Using excess solar energy to heat buried water cistern in Northern Ontario

[sumpower]

The 1500 gallons in our cistern makes for good thermal storage, we have never had any freeze problems.
Lowest temps of 5° F (-15°C) in some years for a couple of weeks. It has never frozen. When we get extened cold spells we pump 2 gallons a minute, recirculating the water in the tank and the circulation keeps it from freezing.

Edit: Our cistern is all above ground with 6" walls, that are filled with wood chips for insulation. Water tank constructed of ferro-cement lining inside of 8" concrete blocks. there is an 18" space between the wood chip insulated walls and the 8" blocks.

 

[OntarioOffgridFarm]

Look up evacuated solar tubes
They heat hot water.
Have some for infloor heat. And preheat domestic hot water

Man you guys are awesome, thanks a lot! I'll look into that

 

[OntarioOffgridFarm]

Man, you are all great. Big thanks to everyone and their inputs. You are a wealth of knowledge. This community rocks

 

[OntarioOffgridFarm]

The 1500 gallons in our cistern makes for good thermal storage, we have never had any freeze problems.
Lowest temps of 5° F (-15°C) in some years for a couple of weeks. It has never frozen. When we get extened cold spells we pump 2 gallons a minute, recirculating the water in the tank and the circulation keeps it from freezing.

Edit: Our cistern is all above ground with 6" walls, that are filled with wood chips for insulation. Water tank constructed of ferro-cement lining inside of 8" concrete blocks. there is an 18" space between the wood chip insulated walls and the 8" blocks.

That's exactly what I was thinking, especially with the massive amount of dirt that I've piled up around it. I like the idea of recirculating the water from indoors, back out into the main tank and I love the idea of building an insulation wall around the tank. Fantastic ideas

 

[McKravitts]

Is it were me this is how I would heat it at 200-400% efficiency

That's the best response so far but not the best solution.
When I first read the OP, I wasn't sure if he was talking about the excess solar radiative energy, or the excess electrical energy from the pv panels.
After reading more of the post it became obvious that the Op and about every response were discussing any excess electricel energy from the pv panels.
As we all know solar pv panels at best only capture 20 % of the total solar radiative energy. In the winter there is less solar radiation and due to the low angle of the sun, the actual solar electrical energy is less than the 20 %.
If he's in Ontario as his name suggest, he'll soon realize that there will be little if any excess solar pv electrical energy in the winter.

A good solution is to get a thermal solar panel. The efficiency at heating water runs 70 % to 90 %.
The efficiency is near the high range if your just trying to keep water from freezing.
One small thermal panel would probably satisfy his requirements.

The best solution is to combine a solar thermal panel with a heat pump. As 1201 suggest that would further increase his efficiency to even more than the solar radiative energy.

 

[OntarioOffgridFarm]

Be careful to not create a bacteria breeding ground. It's really easy to screw up water systems.

It's okay, the water will be salted, occasionally bleach treated, and tested regularly. We will only be using this water for dishes, showers, etc. All our drinking water comes from a spring down the road

 

[max2k]

I bet you don't have a solar direct hot water system, but keep drinking that Cool Aid. You are just moving the freezing problem.

I don't have it and never will- if by 'direct hot water system' is meant a system that uses solar panels DC output to heat water directly. I consider such systems terribly inefficient. If I wanted to heat water with solar I'd probably look at 'heat pump' based systems or utilize those heat tubes, depending how much water needs to be heated.

 

[OntarioOffgridFarm]

That's the best response so far but not the best solution.
When I first read the OP, I wasn't sure if he was talking about the excess solar radiative energy, or the excess electrical energy from the pv panels.
After reading more of the post it became obvious that the Op and about every response were discussing any excess electricel energy from the pv panels.
As we all know solar pv panels at best only capture 20 % of the total solar radiative energy. In the winter there is less solar radiation and due to the low angle of the sun, the actual solar electrical energy is less than the 20 %.
If he's in Ontario as his name suggest, he'll soon realize that there will be little if any excess solar electrical energy energy in the winter.

A good solution is to get a thermal solar panel. The efficiency at heating water runs 70 % to 90 %.
The efficiency is near the high range if your just trying to keep water from freezing.
One small thermal panel would probably satisfy his requirements.

The best solution is to combine a solar thermal panel with a heat pump. As 1201 suggest that would further increase his efficiency to even more than the solar radiative energy.

Fantastic, I think you gentlemen have found my solutions! I greatly appreciate the feedback

 

[1201]

.

