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I need your thoughts and advice on thermal energy storage.

 
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So I'm building a house and planning to go off-grid.  I briefly explained some of my reasons here.
My gut is telling me to be practical and get something off the shelf that is easy to put together; however, this idea keeps nagging at me.

The idea is this:
Heat engine + very large insulated storage tank + very large array of homemade flat-plate collectors = Long term energy independence
I know this is an old idea with a high upfront cost.

Could someone check my math?  I compared lithium-ion batteries because that seems to be where the technology is going.

The energy density of water at a practical 50 degrees C temperature difference is 0.2093 MJ/L.
However, taking the highest possible conversion efficiency into account, it is 0.0314 MJ/L. I bet in reality it will be half this.
Lithium-ion batteries are 0.9 MJ/L.
This makes lithium-ion at least 29 times more dense than hot water.
Lithium-ion batteries are definately more compact than hot water, but cost, fires, and longevity of any chemical battery are concerns.
At some scale heat storage makes more sense than battery storage.
Heat engines at these temperatures can be designed to last lifetimes.  This is one of my favorite aspects, set it and mostly forget it.
The average home in the US uses ~900 kWh per month, this is ~3.24GJ.
This means you would need 103144  liters of hot water to power a home for a month.
This is 27248 gallons, or 3643 cubic feet.
This is a square pond 30.18' long and 4' deep.
More efficiently a cube tank with a length of 15.39', but hydrostatic pressure makes shorter tanks more practical.


In reality I would build a much larger tank(s).  I've built a couple 100k gallon ones before and I have plenty of space.
The thing is that the vast majority of my energy usage will be for heating, not electrical appliances.  Cooling is another big user, but I want to build an icehouse too.
I am already going with hydronic heating and I won't have to deal with conversion inefficiencies.
With a large enough heat reservoir I could even capture excess summer sun for the winter.
Still toying with flat-plate designs, but think I will try something like this.  We have very similar property and I will learn from his mistakes and keep them vertical.
My plan was to find a couple student mechanical engineers at a local college and pay them to design and help build the heat engine.  I'm talking something big and slow and steady, like 60 RPMs.

So what do you think? No nasty PV manufacturing and degradation and few if any batteries to go bad.   Terrible efficiencies, but that's the price you pay I guess.
 
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Are you talking about a space heating and hot water heating setup like below.


They have a pretty nice size storage tank
 
S Bengi
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Or is it that you want to heat some water up to 130F and then connect some type of device to that and turn it into electricity to power a drill/computer/blender/etc?
 
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I would keep heat and electricity separated.
- A LiFePo4 or Lead-Acid battery bank with solar panels for as little electricity as you need.
- An isolated hot water tank that is heated by solar collectors.
- If you do need to cool during a hot day, there is plenty of electricity from the solar panels to run a heat pump.

Storing 100 m³ of hot water sounds really impractical to me… I suppose if you use the entire basement as a tank… But that sounds like another world of trouble if something leaks.
Another major problem is that a heat-engine will generate a roughly constant output. The typical household has anything but a constant power requirement.
 
S Bengi
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Solar Thermal collectors requires fossil fuel and energy to manufacturer and the they do break/degrade over time.  

2kW of solar panel at less than $1 per watt, will heat a 64 gallon water tank for a family of 4. https://www.greenbuildingadvisor.com/article/solar-thermal-is-dead

If you build some thermal mass into your floor, you could use resistive wire or pex piping to store the heat in your house. http://electrodacus.com/DMPPT450/dmppt-presentation-v01.pdf

So less need for electro-chemical storage just straight thermal storage.

 
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I admire your desire to build something epic but solar electric combined with air to air heat pumps will give heating or cooling or electricity storage all in the same package at a fraction of the cost without the need to invent anything or store 100000 litres of water. A reasonable amount of storage helps balance out those over hot or cold days. Solar thermal is in severe decline with the drop in solar pv prices. As to a heat engine, while the promise of cheap long lasting heat engines has been a holy grail for 40 years none have ever materialized. Every few years you see prototypes or youtube videos in labs but they fail to launch for some reason. With cheap PV a lot of the energy has gone out of that work as well. I tend to favour simple hybrid systems myself with multiple elements for redundancy. I don't want to be a downer though. Your math works, the cost and complexity though...
Cheers, David
 
Daniel Richardson
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S Bengi, nice sketch! That is exactly what I'm considering, but with adding a mechanical heat engine.  So Thermal > Mechanical > Electrical.  

