lecture 3 2015_2016
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
Renewable Energy
Systems3
Dr. Caroline Dong
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
3Solar Photovoltaics
3-1 THE PN JUNCTION3-2 PHOTOVOLTAIC CELL STRUCTURE AND OPERATION
3-3 TYPES OF PHOTOVOLTAIC TECHNOLOGIES
3-4 MULTI-JUNCTION THIN-FILM
3-5 PV CELL CHARACTERISTICS AND PARAMETERS
3-6 SOLAR MODULES AND ARRAYS
3-7 SOLAR MODULE DATA SHEET PARAMETERS
3-8 CONCENTRATING PHOTOVOLTAICS
Chapter Outline
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
Silicon (Si)
The most common element used in the construction of Photovoltaic solar cells
The fact that certain solid materials are sensitive to light was a chance discovery during an investigation of silicon as a radar detector by Russell Ohl in 1940
Two types of silicon:
• Amorphous silicon a-Si
• Crystalline silicon c-Si
3
3-1 Silicon Atom and Crystal Structure
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Crystalline silicon (c-Si) is the major material used in semiconductors. The neutral silicon atom has 14 protons, 14-16 neutrons, and 14 electrons.
3-1 The PN Junction
Four electrons
in valence 價shell
Valence electrons
It is these outer shell electrons
that are involved in bonding with
other Si atoms, forming c-Si.
Bohr model
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The process of doping 興奮劑 is to add impurity materials to create nand p materials, that have either an excess of electrons in the crystalline structure or a deficiency of electrons.
3-1 The PN Junction
Pentavalent 五價 impurity n materials: antimony (Sb), phosphorous
(P), arsenic (As), bismuth (Bi)
Trivalent 三價 impurityp materials: boron (B), indium
(In), gallium (Ga)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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When a pn junction is formed from a single crystal of Si, an n region is on one side and a p region is on the other side. A depletion region is formed at the boundary.
3-1 The PN Junction
The pn junction is the key property of ordinary semiconductor diodes and allows the diode to pass current in one direction only.
Phosphorus
Boron
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A PV cell is a specialized diode that has a window into a thin n region. At the junction between the regions a depletion layer forms as in an ordinary diode.
3-2 Photovoltaic Cell Structure and Operation
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3-2 Photovoltaic Cell Structure and Operation
Light enters the PV cell through the window on top. If the photon energy of the light is higher than the band gap energy of 1.12 eV, it will knock out electrons from the valence band of the Si into the conduction band, creating an electron-hole pair. These electrons move across the depletion region and develop a voltage across the cell.
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1 eV = 1.6E-19 Joule
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Photon energy
A photon 光子 has an energy related to its wavelength:
𝐸 =ℎ𝑐
𝜆
E: photon energy
h: Planck constant
c: speed of light
𝜆: photon’s wavelength
h and c are constants:
𝐸 =1240 𝑒𝑉 ∙ 𝑛𝑚
𝜆
9
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Photon energy
Example:
Determine the maximum wavelength of sunlight that can be absorbed by silicon PV cell?
Solution:
E = 1.12 eV
𝜆 =1240 𝑒𝑉∙𝑛𝑚
𝐸=
1240 𝑒𝑉∙𝑛𝑚
1.12 𝑒𝑉≈ 1100 nm
10
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-3 Types of PV Technologies
PV cells
Various technologies for PV cells are available. Three types are monocrystalline cells, polycrystalline cells, and thin-film cells. • monocrystalline cells are
single crystal wafer 晶圓cells and have the highest
efficiency. (14%-20%)
• polycrystalline cells are
composed of numerous smaller crystals and are less
efficient. (13%-15%)
• thin-film cells can be made from
amorphous Si and have the
lowest cost but also the lowest
efficiency. (5%-7%)
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-3 Types of PV Technologies
A useful type of monocrystalline semiconductor is Gallium Arsenide (GaAs). It is a higher efficiency cell with high heat resistance.
The Sunraycer car is
powered by more
than 1,400 silicon and
3,800 gallium arsenide
solar cells. It recently
won the World Solar
Challenge race in
Australia.
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e: N
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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Another PV technology is the dye-sensitized solar cell (DSSC).
