# 4.1 Conversion between units 10 5. Temperature and heat 6. Energy

```Unit 2
Energy Basics, Energy Services and Demand
Introduction
3
1.
Energy:
2.
Forms of energy
3.
4.
3
3
Units of energy
Power
4.1
Conversion between units
5.
6.
What is it?
Temperature
5
6
10
and heat
11
Energy conversion
6.1
Conservation of energy - the first law of thermodynamics
6.2
Efficiency
6.3
Primary and end use energy
6.4
Maximising energy efficiency
6.4.1
6.4.2
13
13
14
15
18
Energy conservation
Energy efficient appliances
18
19
6.5
The second law of thermodynamics
7.
Energy services
7.1
Matching the energy source to the service
20
23
25
8.
27
Home energy audit
Identify all energy services and their energy sources
Obtain historical records of energy usage
Determine the energy consumption for each service
8.1
8.2
8.3
8.3.1
8.3.2
Estimating electrical energy consumption
Electrical energy consumption for a battery charging stand-alone power
system
Estimating energy for hot water
8.3.3
8.4
Analyse the information, draw conclusions and make recommendations
8.4.1
8.4.2
8.4.3
8.5
8.6
8.7
9.
28
28
31
32
34
35
36
Calculating primary energy
Major energy use areas
Identifying ways to reduce energy use
36
39
40
Implementing the Recommendations
Analyse the result of the improvement.
Do it all again!
43
43
43
Summary
Summary of quantities, units and their symbols used in this unit:
Standard metric prefixes
9.3
Summary of equations used in this unit:
44
46
46
47
9.1
9.2
1O.
Glossa ry
48
11.
Bibliography
50
Home Energy Audit - Sample Worksheets
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Renewable Energy Centre
51
Unit
2,
1
Resource Book
In the natural world, as well as in our constructed energy systems, energy is constantly
being converted from one form to another. For example, plants convert light energy
from the sun into chemical energy which it uses to grow. When we turn on a light
switch, we allow the conversion of electricity into useful energy as light. There are two
laws which describe what happens in every energy conversion process, regardless of
what kind it is. These laws are called the Laws of Thermodynamics.
Thermodynamics is simply a branch of science which looks at how heat and other
forms of energy are related.
In tlle case of turning on the electric light, the electrical energy is converted to light, but
some of it is also being converted into heat. This heat constitutes an energy &quot;loss&quot; in
tlle conversion process. Note that the energy tllat is &quot;lost&quot; does not actually disappear,
but it is no longer available to do the task we wanted. It is no longer in a useful form.
6.1
Conservation of energy - the first law of thermodynamics
The First Law of Thermodynamics is also known as the Law of &quot;Conservation of
Energy&quot;. It causes us to examine the proportions of useful eJlergy output and eJler:?'J
losses in any energy conversion process.
The First Law states: Energy cannot be created or destroyed; whenever
energy is transformed from one form to another, the total quantity of energy
remains the same.
This is shown in the Figure 2 below. In a light bulb, the input eJlergy is the electricity.
The useful energy output is light energy (tllis is also called the &quot;useful work&quot;). The
energy loss is the heat given off from the bulb.
/
Input Energy
~
~
Useful Energy
Output (Light)
(Electrical)
~
Energy
Loss
(Heat)
Figure 2
A light bulb provides a simple example of what happens in every
energy conversion process.
This can be represented mathematically by the following equation:
input energtJ = useful energy output + energtJ lasses
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EnErgy Cenhoe
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Introduction
to Renewable Energy Ted1llologies
or
Eour + Ewss
EIN
where
EIN
.............................................................
(Eq 6.1)
the total energy input Goules, watt-hours, etc.)
the useful energy output Goules, watt-hours, etc.)
the energy losses from the conversion process Goules, watthours, etc.)
Eour
Ewss
Energy losses can take a number of forms. Heat is always one form, others examples
are air movement (e.g. from a built in cooling fan), noise and vibration.
The fact that the First Law operates with every energy conversion that has ever been
witnessed (as far as tlie author knows) is why perpetual motion machines never work!
Perpetual motion machines are supposed to prOduce more energy---outpUt than what
goes into them.
-
6.2
Efficiency
The effidellctj of an energy conversion process is the proportion of input energy which
is converted to useful energy output. It is defined mathematically as:
useful eneI'm output
input energy
efficiency
Using symbols, this equation is written as:
Eour
1]
where
..........................................................
