Backyard Aquaponics
A place to discuss aquaponics

Hi DD,
I can understand your cold weather dilemma, a similar problem here in that we are limited in the size of greenhouse we can erect.

Another problem i`ve considered is, if the outside beds are at 4C, the plants aren`t going to be growing too fast and the nitrates in the return water will still be quite high.
Logically the growbeds in the greenhouse will be sufficient for sustaining the water quality, as at 4C the external beds aren`t going to be bacterially active to help out.
You may be better off shutting down the external beds for the winter as production will not necessarily offset the extra cost/complexity involved.

The only other thing is to run the supply/return pipes underground and fit a double skin cover to each exterior bed, (excess humidity could be an issue here).
The gravel in the bed is thermal mass and the covering affords some insulation value against the outside temps. On sunny winter days the gravel will store at least some of the solar gain.

Nothing is ever easy is it

have you considered including aquatic plants in winter - they will use up nutes and you can compost them as soon as the waether warms up...

Another day and its a lot clearer. (Due to the excellent drawings)
Verbal pondering.
1. The heat transfer will only be in one direction.
2. Fish tank water would have some of its heat transfered into the returning green house water (thus preheating it prior to its being returned to the fish tank).
3. and the loop continues.
I guess its a build and see what your losses are Dave, it could very well be viable.
As far as how to make build the heat pump, that is beyond me.
Steve, can you find the time to do a step by step?

Tomo, thanks for the excellent suggestion!

Hex, yep it's a dilemma all right. I would really like to have more options in the sub-tropical times of the year without going broke trying to heat up the world 4 months early. We just went through February, and there are some really sunny days, and it chafes my butt that I am spending money on an electric light instead (because it's in a dark basement). I'd also like to have more room for plants. If the temperature management weren't an issue it might be an option to plant more to make up for the slow microbial activity in the cool weather. The temperate non-freezing times is where I think this option could really give things a boost - when it's sunny and warm out but not "warm enough".

AM, one thing I'm trying to improve is not having much space for plants in the basement. If I could use the outside in spite of the cold then it would free up a lot more options on what the system could do.

Hi Dave,
I hope my reply came across as constructive rather than sounding like the ultimate pessimist.. it`s hard for me to type what i actually mean to say sometimes.

If space wasn`t the issue.. it might actually work out cheaper to run hid lights for 12 hours a day for both heat and light and insulate to keep the heat in for the other 12 hours.
In the middle of winter, heating is probably only half the battle

It did Hex, I think us cold weather folks all know the issues here. Sometimes around here sunlight and solar thermal tech would work, but not always and you'd have to fall back on electricity for both.

I'll join that club as well. Communication in this medium is a challenge, unable to view body language, voice tone, etc. We can only use our individual filters as we attempt to understand what is being said.
Say What?
Dave could you provide an outside picture of where the Green House will be placed? Going just over your last years growing area, yes?

Yep, same place. I can pump water up there easily.

I realized from Tomo's suggestion that it might be possible to do it like an on-demand tankless water heater. You could use a long pipe-in-pipe with hot in the center and cold on the outside to equalize the water temp, then after that have two pipes with coiled refrigerant tubing around the pipes to heat/cool the water, without using the barrels. The pumping rate would have to be slow enough that the heat pump could cope. In order to do the two barrel thing you'd need either a bunch of pumps or some kind of switched valves or flaps to direct the water hither and thither.

DD & All, to be honest i haven't had a change to look at the drawing properly yet due to birthay drinks (BUT I WILL).

Oh dear, now its only me that doen't understand, and the fact that tomo gets it and has more years under his belt isn't looking good.

Talk amongst youselves i'll be back later tonight

Have fun Steve!

My Mom doesn't limit herself to just a birth-day. She celebrates a birth week!

i'm on birthday recovery day now.................and wedding duties ;(

It's called a heat exchanger. Pipe in pipe would work fine - 40mm outside pipe and 19mm inside. Run the water in opposite directions (counter-flow) for maximum efficiency. No idea how to maintain the inner pipe in the centre - it might not matter. You could wind the whole thing up in a spiral or cylinder if you need to, but insulate each turn (air would probably suffice). Getting the water to the inner and outer pipes is left as an exercise (perhaps a tee with copious silicone). Faster water flow reduces boundary layer, but also reduces the time in contact with the surface, so there is an optimum speed (you could use a computer to vary the pump speed to maximise value). Changing the temperature differential will change the optimum flow rate - bigger differentials require low flow rates.

Efficiency is dependent on conductivity of the pipes, boundary layer and total length. Inefficiency decreases exponentially (A^(-x)) with length, so if it ain't working well, make it longer. I'd do the calcs, but in the past I've just confused people with that.

Insulate the pipe.

No need for heat pumps and other such messy things.

A heat exchanger is definitely a good plan, but it won`t be able to do the whole job.
Disregarding any type of losses, the main limiting factor is the 2 barrels of water at set temperatures, 40F and 85F.

The cold 40F barrel will likely never get close to 60F as there will only be a 5F temperature difference driving the heat transfer process at that point, the warm 85F barrel having dropped to 65F.

The barrel of water (assume 500lbs) now at 60F needs to find an extra 25F (12,500 btu or 3.66kw) from somewhere to reach the target 85F fishtank temp.

If, as Steve said earlier, a heat pump is 2.5x more efficient than using direct electrical heat, it could be more economical (1.5kw) to heat the barrel

Exotic phase change and hvac type systems are beyond my experiences but may well be one solution.
Not something i`d consider though ..with my luck it would definitely fall foul of good old Murphy`s Law

You are not understanding counter-flow heat exchangers. You can get the input and output temperatures arbitrarily close in return for an arbitrarily large heat exchanger.

http://en.wikipedia.org/wiki/Countercurrent_exchange

I think Hex is correct in many respects. We will never have arbitrarily long exchangers. Getting within 5 degrees is optimistic in the real world.

This whole problem is why I ended up with cool water fish. If your warm water fish are going full tilt metabolism and the cold plants are not pulling out enough nutrients, you will have nutrient removal problems.

Dave, I understand the system you are proposing. If you send cold water to a cold greenhouse, then it will not lose much heat out there. Plants would do better in the greenhouse with 50F on the roots. Consider making that temp your goal for water to the greenhouse. If you do manage enough heat exchanger to get 85 down to 65 and 40 upto 60, then your heat pump would handle the lift from 60 to 85. Looking at that system, you would have 65F water into grow beds. It would cool in the growbed and heatup to 60F in the return pipe(heat exchanger). The outside plants would thrive with that root temp. I still think you will be paying big bucks for energy.

Consider slowing down the Tilapia. How about 70-75F for fish water? Now would that balance with plant growth with roots at 55-60F? I think that would be fun to try. Of course, I would have to use a computer to run the system...cause thats fun for me...but not at all necessary.

Wow, I am long winded! But I see something working out. Balancing fish/bacteria/plants is always the key to a prosperous system. I do not see running the fish at 85F as reaching a balance without large energy input. Slow all legs of the system equally in the winter.