The best solution is to combine a solar thermal panel with a heat pump. As 1201 suggest that would further increase his efficiency to even more than the solar radiative energy.

I think both approaches are great.

A solar thermal panel can reach about 75% efficiency.

Using PV at 20% with a mini split at 300% efficiency would be around 60% overall but I think it may be better since the goal is to use excess solar they would otherwise be lost.

 

[OntarioOffgridFarm]

I think both approaches are great.

A solar thermal panel can reach about 75% efficiency.

Using PV at 20% with a mini split at 300% efficiency would be around 60% overall but I think it may be better since the goal is to use excess solar they would otherwise be lost.

This

 

[Brucey]

If it were me this is how I would heat it at 200-400% efficiency

He's not going to get 200-400% efficiency when its -30F to -40F

Prob better off with resistive elements at those temps.

 

[McKravitts]

He's not going to get 200-400% efficiency when its -30F to -40F

Prob better off with resistive elements at those temps.

I think with a good thermal panel you could get well over 100 % thermal efficiency even at very low temperatures.

Good thermal panels are built to minimize both convective and conductive heat transfer.
Evacuated tube models are virtually immune to convective heat transfer and if built properly have very low conductive heat transfer.
That just leaves radiative heat transfer and the suns spectrum is almost totally absorbed by a dull black surface.

Granted a dull black surface will radiate heat away in the infrared spectrum but much of this can be blocked with proper choice of glass or glass coatings.

Even the surface of the moon has been measured at over 200 F in direct sunlight, even though it’s in the fridged cold of space well below -40F.

 

[doing it different]

I would totally make it a bit more insulated.... but salt water????? That's a problem waiting to happen, don't like if for concrete, pumps fauset, anything that the water touches....don't like it at all.


Best system is the one that needs no energy, KISS,
unless your in permafrost,
you probably should have no problem if its decently covered, with insulation and dirt.
Get a wireless thermometer and mount it in side the top of the tank...

 

[efficientPV]

Spending money to heat the dirt! I don't think so. A pump is the lowest cost solution. Circulating water will not freeze. There are boat systems that bubble the surface around the boat to keep ice from forming. A tank it doesn't have to keep all the surface from freezing, just enough to allow expansion. As an extra, consider adding ping pong balls to cover the surface of the water. This is common practice in industry.

 

[McKravitts]

I think both approaches are great.

A solar thermal panel can reach about 75% efficiency.

Using PV at 20% with a mini split at 300% efficiency would be around 60% overall but I think it may be better since the goal is to use excess solar they would otherwise be lost.

I was refering to combining both aproaches into one system.
A heat pump pumping heat out of a solar thermal loop and into, either directly or indirectly, the hot water tank.

 

[1201]

I was refering to combining both aproaches into one system.
A heat pump pumping heat out of a solar thermal loop and into, either directly or indirectly, the hot water tank.

You mean like a water to water heat pump?

 

[OntarioOffgridFarm]

I would totally make it a bit more insulated.... but salt water????? That's a problem waiting to happen, don't like if for concrete, pumps fauset, anything that the water touches....don't like it at all.


Best system is the one that needs no energy, KISS,
unless your in permafrost,
you probably should have no problem if its decently covered, with insulation and dirt.
Get a wireless thermometer and mount it in side the top of the tank...

I hadn't thought of the impact on the pumps, maybe I'll hold off on the salt. Shouldn't need it anyways really, not with all the great suggestions here. Truly, a goldmine of ideas

 

[max2k]

I hadn't thought of the impact on the pumps, maybe I'll hold off on the salt. Shouldn't need it anyways really, not with all the great suggestions here. Truly, a goldmine of ideas

please be careful with salt, it is highly corrosive. If anything, I'd rather put a temperature sensor on the water coming from cistern to see how much 'room' I have there temperature wise. As you noted, problem might not even exist. Sensor would provide visibility and if temperature would go close to some dangerous limit, say +5C would leave enough time to react.

 

[OntarioOffgridFarm]

please be careful with salt, it is highly corrosive. If anything, I'd rather put a temperature sensor on the water coming from cistern to see how much 'room' I have there temperature wise. As you noted, problem might not even exist. Sensor would provide visibility and if temperature would go close to some dangerous limit, say +5C would leave enough time to react.

That's also a good idea, definitely

 

[McKravitts]

You mean like a water to water heat pump?

Exactly.