EIN
17
(Eq 6.2a)
efficiency (no units, but is often expressed as a percentage).
is the Greek letter&quot; eta&quot;.
the useful energy output Goules, watt-hours, etc.)
the total energy input Goules, watt-hours, etc.)
11
Eoul'
EIN
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TI1e definition of efficiency given above relates to Energy efficiency i.e. it relates to the
total amount of energy into and out of a process over a period of time. It is also useful
to define an instantaneous efficiency i.e. Pawer efficiency. This has exactly the same
form, but applies to the input and output power flowing at any instant.
Pour
1]
.........................................................
PIN
where
1]
(Eq 6.2b)
efficiency (no units, but is often expressed as a percentage).
the useful pawer output (watts, kilowatts, etc.)
the total pawer input (watts, kilowatts, etc.)
Pour
PIN
Note that the same symbol has been used here for power efficiency and energy
efficiency. They will be the same, unless the efficiency changes over time.
Where there are a number of steps in an energy conversion process, the total efficiency
of the process is equal to the product of the efficiencies of each step. Mathematically:
1]TOT
=
..........................................................
1]1 X 1]2 X ••. X 1]n
Where
(Eq 6.2c)
the total efficiency of the process
the efficiency of the first step
the efficiency of the second step
the efficiency of the n th (last) step.
-
6.3
Primary and end use energy
e1lergy is the energy that we collect from our original energy source. The
energy sources shown in Table 1 of Unit 1 (e.g. solar, wind, oil, coal etc.) are all
primary energy sources. E1ld-use energJ} is the energy at the point of use - the energy
finally consumed by the appliance. This distinction is important because of the
significant inefficiencies in our energy systems.
Primary
End-use energy is the energy consumed by an appliance at the
point of use e.g. in the home, office or work-site.
In most energy systems, energy is converted from one form to another several times.
The efficiency of each energy conversion step can be assessed. The overall efficiency of
the complete system is then the multiplication of the efficiencies of all energy
conversion steps.
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Renewable
Energy
Cenb-e
Unit 2, 15
o0&deg;0
[l]~~.
_&quot;,A
~
'~?~~,r:;TD/
Pr,XjuctiDn
of C::&gt;al
Tran:.portalion
of
Coal
~
BOllef
powe~n~tatjon
TurLJinc
Auxiliaries
Tr~~~~~on
Primary
Energy
End
Us.
Energy
Energy less
Energy loss
Energy used
Figure 3
Energy losses in the production of electricity from coal. Note that these
figures will vary from state to state, with total cumulative efficiency in
the range 0.27 to 0.32.
In the above example, the energy in the coal is the primary energy. The electrical energy
being used in our home is the end use energy. For our purposes we wi1l refer to
electricity as an end-use energy source. (It is sometimes referred to as a secondary
energJj source). The cumulative efficiency is simply the product of the efficiency of all
the previous steps multiplied together. Note that energy &quot;losses&quot; in the first two steps
represent the amount of energy used for production and transportation. This energy
does not actually come from the coal itself.
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Only about 28 percent (0.28) of the energy content of the coal is being delivered to the
point of use in our home, factory, or business. The remaining 72 percent represents
energy losses in waste heat. There will be further energy losses depending on the
efficiency of the appliance or machine that is driven by the electricity to perform work
for us.
It is important to note that while the amount of energy that we pay for on our
electricity and gas bills is the end use energtj, the pollution and depletion of resources
that we cause is related to the amount of primary energy* consumed. The two quantities
are related by a form of the efficiency equation:
EENDUSE
............................................................
17PR-EU
where
(Eq 6.3)
primary energy Goules, watt-hours, etc.)
end use energy Goules, watt-hours, etc.)
efficiency of conversion from primary to end use energy.
EpRlMARY
EENDUSE
17PR-EU
*It is common to refer to a fuel such as coal as the primary energy source, though U1isis not stricUy
correct The sun is really the primary source of energy. The efficiency of U1ewide range of energy
conversions required to convert plant matter to coal are NOT taken into account (e.g. U1Camount of
solar radiation converted in photosynthesis; the gravitational and heat energy required to compress
U1eplant matter and convert it to coal over millions of years ... ) It is important to keep this point in
nLind when making comparisons between electricity generated from coal and electricity generated
directly from U1esun!
Some typical efficiency factors
(T)m-ill)
End-use
Energy Source
electricity
(from coal)
electricity
(from
diesel genset)
T)m-ill
0.27
0.32
-
for various fuels are provided below.
Depends on coal type, power station technology
and transmission distance.