To be clear, I am considering installing a 100,000 gallon(378541 liter) hot water tank.  I am on a well and want a cistern anyways, so dual purpose.  The tank in your picture looks like it could be 40,000 gallons?  Just a guess.  It would be out in the field, not under the house, though that might not be so bad in the winter.  The house is all concrete, along with the floors, so there is a lot of thermal mass. I am aware that solar thermal collectors do wear out, usually from overheating or freezing.  There have been a lot of lessons learned in this field and maybe I won't have to learn too much the hard way.  I was considering using regular 11/16" clear insulated glass. I'm pretty sure the manufacturing and recycling is a lot easier than PV panels and doesn't produce silicon tetrachloride.  What is nice is that I could use flat-plate thermal, PV, wind, or fossil fuels to heat the water if need be.  Flexibility is nice and I have survivalist leanings sometimes.  I'm less hung up about how to heat the reservoir and more hooked on not using chemical batteries that require a lot of care to maximize a fairly short life.

Sebastian, I was thinking of sizing the heat engine to something like 4kW of constant electrical power and feeding any unused back into the heat reservoir by ohmic heating.  A large flywheel could help with surges, and some chemical batteries might be needed.  I would ultimately go with what the engineers think is best.  Large, low temperature differential heat engines are almost unheard of because they have such low power density and efficiency; the economics don't work out for commercial applications.  There certainly isn't anything mysterious or exotic about them.  I'm curious about how big it could be, I'm guessing maybe the size of a small room.  I would require pumps to move the water around, and it might make some sense to use some mechanical energy from the heat engine to run them.  

David, you're right of course about the complexity and cost.  Out of the 3 of you who've replied thus far, everyone is on the side of impractical.  Give me a week or so to get in touch with some engineers to find out just how impractical this idea is and if I can live with it.  I'll let you know what I find and thanks for the input!  Thinking about the heat reservoir, what if I just used wet soil rather than a tank and sealed and insulated around it?  My soil is very expansive and holds a lot of water, at least %40 by volume while still retaining shape.  This could be cheaper with less maintenance than a tank.  Ever heard of anyone doing this?  
 
S Bengi
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This annualized solar banking is wonderful for the arctic circle/alaska with 1hrs of sunlight in the winter vs my 9 hrs of winter sunlight.

Check out this 1kw, 24kWHr per day stirling generator with 35% thermal efficiency. But that is with a 2000C/4000F flame.
https://arpa-e.energy.gov/?q=slick-sheet-project/free-piston-stirling-engine-based-1kw-generator
https://sunpowerinc.com/1kw-stirling-engine/

 
David Baillie
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S Bengi wrote:This annualized solar banking is wonderful for the arctic circle/alaska with 1hrs of sunlight in the winter vs my 9 hrs of winter sunlight.

Check out this 1kw, 24kWHr per day stirling generator with 35% thermal efficiency. But that is with a 2000C/4000F flame.
https://arpa-e.energy.gov/?q=slick-sheet-project/free-piston-stirling-engine-based-1kw-generator
https://sunpowerinc.com/1kw-stirling-engine/

it would be interesting to know how that project is going. That one and was it whispergen? Made a lot of headline 3 or 4 years ago. So we are talking apples and apples the 35 percent number is supposed to be the amount of energy converted to electricity? Or the electricity plus recovered heat added together? I ask because you see the 80 percent claim sometimes and that is what they do and it kind of fudges the numbers. Complexity rears its head there again since those high temps would mean exotic materials, aerospace level machining, high cost etc. Those high temp stirlings were what I had in mind when I talked about cheap solar knocking a lot of wind out of the sails of competing technologies.
Fascinating conversation. I'd love to know what the engineers can come up with. There was some videos of a large displacement low temp differential Stirling posted by a Japanese university... no link handy.
 
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A large thermal store could be useful during the winter, but not so useful the rest of the year.  Batteries are useful all year round.

Plus a large, well insulated, holding tank is not going to be cheap.  Batteries might actually be cheaper.

Something else to think about, as your thermal store starts cooling off, it becomes more and more difficult to extract energy from it.  With Lithium type batteries the energy output is pretty flat until close to the end.

Adding a heat-pump to the battery system can further improve its advantages.  With the right pump you can get heating AND cooling, either way they tend to have a COP of 3-4, which means you get 3-4 kwh of heat/cooling for every 1kwh of electricity.

