Vehicle efficiency
My friend Mark sent me a link to a book in progress about sustainable energy. One of the more interesting sections is on vehicle efficiency. The author stresses that, while some kinds of efficiency gains are physically possible, others are not:
Could we make a new car that consumes 100 times less energy and still goes at 70mph? No. Not if the car has the same shape. The energy is going mainly into making air swirl. Changing the materials the car is made from makes no difference to that. A miraculous improvement to the engine could perhaps boost its efficiency from 25% to 50%. But the energy consumption of a car is still going to be roughly 40 kWh per 100 km.
The story is a familiar one: efficiency can get you a long way, but there are no free rides. Another interesting comment from this chapter is the major design differences between an efficient city car and an efficient highway car. Since the former is always stopping and starting, low weight is really important. Brakes that regenerate energy also make a big difference. For a highway car that avoids major acceleration and deceleration, the most important thing is reducing drag. Weight is comparatively trivial.
One other interesting assertion is that the energy involved in making a car is actually pretty trivial compared to the amount used in driving it around:
The energy cost of making the raw materials for a one tonne car is thus equivalent to about 3000 km of driving; an appreciable cost, but probably only 1% of the lifetime energy-cost of the car’s fuel.
If correct, that makes it seem a lot more reasonable to upgrade from an old and inefficient vehicle to a newer and less gas-thirsty model. It also suggests that government programs to replace inefficient cars with better ones might have strong justification, in terms of climate change mitigation potential.
In order to move to a low carbon society, we need to do a slew of things. We definitely need to increase the energy efficiency of accomplishing most tasks. We definitely need to reduce the quantity of greenhouse gas produced in the process of generating a unit of energy. We probably need to significantly reduce total energy consumption. Finally, we need to take actions that manage the greenhouse gasses that will inevitably be produced by some actions. The protection and enhancement of carbon sinks (mostly forests and soils) are essential for this.
When it comes to reducing total energy usage, the chapter does make one excellent suggestion: “a cyclist at 21 km/h consumes about 30 times less energy per kilometre than a lone car-driver on the motorway: about 2.4 kWh per 100 km.” Those who cycle more slowly are likely to be even more efficient, since doubling the time it takes to travel somewhere apparently reduces energy usage by three quarters.
May 9th, 2008 at 11:55 am
Planes
“Half of the work done by a plane goes into staying up; the other half goes into keeping going. The fuel efficiency at the optimal speed, expressed as an energy-per-distance-travelled, was found in the force (C.18), and it was simply proportional to the weight of the plane; the constant of proportionality is the drag-to-lift ratio, which is determined by the shape of the plane. So whereas lowering speed-limits for cars would reduce the energy consumed per distance travelled, there is no point in considering speedlimits for planes. Planes that are up in the air have optimal speeds, different for each plane, depending on its weight, and they already go at their optimal speeds. The only way to make a plane consume less fuel is to put it on the ground and stop it. Planes have been fantastically optimized, and there is no prospect of significant improvements in plane efficiency.”
…
“Possible areas for improvement of plane efficiency
‘Laminar flow control’ (cunning trick for reducing drag a little). Flying wings: said to be 25% more fuel efficient. Propfans instead of turbofans? Said to be 12% more efficient for short journeys (less than 3000 km), but not for long journeys. They’re more efficient because the engine efficiency is greater.
Formation flying in the style of geese could give a 10% improvement in fuel efficiency (because the lift-to-drag ratio of the formation is higher than that of a single aircraft), but this trick relies, of course, on the geese wanting to migrate to the same destination at the same time.
Optimizing the hop lengths: long-range planes (designed for a range of say 15 000 km) are not quite as fuel-efficient as shorter-range planes, because they have to carry extra fuel, which makes less space for cargo and passengers. It would be more energy efficient to fly shorter hops in shorter-range planes. The sweet spot is when the hops are about 5000 km long, so typical long-distance journeys would have one or two refuelling stops. Multi-stage long distance flying might be abou”
…
“Earlier in this chapter, however, our cartoon made the assertion that the transport efficiency of any plane is about
0.3 kWh/tonne-km.
According to the cartoon, the only ways in which a plane could significantly improve on this figure are to reduce air resistance (perhaps by some newfangled vacuum-cleaners-in-the-wings trick) or to change the geometry of the plane (making it look more like a glider, with immensely wide wings compared.”
May 10th, 2008 at 9:49 am
“40 kWh per 100 km”
This strikes me as very high, which seems to be the trend for environmentalists talking about cars, they just don’t seem to know anything about cars so when they come up with these estimates they don’t question them. What is a kwh? 746watts for an hour. Now, it’s a bit strange to talk about kwh per 100km, it would be easier to talk about kw per km/h, and since all I’m doing is taking the “h” and moving it across the equation (it becomes a division), there is no issue. Of course, you can no longer increase the other side of the equation because a cars resistence will go up logrithmically with speed, but then again, it’s impossible to give an “average” kwh per km because the kwh varies so drastically with speed.
For example, a really awful car like the Jaguar XJ-S (don’t worry Benn if you’re reading, I still think it’s pretty), uses about 50 “road horsepower” at 50miles per hour, and 80 “road horsepower” at 70mph. What is a road horsepower? It is the imperial unit equivalent to watts per hour. More accurately, 746 watts per hour.
So what’s 40kw for 100km/h? it would be 40/0.746 road horsepowers at 100km/h. That’s 53 road horsepower. Ok, so, fair enough. Maybe it is a reasonable number.
