Power Consumption in Transportation
To this point in the class, we've talked about:
Electrical science
"The Grid"
Power Production technologies, including:
Alternative means of producing power
Total resource costs of that power
Possible breakthroughs in power production
The surprisingly strong need for energy storage
The quirks of attaching renewable power sources to the grid
Today we FINALLY turn to Power Consumption
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
World per capita energy consumption:
From the International Energy Agency (2003):
http://en.wikipedia.org/wiki/List_of_countries_by_energy_consumption_per_capita
More quantitatively:
For perspective, an abbreviated bar graph:
Or, in rank order:
(World Bank 2011)
Some lower ranked but affluent industrial economies:
Germany
4003.3
France
4030.5
Japan
3898.4
http://en.wikipedia.org/wiki/List_of_countries_by_energy_consumption_per_capita
Message received?
We North Americans are profligate energy consumers
But, yes, our Canadian neighbors have hung right in there with us!
We're in a club whose other members are either the world's oil producers
Or where something really strange is going on:
Iceland: Cold as heck + most abundant/cheap geothermal in the world?
Finland: Damn cold (=> saunas?), but how on earth can they afford it?
Trinidad and Tobago: I haven't a clue
So not only are we the climate change bad guys
But maintaining our economy and lifestyle is going to be particularly difficult
E.G., about 75% more difficult than for even economic leader, Germany!
http://en.wikipedia.org/wiki/List_of_countries_by_energy_consumption_per_capita
So how do we in the U.S. consume so much energy?
And are THEY the big culprits? For example "they" = the military industrial complex
Or some other "they" over which you feel almost no personal control?
Or are WE the big culprits?
We humble U.S. citizens in our apartments & 3 BDRM homes
The U.S. Energy Information Agency's Breakdown (as of 2014):
Supply
Consumption
Source: EIA 2014 - http://www.eia.gov/todayinenergy/detail.cfm?id=16511&src=Total-b1
Converting the consumption end to an easier to read pie chart:
(also omitting the EIA's embarrassing use of antiquated BTU units)
Exports (fuels):
12.01%
Commerce:
17.93%
Residential:
21.13%
Transportation:
27.01%
Residential + Transportation = 44%
Industrial:
31.48%
Big Surprise! / Big Deal!
Industry + Commerce = 45%
No big surprise
But is that really about me (personally)?
The 21.13% residential figure is certainly all about your personal choices
And as for how the 27.01% transportation figure breaks down
Plotted from 2009 EIA data:
Whoops!
59.7% = "Light Vehicle"
= You and Me (directly!)
Plus 7.7% Air
Probably still pretty much us directly
Plus 22.1% Truck
Us directly/indirectly
http://news.thomasnet.com/IMT/2012/03/12/the-damage-done-gas-addiction-edition-how-detrimental-is-petrol/
As immortalized in the 1971 Earth Day comic strip Pogo, by Walt Kelly:
The complete strip of that day:
Pogo, by Walt Kelly, Post Hall Syndicate
Even more iconic for older Americans: Images from the "Gas Crises"
President Carter's "sweater" speech:
New national speed limit:
The seemingly incontrovertible messages (loathed by many Americans to this day):
Energy Conservation = Being cold
Energy Conservation = Going really slow
Yes, if (absent foresight) you must cut your energy consumption right now!
But DO note that above was the latest in decade long series of gas crises!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
However, (at least in the U.S.) cold and slow aren't really required:
Because in the last century we got really used to being a big energy producer
Not just in isolated TX or PA oilfields, but in even "downtown" LA !
Long Beach CA
Signal Hill CA
Venice CA
Beverly Hills High School
Placentia CA
Photos from: http://www.theatlantic.com/photo/2014/08/the-urban-oil-fields-of-los-angeles/100799/
Which led us to becoming really great . . .. well . . . energy wasters
Which, ironically, means that we now have an exceptionally large number,
of often painless (or almost painless) ways
in which we can drastically reduce our energy consumption
While not only remaining warm, but living in more comfortable homes
While still driving quickly, alone, in our big cars
While personally saving a lot of money doing this
While simultaneously creating jobs
While stimulating our economy
Indeed, reducing energy consumption is easy compared to increasing production!