Mimic the plants to absorb sunlight in a dye (chlorophyll)
Dye cells are formed as a thin film semiconductor that has a light absorbing layer of Titanium oxide (TiO2) over a liquid electrolyte on a very thin layer of Platinum (Pt) and a substrate.
Dye cells are inexpensive and simple to make but have low efficiency and problems with the liquid electrolyte for long term
operational use. There are new fabrication techniques and
technologies that may improve these cells in the future.
3-3 Types of PV Technologies
Sourc
e:
David
Buchla
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-4 Multijunction Thin-Film
A multijunction thin-film PV cell is two or more types of
single-junction cells arranged in descending order of
band gap. Each cell is designed to absorb photons over
a specific portion of the electromagnetic spectrum that
matches the band gap.
Matching the photon energy to the band gap greatly increases the overall efficiency. A recent triple junction cell was tested at 44% efficient, with the prospects for 50% in the future.
Cell 1
Cell 2
Cell 3
i-type a-Si
p-type mc-Si
n-type a-Si
Transparent conducting
oxide (TCO)
Back contact
Substrate
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3-5 PV Cell Characteristics and Parameters
The I-V characteristic for a solar cell is essentially
constant over a range of output voltages for a specified
incident light energy.
Isc = 2 A
Voc = 0.6 V
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-5 PV Cell Characteristics and Parameters
Maximum power occurs on the “knee” of the I-V curve.
P = VI
MPP = 0.53 V* 2 A
= 1.06 W
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3-5 PV Cell Characteristics and Parameters
Current from a cell is proportional to the irradiance.
There are several factors that determine the efficiency of a PV cell: the type of cell,
the reflectance efficiency of the cell’s surface, the thermodynamic efficiency limit, the
quantum efficiency, the maximum power point, and internal resistances.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-5 PV Cell Characteristics and Parameters
The fill factor is the ratio of the cell's actual maximum
power output (VMPP x IMPP) to its theoretical power output
(VOC x ISC).
FF = (VMPP)(IMPP) / (VOC)(ISC)
Typical fill factor is 0.7
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-5 PV Cell Characteristics and Parameters
PV cells also have a temperature dependence.
Increasing temperature decreases the band gap and
decreases the open-circuit voltage. Current changes
only slightly with temperature.
Performs better in
cooler temperature
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-6 Solar Modules and Arrays
Courtesy of NREL
Modules come in a variety of sizes, types, and ratings.
The performance of PV modules are usually rated
according to their maximum dc power output (watts)
under Standard Test Conditions (STC). The specific
output depends on the size and the internal wiring of the
module.
So
urc
e: N
REL
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-6 Solar Modules and Arrays
One type of module can blend in with roof shingles.
Each module is rated for 17 W.
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REL
So
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Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-6 Solar Modules and Arrays
Most solar systems consist of multiple modules that are
combined into arrays. The outputs have standard
connectors for ease of connection with other panels,
generally in a unit called a combiner.
Very large systems are
used by utilities to
provide power to the
electrical grid. This one
is in Arizona.
So
urc
e: N
REL
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-7 Solar Module Data Sheet Parameters
Solar module data sheets are divided into several sections: Electrical data, Mechanical data, I-V curve, tested operating conditions,
certifications, etc.
A sample of electrical specifications are:
Electrical Data
Peak power Pmax 215 W
Rated voltage Vmpp 39.8 V
Rated current Impp 5.40 A
Open circuit voltage VOC 48.3 V
Short circuit current ISC 5.80 A
Series fuse rating 15 A
In addition, temperature coefficients (e.g. current/degree) should included.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-7 Solar Module Data Sheet Parameters
Mechanical data includes physical characteristics of
the module. A sample of mechanical specifications
are:
Mechanical Data
Solar cells 72 monocrystalline
Front glass High transmission tempered
Junction box IP-65 with 3 bypass diodes
Dimentions 32 X 155 X 28 mm
Output cables 1000 mm length/ MC-4 connectors
Frame Anodized aluminum
Weight 33.1 lbs (15.0 kg)
In addition, a drawing of the module will be given with dimensions.
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-7 Solar Module Data Sheet Parameters
The I-V curve as a function of irradiance and
temperature will be given. For example, for a module,
the I-V curve may look like the following:
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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3-8 Concentrating Photovoltaics (CPV)
CPV systems use lenses, mirrors, or a combination of
both to concentrate sunlight on a small area of PV
cells. CPV systems work best with direct rather than
diffuse light, so tracking systems are generally
employed with CPV systems.
The cost for high efficiency
multijunction cells such as
GaAs is higher than
conventional cells, but the
higher solar efficiency tends
to offset this cost.
Sourc
e:
David
Buchla
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4Solar Power Systems
4-1 STAND-ALONE PV SOLAR POWER SYSTEMS
4-2 SIZING THE STAND-ALONE SYSTEM
4-3 GRID-TIE PV SOLAR POWER SYSTEMS
4-4 SOLAR CONCENTRATORS
4-5 SOLAR HOT WATER SYSTEMS
Chapter Outline
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
Copyright © 2015 by Pearson Education, Inc.All Rights Reserved
Stand-alone solar electric systems do not connect to the grid but in general supply electricity for smaller or remote applications or to supplement the grid. Many are strictly dc systems, running 12 V, 24 V or 48 V.
4-1 Stand-Alone Solar Power Systems
This stand-alone traffic signal is
an example of a low voltage
application, in which the PV
module keeps a battery
charged using a charge
controller to regulate and limit
charging current to prevent
overcharging the batteries.
So
urc
e:
David
Bu
ch
la
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-1 Stand-Alone Solar Power Systems
Larger stand-alone solar electric systems include ac as an output. In this case an inverter to convert dc to ac is used. A representative system is shown:
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4-1 Stand-Alone Solar Power Systems
The cost for a solar electrical system is done as a “life-cycle” cost (LCC) that includes purchase price, operating, maintenance, energy costs, and recycling costs.
To understand the life-cycle costs, the expected life and long-term reliability has to be understood. Some countries (all of Europe, Japan, and parts of Asia require testing for product reliability and safety.
© il-
fed
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lia
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-1 Stand-Alone Solar Power Systems
Costs including capital and operating expenses can be evaluated with the help of software such as HOMER, a computer simulation tool for designing and analyzing power systems that have various resources (PV, wind, generators, etc.)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-1 Stand-Alone Solar Power Systems
HOMER, simplifies the task of evaluating designs for both stand alone and grid-tied systems for a variety of applications. Some of the issues HOMER can address are:
• the components to include in the design
• quantity and sizes of components
• variations of the resource
• costs including capital cost and operating
cost
• sensitivity of variables to changes (for
example, how does a change in fuel cost
affect the system choice?)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-1 Stand-Alone Solar Power Systems
Many developing countries do not have power in remote villages. Important projects for solar electricity include projects for hospitals and health centers, pumping clean water, and providing power for schools.
Another application in
developing countries is
solar-driven refrigeration
systems based on a solid-
absorption (CaCl2/NH3)
cycle. The benefit include
food and vaccine storage.
Sourc
e:
David
Buchla
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-1 Stand-Alone Solar Power Systems
Codes are standards for the building industry and include the International Building code (IBC) and the National Electric Code (NEC) in the U.S. The NEC has standards for electrical design, installation, and inspection of electrical installations including solar electric systems.
There are NEC standards for
wiring (sunlight, moisture,
etc.) as well as protection
circuits, grounding, surge
arresters, conduit, boxes, and
more. Sourc
e:
NR
EL
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-2 Sizing the Stand-Alone System
Steps:
Site evaluation
Energy audit Initial
concept
Evaluatecabling and
batteries
Determinearray size
Select components
Review design
Sourc
e:
NR
EL
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-2 Sizing the Stand-Alone System
AC Load description Quantity
Power rating
(W)
Time on per da
(h/da)
Energy used
(Wh/da)
Refrigerator 1 450 8 3600
Washing machine 1 500 0.5 250
TV 2 100 3 600
Lights (incandescent) 4 60 6 1440
Lights (fluorescent) 5 30 10 1500
Toaster oven 1 1500 0.5 750
Microwave oven 1 1000 0.4 400
Ceiling fans (medium speed) 3 25 10 750
Computer 2 125 4 1000
Printer 1 400 0.25 100
Miscellaneous loads 1 200 2 400
TOTAL= 10790
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4-2 Sizing the Stand-Alone System
Site evaluation includes insolation data for the site
and other installations issues such as snow or wind
loading, support requirements, shading issues, steep
ground, etc.
