Topic 8: Energy, power and climate change

1. 8.4 Non-fossil Fuel Production Topic 8: Energy, power and climate change

2. Nuclear Power Solar Power Hydroelectric Power Wind Power Wave Power Non-Fossil fuel production

3. Chain reactions

4. Controlled fission To maintain a sustained controlled nuclear reaction, there must be at least one neutron from each fission being absorbed by another fissionable nucleus. The reaction can be controlled by using control rods of material which absorbs neutrons. Control rods are commonly made of a strongly neutron-absorbent material such as boron or cadmium.

5. Uncontrolled fission A fission reaction whereby the reaction is allowed to proceed without any moderation or control rods is called an uncontrolled fission reaction . If there are too many neutrons, the chain reaction would proceed at tremendous pace and result in an explosion. An example would be in an atomic bomb where the reactions are uncontrolled. In a nuclear reactor, if the fission process is not well controlled, the large amounts of energy would cause the fuel to melt and set fire to the reactor in what is called a meltdown.

6. Fuel enrichment Uranium found in nature consists largely of two isotopes, U-235 and U-238. The production of energy in nuclear reactors is from the 'fission' or splitting of the U-235 atoms, a process which releases energy in the form of heat. U-235 is the main fissile isotope of uranium. Natural uranium contains 0.7% of the U-235 isotope. The remaining 99.3% is mostly the U-238 isotope which does not contribute directly to the fission process (though it does so indirectly by the formation of fissile isotopes of plutonium).

7. Role of control rods The control rods, an important part of the reactor, regulate or control the speed of the nuclear chain reaction, by sliding up and down between the fuel rods or fuel assemblies in the reactor core.  • The control rods contain material such as cadmium and boron.  Because of their atomic structure cadmium and boron absorb neutrons, but do not fission or split.

8. Role of moderator In addition to the need to capture neturons, the neutrons often have too much kinetic energy. These fast neutrons are slowed through the use of a moderator such as heavy water and ordinary water. Some reactors use graphite as a moderator, but this design has several problems. Once the fast neutrons have been slowed, they are more likely to produce further nuclear fissions or be absorbed by the control rod. Java applet nuclear reaction http://library.thinkquest.org/17940/texts/java/Reaction.html

9. A nuclear reactor Inside the "core" where the nuclear reactions take place are the fuel rods and assemblies, the control rods, the moderator, and the coolant. Outside the core are the turbines, the heat exchanger, and part of the cooling system.

10. Heat exchanger A heat exchanger is a device built for efficient heat transfer from one medium to another The heated water rises up and passes through another part of the reactor, the heat exchanger. The moderator/coolant water is radioactive, so it can not leave the inner reactor containment. Its heat must be transferred to non-radioactive water, which can then be sent out of the reactor shielding.

11. This is done through the heat exchanger, which works by moving the radioactive water through a series of pipes that are wrapped around other pipes. The metallic pipes conduct the heat from the moderator to the normal water. Then, the normal water (now in steam form and intensely hot) moves to the turbine, where electricity is produced.

12. Plutonium-239 U-238 is not fissile but it is useful because it can be used to produced Pu-239, a fissionable isotope. First, U-238 becomes U-239 by neutron capture: Then U-239 goes through beta decay to become Neptunium

13. Then Neptunium beta decays into Plutonium And Pu-239 is fissionable and large amounts of energy is released

14. Plutonium-239 as a nuclear fuel U-238 is 140 times more abundant than U-235. The neutrons given off in a U-235 reaction can be used to “breed” more fuel if the non-fissionable U-238 is placed in a “blanket” around the control rods containing U-235. On average, 2.4 neutrons are produced in a U-235 reaction with 1 neutron required for the next fission and 1.4 left for neutron capture by U-238.

15. Safety and risks of nuclear power Problems associated with mining of Uranium Problems with disposal Risk of thermal meltdown Risk of nuclear programs as means of nuclear weapon production

16. Biggest risk for mining of uranium is the exposure of miners to radon-222 gas and other highly radioactive products, as well as water containing radioactive and toxic materials In 1950s, a significant number of american miners developed small cell lung cancer due to radon which was the cancer causing agent.

17. The biggest concern is Pu-239 which has a half-life of approx 24,000 years. • It is also used in nuclear warheads • Presently the disposal methods include deep storage underground. • If these methods fail, there would be catastrophic consequences • Radioactive waste would find its way into the food chain and underground water would become contaminated.

18. Nuclear power using nuclear fusion The most probable way is to fuse deuterium and tritium. Deuterium atoms can be extracted from seawater and tritium can be bred from lithium.

19. Nuclear power using nuclear fusion? The basic problems in attaining useful nuclear fusion conditions are to heat the gas to these very high temperatures and to confine a sufficient quantity of the reacting nuclei for a long enough time to permit the release of more energy than is needed to heat and confine the gas. the capture of this energy and its conversion to electricity.