Photos from: http://www.theatlantic.com/photo/2014/08/the-urban-oil-fields-of-los-angeles/100799/
Today let's focus on transportation (with a later lecture on housing):
Look again at that plot of U.S. energy consumption in transportation:
As engineers our first instinct is to dive into efficiencies
Especially efficiencies of things affected by our engineering field
But for maximum impact, we must address technologies using the most energy
Regardless of whether they are now efficiency winners or losers
http://news.thomasnet.com/IMT/2012/03/12/the-damage-done-gas-addiction-edition-how-detrimental-is-petrol/
So focusing on impact:
For best perspective we might convert above data to "per-ton" or "person-mile"
But even without such corrections, the message is crystal clear:
Don't focus on buses or trains - They are doing just fine!
Shipping (despite turbulent wakes) also looks pretty damned good
So we can't just blame it all on importing too much from China
And I don't know what can be done to improve pipelines
As turbulence and oil/fuel viscosity are hard to beat
And vehicular transport alternatives (below) are hugely worse
Leaving us with the inescapable baddies: Cars, Trucks and Planes
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
So how could you improve the efficiency of automobiles?
The list of possibilities is very long
And it can be very hard to sort which ideas offer the greatest impact
Especially as many of those ideas are just that: ideas
Or are clouded by marketing hype
But there's one sure way of gaining perspective: Old fashioned Newtonian Physics
It can't tell us what technology will achieve
But it will tell us what technology cannot possibly achieve
So to identify mechanisms and limits, I am going to follow our textbook's lead
And do some calculations on both auto and airplane physics
Which, even if you studied the book's technical chapters,
Might still be helped by a little review and clarification
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
The physics of automobile transportation:
City driving (stop and go)
Here power goes mostly into the car's own kinetic energy
Which is SPENT on every acceleration, then LOST on every braking
Approximate the car's city driving (stop and go) as:
Driving a distance d, at velocity vcity, stopping, then all of this repeating
Ignoring lesser energy losses to air resistance and rolling friction
In each of those intervals, fuel energy goes into kinetic energy of:
Ekcar_kinetic = ½ Mcar vcity
2
It spends that much energy (getting going) once every time interval = d / vcity
So average power (energy/time) = Pcity = Mcar vcity 3 / 2 d
So how do you increase car's city driving efficiency?
Answers are going to come right out of:
Pcity = Mcar vcity 3 / 2 d
1) Slow down (reduce vcity)
2) Find route with fewer stop signs / stop lights (increase d)
3) Decrease car's weight:
Buy a smaller car ("But that's downright un-American!!")
AND/OR buy a car built using lighter materials
Electric car's heavy batteries are going to be a problem!
4) Don't throw kinetic energy away every time you stop!
That is, rather than heating brake discs/shoes, RECAPTURE that energy
Its called "Regenerative Braking" - I'll come back to this a bit later
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
CASE 2) Highway driving:
The interval between accelerations is now vastly stretched out
Diluting the (acceleration) kinetic energy expenditures of CASE 1
Dominant energy loss then becomes the loss to air friction (a.k.a. "drag")
That is, passage of car accelerates a volume of air up to almost the car's speed:
=> Transferring kinetic energy to individual air molecules
Volume then gradually slows and expands, as Ekinetic of 1st air molecules
is then shared with vastly larger number of air molecules
Car image from: www.clipartlord.com/category/transportation-clip-art/
Energy loss to drag can be modeled as follows:
Consider cylinder of air dragged immediately behind car
It will attain almost the car's velocity
But its cross-section will depend on car's streamlining
Better streamlining, less air accelerated => smaller cross-section (A):
Aair = cdrag Acar That is, it will be car's frontal cross-section x cdrag
With cdrag likely being < 1 and decreasing with streamlining
Behind car, in time t, accelerated air volume = Aair (vhighway t), moving at ~ vhighway
Then, for air of density rair, can calculate that air's gained kinetic energy
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Kinetic energy gained by car's trailing cylinder of air:
Eair_kinetic = Edrag = ½ Mair vhighway 2 = ½ rair (volume of air) vair2 which becomes
= ½ rair (Aair vcar t ) vhighway2 => ½ rair (cdrag Acar vcar t ) vhighway3
Power = Energy / time = (equation above) / t: Pdrag = ½ rair cdrag Acar vhighway3
From this, how do you increase car's highway efficiency?
1) Slow down
Reduce vhighway
2) Reduce the car's drag
By reducing cdrag (involving the shape of the car)
Or by reducing Acar (involving the size of the car)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Two ways of reducing a car's drag:
1) Drag Coefficient (cdrag), relating to the streamlining of the car:
For modern cars (≠ Citroen 2CV):
Honda Insight
0.25
Prius
0.26
Renault 25
0.28
- Somewhat boxy Polo: cdrag = 0.32
Honda Civic
0.31
- Teardrop shaped Insight: cdrag = 0.25
Volkswagen Polo 0.32
Streamlining => diminishing returns
- Total range: Ratio of 1.5 to 1
Peugeot 206
0.33
So more severe, streamlining is not
Ford Siesta
0.34
likely to be a "silver bullet"
Audi TT
0.35
Honda Civic
0.36
Citroen 2CV
0.51
Especially as we are already cutting into:
head &, cargo room, utility
After "Sustainable Energy without the Hot Air" (page 257)
Second way of reducing a car's drag:
1) Drag Area (cdrag Acar) in m2:
Honda Insight
0.47
Volkswagen Polo 0.65
Honda Civic
0.68
"Typical Car"
0.8
Volvo 740
0.81
Land Rover
Discovery
For modern cars (≠ Citroen 2CV):
SIZE MATTERS A LOT!
- Small Honda Insight: 0.47
- "Typical Car:" 0.8
Sub-range: Ratio of almost 2 to 1
- Land Rover pushes ratio to almost 3:1
1.6
- As would popular large U.S. SUV's
Full range: Ratio of 3 to 1
("We have met the enemy, and he is us")
After "Sustainable Energy without the Hot Air" (page 257)
OK, but that physics ignores energy losses IN the car:
That is, preceding accounts only for energy used in ideal movement of the car
Equaling the kinetic energies imparted to the masses of the car and air
But to supply that energy, we are going to have to use a lot more energy
With difference going into inefficiencies of engine, drive train . . .
As manifested by all sorts of things heating up
With that waste buying us nothing in the way of movement
So where IS car's input energy (from gas, diesel or batteries) used?
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
For city driving:
Where the energy goes in a 2005 Toyota Camry:
POWERTRAIN
VEHICLE-Related
Idling
8%
Fuel Tank
100%
Engine
Engine Loss
76%
Aero
3%
16%
Driveline
Driveline
Losses
3%
13%
Rolling
4%
Braking
6%
Only 13% of the fuel's energy makes it into things we calculated above!
From MIT lecture "Electric Cars 101" by Dan Lauber:
http://www.slidefinder.net/e/electric_vehicles_101_introduction_dan/evs101-11-13-09%28web%29/10697203
For highway driving:
Where the energy goes in a 2005 Toyota Camry:
POWERTRAIN
VEHICLE-Related
Idling
0%
Fuel Tank
100%
Engine
Engine Loss
77%
Aero
10%
23%
Driveline
19%
Driveline
Losses
4%
Rolling
7%
Braking
2%
Still use only 19% of the fuel's energy!
From MIT lecture "Electric Cars 101" by Dan Lauber:
http://www.slidefinder.net/e/electric_vehicles_101_introduction_dan/evs101-11-13-09%28web%29/10697203
Internal combustion engines waste ¾ of the energy!!!
Question #1) Where the heck is all of that energy going?
Into heat of exhaust gas + Heat of that massive engine system itself
Question #2) Isn't this a one line (killer) argument for electric cars?
After all, aren't electric motors more like 80-90% efficient?
Department of Energy / EPA webpage seems to support that conclusion:1
Electric vehicles convert about 59%–62% of the electrical energy from the grid to power at the
wheels—conventional gasoline vehicles only convert about 17%–21% of the energy stored in
gasoline to power at the wheels.
But let's think about this a little more deeply:
- Car's battery is charged from our largely hydrocarbon powered grid
- Power plants convert hydrocarbon fuel to electricity at efficiency ~ 33%
1) http://www.fueleconomy.gov/feg/evtech.shtml
2) http://en.wikipedia.org/wiki/Fossil-fuel_power_station
2
So the true "apples to apples" comparison should be:
Hydrocarbon energy to car movement energy conversion efficiency:
Gas powered cars: 17-21%
Electric cars: (59-62%) x (33%) = 19.5 – 20.5%
Add to that the facts that:
For decades we have worked hard to clean up automobile exhaust
But put FAR less effort into cleaning up fossil fuel power plant exhaust
Result: Per gram hydrocarbon burned, cars release less CO2 than power plants
Putting this all together (for our current types of power plants):
Total energy efficiency of gas vs. electric cars is a dead heat
With gasoline cars now having a significantly lower net carbon footprint
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
What I just did, shown diagrammatically:
x 17 – 21%
Gasoline Car
Efficiency:
17 – 21%
+
=1
OR
x 33%
x 59 – 62%
Electric Car
Efficiency:
19.5-20.5%
It's as if both cars come attached to an oil well + refinery OR a coal mine
But electric car then adds a fossil fuel electric power plant
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
What about "hybrid" cars like the Prius?
