the electric field,
or if free electron and free hole happen to wander into its range of influence,
the field will send the electron to the N side and the hole to the P side. This
causes further disruption of electrical neutrality, and if we provide an external
current path, electrons will flow through the path to their original side (the P
side) to unite with holes that the electric field sent there, doing work for us
along the way. The electron flow provides the current, and the cell's electric
field causes a voltage. With both current and voltage, we have power, which
is the product of the two. There are a few more steps left before we can really use
our cell. Silicon happens to be a very shiny material, which means that it is very
reflective. Photons that are reflected can't be used by the cell. For that reason, an
antireflective coating is applied to the top of the cell to reduce reflection losses to
less than 5 percent.
The final step is the glass cover plate that protects the cell from the elements. PV
modules are made by connecting several cells (usually 36) in series and parallel to
achieve useful levels of voltage and current, and putting them in a sturdy frame
complete with a glass cover and positive and negative terminals on the
back.
Photovoltaic Cells (Solar Cells), How They Work
d N-type silicon is created by doping
(contaminating) the Si with compounds that
contain one more valance electrons than Si does,
such as with either Phosphorus or Arsenic. Since
only four electrons are required to bond with the
four adjacent silicon atoms, the fifth valance
electron is available for conduction.
e P-type silicon is created by doping with
a The encapsulate, made of glass or other clear
material such clear plastic, seals the cell from the
external environment.
b
The contact grid is made of a good conductor,
such as a metal, and it serves as a collector of
electrons.
c Through a combination of a favorable refractive
index, and thickness, this layer serves to guide light
into the PV Cell. Without this layer, much of the
light would bounce off the surface of the cell. The
RTWCG method of depositing this AR Coating is by
far the most desirable technique known to us.
compounds containing one less valance electrons
than Si does, such as with Boron. When silicon (four
valance electrons) is doped with atoms that have
one less valance electrons (three valance electrons),
only three electrons are available for bonding with
four adjacent silicon atoms, therefore an
incomplete bond (hole) exists which can attract an
electron from a nearby atom. Filling one hole
creates another hole in a different Si atom. This
movement of holes is available for conduction.
The back contact, made out of a metal, covers the
entire back surface and acts as a conductor.
The path of the photon.
After a photon makes it's way through the encapsulate it encounters the antireflective layer. The antireflective layer channels the photon into the
lower layers of the solar cell. Click on the following link if you would like to learn about our novel room temperature wet chemical growth
antireflective layer (RTWCG - AR).
Once the photon passes the AR coating, it will either hit the silicon surface or the contact grid metallization. The metallization, being opaque,
lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet cover as little of the solar
cell's surface, allowing more photons to penetrate.
A photon causes the photoelectric effect.
The photon's energy transfers to the valance electron of an atom in the n-type Si layer. That energy allows the valance electron to escape its orbit
leaving behind a hole. In the n-type silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As
the term "carrier" implies, both are able to move throughout the silicon layer, and so are said to be mobile. Inversely, in the p-type Si layer,
electrons are termed minority carriers and holes are termed majority carriers, and of course are also mobile.
The pn-junction.
The region in the solar cell where the n-type and p-type Si layers meet is called the pn-junction.
As you may have already guessed, the p-type Si layer contains more positive charges, called holes, and the n-type Si layer contains more negative
charges, or electrons. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward
biased) but not in the other (reverse biased).
An interesting interaction occurs at the pn-junction of a darkened photovoltaic cell. Extra valance electrons in the n-type layer move into the ptype layer filling the holes in the p-type layer forming what is called a depletion zone. The depletion zone does not contain any mobile positive or
negative charges. Moreover, this zone keeps other charges from the p and n-type layers from moving across it.
So, to recap, a region depleted of carriers is left around the junction, and a small electrical imbalance exists inside the solar cell. This electrical
imbalance amounts to about 0.6 to 0.7 volts. So due to the pn-junction, a built in electric field is always present across the solar cell.