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re: concentrator questions & clarifications
26 apr 1997
david wells wrote:
>nick pine wrote:
>> >> aerospace engineer rudy behrens (aurora farms, 1547 n trooper road,
>> >> norristown, pa 19403/(610) 489-6356) says making a small steam engine
>> >> is a piece of cake, very old technology involving about a half-hour of
>> >> calculations on the back of an envelope. start with a briggs and stratton
>> >> 5 hp gasoline engine, add oil to 150 psi steam, modify the timing to get
>> >> 30-40% efficiency, watch out for explosions, $250 or so... who needs pvs?
>nick-
>i bend down, offering my humble apologies!!(and i am not kidding)
>you are absolutely correct in bringing up steam (heat) engines as a good
>competitor to pv....
thanks for the prostration dave, but its enough to hear someone say
i may be right :-)
>i've just been focused too much reecently, i think, on pv.
a lot of people seem to be. a curious and expensive hobby. lots of people seem
too focused on electricity too, vs house heating, since the price and amount
of electricity used in an most houses is small and decreasing, vs the price
and amount of heating oil.
and are we overcomplicating all this, and pursuing unreasonable technical
sophistication in search of a small gains in efficiency, if not for its
own sake...
>of course, non of the technical history on solar heat engines is
>concisely available in any little handbook anywhere, but i did find a
>very interesting bit of historical data in lof's book active solar
>systems (mit press, london, england, 1993).
i've never seen that book. i'd like to look at it sometime. the steam
engine part of this problem seems straightforward. the sun is just another
heat source, usually uneconomical compared to coal, oil, etc, because
the collection system costs a lot.
>it should be pointed out that most of the systems lof discusses are
>relatively low-temperature systems as concentrators go.
that's good, for simple hardware and low thermal losses and high solar
collection efficiency on hazy days. not so good for carnot efficiency, say
(450-150)/(460+450) = 33%, if 150 psi steam is 450 f, not bad, especially if
the fuel is free, and the "waste heat" has some value, eg to heat a house.
>here is a summary of info from table 19.2, from page 732 of this 962
>page textbook:
p/ppump efficiency
>vendor/type expander pwr(kw) temp(c) press(atm) expand cycle
>airresearch/turbine 1.7/.3 88 6 .81 .08
>"" "" 12/.7 93 6 .85 .1
>"" "" 37/4 93 6 .85 .09
>barber/nichols turb 34/1.1 93 6 .75 .1
>carrier/turb(r113) 20/1.3 149 9 .8 .14
>energytech(water) 22/.1 160 4.6 .78 .20
>g.e./rotaryvane(fc-88) 2.3/.4 149 13.8 .83 .13
>"" "" "" 7.9/.8 149 13.8 .83 .15
>barb/nichols(r-113) 1.8/.1 91 2.6 .67 .08
>utrc/r-11 16/1.2 149 17 .76 .13
what's the first column? heat/electrical output? is the last overall
efficiency, sun in/mechanical power out? how about explaining the first line?
what does 1.7/.3 mean? is the working fluid at 6 atm and 88 c?
>i will add to this list also the data from the jpl conference
>proceedings in 1982 and 1982 about the organic rankine turbine
>
>vendor/type expander pwr/pp temp(c) press(atm) eff,ex eff,cyc
>barb/nich(turb,toluene) 16/.7 400 40 .83 .23
too exotic, expensive and dangerous.
better to modify a $250 5 or 10 hp briggs and stratton lawn mower engine. or
start with a $1k turbocharger, with 250 psi steam, if you want more like 25 hp.
>...the turbine quoted above was spinning at 60,000 rpm, and coupled to a
>samarium-cobalt generator...
seems like the wrong track, for small-scale production, altho fast-spinning
things like dental drills tend to weigh less than those that spin more slowly,
for the same power, as do power conversion things at higher frequencies, eg
switching power supplies..
>at united stirling, we used an induction generator to produce a.c. power
>to the grid...
good idea, but non-mass-produced stirling engines seem like a bad idea,
economically. why are they not mass-produced? the demand isn't there. why no
demand for solar stirling engines? grid electricity is so cheap: 2 cents/kwh.
>i pick up my handy-dandy grainger catalog
good idea :-)
>and look up high efficiency 3-phase motors and get the following for 1725 rpm
>motors(which are cheaper and more efficient than any other speed):
>hp kw eff(%) cost cost/kw
>5 3.73 90.2 $506 $135
>10 7.46 91.7 $824 $110
>20 14.9 93.0 $1361 $91
>50 37.3 94.5 $3052 $82
>100 74.6 95.4 $6845 $92
>
>...the induction motor is king in both cost and efficiency for producing ac!
take a look at these "low-efficiency" motors on pages 13-15 of
grainger's 387 catalog...
hp stock # eff(%) cost cost/kw
5 3n659 85.5 $205 $60
10 3n661 87.5 $359 $48
20 5n364 88.5 $577 $39
50 5n901 91 $1379 $37
>...think 1800 rpm.
good idea, altho some of grainger's 3600 rpm motors like the 20 hp one above
are cheaper than the 1800 rpm equivalents, with half the torque, and smaller
belts and shafts and pulleys required, and less needed gear reduction from
a turbine.
>would you believe that in fact there are off-the-shelf, 1800 rpm
>expansion "engines" available now?
sure.
>there are a boatload of manufacturers of compressors, air compressors,
>refrigerant compressors. most of them will laugh at you if you call and
>mention the word solar.
with good reason.
>but you might get one who will listen to your marketing pitch, and maybe help.
why should you need "help," if these things can pay for themselves. almost all
of those 19th century solar steam systems were technical successes and
economic failures. (pvs are a lot more expensive.)
think rotary. this gives you the options as follows:
>piston
>rotary
> vane
> scroll
> screw
>
>now, you can't use the piston as-is.
from what rudy says, piston compressors are the most suitable, and only minor
modifications are required, and pistons are about twice as efficient as
turbines when using less than 250 psi steam, at which point turbines become
more efficient. i think it's important to minimize the steam volume, ie
"the pressure vessel," for safety's sake.
>but you can use the typical rotary compressor in reverse.
hmmm...
>and, of course, nobody wants to pay for the development. but then,
>there is you.
right. that's what tinkerers are for. this is not rocket science, but it helps
to know some thermodynamics. neither is it economical, with grid power at
2 cents/kwh.
>wanna get started developing a 1800 rpm expansion engine?
no. i'd like to help design more 100% solar heated houses.
>i'd start by calling earl knox company (800)228-9167. you can
>get a rebuilt rotary screw "air end", which is used on utility trucks
>for compressing air, for the following bargain prices:
>
>hp sale price
>7.5 $1045
>20-25 $1265
>25 $1375
>50 $2,491
more than a lawn mower engine...
nick
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