x 25 - 35%
+
x 17 – 21%
Hybrid Efficiency:
25 - 35%
Gas Car Efficiency:
17 – 21%
=1
OR
x 33%
x 59 – 62%
Electric Car
Efficiency:
19.5-20.5%
Hybrids ALSO use gas as their energy source, bypassing coal power plant
AND they use that gas more efficiently (via some of the tricks that follow)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Yes, when our power plants are more efficient and cleaner
ELECTRIC CAR'S total efficiency and cleanliness WILL improve!
For example, if all our power came from hydro, solar, wind and nuclear
THEN electric car's energy/greenhouse gas footprints will be due to only:
Getting materials for, and manufacture of, the power plants and the car
=>"Life cycle carbon & energy footprints" = Topic of a later lecture
But, at least for now, we cannot write off the good old internal combustion engine
For which the obvious action is to, somehow, make it more efficient!
But during "gas crises" of 70's & 80's, manufacturers swore little could be done
Were they right?
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
EPA data on gasoline IC engine carbon footprint & fuel economy:
1975-2013: Internal combustion car's CO2 emissions/mile fell by 50%
1975-2013: Fuel economy figures rose by 100%
(Although, in fine print, EPA confesses that gas mileages are all inflated by 20%)
Source: www.epa.gov/otaq/fetrends.htm
With shape of the data telling a really interesting story:
Gas Crises => Action
Neglect
Oil state wars etc. => Renewed Action
With clear implications:
- Technological fixes were pretty EASY
- Willpower fixes were the hard part
Leading to:
New CAFE (Corporate Average Fuel Economy)
standards requiring another 100%
improvement in fuel economy (to 55 mpg!)
This time the industry did NOT oppose and
instead only wanted to ensure a level playing
field between competing companies
Source: www.epa.gov/otaq/fetrends.htm
A problem(?): As engines get more efficient, they also get quieter
From the 21 January 2015 business section of the Washington Post:
“America’s best-selling cars and trucks are built on lies:
The rise of fake engine noise”
“Stomp on the gas of a new Ford Mustang or F-150 an you’ll hear a throaty rumble – the
same style of roar that Americans have associated with auto power and performance for
decades. It’s a sham.”
Fearing a loss of vroom-vroom (male?) customers:
Ford has created artificial engine noises and is secretly
piping them in through the vehicle’s hi-fi speakers
http://www.washingtonpost.com/business/economy/americas-best-selling-cars-and-trucks-are-built-on-lies-the-rise-of-fake-enginenoise/2015/01/21/6db09a10-a0ba-11e4-b146-577832eafcb4_story.html
Vs. a serious Idea: Keep engine nearer to its most efficient speed
This relates to what is called an engine's "Torque Band," with possibilities such as:
For two alternative engines:
<= RPM for max power OR max torque
For optimum power and efficiency let's
guess that sweet spot is ~ 4000 RPM
http://en.wikipedia.org/wiki/Power_band
But a CAR must go a full range of speeds, so invoke gear ratios of a transmission:
Engine's Speed
1st Gear
2nd Gear
3rd Gear
4th Gear
4000 RPM
Car's Speed
0 MPH
70 MPH
That only sort of works:
With acceleration, transmission allows us to repeatedly drop down engine speed
So engine spends more of its time vaguely near target of 4000 RPM
But what if we had an infinite number of possible gear ratios?
So that whatever the car's speed, motor could stay at ~ optimum speed?
