CH2 - Energy Fundamentals
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Transcript of CH2 - Energy Fundamentals
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CHAPTER 2
Energy
Fundamentalsfor Energy Auditors
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What is Energy?
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Energy Lets Us Do Work Energy is the ability to do work.
As such, energy is important to all living things in
order to maintain life functions from the smallestpart of a cell to the organism as a whole.Humans also use energy to modify their
environment and perform work. Energy is measured by the amount of work it is
able to do. The units for measuring energy are
Joules (J). One Joule is a very small amount of energy,
but 1000 Joules is roughly the heat energy
produced by burning a blue tip kitchen match.
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Sources of Our Energy The energy sources that we use every day are
divided into two groups: Renewable an energy source that we can
use over and over again, and can be replaced
naturally in a short period of time. Non-renewable an energy source that we
are using up and cannot recreate in a short
period of time.
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Sources of Our Energy
The renewable energy sourcesinclude solar energy, which comesfrom the sun and can be turned
into electricity and heat. Wind,geothermal energy from inside the
earth, biomass from plants, andhydropower from water are also
renewable energy sources.
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What Is Power? Work can be done at different rates,
sometimes slow, sometimes fast. Sincework involves the transformation ofenergy, the faster the work is done, thequicker energy must be transformed.
Poweris the term used for the measure of
how fast work can be done. Or in otherterms, power is defined as the rate atwhich work is done.
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What Is Power? In mathematical terms power equals work done
divided by time required, so the units of powerwould be Joules per unit time, most commonlyJoules per second, or watts:
Power is an important concept because it ties
the dimension of time into the energy picture.
RequiredTimeDoneWorkPower
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Almost all mechanical and electricalequipment have nameplate ratings in
terms of the maximum power that theycan supply, not the energy they cansupply.
As we will see later, almost allmechanical devices like motors are ratedin terms of their maximum poweroutput,
while almost all purely electrical devicesare rated in terms of their maximumpower input.
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Energy and power are often confusedwith each other.
A useful analogy can be found in ourcar where we have both thespeedometer that tells us how fast we
are going in kmph, and we also havean odometer which tells us how farweve gone in km.
KMPH
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The odometer is like an energy
meter that tells us the totalamount of energy in Joulesthat weve used.
The speedometer is like a powermeter that tells us the rate at whichwe have used that amount of energy in
Joules per second, or watts.
With our car, the quantities of interest
are km and kmph. With our electricaland mechanical equipment, thequantities of interest are Joules and
Joules per second, or watts.
KMPH
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Electrical Power When dealing with electricity, power isdefined in the same way.
Electrical devices provide resistance whichdescribes the amount of work that needs tobe done for a specific task.
A certain amount of work must be done tomove electrons through the resistance.
More resistance means more work mustbe done to move electrons through theresistance and allow the device tooperate.
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Electrical Power The rate which that work is accomplished isrelated to the power applied. More electrical
power means energy is being converted at afaster rate.
This electrical energy is supplied by the source
of the electrical current like a battery or electricalgenerator.
Electrical power is measured in units calledwatts, which are related to the number of Joules
of energy expended per second.
1 watt = 1 Joule/second
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This means the energy can be expended atdifferent rates depending on how fast thework needs to be done.
Some devices use more power toaccomplish a task that others do with muchless power.
For example light bulbs come in differentsizes meaning different wattages. Somelight bulbs are rated 60 watts while others
are rated 100 watts. The 100 watt bulb willgive off more light than the 60 watt bulb butif you only need the amount of light from the60 watt light bulb, you are using more powerthan necessary.
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Units of Electrical Energy The watt (W) is a physical unit which is
named for James Watt, the inventor of
the steam engine. Since the unit refers to a personsname, we abbreviate it with a capital W.
The basic unit of electrical energy is the watt-hour, orWh.
