lecture_17_WEEK_10 - Energy Systems in a Sustainable World...

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Unformatted text preview: Energy Systems in a Sustainable World Conserva6on: Be9er Transport Dr. Francesco Ciucci Assistant Professor Mechanical Engineering Chemical and Biomolecular Engineering Francesco Ciucci 1 Why are we here? •  We have seen that energy systems strongly affect global warming •  To avoid global warming we need to go to 0 Carbon technology & we need to do it now!!! •  How can we address the problem ra#onally? –  Compute energy expenses (how can we reduce them?) –  Compute energy resources (which are are be9er?) –  Compute land usage (is there enough land?) Act Now! Why? Because we want to select sustainable and impacSul technologies! Francesco CIUCCI 2 Course Structure Mo6va6on 1.  Environmental impact 2.  Dwindling resources 3.  Energy security Reduce expenses & boost resources (how? EROI & costs) How do we spend energy when we drive a car? make decisions and sustain a discussion on energy Expenses Resources Select most promising! Francesco CIUCCI 3 Balance Sheet 195 kWh/day/person •  •  •  •  •  •  178 kWh/day/person Consump7on Transporta7on Hea7ng and cooling Ligh7ng Manufacturing Food Informa7on systems and other gadgets •  •  •  •  •  •  •  Sustainable Resources Wind Solar (photovoltaic, thermal, biomass) Hydroelectric Wave Tide Geothermal (Nuclear????) REDUCE EXPENSES!!! Francesco CIUCCI 4 Important conclusions we can draw from our es7mates TOTAL: 197 kWh/day/person Roughly 1/3 of our energy expenses go into transporta7on (cars, airplanes, products transport) 70 kWh/day/person Goals: 1.  Achieve beRer efficiency in transporta6on’s energy use 2.  Eliminate fossil fuel use in transport (in principle reduce CO2 emissions to zero) Can technology deliver a reduc6on in energy consump6on from transporta6on? Francesco CIUCCI 5 Cars! Or How do you win the Shell ECO Marathon: h9p://youtu.be/ID4JW-­‐20OSA Francesco Ciucci 6 Using Cars •  Energy from fossil-­‐fuel (gasoline, diesel etc) goes to Form of Energy: –  Speeding up then slowing down (with brakes) –  Air resistance (aerodynamic forces) –  Rolling resistance (rolling against ground) –  Heat (75% of the energy is thrown away as heat, because the energy-­‐conversion is inefficient) –  Electrical systems (headlights, sensors etc.) Mechanical Thermal Electrical •  Which factors impact the efficiency of the car? Francesco Ciucci 7 Using Cars Energy from fossil-­‐fuel (gasoline, diesel etc) goes to –  Speeding up then slowing down (with brakes) –  Air resistance (aerodynamic forces) –  Rolling resistance (rolling against ground) –  Heat (75% of the energy is thrown away as heat, because the energy-­‐conversion is inefficient) –  Electrical systems (headlights, sensors etc.) Francesco Ciucci 8 Driving Scenario (Braking) (a) Driver accelerates his car (Mass = Mcar) from rest 0 to speed V Start at Kine6c Energy = 0 1 Reach Kine6c Energy = M carV 2 2 (b) Maintains go back to V=0 aler a distance d 1 Kinetic Energy = MV 2 2 d = distance between traffic lights, stop signs or traffic jam) (c) Driver Breaks and goes to velocity V=0 (Kine6c Energy = 0) (d) Since Energy is Conserved change in kine6c energy needs to go into either work done or heat dissipated (heat in brakes + vor7ces around car) Francesco Ciucci ΔU = W + Q Kinetic Energy if Part of ΔU 9 Power Dissipated Car speeds up (from V=0 to V) and slows down (from V to V=0) within each distance d then power dissipated is then given by 1 M carV 2 1 M V 3 Kinetic Energy Change car =2 = d 2 d Time Between Braking Events V Francesco Ciucci 10 Using Cars Energy from fossil-­‐fuel (gasoline, diesel etc) goes to –  Speeding up then slowing down (with brakes) –  Air resistance (aerodynamic forces) –  