0.05 - Small genset efficiencies are 15-20% under
0.10
optimum conditions, and when supplying
household
dummy loads), the efficiency is very low.
Natural gas 0.9
Takes into account extraction, transport or
piped**
transnussion, processing and leakage.
8.
petrol, diesel
0.9
as for gas
LPG (for cars and 0.87
Lower eff. than natural gas because it needs to
bottles)
be compressed for storage in bottles/ tanks
kerosene
0.9
as for gas
wood
0.9
Accounts for energy in collection and transport.
**Natural gas (boWed and for cars) = 0.7 due to high pressures required for bottling.
Table 3
Energy conversion efficiencies for various end-use energy forms.
Note: If fuel is transported over long distances, T)PR-EU will be lower.
-
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Unit 2, 17
Introdllctiml
6.4
to Renewable Enelgy Tedlllologies
Maximising energy efficiency
In order to improve the overall efficiency of an energy system, we need to:
Minimise the number of energy conversion steps
Maximise the efficiency of each energy conversion step
For example, consider the provision of hot water to a home. Two possible choices of
technology are: an electric hot water system, and a gas hot water system. For the
electric system, the overall efficiency of energy conversion from coal to hot water is
about 25%. For a gas system, the overall efficiency is about 54% (or higher).
The efficiency of the electric hot water system alone is fairly high - around 75 % (better
insulation could improve this considerably), but the efficiency of conversion from coal
to electricity is low because of the number of steps involved, and the large inefficiency
in some steps. The overall efficiency of the gas system is much better because the fuel
is burnt directly at the appliance, so there is only one major energy conversion step.
The use of a gas hot water system will result in a primary energy consumption of less
than half that of the electric system. The use of a solar hot water system would reduce
fossil fuel consumption even further, and has other advantages which are discussed in
the following section.
6.4.1
Energyconservation
Energy conservation means using only as much energy as necessary. For example,
turn off lights when you leave a room; put on an extra jumper instead of turning up
the heater. There is another important side to energy conservation. As you've seen in
Example 13, you could waste over 72 % of the energy of brown coal before you get
electrical energy to your house. Therefore, saving a little energy at the house results in
a far greater saving at the energy source.
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When you use 100 Wh of energy at home (the
amount a standard light globe requires) we
must produce 340 Wh of generation at the
power station to supply this lOOWh.
100Wh
&lt;
340 Wh
I
Using 20 Wh at home (the incandescent light
that gives the same light output as the
standard
globe)
requires 70 Wh of
generation at the power station.
20 Wh
&lt;
o
o
o
o
70 Vv'h
I
The simple act of using a different light bulb in one room of
your house results in the power station needing to generate 270
Wh LESS!
Figure 4
energy.
6.4.2
Small savings in end-use energy create much larger savings in primary
Equation 6.3 is used to calculate the primary energy. A conversion efficiency of .29 is assumed.
Energy efficient appliances
Part of an overall approach to energy conservation is the use of energy efficient
appliances. Many electrical appliances wiI1 have an ellergJj star rati1lg based on the
energy consumption of the appliance under standard test conditions. Quite simply, the
more stars, the more energy efficient the appliance. The star rating as well as the total
predicted yearly energy consumption is usua]]y prominently displayed on the
appliance.
Carefully note the settings at which each appliance is rated as these can vary
considerably between appliances. For instance, one washing machine may be rated on
a heavy duty cycle, whereas anotl1er at a less energy consuming setting. If possible,
note the predicted energy consumption at the setting you would be using most often.
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Introduction
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Figure 5
Example of appliance energy star rating labels.
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6.5
The second law of thermodynamics
The Second Law of Thermodynamics has significant implications for the way our
soc.iety uses energy. TI1e depletion of our energy resources in the next few years is of
great concern, yet from the First Law, we know that we are not running out of energy
as such: it can neither be created nor destroyed. The problem lies in the way we use
energy. The Second Law relates to the quality of an energy source: its potential for
useful conversion.
\ The First Law talks about the quantihj of energy that we may have. The Second Law
Ienergy
talks about ellergtj quality. Energy quality depends on the degree of order in the
form. (An inverse measure of the quality of an energy source is ClltfOp1j, which
I is a measure
of the randomness or state of chaos of an energy form).
High quality energy can be easily converted into other forms, and may be used for
many useful purposes. Low quality energy is much more limited in its usefulness. For
example, the gravitational potential energy of stored water in a dam, and the chemical
energy which bonds the atoms in coal are both examples of energy in a highly ordered
state: i.e. high quality energy.
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