by Doug Coulter » Sun Jan 09, 2011 12:51 pm
I've sometimes traded stock in Capstone Turbine, a little company that seems to have a novel approach to all this, and I even considered buying one for backup power use here. Their thing has air bearings and eats a variety of different fuels, a do-all engine. They don't make ones small enough for me, though, and I'd guess there are scaling issues with making one both efficient and small, the old surface area to volume ratio thing again, or something like it.
Does anyone know offhand the basic efficiency of a gas turbine in say BSFC or pounds per horsepower-hour so I could compare it with a good IC engine? I know a small, high compression Honda fixed engine is really pretty good there with a little bit of tweaking, which is what I used in my high efficiency generator. Which needs repair next time the weather is permitting. Blew a head gasket (this time). Running one of these wide open to get the thermodynamic efficiency high means they tend to not have a super long service life, though. What usually does them in though is spinning the seals on the crank bearings (Honda uses ball bearings there!), or the doggone cam drive gear slips, it's only pressed on the crank, and it's not a clearance engine, so that's instant total destruction. Next one I buy, I might take apart and weld that sucker on, instead.
Of course, the basic objection to IC engines (other than wearing out) is the noise they make. It's very quiet here, and I like that. Diesels stink and sound like they are gargling rocks -- I prefer the swiss watch sound myself, and no water cooled auto/tractor type engines seem small enough to do well in efficiency for this application. And are picky about feeding -- gotta have the right fuel. Smallest one of those I've seen is about 20hp (and it's on my tractor) which is about 4 or 5x what my batteries can eat on charge easily.
Ideal in some ways would be a small, water cooled, fuel injected liquid fuel engine that made about 5-6 hp, but of course no one makes that. I'd guess one reason is that with the small displacement, the surface area to volume ratio in the cylinders gets high and heat losses from that kill the efficiency. But such a thing could probably be made to eat almost any liquid fuel and live. BillF sent me a link once to a home made engine that used solenoids for the valves, so you could effectively adjust compression ratio on the fly with valve timing!
Some alcohols have syntheses that mainly need heat to make them go, which makes them solar-power-able. That would make for an unlimited capacity "battery" which would be super nice to have. Till then, gotta get out that vanadium pentoxide and try to make that redox battery, but the membrane they need is super expensive and not found on street corners. It's not that I don't get enough kilowatt hours out of my PV panels, it's that they don't come when I need them. Storage is the issue there.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.