Shading problems can be
investigated with a device like
the Solar Pathfinder™. The
Solar Pathfinder™ uses a
polished transparent, dome
that shows a reflected
panoramic view of the site.
Co
urt
esy
of
The
So
lar P
ath
fin
de
r C
om
pa
ny
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-2 Sizing the Stand-Alone System
The size of an array can be determined starting with
the energy audit. The power, in watts, is estimated for
each month by:array
solar sys
WP
t
A site has 6 hours of peak sunlight per day in March.
If 15 kWh is required on an average March day from
a grid-free system, what power is required from the
array for this month? (Assume 65% efficiency.)
15,000 Wh
3846 W = 3.85 kW6 h 0.65
array
solar sys
array
WP
t
P
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4-2 Sizing the Stand-Alone System
Most stand-alone systems require a battery backup.
Battery voltage gradually decreases as the battery
discharges, so there is a minimum voltage that is
useable.
Ahday store
dod inv
W t
VB
The ampere hour requirement can be
estimated with the following formula:
Court
esy o
f S
ola
r D
irect
Ah the required ampere-hours from the batteries,
Wday the daily energy requirement per day in W-h/d,
tstore the backup time required in days,
V the dc system voltage to the inverter in volts,
Bdod the battery’s maximum depth of discharge, expressed
as a fraction, and
hinv the efficiency of the inverter and cabling, also
expressed as a fraction.
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4-2 Sizing the Stand-Alone System
Batteries in solar electric systems should be deep-
cycle types. They must be checked regularly for fluid
level and any potential problem like corrosion or
sulfation (lead sulfate crystals on the positive terminal).
Each day that is sunny, there is daily charging period
and a discharge period. Because of variations in
weather and rate of energy use, the depth of
discharge will vary seasonally but having ample
battery backup will extend the life of the batteries.
The graph on the next slide illustrates the expected
number of cycles versus depth of discharge.
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4-2 Sizing the Stand-Alone System
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-3 Grid-tie PV Solar Power Systems
Grid-tie systems can be set up with or without a
battery backup. The simplest grid-tie system
supplements some fraction of the utility power with
solar power. The major components in this system
are the PV modules and an inverter 逆變器.
For systems that are set up to
send excess power to the grid,
a special inverter is required
that includes a transfer switch.
( S
ourc
e:
NR
EL)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-3 Grid-tie PV Solar Power Systems
A battery-free system is less expensive and easier to
install and virtually maintenance-free. It can offset
any fraction of the utility power and have the utility
make up the difference
The block diagram for a basic grid-tied system is
shown:
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4-3 Grid-tie PV Solar Power Systems
The block diagram for a basic grid-tied system is
shown:
Some loads are backed-up; others are not. This saves
the number of batteries required for backup, reducing
capital and maintenance cost as well as space.
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4-3 Grid-tie PV Solar Power Systems
Parking shelters with PV panels on the roof offer an
excellent match of the need to the resource and
provide power for electric vehicles and for offices
during the day.
Parking shelters
can be grid-tied
systems for the
charging stations
to provide reliable
power on cloudy
days.
( S
ourc
e:
NR
EL)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-3 Grid-tie PV Solar Power Systems
A number of power companies have implemented
solar farms using large PV arrays. An advantage for
utilities is that the rest of their system can act as
backup for when solar power is not available.
Utilities must consider load
balancing, equipment
loading, and power quality
issues, transmission system,
distribution requirements,
and the impact on existing
facilities. ( S
ourc
e:
NR
EL)
Florida Power and Light’s Company’s
DeSoto plant
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4-4 Solar Concentrators
A solar concentrator uses either a mirror or lens to focus
light. With a mirror, on-axis light from infinity reflects to the
focal point(fp), which is ½ the radius of curvature (rc).
