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Storing heat in concrete/masonry with PV

 
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Here's an example on an industrial scale where this works out - link

Any thoughts on a domestic equivalent? I've been looking at dumping PV direct into some kind of highly insulated thermal store during the summer - for both heating water and cooking (see here).

I've been unable to find any concrete/brick storage batteries that are used for hot water. I imagine there's an obvious reason but my basic reasoning leads me to think as brick can reach a higher temperature than water (before it boils...), there'd be less mass needed... waiting for someone to slap me down with some knowledge
 
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Sean, the article is about solar thermal collection, not PV. It would work with PV, perhaps in an off-grid scenario, but there’s lots of reasons to maybe do it differently.
The top one for me is lesser efficiency of PV vs. thermal when heat is the goal.
Next would be net-metered grid-tie PV, where over production (summer in Northern hemisphere) could be “saved” as credit with utility company or stored as cash payments (and used for anything you wish , even heating).
Then there’s the time scale... these systems are generally for balancing production and loads for electric utility plants in the scale of hours and days, not months. And they’re working with steam.

There are underground Annualized Thermal Inertia systems that use a heat pump for storage and delivery. If you have surplus PV it could power the heat pump, and you’d get cooling in the summertime as well. (Not just a stack of hot bricks)
 
Sean Kettle
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Kenneth Elwell wrote:Sean, the article is about solar thermal collection, not PV. It would work with PV, perhaps in an off-grid scenario, but there’s lots of reasons to maybe do it differently.
The top one for me is lesser efficiency of PV vs. thermal when heat is the goal.
Next would be net-metered grid-tie PV, where over production (summer in Northern hemisphere) could be “saved” as credit with utility company or stored as cash payments (and used for anything you wish , even heating).
Then there’s the time scale... these systems are generally for balancing production and loads for electric utility plants in the scale of hours and days, not months. And they’re working with steam.

There are underground Annualized Thermal Inertia systems that use a heat pump for storage and delivery. If you have surplus PV it could power the heat pump, and you’d get cooling in the summertime as well. (Not just a stack of hot bricks)



You're quite right, my apologies.

In our context - we would be off-grid. I'm thinking of a standalone set of panels for this purpose. The approach of using PV over thermal appeals in that it's a simpler system with less (or no) moving parts.

Keen to know if anyone has tried something along these lines, but as I wrote before - there's likely a good reason I'm struggling to find examples.

 
Sean Kettle
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One problem with this idea... a hot water coil running through a thermal mass with a temperature exceeding 100c... is probably not that safe. Does anyone know of safe means to transfer heat in this situation?
 
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The problem with using concrete/masonry is two fold.
First the improved efficiency comes from heating the material to an extremely high point, so how will it be safely contained?
Second, how does one move the heat to the point of use?

Heating water solves both problems.
The existing systems for moving storing and utilizing hot water are extremely cheap and robust.
Space heating/ water heating via solar PV is often accomplished via heat pump water heaters
You might need three times the volume of water to store the same amount of heat,  but broken water heaters are cheap or free and come with insulation and plumbed connection points.
Concrete or masonry  requires you to make all of those systems yourself.

Unless one is really pressed for space,  storing  PV collected energy in water makes more sense than storing it in cement or masonry.
 
William Bronson
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OK,  I just realized I had shared my reaction but failed to offer any useful advice.

To avoid the problem of water in a coil running through a mass that's hotter than the 100c,  you can set an   tank of  water into the mass and run a coil through that.
Leave that tank unpressurized and open to the atmosphere and add a fill valve to keep it topped up.

You will lose water and energy  to the atmosphere through evaporation,  but a tall enough  column (pipe for steam exhaust) would give the steam time to condense and return to the tank.

Alternatively keep an empty tank in the heated mass and add water when you need it.
It will flash into steam, which is very dangerous.
Don't close in the tank,  leave it open to the atmosphere.
Instead run a large (huge!) diameter  pipe from the tank through cooler space to a second(header) tank, set as high as possible  where the condensed water can gather.
This set up will reduce the amount of energy lost on an ongoing basis.
 
Kenneth Elwell
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Sean, I get the idea of the "no moving parts", and that wires don't freeze. I also understand that the cost of PV panels has dropped quite a lot over the years.
Here's what I'm not quite getting: Solar PV is something like 15% efficient, and Solar Thermal is something like 85% efficient, so why use PV to make heat?
Do you have another need for that much electric that is a factor? Like a workshop or car charging? And then, why not store the electricity in batteries? That way you have the option to use electricity for anything: lighting, electronics/appliances, even for heat/hot water.

Have you looked at what has been done at Tamera in Portugal? They are using hot vegetable oil as a working fluid in a solar thermal system, with an insulated storage tank. That allows for >100C temps. (200C I think) and can be used on demand in a jacketed kettle to boil water for cooking, and for steaming.
 
Sean Kettle
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William Bronson wrote:OK,  I just realized I had shared my reaction but failed to offer any useful advice.

To avoid the problem of water in a coil running through a mass that's hotter than the 100c,  you can set an   tank of  water into the mass and run a coil through that.
Leave that tank unpressurized and open to the atmosphere and add a fill valve to keep it topped up.

You will lose water and energy  to the atmosphere through evaporation,  but a tall enough  column (pipe for steam exhaust) would give the steam time to condense and return to the tank.

