buried solar tank + heat pump, insulate?
 

Conventional practice is to highly insulate a solar storage tank.

But, when this solar storage tank sources a W/W heat pump it may be more efficient & cheaper to NOT insulate the tank.

Heat moves from an uninsulated tank into the surronding earth MUCH faster
(10-20X) or so depending on soil type & ground water..

But it's not all lost, when the HP draws the tank temperature down to below the surronding earth temperature, heat moves back into tank, possibly 25% of it.

This still sounds like a loss, but consider that MY solar panel array will be WAY oversized (10-15X) vs. summer DHW use.

The surplus, solar heat in warm weather can be dumped into the earth with part of it being used in cold months. Plus a cooler tank will allow the panels to be more efficient.

Edit: They insulate the top only.

See:
Laramie, Wyoming - Thermal Battery Systems | Thermal Battery Systems

How about using the two methods at the same time / in conjunction.

I noticed you would be losing the greater amount of the possible stored heat by not insulating that section of the system.


Have one system in the soil as you have described with a second heat collector system heavily insulated and buried to contain / store the heat for later use..

Found another website from Austria that uses Solar, Heat Pump, large uninsulated underground storage tank, which is allowed to freeze some (latent heat).

Nice simple design presented well.

Their house needs 18,000 KWH of heat (61.42 million btus) per year, about 1.3 x my house design.

http://punktwissen.at/pdf/punktwisse...System-LEO.pdf

and

https://elkement.wordpress.com/tag/heat-pump/

and

https://elkement.wordpress.com/2013/...umps-are-cool/

They also have a DHW tank with internal HX kept @ 50C (122F), which can be heated at a fairly high COP by Heat Pump, without fear of Legionella.

"Regarding the hydraulic design a question that comes up very often is about hot water heating:

You heat hot water indirectly by using a tank at 50°C? I don’t believe you that this is sufficient.

Believe me, it is. My very own very long and very hot showering – elementary showering as I call it – is a worst case test. The heat exchanger in this hygienic storage tank has an effective area of nearly 6m2 - that’s rather large, and this is crucial for a heat-pump-powered system.

The operating temperature of the heat pump should be kept as low as possible in order to obtain high coefficients of performance. Thus the temperature difference between tap water and heating water is rather low, and in order to compensate for that and still get reasonable heating powers the area of the heat exchanger should be big. The effective heating power of this heat exchanger is 12kW."

I wonder if you could use an in ground pool as the thermal battery.. something in the 15-20k gallon range..

Fordguy64, I think so, depends on climate & if you want a cold pool.

Thermal Battery Systems Inc.

"For the low temp applications we are pursuing water is working out very well as the storage medium. We find the following benefits with using water.

1: It is basically free and available everywhere.
2: It is a good conductor into heat exchangers that are placed in it.
3. High specific heat.
4. A latent heat capacity that is 143x the specific heat and this latent aspect is at the "perfect" temperature because it "protects the bottom" of the acceptable entering water temperature to the heat pump.

The other thing to mention is that these water based "thermal batteries" are buried in uninsulated tanks. (The top is insulated) So they are thermally interacting with the surrounding earth. If the application intended to store high temp in the system this would obviously result in net losses to the ground. In low temp applications the uninsulated aspect results in net gains from the ground. In other words it acts like a geothermal ground loop which absorbs heat from the earth when cold water is circulated through it.

The idea of thermal storage is an interesting one if you think about it from the perspective of a conventional "ground source" heat pump system. In that case the design are focused towards the un-containment of thermal energy. Too small of an area results in a failed system from a depleted or a temperature saturated loop. All the heat is gone / bottled up!

The solar thermal approach is the opposite. Everything is done to contain thermal energy. The idea of a non-insulated tank being buried in the earth is preposterous to many in the solar thermal trade. All the heat will go away!

The application is the aspect that defines the design. If the application concept is presumed to be something other than what it is then the design approach will be off."

This approach could be very efficient in winter conditions if your ground temperature is not too close to freezing. As long as there is heat to be gained, this tank could be an awesome buffer against freezing your ground loop. However, using a storage tank in this manner requires a different set of operating parameters than a traditional heat store. Needless to say, it would do a better job at space heating than for dhw.

A compromise could be to just insulate the top of the tank. It isn't very easy to insulate the bottom anyway because moisture in the ground will accelerate cooling. The top of the store needs to be protected from rainwater to avoid wetting the insulation.

I have more or less this arrangement except that the upper part of the store is above ground level. It needs to be because the insulation is straw bales. In the coldest part of the winter heat rising from the ground under the store sets a lower limit for temperature. At the coldest point last winter, end of February, the store temperatures dropped to about 6C and at that point the upper layers of the store were slightly colder than the bottom. My store uses clay and rock for storage so that heat transmission downwards is quite slow but only where the clay is dry. Water will conduct heat downwards fairly rapidly but the water at the bottom will stay fairly cool.

I'm leaning toward using NG boiler along with HP DSH to heat DHW.
COP balance point looks to be about 3.0

If I use HP to only heat the radiant floors via ST120 buffer tank, (ODR controlled temp).

I would use my 360ft2 solar array to heat the 1,000 gal. solar storage tank, insulate top.

Tank has surface area of sides + bottom = 120ft2
500' 3/4 pipe has surface area = 115ft2
So about 580ft2 total in direct contact with my crappy heat transfer gravel soil type GP.

I expect to have an abundance of solar heat even in winter.

Heat from this solar storage tank would be pumped out through the 4 x 500' slinky loops, I read that I need about 6gpm for my 2T HP. That would be 1.5 gpm through each loop, about 10' head loss. A Taco brass 008 should work fine here.

The question is: must I use antifreeze in loops?

Cold water from HP would 1st go through the solar storage tank, pick up heat & then distribute it in slinky loops. water back from slinky loops source HP.

My choice would be PG or Etheyl alcohol in loops (I don't want to accidently poison my water well.)

If antifreeze isn't required in my abundant solar situation, then I would not need the HX in solar tank. Water only needs about 1 GPM to start turbulent flow in 3/4 pipe , but glycol or alcohol needs about 3.5 GPM.

5 Tips on Designing Vertical or Slinky Geothermal Loop Fields | HeatSpring Magazine

It is safer to use anti freeze. Heat pumps usually sense input temperature and cut out at around freezing but at that stage the return water will be below zero. It can happen if there is flow restriction. Propylene Glycol should be safe enough and would only need about 10% if the pipes are reasonably protected from frost.