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re: wanted: 5-10 hp steam engine
15 dec 2001
fred b mcgalliard wrote:
>dear nick. exactly what were you expecting?
less handwaving.
>i provided the raw assumptions solar insolation = 1kw/m^2
ok for the sahara at noon. how about am2 (800 w/m^2)?
>collection efficiency = 80% (...a bit high for a small home built system)
what's the collection temperature and how is the collector insulated?
what's the concentration ratio? is there a selective surface? what kind?
>steam engine efficiency = 40% (assumes a very good engine but no million
>dollar commercial turbine)
i've heard 6% for small steam engines. what's your cold source temp, and
where's your carnot calc, and theoretical engine output calc? the carnot
calc assumes 0 output...
for example, starting with an 80 c fluid and a 30 c heat sink, the carnot
limit for a heat engine is 1-tc/th = 1-(273+30)/(273+80) = 14%, using
absolute temperatures, but that only applies if the engine is not making
any usable power, ie if it isn't cooling the hot fluid at all.
the maximum theoretical usable output from a heat engine at temp te is
proportional to the power obtained by cooling the hot fluid times the
carnot limit, eg (353-te)x(1-303/te), maximized when te = sqrt(353x303)
= 327 k or 54 c, when the input/output efficiency is (80-54)/(80-30)
= 0.52 (since we only cool the hot fluid to 54 vs 20 c) times carnot,
(1-303/327) = 0.073 to make 0.52x0.073 = 0.038 overall, ie 3.8%.
and reasonably-priced engines only achieve half of that theoretical max,
so a system like this might be less than 2% efficient. a solar trough
built into an attic with a 30'x60' south roof that collects 250 kwh/day
of heat might only make 5 kwh/day of electrical energy.
>alternator =90-95% (not too hard to achieve...
where do we buy one of these?
>the arithmetic is obvious isn't it? figure out how much power you want out
>of the alternator, divide by all the efficiency numbers and you get the
>insolation input you need in kw.
let's try that, with all the efficiency numbers, and off-the-shelf parts.
nick
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