Thus my wife's new Subaru has a "Continuously Variable Transmission" (CVT)
It uses a chain belt running between two continuously adjustable pulleys:
5:1 Ratio
http://nissanaltimaaustin.com/altimascvt-keeps-moving/
1:1 Ratio
Gear equivalents at bottom
Another serious idea: Regenerative Braking:
Above, "city driving" figure stated ~6% of power went into heating of the brakes
Physics said that in city driving, air turbulence, friction . . . were insignificant
Energy use was ALL about putting kinetic energy into the motion of the car
And then throwing it away every time you stopped!
What if, instead, you could STORE that energy as you stopped
And then just reapply it, to accelerate back up to speed?
You'd get back the 6% now lost to heat in the brakes
With simpler electric motor drive chain, you might actually save full 13%
Plus, if you shut down the motor when stopped, save 8% of power lost to idling
Total possible savings => ~ 20% !
How might you store that energy as you stop?
In an electric car, or hybrid electric car, the answer is really simple:
Use torque from slowing wheels to turn electric motor into a generator
As is done in gas-powered HYBRID CARS, such as the Prius
Which also switches off its engine when stationary:
http://www.cvel.clemson.edu/auto/systems/regenerative_braking.html
But what if it's not an electric car?
Kinetic Energy Recovery Systems (KERS) of Formula I race cars store energy in:
Heavy, metal, flywheels
A new flywheel-based KERS system being tested in Jaguar's cars:
Differential / Coupling Gear
Flywheel
Continuously Variable Transmission
http://www.wired.com/2010/10/flywheel-hybrid-system-for-premium-vehicles/
Or Mazda's unique approach: "i-ELOOP"
Car engines normally drive car PLUS an "alternator" to charge the battery
Battery then runs the starter PLUS air-conditioner, HiFi, WiFi, GPS, videos . . .
An "alternator" is just another name for a generator/motor
But driving it continuously adds to the load on the engine (eating fuel)
With i-ELOOP, the alternator is driven only when foot is off the accelerator
When foot returns to accelerator, super-capacitor energy is transferred to battery
Mazda6 with i-ELOOP => 40 MPG highway / 28 MPG city:
http://www.mazda.com/technology/env/i-eloop/
While accelerator is off, the alternator quickly charges a super-capacitor
Can do same things for Trucks - So let's move on to airplanes:
Does air travel being such a "bad actor" somewhat surprise YOU?
After all, how many trips do we (or "average U.S. citizens") take per year?
One, a few, a half dozen? (even including business travelers?)
Or could all of our Amazon.com airfreight deliveries have really hurt that much?
I made a half-hearted (but unsuccessful) try at answering both questions
But I'm pretty sure real answer is just that Flight uses LOTs of fuel!
(Which will come back to haunt us in the Carbon Footprint lecture)
So let's again follow Sustainable Energy Without the Hot Air's lead
and now learn a bit more about the physics of flight:
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
First, to offset gravity, airplanes REALLY want to have lift:
To offset force of gravity, exploiting Newton's Action = Reaction:
An airplane MUST steadily push LARGE volume of air downward (caused by
wings)
Omitting details (such as wingtip vortices) and approximating as a simple cylinder:
Masscylinder = density x volume = ρair x (vplane t Arealift_cylinder)
Where ρ is air density, A is cylinder's cross-sectional area
And (vplane t ) is the distance the plane flies in a time t
Jet image from: www.clipartlord.com/category/transportation-clip-art/
(continuing)
If, due to plane's passage, that air is forced downward at a velocity uair:
Cylinder's downward momentum = Mair uair = ρair vplane Alift uair t
Downward force = D Momentum per time which is then:
Forceair = ρair vplane Alift uair
Which must be in balance with the force of gravity = Mplane g
Equating and solving for uair = Mplane g / (ρair vplane Alift)
Use this to calculate the kinetic energy lost to that now downward moving air:
½ Mair uair2 = ½ (ρair vplane t Areacylinder)(Mplane g / (ρair vplane Alift))2
= t (Mplane g)2 / (2 ρair vplane Alift)
So power (energy/time) in that lift air is Plift = (Mplane g)2 / (2 ρair vplane Alift)
But in addition to energy expended on lift is energy to drag:
For which the analysis is just like that for the earlier highway-driving car
And which (after updating the subscripts) gives us:
Pdrag = ½ ρair cdrag Aplane vplane3
Combining (and color coding) both LIFT and DRAG power expenditures:
Ptotal = Plift + Pdrag = (Mplane g)2 / 2 ρair vplane Alift + ½ cdrag ρair Aplane vplane3
With: Alift: Circle ~ size of plane's wingspan, Aplane: Circle ~ size of fuselage Xsec
Converting power per time to energy per distance:
Energy / distance = (Energy / time) (time / distance) = Power / velocity =>
Eper_distance = (Mplane g)2 / 2 ρair vplane2 Alift + ½ cdrag ρair Aplane vplane3
(To figure out fuel used, we'll need to factor in jet engine's efficiency e)
To minimize this (absolutely essential for airline's economic survival!) must have:
d/dvplane (Energy / distance) => 0
After some math (which I did, to check the book) you find that:
ρair vplane_optimum2 = Mplane g / (cdrag Aplane Alift)1/2
Left side = Things WE can tweak: speed and altitude (which affects air density)
Right side = Fixed things + plane's decreasing weight as it burns off fuel
Which brings us to some surprising conclusions:
From optimization:
ρair vplane_optimum2 = Mplane g / (cdrag Aplane Alift)1/2
Conclude (unlike cars) that for planes going slower is NOT better
In fact, if go higher (into thinner air) plane should speed up!
