Use for excess energy without sending to grid?

[hwy17]

Basement thermal batteries were a proven and implemented technology during the 70's oil crisis solar frenzy. I saw someone on reddit recently pulling out the many tons of large rock under their house that was part of one. It used air tube collectors outside to bring the heat into the rock. I don't know if it was interseasonal or not.

 

[justinm001]

I suppose all of those things are possible.
I'm only concerned with using excess energy from summer. To heat my house in the winter.
I think it is simpler to install a stationary system.
The earth will be my insulation.

So you're saying you're going to dig a hole, fill it with sand and tubing then pump hot air to heat the sand all summer long. Let it sit for a couple months then pump that heat out for a couple month?

How much sand do you think, how deep, how hot and what percent of hear do you think will still be there after 30 days? Because in the middle you're heating a few feet of pipe 600C but the entire surface area of the hole is constantly cooling it to 14C. im sure with specific specs there's someone smarter than me on here that can calculate the losses per day you'll need to account.


Idk if you've ever been a cavern but we have a few in OH and there's some in KY too. They're cold. 57deg cold all year round with water dripping through.

 

[justinm001]

Here's a great white paper for thermal losses for underground pipes used in district cooling/heating. Haven't read through it all but it has all the formulas and is for fluid in transit so we can multiply to get the per month losses.

Heat transfer analysis of underground heat and chilled-water distribution systems - GovInfo https://www.govinfo.gov/content/pkg...VPUB-C13-8bce7c0cd230f7b43ab3836067612994.pdf

 

[timselectric]

So you're saying you're going to dig a hole, fill it with sand and tubing then pump hot air to heat the sand all summer long. Let it sit for a couple months then pump that heat out for a couple month?

Yes

How much sand do you think, how deep, how hot and what percent of hear do you think will still be there after 30 days?

Nothing exact.
Big hole, lots of sand, pretty dang hot, and hopefully enough. lol

It really doesn't matter. If I get enough to make it worth the investment. I will be winning.

 

[timselectric]

Here's a great white paper for thermal losses for underground pipes used in district cooling/heating. Haven't read through it all but it has all the formulas and is for fluid in transit so we can multiply to get the per month losses.

Heat transfer analysis of underground heat and chilled-water distribution systems - GovInfo https://www.govinfo.gov/content/pkg...VPUB-C13-8bce7c0cd230f7b43ab3836067612994.pdf

In this case the heat leaving the pipe is what I want.
So, it's not a loss.
The only losses are at the outer edge. Where the sand meets the insulation.
I'm heating the center. And drawing from the center.

 

[Zwy]

I don't have it with me. (I'm on vacation)
But basically it's a big hole in the ground with large steel piping. Then filled with sand. You heat the sand during the summer. And use it to heat your home in the winter.

You would do better using the excess production to produce hydrogen from water and store the hydrogen for heating use.

Don't have any idea what size jet you might use. I'll let you experiment and get back to me.

 

[justinm001]

In this case the heat leaving the pipe is what I want.
So, it's not a loss.
The only losses are at the outer edge. Where the sand meets the insulation.
I'm heating the center. And drawing from the center.

Ahh I think you're missing the point. Your sand pit will be the pipe and the losses is the heat leaving the sand and into the dirt. I'm assuming a 100% efficiency electricity to heat and 100% efficiency from pipe heating to sand.

Yes you're heating a small sqft of the center and the ground is cooling a much larger sqft all around it so if it's 10x the size you'll need 10x the heat temp just to break even. But its cooling 24x7x365 and you're not

 

[timselectric]

You would do better using the excess production to produce hydrogen from water and store the hydrogen for heating use.

Don't have any idea what size jet you might use. I'll let you experiment and get back to me.

That's a little more complicated. But, I have looked into it.

 

[timselectric]

Ahh I think you're missing the point. Your sand pit will be the pipe and the losses is the heat leaving the sand and into the dirt. I'm assuming a 100% efficiency electricity to heat and 100% efficiency from pipe heating to sand.

