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Author Topic: What is the best medium for storing solar heat?  (Read 14153 times)

Offline snooperay

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What is the best medium for storing solar heat?
« on: 07/04/2010 19:29:07 »
Hi. First post. I know of a man in Minnesota who made his own solar thermal collector panels. He moved the heat collected through insulated ducts to a 60'x 6'x 6' insulated box filled with 1" gravel. The box was buried below ground and the heated air traveled through the box coming out the other end where it was ducted to the house through a thermostatically controlled fireplace. When the heat was not being blown into the house it was routed back through the collectors. He claims that he heated the rock all summer long and used the heat from the rocks throughout the fall and winter until he finally had to turn on his furnace in late February. I would like to build a similar system, but wonder if there might be a better medium for retaining the heat collected. Any ideas? Thank you.
Snooperay


 

Offline Geezer

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What is the best medium for storing solar heat?
« Reply #1 on: 07/04/2010 19:45:41 »
Actually, water is pretty good, but it depends on a lot of factors like available space, weight constraints etc. etc.

I suspect the guy in MN was really getting a lot of the heat from solar energy during the winter. The mass required to store much of the energy for a winter would be very large.

Here's a link to information on heat capacities. There are tables at the end that list the specific heats of various materials.

http://en.wikipedia.org/wiki/Specific_heat_capacity
 

Offline Soul Surfer

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What is the best medium for storing solar heat?
« Reply #2 on: 07/04/2010 22:46:46 »
Water has just about the highest thermal capacity of anything but unless you pressurise the system you are limited to getting it up to boiling point so a large boiling swimming pool in the basement might be a good idea :-)  hot rocks can get very hot and are used in electrical storage radiators but you need to get them much hotter and that may be difficult with solar thermal generators although the modern vacuum tube ones do much better than the old fashioned blackened tubes in a class box.  so it probably depends a bit on your collector technology
 

Offline JimBob

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What is the best medium for storing solar heat?
« Reply #3 on: 07/04/2010 23:49:32 »
The guy was heating the gravel, not water. Depending on the type of material the rock was made from, it can hold a massive amount of heat, especially if isolated from the rest of the environment.
 

Offline Geezer

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What is the best medium for storing solar heat?
« Reply #4 on: 08/04/2010 01:25:45 »
Yes, but the OP asked which was the best medium, and water holds about four times more heat than rock (by weight). If you had bothered to check out the reference I provided, you might have learned that.

(Typical geologist - they seem to think the World is made of rock.)
 

Offline snooperay

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What is the best medium for storing solar heat?
« Reply #5 on: 08/04/2010 15:27:48 »
Thanks all. Geezer I apreciate the info. Looks like if I go with water there are better water heating technologies with soloar than what I described. I would also probably need to change my forced air system to a baseboard type heating system or underfloor heating. Guess I will have to rethink. Thanks again.
 

Offline JimBob

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What is the best medium for storing solar heat?
« Reply #6 on: 08/04/2010 23:10:40 »
I believe snoopray is asking for usable solution, not a theoretical one. Besides water stored in the ground will quickly reach the temperature of the surrounding earth.

Unlike the typical Scotsman you are Geezer, if you would spend a penny you could Google "heat +sinks +green +homes +Minnesota" you will find that the solutions provided are earth based. A thermal heat pump using gravel is a very effective means of isolating heat for home use.

Below about 6-10 feet, the earth remains almost a constant temperature year round - about 60 to 70 degrees, depending on the latitude. If the heat pump is set deeper a more efficient heat pump is obtained.

(Jees - you would  think a guy living in the mountains with snow still on the ground would have found ways to save on heating bills. ESPECIALLY ONE BORN IN HAGIS-SWALLOWING LAND! I know of what I speak - listen up or I’ll gie ye a skelpit lug!)

« Last Edit: 08/04/2010 23:13:30 by JimBob »
 

Offline Geezer

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What is the best medium for storing solar heat?
« Reply #7 on: 09/04/2010 00:08:09 »
Nothing impractical about water. You just need to insulate the container. You'd have to do the same thing with rock anyway.

It's also a lot simpler to use the heat with water. You simply circulate it through radiators in the home using thermostatic valves to control the temperature in each room.

