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Energy Conversion and

Efficiency

A course for Engineers

By

Dr. Hasan Maridi Assistant Professor at alternative energy technology,

Faculty of Engineering and Technology, Philadelphia University

https://sites.google.com/site/hasanmaridi http://www.philadelphia.edu.jo/academics/hmaridi/

2nd edition, 2019

Energy Conversion and Efficiency Dr Hasan Maridi https://sites.google.com/site/hasanmaridi/

http://www.philadelphia.edu.jo/academics/hmaridi/

Course Description: A course developing the basic ideas of energy and power and their applications from a quantitative and qualitative viewpoint. The fossil fuels (coal, oil, natural gas) are studied together with their societal limitations. Nuclear power is similarly studied in the context of the societal concerns that arise (radiation, reactor accidents, nuclear weapons proliferation, high-level waste disposal). The opportunities as well as the risks presented by alternative energy resources, in particular solar energy, wind, geothermal, and hydropower, together with various aspects of energy storage and conservation, are developed and discussed. 2

References 1. Energy: Production, Conversion, Storage, Conservation, and

Coupling, by Yaşar Demirel, Springer, Berlin Heidelberg, 2nd

edition, 2016.

2. Energy Conversion, by Archie Culp, McGraw-Hill,1991

3. Energy Conversion, edited by D. Yogi Goswami and Frank Kreith,

CRC Press, Taylor & Francis Group,©2008.

4. Energy Conversion: Systems, Flow Physics and Engineering, by

Reiner Decher. (Oxford Engineering Science Series),

5. Energy and Fuels in Society, by Ljubisa R. Radovic, McGraw-

Hill,1997.

6. Lecture notes “Energy and Society”, by Professor Ani

Aprahamian, 2012

7. Lecture notes “Energy Engineering”, by . Subramanian, 2010.

8. Lecture notes “Energy Conversion and utilization” Mohsen F.

Morsi, 2014.

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Course Topics

Grading policy: First Exam 20% Second Exam 20% Homeworks 5% Quizzes 7% Project 8% Final Exam 40%

1-Introduction: Energy units, types, and sources

2- Energy conversion laws and thermal Energy,

Efficiency of energy conversion, Heat engines

3- Fossil Fuels: Coal, Petroleum, Natural gas

4- Nuclear: Energy from Fission

5- Renewable energy I: Hydro, Tidal, Wavepower,

Biomass, Geothermal

6- Renewable energy II: Solar: PV and thermal

7- Renewable energy III: Wind

8- Direct energy conversion methods, including,

Photovoltaics, Fuel Cells, Thermoelectric

conversion, Thermionics and MHD.

9- Energy storage categories: electrical,

electromechanical, mechanical, direct thermal,

and thermochemical

10-Energy Conservation, increasing of efficiency.

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Aspects of Energy Definition Types sources Supply and demand efficiency conversion Energy demands in industry, Fossil Fuels Coal Natural gas oil

What about Nuclear Energy?

Alternative energy resources and resources needed solar energy wind geothermal hydropower new discoveries that change everything?

energy storage and conservation as a solution? 5

Section 1 Energy definition Energy types

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What is the Energy?

Energy is involved in all life cycles, and it is essential in Engineering

as much as in all other productive activities. An elementary food

chain already shows the need for energy: crops need energy from

solar radiation to grow, harvesting needs energy from the human

body in work, and cooking needs energy from biomass in a fire. The

food, in its turn, provides the human body with energy.

In order to support any industrial operation, equipments and tools

are used, the production of which also requires energy.

Energy lights our cites, power our vehicles, runs machinery in

factories, Energy warms and cools our homes, cooks our food,

plays our music, …. Energy is defined as the ability to do work 7

Velocity (v)= Δx/Δt Acceleration(a)= Δv/Δt Force(F)= ma=massx acceleration Force required to change motion

Motion …

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Energy Basics…

first displacement, velocity, acceleration

Energy is the ability/capacity to do work Work is the transfer of energy ……..

Work = force x distance Angle between force and distance

Units: newton-meters,

1 J = 1 N m

A force of 1 N moving a body over 1 m does 1 J of work.