Yes you're heating a small sqft of the center and the ground is cooling a much larger sqft all around it so if it's 10x the size you'll need 10x the heat temp just to break even. But its cooling 24x7x365 and you're not

Just finish the videos, already. lol

 

[SeaGal]

So you're saying you're going to dig a hole, fill it with sand and tubing then pump hot air to heat the sand all summer long. Let it sit for a couple months then pump that heat out for a couple month?

Exactly!... Sounds fantasial in my mind that doing that would be cost-effective... (but I'm no geo-scientist)!

Buying a property in the southern hemisphere ?? ?? ?? to occupy for the winter may have a better ROI?

 

[Hedges]

Resistive heating is 100% efficient. Because the exact same amount of energy (in electricity) you put in, is produced (in heat) on the output.

Losses are in the wires outside the thermal battery.



For the sand battery, volume vs. surface area would drive efficiency, so the larger the better.
The math for energy storage vs. amount of sand and temperature rise is easy. losses through dirt more difficult, but given the problem of ground water, just assume it has perfect conductivity (zero insulation) and is always 50 degrees F.

The key design and materials aspect would be selection of insulation to withstand temperature and weight. You might use a couple layers, inches of styrofoam in contact with the dirt followed by one or more layers able to handle higher temperatures. Trick here is that heat conducted through inner higher temperature layers must be low enough that it can escape through outer layers without driving them hot enough to degrade.

The other big problem is keeping water intrusion out. Barrier, sump with pump, another barrier.

 

[timselectric]

Exactly!... Sounds fantasial in my mind that doing that would be cost-effective... (but I'm no geo-scientist)!

Buying a property in the southern hemisphere ?? ?? ?? to occupy for the winter may have a better ROI?

I have thought about doing that, but.
I enjoy the different seasons. And I don't want the upkeep of two properties in my Retirement.

 

[timselectric]

The other big problem is keeping water intrusion out. Barrier, sump with pump, another barrier.

I'm on a hillside. Most rain water runs off the top.
I might add a vapor barrier on the top and sides, just for additional insurance.

 

[fromport]

Using electric water heaters to store renewable energy could do the work of 2 million home batteries – and save us billions

A heater with a 300-litre tank can store as much energy as a home battery at a fraction of the cost. Being able to store surplus solar energy at the right times helps grid stability and cuts emissions.

theconversation.com

 

[Hedges]

But there's no money to be made doing that ...

How about a business that installs pagers on water heater thermostats, and sells load decrease/increase services to the utility?

I've been saying for years now that load control is more important than storage.

 

[OlderDIYer]

I find this discussion fascinating and can sort of see both sides of the two sides of sand storage. I have a 30 x 48 garage that I had insulated very well with spray foam when it was built and I brag that the inside never gets below 45 degrees even when it's 5 degrees outside with no heat added. I've always assumed this is because of the large amount of thermal storage from the ground at about 55 degrees. I had also heard the ground temperature is a yearly average of the winter and summer temperatures because it is so good at long term storage. And I'm sure it has some insulating properties as well. I'm guessing the conductance is so slow and the storage is so great is why it can average temperatures between winter and summer. So the sand storage makes sense in a way. But I see the other side too with the massive amounts of ground around the sand sucking away any heat that's over 55 degrees. To me it would be ideal to dig a large pit then spray foam the pit with 2" of closed cell foam before backfilling the pit with sand to about 2 feet below ground level. Then spaying over the top before backfilling the rest with some of the dirt that was dug out. Realistically, you could probably just use the dirt you dug out instead of sand. With some engineering math you could probably calculate the size of the pit needed to accomplish the task so you don't waste money on extra spray foam. Go for it timselectric!