FWIW I did a lot of research into thermal storage systems, including phase change systems that fix some of the issues with using something like water.  After crunching all the numbers I decided it was much cheaper just to use a mini-split heat pump and electricity.  With a well insulated house you can get by with a surprisingly small heat-pump.  I use a 12,000 btu unit to heat/cool my 1500 sq ft house.
 
S Bengi
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If anyone can pull off that 1kw stirling gen jt is those guys.
The $3.5 grant is up November of this year so, hopefully some more news soon. That said they are apart of the space industry so, I don't see the device being cheap, at lease not anytime soon.
 
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I think Peter hit the solution squarely on the head with a pretty good sized Eswing. Rather then devise complex and elaborate measures that seldom come to fruition, why not just work on conservation measures? Building a home is a lofty endeavor and there are ample things to do properly that cannot be done after certain phases of the build are done.

I have always told people to focus on conservation because it is a one for one deal. That is, stop spending money on energy, and it is a dollar that is kept in your pocket. When you build things, there is life expectancy and cost, so the return is never as good as conservation.
 
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I am going to go the other way on this issue.  Heat energy done right looks so much better I would say.  A good large tank designed to encourage stratification and it should be able to do hot water heating, household heat and air conditioning all in the same system.  During the winter store all heat using the lower heat for household heating and the hot heat for hot water heating.  In the summer start throwing the low level heat away every night using the solar collector as radiant cooler and the stratification to keep the cold water from hurting the hot water so they can be stored in the same tank.  During the day use the cold of the tank to cool the brine before it comes back into the house to gather humidity.  Neat thing is the low power needs of this system and lack of toxic stuff making it.  And as quality of glass etc gets better there is no reason why such systems shouldn't run for a century or more with minimal maintenance.  

Here is something I wrote up else where discussing the beginnings of even how to maybe do this all on a real budget.

Here is the link to the dream storage system It starts at about 1 hour and 11 minutes
https://www.youtube.com/watch?time_continue=4268&v=AtHkvpRI6fc&feature=emb_logo
Then about 6 weeks ago I tripped over a DYI storage tank that might be in my budget and able to be fitted to my space. That one lead eventually lead me to this video that best explains it. Now my tank shape would be different.
https://www.youtube.com/watch?v=bI0T7ZegAPU
Combine that with a drain back collector so no antifreeze needed and the above article that would indicate that pex would work for the collector tubing as plastic and suddenly the numbers look way more doable cost wise. Now PEX is very UV sensitive but apparently they are getting far longer life expectancies out of it by a simple coat of black paint to protect it from the UV that also increases its absorbtion. When I first started looking Walmart had one of the rubber membrane sheets(large enough) special for $53. So with some hunting I assume it is possible to get down in that sort of price range again. 300 ft of pex for $100 using 200 of it in the collector and 100 ft of it in the tank as a heat exchanger to preheat the city water going into the existing water heater. Another hundred for the pump. If I could do the insulation, wood work and misc for $500 to $600. Most of the wood work could be reduced in cost if I can get pallet topper plywood sheets free and piece and laminate them to bigger sheets to build the hot water tank box inside and the outdoor collectors. And the final big cost would be for a tempering/mixing valve for the outlet of the water heater to keep the hotter hot water from scalding anyone. I have already gathered the glass to cover the collectors and all I have in it is some fuel to go get the glass.
Now why do it? That sounds sort of expensive compared to just buying a hot water heater and it certainly is going to eat a fairly large piece of floor for an insulated tank holding 300 gallons of hot water. (roughly 24 square feet of floor space) Well I burn roughly 250 gallons of year of propane doing hot water, clothes drier and cook stove. Say I could knock 150 gallons off that. If propane was $2 a gallon(keeping the math easy) that would be $300 a year savings and the pay back period would be less than 3 years likely. And even if propane fell to $1 a gallon it would still pay back in 6 years. The membrane they are listing as a 20 or 30 year life expectancy. Pretend it only does 10 in this application Pex as a collector rating unknown but lets pretend we can get 10 out of it. The pump there again should be 10 plus. The rest of the system should be good for many decades. So potentially looking at say $300 once a decade on materials for repair. The numbers now look really good. And reducing the second highest single energy use in a normal household is good for the environment too. Total energy input would be say 50 watts 5 to 7 hours a day 4 or 5 days a week. There again very doable. More importantly this one only needs to run while the sun shines making it ideal for photovoltaic solar drive..
In the process of researching this I also tripped over desicant air condition and it is possible to possibly combine all of this into a single system.   Now things are better still.
 
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