Compressed Air Hybrid

Has anyone else used this approach?
Yes.
Did somebody find a significant flaw like power density, or size restrictions that make this approach impractical?

Blink has pointed out the weakness in this approach on several threads on this topic....search CR4 for air powered vehicles or suchlike. (His profile [to which I have linked] will have links to posts he has made.)

Del

The obvious flaw is that the compressor gets hot and must be cooled. The heat removed is wasted, warming up the atmosphere. Result: overall efficiency, energy in to energy out, is about 15 per cent, OK for jack hammers and mining equipment but not good for vehicles.

Fortunately, the solution to the flaw is simple; don't waste the heat. Cool the compressor by injecting water, producing a mixture of compressed air and steam. When the air expands in the expander ("steam engine"), the steam condenses to reheat the air. Overall efficiency is better than an electric generator and motor, as proven by dilligent German scietists in 1930. My opinion is that a pneumatic hybrid is preferable to, likely less costly than an electric hybrid. See US Patent 5,832,728.

Thanks for the insightful reply, esbuck. The patent you sight looks intriguing.

I see one reoccuring flaw in many people's complaint of all hybrid engines. Energy lost to heat, or any other energy conversion will always be less than the energy lost to the friction of a brake. Of course when one ignores all other concerns, higher energy transfer efficiencies are prefered. But lets not forget the hybrid part of this puzzle recovers energy that would be lost by friction in the first place.

Pneumatics for energy recovery seems to me to produce the least amount of other concerns.

HMMMMNNNNN... Sounds nice but the challenge will be to addapt with a gear set up some kind of recharging air pump that work on the fly. This way you'll have the back-up air reserve ready to 'crank that puppy up' back in motion from stops. See like an autorechargeable loop 'technically simplified' then the compressor heat may probably have some good use by transformed it to electrical supply for any other gadgets onboard. Hey who know's?

Back on Bussines,

MC

They DO work well in selected applications. For example, Ethyl Corp used robotic air motor powered gondolas hanging from a mono-rail to move reactants for TEL from one building over 300 yards to another, with 3 stories of vertical movement on each end. The gondolas even had to stop and 're-fuel' in the middle of the run as the air tank was limited to the 150 psig 'plant air'. (This was implemented 50 years ago). There were no computers or sophisticated electrical or electronic control systems, just basic and reliable air powered position switches. The gondolas would connect/disconnect automatically from both air and power connections at each end, receive the load (weighed) close the transport vessel, take off and arrive at destination , open the pressure vessel receiver, deliver their load (weighed again), close up everything, and then go back for another load. They would even switch destination receivers to the next of 10 reactors available.

However, air for automotive power source is not nearly efficient enough to compete.

"However, air for automotive power source is not nearly efficient enough to compete."

IMHO, if it's more efficient than a generator, battery, and motor, and cheaper, too, it's efficient enough to compete.

Back in 1930, the German railways methodically studied the most efficient way to couple a diesel engine to the wheels of a locomotive. They tried a mechanical transmission, a hydraulic transmission, a generator with electric motors driving the wheels, and a diesel-pneumatic drive. The diesel engine drove a two-cylinder air compressor which fed a conventional steam locomotive chassis. The diesel- pneumatic used 25 per cent less fuel than the diesel electric, pulling the same train on the same schedule over the same route. Of course, in 1930, the German railways were not about to dieselize, but the fact is that the pneumatic drive was shown to be efficient. Again, the trick is to conserve heat, not thowing it away. It's perfectly straight forward engineering, if you are willing to ignor common practice (air cooled compressors). Experimental evidence trumps academic analysis every time, and the fact that most people do it the "wrong" way doesn't make it the best way.

Ingersol-Rand isn't about to scrap their line of air-cooled compressors. The Dept. of Energy, which has spent hundreds of millions of dollars on battery research, isn't about to admit that a simple air tank is more efficient. (Presented with my patent, they said it would work, but they had "something better." They would not reveal what that was) The Swiss are researching adiabatic air compression (by definition, without loss of heat), but otherwise it seems everybody believes they know the solution to the problem of energy storage. Quite beyond the question of vehicles, there are problems like electric utility load leveling. Is it conceivable that a sane engineer would propose to store many megaWatt-hrs in batteries or that you could buy enough of them? In contrast, large air tanks (think of a cavity in bedrock, an old mine, perhaps) are dirt cheap and don't use scarce materials.

Consider this scenario. You have a dirty old coal-fired electric utility plant, and you would like a "greener" source at a reasonable price. The turbines, generators, switch gear, etc. all work well and are paid for, but the environmental costs are high. You put up a wind farm of wind turbines, and they drive simple compressors (heck, make them from old diesel truck engines), steam cooled, and pipe the air-steam output in insulated pipes down to the old steam boiler, saving some in a convenient container, like a hole in the ground. The plant works as before, tied into the grid, but nothing comes out of the stack; there is zero polution. (Sell the old coal pile) There is no thermal pollution, either, as condensers aren't necessary. If the electricity demand goes down, more air-steam mixture (I call it "wet compressed air") is stored, and if the electricity demand exceeds the wind turbine output, draw on the stored energy.