Rolling resistance (rolling against ground) –  Heat (75% of the energy is thrown away as heat, because the energy-­‐conversion is inefficient) –  Electrical systems (headlights, sensors etc.) Francesco Ciucci 11 Kine6c Energy of Swirling air (poor man’s Aerodynamics) Car moving at speed V creates a cylinder of turbulent air, we take that V is also the velocity of the air in the tube h9p://www.youtube.com/watch? v=xbvVUrI_IDg&feature=share&list=UUyUjgCTvvFuzAcZf3IITffg Francesco Ciucci 12 Compute the Kine6c Energy of Swirling air Length of tube = L Area of tube = A Air Density Volume of tube = A L =cD Acar L ρ Mass of tube =ρ cD Acar L cD = drag coefficient 1 1 2 Kinetic Energy = MV = ρ cD Acar L V 2 2 2 Francesco Ciucci 13 Compute the Kine6c Power of Swirling air 1 1 2 Kinetic Energy = MV = ρ cD Acar L V 2 2 2 1 1 Kinetic Energy 2 L 2 L = ρ cD AcarV = ρ cD AcarV Time time 2 time 2 1 Power Dissipated = ρ cD AcarV 3 2 Francesco Ciucci V 14 Power Used by Car Opera6on According to our very simple model, the power used is the sum of two contribu6ons: 1 M V 3 car 2 d 1.  Power dissipated to braking pads 1 2.  Power dissipated to aerodynamics 2 ρcD AcarV 3 11 1 3 3 Power Dissipated = M carV + ρ cd AcarV 2d 2 Note: Cube Velocity Dependence DRIVE SLOWER!!! Francesco Ciucci 15 Ra6o Power Losses Pbraking 11 3 = M carV 2d Paerodynamics 1 3 = ρ cd AcarV 2 Ra6o between those two terms is given by Pbraking Paerodynamics 11 3 M carV M car 2 d = = 1 ρ c A d 3 d car ρ cd AcarV 2 MASS CAR MASS AIR Note: Energy dissipa6on is dominated by braking losses if the mass of the car is bigger than the mass of the tube of air from one stop sign to the next Francesco Ciucci 16 Cut-­‐off distance Find the distance d* for which the two terms are equal Mcar = 1000 kg ρ=1.3 kg/m3 1= Pbraking Paerodynamics M car = ∗ ρ cd Acar d cd = 1/3 Acar = frontal area = 2m wide x 1.5 m high = 3 m2 d* = 750 m Francesco Ciucci 17 City Driving (d<750 m) Pbraking 11 3 = M carV 2d DOMINATES If one wishes to reduce expenses, one needs to: 1.  Reduce the mass of the car 2.  Move slower 3.  Get a car with regenera6ve brakes Francesco Ciucci 18 Highway Driving (d>750 m) Paerodynamics 1 3 = ρ cd AcarV 2 DOMINATES If one wishes to reduce expenses, one needs to: 1.  Reduce drag coefficient cd (get a more aerodynamic car) 2.  Get a car with reduced cross-­‐sec6onal area 3.  Move slower! Francesco Ciucci 19 Reality Check Total Power consumed by car 11 1 3 3 Power Dissipated ≈ M carV + ρ cd AcarV 2d 2 1⎛1 1 1 3 3⎞ Power Consumed by Car ≈ ⎜ M carV + ρ cd AcarV ⎟ η⎝2 d 2 ⎠ V = 70 mph = 110 km/h = 31 m/s cd = 1/3 Acar = 3 m2 η = 25% d→∞ (Highway condi6ons) ENGINE EFFICIENCY 3 11 kg m ⎞ 3 2 ⎛ Power ≈ ρ cd AcarV = 2 ×1.3 3 ×1 m × ⎜ 31 ⎟ = 80 kW η2 m ⎝ s ⎠ Francesco Ciucci 20 Notes •  If you drive at 110 km/h for one hour, then you consume 80 kWh •  If you drive at 55 km/h for two hour (note same distance!) then you consume 20 kWh •  This theory seems quite consistent with the es6mates given earlier in the course (40 kWh for a 25 km commute) Francesco Ciucci 21 Shell Eco Marathon “The Shell Eco-­‐marathon … winners are the teams that go the farthest distance using the least amount of energy” Team Crocodile does 2184 miles per gallon (1.3 kWh per 100 km per person) at a speed of 15 mph (24 km/h). Bet: who’s the driver? h9p://www.shell.com/home/content/ecomarathon/ Francesco Ciucci 22 Summary Strategies for be9er transporta6on 1.  Short-­‐distance travel -­‐ reduce the vehicle’s weight per person, go at a steady speed and do not brake, go slower, use regenera6ve braking 2.  Long-­‐distance travel -­‐ move slower, improve aerodynamics by reducing cD and frontal area 3.  