Transcript Fusion
Fusion • Powers the sun • In fusion, we combine two atoms and release energy • Easiest to do this with H or its isotopes – We already talked about the proton-proton chain in the sun • D-T reaction-takes deuterium and tritium and creates He Fusion • In order for these reactions to occur, one needs the deuterium and tritium at high temperatures • For the DT reaction, T = 40 x 106 K • For DD reaction, T = 100 x 106 K • DT is good for bombs, not so good for long term power, and tritium has a half life of 12 years Thermonuclear devices • Also called H bombs, these are fusion bombs • Require fission to compress and heat the fusion fuel. • The fusion releases enormous amounts of high speed neutrons which are then often used to induce fission in matter that it is normally difficult to induce fission in (such as depleted Uranium, Uranium composed mostly of 238U). • This adds to the radioactive fallout of the bomb Fusion reactors • Main problem is maintaining the fusion material at high enough T so that fusion produces the bulk of the energy (break even) • Confined plasma via a magnetic field so it does not contact the container walls (which would cool it and quench the reaction) • Not nearly as hazardous as fission reactors, most studies show severe failures to be contained within the plants themselves. Barriers to fusion power • Scientific feasibility of the reactions • Economics • Damage to reactor components due to large flux of high energy neutrons • Availability of materials to build the reactors • Maybe in 50 years -but we have been saying that for 50 years! Planes, trains and automobiles • 27% of the total national energy budget goes into transportation • Of this 27%, 35% is used by automobiles • Autos are among the least energy efficient modes of transportation (Bicycles are number 1) • Rely in the internal combustion engine What does it take to move a car? • Four force terms need to be considered: • Force needed to accelerate the vehicle – Fa = ma • Force needed to climb any hills – Fh=msg, where s is the slope of the hill • Force needed to overcome internal energy losses (tire flexure, wheel bearings, friction with the road surface, etc) – Fr = Crmv, where Cr is a constant term • Force needed to overcome aerodynamic drag on the vehicle, depends upon speed. – Fad = CD Af v2 /370 where CD is the aerodynamic drag coefficient, Af is the frontal area of the vehicle. • So the total force required is the sum of these 4 terms: – FT = Fa + Fh + Fr + Fad Energy required • The energy required will be equal to the work done by the force over a given distance or – E = W = Fd or – E = Pt, where P is the power output and t is the time the vehicle is operated or – E = Fvt • So to minimize energy, you need to minimize the forces. Making current cars more efficient • Minimize the force required: – ma+msg+ Crmv+CD Af v2 /370 • • • • • • Make m small Make Cr small Make CD small Make Af small Make v small Or any combination of reducing these values Alternatives to the internal combustion engine • • • • • Flywheels Electric batteries Hybrids Alcohol Hydrogen Flywheels • Energy storage device • Flywheel is spun up and the energy is stored as rotational energy to be used at a later time • Designed to resist losses of rotational energy due to friction, etc • Energy stored is given by • Ek = Iω2 where I = moment of inertial of the flywheel, and ω is the angular velocity. • The moment of inertial is a function of the mass and the distance from the center of rotation • So the structure of the flywheel and the rotational rate determine the amount of energy stored. • Ultimate limit on the energy storage is the strength of the flywheel. Spin it too fast, and it will tear itself apart. Flywheel vehicles • • • • • • Could extract energy from braking-rather than waste the energy into frictional heating of brakepads, reverse the engine and spin up the flywheel. Need to be recharged on the power gird, saves gas, but drains electricity The big implementation problem is materials which can withstand the stress needed to spin the flywheel fast enough to make this a worthwhile alternative. Prototype mass transportation vehicles have been built (In Sweden and by Lockheed) Used in Formula 1 racing to recover energy lost in braking and along with a continuously variable transmission to improve Formula one car acceleration. Also used in the incredible hulk roller coaster at Universal Islands of Adventure in Orlando, Fl. – Ride starts with an uphill acceleration, rather than a gravity drop. – Flywheels are used to provide the initial energy impulse, otherwise the park would brown out the local energy grid everytime the ride began. Hybrids • Still use gasoline powered engines, but combine them with (usually) batteries to achieve better fuel economy. • Idea is to use as small as possible a gasoline engine, and only when it can be run at peak efficiency. • Use excess power to recharge the battery (no need to tap the power grid) • Use energy from braking (regenerative braking) to also charge the battery • Work best in stop and go driving. • Major initiative in the auto industry right now. • Result in using less gas-stretching our fossil fuels Pure electric vehicles • • • • • • • • • • Powered by an electric motor, rather than a gasoline engine Needs batteries – current generation of batteries have 520 times less energy density than gasoline. Need to be charged from the power grid If all the vehicles in the US were converted to electric cars, it would triple the current electric energy generation Recharging electric vehicles takes time- several hours, whereas it takes minutes to refill your gas tank Batteries have a finite lifetime, need to be replaced every 2-3 years at a current cost of 1000 Limited range (less than 100 miles before recharging is needed) Ultimate limit is current battery technology-current lead acid batteries have not changed much in 100 years. Environmental effects from the disposal of lead acid batteries No new promising battery technologies on the horizon to substantially help electric cars Fuel cells • An electrochemical conversion device • Chemical reactions cause electrons (current) to flow • Requires a fuel, an oxidant and an electrolyte ( a substance that contains free ions and acts as a conductor) • Typical type of fuel cell is called a proton exchange membrane fuel cell (PEMFC)