lecture 2 2015_2016

Renewable Energy SystemsDavid Buchla | Thomas Kissell | Thomas Floyd

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

Module:SEV5371:

Sustainable Energy Systems

1

Dr. Caroline Dong

PhD BEAM Pro





Renewable Energy

Systems2





2Electrical Fundamentals

2-1 ENERGY, CHARGE, AND VOLTAGE

2-2 ELECTRICAL CURRENT

2-3 RESISTANCE AND OHM'S LAW2-4 POWER AND WATT'S LAW

2-5 SERIES AND PARALLEL CIRCUITS

2-6 CONDUCTORS, INSULATORS, AND SEMICONDUCTORS

2-7 MAGNETISM AND ELECTROMAGNETIC DEVICES

2-8 CAPACITORS , INDUCTORS, AND TRANSFORMERS

2-9 PROTECTIVE DEVICES

2-10 BASIC ELECTRICAL MEASUREMENTS

Chapter Outline



2-1 Energy, Charge and Voltage

• Work is done when a force is applied over a distance. To do work, you must have both force and distance.

4

d

F

The work to move the box was done against friction and is the product of force, F, and distance, d.

You can also do work in lifting the box; in this case the work would be the force exerted against gravity multiplied by the height, h, it is lifted.




• Energy is the ability or capacity for doing work; it comes in three major forms; potential, kinetic, and rest.

Potential energy is stored energy. An example is the

water stored behind a dam because it can do work.

The equation for gravitational

potential energy is WPE = mgh (g=9.8m/s2)

Kinetic energy is the energy of

motion. An example is the motion

of wind or water. The equation for

kinetic energy is WKE = ½ mv2.

Rest energy is the equivalent energy of mass as expressed by E = mc2.

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• Energy is measured in units of the joule. A joule is a small amount of energy; it is the work done in lifting a 1 newton (n) weight (about 3.6 oz) 1 meter (m).

• n = newton; unit of force in SI system.

• From Newton’s second law, W=Fd=(mg)h in a gravitational field

1 n

1 mHow much energy is expended in lifting an 8000 n piano a height of 0.5 m?

The energy is the product of the force and distance. In this case, the distance moved is the height, h. W = Fh = (8000 n)(0.5 m) = 4000 J.




• The basic unit of electrical charge is the coulomb, symbolized by the letter Q.

7

Voltage, symbolized by V, is defined as energy per unit

charge. The volt is the unit of voltage symbolized by V.

The formula for voltage is V = W/Q where W = energy in

joules and Q = charge in coulombs.

a) Battery (b) Graph of voltage versus time (c) Symbol



• Example:

What is the voltage if the energy available is 100 J and the total charge is 5 C?

• Answer:

Q=5 C W=100 J

V = W/Q = 100 J / 5 C = 20 V

8





• AC is alternating current and follows a sinusoidal pattern. In North America, the utility frequency, f, is 60 Hz, meaning there are 60 cycles in one second. In many countries, the frequency is 50 Hz. In Hong Kong, the frequency is 50 Hz. The period, T is 1/f.

9

T

What is the period if f = 60 Hz?

T = 1/f = 1/60 Hz = 16.7 ms



• Example:

If the period of an AC voltage is 0.05s, determine the frequency:

• Solution:

T = 0.05s

f = 1 / T = 20 Hz

10




2-2 Electrical Current

Current is symbolized by I and its unit is the ampere (A). Conventional current is based on the assumption that charge moved from positive to negative by definition. Electron flow is just opposite to this definition and is negative to positive. Either can be used, but it is important in some cases to know which is referred to.

The basic formula for current is I = Q/t, where Q is the

charge in coulombs and t is the time in seconds.



2-2 Electrical Current

• AC can be compared to dc (direct current) by their equivalent heating value (power). If the ac is specified as rms (root-mean square) current or rms voltage, the result is equivalent to a direct current or direct voltage.

12

DC voltage of 1 V

AC voltage of 1 Vrms

Note that the peak is 1.41 times higher than the rms value for the ac waveform.