Getting back to the hybrid vehicle, there are several approaches. I could convert my Prius, using an air motor in place of the electric motor and an air tank in place of the batteries (whch will have to be replaced in a few years -- the air tank won't). Alternatively, I could convert a diesel truck or car, replacing the transmission with a compressor and motor (like the German locomotive), with an air tank to store energy for drag racing Corvettes, pulling boats up Pike's Peak, and other demanding activities. Or, I can leave the air motor in place, put in more tanks, and have the compressor at home, driven electrically. If I have a big storage tank, I can recharge the vehicle in minutes; otherwise, recharge it overnight. A pick-up truck could carry enough tankage (about 70 cubic feet) to go 200 miles on the electrical equivalent of two gallons of gas (about 70 kw-hr) which, if there are 4 seats (crew cab), would win the automotive x-prize (~$10 Million). If you are on a tight budget, the air motor can be a diesel converted to compressed air. A common rail injector system will work with air, and a change in the cam shaft would deliver one power stroke per crank revolution. However, just as the world's railways use diesel-electric locomotives, wasting fuel, because "everyone knows" how to build them, none of the entrants for the auto x-prize will use wet compressed air, because it was "not invented here." I suppose that the postal service, UPS, Fed-EX and school bus operators might like such zero-emission vehicles, but they don't know that yet; they don't listen to suggestions. Ford, GM, and Chrysler have all informed me that, on advice from their lawyers, they do not read suggestions for innovations from outside the corporation.

Why not inject liquid air or nitrogen into the compressor? This boosts efficiency of a gas turbine to around 80%. The coolant is from advanced liquefiers driven by solar and other recoverable intermittent sources. I am trying to start a blog to promote construction of a prototype at; http://liquidaireconomy.blogspot.com/

If the object is to cool the compressor and store energy at a reasonable temperature, water, with the high heat of vaporization, is very good. Perhaps liquid air would do, but it seems rather expensive. As I recall, liquid nitrogen costs about as much as beer, and it seems intuitively strange to expend energy to liquify it, then expend energy to gassify it and compress it. With the water to steam (wet compressed air), the input is air and water (cheap), and the output is air and distilled water. Thermodynamically, it costs no energy to cool the compressor. With liquid air, your expensive coolant goes out the exhaust pipe.

There have been cars powered by liquid air. The air must be warmed, gassified, before use. If you use environmental air as a source of heat, you need a huge heat exchanger, and you may leave a fog bank behind you. When the air is expanded, it cools to a temperature much below the air you used to warm it, so you leave fog and snow behind. You can use fuel to heat the air, but then why not run a diesel?

Feeding liquid air into a gas turbine may indeed increase the power, as you increase the mass of air going through it. One could possibly do that with a conventional IC engine, as for a dragster, a "poor man's supercharger". A somewhat similar scheme is used by some utilities, in which compressed air is stored and injected into a gas turbine when more power is needed. Basically, it relieves the turbine of driving its own compressor, leaving more power for usefull output.

It would be best to burn some fuel in the cryo-GT; liquid air consumption is about one-third as running fuelless. Efficiency is two times as for Diesel and is relatively constant over the load range. It is lighter weight and has longer life. Also, it cleanly burns a greater variety of fuels. Heat exchanger size is reduced in proportion to the efficiency. In Prius vehicle configuration it gets 200 mpg (gasoline) following US-06, the most aggressive standard driving cycle. Relatively little energy is "expended" to make the liquid. Solar, wind, off-peak grid and other energy sources that would be used are like an uncapped oil well.

Let me get this straight. You can start with a Prius which gets about 50 mpg burning gasloline in "free" air (about 25% efficeincy). You can get 200 mpg by using liquid air? What am I missing here? Is the liquid air used in addition to atmospheric air? If so, what is the fuel to air ratio and how does it burn and is it legal, since EPA dictates allowable fuel to air ratios? 200 mpg implies 100%, or more, efficiency, heat from fuel to power, which we all agree, I'm sure, is impossible, so where does the additional energy come from? What will it take to convert my Prius?

You are right. I have actually calculated cycle efficiency of 85 % which is result of low compression work with dense cryogenic air. Mileage in Prius configuration is 153 mpg. Addition of solar and slipstream recovery raises this to over 200 mpg. Atmospheric air is cooled by the liquid. Fuel/air ratio for a 5 bar system is about 0.01. I don't know what EPA rule is, but low fuel consumption improves emissions. Need to get an engine together before taking your Prius apart.

There is no question that it is important to try to recover the braking energy of the vehicle, which otherwise is wasted in form of heat in friction brakes.

All known systems add weight (ΔW), (mass Δm) to the vehicle, which adds to the losses from rolling resistance (ΔW*f [N], f=0.015) and acceleration resistance (Δm*a). These are the powers we are loosing at constant speed V (P [W] = ΔW [N] * f [-] * V [m/s]), or in case of acceleration "a" [m/s²]; (P [W] = Δm [kg] * a [m/s²] * V [m/s]).

Weight (mass) of the vehicle is responsible for ~88% (rolling and acceleration resistances combined) of fuel consumption in city driving, and 25% at constant speed in highway driving, remaining fuel consumption goes to aerodynamic drag; 75%!!! It is therefore important to reduce the mass of the vehicles and radically improve aerodynamics.

We are comparing apples to oranges when we compare the fuel consumption of hybrids (Prius, Insight) with relatively good drag coefficient; C_d=0.25 to ordinary cars with bad aerodynamics; C_d=0.40 or 0.8 for SUVs...

Hybrids are a temporary solution, not the answer to our energy problems. BEVs are the future, but first we will have to improve efficiency of our cars (weight and aerodynamics) and then provide electric energy from [truly] clean sources... not from coal...!

Energy storage of the liquidair/H2 gas turbine is 3.5 times as with lithium batteries, and will increase to over 100 times with development of on-board liquefiers.