- Understand how the energy stored in fuels originates from the Sun.
- Use Sankey diagrams to represent energy flow.
- Calculate the amount of energy release when different fuels are burnt.
- Understand the principal of a heat engine.
- Understand the principal of a generator.
Need to know
Doing this at home
Always a good topic in which to stop and think about your own energy needs and how these are fueled. Are you using household heating? How do you cool or heat ingredients and cook meals? What are your country's leading mechanisms for electricity generation?
Fuels are chemicals that are burnt to produce heat energy which can be converted into mechanical energy by an engine or turbine which is used to turn a generator to produce electricity. The energy in most fuels originates from the Sun.
This can be represented on an energy flow diagram like the one below. Drag and drop the labels to their correct place.
Energy from the Sun
The core of the Sun is so dense and hot that Hydrogen fuses to form Helium. This is a many stage process called the proton - proton chain. Here is an animation from The University of Nebraska - Lincoln.
This process results in a total power of 3.9 x 1026 W
As the distance from the sun increases the power spreads out over a bigger and bigger sphere.
The Earth is 1.5 x 1011 m from the Sun.
- What is the area of the sphere over which the power is spread.
- Show that the power per unit area at the earth is approximately 1400 Wm-2 .
Plants on Earth absorb the energy from the Sun and use it to combine carbon dioxide and water to create carbohydrates. In terms of physics what is happening is charged particles are being pushed together increasing their PE, this microscopic PE is called Chemical energy. When an animal eats plants this chemical energy is converted into heat and work by a process known as respiration.
If a plant has a leaf area of 1m2 approximately how much energy would it absorb in 12 hours (one days worth) of sunlight ?
Intensity x Area = Power =1400 x 1 = 1400 W
Energy = power x time = 1400 x 12 x 60 x 60 = 60.5 MJ
Photosynthesis is about 1% efficient, how much energy would the plant store each day,
A human eats about 2,500 kcal a day, that's 10,500 kJ. How many days would the plant have to grow to store enough energy.
What is the power of a human?
time in days = energy/energy/day = 10.5 MJ/0.6 MJ/day = 17 days
Power = Energy/time
= 10,500 x 103 /(24 x 60 x 60) = 122 W
An evergreen tree with trunk diameter of 1 m is said to increase its mass by 100 kg per year. If wood contains 20 MJ of energy per kg, estimate the leaf area of this tree.
Energy per year = 100 x 20 x 106 = 2 x 109 J
The plant stores 0.6 MJ/day/m2
Energy stored per m3 each year = 0.6 x 365 = 219 MJ/year/m2
To produce 2000 MJ in 1 year requires 2000/219 = 9.1 m2
Plant material such as wood can be burned directly to produce heat. Wood has a specific energy of about 20 MJkg-1 and an energy density of 3 MJL-1.
Dead plants compressed for millions of years turn into coal with a specific energy of 32 MJkg-1. This doesn't appear to be much better than wood but the compression increases the density of the material resulting in an energy density of 72 MJL-1.
- How many times more energy is there in a truck load of coal compared to a truck load of wood?
When the bodies of millions of microscopic animals collect on the seabed, get covered with sand and are compressed for millions of years they turn into oil and gas. This has a higher energy density than coal and has the big advantage of being fluid.
- Why does the fact that oil is a liquid make it more useful than coal?
Wood can be burned in the house to provide heating. A wood burning stove is about 80% efficient which means that 80% of the energy goes to heat the house and 20% goes up the chimney.
This can be represented on a Sankey diagram. Here the width of the arrows is proportional to the amount of energy. Adjust the values below to represent a wood burning stove burning 10 kg of wood.
Heat into work
To convert heat into work requires some sort of engine. To understand the concept of a heat engine let us consider a hypothetical engine powered by a helium balloon.
- Where does the energy given to the car come from?
The car stops after a short while so to go further you would have to pull the balloon back down.
- Why is this engine pointless?
Now, imagine you used a hot air balloon instead of a helium balloon.
- How could you bring the balloon back down without doing any work?
- Where does the energy come from to power the car?
For the balloon engine to work heat must be lost, this is true for all heat engines (if you have done the engineering option you'll know this). This can also be represented on a Sankey diagram.
- Make a Sankey diagram for an engine using 10 kg of wood with an 80% efficient burner and a 60% efficient engine. Note the controls on the diagram are % heat loss not efficiency.
A turbine is a sort of heat engine. Fuel is used to boil water in an enclosed vessel causing the pressure of the steam to increase. The high pressure steam is directed at the vanes of a turbine causing it to turn.
- Why does the steam turn to water after passing through the turbine?
When a magnet is moved in and out of a coil of wire an EMF is induced. HL students will know all about this but SL students can try this with a magnet and coil. You can also try the simulation below.
An EMF is also induced if a coil is rotated in a magnetic field, this is the principle of a generator.
The resistance of the wires and friction leads to energy loss in the generator.
- Sketch a Sankey diagram for the previous wood powered generator with a 75% efficient generator.
A power station produces electricity by burning fuel. The fuel could be coal oil or gas but the principle is the same.
Here is a video explaining the process.