Greg Walker

Dear Marcin,

Thanks for your message. What you are doing looks very exciting. I have been exploring tesla technology, but I’m afraid my findings were that it’s going to be hard to obtain sensible power density. It lends itself to laser cutting methods. You need to watch out for the centripetal forces that build up. Better to build it big-ish – say 10” dia. Spacing plates need to be very fine to get efficient momentum transfer from steam to shaft. Potentially high maintenance. I would love to be proved wrong, but so far I really haven’t seen delivery of decent shaft power from a tesla turbine.

Transfer of solar energy to steam – I found your figure of 1KW/M2 peak solar energy density was very illuminating. I hadn’t worked it out, & think I should have done so. The trick is how much of that can you get into the water? And from the water to the shaft? It’s a lot easier to get the fluid up to 100° C than 400, but to get decent mechanical power out, you really need to be getting well beyond 100.

My steam generator delivers steam at ~80 bar & 400° C. Actually, my thoughts have been turning to using charcoal or pyrolysis oil, derived from wood, to fire this. Charcoal is good because ash content is minimal, pyrolysis oil for more convenient materials handling. Wood has its problems forming voids & avoiding wasted heat from boiling off its moisture content – hard to get it hot enough to get decent heat flow into dry steam. Therefore, potentially worth the intermediate conversion process. Great to get straight from sun to steam, avoiding all that wasteful photosynthesis, drying out, conversion & burning! Interesting metric though, is how many KW of shaft power, or electricity can you (we!) get from 1KW of sunlight? I’ve done the sums for biomass, & it’s not too far off 1 acre of growing power per KW delivered – sustainably. Talking just orders of magnitude & we’re not too far off .01% of solar power gets to the shaft going the biomass route.

When is this guy going to get to the steam engine?

You will have gathered that I want to make an engine that eats 80 bar, 400C steam, or thereabouts. Do you know what your solar generators will produce? How much of the solar power gets into the water? I want to capitalise on the bottom end of an existing 3 cylinder car engine. (eg VW Fox 1.2 litre http://cdn.volkswagen.co.uk/assets/common/pdf/brochures/fox-brochure.pdf ) Remove cylinder head & install single acting steam piston, cylinder and valves in its place. Inlet valves to provide ultra short cut off for efficiency & to get decent revs, and uniflow exhaust. Wrap that all up in a cosy blanket so we don’t waste the heat.

What I had in mind – in fact, what I have in mind, is to build something based on a single cylinder 4 stroke engine to prove the principle, & to get quantitative performance & efficiency metrics. Minimum size something like Honda GC135 http://www.honda-engines-eu.com/en/engines/models_range/gc_range/GC135/index.jsp - that’s 135cc. Maybe anything up to about 400cc. Briggs & Stratton will have something similar. I’m a bit concerned that they all seem to have splash lubrication, I’d prefer to find a small single cylinder engine with pumped lubrication, but that’s looking quite hard to find. I actually think I can get about 5 bhp @ 3600 rpm from the 135cc engine. A motorcycle engine?

So that’s where I intend to be going. The Barrett steam car uses half of a VW beetle boxer twin engine. I really need to get that going. I can do the design work (I use solidworks) on my engine development, but I’m under resourced, so progress is very slow. I’m in the UK. Had it not been for your endeavours, I believe I would have been happy to get around to my own development work once the Barrett is flying, but I’m tempted to try both in parallel if more people power can be thrown at it.

http://firedragon.com/~kap/Barrett/index.html

You have probably realised from the above that I’m really not trying to be secretive about my work, or to make money from it. Were I to try, I have every reason to believe that I, like many people much more talented than me before, would fail. I think the secret is in sharing knowledge & not trying to be greedy. To that end, I applaud the open source approach.

What about stirling engines? But that’s another story! Power density is the Achilles heel, pump it up is the solution.

Kind regards

Greg Walker