2-3 Resistance and Ohm’s Law

Resistance is the opposition to current. Except for superconductors, all materials have resistance.

Fixed resistor Symbol

Fixed resistors are components that have resistance that cannot be altered:

Variable resistors are components with resistance that can be altered:

Variable resistor (to control current) Symbol




Resistance is the opposition to current symbolized by R and the unit ohm, symbolized by Greek letter Ω.

One ohm (1 Ω) of resistance exists when there is one ampere of current in a material with one volt (1 V) applied across the material.




Wire size

AWG

Current

capacity

copper

wire

(amps)

Resistance

per 1000

feet (ohms)

Resistance

per km

(ohms)

16 15 4.016 13.176

14 20 2.525 8.284

12 25 1.588 5.210

10 30 0.9989 3.277

8 40 0.6282 2.061

6 55 0.3951 1.296

4 70 0.2485 0.815

3 85 0.187 0.614

2 95 0.1563 0.513

1 110 0.1239 0.406

1/0 125 0.0983 0.323

2/0 145 0.0779 0.256

3/0 165 0.0618 0.203

4/0 195 0.049 0.161

Wire size and resistance is related to the current carrying capacity of wires. Wire resistance is particularly important when long runs of wire are involved or when a long winding is part of a motor, generator, or other component.

What is resistance of 15 km of 1/0 wire?

R = 0.323 W/km. (0.323 W/km)(15 km) = 4.9 W




Ohm’s law is the most important law in electronics. It

indicates the relationship between voltage (V), current

(I) and resistance (R).

Three forms are illustrated:

To solve for current,

To solve for voltage, V = IR

To solve for resistance,



Example• Determine the voltage when the current is 2 A and

the resistance is 10 W

• Solution

I = 2 A R = 10 W

V = I R = 2 A* 10 W = 20 V

17



2-4 Power and Watt’s Law

Power (P) is the rate at which energy is expended. Rate always involves time (t), so power is expressed as

where P = power in joules when W (energy) is in newton-meters and t is in seconds

1 2

Two identical weights are lifted the same distance but in different times. Compare the energy required and the power expended.

The energy expended is the same; the one lifted in the shortest time requires the greater power.



• Example

Determine the power in watts if 72 kJ of energy are used in 1 hour.

• Solution

• P = W/t = 72 kJ/1 hour = 72,000 J / 3600 seconds

= 20 W

19




In the electrical field, power is often expressed in units of kilowatts(1000 watts) and megawatts (1,000,000 watts).


The power company does not charge for power, but for energy. In the electrical field, energy is expressed as kilowatt-hours (kWh) or megawatt-hours (MWh)

What is the energy used if five 60 W bulbs are on for three hours?

The total power is 300 W. The energy used is (300 W)(3 h) = 900 Wh = 0.90 kWh.

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Watt’s law formulates the relationship between power

voltage and current. Three forms of Watt’s law are:


(a) What is the power used in a heater if 120 V is applied and the current is 8 A?

(a) The power is P = VI = (120 V)(8 A) = 960 W.

(b) What is the resistance of the heater?

(b) The resistance is

2120

15 960

= = WV

W

2VR =

P

P=VI P=I2R P=V2/R V=IR



In a series circuit, there is only one path for current, so current is the same everywhere in the circuit.

2-5 Series and Parallel Circuits

2.0 mA

The reading on the first ammeter is 2.0 mA,

What do the other meters read?

2.0 mA

2.0 mA2.0 mA



The total resistance in series is the sum of the individual

resistances: RT = R1 + R2 + R3 +......+ Rn and the total

voltage is the sum of the individual voltages:

VS = V1 + V2 + V3 +......+ Vn


(a) What is the total resistance?

(a) RT = R1 + R2 + R3 = 6 W + 6 W + 12 W = 24 W

(b) What are the voltage drops?

(b) From Ohm’s law, I = 0.5 A and VS = 12 V.

