Store Excess Solar (and Wind) energy in the ground as heat

[Michelle Konzack]

Good day Off-Grider,

I live in Estonia Off-the-Grid and I am co-owner of a small organic farm
We have rough climate up to -40degC and last year, many of out pre-grown plants died in June (!!!) when we got MINUS Temperatures.
My Farm was not the only one, because an apple farm with several ha has lost all Flowers, hence no fruits in Autumn.

Hence I want to build a WInter-Greenhouse which is roughly 17x7,2m in size (maybe even 4,3m longer).
The Solarpanles (550Wp or 650Wp) are mounted on the very most top.
I will use a Fronius Symo 10kW with a Fronius OhmPilot 9kW and as well a Fronius EnergyMeter which sit directly in front of the ACout of my VictronSystem and should prevent overloading my Victron MultiPlus-II Inverters.

I have plans to dig the Ground under the Greenhouse at least 4m deep out and put 400-500m HDPE Pipes 40x2,4 PN4 (they are specially used for Ground-Water Heating Systems) into and then fill it up with so much sand, that the top level is 1m under the ground, because the Greenhouse is 1m sunken in the ground.

The HeatStorrage will be low temperature, hence only 50-60degC, which hold mostly enough energy to take me over the month from October to January and maybe even February, because my existing 1,9kWp plus new 5,4kWp plus the 10kW Fronius produce in October still so much Energy that I can easily heat electic (Underfloor Heating System) without using my BrickStove. The same count for February already.

Hence, I have to get over November to January and I want it clean.
In addition it would also allow me to go into vacancies, what I currently can not do.

Now it comes where I need help:

What kind of insulation should I use on top of the HeatStorrage?

Ordinary EPS300 with two layers of 100mm would probably not work because EPS is starting structural changes at 60degC and disintegrating at 70degC

What are your oppinions and suggestions?

Thanks in advance
Michelle

Note 1: Polar Night Energy does something similar, but with 600degC
Note 2:
My new 12 Solarpanels LONGI 455Wp cost only 1368€ and the prices for the 550Wp ones are 137€ and the 650Wp only 162€. And this are prices from Estonia!
Hence it make sense to buy them (I think onbuying a whole pallet of 36 and respectiv 31 modules)

 

[OzSolar]

There's several threads on seasonal heat storage and sand batteries. Drop "sand battery" into the search bar.

Lots of folks on this forum talking about how easy it is to do but as far as I can tell none of them have done it yet. There's definitely quite a challenge there, something about breaking the laws of physics and thermodynamics tend to prevent folks from getting very far along with it.

Seriously though, if you're going to try this you do need to get a very good grasp of how much energy you need for a season then compare that to how much you can produce and store minus all losses.

 

[Luk88]

Let's get one thing out of the way very quickly. It is very unlikely you'll store enough heat to heat up the greenhouse for months. Let's say conservatively you need 5kWh of heat per day to stay above freezing. For only 90 days this is 450kWh. A ton of sand at a temp. delta of 50C (heated to 70C, cooled to 20C) stores 11kWh and that is a large temp difference. So you'd need to heat 40 tons to 70C. You'd probably need 200tons more as insulation. So 240t of sand total. This is 12 full truck loads(for the trucks in our part of the world). If you have a sand mine nearby(10km away), maybe it can be 150Eur per load to haul it(and 20eur for the sand). And you need the space to store it, pay the digger to dig the hole, haul the earth away (although often people will take it away for free) and so on.


If you want to keep a greenhouse above freezing IMO there are many better ways (including horizontal geothermal).

Another idea is not seasonal, but daily/weekly heat storage. Why? Because if you're running a heatpump to heat it'll be a lot more efficient during the day when it's - 3C than at night when there is - 30C. So you store the heat during the day, use it during the night.


Or, use an intermittent energy source like a wind turbine. Then you can use normal electric heating elements to heat the sand for a week when wind blows and recover the heat for a week or two after.


So let's say you definitely want it.


One of big advantages of sand heat batteries is that sand is only mediocre as low heat storage medium (it gets a lot better if you go well above 100C), but it has pretty good heat insulation property. Dry sand has heat insulating property about a third of polystyrene. So companies like polar night can get away with not using any additional insulation.

