by Danielle Fong
The clock stuck twelve. It’s October 30th. In a heartbeat I emerged an adult in the eyes of American law. In an alternate universe, I danced the night into a hazy sunrise. But I left celebration to Haight St. patrons, their addled revelry spilling muffled through the crack in my window. Tonight, we work.
Dawn, a night and two weeks later. It was ready; the design for the both the engine and the drivetrain, encoded in a scattered handful of drawings and documents, one wiki, two heads, and a thousand lines of physical simulation code. The first test: powering a scooter through a staccato ride amid frenzied Manhattan traffic, calculating, by the hundredth second, the will of the engine, and the vehicle’s reply. We’d follow a path devised to track emissions from humming, throbbing combustion engines, byproducts of fuel burnt in tiny explosions sparked every second by the thousands.
But nothing save cool air would our machine exhale. Compressed air, ‘a thermomechanical battery’ of sorts, is cheap, long lasting, and quick to recharge (one need only open a valve, and if impatient, run a pump, the tank will fill in seconds.) What’s more, it’s efficient. A batteries charge begins life in mechanical form, in a spinning turbine if charged off the grid, or in the inertia of a vehicle, during regenerative braking. This is then converted into AC electrical current, which is converted into DC current, which, finally, is converted into mechanical energy, losing power at each step. To power the engine this whole process runs in reverse! But compressing or expanding air keeps mechanical energy mechanical (so long as temperature is kept reasonably constant.) In powering vehicles it is superior to the most advanced battery systems known. That is, in every parameter but one.
Historically, the low energy density of compressed air had crippled any attempt to venture further than a couple dozen miles; physics, it seemed, demanded tanks of excessive proportions to travel longer. At 300 bar (‘scuba pressure’), compressed air could release only half a percent as much energy as the same volume of gasoline burnt. We understood, however; it was an efficiency war. We knew that conventional vehicles were incredibly wasteful. There were many battles left to fight.
The Laws of Thermodynamics1
“You can’t win.”
“You can’t break even.”
“You can’t give up.”
We hunted losses relentlessly. We were repaid with a series of compounding improvements, each building upon another, reversing the conventional patterns of efficiency losses endured by vehicles for more than a century. Finally, in a brilliant and unusually compact layout by Steve Crane, we found room to replace the paltry 1.3 gallon gas tank with one ten times its size. Nights yielded to our toil, and, slowly but surely our enemy routed.
“We’ve cracked the code,” we exclaimed. “The city is ours to conquer.” On the highway, whatever benefit earned by our scooter’s light weight, low rolling resistance and ultra-efficient regenerative braking would be dominated by air resistance.2 But air resistance falls quickly with speed, and in the stop-start motion of the city our combined inventions would give our scooter an efficiency historically unmatched.
I keyed in the last few drivecycle parameters, drew a shallow breath, cocked my head, and pressed the enter key. The simulation lasted only a moment, but in that time, my little scooter ran more than one hundred and twenty miles, the equivalent of dozen rides between Wall Street and the Bronx on a single tank. “We’re in business,” I said. With that, and for all of a New York Minute, the questions, worries and restlessness retreated from our hearts. We huffed. “What’s next?”
: To paraphrase C. P. Snow. Hat tip to Jonathan Smith.
: Scooters are not particularly aerodynamic vehicles. Ordinary scooters have a drag coefficient of nearly 0.9, and a frontal area of 0.6 meters squared. We hope to achieve a drag coefficient of 0.6, similar to faired motorcycles ridden upright, but due to the rider’s position this will be difficult: some have described the aerodynamics of a scooter as like a “brick attached to a parachute.”
Congratulations! And happy birthday.
I read the LightSail Designs Technologies page a few days ago. Impressive stuff. Right off the bat you have the double-acting piston. You’re not only working on the basic problem of efficiency but adding amenities, such as the pneumatic suspension and HUD.
This is a great age to be working on real work, when many people our age have no idea what they want to do, and older people have already given up on making anything useful. Onward ho!
Happy belated b-day. Great blog, and very interesting work! You might be interested in an expansion of the implications of thermodynamics that I posted about a while back: (http://quietlightfalling.blogspot.com/2008/08/entropy.html)
Congrats! Sounds cool. Do you mechanically repressurize the tank from your regenerative braking, or does that go to an electric pump?
Ah. NOW I’m somewhat excited.
Let me drop one intriguing tip to consider. If it entices you, you know where to contact me. ^_^
Use a graphite piston perfectly fitted into a smooth fused quartz cylinder. (They have virtually the same thermal expansion coefficients in operational temperature ranges.) No seals. No static friction.
(Chill the piston in dry ice or liquid nitrogen to get it to shrink so you can get it into the cylinder.)
So, I did some calculations of my own out of curiosity, and now I’m not so optimistic about compressed air. As a storage medium, compressed air is not lossless. There are several major losses incurred during the compression of the air: inefficiencies in the compressor, and thermodynamic losses from the heat dissipating from the compressed air, and according to the calculations I did, the thermodynamic losses are huge.
Here’s the premise.
When you compress a gas, it experiences adiabatic heating. For a given reduction in volume, the most work you can do would be adiabatic compression, where none of the heat can dissipate. The least work you can do for the same compression is isothermal compression, where all of the heat due to compression is dissipated as the gas is compressed. If your compressed air tank holds air at scuba-tank pressure (200-300 bar) at room temperature, the compressor would have to dissipate a huge quantity of heat from the compression of the gas; that molar quantity of gas, if compressed adiabatically, would be over thousand degrees. (In fact, at compression above 250 bar, the ideal gas law no longer works as an approximation of the behavior of air.)
All of the heat dissipated during compression is energy lost. The storage tank will be hot after being filled with compressed air, and it will dissipate energy as it cools down until it matches ambient temperatures. However, there are even more losses during decompression: gases experience adiabatic cooling when decompressed, especially when decompressed too quickly to absorb significant quantities of heat from the surrounding, such as when driving pistons in a compressed air engine. Your compressed air storage tank will get extremely cold as you let air out. As you know from thermodynamics, adiabatic expansion is the *least* work a volume of gas can do, yet the entire operation of a compressed air engine is in the realm of adiabatic expansion (if you’re not going to use fuel to heat the compressed air).
If you put heat absorption fins on your compressed air tank to absorb as much ambient heat as possible as it decompresses and somehow manage to achieve near isothermal expansion, at best you lose all the energy dissipated as heat during compression. If your tank is insulated, or doesn’t have a high surface area to volume ratio, at worse you lose all the energy between the adiabatic expansion curve (on a PV diagram, starting at ambient temperature and the compressed volume of a full tank) and the adiabatic compression curve (starting at the intersection of ambient temperature, the original volume of all the air that went into the tank). This is a huge quantity of energy lost. A casual inspection of a graph of these curves shows that much more energy is lost than is stored and recoverable even in the best case scenario.
(If you have a Mac, I can send you the Grapher file with my calculations.)
Have you taken the losses due to dissipation of heat into account?
(Let me correct something I said; the molar quantity of gas in a scuba tank might not heat up to a thousand degrees under adiabatic compression, but it would be several hundred degrees at least.)
The problems with adiabatic compression and expansion were obvious from the outset. We’ve taken these and many other losses into account.
The heat of compression can be stored as well. However, heat is available over a range of temperatures and needs to be stored that way. There are some new patents out there.