Power = the rate at which work is performed. 9

MORE DEFINITIONS:

Familiar units of power: kilo-watt, kW 1hp = 746 W

Energy = power x time =kWh is what you get charged for…

Power: rate at which energy is converted to work

Power =energy time

Units: Joules per second, J/s, or watts (W)

• 1 ton of refrigeration = 50 kcal/min = 1200 BTU/h • 1 Watt = volt x ampere

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Work = force . Distance

NO work is done if the object does not move.

If the force is perpendicular to the direction of motion NO

work is done by the force.

Energy can be kinetic (associated with motion) Energy can be potential (ability to do work stored in some form)

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Potential Energy:

It is the stored energy as a result to an object position or structure,

such as: stretching a rubber band, pulling back on a bow’s arrow,

and lifting a brick high in the air. Also, the water falling from a

definite head to the sea level has a potential energy; in this case it is

called gravitational potential energy. More potential energy is

available when there is more water and when it is at a higher height.

Gravitational Potential Energy PE mgh

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Kinetic Energy:

Is the energy of motion, as in wind or in a water stream. The

faster the stream flows and the more water it has the more energy it

can deliver. Similarly, more wind energy is available at higher wind

speeds, and more of it can be tapped by bigger windmill rotors

dimensions. Kinetic energy depends on both mass and velocity.

K = ½ mv2

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Example

A rubber ball of mass 30kg falls freely from a height of 20

meters, at position- 1, shown above in the previous Fig.

Compute the object speed, KE, and PE at the three positions

indicated above, if position-2 at the mid height between free

falling and impact position. Find the falling time in both

positions.

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Energy Forms and States

There are six different forms of the energy in the nature which are:

1) Chemical Energy: (coal, oil, natural gas) Is the energy that bonds the material atoms together, and when atomic bonds broken, this energy is released.

For example, when an explosive goes off, chemical energy stored in it is transferred to the surroundings as thermal energy, sound energy and kinetic energy.

Also, the dry wood is a store of chemical energy.

As it burns in the fireplace, chemical energy is

released and converted to thermal energy (heat)

and light energy.

Food is also a good example of stored chemical energy. This energy is

released during digestion. Molecules in our food are broken down into

smaller pieces. As the bonds between these atoms break, a chemical

reaction will occur, and the energy is released. 17

2) Thermal Energy-Heat: Is caused by the internal motion of

the atoms,

because moving particles has

friction producing heat. A larger

body, or larger quantity contains

more thermal energy-heat is

obtained.

Note: Thermal energy is

the total kinetic energy of

moving particles of matter.

Also chemical energy is stored Batteries.

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3) Electrical Energy: Is resulting from the motion of electrical charge between two

positions, which is referred to the potential difference of these two

positions. Electrical generator and electric storage battery fed the

required potential difference causing electrical energy. The higher

the potential difference resulting more charges to move, or, the

higher current capacity and the more electrical energy is made

available, electrical energy = qV

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4) Mechanical Energy: Is the energy gained-by or exerted-on an object

to do work. This energy is depending on the

object dimension as well as on the methods of

doing this work (walking, position change, lifting,

etc.),

Note: Mechanical energy is the sum of kinetic

and potential energy in an object that is used to

do work.

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Electromagnetic radiation = hf

5) Electromagnetic or Radiation Energy: Is the energy travelled in the form of waves,

such as the radiation from: the sun, the light or

a fire. These radiant

energies give lighting or warming sensation.

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When nucleons come together, the mass of the product is less than the sum of the masses of individual nucleons.

Mass energy

E = mc2

Energy= (change in mass) c2

Nuclear energy depends on the conversion of mass

into energy

6) Nuclear Energy: This energy is stored in the nucleus of an atom, and when nucleus

splits or fused nuclear energy is released in the form of heat and

light energies. Physics proved that, the sun’s energy is produced

from a nuclear fusion reaction in which hydrogen nuclei fuse to form

helium nuclei.

