eutectic fridge and aircon.

Discussion in 'OnBoard Electronics & Controls' started by discovery, Oct 1, 2013.

  1. discovery
    Joined: Aug 2013
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    discovery Junior Member

    I am unsure if this is possible but would it be feasable to set up 1 large eutectic tank, and use it for fridge, freezer, and cabin aircon?
    My initial idea this way was to use the eutectic tank for normal fridge and freezer operations, but for cabin air con circulate another fluid that could be pumped through an aircon evaporator in the cabin and through the eutectic tank as well to drop the heat picked up in the cabin.
    Another peice tomy puzzle is would this be also possible to utilise a main engine mounted compressor for this purpose, but also have a 12v compressor in parallel to operate when the main engine isnt running?
    I run 360 watts of solar panelling on my boat and so I have 12v power to spare at the moment, also have 450 amp hrs of battrey storage and would like to keep with 12v and not have to go to 240v or add an auxiliary.
     
  2. PAR
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    PAR Yacht Designer/Builder

    A 12 VDC compressor is a pretty heavy draw on resources. I've played with similar arrangements for cabin A/C and the eutectic approach has considerable merit. I'd think you'd want separate fluid tubing in the tank, to flow to a remote evaporator, as pushing air through the frozen block is a bit self defeating. Is this what you're thinking?
     
  3. philSweet
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    philSweet Senior Member

    You're going to loose a great deal of efficiency by trying to operate comfort air off a -30 degree source. Small craft will often run a dozen or more identical little units to run ice, freezer, fridge, and a/c's, but they can be set up at different temps and capacities. The reason to go with holding plates is that you have capacity issues or power availability issues and are willing to ignore efficiency in the simple pursuit of getting the job done. Run the a/c's by day and freeze the plates at night for fridge duty. The term for a that type of comfort air terminal is a fancoil. There isn't any evaporation taking place there, just cold water in a loop.

    For a solar assist boat, I try to use the genset in the morning to get the batts up to 80% and size the solar relative to the battery acceptance at 80 percent charge. This saves you running a genset at low output for long periods just trying to top the batteries. I would dedicate about a 150w panel to a 2 gal plate icebox and use a two-speed compressor that runs on high when the panel is putting out high voltage and runs on low off batteries only if the box temp climbs above what it can hold when running on low.
     
  4. philSweet
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    philSweet Senior Member

    What is done in practice, and this is both efficient and practical, is use a big comfort a/c and then dump the freezer heat load into the cool cabin. That's the opposite of what you are proposing.
     
  5. El_Guero

    El_Guero Previous Member

    What do you gain by dumping the heat load into the cabin?
     
  6. discovery
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    discovery Junior Member

    The seperate fluid tubing in the tank is exactly what I'm thinking. You could possibly still use a normal style evaporator and fan set-up, and just pump fluid through it instead of gas. Of course you wouldn't want one of those annoying TX valves in the equation either.( Not trying to state change the refrigerant at this point). The other part of the equation is the size of the freezer I'm talking about, it aint any little fridge/freezer combo. 2.5 to 3 cubic meters is roughly the size I'm thinking (commercial fishing), but not a full on freezer, more a bloody cold brine tank.( dumping the heat load from a 3 cubic meter freezer to a 15 cubic meter cabin probably wont work in tropical north Australia)
     
  7. discovery
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    discovery Junior Member

    Part of what I'm trying to achieve is to long term avoid the "need " for a full time aux genset in the boat. I dont mind having a smaller inverter style petrol genset to pull down the eutectic tank, provided the solar and battreys (coupled with running the main engine from time to time for moves). Even if the solar could slowly (2-3 days before the trip) pull down the temps, or maintain the eutectic mix from trip to trip this would be the ideal.
    The biggest stumbling block to a 3 or 4 day trip is keeping ice/ catch cold, and maintaining bait, and a good freezer system is mandatory for this. Every other boat in the fleet has aux generators running all day and night, I am mainly thinking that applying a little technology and half a brain (you guys helping me gives me about a half) with the solar array and the sun out the way it is in my neck of the woods, I'd be mad not to try something. The worst that can happen is I do like all the others, and go to 240v marine aux genset. The ideal outcome from this type of thinking is I end up with a "solar fridge/brine/freezer/aircon" that runs flat out all the time, with minimal costs and maintenance. I would be lucky to land somewhere in the middle of those 2 extremes but its worth a go.
     