1 Wh = 3600 Joules1 kWh = 1000 Wh = 3600000 Joules = 3.6 MJ1 MWh = 1000 kWh
1 GWh = 1000 MWh
1 kW = 1000 W
1 MW = 1000 kW
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One barrel of oil produces
about 550 kWh in a thermalpower plant
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One kg of coal produces about
2 kWh
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Energy Units And
Energy Conversions
minutes
degreesCelsius
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Energy Units And Energy
Conversions The basic unit of energy is the Joule (J)
One thousand Joules is about equal to theheat produced by burning an ordinaryblue-tip, kitchen match.
One Joule is not a very large amount ofenergy, so you will often see one of twocommon multipliers of Joules; the kJ, or one
thousand Joules; or the MJ, which is 1000kJ, or one million Joules. For even largeramounts of energy, the GJ = 1000 MJ.
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Word and Numerical
Equivalents
One J = 1 J.
One thousand J = 1000 J = 103 J = 1 kJ
One million J= 1,000,000 J = 106 J = 1 MJ One billion J = 1,000,000,000 = 109J = 1 GJ
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Energy Conversion Unit Table
1 kWh .. 3.6 MJ
1 m3 LPG..25.56 GJ
1 kg #2 fuel oil . 43.3 MJ
1 m3 natural gas.. 37 MJ
1 m3 #2 fuel oil 39.85 GJ
1 litre LPG gas ... 7.1 kWh
1 kg LPG gas12.68 kWh
1 litre #2 fuel oil11.07 kWh1 kg #2 fuel oil..12.03 kWh
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Energy Unit Conversions and
the Railroad Track Method Since we have several different basic energy units
and many different energy unit multipliers, energymanagers must often convert from one set of energyunits to another. There is a very systematicapproach that can be applied to basic conversions,
and also to more complex conversions andcalculations.
The principle of this unit conversion method is simply
to carry out algebraically correct multiplications anddivisions using correct units at each step, startingwith the given piece of information and transformingit into the desired units using one or more conversion
factors.
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For example, if we want to find the number(X) of Joules in 1000 cubic metres of naturalgas, we can use this method as follows:
= 1000 37,000 kJ
= 37,000,000 kJ
= 37 109 J
= 37 GJ
From Table C-20
kJ000,37
m
m1000=
mkJ000,37m1000=gasofGJX
3
3
33
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In the above calculation, cubic metresin the numerator and cubic metres inthe denominator cancel out, and the
remaining unit on the right side of theequation is J.
Our goal was to end up with J as ourdesired unit on the right, and we madeour unit conversions on the right side
until we had the same unit as on theleft side.
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If we ever perform one of these basicunit conversion calculations, and findthat we have different units on the left
and the right we do not have thecorrect answer in terms of the desiredunits.
This method is given the colloquialname Railroad Track Method, because
the vertical separation lines remind usof railroad tracks.
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Find the number (X) of kWh in 1000 cubicmetres of natural gas.
= 1000 37,000 kWh/3600
= 10,278 kWh
Example Problem
3600
kWh1
kJ
kJ000,37
m
m1000=
kJ3600
kWh1
m
kJ000,37m1000=gasofkWhX
3
3
3
3
From Table C-20
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Example Problem
Solution
X J = 10 kWh 3.6 MJkWh
= 36 MJ
In this example, the two kWh unitscancel out, leaving the remaining unit
on the right side as J.
How many J are in 10 kWh?
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Example Problem
How many kWh are in 2500 mJ?Solution:
In this example, the two MJ unitscancel out, leaving the remaining uniton the right side as kWh.
kWh44.694=MJ
kWh6.3MJ2500=kWhX
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Example Problem A tank is filled with 100 litres of Number 2 fuel
oil. How many GJ of energy is contained in thetank of oil?
Solution
From Table C-20, there are 39 MJ per litre of oil.
= 3.9 GJ
In this example, the two litre units cancel out, and thetwo MJ units cancel out, leaving the remaining unit onthe right side as GJ, our desired unit.