With a lens, on-axis light from infinity passes through
the lens to the focal point. Because of costs, lens
systems are not as widely used as mirror systems.
mirror
lens
fp
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4-4 Solar Concentrators
The are a number of variations on mirror systems. Three
types are illustrated here:
In each of these cases, the collector is at the focal point of a
concave mirror. These collectors all work best with direct
sunlight, so use tracking to keep the sun on the target.
Parabolic troughs
used for heating a
fluid
Parabolic mirrors to
focus light to a PV
cell
Parabolic mirrors to
focus light to Stirling
engine.
(Sourc
e:
NR
EL)
(Sourc
e:
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(SolF
ocus, In
c.)
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4-4 Solar Concentrators
The world’s largest solar plant uses a large bank of
heliostats to focus the sun on a tower. The plant is the
Ivanpah Solar Electric Generating System (ISEGS) in
California’s Mojave Desert.
ISESGS is a 370 MW solar
complex of three towers
that receive energy and
use the heat to drive
turbines. The towers are
the same as this one,
photographed in Israel.
Sourc
e:
NR
EL
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4-4 Solar Concentrators
The basic idea of tower power is summarized in the
block diagram:
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4-4 Solar Concentrators
An experimental PV system with circular plastic Fresnel
lenses has been constructed at the University of
Nevada in Las Vegas to focus light onto cells in each
small square. This system is rated at 25 kW for a solar
flux of 850 W/m2.
( S
ourc
e:
NR
EL)
Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd
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4-5 Solar Hot Water Systems
Water heating is a way to significantly reduce energy
consumption and is a proven technology that is a
good match of a resource to a need.
Flat plate collectors
circulate water or fluid in a
manifold. Heat from the sun
is transferred to the fluid.
Normally, pipes are coated
with a material that has high
absorptance and low
emittance.
( S
ourc
e:
David
Buchla
)
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4-5 Solar Hot Water Systems
Solar heat pipes function on an evaporation and
condensation cycle using a non-toxic fluid in the tube.
( S
ourc
e:
David
Buchla
)
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4-5 Solar Hot Water Systems
In areas subject to freezing, a closed-loop pressurized
glycol-water system can be used.
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Selected Key Terms
Band gap
Doping
Fill factor
Hole
PN junction
The amount of energy required to free an
electron from the valence band of an atom. For
silicon, the band gap energy is 1.12 eV.
The process used to increase the conductivity of
a semiconductor in a precise and controlled way.
A vacancy created in an atomic bond when a
valence electron becomes a free electron.
The boundary created between an n-type and
p-type semiconductor.
The ratio of a cell's actual maximum power
output to its theoretical power output.
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Selected Key Terms
Photovoltaic (PV) cell
Solar array
Solar module
Thin-film
A device that converts the energy of sunlight
directly into electricity using a thin layer or wafer
of silicon that has been doped to create a pn
junction.
A combination of solar modules.
Combinations of multiple PV cells connected to
produce a specified power, voltage, and current
output
Types of photovoltaic that use layers of
semiconductor materials from less than a
micrometer (micron) to a few micrometers thick.
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Selected Key Terms
Absorptance
Charge
controller
Combiner box
Depth of
discharge (DOD)
A dimensionless number that the ratio of
absorbed to incident radiation.
A device that regulates and limits charging
current to prevent overcharging batteries.
The ratio, expressed as a percentage, of the
quantity of charge (usually in ampere-hours) removed from a battery to its rated capacity.
A double-insulated box that allows several strings
from modules to be connected together in
parallel; it also houses fuses for the strings and will
include surge and overvoltage protection from
potential lightning strikes.
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Selected Key Terms
Drainback system
Emittance
Ground fault protection
device (GFPD)
Insolation
A solar water heating system in which the
circulating fluid is only circulated when heat is available at the collector ‒ otherwise the
collector and exposed plumbing is drained.
The total flux emitted per unit area from a
material; it is related to the ability of the material
to give off radiant heat.
The word insolation is from “incident solar
radiation” and is a measure of the energy
received on a surface in a specific amount of
time; it can be measured in units of W/m2.
A device that has the following functions: 1)
detect a ground fault, 2) interrupt the current in the line, 3) indicate a fault has occurred with a
visible warning, and 4) disconnect the faulty
module.
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Selected Key Terms
Latent heat of vaporization
Solar
concentrator
Stirling engine
Transfer switch
The heat absorbed or released during a change
of state from a liquid to a gas.
A type of solar collector that collects light over a
certain area and focuses it onto smaller area.
A switch that can switch loads between alternate
power sources without interrupting the current.
A type of heat engine that cools and compresses
a gas in one portion of the engine and expands it
in a hotter portion to obtain mechanical work.
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true/false quiz
1. A pentavalent impurity is added to
silicon to create a p-material.
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true/false quiz
2. The depletion region is between a
p-material and an n-material and is
depleted of charge carriers.
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true/false quiz
3. Photons with less energy than the
band gap for a given material
cannot create an electron-hole pair.
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true/false quiz
4. The highest efficiency PV cells are
multijunction cells.
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true/false quiz
5. Fill factor is the ratio of a cell's actual
maximum power output to its
theoretical maximum power output.
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true/false quiz
6. Dye cells use a heavily doped pn
junction to increase efficiency.
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true/false quiz
7. The cutoff voltage increases for a
solar cell when temperature
increases.
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true/false quiz
8. Current from a solar cell is much
lower with increasing temperature.
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true/false quiz
9. Solar module data sheets show the
rated voltage and current under
standard test conditions.
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true/false quiz
10. CPV systems respond best to diffuse
solar energy.
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true/false quiz
Answers:
1.F
2.T
3.T
4.T
5.T
6.F
7.F
8.F
9.T
10. F
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Multiple choice quiz
1. Silicon can be doped with
A. Boron
B. Gallium
C. Bismuth
D. Any of these
71
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2. The output voltage of a PV cell increases slightly with
A. Surface area
B. Light intensity
C. Wavelength
D. None of these
72
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3. Compared to a single PV cell, 32 PV cells connected in parallel to a specified load means that the
A. Output voltage increases
B. Output current increases
C. Efficiency increases
D. Output power decreases
73
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4. Light with a wavelength of 500 nm is
A. Ultraviolet
B. Visible
C. Infrared
D. Microwave
74
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5. Light energy per photon increases with
A. Lunar position
B. Frequency
C. Wavelength
D. Cloud cover
75
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6. A higher fill factor indicates
A. Higher output power
B. Higher output voltage
C. Larger surface area
D. More semiconductor layers
76
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Answers
1. D
2. B
3. B
4. B
5. B
6. A
77
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Example:
Determine the maximum wavelength of sunlight that can be absorbed by a PV cell if the band gap energy is 1.5 eV?
Solution:
E = 1.5 eV
𝜆 =1240 𝑒𝑉∙𝑛𝑚
𝐸=
1240 𝑒𝑉∙𝑛𝑚
1.5 𝑒𝑉≈ 826.7 nm
78
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Example:
The open circuity voltage is 0.611 V and the short circuit current is 3.5 A. The maximum power of the solar cell is 1.176 W. Determine the fill factor of the solar cell.
Solution:
Voc = 0.611 V
Isc = 3.5 A
MPP = 1.176 W
FF = 𝑀𝑃𝑃
𝑉𝑜𝑐𝐼𝑠𝑐= 1.176/(0.611*3.5) = 0.55
79
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Example:
Compare the life cycle costs of the two PV products and determine which one you want to buy. Assume the life span of the two products is same and it is 20 years. The electricity is 1.5 HK$ / kWh.
80
Item PV 1 PV 2
Price 12,000 HK$ 10,000 HK$
Electricity
generated per
year
1000 kWh 800 kWh
Maintenance
fee per year
100 HK$ 150 HK$
Disposal fee 200 HK$ 250 HK$
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81
Item PV 1 PV 2
Price 12,000 HK$ 10,000 HK$
Electricity fee
saved for 20
years
30000 HK$ 24000 HK$
Maintenance
fee for 20 years
2000 HK$ 3000 HK$
Disposal fee 200 HK$ 250 HK$
LCC -15,800 HK$ -10,750 HK$
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