20. If fusion energy does become practical, it offers the following advantages: a limitless source of fuel, deuterium from the ocean; no possibility of a reactor accident, as the amount of fuel in the system is very small; and waste products much less radioactive and simpler to handle than those from fission systems.

21. Photovoltaic cells Photovoltaic devices make use of the photoelectric effect. Solar photovoltaic modules use solar cells to convert light from the sun into electricity.

22. Solar heating panels Solar thermal panels contain liquid that circulates through special panels and is heated by sunlight, this then passes through a coil in the water tank which in turn heats the water stored in the tank

23. What are the factors that would affect the amount of solar radiation that a place gets?

24. The main factors are: Geographic location Time of day (altitude of the sun from the sky) Season Local landscape Local weather The distance of earth from the sun

25. Because the Earth is round, the sun strikes the surface at different angles ranging from 0º (just above the horizon) to 90º (directly overhead). When the sun's rays are vertical, the Earth's surface gets all the energy possible. The more slanted the sun's rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year

26. Countries like the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun's rays are far more slanted during the shorter days of the winter months. Cities like Denver, Colorado, (near 40º latitude) receive nearly three times more solar energy in June than they do in December

27. 3 main schemes Water storage in lakes Tidal water storage Pump storage

28. Water storage in lakes

29. Water storage in lakes The Three Gorges Dam on the Yangtze River will be the largest hydroelectric dam in the world when it is complete in 2009. It will generate 18200MW The dam is more than 2 km wide and has a height of 185m. Its reservoir will stretch over 600km upstream and force the displacement of more than 1.3million people.

30. Tidal water storage A dam is built to catch the high tide. A sluice gate is opened to let the high tide water in The water is released at low tide, and the gravitational potential energy is used to drive turbines which produce electrical energy

31. Pumped storage Used in off-peak electricity demand period Water is pumped from low reservoir to high reservoir Generating Mode Pumping Mode

32. Energy transformations Water trapped in reservoirs have gravitational potential energy Water falls through a series of pipes where its potential energy gets converted to rotational kinetic energy that drives a series of turbines The rotating turbines drive generators that convert the kinetic energy into electrical energy by electromagnetic induction.

33. Basic features Foundation Tower Nacelle Rotor blades Hub Transformer (not part of wind turbine)

34. 1) Foundation and 2) Tower Guarantee the stability of a wind turbine a pile or flat foundation is used, depending on the consistency of the underlying ground. The tower carry the weight of the nacelle and the rotor blades, AND must also absorb the huge static loads caused by the varying power of the wind. Generally, a tubular construction of concrete or steel is used. An alternative to this is the lattice tower form.

35. 3) Nacelle and 5) Hub The nacelle holds all the turbine machinery. Because it must be able to rotate to follow the wind direction, it is connected to the tower via bearings. The build-up of the nacelle shows how the manufacturer has decided to position the drive train components (rotor shaft with bearings, transmission, generator, coupling and brake) above this machine bearing.

36. 4) Rotor and rotor blades The rotor is the component which, with the help of the rotor blades, converts the energy in the wind into rotary mechanical movement. Currently, the three-blade, horizontal axis rotor dominates. The rotor blades are mainly made of glass-fibre or carbon-fibre reinforced plastics (GRP, CFRP). The blade profile is similar to that of an aeroplane wing. They use the same principle of lift: on the lower side of the wing the passing air generates higher pressure, while the upper side generates a pull. These forces cause the rotor to move to rotate.

37. Power calculation The power in the wind is proportional to: the area of windmill being swept by the wind the cube of the wind speed the air density - which varies with altitude

38. Formula P = 0.5ρAv³ Where P: is power in watts (W) ρ: is the air density in kilograms per cubic metre (kg/m3), (about 1.225 kg/m3 at sea level, less higher up) A: is the swept rotor area in square metres (m2) V: is the windspeed in metres per second (m/s).

39. The actual power that we can extract from the wind is significantly less than what the previous formula suggests. The actual power will depend on several factors, such as the type of machine and rotor used, the sophistication of blade design, friction losses, and the losses in the pump or other equipment connected to the wind machine.

40. Wave Power Describe the principle of operation of an oscillating water column (OWC) ocean-wave energy converter Determine the power per unit length of a wavefront, assuming a rectangular profile for the wave. Solve problems involving wave power.

41. Simple animation of OWC: http://www.daedalus.gr/DAEI/PRODUCTS/RET/General/OWC/OWCsimulation2.htm Offshore OWC Onshore OWC

42. As the wave enters a capture chamber, the air inside the chamber is compressed and the high velocity air provides the kinetic energy needed to drive a turbine connected to a generator. As the captured water level drops, there is a rapid decompression of the air in the chamber which again turns the turbine that has been specially designed with a special valve system which turns in the same direction regardless of the direction of the air flowing across the turbine blades.

43. http://www.darvill.clara.net/altenerg/wave.htm http://www.alternative-energy-news.info/technology/hydro/wave-power/

44. Power Per Unit Length • Power per metre of the wave assuming the wavefront has a rectangular profile • (on the data booklet) • Power per metre = 0.5ρgA2v