Energy Degradation & Power Generation

1. Energy Degradation & Power Generation Topic 8.1 & 8.2

2. Objectives • Explain the meaning of the term energy degradation; • Understand that in the cyclical operation of an engine, not all the available thermal energy can be transformed to mechanical work.

3. The Heat Engine • The difference in temperature between two bodies offers the opportunity to run a ‘heat engine’ between those two temperatures, extracting useful mechanical work in the process. • Some of the thermal energy that is transferred from the hot to the cold body can be transformed into mechanical work.

4. Sankey Diagram Schematic energy flow diagram for a heat engine. Right: Sankey diagram – width of arrow is proportional to the energy it carries. HOT 800 J 200 J 600 J COLD

5. Efficiency • With time, the two bodies will approach the same temperature, and the opportunity for using the two bodies to do work will be lost • Thus the flow of thermal energy from the hot to the colder body tends to equalize the two temperatures and deprives us of the opportunity to do work.

6. The Thermal (Heat) Engine • Two reservoirs at different temperatures • Hot – inside cylinders where fuel is combusted • Cold – outside exhaust system (heat sink) • Any practical heat engine must work in a cycle • Ex: gas absorbs thermal energy & expands, pushing the piston out (mechanical work), piston returned to repeat the process & do more work. • Because of the cycle requirement, not all thermal energy can be converted to mechanical work. • Some must be returned to cold reservoir

7. The Thermal Engine • http://www.animatedengines.com/twostroke.html

8. Energy degradation • Some energy that could have been used for work was returned to the cold reservoir, making it less useful. It can only be used if another colder reservoir is found. This energy has become degraded. • The consequence of losing some energy to the colder reservoir in a cyclic process is a consequence of a fundamental law of physics, the second law of thermodynamics.

9. Energy degradation • Energy, while always being conserved, becomes less useful, i.e. cannot be used to perform mechanical work – this is called energy degradation.

10. Assignment • 1& 2

11. Objectives • Outline how electricity is produced.

12. Electricity Production • (almost universally) happens in electrical generators • Rotate a coil in a magnetic field so that magnetic field lines are cut by the moving coil, which (according to Faraday’s Law) produces an emf (voltage) in the coil, which causes electrons to move and energy can be delivered to consumers. • A generator converts mechanical (rotational energy) to electrical energy. The various other sources of energy are used to provide the mechanical energy for the rotating coil.

13. Electricity Production

14. Electricity Production

15. Assignment • 4 & 5

16. Objectives • Understand the difference between renewable and non-renewable forms of energy.

17. Energy Sources • Civilization has developed hand-in-hand with an increase in the use of energy • Ancient peoples had only food and sunlight & consumed no more than 8 MJ of energy/day. • Modern humans depend on energy for food, shelter, transportation, communication, manufacture of goods, services & entertainment • Average world use - about 300 MJ/person/day in more developed countries; power consumption of about 3.5 kW. (USA = 10 kW, Africa (parts) <0.1 kW). World average ~ 0.8 kW per person

18. Energy Sources • Some interesting math: • 3Kw per peron= living in comfortable conditions • 7 x 109 people = world population • 6.3 x 1020 J= Total annual world production of energy (an overestimate)

19. Energy Sources • Finite; being depleted; will run out • Fossil fuels (oil, natural gas, coal) • Nuclear (uranium) • Energy stored in sources as potential energy, released by human action • Will not run out any time soon, could be replaced quickly • Solar • Wind • Wave/tidal Non-renewable: Renewable:

20. Main Sources of Energy Energy sources and the percentage of the total energy production for each. The third column gives the mass of carbon dioxide emitted per unit of energy produced from a particular fuel. Fossil fuels account for about 86% of total energy production.

21. Assignment • 6 & 7

22. Objectives • State the meaning of the term energy density.

23. Energy Density • the energy that can be obtained from a unit mass of the fuel [units J kg-1] • If the energy is obtained by burning the fuel, the energy density is simply the heat of combustion

24. Energy Density - Nuclear • In a nuclear fission reaction, mass is converted directly into energy through Einstein’s formula E =mc2. • 1 kg uranium-235 releases 7x1013 J = 7x104 GJ • Natural uranium (mainly uranium-238) contains about 0.7% of uranium-235, so the energy density of natural uranium is , substantially higher than fossil fuels. • Enriched uranium contains 3% uranium-235 has an energy density of

25. Energy Density of Water • 1 kg water falls from a height of 100 m and all the kinetic energy is converted to rotational motion of the generator • This implies that the energy density of water used as a ‘fuel’ in a hydroelectric power plant is 103 J kg-1, substantially below the energy density of fossil & nuclear fuels.

26. Energy density & fuel choice • All other factors being equal, the higher the energy density, the more desirable the fuel.

27. Assignment • 3 & 8

28. Fossil Fuel Power Production 8.3

29. Objectives • Outline the historical & geographical reasons for the widespread use of fossil fuels • Discuss the energy density of fossil fuels with respect to the demand of power stations • Discuss the relative advantages & disadvantages associated with the transportation and storage of fossil fuels • State the overall efficiency of power stations fuelled by different fossil fuels • Describe the environmental problems associated with the recovery of fossil fuels and their use in power stations.

30. Fossil fuels • Created over millions of years by the decomposition of buried plant and animal matter under the high pressure of the material on top & bacteria -> pretty much all over the world, more under water because life has existed there longer. • When burned, thermal energy is used to power steam engines and generators. • Generally 30-40% efficient • Responsible for atmospheric pollution & contribute to greenhouse gas accumulation.

31. Fossil fuels – Power Plants • Coal ~ 30% efficient depending on tech level & precise cycles of operation. • Natural gas ~ 42% efficient

32. Example A power plant produces electricity by burning coal, using the thermal energy produced as input to a steam engine which makes a turbine turn, producing electricity. The plant has a power output of 400 MW and operates at an overall efficiency of 35%. • Calculate the rate at which thermal energy is provided by burning the coal.

33. Example The plant has a power output of 400 MW and operates at an overall efficiency of 35%. b. Hence calculate the rate at which coal is being burned (30 MJ kg-1).

34. Example The plant has a power output of 400 MW and operates at an overall efficiency of 35%. c. The thermal energy discarded by the power plant is removed by water. The temperature of the water must not increase by more then 5°C. Calculate the rate at which the water must flow.

35. Fossil fuel mining • Coal mining produces a large number of toxic substances & the coal (stored in large quantities near the mines); coal is high in sulfur content and traces of heavy metals. • Rain can wash the sulfur and heavy metal traces which could eventually reach underground water reserves (HUGE environmental issue). • Coal mining sites classified as environmental disaster areas – companies need to have plans to reclaim the area after mining

36. Pros & Cons of Fossil Fuel Use • Relatively cheap (while they last) • High power output (high energy density)\ • Variety of engines and devices use them directly and easily • Extensive distribution network in place • Will run out • Pollute the environment • Contribute to greenhouse effect by releasing greenhouse gases into atmosphere Advantages Disadvantages

37. Fossil fuels & fuel choice • Account for cost of transportation (production -> distribution), storage, and environmental costs. • Fossil fuels tend to have high costs because of large mass and volume • Extensive storage facilities • Leakages in production & distribution line = serious environmental problems

38. Fuel Choices

39. Fuel Choices