Energy conversion step -­‐ use be9er engines Francesco Ciucci 23 Ques6on 1: Can you make a car that consumes 100 6mes less energy and goes at 110 km/h? Nope unless you: •  Significantly reduce the frontal area •  Change the shape of the car so that it is very efficient aerodynamically (low drag coefficient) •  Pick more efficient engine η = 50% reduces by 50% the consump6on Francesco Ciucci 24 Ques6on 2: And what about electric cars? Electric vehicles have some nega6ves and posi6ves: •  Weight of the energy stored is 25 6mes bigger than gasoline! (nega6ve or posi6ve?) •  Weight of electric engine is 8 6mes smaller than fossil-­‐fuel engine! (nega6ve or posi6ve?) •  Electric motors have η = 90%! Francesco Ciucci 25 XL1 Volkswagen’s Concept Car h9p://youtu.be/m75t4UtX93c (>1.4 mil hkd) h9p://youtu.be/EzJNTC0GxO8 (Most efficient Car in the world) REVOLUTION IN 1986 h9p://youtu.be/69nEN6ZU1iE 26 Pair/Share Exercise (on flash card) •  What policies would you implement to reduce car energy consump6on? List and comment on the pros and cons (2 mins) •  Pair up with neighbor (3 mins) •  Form a two pair group (2 mins) •  Hand in your cards! Francesco Ciucci 27 Opportuni6es for Policies •  Fast: Cars that travel slower & steadily use less energy. •  If you want to drive more economically, then –  Do not step on the accelerator –  Brake less –  Driving in the highest possible gear •  Fuel consump7on is reduced if there is less conges7on. Stopping and star6ng, speeding up and slowing down, is less efficient than driving smoothly. (Idling in traffic is the worse!!) Francesco Ciucci 28 Let’s compare biking and driving The energy consumed by a car, per distance travelled, is the power-­‐consump6on divided by the speed 1 1 car Energy consumed per unit distance (car) ≈ ρ cd AcarVcar2 ηcar 2 car 2 η c A V Energy consumed per unit distance (car) d car car ≈ bike bike 2 Energy consumed per unit distance (bike) ηcar cd AbikeVbike Francesco Ciucci 29 Drag Coefficients VEHICLE VW Polo GTi Masera6 Granturismo Cyclist Long-­‐distance bus DRAG COEFFICIENT 0.32 0.3 0.9 0.425 Francesco Ciucci 30 Let’s compare biking and driving (cont’d) ηbike ≈1 ηcar car d bike d c c 1 = 3 2 Energy consumed per unit distance (car) Energy consumed per unit distance (bike) ηbike cdcar AcarVcar2 4 2 100 = ≈ ×5 = bike 2 ηcar cd AbikeVbike 3 3 ⎛ Vcar ⎞ ⎛ 100km/h ⎞ 2 = = 5 ⎜ V ⎟ ⎜⎝ 20km/h ⎟⎠ ⎝ bike ⎠ Cyclist at 20 km/h consumes about 3% of the energy per km of a lone car-­‐ driver in the highway Acar =4 Abike Francesco Ciucci 31 Rolling Resistance Energy consumed due to 6res and bearings of the car, the energy that goes into noise of wheels, and the energy that vehicles put into shaking the ground Frolling = Crr M car g Prolling = FrollingV = Crr M car gV Francesco Ciucci Inflate your Tires!!! Drive Light!!! 32 Summary 1.  short-­‐distance travel with lots of star6ng and stopping, the energy goes into speeding up the vehicle and payload 2.  long-­‐distance travel at steady speed most energy goes into aerodynamic losses 3.  all forms of travel require energy conversion: energy from fuel is used to accelerate the vehicle or to keep it going. Standard car most of the energy is dissipated as heat. Francesco Ciucci 33 Summary (cont’d) •  We looked at ways to reduce transporta6on costs and devised a few design principles (low frontal area, low speed, travel less, use be9er motor etc.) •  Compared various forms of transporta6on NEXT TIME: More Public Transporta6on and Storage Francesco Ciucci 34 Summary (cont’d) •  Passenger transport: passenger-­‐kilometer (p-­‐km): Examples: Passengers * kilometer •  Car carries one person for 100 km => 100 p-­‐km •  Car carries four people for 100 km=> 400 p-­‐km •  High-­‐speed trains violate many of the “design” principle men6oned above –  go fast –  weigh a lot Francesco Ciucci 3 kWh per 100 p-­‐km 35...
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