VS = V1 + V2 + V3

= IR1 + IR2 + IR3

= (0.5 A)(6 W) + (0.5 A)(6 W) + (0.5 A)(12 W)

V1 = 3 V, V2= 3 V, V3 = 6 V

I=V/R




VT = 6 (18 V) = 108 V

The total voltage from solar modules or batteries is

the sum of the individual voltages. If six modules are

wired in series and each module has 18 V output,

what is the total output voltage?

When you need to increase the output voltage, connect sources in series, but be aware of safety issues.



The total resistance in parallel is the reciprocal of the

sum of the reciprocals of the individual resistances:


(a) What is the total resistance?

(a)

(b) What is the current in each resistor?

(b) From Ohm’s law, I1 = 2.0 A, I2 = 0.625 A, I3 = 1.0 A,

= = W

W W W

T

1 2 3

1 12.76

1 1 1 1 1 1 1

5.0 16 10 n

R =

+ + + ...+ + +R R R R

T

1 2 3

1

1 1 1 1

n

R =

+ + + ...+R R R R



If equal sources are wired in parallel, the total output

voltage is the same as any one source. The advantage

to parallel wiring is an increase in ability to supply

current.


Assume six 18 V solar modules are wired in parallel. What is the output voltage? How will the ability to supply current change?

The output voltage is 18 V,

but taken together, the

ability to supply current

has increased by a factor

of six over a single module.



Conductors are materials that allow the free movement

of charge. Metals tend to be good conductors

because many electrons can move freely in the

metallic crystal. These electrons are called conduction

electrons and they are not bound to a particular atom.

2-6 Conductors, Insulators and Semiconductors

In liquids, the moving charge is composed of positive and negative ions, never electrons. Materials known as electrolytes(電解質) form ions in water solution and are good conductors.



Insulators are materials that prevent the free movement

of charge. The outer shell electrons that are normally

involved in chemical bonds are called valence

electrons (價電子). These electrons are generally not involved in conducting charge in the solid.


Comparing the energy

diagrams of conductors and

insulators reveals that the

electrons must acquire much

more energy to be in the

conduction band of insulators

than in conductors. In metals, electrons can easily acquire

sufficient energy to become

conduction electrons.Conductor Insulator




Cables and wire are examples

where high quality conductors

and insulators are needed.

Insulators are used as a

protective covering for cables

and wires. Coaxial cable (同軸電纜) is an example; it has an inner conductor, an insulation

layer, an outer braided

conductor that normally is

connected to ground, and a

outer insulator.

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A semiconductor is a crystalline material that has

properties between those of conductors (metals) and

insulators (nonmetals). For electronics, silicon (Si) is the

most widely used semiconductor. For semiconductors

to be useful, impurities are added creating two

important classes of materials: p-materials (positive)

and n-materials (negative).


An important semiconductor device is a diode (二極管), which has a p-material on one side and an n-material on the other side in one crystal. A few representative diodes are shown.



Diodes allow current in one direction only, so are important in converting ac to dc.


There are several types of specialized

diodes. A PV cell is a special diode

that converts sunlight to electricity.

PV cells form the basis of larger

modules, which are connected

together in many solar energy

systems.

Symbol for a PV cell

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David

Bu

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la

© 2015 by Pearson Higher Education, Inc.

http://en.wikipedia.org/wiki/File:Photovoltaic_cell.svg

http://en.wikipedia.org/wiki/File:Photovoltaic_cell.svg



An important semiconductor is the transistor (晶體管). A basic bipolar transistor is a sandwich of alternating n-and p-material. It can amplify signals or is often used in switching applications.


Another important semiconductor is the thyristor (晶閘管). These are generally four-layer devices that are used to control power. In renewable energy systems, they are frequently used in charging circuits.



All magnetic fields have their origin in moving

charge, which in solid materials is caused by moving

electrons. In certain materials, such as iron, atoms

can be aligned so that the electron motion is

reinforced, creating an observable field that extends

in three dimensions.

2-7 Magnetism and Electromagnetic Devices

Magnetic fields are

described by flux lines.