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Section 2 Energy units

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Traditional energy, Renewable energy, New and renewable, Commercial and non-commercial energy, which will be defined and explained in details in next chapters.

There are many other important terminologies which are:

Energy Units

The physical quantities classified into basic and derived quantities. The International system of units (SI) of the fundamental basic quantities is shown in Table.

Table, Basic quantities and its SI units

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Rather than the above quantities, any another physical one can be derived from the above basic quantities, as well as its unit. The relationship between some derived SI units and the basic SI units are represented in Table.

Table, Engineering derived quantities and its SI units

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Rather than the “Joule”, and in some countries, or in a particular context and systems, different units are used. These particular units and their conversion factors to-or-from SI units are given in Table.

Energy conversion factors to SI units

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Power conversion factors to SI units

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The powers of ten are often abbreviated by writing prefixes before the unit. For

instance, the symbol G stands for Giga, which means 10 to the power 9, i.e. a

billion. One billion W is then written as 1 GW (one Giga Watt). Common prefixes of

the power of 10 are given in Table.

Prefixes of the power 10

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In addition to the previous energy unit, the energy as heat measured in:

■ Calorie (C): the amount of heat energy required to raise the temperature

of 1 gram of H2O from 14.5oC to 15.5oC, 1C = 4.187 J

■ A food calorie (c): is 103 calories, 1 c = 1000C = 103C = 4.187 KJ

■ BTU(British Thermal Units): the amount of heat energy required to

raise the temperature of 1 lb. of H2O from 63oF to 64oF, 1 BTU

=1.055 x 103 J = 1.055 KJ

■ The specific heat capacity (C) is defined as: the energy per kilogram of

material, per the incremental change in the temperature, i.e.

■ The tonne of oil equivalent (toe) is a unit of energy: the amount of

energy released by burning one tonne of crude oil, equals 42.6 GJ

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Example-2

Compute the energy required to raise the temperature of 1Kg of wood and water from 20oC to 25oC, if Cwood = 1700, Cwater = 4200 J/Kg oC. Compare the obtained results.

Solution

TEwood = C x m x = 1700 x 1 x 5 = 8500 J = 8.5 kJ

TEwater = C x m x = 4200 x 1 x 5 = 21000 J = 21 kJ

As computed the water need more energy, approximately 2.5

times.

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Example-3 A student eats 1000 food calorie meal consisting of pizza, soda, and snacks. How many 100kg must to be lifted based on this meal (just to lift the weight assuming no losses)?

Solution: Total energy eaten (ET) = 1000 food cal. x 103 cal. = 1 x 106 cal. = 4.186x106 J The work done to lift a weight (m) is against gravity: WD = mxgxh Assume: h = 2.0 meters. Hence, the work done to lift 100Kg is: WD=100x9.81x2=1962 Nm (J) This mail enough to lift this weight n-times, n = ET/ WD = 2133 times

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Example-4 For the student of Example-3, if the training course time is 3-hrs. Compute the time of one lift. If replacing the student by a robot equipped by an electric motor to do this job, find the motor required horsepower.

Solution

The numbers of lifting is approximately 2133 per 3-hrs,

Then the time required for one lifting assuming regular potential is

tLift = (3x60x60) / 2133 = 5.06 sec

The power required for the robot’s motor, P = Energy/ time

For No Losses P = 4.186x106 / (3x60x60) = 387.6 watt = 0.52 Hp « 0.5 Hp

Assuming there are losses (effic.=80%),P = 0.52/0.8 = 0.65 Hp » 0.75 Hp

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Example-5 A man does very little physical work needs about 2,000 Kcal of energy in his daily food, the energy of this food converted into thermal energy and heat. Compute the equivalent power in watt required to feed this energy.

Solution

The daily energy needed is:

ETC= 2000 Kcal = 2x106Cal = 2x106 x 4.184 J

The power is defined as the energy per unit time, hence the equivalent

power is computed as:

PT= Ej/time = (2x106x4.184 J) / (24x60x60) sec = 96.852W

It is see that a human body doing no work is equivalent to a

power-heat source of about 100 W, which is very little power required (no

load). 33

Example-6

A mountaineer has 100Kg, can climb 300m uphill in 1-hr. Compute

the power and consumed energy if he is climbing for 3-continuos

hours.

