More On Compressed Air

Here's a followup to the earlier entry on compressed air cars.

It turns out lots of people are looking into this. Ford and UCLA have been looking at compressed air hybrid vehicles. Here, a conventional internal combustion engine is modified (through camless valves) so some or all of its cylinders can be made to act as a compressor. This enables engine braking to convert vehicle kinetic energy into energy of compressed air, which is stored in a tank. When extra power is needed, the valves are again tweaked, allowing stored air into the cylinder. This enables the engine to operate without having to (at that moment) expend energy in a compression stroke.

Prof. Tsao at UCLA has an abstract on their work here (go down to D.19). The relevant paragraph:

The latest development in camless engine technology makes compressed air hybrid vehicle feasible. By variable valve control, the engine is configured to work in 4 different modes. During deceleration, the engine absorbs the energy of vehicle motion by operating as a compressor, charging an air reservoir with compressed air. This is the compressed braking mode (CB). Air motor (AM) mode and air-power-assisted (APA) mode allow the engine to recover the stored energy during acceleration. The other times the engine works in conventional unthrottled (CU) mode. We conducted a EPA cycle simulation and showed a 52.3% fuel consumption reduction, compared to conventional engine.

See also this page.

One issue I see with this idea is adiabatic heating of the air. If it's too hot, that complicates the design of the storage tank and reduces its capacity; also, if the air must be cooled it will lose energy. You could get around that by interposing a thermal store between the engine and tank; air flowing through the store would be cooled on the way to the tank and reheated on the way back to the engine (an example of a countercurrent exchange system.)

Storing energy as sensible heat in a material can actually be very effective. Active Power, an Austin company, sells compressed air UPS systems. Their TACAS system extends the energy density by electrically heating a block of steel. This block is used to heat the stored air before it is run through a turboexpander to drive a generator.

I wonder how expensive a small diurnal compressed air storage system for homes or small businesses would be. Many utilities have time-sensitive electricity rates, with off-peak rates a factor of five or more cheaper than peak rates. Not only could such a system produce electricity at peak times, it could also be used directly for air conditioning -- the air coming out of the expander will be very cold. And, if the owner also had a compressed air vehicle, the stationary tank could be used to at least partially 'refuel' the vehicle in a few minutes.

Improved Nuclear Fuel

Today's nuclear power reactors overwhelmingly use fuels based on uranium oxide. It's a technology that works well, but it could use improvement. Power density is limited by thermal conductivity of the oxide -- if the power is too high, the temperature at the center of the UO2 fuel pellets becomes too high, and they can be damaged or even melt. This also affects fuel performance in accidents, making it easier to keep the pellets from being damaged when cooling is impaired.

Now, researchers at Purdue have come up with an improved oxide fuel with thermal conductivity 50% higher than before. The trick is to make a ceramic composite in which the uranium oxide is mixed with beryllium oxide, which has a thermal conductivity six times that of UO2 at 1000 C. The BeO forms a network that efficiently carries heat out of the pellet.

Burnup in oxide fuels is limited by degradation of thermal conductivity with time. If the separate BeO phase does not suffer the same degradation (and remember, the fission isn't occuring in that phase), then it's possible that fuel could be left in a reactor for much longer times before being replaced, reducing both cost and waste volume.

Compressed Air Cars

Even before the electric car fad was stillborn, aborted by high costs, short range, and slow recharging, I've thought there'd be a market for cars using compressed air energy storage. Now that gas is $2.55/gallon in my neighborhood, the opportunity seems even more interesting.

The advantages over electric cars are compelling. Compressed air, expanded near-isothermally, has a considerably higher specific energy than batteries (especially if one takes into account the loss of mass of the air as it is vented). Tanks seem likely to be cheaper and longer lived than sophisticated batteries. Power can be extracted from the air and supplied to the drivetrain without electric energy conversion.

So, I was pleased to see that a company in Europe, MDI, has designed two compressed air cars and is licensing their manufacture around the world. The designs reflect experience with prototypes over more than a decade of development and show some interesting refinements.

In these vehicles, air is stored at 300 bar (4500 psia) in carbon-fiber wrapped plastic tanks. The tanks are under the floor, and in tests fail at 740 bar -- not too bad a safety factor. The tanks can be refilled in a few minutes from a stationary compressed air source, or in 3 to 4 hours at home (on 240 V AC) using a built-in electric motor to drive the car's engine as a compressor.

The first car is air-only, and has a range of 200 km on a single charge, about twice the range of electric cars. The engine has three expansion units with heat exchangers after the first two to reheat the air using ambient atmospheric heat. In the second car, the air can be preheated by an external combustion heat source. This extends the range of the vehicle and also greatly increases the amount of work that can be produced per unit of heat energy, since, unlike in a normal engine, none of that work is used to compress the working fluid. However, since the car also has a compressor, it can also operate in a mode where the engine powers the compressor to refill the tanks, operating as a kind of external-combustion air engine. So, if you really need to drive long distances, the car naturally converts to fuel-burner with very long range (2000 km on 50 liters of gasoline). In this sense it's alot like a 'plug-in hybrid'.

The price appears to be very reasonable -- 6840 and 9460 euros, respectively. I'm looking forward to hearing more about these as they move into production. Assuming they're not too flimsy for US crash standards, maybe someone will sell them here.

A Near-Term Application of 'Nanotechnology': Nanofluids

Often when we see the term nanotechnology, we think of utopian visions of submicroscopic robots, cleaning out our cells and bringing a new age of immortality and prosperity (or, an unending dark age of repression and torment, if you're being pessimistic.)

Much of the work that's being done under the rubric of nanotech is much more mundane, and might even be applied in the near term. I've just come across one of the more exciting possibilities, one that is attracting a lot of research effort. It's been discovered that suspensions of nanoparticles (of size less than 100 nm) can dramatically alter the thermal properties of liquids.

It's been found that adding 0.3% copper nanoparticles to liquids can increase the thermal conductivity by 40%. Higher thermal conductivity increases the efficiency of heat transfer in heat exchangers. This is attracting the attention of companies such as Valvoline (nanotech radiator fluid, nanotech engine oil), as well as nuclear engineers, who envision retrofitting existing PWRs with a nanofluid as the primary coolant in order to increase the thermal capacity of the reactors (new nuclear fuels with enhanced internal thermal conductivity would also help here.)

Another interesting application would be in rocket fuel. Suspend a small quantity of nanometer carbon (diamond would be particularly good, or possibly short carbon nanotubes) in RP-1 and the fuel would become a better coolant in rocket thrust chambers. This would enable the engine to operate at higher chamber pressure without excessive wall heating. If the nanoparticles were aluminum, the specific impulse of the engine would increase even without a change in the chamber pressure or geometry.

As far as I can tell they don't yet have a good understanding of why nanoparticles have this effect. It'll be interesting to see what's going on here.