Low-energy air conditioning with seawater

[Larry Shick]

Over a decade back there was a company that sold air cooling devices for cruising sailboats. A sailboat at anchor is chronically short of electrical energy unless one runs a generator, which has its own trade-offs. The essence of operation of these devices was to pump seawater up from some small depth (maybe 15' below the surface), through a radiator, and back over the side. The radiator had fans to blow outside air through the radiator and into the cabin.

The company subsequently went out of business as far as I can tell. I'm thinking of trying a DIY implementation, but before I do, I thought I ought to check whether they went bust because of bad engineering/physics or bad business judgement.

Assuming the following does not look too much like "homework": Suppose that I have a standard automotive radiator, copper (because of the sea water if for no other reason), and I pump (say) 4GPM (15LPM) of seawater through the radiator, where the seawater is 10°F (6°C) below the air temperature. Air would be moved by some small "muffin" fans. What is a reasonable expectation for heat extraction or, alternatively, what kind of temperature drop is it reasonable to expect from ambient air to exhaust air? If I see 5°F temperature drop, it might be worth doing. The water flow rate above is the flow rate of a common marine pump and is arbitrary. I kind of suspect that building some kind of shroud that would pass the air multiple times through the radiator would not pay off, but would appreciate confirmation or correction.

I have some college physics, but the textbook was printed on parchment.

Thank you.

 

[Larry Shick]

For what it's worth, this seems to be classified as an "open-loop direct surface-water cooling" (DSWC) system. There's a writeup at http://www.hvac.okstate.edu/Papers/Mitchell_and_Spitler_2013.pdf which indicates (p.126) that the upper limit for input water temperature for "sensible cooling" is about 65°F (18°C). My intended usage (Chesapeake Bay in July) would not come close to meeting that requirement.

 

[jim hardy]

i think that's right - we perceive dewpoint as much as temperature
and dewpoint above ~65 F is 'muggy'
and to produce that dewpoint you'd have cool your air to that temperature or lower.
I'd wager that's why the limit in your link. You can't reduce dewpoint below your inlet water temperature.




Temperatures in Chesapeake Bay:
http://mddnr.chesapeakebay.net/bay_cond/bay_cond.cfm?param=wt&station=cb52
http://mddnr.chesapeakebay.net/cfxgraphicsserver/2778.jpg

and comfort vs dewpoint:
http://www.crh.noaa.gov/lmk/?n=humidity

Meteorologists routinely consider the "dewpoint" temperature (instead of, but analogous to absolute humidity) to evaluate moisture, especially in the spring and summer. The dewpoint temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated. Although weather conditions affect people differently, in general in the spring and summer, surface dewpoint temperatures in the 50s usually are comfortable to most people, in the 60s are somewhat uncomfortable (humid), and in the 70s are quite uncomfortable (very humid). In the Ohio Valley (including Kentucky), common dewpoints during the summer range from the middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have been recorded, which is very oppressive but fortunately relatively rare.

nifty 'heat index' calculator here
http://www.crh.noaa.gov/arx/heatindex.php