If this optimum speed condition is substituted into energy consumption equation:
Eper_distance = (Mplane g)2 / 2 ρair vplane2 Alift + ½ cdrag ρair Aplane vplane3
Find that, at optimum speed, first (lift) and second (drag) term are equal, thus:
At most efficient speed, plane spends ½ its power on lift, ½ on drag
With the total energy spent per distance then becoming:
Eper_distance_at optimum_speed = (cdrag Aplane / Alift)1/2 x Mplane g
Working from that final optimized equation:
Eper_distance_at optimum_speed = (cdrag Aplane / Alift)1/2 x Mplane g
Plane's ENERGY EFFICIENCY is not improved by:
- Making plane bigger or smaller: Changes in A's cancel, negating effect
- Changing altitude: Because air density has dropped out!
(But at higher altitude can go faster for same energy per mile)
Plane's ENERGY EFFICIENCY is improved by:
- Decreasing drag coefficient by making plane more "streamlined"
Limited by need to retain space for paying passengers / cargo!
- Making the plane lighter, which could be done three ways
By building it with lighter structural materials OR
Or hauling less/fewer passengers, cargo, bags OR lighter fuel
And they ARE indeed working on making planes lighter:
By (where possible) removing metal in favor of lightweight plastic composites
And discouraging (or charging for!) excess luggage
However, short of ALSO charging passengers for their weight
Which HAS been suggested and would reflect real airline costs!
The only remaining "knob to turn" would seem to be the fuel weight:
But to travel the same passenger/cargo distance,
You can't just cut the amount of fuel, unless:
You get more energy per kilogram out of that fuel
Leading us to this table and its rather surprising (and disappointing) entries:
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Energy density of fuels (and would-be fuels / energy sources):
Conventional Battery
=
0.001 x Gasoline's energy density
PC Battery
=
0.01 x Gasoline's energy density
TNT (0.65 Cal/gm)
=
1/15 x Gasoline's energy density
Butyl alcohol
=
0.9 x Gasoline's energy density
Kerosene / JP-4 / Jet Fuel
=
0.93 x Gasoline's energy density
Gas / Diesel
=
1 x Gasoline's energy density
Liquid Natural Gas
=
1.3 x Gasoline's energy density
Hydrogen
=
2.6 x Gasoline's energy density
(Uranium or Plutonium
=
106 x Gasoline's energy density)
Only significant improvements to kerosene/JP-4 are:
Liquid or pressurized gases!
Which would THEN require MUCH heavier fuel tanks (!@#!!$#!)
Data mostly from from Richard A. Muller's book "Physics for Future Presidents"
OK, but what about engine efficiency (as overlooked by physics)?