Solution

The energy:Ej = WD = 100x9.81 x300 x3= 882.9 KJ

The power is:P = Ej / time = (882.9) / (3x60x60) = 0.0818 kW=81.8W

This amount of energy is equivalent to food calories

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Example-7 A person uses on average nearly 1000MJ/day. This energy used for motion, transport, heating and domestic purposes. Compute his daily power need, his consumed energy per month. How many dollar he pay for this energy if the average cost of electricity is $0.1/KWh.

Solution

The daily total energy: ED = 1000MJ= 109 J/day

The needed power is: P = ET / time= (109) / (24x60x60) = 11574.1 W

The energy per month is: ED = 109 J/day x 30 / (1000x60x60) = 8333.4 KWh

The monthly cost of this energy is: cost = 8333.4 x 0.1 = 833.34 $

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Example-8 An electric lift rated at 110 V, 30 A raises a 700 kg load a height of 20m in two minutes. Considering the electrical energy input and the potential energy gained by the mass is the useful output energy, determine the percentage efficiency of this energy transformation.

Solution

Energy input to the system is: Ein = V*I*t =110x30x120= 396 kJ

Energy output is the potential energy is:

Eou = PE = MxgxH = 700x9.81x20 = 137.3 kJ

The system efficiency is: n = Eou / Ein = 137.3 / 396 = 34.7%

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Example-9 In order to estimate the efficiency of an electric light bulb, the bulb is immersed into a beaker of water as shown in Fig. Assuming all the heat energy generated by the bulb is transferred to the water, use the data provided to calculate the efficiency of the light bulb as a light energy producer. Calculate also the amount of waste energy. The supply voltage = 12 V, the current drawn = 5A, Water volume = 0.5 liters, Initial and final temperatures are: 18o C, 30o C, within 10 minutes.

Notice that: Cwater = 4200 J/ Kg oC, water density= 1.0 Kg/ liter

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Solution

Energy input to the system is electrical and equal to:

Ein = V * I * t = 12 x 5 x 10 x 60 = 36000 J = 36 KJ

The thermal energy absorbed by the water and heat it computed as:

TEwater = C x m x AT = 4200 x 0.5 x (30-18) = 25200 J = 25.2 KJ

In the usual operation shown in Fig., the useful output from a lamp is the light, while

the energy losses radiated to the surrounding medium gives a warm feeling, i.e.

The computed thermal energy is the energy losses, ELoss= TE = 25.2 KJ

From the balance equation: Ein = Eou + ELoss

Eou = Ein - ELoss= 10.8 KJ

The bulb efficiency is: n = Eou / Ein = 10.8 / 36 = 30%

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Example Each person in the USA consumes an energy

equivalent of 58 barrels of oil/year. How many tons

of coal is that?

1 barrel of oil=5.8 x106 Btu

58 barrels= 58 x 5.8 x106 Btu=3.36x108 Btu

1 ton of coal (2000lbs) = 2.66x107 Btu

3.36x108 Btu /2.66x107 Btu/ton= 12.6 tons or 25,260 lbs of coal 11,482 kg

How many grams of Uranium is that? 1 gram of Uranium= 7.84x107 Btu 3.36x108 Btu/7.84x107 Btu/g of U =4.3g

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Example: A bicyclist on a flat road expends energy at the rate of 100 watts. How many calories of energy are expended in five minutes of peddling?

100 watts = 100 J/s

5 minutes x 60 s/min = 300 seconds

100 J/s x 300 s= 30,000 J

1 calorie= 4.184 J 30,000 J/ (4.184 J/cal)= 7170 calories or 7.2 Calories

So…is this a good way to lose weight?

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Equivalence and Replacement of Energy Forms

Comparing sources of energy, we are more interested in the replacement

value of the energy form, i.e. how much of that energy form is required to

do the same job as another energy form or fuel.