Here the lines surrounding

two magnets are

visualized with iron filings

and tend to reinforce.

Sourc

e:

David

Buchla



A magnetic field surrounds current carrying wire, forming a circular pattern. It can be visualized with iron filings. Here the wire goes through a paper plate with iron filings in it.


To produce the pattern on the plate, a very high current is required.

By forming the wire into a coil, and placing it into a magnetic material, useful devices can be formed.

Sourc

e:

David

Buchla



Generators are electromagnetic devices of great importance in renewable energy systems, so will be covered in Chapter 13. Generators are spun by an energy source and produce electricity.


Generators produce electricity when a conductor moves perpendicularly to a magnetic field. A common type of generator rotates a coil in a magnetic field to produce a sine wave.

Generators at Hoover Dam © tomalu/Fotolia



2-8 Capacitors, Inductors, and Transformers

A capacitor is an electrical device that stores energy

in the form of an electric field established by electrical

charge. In its most basic form, the capacitor is

constructed of two conductive plates placed

physically in parallel and separated by an insulating

material called the dielectric (電介質).

A representative capacitor is shown. This is a mica capacitor consisting of alternating conductive and dielectric layers.




The amount of charge that a capacitor can store per unit of voltage across its plates is its capacitance (C).C = Q/Vwhere C is capacitance in farads, Q is charge in coulombs, and V is voltage in volts.

The energy stored is

W = (1/2)CV2

where C is capacitance in farads, V is voltage in volts

and W is energy in joules.

Currently, there is research on supercapacitors for energy storage; they have significant advantages over batteries, with much longer lifespans.



• Example

• Determine the energy stored by a 10 uF capacitor with 24 V across it

• Solution

• W = ½ (CV2) = ½ (10* 10-6 F * (24V)2) = 2.88 *10-3 J

38




Supercapacitors (AKA ultracapacitors) can be thought of as two nonreactive porous carbon electrodes suspended within

an electrolyte. The electrodes are

made from porous carbon

separated by about 1 nm!

Researchers at Vanderbilt University have

reported they have found a novel way to construct silicon-based

supercapacitors. The supercapacitors might be integrated into

solar cells along with the microelectronic circuitry that it powers them. This could lead solar cells that can store energy.




Inductance (電感) is the property of a wire conductor to oppose a change in current. An inductor is basically a length of insulated wire formed into a coil that intensifies the magnetic field.

When the current through a coil changes, an induced voltage is created across the coil in a direction that always opposes the change in the current.

© zigzagmtart/Fotolia




A transformer is a device formed by two or more coils (windings 繞組) magnetically coupled to each other to provide for transfer of ac power electromagnetically from one winding to the other.

In power applications,

transformers are used

to change ac voltage

levels from one value

to another.

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Turns Ratio

• The secondary voltage may be larger, smaller or equal to the primary voltage; it depends on the ratio of the number of turns in the secondary winding to the number of turns in the primary winding.

• Turns ratio: the number of turns in the given secondary divided by the number of turns in the primary

• n = Nsec / Npri

42



2-9 Protective Devices

Fuses (保險絲) and circuit breakers (斷路器) are placed in series with circuits and are used to create an open (break in the circuit) when the current exceeds a specified number of amperes.

A fuse is a one-time device that must be

replaced when it overheats and “blows”.

Fuses come in a large variety of sizes.

A circuit breaker will “trip” when

excess current is detected. After the

condition is corrected, the circuit

breaker can be reset manually.




A GFCI (ground fault circuit interrupter) is a circuit breaker that is used to protect from severe or fatal electric shock.

The GFCI monitors the difference

between the hot and neutral

currents and trips the breaker if

they differ because the current is

returning to the source via a

ground connection.

Sourc

e:

David

Buchla




A switch is a device that controls a circuit by opening

or closing contacts. A pole is a contact and a throw is

the movable part. Switches are classified by the

number of poles and throws. Pushbutton switches are

either normally open (NO) or normally closed (NC).