They are certainly working on jet engine efficiencies
Often via better materials that are lighter and/or more heat tolerant
But they've also been tinkering with the basic operation of the jet engine
E.G. by increasing the size of the up front air-driving "fan" blades:
Early "turbojet" jet engine
Later "bypass turbofan" jet engine
Modern turbofan with even more pronounced enlargement of front fan blades:
http://en.wikipedia.org/wiki/Jet_engine
http://aviation.stackexchange.com/questions/11586/what-is-a-high-bypass-geared-turbofan-and-why-is-it-so-much-more-efficient
Then there is United Technologies' "Billion Dollar Bet" 1 on:
The Geared Turbofan
The idea:
- You push more air by making the front blades bigger than the rest of the engine
- But those "fan" blades push most efficiently when turning at modest speeds
- Where later blades of fuel-burning section of engine work best at higher speeds
- But in conventional turbofan, they are on the same shaft, turning at same speed
So add a gearbox between them (allowing front fan blades to slow down):
2
1) http://www.forbes.com/sites/danielfisher/2013/01/23/the-billion-dollar-bet-on-jet-tech-thats-making-flying-more-efficient/
2) http://www.airplanegeeks.com/2012/01/24/episode-182-alan-epstein-and-the-geared-turbofan-engine/
Or, if you want all of the details:
3
The goal:
"16% greater fuel efficiency while reducing the noise footprint by up to 75% 4
3) http://airinsight.com/2012/10/02/airinsight-technical-analysis-the-core-of-the-pw1000g-geared-turbo-fan/#.VN40n8bPeEQ
4) United Technologies full page ads running in the Washington Post, February 2015
16% would be nice, but it's still not an air travel game changer
So how DOES flight consume ~ 1/3 of truck OR ~ 1/10 of car fuel total?
Well let's work out some simple fuel consumption numbers:
Say as "average American" I drive my car (mostly alone) 12,000 miles per year:
Which (at least if its a new car) is supposed to get ~ 24 miles /gal*
I would then personally consume 508 gallons of gasoline per year
Say, in a plane, I also took one cross-country flight per year (not alone!)
JFK to LAX to JFK = 2 x 2775 miles = 5550 miles = 8880 km
With "Hot Air's" 747 plane number of 25 passenger-km / liter of fuel =>
=> 355 liters ~ 80 gal which is already over 1/10 of my car's consumption!
NOTE HOWEVER PER KM:
Car ~ 9.4 km/liter
Plane ~ 25 km / liter
*Source: EPA automobile fuel economy chart from an earlier slide
What about saving energy in other modes of transportation?
I dismissed ships, busses and trains because of their small overall impact
But, shouldn't we still try to make them more efficient? Yes, of course!
For instance, most all of the above suggestions for cars will also apply to busses
But trains are going to be hard to improve, because:
As one long body they are already well streamlined (Pdrag = small)
And they already minimize starting and stopping (Pkinetic = small)
And with steel wheels on steel tracks, running friction is small
And finally, they already avoid going up and down hills
And surprisingly (at least to me) slow cargo ships are almost as efficient
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Yielding energy per freight load mass per distance transported:
After figure in "Sustainable Energy without the Hot air" page 92
Energy (kW-h / tonne / km)
1.5
1.0
0.5
0
Speed (km / h)
25
50
75
Images from: www.clipartlord.com/category/transportation-clip-art/
900
and
https://lionelllc.wordpress.com/page/2/
Or energy per passenger per distance transported:
After figure in "Sustainable Energy without the Hot air" page 128
Energy (kW-h / 100 passengers / km)
150
Cars (gas or electric): Assuming single passenger
Plane/Ship: Assuming full-ish passenger load
75
50
25
0
Speed (km / h)
50
100
150
200
900
Images from: www.clipartlord.com/category/transportation-clip-art/
SUMMARIZING: To cut energy in transportation, priorities should be:
Cars and Trucks (assuming we will resist buying smaller and/or slowing down):
Streamline (where possible) or at least reduce forward cross-section
Continue improving engines / transmissions
Add regenerative braking
Airplanes: BIGGEST IMPACT = Fly less
Teleconferences! / Local Vacations!
Short term (small gain): Further trim plane's weight / develop engines
Medium term: An opportunity for otherwise questionable biofuels?
Long term: Figure out lightweight tanks for compressed gasses?
Trains: Damned good as they are but we should certainly USE THESE MORE!
Ships: Sure, some gain from less cross-oceanic importing, however:
Impact on total energy spent is still small compared to things above
Credits / Acknowledgements
Some materials used in this class were developed under a National Science Foundation "Research
Initiation Grant in Engineering Education" (RIGEE).
Other materials, including the "UVA Virtual Lab" science education website, were developed under even
earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate
Education" (NUE) awards.
This set of notes was authored by John C. Bean who also created all figures not explicitly credited above.
Copyright John C. Bean (2015)
(However, permission is granted for use by individual instructors in non-profit academic institutions)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm