Hi, @ronv, and welcome to the forum. Good to have a new International member as well.

I'm certainly no professional but at a first glance I think you may have an issue with scale. In effect, by using an underwater tank, you are designing a water source heat pump rather than a ground source one, and in the UK the recommendation for a water source for a typical domestic installation is a body of water that's at least 1.5 - 2 metres deep and 0.5 to 1 acre in size to ensure the heat pump doesn't start freezing up the pond/lake. Doing the maths, that means a realistic minimum of 3 million litres so even if the minimum recommendation is wildly cautious a 20,000 litre tank is still orders of magnitude too small.

I'd also be interested to know what kind of costs would be involved to excavate a hole large enough to accommodate a 20,000 litre tank compared with the costs to sink a vertical coil for a typical ground source heat pump. In the UK, garden space is at a real premium and access is often restricted so I suspect the costs here would be far higher than in many sites in Canada, so perhaps what would be financially unlikely here might be perfectly feasible for you.

As I said, I'm purely an interested layman and so I could well be wrong. However, it's hard for me to look past the disparity in liquid volumes.

Posted by: @ronv

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My specific idea is (for a 3 Ton pump) to have two ±20,000 liter underground tanks, at a good distance from each other. Instead of using one source for all temperatures, have one designated hot liquid tank, and one cold / ice tank.

Worth a back of an envelope calculation.

The specific heat capacity of water is 4.2 kJ/kgK.

So if I heat at 20000 l tank by say 20C it takes 4.2*20000*20 kJ=1.7 GJ.  1 kWh is 3.6MJ so 1.7GJ is about 500kWh.

The typical UK house consumes 11MWh of gas annually, almost all for space heating.

So (unless I made a decimal point error) a 20000 l tank is nothing like big enough without drawing in heat from the surrounding soil.

That said regenerative heating/cooling does exist as a concept, using the ground as a big store.  Cool it in winter, heat it in summer, both made more efficient by the previous seasons escapades.

20,000 litres of glycol laced water will have the environment agency on your back pretty damned fast.

There are systems that do this in production today. It's used for greenhouses where many of them otherwise swing wildly between overheating and freezing cold over a 24 hour cycle.

Posted by: @etchedpixels

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20,000 litres of glycol laced water will have the environment agency on your back pretty damned fast.

...

Although regulations may well vary from country to country. Canada's equivalent to the Environment Agency may be more lax than the UK or, indeed, stricter.

@majordennisbloodnok 

Tanks / blatters are to be buried in the ground, they're about $5k each. I know how to play with excavator, so it will cost me a weekends rent at about $2k. Boreholes for wells are pretty expensive at $20 - $30k each (seems to be the going rate here) and you don't get the advantage of storing the heat and cold energy underground, which is kind of the idea behind the use of the two separate designated storage/water tanks.

@jamespa 

I figured by adding the evacuated solar tubes, and mixing valves,  there should be enough heat energy to prevent the water in the cold tank from freezing, and keep it in an optimal efficiency temperature range. (heat from solar tubes, flow rates, needed tank size etc. to be calculated by someone with much more specific knowledge of applicable formulas) I figure that if my tank needs an x amount of energy additional to the ground exchange, it's just a matter of finding the balance between tank size, and amount of solar tubes.

@etchedpixels 

I thought about that too, and between the cost of the glycol, and potential regulatory problems, I'm actually thinking of having pure water in the tanks, and run an x amount of glycol filled coils in the tank. This will also make it easier to freeze the cold tank into a solid clump of ice at the end of the winter, so this cold energy can be used for airco in spring and early summer. At the end of the summer you dump excess heat into the cold tank, so it takes a long time when the system starts using it for heating before it gets to freezing point.

So there would be one glycol loop through cold tank, one glycol  loop through hot tank, and one glycol loop from evacuated solar tubes..... Thinking that loop should heat a hot water storage tank (domestic water & floor heating), and then loop through to the hot tank. When the system is in heating mode all stored hot energy can be used (with mixing pump / valves) to keep the water in the cold tank at optimal temp during winter use. From what I read evacuated solar tubes still produce a large amount of heat, even on the coldest winter days.

If you freeze water it expands. That's been a big problem people have hit trying to do this on a smaller scale freezing 1m cubes of ice in their basement (which is a colossal amount of energy). The other one being getting it back out at a sensible rate.

It's a great concept and ice melting takes insane amounts of energy per cubic metre.

To use ice as a storage medium, consider the latent heat of fusion (334 kJ/kg) which is about the same as 80°C worth of thermal capacity.

A "3 ton" AC (cooling capacity 10kW) has daily energy transfer equivalent to freezing 3 tons of ice, so in 2 months it cools by an equivalent of melting 180 tons of ice. So you need a lot of ice. It would be doable if you own a floodable cavern or something of the sort.

It is possible to store cold in the winter and heat in the summer, but this requires an enormous mass, which must be free, so it's usually earth with pipes running through it. In other words geothermal.

I recommend comparing the return on investment of any project against: photovoltaics + standard AC for cooling, and insulation for heating.

@etchedpixels I read something about that years ago. I guess it's what planted the seed in my mind🙂

@bobflux

Thank you for your response. I am trying to get away from the large area needed for ground loops, putting them in a vessel instead which will extract or dump energy into soil around it. The heat generated in the summer will "preheat" the cold tank surroundings, so it should be quite  a while until all that heat has been extracted, and the water gets to the freezing point. The other "help" are the evacuated solar tubes which provide "free heat" even in winter to stop the tank from freezing. (idea is a water to water floor heating system, where heat generated by solar tubes get first used for heating, and when this not sufficient have the heat pump kick in. According to AI: 

One rack of ten standard (58mm x 1800mm) evacuated solar tubes in Eastern Canada will generate approximately 400 to 550 kWh (1.36 to 1.88 million BTUs) of usable heat energy per year. See below message for more info.

Keeping this in mind, and let's say both the hot and cold tanks (and surrounding soils) are heated to 30* C. Do you still think that if the heat pump gets to work around the middle of December, the tanks will all be frozen solid by the end of March? If so, hypothetically, how many racks of tubes would I need to install to balance the demand?