 

[justinm001]

I find this discussion fascinating and can sort of see both sides of the two sides of sand storage. I have a 30 x 48 garage that I had insulated very well with spray foam when it was built and I brag that the inside never gets below 45 degrees even when it's 5 degrees outside with no heat added. I've always assumed this is because of the large amount of thermal storage from the ground at about 55 degrees. I had also heard the ground temperature is a yearly average of the winter and summer temperatures because it is so good at long term storage. And I'm sure it has some insulating properties as well. I'm guessing the conductance is so slow and the storage is so great is why it can average temperatures between winter and summer. So the sand storage makes sense in a way. But I see the other side too with the massive amounts of ground around the sand sucking away any heat that's over 55 degrees. To me it would be ideal to dig a large pit then spray foam the pit with 2" of closed cell foam before backfilling the pit with sand to about 2 feet below ground level. Then spaying over the top before backfilling the rest with some of the dirt that was dug out. Realistically, you could probably just use the dirt you dug out instead of sand. With some engineering math you could probably calculate the size of the pit needed to accomplish the task so you don't waste money on extra spray foam. Go for it timselectric!

Closed cell foam has an R value of 6.5-7 per inch and if the sand is 1000 degrees (around 550C) then dirt is 50 degrees with a box thats 10sqft then 1357=10*950/7 or you'll be losing 1357BTU per hour just from the insulation. This is assuming closed cell can withstand 1000F temps. This means you'll be losing about 400w of energy constantly assuming a 100% efficiency of heat transfer 3.412BTU/H. Or 288,000 watts per month If my calculations are correct.

Sand has an R value of .5 or so per inch. so the ideal of only the middle would be hot and outsides would be cold means you'd need 158ft of sand on all sides around the heat source. So every foot further would be about 6 degrees colder or 1/2 degree per inch. Now because each foot would be cooler than the one before it'll be a bit less than that.

The following formula is used to calculate the Heat Loss from R-Value.
HLR=SA∗dT/R
Variables:

• HLR is the Heat Loss from R-Value (BTU/hr)
• SA is the surface area (ft^2)
• dT is the temperature differential (F)
• R is the R-value

 

[OlderDIYer]

Justin, I was thinking of a much larger pit and much lower temperature to store in the sand. Does that help? I know it makes the foam much more expensive but by keeping the temperature difference down you will lose heat many time slower. I could see a simple formula that says if you have a 11,000 cubic ft in the house you need the same for the pit, or maybe it works out to 80% or 60%, whatever. The idea is to have a large amount of heat holding mass to slowly charge with excess solar during the summer and then draw from it slowly during the winter. I think it would work. Probably not to the point of a big positive ROI, but a some of us look beyond that to an elegant solution to a perceived problem.

I think about an underground house. You wouldn't build one without insulating the walls. Just because the dirt stays at 55 year round doesn't mean you want to be uninsulated like timselectric is considering. That's where I agree with you. But the foam would keep the heat inside against the smaller temperature delta more efficiently. And the large amount of sand (or dirt) would do the rest!

 

[timselectric]

I think that everyone is over thinking it.
It's a simple process. That doesn't need to be complicated.
I'm going to do it. You don't have to.
And no, it's not going to be a tiny (10' by 10') amount of storage. I need to store enough heat to heat my house throughout the winter season. And a little in spring and fall.

 

[justinm001]

We're not over thinking it, its just simple thermodynamics on insulation efficiency. The problem isn't creating the heat but sustaining those heat losses that you're not accounting for. I hope you do it and I hope it works, I love the idea but please get some data and document your build.

You can easily test your theory by cooking a brick or something at a high temperature then submersing in bunch of sand, then calculating the temperature difference after X hours of the brick and the exterior of the sand. Or submerse a heating element that gets to a specific degree in sand for X hours/days then recording the change in temp of the sand at various lengths away from the element. Turn off the heater then record over x hours/days the temp changes.

With this scale doesn't matter so it doesn't matter if you're heating a bucket, a house or an entire country its just limited on size and power.