Backyard Aquaponics
A place to discuss aquaponics

good to see you experimenting. Its the best way after all is said and done.

Also COP increases with a lowering of compression ration (difference between high side and low side, or hot temp and cold temp)

Just be careful with taking too much heat out of the high side, yuo MAY get refrigerant flood back to the compressor, but probably not becasue you have converted an air cooled evaporator to a liquid cooled.

Steve

Don't know because its not something I've ever done but based on the theory I reckon you'll have better luck with NJH's suggestion of the counter current system.

Something I do know a little about is the circulatory systems of penguins. They have the challenge of maintaining a warm core temperature while supplying their uninsulated feet with blood so they don't get frost bite. In principle exactly the same problem your trying to solve. If you got the length and flow rate of your counter current heat exchanger running you could have it operate continously at a pretty effcient rate.

Stuart

Experiment #2:

I ran the dehumidifier continuously for 1 hour, tried to get warmer starting temp but I'd have liked to have it closer to the target of 60F. I kept foam around and on top of the tubs and stirred them every 5 or 10 minutes.

Cold tub start: 69.3F/20.7C Hot tub start: 68.5/20.2C
Cold tub end: 49.5F/9.7C Hot tub end: 90.8F/32.6C

Spread of 41.3F/22.9C

SC, A heat exchanger alone will not do what I am trying to do, although several will be involved and efficient ones will be welcome. Looked at some cool ones here yesterday: http://www.turbotecproducts.com/product2_1.htm

I am thinking a room air conditioner will work well, because I am trying to put the two coils in different tubs without breaking the tubes open and having to recharge them and there is more separation between the coils in a room air conditioner than a dehumidifer (they are stacked one after the other). I would like to do this with found items, and without making custom heat pumps if possible, although it would be a lot easier if I could just get a coil of copper tubing and coil it inside a barrel.

Since these coils are made out of aluminum and copper I will need to isolate the fish water from them somehow. I am thinking of running plastic or rubber tubing coiled in the heated or cooled tubs, but this is added complexity. This part becomes strict heat exchanger tech, the only complication is that I can't just use copper or aluminum because I don't want to use those metals with the fish.

Tomo and Steve, what problems could you expect from using an air-air device as water-water again? Should I be concerned about the coils or pipes popping open from high pressures or something? Can these things run at weird angles? These tests were done with the compressor on its side and almost upside-down; other than vibration it still seemed to work OK for awhile anyway.

DD, LOL!

The compressor will run on its side and upside down, but not for long

they are oil filled for lubrication, and bust be upright. I read of a heap of box airconditioners being fitted upside down so the short people could reach the nobs (seriously, it was in one of the text books!) they rand for a few months before the compressors seized up!

only problem i see with using an air-air as water water is the capacity of the coils will be much greater so no longer matched to the compressor.

a problem with capillary (that tiny thin tube) systems as found in the box a/c, fridges and you humidifier. That system doesn't lend itself to load changes. but yours will be operating on a fairly constant load.

Ummmmm.....................if you're running at the temps you've listed i think you'll be ok..............i'd just watch for signs of ice on the return pipe to the compressor. in an ac systems temp and pressure that will mean that your getting liquid refrigerant back to the compressor. Tomo sounds like he has more years on him, so i'd be happy to defer to him

With the heat exchanger, you'll be fighting an uphill battle using plastics and rubber. i'd go a S/S exchanger

Whats a S/S exchanger?

Stainless Steel

steve the last time i worked on refrigeration as a job was the year you were born i have probably forgotten more than i was ever taught so my suggestion to dave is that he follow your advice

you sure know how to make youself feel old!

Great, thanks for the info!

As I said, the efficiency of a counterflow heat exchanger improves exponentially (every extra metre will multiply your inefficiency by some constant less than 1). This means that even polypipe (we've discussed this before) makes a perfectly good water-water heat exchanger. You can now buy low pressure tube that comes in rolls with very thin walls (rated to something like 10kPa). This would be ideal for the inner tube.

I'd be guessing the COP of such a system would be at least the order of 100. (A 5W pump could easily move 500W of heat)

Read a bit just a second ago; is this what you're talking about NJH and SC?

http://www.answers.com/topic/countercurrent-exchange

This actually makes a lot of sense, the input end of the one fluid is the output end of the other fluid, if the tubes are long enough the temps are exchanged fully, they don't just mix and reach an equilibrium temp because they are going in opposite directions.

I guess I should invest in some rubber tubing and garden hose to do experiments again this weekend? This would be a lot easier and simpler. Thanks for the info!

I just had a thought where this heat pump idea might work. Warm greenhouse, cold water fish -- trout for instance. Trout grow fast and have good feed conversion but they need very high DO. Cold water dissolves more oxygen so that would be better for the trout. Warmer water would be better for most plants -- faster uptake of nutrients...

I suppose with intensive tank culture of warm water fish, a heat pump/exchanger system could be set up so the water is cooled before pure oxygen is introduced under pressure -- and then the water warmed back up. It might be possible in that way to get water supersaturated in oxygen -- whatever that's worth.

I bet you'd lose the oxygen when the water was warmed up. I think I may have seen this during my experiment, because the hot coils developed a ton of tiny bubbles on them, but the cold coils did not. Your mention of the gas dissolving makes me think that was where the bubbles came from!

I have been thinking that I could have tilapia inside and some cool water fish outside (yellow perch?), in the same system (water mass).

I think you'd have to have a powered heat pump in order to bring the water temp down to less than ambient.

DD- Yes, that's what njh has been talking about, but why not combine the systems?
I think that would be a good system for your heat exchange out to the GH. If you want to, you could combine the counter-flow with your dehumidifier loop by running your counterflow in a U formation, having your exchanger at the outflow of each. Since the dehumidifier isn't 100% efficient, it will bring up your overall temp, but concentrate your heat at the flow back into the fish tank.
For example, if your fish tank water is at 75F at the outflow you can run the counterflow out to your 50F water outside. If you get 100% heat exchange, your outflow from the exchanger will also be at 75F on the hot side, and 50F on the cold side. Use the dehumidifier as your tank heater, and it will jack your interior heat to, say, around 80F, and drop your outside to 46F, (or some such temp change). Thus, you can keep your fish water reasonably warm with little additional energy.

As you move towards summer, the outside water will warm and the heat exchange done in the counterflow will be reduced. Also, your inside temp will be maintained, since if your tank gets too cold, your humidifier will kick on, and warm your tank water (at the expense of your GH water). If your GH starts overheating the water, you can run a fountain in the GH, using evaporative cooling to cool your GH water.

All you would have to do is change the humidostat to a thermostat, and set it to slightly above your lower pain threshold for the Tilapia.

Cool stuff!

Now you see why I was not sleeping that night!

The dehumidifier is more efficient when the temps are closer then when they are farther apart - I wonder if it could be configured to take advantage of that? Put the heat pump coils at the halfway point of the heat exchanger loop?