MJ1000
GJ1
L1
MJ39L100=GJX
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Benchmarking
A benchmark is a value youcompare something against.
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Energy Benchmarking for
Buildings Building energy benchmarking is the
comparison of whole-building energy userelative to a set of similar buildings.
It provides a useful starting point for individualenergy audits and for targeting buildings forenergy-saving measures in multiple-site audits.
Benchmarking is of interest and practical use toa number of groups.
Energy service companies and performancecontractors communicate energy savingspotential with typical and best-practice
benchmarks.
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Control companies and utilities can providedirect tracking of energy use and combinedata from multiple buildings forbenchmarking.
Benchmarking is also useful in the designstage of a new building or retrofit todetermine if a design is relatively efficient.
Energy managers and building owners havean ongoing interest in comparing energyperformance to others.
Large corporations, schools, andgovernment agencies with numerousfacilities also use benchmarking methods tocompare their buildings to each other.
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Benchmarking Audit Benchmarking Audits are associated with
the idea that after the energy bill data iscollected and processed, some facilityinformation will be collected on a walk-through, and the data will be run through
some benchmark to determine if there is apotential for significant improvement inenergy efficiency and reduction in energy
operating cost.
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Benchmarking Criteria
Energy Use Index - MJ/m2/year, kWh/m2/year Total, Electric, Gas, Oil
Energy Cost Index - $/m2/year Total, Electric, Gas, Oil
Productivity Index
kJ/kg, kJ/person, kJ/student, kJ/tonne, kJ/item kWh/kg, kWh/person/ kWh/tonne, kWh/item L H2O/kg, or /student, or /item (also sewer)
System performances kWe/kW cooling, LPS/kW air, kWh/L pumping
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Basic Energy Accounting
Basic energy accounting deals with thefollowing ideas:
Recognizing different energy and fuel types Electricity, gas, light oil, steam, chilled water
Understanding energy related units
kWh, kJ, MJ, kW, kJ/h, L or kg of oil, m3 ofgas
Performing conversions to different energyrelated units
For example, 1 kWh = 3600 kJ = 3.6 MJ
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The Facility Energy Use Index The facility Energy Use Index (EUI) is a
statement of the number of MJs (or kWh) of
energy used annually per square metre ofconditioned space (heated or cooled, or both).It is a basic measure of the facilitys energyperformance the lower, the better.
To compute a facilitys EUI Identify all the energy used in the facility
Add up all the MJs (or kWh) of energy
Find the total square metres of conditioned space
Divide the total MJ (or kWh) used per year by thesquare metres of space.
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ExampleA facility with 1,000 square metres of
conditioned space uses 100 GJ ofgas and 150,000 kWh of electrical
energy in one year. What is thefacilitys EUI?
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Solution (in MJ/m2
yr)
yrm/MJ640=yrm000,1
MJ)000,540+000,100(=EUI 22
yr/MJ000,100=GJ
MJ1000
yr
GJ100
=MJgas
yr/MJ000,540=kWh
MJ6.3
yr
kWh000,150=MJelect
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Solution (in kWh/m2
yr)
yrm/kWh8.177=
MJ6.3kWh1
yrmMJ640=EUI
2
2
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1999 CBECS EUI Data - USA
kWh/m2/yrAll Bldgs 236
Education 208 Vacant 44.5Food Sales 561 Food Service 669
Health Care 490 Lodging 278
Retail Stores 200 Office 251
Assembly 228 Safety 242
Churches 88.9 Service 346
Warehouse 122 Other 400
E U I d f C i l B ildi
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Energy Use Index for Commercial Buildings
kWh/sq metre/yr
0
100
200
300
400
500
600
700
AllBldgs
Education
FoodSales
FoodServ
Health
Lodging
Retail
Office
Assembly
Safety
Churches
Service
Warehouse
Other
Vacant