Solar Cells

kitteninterestΤεχνίτη Νοημοσύνη και Ρομποτική

15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

36 εμφανίσεις

Solar Cells


Typically 2 inches in diameter and 1/16 of an inch
thick


Produces 0.5 volts, so they are grouped together
to produce higher voltages. These groups can
then be connected to produce even more output.


In 1883 the first solar cell was built by Charles
Fritts. He coated the semiconductor selenium
with an extremely thin layer of gold to form the
junctions. The device was only around 1%
efficient.


Generations of Solar cells


First generation


large
-
area, high quality and single junction devices.


involve high energy and labor inputs which prevent
any significant progress in reducing production costs.


They are approaching the theoretical limiting
efficiency of 33%


achieve cost parity with fossil fuel energy generation
after a payback period of 5
-
7 years.


Cost is not likely to get lower than $1/W.

Generations of Solar cells


Second generation
-
Thin Film Cells


made by depositing one or more thin layers (thin film)
of photovoltaic material on a substrate.


thickness range of such a layer varies from a few
nanometers to tens of micrometers.


Involve different methods of deposition:


Chemical Vapor deposition the wafer (substrate) is exposed
to one or more volatile precursors, which react and/or
decompose on the substrate surface to produce the desired
deposit. Frequently, volatile by
-
products are also produced,
which are removed by gas flow through the reaction
chamber.

Thin Film deposition techniques


Electroplating


electrical current is used to reduce cations (positively
charged ions) of a desired material from a solution
and coat a conductive object with a thin layer of the
material.


Ultrasonic nozzle


spray nozzle that utilizes a high (20 kHz to 50 kHz)
frequency vibration to produce a narrow drop size
distribution and low velocity spray over the wafer


These cells are low cost, but also low efficiency

The Third Generation


Also called advanced thin
-
film photovoltaic
cell


range of novel alternatives to "first
generation” and "second generation” cells.


more advanced version of the thin
-
film cell.


Third generation alternatives


non
-
semiconductor technologies (including polymer cells and biomimetics)


quantum dot technologies



also known as nanocrystals, are a special class semiconductors. which are crystals composed
of specific periodic table groups. Size is small, ranging from 2
-
10 nanometers (10
-
50 atoms) in
diameter.


tandem/multi
-
junction cells


multijunction device is a stack of individual single
-
junction cells


hot
-
carrier cells


Reduce energy losses from the absorption of photons in the lattice


upconversion and downconversion technologies


Put a substance in front of the cell that converts low energy photons to higher energy ones or
higher energy photons to lower energy ones that the solar cells can convert to electricity.


solar thermal technologies, such as thermophotonics(TPX)


A TPX system consists of a light
-
emitting diode (LED) (though other types of emitters are
conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic
control circuit. The LED is heated to a temperature higher than the PV temperature by an
external heat source. If power is applied to the LED, , an increased number of electron
-
hole
pairs (EHPs) are created.These EHPs can then recombine radiatively so that the LED emits light
at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then
delivered to the cooler PV cell over the optical coupling and converted to electricity.


Efficiency and cost factors


In 2002 average cost per peak watt was $2.90
-
$4.00.
Coal fired plant is $1.00/watt.


Efficiency is not great.


Recall, 77% of the incident sunlight can be used by the cell.


43% goes into heating the crystal.


Remaining efficiency is temperature dependent


Average efficiency of a silicon solar cell is 14
-
17%


The second and third generation technologies
discussed are designed to increase these efficiency
numbers and reduce manufacturing costs

Solar Cooling


Consider a refrigeration system with no moving parts.


Heat the coolant (say ammonia gas dissolved in water) and force it via a
generator into an evaporator chamber where it expands into a gas and cools.
Move it to a condenser and cool it back to a liquid and repeat the process.


These systems actually have existed for a number of years, refrigerators in
the 1950s were sold with this technology (gas powered and there was/is a
danger of CO emissions).


Energy to heat the coolant and drive it through the system comes from
burning fuel or a solar cell to provide electricity to do the heating.


Need what is called a concentrating collector (lens or other system to
concentrate more light on the solar cell).


Ideally, you could do this with a flat plate collector system, though you do
not obtain as much cooling.


