Posted by Neon John on February 21, 2007, 9:39 pm
On 21 Feb 2007 12:13:25 -0800, email@example.com wrote:
Actually you won't. An automotive compressor is similar to an
automotive alternator - it is designed to work fairly well over a wide
speed range but it doesn't work really well at any speed. Just as a
car alternator makes a pretty poor generator when efficiency matters,
so too does an automotive compressor.
To be able to supply useful cooling at idle speed (perhaps 1000 RPM
compressor shaft speed), to function well at twice-three times that
for cruising and not overheat or otherwise malfunction at maximum
engine speed (perhaps 8000 RPM shaft speed) requires several design
compromises that adversely affect efficiency.
Then there is the whole matter of frictional losses in the shaft seal.
This is considerable, especially with later model zero-leakage seals.
You'll find that many watts of energy are dissipated as heat in the
seal. I suggest getting an old compressor, gutting it of everything
but the shaft and seal, reassemble and pressurize the housing, lash it
up to an electric motor and measure the losses. BTDT but you ought to
do it too for the experience.
Frankly, you're not going to jack-sh*t with 200 watts of power. A
horsepower is 746 watts and roughly, one HP is about equivalent to 1
ton of cooling in a conventional HVAC unit. 1 ton is 12,000 BTU/hr.
At 200 watts, that's 200/746 or about 1/4 hp, ignoring losses. That's
about 3,000 BTU. After losses, figure maybe 2,000 BTU. Even the
tiniest single-room window unit can move 5,000 BTU. If everything is
well-optimized (not likely for a shadetree hack) then you might be
able to air condition a closet.
BTW, don't expect a Coefficient of Performance of 3 if you're trying
to heat with the unit. That is, pull heat from a cold area and pump
it into a hotter one. Even well-designed heat pumps rarely do much
better than 2.5 COP in the heating mode. You won't get anywhere close
John De Armond
See my website for my current email address
Cleveland, Occupied TN
Don't let your schooling interfere with your education-Mark Twain
Posted by markzoom on February 21, 2007, 11:36 pm
Can't comment on automotive compressors as I've never seen one.
I do know that alternators are obviously crap in small scale wind
(as I said, I'm not interested in cooling). As far as I can deduce,
the only change needed to convert a standard speed refrigeration
compressor to variable speed is a pressure valve instead of a fixed
As it happens a friend of mine invented a zero leakage low-friction
seal, used by nasa now, amongst others, I gather.
Thanks, but I get the jist.
It might do for testing.
Like I said, I need between 3-4kw
I don't, because 1/3 of that is usually heat from the electric motor,
which we won't have.
I expect about 2, more if I re-use some of the pressure instead of
wasting it by forcing it through a restrictor nozzle/pressure valve,
but that's another discussion.
Posted by AJH on February 23, 2007, 10:44 am
I agree which is why when he previously enquired I mooted the idea of
using a small battery as a temporary buffer. I think one of the
problems with grid tied turbines is that they don't make use of the
small amount of power they could generate below cut in speed. It could
be insignificant but a small permanent magnet alternator and a "joule
thief" type circuit could still be charging a battery at low speeds.
The compressor then only cuts in when there is enough charge in the
battery and runs for a period depending on battery capacity rather
than attempting to short cycle whenever a gust is strong enough.
The other advantage is that the market for electrical ac units is
large and competitive, so capital cost is a lot lower for a
A recent report in UK has shown an overall capacity factor below 30%
for wind power with some turbines in prominent (green sector) but
inappropriate sites generating as low as 10% of installed capacity. So
it must make sense to get the best use of the output, which in many
cases will be a non heating electrical use.
Posted by Snap Whipcrack.............. on February 22, 2007, 4:23 pm
Try a water wheel. All early settlers located near streams and rivers to
capture power from water wheels. Grain mills, power plants, factories,
saw mills, you name it. The age before steam was the most efficient.
Maybe we can go back there again when the oil runs out.
Posted by Tony Bryer on February 22, 2007, 5:11 pm
On Thu, 22 Feb 2007 16:23:28 GMT Snap Whipcrack.............. wrote :
Hydro power is the same thing on a larger scale. But unlike wind you can
store it up and then release it at controlled rate.
Tony Bryer SDA UK