Lecture 3 2015_2016

Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd

Copyright © 2015 by Pearson Education, Inc.All Rights Reserved

Renewable Energy

Systems3

Dr. Caroline Dong



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



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



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



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)



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



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



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.

So

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1 eV = 1.6E-19 Joule



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



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



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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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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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.


Sourc

e:

David

Buchla



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



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




Maximum power occurs on the “knee” of the I-V curve.

P = VI

MPP = 0.53 V* 2 A

= 1.06 W




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.




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




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



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.

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One type of module can blend in with roof shingles.

Each module is rated for 17 W.

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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.

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REL



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.




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.




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:



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



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



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




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:




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-

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oto

lia




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.)




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?)




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




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



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




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




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




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




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.




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.






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)




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:




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.




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)




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



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




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:

NR

EL)

(SolF

ocus, In

c.)




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




The basic idea of tower power is summarized in the

block diagram:




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)



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

)




Solar heat pipes function on an evaporation and

condensation cycle using a non-toxic fluid in the tube.

( S

ourc

e:

David

Buchla

)




In areas subject to freezing, a closed-loop pressurized

glycol-water system can be used.

© 2015 by Pearson Higher Education, Inc.



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.



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.



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.



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.



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.



true/false quiz

1. A pentavalent impurity is added to

silicon to create a p-material.



true/false quiz

2. The depletion region is between a

p-material and an n-material and is

depleted of charge carriers.



true/false quiz

3. Photons with less energy than the

band gap for a given material

cannot create an electron-hole pair.



true/false quiz

4. The highest efficiency PV cells are

multijunction cells.



true/false quiz

5. Fill factor is the ratio of a cell's actual

maximum power output to its

theoretical maximum power output.



true/false quiz

6. Dye cells use a heavily doped pn

junction to increase efficiency.



true/false quiz

7. The cutoff voltage increases for a

solar cell when temperature

increases.



true/false quiz

8. Current from a solar cell is much

lower with increasing temperature.



true/false quiz

9. Solar module data sheets show the

rated voltage and current under

standard test conditions.



true/false quiz

10. CPV systems respond best to diffuse

solar energy.



true/false quiz

Answers:

1.F

2.T

3.T

4.T

5.T

6.F

7.F

8.F

9.T

10. F



Multiple choice quiz

1. Silicon can be doped with

A. Boron

B. Gallium

C. Bismuth

D. Any of these

71



2. The output voltage of a PV cell increases slightly with

A. Surface area

B. Light intensity

C. Wavelength

D. None of these

72



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



4. Light with a wavelength of 500 nm is

A. Ultraviolet

B. Visible

C. Infrared

D. Microwave

74



5. Light energy per photon increases with

A. Lunar position

B. Frequency

C. Wavelength

D. Cloud cover

75



6. A higher fill factor indicates

A. Higher output power

B. Higher output voltage

C. Larger surface area

D. More semiconductor layers

76



Answers

1. D

2. B

3. B

4. B

5. B

6. A

77



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



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



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$



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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