Devices are not widely used, due to the intermittency of sunlight


Other renewable energy sources


Hydropower


Wind energy


Ocean Thermal


Biomass


Geothermal


Tidal

Hydropower


Well established electrical generation technology, about 100 years
old


Known for over 2000 years that the force of moving water on a
water wheel could save human labor


13
th

century
-
hammers in iron works in Europe were operated with
waterwheels.


16
th

century

primary source of industrial power in Europe


In the US, mills were established at sites with reliable water flow
and dams were constructed to regulate water flow.


Cave mill here in BG. Several hydro powered mills for corn, flour and
sawing in the 19
th

century existed on this site at different times.


With the advent of electricity, water wheels were used to drive
electricity generation.


About 7% of US energy generation is from hydroelectric plants


The physics of Hydropower


Gravitational potential energy in the water at a height h above the wheel
is converted to kinetic energy of the wheel which drives a turbine and
generates electricity.


So each mass element of water, m, falls a distance h and attain a velocity v.
So its initial potential energy is mgh, where g is the acceleration due to
gravity (9.8m/s
2
) and the kinetic energy is 1/2mv
2
.


This tells us the amount of potential energy available to be converted to
kinetic energy is 9.8 joules per kilogram of water per meter of height
above the wheel.


h is often called the head.


Efficiencies of 80
-
90% can be achieved.


Power = (Height of Dam (distance the water falls)) x (River Flow) x
(Efficiency) / 11.8


where the height is in feet, river flow is in cubic feet per second ,
efficiency is what you expect and 11.8 converts from feet and seconds to
killowatts


Plant operation


Hydro Turbine


Advantages


No Pollution


No waste heat


High efficiency


Plants have decades long lifetimes and low
maintenance costs


Good response to changing electricity demands


Damming of rivers can serve other purposes:
flood control, irrigation, drinking water supply

Limitations


About 50% of the US capacity for Hydro is developed


Limited lifetimes for certain reservoirs
-
as the fill with silt,
they become less useful for water storage. But the dam
must be maintained long term, if it fails, communities
downstream are in danger from the tremendous volume of
silt that would be released.


Loss of free flowing streams due to damming and the loss
of the lands flooded by damming a river
-
environmental
impacts


Salmon population in the Nothwest has been impacted


Flood risk due to dam failures


Currently hundreds thousands of people in danger if dam
failures occur

Fish Ladders


Solution to the salmon problem
-

have not
been very effective

Wind power


Not subject to day night cycles


Direct result of solar heating of the Earth’s
atmosphere


Use of wind for energy first noticed by sailors the
old sailing ships could extract the equivalent of
10,000 hp from the wind!


Windmills were prevalent in Europe in the 19
th

century


Several million were pumping water in the US in
the early 1900s

WHAT YOU’RE PROBABLY THINKING
OF….


Power in a windmill


The power in the wind can be calculated by P/m
2
=6.1 X
10
-
4
v
3


This gives the power in kilowatts per meter squared, where
the cross sectional area is oriented perpendicular to the
wind direction.


This is the total power, of course not all of it can be
extracted. According to Betz’s Law, developed in 1919 by
German physicist Albert Betz, no turbine can capture more
than 59.3 percent of the potential energy in wind.


However, the total amount of economically extractable
power available from the wind is considerably more than
present human power use from all sources!


Extracting the energy: The turbine


The world's first
automatically operated
wind turbine was built
in Cleveland in 1888 by
Charles F. Brush. It was
60 feet tall, weighed
four tons and had 12kW
turbine.


Turbine types:


2 types, based on the
direction of the axis that
the turbine rotates about.


Horizontal axis wind
turbines (HAWT)
-
the
turbine rotates around an
axis that is horizontal.


Vertical Axis Wind
Turbines (VAWT)

the
turbine rotates around a
vertical axis



HAWT


Horizontal Axis Wind Turbines


main rotor shaft and electrical generator are
locate at at the top of a tower, and must be
pointed into the wind.



Small turbines are pointed by a simple wind
vane, while large turbines generally use a wind
sensor coupled with a servo motor.



Most have a gearbox, which turns the slow
rotation of the blades into a quicker rotation that
is more suitable to drive an electrical generator.

HAWT


the turbine is usually pointed upwind of the
tower since it creates turbulence behind it.


Turbine blades are made stiff to prevent the
blades from being pushed into the tower by
high winds.


The blades are placed a considerable distance
in front of the tower and are sometimes tilted
up a small amount.