How to Sleep Comfortably on a Hot Night • Hacker News Board

Re: How to Sleep Comfortably on a Hot Night

#31

Post by bootload » Mon, Aug 17, 2009, 9:44 PM UTC

nomoresecrets wrote: post #25

bootload wrote: post #13

"... How to Sleep Comfortably on a Hot Night ..." hotest it got here in Melb last year was 46C and multiple days above 43C with extreme fire weather ~ http://www.flickr.com/photos/bootload/sets/72157614397725714... Couple of measures you take: - open windows - multiple 4Ltr freezer blocks work well - stay at the pool till late - sleep on concrete or tiles - use fans - wet towels & buckets of water for feet are pretty…

I think anyone who reads HN who needs to be told to open windows in order to keep cool should maybe hand in their hacker badge.

"... I think anyone who reads HN who needs to be told to open windows in order to keep cool should maybe hand in their hacker badge. ..."

It really depends. On the really hot days having a window open increases the interior temperature if the previous days have been cooler. On successively hot days leaving the window open decreases the interior temp but only if there is a wind.

I don't know what latitude where you come from since you don't have any identifying information but being smart has nothing to do with understanding how to keep cool. It comes from local knowledge. If you come from the continental Europe or the UK I'd guarantee you'd miss half these hacks due to the heat.

Re: How to Sleep Comfortably on a Hot Night

#32

Post by derobert » Tue, Aug 18, 2009, 7:04 PM UTC

baddox wrote: post #30

calambrac wrote: post #20

Earlier quoted context omitted.

Of course, but is the marginal cost of adding a few trays of ice really more expensive than running an air conditioner? The problem is so under-specified at this point it's kind of silly to pretend we could get a meaningful answer yet. Are we talking about the ice/fan and a/c cooling the room to the same temperature? Or are we just talking about the sleeper's comfort, in which case we should take into account that i/…

Well, that isn't quite a fair comparison, since the ice-fan only cools the small column of air between the fan and your bed, while an air conditioner cools all the air in a room or building. That said, the ice-fan is probably more efficient than a very small air conditioner than performs the same level of cooling (not that such small air conditioners exist), whereas an air conditioner is surely more efficient than th…

With the ice-fan, the heat from the room is going into melting the ice (and, eventually, warming that water to room temperature).

The freezer, of course, works just like an AC: it moves the heat from inside the freezer to outside the freezer — i.e., to the room its sitting in (some large commercial freezers actually use outdoor parts so then can dump the heat outdoors). Assumably, during the day, the windows are open, so that heat will escape outdoors.

Random note: If you could prevent all heat transfer with the outdoors, the freezer/fan/ice cycle would actually heat the room.

Re: How to Sleep Comfortably on a Hot Night

#33

Post by derobert » Tue, Aug 18, 2009, 7:06 PM UTC

jacquesm wrote: post #17

Elepsis wrote: post #10

Earlier quoted context omitted.

I'm not so sure this logic makes sense -- in fact I think it might be entirely backwards. I've always been under the impression that though you pay a penalty for freezing things the first time you put them in, thereafter a full refrigerator (and freezer) is actually much more efficient because it's more difficult to keep air cold than it is to keep any variety of items that are more dense. If we're talking about leak…

reduce it to an absurdity: If you imagine a giant freezer filled with ice it will eventually melt. This takes energy, a larger freezer will take more, an empty one will take less than a full one. Counteracting the heat transferred through the freezer walls from the outside in will require an amount of energy proportional to the mass in the freezer and the leakage from the outside. It takes energy to maintain that imb…

I believe the inefficiency of an empty freezer/fridge actually comes from it being inefficient to cycle the compressor. The low heat capacity of air leads to the compressor cycling often.

Re: How to Sleep Comfortably on a Hot Night

#34

Post by derobert » Tue, Aug 18, 2009, 7:12 PM UTC

calambrac wrote: post #19

swombat wrote: post #14

Earlier quoted context omitted.

Air has a very low heat capacity. It doesn't take much energy to cool a cubic metre of air quickly (see air con, for example). On the other hand, cooling a cubic metre of water and other watery solids takes a load more energy. In terms of keeping it cool, that probably depends on how good your freezer's insulation is. If it's really good, it probably doesn't take too much energy to keep things cool (air or otherwise)…

No doubt there's still an additional cost, I just don't think it would be high enough to make it more expensive than running an air conditioner, given that you're going to be running the freezer regardless (depending, of course, on how much we're talking about. I'm picturing a window unit vs. a window fan with a baking pan's worth of ice). Any ideas on how to be more rigorous about thinking this through or running an…

An experiment is easy to run. Obtain a Kill-a-Watt or other power meter, then measure (a) how much power the freezer uses over a day w/o freezing extra ice for fan-ice cooling, repeating several times; (b) how much power the freezer uses over a day with freezing extra ice for fan-ice cooling, repeating several times; (c) how much power the fan uses blowing over the ice; (d) how much power an AC uses to keep the room comfortable, repeated several times; (e) [bonus] how much power an AC set to a higher temp. + a fan blowing on you uses to keep the room comfortable, repeated several times.

You can then compare (b)-(a)+(c) vs. (d) or (e).

Ideally, you'd do these experiments in identical conditions (temp., sunlight, wind, humidity, etc.), of course.