Alternatively keep an empty tank in the heated mass and add water when you need it.
It will flash into steam, which is very dangerous.
Don't close in the tank,  leave it open to the atmosphere.
Instead run a large (huge!) diameter  pipe from the tank through cooler space to a second(header) tank, set as high as possible  where the condensed water can gather.
This set up will reduce the amount of energy lost on an ongoing basis.



Thanks for sharing this William, a great help. A neat little solution, but when considering all the points you make I'm with you - keep it simple, use water as the storage medium!

I will continue to explore the possibility of dumping into masonry for a summer cookstove however...

Kenneth Elwell wrote:Sean, I get the idea of the "no moving parts", and that wires don't freeze. I also understand that the cost of PV panels has dropped quite a lot over the years.
Here's what I'm not quite getting: Solar PV is something like 15% efficient, and Solar Thermal is something like 85% efficient, so why use PV to make heat?
Do you have another need for that much electric that is a factor? Like a workshop or car charging? And then, why not store the electricity in batteries? That way you have the option to use electricity for anything: lighting, electronics/appliances, even for heat/hot water.

Have you looked at what has been done at Tamera in Portugal? They are using hot vegetable oil as a working fluid in a solar thermal system, with an insulated storage tank. That allows for >100C temps. (200C I think) and can be used on demand in a jacketed kettle to boil water for cooking, and for steaming.



Thanks Kenneth for your thoughts and for highlighting what Tamera are up to, amazing work. I will delve into it further.

Regards PV vs Solar Thermal, the price, reduced complexity and plumbing and lifespan of PV sway me. There are some great MPPT controllers out there designed to hook up to 2kW of PV straight to an immersion heater. This would cover our hot water needs completely from March to the middle of October and wouldn't break the bank.

We'll have other PV panels, I'm keen to build a "multi linear" system akin to the one at Living Energy Farm - these folk walk the talk (granted they use Solar Thermal for their hot water!).
 
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Solar Thermal is twice as efficient as solar-electric. Thus you will need twice the amount of covered space/panels
Solar Thermal is twice the price of solar-electric per panel. So they work out to being about equal in price.

Here is a guy who used solar electric for heating. It was used to heat his water, to heat his radiant floor and to heat the incoming air from his ERV.
http://electrodacus.com/DMPPT450/DMPPT450.pdf


 
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I have considered the possibility of embedding nickel chromium wire as a heating element in concrete in lieu of water as a manner of storing PV energy or Wind energy as heat in a structural mass.  One would certainly have to make certain as to not overwhelm the wires and lose continuity.  That would be hard to repair.  Then again, the idea of it seems dead simple.
 
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It's an interesting idea. I wonder how efficient the heat transfer would be for directly embedded wire.

The problem I see is that concrete is porous. Moisture gets in unless significant measures are taken to seal it on all sides. And moisture provides potential short-circuits for the wire, threatening the integrity of some sections. Unless the wire is insulated, which affects its heat transfer. Hmm.
 
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I'm skeptical but...

This could be an ideal use case for night storage heaters.

In the UK there are electricity plans that supply cheaper electricity at night. See here Economy 7

A night storage heater gets powered by a seperate circuit that turns on automatically when the cheaper rate starts. They're full of specially made bricks with heating elements in between them and vent controls to set the heat release rate.

The main problem with them is that they get heated and release it at the wrong time of day. You end up with a hot house in the morning and cold in the evening and night. Direct solar supply would solve that.

storage4.jpg
[Thumbnail for storage4.jpg]
 
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In Australia we had 'Heat Banks' which used night rate electricity to heat clay bricks moulded so the heating elements
wove through them. They disappeared about 20 -30 yeas ago when gas prices were very low.

I saw one pulled out very recently and the folks had solar PV but did not realise they could have used the old Heat Bank.
Today, using excess or deliberate PV power I thick they would be ideal.
Daytime power will heat them for use later.
I guess in winter the issue maybe available sunlight to drive the PV panels.
 
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I have been interested in some heat storage using PV also. A month ago I installed a heat pump water heater to use excess net metered PV and to eliminate the use of gas to heat hot water. The heat pump water heater uses one to three KWH a day. This works out to one or two 4oo watt panels to make hot water. The PV panels make power in lower light and cooler days when a solar thermal panel would not. It makes sense with the hybrid water heater, but i suppose there is the cost of the water heater every ten years or so and having to add some heat to the house in the winter. The basement is less funky and I am in the process of ducting the cool air into a small room to use as a cold cellar.
A small heat pump or ductless split would probably make sense, but how to store the heat for cloudy days.

I imagine a storage tank with a removable piece of foam on top and a small heat pump section from a water heater or such mounted outside to run when the sun is shining(which would be the warmer days when the COP/ efficiency is higher). At night or in the morning the foam could be removed from the storage tank for a little space heat. Seasonal storage could be done with a small storage tank, radiant floor tubing and a rug or removable foam panel on the floor. Maybe an uninsulated storage tank could be buried under the floor or boxed up in foam with sand or dirt around it.  A person would need to do some math and figure out the timing on when to start the seasonal storage. Might be more trouble than its worth.
 
Thomas Tipton
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Looks like Nichrome wire is resistant to corrosion.  Someone should take this up as a project.   The longer the wire, the more amperage it can handle.  Cast it in concrete, or bury it in cobb.  I see 18 gauge nichrome wire, 50 feet, for $20 or less on Amazon.  I imagine this would even provide an alternative means by which one could preheat a rocket stove exhaust flue to get it primed.  Seriously, has anyone tried this?
 
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