Tonnes of coal equivalent (tce):

The coal is used as a reference, and the unit for comparison. The tce, is

representing the energy generated by burning one metric tonne (1 tonne

= 1000Kg = 2204.62 pounds) of coal, equivalent to the energy obtained

from burning (5.2 barrels=700Kg) of oil

1tce=29.39GJ=8.14MWh=27.78x106BTU= 7x106Kcal

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The following table gives the equivalent values of some fuels.

Energy equivalent values of some fuels

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Magnitudes of Energy

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Magnitudes of Power

:50 – 500 W

:50 – 100 kW

:0.2 – 1 MW

:500 – 800 MW

Kitchen appliances

Passenger cars

Wind turbine

Large steam and

water driven turbo turbines

Modern fossil-fuel based

thermal power plant :1000 MW

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Units Conversion - example problem • If you turn on 4 light bulbs, each rated at 40 W, how long can

they be on before you reach 1 kWh?

4 bulbs x 40W/bulb = 160 W

E=P x t => t=E/P = 1 kWh/160W = 1kWh/0.16 W = 6.25 h

Units Conversion - exercise problems

1. In 2004, for the World-wide generation of 16,074 terawatt hours of electricity, 3.7 billion tons of oil equivalent was used. Calculate the efficiency of thermal energy conversion to electricity. (1 toe = 42 GJ)

2. In 2006, India had 144 GW of installed electric capacity and

generated 703 billion kWh. What is the percent capacity utilization of electric power stations?

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Example-10

Compute the energy of the following systems:

a) A large object of mass (1,000 kg) at a height of 10 m from sea level,

b) The energy stored in the mass of a car (1,000 kg) moving at 50 km/h, c) Cooling three cups of coffee of mass 0.4 kg from 80°C to 20°C;

d) Melt 3 Kg ice, assuming Cice=2110, At=15 °C (from -10 to 5°C)

e) Electric light bulb consumes 100 W in 17 minutes,

f) Burning 3.5 g coal,

Solution

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Section 4 Energy conversion Energy efficiency

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Energy Transducers

The device used to convert energy from one form to another is

called transducer.

There are many different types of transducers such as:

Electric generator (convert kinetic mechanical energy to

electricity),

Nuclear power plant (convert nuclear energy to heat,

then to electricity),

Fossil fuel power plant (convert chemical to heat, then to

electricity).

Solar cells and solar thermal (convert sunlight to electricity and

heat),

Windmills (convert wind energy to electricity or mechanical energy),

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Hydroelectric dams (convert gravitational potential energy to

electricity),

Fuel cells (convert chemical energy through reactions to

electricity),

Wave power station (convert wave mechanical energy to

electricity),

Geothermal power (convert internal Earth heat to electricity),

Thermoelectric (convert temperature difference to

electricity),

Electric lamp (convert electricity to light),

Microphone and Acoustoelectric (converts sound into an

electrical signal)

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Energy Conversion and Energy Conservation

Energy Conversion means converting energy from one form

into another through a transducer.

Energy Conservation law state that: energy can’t be

destroyed-to or created from nothing, but it is converting from

one form to another.

Energy Generation: means converting energy from one form

into another, For example, a diesel engine generates energy

means that, this engine converts chemical energy of oil into

mechanical energy. Also, a wind turbine generates energy

means converting kinetic wind energy into mechanical

energy. A solar cell generates energy by converting sunlight

radiant energy into electricity,

whereas with light bulbs we do the reverse. 50

Utilizing energy: the energy conversion process is very important

step to utilize energy, without converting energy it can’t be used

directly in its original state. For example, to heat: we convert

chemical energy of wood into

heat, or, convert electrical energy to heat through heater. to irrigate

crops: a diesel engine converts chemical energy of oil into

mechanical energy drive a pump which, converts this mechanical

power to bringing the water from well to a higher height (work done),

and to light: electrical energy converted into radiation and lighting

energy through electrical lamp, etc.

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Energy Loss, Energy Balance and Energy Efficiency

Energy Loss In all the previous conversion process, we find that part

of the energy is lost. This does not mean energy is destroyed, but

these losses occurring as a small price for the conversion purposes

and does a work. The energy loss appears in the form of radiant

heat in the surrounding environment. The conversion process in a

block diagram is shown in Fig.