From the definition, determine what each type of switch in terms of poles and throws: (S: single, D: double, P: pole, T: throw, PB: push button)

SPST SPDT DPST

DPDT NOPB NCPB



2-10 Basic Electrical Measurements

The digital multimeter DMM is the most widely used electronic-

measuring instrument. It can be used to measure voltage, current

and resistance. Many DMMs can measure other quantities as well.

To use a DMM. first select the quantity to be measured. For current

or voltage, select ac or dc.

To measure voltage, connect the meter in parallel with the voltage to be measured.

To measure current, connect the meter in series with the component to be measured.

To measure resistance, disconnect the resistor from the circuit and place the leads in parallel with the resistor.



2-10 Basic Electrical Measurements

The clamp meter is a type of DMM that does not require that the circuit is opened for current measurements. The sensing element is a set of jaws that are opened or closed around a single conductor.

As in the case of a standard DMM, the

quantity to be measured is selected.

For current or voltage measurements,

either ac or dc is selected.

Source: Fluke Corp.



Assignment 1

• Moodle system

• Submission deadline: Sept 30 (Wed) 2015

48





Selected Key Terms

Ampere

Current

Digital multimeter

(DMM)

Energy

Joule

The unit of current symbolized by I.

The flow of electrical charge past a specified point in a circuit.

The ability or capacity for doing work.

The SI unit of energy. The work done when 1 newton of mechanical force is applied over a distance of 1 meter.

An instrument that can measure voltage, current, and resistance.





Selected Key Terms

Kilowatt-hour

magnetic flux

density

Ohm

Ohm's law

Parallel circuit

A unit energy. The energy used when one thousand watts of power are expended in one hour.

The amount of flux, f, per unit area (A) perpendicular to the magnetic field.

The unit of resistance.

A type of circuit connection where two or more components or loads are connected across a common voltage source.

A circuit law that specifies the relationship between voltage, current and resistance as a mathematical formula.





Selected Key Terms

Power

Resistance

Series circuit

Sinusoidal wave

Voltage

Watt’s law

The rate at which energy is expended.

A circuit law that expresses the relationship of voltage, current, resistance, and power as a formula.

The opposition to current.

The cyclic pattern of ac voltage or current. Also known as a sine wave.

Energy per unit charge

A type of circuit connection in which there is a single complete path (forming a string) from the voltage source and through the load (or loads) and back.





true/false quiz

1. You are doing work if you push on a

car, but it won’t budge.





true/false quiz

2. Three forms of energy are potential, kinetic, and rest.





true/false quiz

3. The unit of current is the coulomb.





true/false quiz

4. One form of Watt’s law can be

expressed as W = IR2.





true/false quiz

5. AWG #6 wire is smaller than AWG #4

wire.





true/false quiz

6. Ohm’s law can be written as V= IR.





true/false quiz

7. The current in a parallel circuit is the same everywhere.





true/false quiz

8. Diodes allow current in one direction only.





true/false quiz

9. When there is current in a wire, there

is always a magnetic field present.





true/false quiz

10. A capacitor can store more charge if it is charged to higher voltage.





true/false quiz

Answers:

1.F

2.T

3.F

4.F

5.T

6.T

7.F

8.T

9.T

10. T



Multiple choice

1. The unit of energy is

A. Watt

B. Joule

C. Coulomb

D. kilowatt

63



2. A coulomb is the unit of

A. Current

B. Energy

C. Power

D. charge

64



3. The unit of current is

A. Ohm

B. Joule

C. Ampere

D. Watt

65



4. Power is the

A. The same as energy

B. Rate at which energy is used

C. Same as force

66



5. When several solar cells are connected in parallel

A. The voltage is increased

B. The available current is increased

C. Both the voltage and current is increased

67



6. A transformer can

A. Step up ac voltage

B. Step down ac voltage

C. Operate with dc voltage

D. Both A and B

68



1. B

2. D

3. C

4. B

5. B

6. D

69



•Any questions regarding the Assignment 1?

70

lecture 2 2015_2016

Documents

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energy of motion

rest energy

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8ms2kinetic energy

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