Still, there are vehicles today which use drastically less road horsepower at that speed. the LOREMO is a good example, its road horsepower figure isn’t quoted, but on the other hand it can achieve 160km/h with 20horsepower, which logically means its road horsepower at 100km/h is less than 10.
http://en.wikipedia.org/wiki/Loremo
We can expect that other cars in the extremely high miles per gallon range to have similar highway figures. The interesting thing about highway mileage is no tricks with batteries and electric motors tacked on can increase mileage - those help only when one is speeding up and slowing down again. Steady state motoring are a fairer test of a cars ability to pass efficiently through the air.
One more thing, “The energy cost of making the raw materials for a one tonne car is thus equivalent to about 3000 km of driving; an appreciable cost, but probably only 1% of the lifetime energy-cost of the car’s fuel.”
How many 1 ton cars do you know run for 300 000 km? 300 000 might be easy for a big family car that’s well maintained, but those weigh at least 1.5 tons. a 1 ton car is a city runabout in our world. The Loremo, however, might be an exception depending on its maintenance costs.
May 10th, 2008 at 12:03 pm
What… is the air speed velocity of an unladen swallow?
May 10th, 2008 at 12:13 pm
R.K.,
Estimating the Airspeed Velocity of an Unladen Swallow
Hashing out the classic question with Strouhal numbers and simplified flight waveforms.
Although a definitive answer would of course require further measurements, published species-wide averages of wing length and body mass, initial Strouhal estimates based on those averages and cross-species comparisons, the Lund wind tunnel study of birds flying at a range of speeds, and revised Strouhal numbers based on that study all lead me to estimate that the average cruising airspeed velocity of an unladen European Swallow is roughly 11 meters per second, or 24 miles an hour.
May 10th, 2008 at 12:22 pm
Tristan,
1. All the calculations are at the beginning of this chapter.
“Now, Ad is the volume of the tube of air swept out from one stop sign to the next. And ρAd is the mass of that tube of air. So we have a very simple situation: energy dissipation is dominated by kinetic-energy-being-dumped-into-the-brakes if the mass of the car is bigger than the mass of the tube of air from one stop sign to the next; and energy dissipation is dominated by making-air-swirl if the mass of the car is smaller”
2. As mentioned above, the book is still in progress. I am sure the author would be happy to correspond with you about such things.
3. One kilowatt-hour (kWh) is actually 1000 watt-hours, or 3,600,000 joules, or 3.6 megajoules.
May 10th, 2008 at 12:26 pm
Some comparative numbers from the book:
Car at 110 km/h: 80 kWh/(100 km)
Bike at 21 km/h: 2.4 kWh/(100 km)
Internal combustion engine train at 200 km/h: 2.9 kWh/(100 seat-km)
Transrapid train at 200 km/h: 2.2 kWh/(100 seat-km)
Airbus A380 at 900 km/h: 27 kWh/(100 seat-km)
May 10th, 2008 at 12:32 pm
Related posts: ground transportation.
May 10th, 2008 at 5:03 pm
The unit KWh per 100km is a bit strange. Why not just express the energy consumption in kilowatts? Why does everything have to be a static unit rather than a variable one?
I do understand what KW hour is, I just prefer to divide out hours and talk in kilowatts.
May 10th, 2008 at 9:29 pm
At the start of his book, he explains that he wants to explain all energy use in the unit most familiar to consumers. Since kilowatts are the only unit of energy we buy directly, it is what was selected.
May 11th, 2008 at 12:36 am
“Since kilowatts are the only unit of energy we buy directly, it is what was selected.”
No, he is using Kilowatt hours, where he could be using kilowatts. Kilowatts are a power unit, and kiowatt hours are an energy unit. It is much easier to use units of power when we are talking about what it takes to push something along.
“Could we make a new car that consumes 100 times less energy and still goes at 70mph? No. Not if the car has the same shape.”
Anyone who thinks efficient cars are the same shape as non fuel efficient cars probably works for Chrysler and can be safely ignored. It seems to me that this question is deeply flawed. What does it mean for a car to consume 100 times less energy?
A gasoline engine converts the energy in the fuel into mechanical forward thrust with much heat loss - so it is inefficient. A gas turbine engine is much more efficient, but doesn’t adapt well to variable use situations. If we were happy to have slowly accelerating cars with a top speed of only 120km/h, we could run gas turbine engines and have huge increases in efficiency simply because the motor itself burned fuel more efficiently.
An electric engine in the wheel is very efficient at turning electricity into motor force, but batteries are heavy which means there is more car to “motor force” along.
Certainly the only non-nonsense way of interpreting the question is can we make vehicles which can travel along highways at seventy miles per hour while using an amount of energy which we can reasonably forcast to be cheap and plentiful. And to this question the answer seems to be an obvious yes. Cars like the loremo already achieve 5 times the miles per gallon of a normal family car. Thus, they could be operated if fuel prices were 20$ per gallon. Equiped with electric propulsion, the amount that electricity would have to cost to make it expensive to run this machine, would be the state of things only in a world where we used electricity to do almost nothing.
Cars that use around 10 horsepower, or 7kilowatts, to move them at a steady state of 100km/h, should cost 7 times the price of kilowatt hour to run. Currently, a kw/h costs a few cents. Even if it cost 2 dollars, it would cost 14$ to travel 100km. That is very similar to what it costs to run an SUV today (say 12 liters per 100km, at 1.20$ a liter - about 14$ per hundred clicks).
But, what would a world look like where the price of electricity was two dollars per kilowatt hour? Running a home computer, which draws say half a kilowatt, would cost a dollar an hour to run - that’s 12$ a day if it’s off half the time. Of course you can say the computer is effectively free to run because its a heater and its heating the room exactly as efficiently as a 500watt heater and that may be true, but who could afford to heat their house with electricity at this price? No one presumably.
The point of this argument is just to show that with existing technology, the idea that we can be priced out of driving is unfathomable.