  8. discovery
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    discovery Junior Member

    Air conditioning units are designed to shift a lot of btu's but not by a huge difference in temperature and for a large volume, eg, dropping a 30 degree cabin to 24 degrees takes a lot of btu's butin a freezer it takes a lot of btu's and drops that 30 degree freezer to -10 degrees but can only do it in a very small volume (freezer size). Because of the volume of air the a/c treats, it can absorb the freezers load better, and this is seen as the more efficient way to do things.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Yep, that's what I figured. I've looked at this on a smaller scale then what you're proposing, mostly because of the sub tropical climate I live in. Have you run any numbers on this yet, as I'd be interested. The evaporator/fluid/air handler idea is how I'd go, possibly with just a couple of pressure switches and a reversing valve to shove the fluid back and forth.
     
  10. discovery
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    discovery Junior Member

    Florida probably not too far off what I deal with here in Queensland as far as temperature and climate go. I havent worked anything out yet as far as sizing goes and I haven't really even thought about the eutectic fluids and stuff as for the A/C part of it, you will want a fluid that freezes far lower than the main tank, and stays very low viscosity at low temps to make it easier to pump. I dont imagine there would be any real great pressures involved with that side of the set-up. I have thought a little about the physical set-up required though and I am beginning to think it could end up fairly simple to set up in my boat. I think the key is to set up the refrigeration correctly first though and working out the actual number of btu's you'll be shifting will be the hard part.
     
  11. philSweet
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    philSweet Senior Member

    I have a little better picture now. Lets compare-

    One ton of eutectic weighs a ton plus the weight of the tank and insulation, plus the amount of extra boat you need to carry it. Lets call it 1.4 tons.

    One ton of capacity weighs about 100 pounds for the refer gear, 100 pounds for the genny gear, and will need about 12 gallons of fuel plus a tank. Lets call it three hundred pounds plus some extra boat so maybe 400 pounds.

    In round numbers, the eutectic weighs about 2700 pounds more per ton of capacity. The questions now become - what is the initial cost difference between the two craft, what is the difference in operating cost, and what is the difference in value between the two systems in your opinion.
     
  12. jonr
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    jonr Senior Member

    I'd focus on better insulation and air sealing. Consider vacuum insulation for a freezer.

    Keeping the topsides wet for evaporative and convective cooling (reducing A/C load) might be worth investigating.
     
  13. discovery
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    discovery Junior Member

    I'm taking it from your post, you'd expect I'd need 1 ton of eutectic fluid. I currently have no refrigeration gear on the boat and so take on a 1/2 ton of ice (sometimes more) to do a 3-4 day trip. Have you ever paid for a 1/2 ton of ice? And this is still without any A/C comfort to get a good sleep in at night. To add a marine genset style auxillary (the only ones that handle the constant running all the time), I'll be adding a ton anyway, by the time I fit the unit, the refer gear, the extra tankage for the genny, and then I'm just like everyone else out there with a mind numbing genset rumbling away in the background.
    Anyway even if I drop a freezer off the list, and use the eutectic to pull a brine down for the fish, that would still offer a lot in terms of assistance and cooling.
     
  14. El_Guero

    El_Guero Previous Member

    Discovery

    Thanks!

    Have you tried to double insulate your ice? And do you buy cold ice, or warm ice? Starting at -30c is better than -10c .... did I get that right?

    wayne
     

  15. jonr
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    jonr Senior Member

    Something more moderate, say 500 pounds of ice to provide for fewer generator hours and a quiet night makes sense to me.
     
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