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Energy Awareness
C2: MY ENERGY AUDIT
C2 ACTIVITY 4: ENERGY SANKEYS
Background
Sankey diagrams represent the flow of energy visually by identifying energy stores, energy transfers, and points where energy could be wasted. It is important that the energy we use is not wasted, and knowing the energy transfer helps us to determine the efficiency of a device. Students may be familiar with various graphic representations of data such as bar charts, pie charts and scatter graphs. However, these representations often depend on the interpretation of the reader as well as the quantity of data used.
In 1898 an Irish man called Captain Matthew Sankey used a flow chart to show the energy efficiency of a steam engine. This type of flow chart is now referred to as a Sankey diagram, and is used to investigate the energy efficiencies of systems as well as the cash flow of businesses. The diagrams are constructed from data and represent the energy transfers involved, quantifying these transfers and thus highlighting the efficiency of the system in question.
A Sankey diagram is shown in Figure 4. The width of the arrows represents the quantity of energies involved, and their directions indicate where the energy flows. In Figure 4, the arrow to the right represents useful output and the downward arrow represents output of wasted energy. It also shows the conservation of energy: an input of 5 J results in a total output of 3.9 J + 1.1 J.
Suggested approaches:
- As an introduction to Sankey diagrams, ask students to describe the various types of graphs they use in other subjects such as maths, geography and business studies. Raise the following questions:
? Why^ are^ these^ graphs^ used?
? What^ type^ of^ information^ do^ they^ give? ? What^ shapes^ do^ these^ graphs^ take? ? How do we interpret the resultant patterns?
? How^ useful^ are^ these^ graphs?
Figure 4
Energy Awareness
C2: MY ENERGY AUDIT
Input Output – useful
Output – waste
Figure 5
- Show Figure 5 to the class and ask for comments and questions. Ask a group of two or three students to compile a list of the questions that arise. This is a good opportunity to show the students how to construct a Sankey diagram using C2 ACTIVITY 4 (I): CONSTRUCTING A SANKEY DIAGRAM as a teaching aid. Afterwards, review the list of questions and ask the students if there are any they can now answer themselves.
- Ask the students to tell you what they understand by the terms efficiency and energy efficient. Put the following questions to them:
? In^ what^ circumstance^ might^ a^ microwave^ be^ more^ efficient^ than^ a^ cooker?
? When^ would^ a^ microwave^ be^ more^ efficient^ than^ a^ kettle?
- The efficiency of a device that transfers energy – its ‘energy efficiency’ – refers to the amount of the energy supplied (input) that is transferred into usable energy (output). The efficiency of an appliance is usually calculated as the percentage efficiency. Not all the input energy is transferred into usable energy. There will be some loss of energy through unwanted heat, sound, etc. Because of this energy loss an appliance will never be 100% efficient.
The energy efficiency of a device can be calculated using the following formula:
% Efficiency Useful energy output x 100 Total energy input
What Next:
- After establishing these basics, you may wish to move to the analysis and use it to prompt a discussion about energy efficient light bulbs. The following questions could be investigated, thus enabling students to hone their research and practise their presentation skills. ? Why has the EU eliminated the use of incandescent (filament) light bulbs? ? What are the consequences of this law, intended or otherwise? ? (^) What about catalytic converters – are they efficient or do they simply reduce the emission of noxious gases?
Resources:
- See SEAI website for tips on energy efficiency.
- See energy.gov for information on the history of the light bulb.
Energy Awareness
C2: MY ENERGY AUDIT
100J – Amount of energy that is put in
20J – Amount wasted output energy
Note: Conservation of energy... total Input (100J) Total Output (80J – 20J)
80J – Amount of useful energy
Figure 6 – Examples of Sankey diagrams
Energy Awareness
C2: MY ENERGY AUDIT
C2.4 WORKSHEET H: READING A SANKEY DIAGRAM
- How much thermal energy does the motor produce?
a) 62 joules b) 18 joules c) 60 joules d) 42 joules
Answer:
Electrical Energy 60 joules
Mechanical Energy 42 joules
Thermal
Energy Awareness
C2: MY ENERGY AUDIT
- How much heat energy is emitted by each of the light bulbs in question 2?
A: B: C:
- Which of the light bulbs in question 2 is most efficient? Give a reason for your answer.
- What is a filament bulb?
- Explain what the letters CFL represent.
- Is this filament lamp efficient? Give a reason for your answer.
Electrical Energy 50 joules
Light Energy 1.1 joules
Thermal Energy 48.9 joules
Energy Awareness
C2: MY ENERGY AUDIT
- What is the efficiency of the CFL lamp in the diagram below? Suggest another phrase instead of ‘thermal’ which could be substituted and still retain the same meaning.
- How much useful energy is produced by this power station? [Hint: check the scale.]
- What is the efficiency of this power station?
Electrical Energy 10 joules
Light Energy 1.1 joules
Thermal Energy 8.9 joules
Chemical Energy 7000 joules
Electrical Energy
1 box = 280 J
Thermal Energy Waste Gases
Thermal Energy Cooling Water
Energy Awareness
C2: MY ENERGY AUDIT
C2 ACTIVITY 4 (III): EXAMPLES OF ANALYSING SYSTEMS FOR
ENERGY EFFICIENCY
Car
A car engine transfers chemical energy which is stored in the fuel (petrol) into kinetic energy in the engine and wheels.
Energy input
Chemical energy in the fuel
Energy output
CAR ENGINE
Kinetic energy in the engine and wheels
100% Chemical energy in petrol
25% Kinetic energy to drive
75% Energy lost, e.g. heat
Figure 9: Energy transfer diagram for a car engine
Figure 10: Sankey diagram for a car engine
Energy Awareness
C2: MY ENERGY AUDIT
Power Stations
Figure 11: Power station (Edenderry Power Plant, Bord na Móna)
Figure 12: Sankey diagram for a power station
The IEA makes extensive use of Sankey diagrams to illustrate the energy usage of various countries. Check out how the IEA maps Ireland’s energy usage.
Energy in fuel 100 J Electrical Energy 35 J
Heat loss 20 J
Steam cools to water 40 J