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Energy balance equation: The basic equation of an energy balance

can be written in different forms as follows:

Energy input = Energy output + Energy losses,

or, Input = Output + Variation in the stored Energy + Losses,

Losses can classified to

Technical losses are due to conversions and transport or

transmission,

Administrative losses are due to non-registered consumptions

(Leakage).

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Energy efficiency: As stated above, energy conversions process

always imply energy losses, this leads to the concept of efficiency

as: If a quantity of energy in a certain form is fed to a transducer,

for conversion into another form of energy. The output energy in

the desired form is only a part of the input energy, and the energy

loss, usually radiated in the form of diffused heat. It means that,

the converter has efficiency less than 100%. The efficiency of an

energy converter is now defined as the quantity of the output

energy divided by the quantity of input energy, and it symbolic by

.

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Table 1 , shows some typical efficiencies of practical energy converters:

Note: Petrol engine =gasoline engine

Energy may be transformed from one type to another 55

Notice that:

When thermal energy is involved either as the input or as an intermediate

form, the efficiency is generally low.

■ High efficiency can be obtained when the losses from one converter are re-utilized as input in another.

An example, the efficiency of a diesel engine used to drive an electrical pump is given in Fig. The overall efficiency equals the product of the efficiencies of the various intermittent subsystem of the system.

Diesel engine

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Energy Terminology

The energy field has its own specified terminologies and units, The

terminologies and units are used specially in this scientific area.

Energy terminologies include

Primary energy is the energy as it is available in the natural

environment, i.e. the primary source of energy,

Secondary energy is the energy ready for transport or transmission,

Final energy is the energy which the consumer buys or receives,

Useful energy is the energy which is an input in an end-use

application (light, heat, etc.).

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An example to well define the above 4-terms is shown in Fig. The primary

energy form is the fossil energy. This fossil fuel is converted into electrical

energy at the power station, which is the secondary energy source.

Electrical energy transformed to the consumers through transmission

lines and cable, this is the final energy. The consumer eventually converts

this electrical energy into useful work (light, heat, etc.), this is the useful

energy.

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Ways to Transfer Energy Into or Out of A System

In non-isolated systems, energy crosses the boundary of the system during some time interval due to an interaction with the environment. Work – transfers energy by applying a force and causing a displacement of the point of application of the force. Mechanical Wave – transfers energy by allowing a disturbance to propagate through a medium. Heat – the mechanism of energy transfer that is driven by a temperature difference between two regions in space. Matter Transfer – matter physically crosses the boundary of the system, carrying energy with it. Electrical Transmission – energy transfer into or out of a system by electric current. Electromagnetic Radiation – energy is transferred by electromagnetic waves.

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Examples of Ways to Transfer Energy

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Classification of Energy Sources • Primary and secondary energy

– Primary energy sources are those that are either found or stored in nature (coal, oil, natural gas, biomass, nuclear, geothermal, wind, solar, etc.)

– Secondary energy sources such as steam and

electricity are obtained from conversion of primary

energy sources in industrial utilities.

• Commercial and non-commercial energy

– Commercial energy: the sources that are available in the

market for a definite price (electricity, lignite, coal, oil,

etc).

– Non commercial energy: firewood, cattle-dung, agricultural wastes, etc.

• Renewable and non-renewable energy

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Conversion of

primary energy

into useful

energy requires

many steps:

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Five Energy Sources .Six Energy Classes

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Global Energy Sources

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1

2

3

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Energy Conversion Efficiency

World Energy in 4 minutes – ENERGY TRILEMA

https://www.youtube.com/watch?v=90J0Gl-5ggs

Renewables rely on varying sources (wind, tides, daytime.)

Fossil fuel power plants: 60%, nuclear around 90%, wind 20-40%,

solar 20%.

CAPACITY FACTORS

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Exercise 1:

Average daily human diet has an energy content of 2000 kcal. Convert this 2000 kcal per day into watts.