Regarding your EPS foam question. I'd not use it. Instead just use more sand. If you want to use EPS foam bear in mind there will be a heat gradient within the sand. Even if you heat up the core to high temperature it will travel through the sand very slowly so it is likely your EPS foam will never see temperatures anywhere near what the sand in the core will have.

If you want to limit yourself to below 100C (for the ease of extracting the heat etc) a very good idea is to dig a large hole, fill it a third of the way with sand. Put an at least 1k liter water tank in there and fill it with sand the rest of the way. If you have 1.5m of sand surrounding the tank you have best of both. High heat capacity of water and good(cheap) insulation of sand. However a water tank like this is relatively expensive.


So just the sand is fine too, but you can't just put heaters directly in it. They will overheat. A well tested method is to take a steel barrel. Attach the heating elements to it and submerge it in the sand so it becomes a heat spreader.


Many people think about these things, but not a lot of them get made. Why? Because they are bloody huge and although the sand is cheap once you take into account hauling and digging it becomes cheaper to do something else.

 

[Michelle Konzack]

My SandBattery/ThermalStorrage is the WHOLE Surface of the GreenHouse.

Hence probably 21,5x7,2m and under the floor at least 2,5m deep.

In Sand you can store 0,3kWh/degK/m3.

The ground temperature is in the FIRST year 10degC.

We want to archive a storrage temperature of at least 50degC.

Then let us calculate:

21,5m x 7,2m x 2,5m x 0,3kWh/deK/m3 x (50degC - 10degC) = 4644kWh

To archive this LDHS you have to calculate an efficiency of under 50%, hence you have to put 9288kWh into.

Since I install 18 Solarpanels Leapton Energ 650Wp, a Fronius Symo 10kW, a Fronius SmartMeter, a Fronius Ohmpilot and an 9kW In-Flow-Heater I get with 1200 Solar Hours a year in my Latitude around 12.000kWh of Energy, hence enough to heat up this monster in a single year.

But this is only the theory, because you do not only heat up a cube of 400m3 but also create a heat-cloud around.

Hence, the Energy LDHS will only work properly after 2-3 years preheating.

But now it comes:

Keeping the Greenhouse in tropic climat is no problem at all, but I want to heat my house using an Underfloor Heating System.

OK, from the theory we have a LDHS with 50degC and the UfHS need at least 30degC which calculates

21,5m x 7,2m x 2,5m x 0,3kWh/deK/m3 x (50degC - 30degC) = 2322kWh

My House need under 50kWh a day if we get -20degC here, but in October I need not more then 10kWh and in November probably 20-30kWh.

However, if I take the maximum of 50kWh/d then I get 46days.

But in October I still get much more solar energy as I need for heating and my Battery are still 100% full

I need in general the LDHS only in November-January, but even this is not really true, because in January I get already more Solar energy as in November, AND from January to February I get an enormous amount of wind energy.

I do not even need a HeatPump to get over the critical time...

Hence, even if it fail to heat the house, I have a realy nice Winter Greenhouse...

What do you think?

I will build the Greenhouse anyway and digging (and refilling) 2,5m deeper cost me under 1000€. 500m HDPE Pipe 32x2,4mm PN8 for Earth-to-Water Heatpumps is under 600€. What is a little bit expensive is the massive foundation and the outher EPS insulation. I assume, it will be around 4000€. Hence only 6600€ more then the Winter Greenhouse itself.

Oh, the floor of the Winter Greenhouse should also get at least 100m EPS insulation, otherwise you will be slowly cooked with 50degC and I do not want to grow Kaktees , Ananas, or something similar in the Winter Greenhouse

 

[Michelle Konzack]

If you want to keep a greenhouse above freezing IMO Another idea is not seasonal, but daily/weekly heat storage. Why? Because if you're running a heatpump to heat it'll be a lot more efficient during the day when it's - 3C than at night when there is - 30C. So you store the heat during the day, use it during the night.

How do you archive this Off-the-Grid?
To run a Heatpump in an Estonan DecemberI would need at least 100kWp of Solarpanels

Or, use an intermittent energy source like a wind turbine. Then you can use normal electric heating elements to heat the sand for a week when wind blows and recover the heat for a week or two after.

