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  1. #11
    Jon Goodman's Avatar
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    Ah Corona, la cerveza mas fina. Have you ever tried Modelo Especial or Carta Blanca? Corona beer is a sad story for me. In the 1960s (when Corona was totally unknown in the USA) a friend and I had gotten in the habit of drinking it. We were buying it (in Mexico) for 60 cents a six pack. One day we were having lunch and I said "you know this beer we've been drinking all summer is very good and I think it would sell in America. Maybe we should see about getting the rights to import it." He agreed and then we did nothing at all. Well, that's not entirely true. At the end of the day we did meet some girls and go over to the Caverns of Music (Juarez) and drink salty dogs and eat dried shrimp until about midnight, but we never went forward with the Corona idea. And we never saw those girls again, either. Que lamentable es!
    Jon

  2. #12
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    A flat surface cooler like this might be nice for pouring carbon tissue in warm climates. Turn on the juice after one pours the pigmented gelatin, quickly cool down the pouring surface (nowhere near freezing is needed...40 to 50F) so that the gelatin sets, turn off the juice, and lift off the tissue.

    One might be able to rig the surface to be cooled or warmed (80 to 90F) as needed. Warm for the pour, cool to set. Pour tissues year-round no matter what the room temp is!

    However, my low-tech way works in my mild climate: opening and/or closing the window until the room is the right temp.
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

  3. #13
    Jon Goodman's Avatar
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    That is actually a very good idea, Vaughn. You can warm by reversing polarity (a simple switching function) and you could then reverse to cool. Aluminum conducts heat (or lack of it) uniformly. It should work.
    Jon

  4. #14
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    Many of the cheaper car-fridges are already based on these.

    Efficiency of a peltier is inherently low, before you even get started on inefficiencies of the power supply. Example characteristic curves and a good FAQ. A typical "freezer" application requires a temperature difference of about 40C to ambient, and many devices will have an efficiency coefficient down near 0.5 for that temperature gradient, i.e. you need to put in 60W of electrical power to suck 30W of thermal power from the cold side, then dissipate 90W from the hot-side. A good compressor-based system will achieve an efficiency coefficient of about 3 though, e.g. put in just 10W to transfer 30W and then dissipate the 40W total from the hot side.

    A 12V 4A peltier is about 50W (plus power supply inefficiencies), which is 438kWh/year and it will struggle to keep a few drinks cold. In comparison, my 150L chest freezer uses only 220kWh/year running at -20C.

    A bang-bang thermostat running very slowly (large hysteresis) could probably work well for lack of thermal shock. I set my first attempt to have a 5C window and it was sitting on top of a 65W CPU with only about 50g of Cu as thermal mass, so it was cycling every few seconds. The thermal cycling stress cracked one (of the about 400) peltier elements which meant the whole thing went open-circuit and ceased to cool. Bad-times. I put in a PID controller running in a PIC16F84 and PWM'd the current to the peltier at about 30Hz; it was all good after that and held temp within 1C regardless of CPU load.

    You really don't want to know the power consumption of that PC - turning it off paid for my next machine The heat exhaust was a 4x6" car-heater radiator core and there was a continuous warm blast issuing from it. Great in winter and with near-unbeatable bragging rights on 15C core temperature (above dewpoint!) under load.

    An often-overlooked feature is that they're physically symmetrical, i.e. pushing heat through a Peltier device gives you electrical power! Attach one side to a big heatsink and the other to a few candles and you can get measurable power from it to run LEDs and the like. I believe you can now buy portable wood-burning cooking stoves that produce 5V USB-power for phone-charging while camping. Most phone chargers are only 10W and a small woodfire is a couple kW, so a bit of waste-heat off the side will do that nicely.


    Edit: what I *meant* to say before all that bullshit is, "these things are huge fun".
    Last edited by polyglot; 08-08-2012 at 03:50 AM. Click to view previous post history.

  5. #15
    Jon Goodman's Avatar
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    Polyglot has made some good points...especially the final part about "huge fun". They're definitely fun. As I said early on, these are at once simple and under the surface more complex. If not abused, they will work for a long, long time. I would personally suggest that a person design a machine or appliance which would be an occasional use thing...that being something you need to turn on to accomplish a chore and then turn off when no longer needed. Efficiency can be a slippery thing when you step into the world of refrigeration, and common sense is a good yardstick to remember. Example: I showed you how I created ice in 5 minutes. This also included the burden of 80 degree ambient air and cooling about 4 cubic inches of aluminum to a temperature below freezing. Now, let's consider this: the ambient temperature inside the freezer compartment of your refrigerator is ideally 0 degrees (f) but more realistically it varies between around 10 to 18 degrees (f) ... in a typical appliance, that is. Mine hovers around 14 degrees most of the time. How long will it take to make ice if you place the same quantity of water I froze in 5 minutes in an aluminum cup in there and then set that aluminum cup on top of another piece of aluminum which is at a temperature of 80 degrees (f)? Don't worry about going to try it...I can answer that question right now. It took me over 25 minutes. How about if we forget the 1" thick aluminum spacer? How about if we just put the same quantity of water in the aluminum cup and set it in there? With the fan running (which will improve efficiency of your freezer obviously), it took me almost 15 minutes to get the ice as firm as it was on the test yesterday. So, if it takes you between three and five times longer to freeze that quantity of water using a more traditional appliance, which approach is more efficient? I'm not arguing...I'm just adding the idea that factors other than a simple measure of wattage should be included. You know, often it will take one man all day to do what 5 men can do in a couple of hours.

    The "thermoelectric effect" is really 3 separate effects. The Seebeck effect which has to do with temperature differences converting into electricity, the Peltier effect (which we've been discussing to this point)...the heat transfer and all and the Thomson effect which as I recall wasn't discovered by Thomson but Kelvin. It has more to do with coefficients and is more complex than I care to wander into this early in the morning.

    But yes...you definitely can generate electricity by moving heat through the device and waste heat can be put to good use that way. Thanks for pointing that out, Polyglot.
    Jon

  6. #16

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    Quote Originally Posted by David Brown View Post
    For those of us that live near Hell in the Summer and have water coming out of the tap at 85 degrees (F) or so, could one build an efficient in-line chiller with one of these?
    Practically speaking, no. The amount of heat in water and the speed at which you need cold water to flow thru a line would necessitate a compressor. Remember the line will even have heat in it that must be removed. Even with a loop to a cold water storage box it would take time for a compressor, of course depending on the amount of water and the temperature needed. Small chillers are available in different capacities and in small sizes like cold water drinking fountains.

    For peltier cooling buy one of those ice chests that run on ac/dc or just use your refrigerator or ice.
    W.A. Crider

  7. #17

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    I need something that will keep a substantial mass of fish frozen on my annual migration from Newfoundland to Ontario. Packaging densely without air pockets in stryrofoam sealed boxes gets me about 48 hours or a little more. With good planning of ferry, etc. that will almost get me there. However, it doesn't allow any stops enroute to visit friends.

    Can I build something that will work? Can you estimate the number of hours that I would need to run the vehicle every 24 hours to maintain frozen fish?

  8. #18
    Jon Goodman's Avatar
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    This would not be a good option for keeping a substantial mass of fish frozen, Ralph...the challenge is size and efficiency. I would suggest dry ice and good ice chests might be a better and more economical alternative. Sorry, but I would not encourage you down the wrong path.
    Jon

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