I've used this method successfully for both heating and cooling.
Unfortunately, I have yet to find an inexpensive water to water heat pump.

If you are aware of any please inform me.

I’ve had to build my own several times in the past but I’m getting too old to do brazing, evacuating, charging and optimizing.

I more than tripled the overall efficiency of ground water cooling with a small water to water heat pump. I use one on my solar hot water panel and with excellent results.

It’s a win, win, win in most cases.

If you’re interested in specific details, I’ll be happy to share.

 

[doing it different]

Could always spray water on the outer unit all winter long.

 

[J_W_T]

About 2 decades ago I did some serious research into using a cistern as a heat battery. Heat it by vacuum heat tubes or solar PV. At that time the solar panel technology losses made heat tubes the winner by a mile for just pure capture the energy and heat the water perspective. You need to perform some heat loss calculations of your cistern and appropriate insulation just to get a sense of what losses you will incur daily throughout the year but lots of online calculators will give you a rough estimate.

In my case I was considering using a glycol mix pumped to underfloor heating so no worries about freezing, bacteria etc.

But like most things in life the true issues arose not from the basic calculations of energy captured versus energy lost to heat lost, pumping electricity losses etc. They all made good sense, the heat gained versus the cost of running, build, glycol etc. amortized over 5 years was break even and beyond that was win win. In my use case.

Where it all got gnarly was building something functional. Heat exchangers (radiators repurposed or copper coils etc) dissimilar metals, electrolysis, sizing the piping to reduce pumping losses, sealing the cistern so reduce oxygen transfer, acidification of the glycol mix due to aging etc. When all was said and done it was cheaper and more efficient to use straight vacuum heat tubes to heat the glycol mix for the underfloor heating using a standard pressurized sealed hydroponic underfloor heating circuit. Use the slab thermal mass as a "battery" and although it wouldn't actually keep the building warm throughout the night is was good enough. Cloudy days and snow were still an issue so backup heat was required.

Anyway, not what you were exactly planning but just my 2 cents, its not the math's that will stop you it's the real world issues. And remember this was 20 odd years ago so the options today may be way better for off the shelf solutions but DIY'ing it will give you headaches.

So work out your volume, work out your surface area, run it through some calculators to figure out your min power input to keep water above freezing, now figure out how much above freezing it needs to be to be above freezing after 5 days of no power. Look at your unused PV and see if that aligns and if it does then go to the next step. A heating element doesn't care about DC vs AC. But honestly the conversion of irradiance to heat is better served by vacuum tube heaters than PV IMHO.

something like https://www.siliconsolar.com/product/20-vacuum-direct-flow-solar-collector/ not a recommendation, first one I saw in search.

 

[doing it different]

I love infloor heat.... but plumbing valves pumps controllers, wood boiler. Other boiler, starts to get very complicated, expensive, and a bit of a big mess.

I built a small version of your description... 15years ago....
it would be alot different if I was going to do it again... man price of solar Panels are cheap...

I would even think about running electric heat wire in the concrete with the tubing.

I probably wouldn't use evacuated tubes.. and go straight up elec Panels...
Switches are alot easier and less messy then 3way valves that still need wires run all over the place.

All electric instead of elec controll over hydronic...

 

[max2k]

I love infloor heat.... but plumbing valves pumps controllers, wood boiler. Other boiler, starts to get very complicated, expensive, and a bit of a big mess.

I built a small version of your description... 15years ago....
it would be alot different if I was going to do it again... man price of solar Panels are cheap...

I would even think about running electric heat wire in the concrete with the tubing.

I probably wouldn't use evacuated tubes.. and go straight up elec Panels...
Switches are alot easier and less messy then 3way valves that still need wires run all over the place.

All electric instead of elec controll over hydronic...

it sounds like modern version of the system would utilize electricity to run heat pump instead of resistive heating elements. Otherwise PV is still 4x time less efficient than those tubes, heat pump levels them up with additional flexibility electricity provides.

 

[McKravitts]

Could always spray water on the outer unit all winter long.

Spraying water on the condensor coils does help alot.
If your cooling you want cold water. I used 52 degree F gound water and increased the SEER of a small heat pump by about 75% by spraying it on the condensor coil.
In the winter, you have the problem of the water freezing. If you pump heat out of say 52 degree water you cool the water down and if you get it below 32 it freezes. Plus you are confronted with what to do with the water afterwards. If the outdoor temp is below freezing you'll get an ice buildup.
In the summer months when you're air conditioning, you could use the water for irrigation.