Drain water from 5" pipe buried 9ft underground

[sophiecentaur]

No, earth tubing with air is less effective.

So why choose air? Cheaper and less trouble, of course but a 'heat exchanger' of some kind at the top would help get better results.
I was browsing through Amazon to get some background knowledge about greenhouse heaters. They start at about 60W and go up to around 200W so that's the equivalent of power you'd be needing (I assume). I also assume that the greenhouse is draught proof. The roof looks as though it's insulated

I re-read your posts and you mention 'raised beds'. If you used a water system, you could run hoses in the soil in the beds. Warm (not freezing) soil is always good for starting off plants and your heat would be going where it's needed most. But that could complicate things because you would be better by getting the heat not from the beds so your control system would need electric valves (fairly cheap in plumbing stores.) to bring your heat absorbing CH radiators into play.

At this point people tend to start suggesting Raspberry Pie processors for controlling the system. But you may not be into that stuff.

Possible experiment would be to use a small fan, blowing down your pipe and see the temperature difference between input air and exit air. ( very low power laptop cooling fan)

 

[druidhr]

So why choose air? Cheaper and less trouble, of course but a 'heat exchanger' of some kind at the top would help get better results.

Isn't that enough to go for another solution?

But thx for all the suggestions, earth tubing with water for heat extracting is very good solution. Will think about it for my next project.

 

[sophiecentaur]

Isn't that enough to go for another solution?

But thx for all the suggestions, earth tubing with water for heat extracting is very good solution. Will think about it for my next project.

I'd be tempted to go for a water-based solution for the reasons I've given but, if you do some measurements - even just to feel a difference in the air that comes out - and if it 'feels right' then use air. You can always change to water if this winter's experience is not satisfactory.

Maybe your raised beds should have some insulation underneath.Enjoy your out of season veg!

 

[druidhr]

You can always change to water if this winter's experience is not satisfactory.

Yes, if air fails as heat extractor, I could pull 1/2" HDPE pipe through 5" pipe and fill it with water.

 

[sophiecentaur]

Yes, if air fails as heat extractor, I could pull 1/2" HDPE pipe through 5" pipe and fill it with water.

It should be worth while filling the outer pipe with water - both for good thermal contact with where the heat is transferred from and to and for thermal capacity. On that topic, if you want good heat transfer on hot days, you would benefit from a number of heat exchangers above ground. That would make the best of any available heat and reduce unwanted leakage at night. I mean a sort of thermal ratchet. A simple (CH ) pump could control heat transfer by using a thermostat.
A greenhouse already needs some way to avoid overheating so radiators (the pump) could be switched on before the windows / vents are opened..

You really need a thermometer (or several) for this to allow you to get best performance or at least to convince yourself that the system is working. Shame they are soo expensive!

 

[druidhr]

On that topic, if you want good heat transfer on hot days, you would benefit from a number of heat exchangers above ground. That would make the best of any available heat and reduce unwanted leakage at night. I mean a sort of thermal ratchet.

Do you think these radiators would be good for above ground exchangers.

 

[sophiecentaur]

Do you think these radiators would be good

I've no idea from your photo, how big they are b
ut if they're free then go for them.
Problem is that the details are really important here and you need to measure temperatures 'everywhere'. It's not a trivial bit of engineering and you have to give yourself as good a chance as possible to justify all the work involved. I don't think it would be worth while just to 'suck it and see'.

You need to know the mass of water underground and the temperature you can expect down there. That will tell you how much Energy is being stored. Without knowing the temperature you are getting down there you can't work out how much energy is stored.

There are two things to estimate, how much heat the rads can capture and also the heat capacity of all the underground mass. Radiant heat from the Sun will be the best source. You can expect at least a few tens of Watts getting into each small black painted CH radiator whenever the Sun's out. Croatia is warm in the summer iirc. You need to measure the temperature of the water (contact thermometer on a rad?). Even in UK, a coiled up hose, out in the sun can reach at least 40C on the outer coils.

Without the pump, the static water in the rads would be 'warmish' by the end of the day. Just experiment with a single radiator full of water and see how warm it gets on a summer day. Turn on the pump and the whole mass of water would be absorbing the heat so the temperature wouldn't be very high after one summer day. Obviously, you need as much heat as possible from the system when it's needed so controlling the pump is vital. Only have it running when the soil temperature in your beds is low.
How much heat would be available on a winter's night? That depends on the size of the rads and the catalogues tell you how many Watts for a temperature difference of 50C. You will be expecting only a few degrees difference.

How much stored heat? E = M T 4200 where M is the total mass of water, T is the temperature difference and 4200 is the specific heat of water. That's in SI units but there are many sources of the same information in imperial units. You can estimate the mass of water from the length and radius of the big pipe times the density.

Water circulation: I think the solutions higher up the thread could well be overkill. Air locks could be a problem but only the ones in the inner pipe in the narrow inner pipe. Air (static) in the large pipe wouldn't matter.. If you feed the small pipe through and see if mains pressure water will flow through steadily (after a bit of initial bubbling) I can't see the need for a high pressure pump of the kind discussed at the top of the thread. A cheap CH pump should do the job of circulating the water. You would need a vapour trap at the highest point to avoid build up of air bubbles. A pump above ground would do the job and be a lot cheaper.

 

[SredniVashtar]

The temperature at a given depth can be estimated by solving Fourier's equation. Sommerfeld has a solved problem exactly about that in his theoretical physics books.

When assuming a sinusoidal temporal dependence due to the seasons, the temperature at a given depth is sinusoidal, with a phase difference in time and an exponential decay with depth towards the average value over the whole year.

At about 3 meters of depth we have an inversion between summer and winter, and that's what makes cellars cool in the summer and warm in the winter.

 

[sophiecentaur]

The temperature at a given depth can be estimated by solving Fourier's equation.

But you can do better than that by active heat transfer