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Example USA imports about 12 million barrels of oil per day.

a)what is this in terms of equivalent power?

b)suppose we wanted to replace all the imported oil with energy produced by fission from domestic uranium. How many 1000 MW nuclear power plants would we have to build?

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Example An energy efficient refrigerator consumes energy at the rate of 280W when it is actually running, but it is so well insulated that it runs only about one-sixth of the time. You pay for that efficiency up front: it costs $950 to buy the refrigerator.

You can buy a conventional refrigerator at $700. However, it consumes 400 W when running, and it runs one-fourth of the time. Calculate the total energy used by each refrigerator over a 10 year lifetime and then compare the total costs-purchase price + energy cost- assuming electricity costs 10 cents per kilowatt-hour.

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Example 2.1 Energy consumption by a car

An average car consumes 50 gallons gasoline per month. Estimate the

energy consumed by the car per year.

Solution:

Assume that gasoline has an average density of 0.72 g/cm3 and the

heating value of 47.3 MJ/kg Data: V = 50 gallons/month = 189.25 l/month, 2271.01/year (3.785 l = 1 gallon) (gas) = 0.72 g/cm3 = 0.72 kg/l Mass of gasoline: mgas ¼ qV = 1635.1 kg/year Energy consumed per year: Egas = 1635.1 kg/year (47,300 kJ/kg) = 77,340,230 kJ/year = 77,340.2 MJ/year

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Example 2.2 Fuel consumption by a low and a high-mileage car An average daily traveling distance is about 40 miles/day. A car has a city-mileage of 20 miles/gal. If the car is replaced with a new car with a city-mileage of 30 miles/gal and the average cost of gasoline is $3.50/gal, estimate the amount of fuel, energy, and money conserved with the new car per year. Assume: The gasoline is incompressible with qav = 0.75 kg/l. Sol: Lower heating value (LHV) = 44000 kJ/kg; Fuel needed for the old car: (40 miles/day)/(20 miles/gal) = 2 gal/day Fuel needed for the new car: (40 miles/day)/(30 miles/gal) = 1.34 gal/day Old car: Mass of gasoline: mgas = qav (Volume) = (0.75 kg/l)(2.0gal/day)(3.785 l/gal) = 5.7 kg/day Energy of gasoline: Egas (LHV) = (5.7 kg/day) (44000 kJ/kg) = 250800 kJ/day (365 day/year) = 91542,000 kJ/year = 91,542 MJ/year Cost: ($3.50/gal)(2 gal/day)(365 day/year) = $2555/year New car: Mass of gasoline:mgas = qav (Volume) = (0.75 kg/l)(1.34gal/day)(3.785 l/gal) = 3.8 kg/day Energy of gasoline: Egas (LHV) = (3.8 kg/day) (44000 kJ/kg) = 167,200 kJ/day (365 day/year) = 61,028,000 kJ/year = 61,028 MJ/year Cost: ($3.50/gal)(1.34 gal/day)(365 day/year) = $1712/year The new car reduces the fuel consumption by around 33%, which is significiant.

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Example 2.6 Electricity consumption of a laptop computer A laptop consuming 90 Watt is used on average 10 h per day. The laptop costs $500 and will be used for 4 years. Electricity cost is $0.15/kWh. Estimate the total electricity cost in four years for the laptop. Solution:

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Example 7.7 Heat loss in an electric motor An electric motor attached to a pump draws 10.0Aat 110 V. At steady load the motor delivers 1.32 hp of mechanical energy. Estimate the heat loss from the motor. Solution: Assume that the load to motor is steady. The electric power received by the motor is used to create a pump work and heat: Only 983 kW of the 1,100 kW is converted to the pump work.

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Example 7.9 Thermal efficiency of a heat engine Heat is transferred to a heat engine from a furnace at a rate of 90 MW. The waste heat is discharged to the surroundings at a rate of 55 MW. Estimate the net power output and thermal efficiency of the engine if all the other power losses are neglected. Solution: Assume: steady-state process with negligible heat losses. The heat engine can convert 38.8% of the heat transferred from the furnace.

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