By the way laddie, don't knock the Scots. If it had not been for two Scotsmen finding a penny at the same time, we would not have copper wire.
 

Offline JimBob

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What is the best medium for storing solar heat?
« Reply #8 on: 09/04/2010 16:58:19 »
Nothing impractical about water. You just need to insulate the container. You'd have to do the same thing with rock anyway.

It's also a lot simpler to use the heat with water. You simply circulate it through radiators in the home using thermostatic valves to control the temperature in each room.

By the way laddie, don't knock the Scots. If it had not been for two Scotsmen finding a penny at the same time, we would not have copper wire.

Gosh - you just described a heat pump!!  Ya catch on fast, ya do!
 

Offline Geezer

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What is the best medium for storing solar heat?
« Reply #9 on: 09/04/2010 18:03:19 »
Nothing impractical about water. You just need to insulate the container. You'd have to do the same thing with rock anyway.

It's also a lot simpler to use the heat with water. You simply circulate it through radiators in the home using thermostatic valves to control the temperature in each room.

By the way laddie, don't knock the Scots. If it had not been for two Scotsmen finding a penny at the same time, we would not have copper wire.

Gosh - you just described a heat pump!!  Ya catch on fast, ya do!

Er, bollocks! No heat pump involved. It would only be a heat pump if the working fluid went through phase changes (liquid/gas/liquid)
 

Offline snooperay

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What is the best medium for storing solar heat?
« Reply #10 on: 09/04/2010 19:18:03 »
You guys must be old friends. OK, another guy in Minnesota built a system that pumped hydraulic fluid through pipes that had small orifices every so often. The oil was pumped under high enough pressure that it was heated as it passed through the orifice at a higher velocity than when it was traveling through the larger pipe. these pipes ran through heat radiators in each room and then completed the circuit as it returned to the pressure tank. only power it takes is to run the pump. Said God gave him the plans.
 

Offline Geezer

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What is the best medium for storing solar heat?
« Reply #11 on: 09/04/2010 19:41:59 »
LOL! I best not write what I'm thinking.

There is a problem with that one. It will work, but it sounds like the heat is being produced by friction from the high velocity fluid. There is no free lunch, so the energy to produce the heat is being produced by the pump, which gets its energy from the electricity supply. He might as well be using electric heating elements.

BTW - pay no attention to "Rocky".
 

Offline JimBob

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What is the best medium for storing solar heat?
« Reply #12 on: 10/04/2010 17:53:29 »
You guys must be old friends.

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Miscible binary fluids are the most common fluids used in heat pumps. They are designed to have a much lower boiling point than water so that the heat exchange can be done more efficiently. Whether this is actually more efficient is a category of science I avoid like the plague - thermodynamics. If someone says they understand thermodynamics completely I am looking at either a mentally deranged person or a person with a pocket protector and no life outside of their existence in a lab.

 

Online syhprum

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What is the best medium for storing solar heat?
« Reply #13 on: 15/04/2010 16:47:19 »
I recall reading in a pre war book on the subject that the best heat storage medium was a salt that was melted by the heat and then gave it up when it solidified.
Commercial solar thermal plants use oil that is heated during the day and is used to power the turbines or Stirling engines over night.
 

Offline Geezer

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What is the best medium for storing solar heat?
« Reply #14 on: 15/04/2010 18:05:59 »
I recall reading in a pre war book on the subject that the best heat storage medium was a salt that was melted by the heat and then gave it up when it solidified.

That sounds like a good way to go. I suppose you could do the same thing with a liquid with a lowish boiling point.

I seem to remember some storage heaters in the UK used a salt like that. I bought a house that had storage heaters, but the ones I had were just full of big bricks!
 

Online syhprum

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What is the best medium for storing solar heat?
« Reply #15 on: 16/04/2010 10:19:34 »
The big advantage of using a medium that changes its state from solid to liquid is that it happens at a near constant temperature I believe it was about 50°C with the chosen salt.
You could use Ice that turns into water but the transition temperature is rather low so you would need to use heat pumps.
« Last Edit: 16/04/2010 10:23:37 by syhprum »
 

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What is the best medium for storing solar heat?
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