On Tue, 20 Sep 2005, James Newtons Massmind wrote: > Yes, and I thought about building a concrete containment room for a steam > engine in my back yard... My wife loves me, but... The containment is the steel casing of the machine. Don't worry too much about it, there is not that much energy in that small reservoir. An old book about steam engines gone bad showed extensive damage from ... an exploding flywheel (overrevved). As long as you are not standing directly next to the exploding espresso machine you should be fine. The biggest danger is the big cloud of steam that could cause burns. >> Since you inject the water into the cylinder and it becomes >> moving mass and thermal inertia (and heat absorber i.e. >> cooler for the steam that already flashed) you want it all flashed. > > Good point. Best to really "atomize" the water spray and get all of it > flashed. Or spray in less to match the available heat. First the available heat in the air in the cylinder will not be much. The heat capacity of air compared to water is negligible. Second very small droplets as you would like to make do not travel far, they decelerate very fast and form a 'cloud'. They may not hit the wall. Third, you are talking about injection pressures on par with those in a Diesel engine to make fine mist. The price of such a pump will likely kill your project very dead, at least financially, even before it starts having problems due to impurities in water and corrosion. >> In fact, why not inject a gas ? Like CO2 ? Then you have 1/2 >> of a stirling engine (well a hot air/gas engine). The only >> thing you want from water is its pressure increase when >> heated. A gas can do better here. > > Err... Are you sure? First, water is commonly available and therefore ideal > for use in a low cost system. Second, I thought the state change was what > really made the big difference in volume. Water to steam is approx 300 to > 1... Or rather 1 to 300. The point is you want work done. Volume does not do work, pressure + swept volume does. If you look at a steam calculator you will see that the pressure rises fast *but* the heat input required to achieve it is very high! Taintstaafl! Example: http://www.efunda.com/materials/water/steamtable_sat.cfm A normal gas will increase pressure according to pV/T = CvR (per Mole and standard Mole volume). Water is a 'normal' gas once it is steam, but its Cv*R is large so the pressure increase is large (but hard to bring by). Nitrogen (air with the oxygen reduced out of it) should do as well when compressed sufficiently. The difference lies in the energy density. Steam has a lot of it because of its high density. If you compress a gas to similar density then it should also work as well or better. Helium is a good choice because it has very high heat conductivity so it is easy to heat and cool in heat exchangers. The energy density of a gas under pressure is rho*P. Obviously high rho will give a high energy density for a given pressure. > What sort of expansion can you get without phase change? More than enough. The Carnot cycle, which represents the most efficient heat engine cycle does not use a phase change. Again, the best Stirling engines use Helium as it is the closest known to an ideal gas and has high thermal conductivity and low flow resistance even at high pressure. > Also note that the flashing of water to steam will not happen if the > cylinder is pressurized by the piston on the upstroke. The valving needs to Not true. The flashing is caused by the temperatue and pressure difference. You can flash 370 degree C pressurised water (at 210 bar) in a 350 degree C container by just dropping the pressure (to 160 bar). I think that you are confusing the *delayed* flashing of superheated water with normal evaporation by boiling. Superheated water will not boil before it is given a nucleation point even if it should have been steam already at that pressure. This is a way to transport energy in a compact form to where it is needed, nothing else. > In any case getting even 100W of power out of an old motor cycle engine with > a batch of mirrors pointed at it would be a great start! I think that you underestimate the losses. A small IC engine is only about 70% efficient mechanically. A small motorcycle engine may not even turn over with 100W input. Typical starter power for such a motor is 30A@12V (~360W or ~1/2hp). By scaling some numbers from: http://www.johnsboat.com/calcs-web.html 350W = ~0.5hp is made from ~0.65lb H2O/minute which requires a heating power of about 11000 Watts, just to boil it (disregarding the heat needed for superheating, which is considerable). This is the output of a 11m^2 high efficiency (>>95%) mirror in full sunlight (3.8 meter diameter parabolic dish). So your efficiency will be less than 3% before energy conversion losses. Remember the Victorians who liked steam so much had plenty of coal and wood. If you find a reproduction of that solar driven printing press from the Paris exhibition in the pre-previous century you will see that they used a mirror about as large. Back to 350 Watts that's about one man-power on a bicycle, at least at the beginning of the ride ;-) > Thanks for the conversation, I'm really enjoying it. Does anyone think they > could find the time to do a few tests with something like this? I'd be > willing to pay for results, positive or negative, given some good > photographic and metric documentation. I could suggest that you convert a small air-powered tool into a generator and try to power it with steam to see what happens. That would be the shortest way to get it going somehow. A handheld grinder would be a good start imho (high rpm and low losses, also cheap, you can get one for $30 and it will run fine on 28psi (2bar) pressure). Peter -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist