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4. (10 points)You can turn coal into electricity with 30% efficiency,

3. (10 points) If that 30% in a coal-burning power plant is 75% of the best possible (Carnot) efficiency, and the plant exhausts its heat into a river at 10 deg C, how hot was the fire in deg C? actual efficiency = 30% = 0.30 = 75% of the best = 0.75 e Carnot ,

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4. (10 points)You can turn coal into electricity with 30% efficiency,

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  1. 3. (10 points) If that 30% in a coal-burning power plant is 75% of the best possible (Carnot) efficiency, and the plant exhausts its heat into a river at 10 deg C, how hot was the fire in deg C? actual efficiency = 30% = 0.30 = 75% of the best = 0.75 eCarnot, so eCarnot= 0.30/0.75 = 0.40 (40%) and ecarnot = 1 – Tcold/Thot, with Tcold = 10 deg C = 283 deg K, and Thot=??? Re-arrange—Thot=283/(1-0.40) =283/0.60 = 472 deg K = 199 deg C

  2. 4. (10 points)You can turn coal into electricity with 30% efficiency, but then lose 10% in the transmission and 5% in each of three transformers to get that energy to the customer. What is the net or total efficiency to deliver 50 kW to that customer? Total efficiency = product of each efficiency = 0.30 * (1-0.10)*(1-0.05)3 =0.2315 = 23%

  3. 6. (30 points) The US will eventually face a shortage of transportation fuel. Give advice to a national decision-maker on each of three points, with only one plus and one minus per point. Choose your points with care! It will be necessary to use numbers from your text or elsewhere that support your points. a)Tax petroleum consumption heavily, starting now. b)Make gasoline from coal. c)Use natural gas to make electricity for an all-electric vehicle fleet.

  4. 3070

  5. Today Transportation and energy. Just where does the energy go? Gravitational kinetic energy? But then you come back down. Kinetic energy? But then you stop again. NO—it’s just lost to all kinds of friction! And friction is a force which opposes the motion you want to do, for instance in pounds or Newtons. All power point images are only for the exclusive use of Phys3070/Envs3070 Spring term 2014

  6. Drag friction • Friction from the road, bearings, etc-- ‘rolling friction’ Frolling = Cr *m * v • Friction from the air, aerodynamic drag Fad = Cd * Af * v2 / 370 KEY—be consistent, in funny units

  7. Rolling friction • Fr = Cr m v, With Fr in pounds, v in miles per hour, and then Cr is about = 0.01 , and m in (POUNDS/ g=32 ft/sec2) • Cr is determined by tire type, lubrication, tire pressure, road surface…… • Example: Cr=0.01, M = 2500 pounds/32, v=30 miles/hour • Fr= 0.01*2500/32 *30 = 23.4 pounds. • Twice as fast? Twice as much force/push is needed to overcome rolling friction.

  8. Aerodynamic drag Fad = CD *Af *v2 / 370 With CD the ‘drag coefficient’, in Table 8.3 With v in miles per hour and Af the frontal area pushing through the air, in square feet, to give Fad in pounds. Example: hand area =Af =6” by 6” = ¼ square foot CD (flat)= 1.17 (worst), v= 30 mph Fad= 1.17 * 0.25 * 302 / 370 = 0.71 pounds Double the speed? FOUR times the force. For a car, Fad>Fr faster than about 40 mph

  9. Work(energy) and power,the cost of the force needed to keep going • Work = force * distance • Power = work / time = force*distance/time = force * speed (to overcome friction and stay at speed v) Power to continue at speed v ~ drag force*speed ~ speed 3 (for air friction) • Example– a car needs 23 =8 time the power to keep going twice as fast!

  10. Extend the logic The CAFÉ standards are mpg=miles per gallon =(miles/energy=work) / (gallons/energy) =(miles/force*miles) * (energy/gallon--fixed) =1/ friction force * energy/gallon—fixed. Ergo--milage (mpg) is proportional to one over the drag or friction force, which depends on mass, size, shape, and speed.

  11. Current CAFÉ standards Corporate Average Fuel Economy standards. Enacted by Congress in 1975, regulated by the National Highway Traffic Safety Administration, measured by the Environmental Protection Agency (EPA). If failed, manufacturer must pay $5.50/day per 0.1 mpg missed, for each car.

  12. Prius hybrid Honda Fit Ford F150

  13. Takeaway message? • Decrease friction 1. lighter vehicles (rolling) 2. streamline (aerodynamic) 3.smaller frontal area (aerodynamic) 4. slow down! (both) • Drive shorter distances Gallons = miles / (mpg)

  14. Measures • For people—the passenger-mile Or passenger mile / Btu of energy (Table 8.2) • For freight, the ton-mile Or ton-mile per Btu of energy (Table 8.2) Then—amount of fuel, cost of fuel

  15. Example- • Boeing 727 with 100 passengers going 1000 miles, at 270 passenger-miles / 106 Btu (Table 8.2). • Energy needed = 100 passengers * 1000 miles /(270 passenger-miles /106 Btu) = 102+3=5 *106 / 270 = 3.7 *108 Btu • If jet fuel = gasoline, at 1.25 *105 Btu/gallon, The trip burns 3.7*108Btu / (1.25*105 Btu/gal) = 2960 gallons of fuel

  16. Example --HW #9, March 15, 2010 Fly one liter of Yuppie Water the 6329 miles from Suva, Fiji to Denver. How many barrels and gallons of jet fuel (=crude oil) will this require? Ton-miles? One liter = 1 kg = 2.2046 pounds Times 6329 miles 13,953 pound-miles =6.976 ton-miles. Then ton-miles/ (ton-miles/ Btu) =6.976 / (32/106) (Table 8.2) = 0.218 *106 Btu Then bbl = Btu / (Btu/bbl) =0.218*106 / (5.80*106) =0.0376 bbl = 1.58 gallons ( at several $/gal) (ocean oil tanker gets 12,500 ton-miles/106 Btu)

  17. How to make this more efficient? Use a slower airplane! If half as fast, drag force is (½)2 = ¼ as much, and the energy (force times distance)required is (1/2)2 = 1/4 as much, and the engine need be only (1/2)3=1/8 as powerful.

  18. CAPA #3

  19. Practice– assume diesel fuel has the energy content of crude oil, and compute ton-miles per 106 Btu. Consistent with Table 8.2?

  20. Next week Monday: CAPA #3 is due Transportation, energy and safety. Lots in Chapter 8 Wednesday: Electric, bio, and hydrogen cars. No CO2! Chapter 8.3 Friday: Air pollution, but not CO2, sulfur, or lead.

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