The problem is only, if you have more or less no sun in December, you have no wind in the same time.
In my last 7 year in Estonia, I have never gotten from my 5 Windmills (7,4kW together) more then 25kW the WHOLE month

Many people think about these things, but not a lot of them get made. Why? Because they are bloody huge and although the sand is cheap once you take into account hauling and digging it becomes cheaper to do something else.

;-) It is time to do it!

 

[Luk88]

Somehow I missed your first reply, but it is an interesting conversation so I'll reply to it first.

My SandBattery/ThermalStorrage is the WHOLE Surface of the GreenHouse.

(...)

21,5m x 7,2m x 2,5m x 0,3kWh/deK/m3 x (50degC - 10degC) = 4644kWh

To archive this LDHS you have to calculate an efficiency of under 50%, hence you have to put 9288kWh into.

(...)

But this is only the theory, because you do not only heat up a cube of 400m3 but also create a heat-cloud around.

Yes, this an important factor. We know the heat capacity and thermal conductivity of the sand, but the surrounding soil? I don't think you mentioned soil type (I might have missed it). It is unlikely to be dry below a greenhouse and at depths under 1m. Moist clay is almost twice worst as heat instalation as sand, however moist sandy gravel is really bad, about 5 times worse. So this is an important factor.

Then, how much energy loss there will be through the greenhouse itself (in varying external temperatures). But there will be insulation there.

I think sand is chosen for these types of heat batteries and not let's say water because of its great insulation properties too. Your current design is not really using that feature in your favour.

Let's do a bit of "back of the napkin math". You mentioned entire footprint of the greenhouse and depth of 2.5m. This is 145m2 outside surface area ((21.5+7.2)*2*2.5). Let's say you start heating and you achieve 20C temperature difference at the interface (it will be a gradient, but this kind of calculation is easier and approximates at least roughly). Let's take moist clay* as soil at 0.78 W/K*m2. That is 2262W. You doubled your power budget so let's say your power is good for about 40C of difference, but the moment you stop heating the surrounding earth will still continue to pull the heat out.

So in this case I think if we disregard the loss through the greenhouse itself your almost 10kW of heating could heat up that sand to 50C, but how long will that last? Let's take 2262W as an average cooling power (it's in between 40C and 0C). That's 54kWh of self loss. You have 4644kwh in the sand so 85 days self "discharge". All this if you have moist clay.

Of course you plan to heat for a number of seasons. One would have to calculate this properly to know how its going to perform.

*Here are different soil heat conductivity numbers:https://www.cableizer.com/documentation/k_4/

What do you think?

I think a lot will depend on soil type and the greenhouse construction/insulation.

It will be difficult to get dry sand at such amounts so we have to assume moist. This makes thermal insulation values 20% worse (if you use it as insulation, which in your design is not used as such).

I think it would be much better to increase the footprint and give it at least 2m more of sand on each side as extra insulation.

Either way I'm looking forward to seeing the build and results. I don't think anyone has documented such a thing online yet.

I will build the Greenhouse anyway and digging (and refilling) 2,5m deeper cost me under 1000€. 500m HDPE Pipe 32x2,4mm PN8 for Earth-to-Water Heatpumps is under 600€.

500m of pipe, so I assume you took 20W per running meter of heat exchange. For moist sand that may well be fine. I've seen estimates as low as 10W in sandy soil which would double the pipe required, but I think we need a ground loop heat pump expert to confirm here.

How do you archive this Off-the-Grid?
To run a Heatpump in an Estonan DecemberI would need at least 100kWp of Solarpanels

Yes, usually people that use it that way have a wind turbine and are in high winter wind areas.

The problem is only, if you have more or less no sun in December, you have no wind in the same time.
In my last 7 year in Estonia, I have never gotten from my 5 Windmills (7,4kW together) more then 25kW the WHOLE month

;-) It is time to do it!

It is a big challenge. Good luck.

 

[skyking1]

This is a project that I too am working on, not for a greenhouse but to heat our home. We can confab about this!
I suggest that you insulate the sides and bottom much more than the top of the mass. You want the heat in the greenhouse and not anywhere else.
I also suggest that EPS will be fine. It will be difficult to exceed 65C in the storage mass. I will link my thread and the math later.
EDIT:
Here is the post with most of the math in it.
https://diysolarforum.com/threads/p...al-thermal-energy-storage.81493/#post-1055980

I am proposing using fluid heat collection, but the storage and heat recovery/usage will be the same.

 

[Michelle Konzack]

Thanks ,

I will Insulate the sides over the whole surface, But also around 0,5m deep Horizontal around the Greenhose with a width of 2,4m (two times the length of an EPS Plate)

Since the GreenHouse is a massive building (we can get very nice wind speeds here and the Greenhouse is free standing) I need a Foundation which go deep in the Ground like a basement. Hence outside insulation is no problem.

Since I have on one spot of the Farm a place, where I can get pure sand out (I had planes to build a pond there, but it was dropped because of the sand). Hence I need around 400m3 of sand and was thinking to dry it, but it seems, if the temperature of the LDHS is under 70degC, moist sand is an advantage.

Speak, I can simply fill up the Foundation with sand.

The question is now, which Pipes I should use, because the two Manufacturers PipeLife and HakaPlast have not answered to my question regarding the HDPE Pipe 40x2,4 PN8 which is used for the Ground-Waer Heatpumps. AFAIK the maximum temperature of HDPE is 70degC but when does it start to deform?

One person suggested to use PEX-Al-PEX pipes, because the withstand 105degC but they are more expensive the HDPE.

 

[skyking1]

in the us they call that PE-RT pipe.

 

[OzSolar]

The question is now, which Pipes I should use, because the two Manufacturers PipeLife and HakaPlast have not answered to my question regarding the HDPE Pipe 40x2,4 PN8 which is used for the Ground-Waer Heatpumps. AFAIK the maximum temperature of HDPE is 70degC but when does it start to deform?

In case you hadn't already figured this part out, PN8 is the pressure rating at 20C. From what I've seen on other plastic pipes the warmer you go the lower thier pressure rating. However, Google is not revealing that chart for HDPE for me.

Looks like PE, which may not behave anything HDPE, at 65C is derated down to 20% of nameplate rating. Doesn't exactly answer your question, just offering some "food for thought".

 

[skyking1]

pex fares well on your graph. In my proposed setup I'll have ~20 PSI of head pressure plus resistance.

 

[Michelle Konzack]

pex fares well on your graph. In my proposed setup I'll have ~20 PSI of head pressure plus resistance.

This is roughly 1,4bar and very high.

What inside diameter has your pipe and how long is it?

I was thinking on a 32mm version, which give over 200m with 10l/min which give a pressure loss of lesser then 300mBar and I can put 3 loops in parallel. Maybe I make even only 160m length (or even 120m) because the one roll is 480m.

I am at 08:00 in the city and will check with my supplier, how much it cost.

 

[skyking1]

Head pressure = pressure due to the change in elevation.
My collectors are roughly 10 meters above the lowest point in the storage mass, and about 8 meters above the equipment room and pumps.
10 meters head = 14.5 PSI so my rough guess is not far off.
I am using 19mm pipe, and probably 2 runs @ no more than 400m long.
Two discrete storage masses so I can have a graduated storage scheme.
lower storage from the shoulder seasons, higher storage from the peak 100 days.

 

[Michelle Konzack]

Head pressure = pressure due to the change in elevation.
My collectors are roughly 10 meters above the lowest point in the storage mass, and about 8 meters above the equipment room and pumps.
10 meters head = 14.5 PSI so my rough guess is not far off.
I am using 19mm pipe, and probably 2 runs @ no more than 400m long.
Two discrete storage masses so I can have a graduated storage scheme.
lower storage from the shoulder seasons, higher storage from the peak 100 days.

But the pressure is not really true, because once the system is filled up, the inlets and outlets have the same niveau and you have only to circulate them.

I have a Grundfoss UP15-14B with 5Watt and official it can use only 0,8m, but my small Underfloor Heating in the Bathroom is 3m higher and it works perfect.

However, the manufacturer UPONOR say, that the PE-RT are not allowed to be put in the ground.
The 20mm (European Standard) cost 250€ for 480m and would have been a nice solution