Bucket of concrete as a heat resivoir / exchanger

[pierhogunn]

Hey guys, I got another hairbrained idea that I would like some input form yall on...

here is the idea,

take an old 7'dia or better satellite dish, coat it with Mylar, get it nice and smooth, and put a block of aluminum as it's target at the focal point, and run a coolant, probably antifreeze, or peanut oil through that block, and then run that into a heat reservoir made of 40 - 80 lbs of concrete with a 50' long coil of 1/4" copper tubing spiraled inside of it. inside that same block of concrete is a second copper coil, also 50' to provide heated water to some other application...

okay, places where I need some help:

is concrete suitable for this application if the temperature of the input from the collector is not going to exceede 230*F.. (I believe the stuff would boil at this point, unless it is under pressure)

how do I bend the copper tube so that it doesn't kink

what do I not know to ask about?

will polyurethane foam handle the heat

[jackellis]

Concrete can be used as a heat sink. Good news is that it's really slow to cool down. Bad news is that it's really slow to heat up.

80 pounds of concrete is not going to hold enough heat to be useful unless you want to warm a hamster cage. You needs tons of the stuff.

You might start by experimenting with sand, which is also being tested for use in solar thermal power plants. It's cheaper, easier to get rid of, easier to work with, and may heat and cool a little faster. If you mess up the tubing, you can fix it in sand. If you mess up the tubing in concrete, you're...uh...well you get the idea.

To avoid kinking, I suppose you use a special tool for bending copper tubing, much like a pipe bending tool

What are you planning to do with this? Science experiment? Heat your pool or your home?

[pierhogunn]

I am working with one of the guys involved with http://www.ecobox.me to come up with a dirt-cheap easy to make heat trap to store the days heat for everything from space heating to baking

Molten salts are beyond our ability at the moment, and are out of scope for the project.

what about 4 50 gallon drums filled with sand and insulated?

where could I find the thermal properties of dry playground sand?

[schloff]

If I read this correctly, you want to bend tubing (copper, stainless, whatever) into a helix. I have heard that if you fill the tube with sand, and form it around a mandrel (telephone pole, etc.), this will prevent kinks. You didn't mention the diameter of the helix, the ID, wall thickness, or the length of the tubing.

FWIW.

I have a friend who had an application that warranted this procedure. Not from personal experience.

[LCHIEN]

a satellite dish has a focal point of maybe an inch or two.
Your're going to put a block of aluminum at this focal point (very small) with the tube of coolant and then run it to a heat exchanger where the exchange length is 50 feet?
Small issue of heat transfer.

How about pumps to cuirculate the fluid, and how many GPM should the pumps be?

You need to understand the differences and relationships between heat conductance and heat capacity and heat flow.

Did you just make up the numbers (50 feet, 80 pounds, etc) or is there any basis for their choice?

[pierhogunn]

1/4" copper tubing at Lowes is about $25 for a 50' long bundle, and 2 bags of concrete come in at 80 lbs, with about 20-30 lbs of steel barrel for a housing and a few pounds of insulation so that 2 people could manage to move the completed heat-reservoir around from place to place...

is there a easy to understand source where I can learn about the differences and relationships between heat conductance and heat capacity and heat flow?

[jackellis]

The thermal property you want is specific heat. There are numerous references available but the few I stumbled across are metric so you will have to either do some conversions or work in metric units. I still think in BTUs.

Wet sand, water and concrete or stone are all pretty good, but sand is cheap and it's thermal diffusivity is lower than water so it should lose its heat more slowly.

Forget about cooking with stored heat - the temps will be too low. Space heating is a definite possibility.

To insulate the copper tubing, you want styrofoam or better still spray foam wrapped around the pipe with no gaps and about six inches thick to minimize thermal losses. IOW, build a box around the tube that's 12 inches wider than the tube and carefully fill it with foam. Or, build a smaller box out ofaluminum-faced insulation with the aluminum facing the tube. Make sure there's an air gap between the aluminum and the tube.

Better still, build the coil that picks up heat from your dish with copper, then use appropriately rated plastic tubing to move hot water to the storage medium. It'll probably be cheaper and will also require less insulation.

I agree with Loring that there's more to think about. You might want to build this so that natural convection takes the place of a pump, which means putting the collector below the storage barrels.

[pierhogunn]

how hot can I get the spray foam, the stuff like the great-stuff?

[shortly]

The specific heat for most varieties of stone or concrete is about .19. That's less than one-fifth the specific heat of water. Specific heat is the amount of heat in BTU's required to raise one pound of stuff one degree fahrenheit.

Imagine a bucket with a volume of exactly one cubic foot. If it were filled with water it would weigh about 62.3 pounds and would absorb 62.3 BTU's for each degree fahrenheit that its temperature increased. To raise the temperature of this bucket o' water 100 degrees would require 6,230 BTUs. Conversely, it would give up 6,230 BTUs as it cooled by 100°.

That same bucket filled with concrete on the other hand, would weigh about 144 pounds. Remember that the specific heat for concrete is only .19 so it would only take 2,736 BTU to raise its temperature 100° (144 x .19 x 100). As it cooled (giving up its heat to the space you are tying to heat for example) it would only give back - you guessed it - 2,736 BTU as it cooled.

Even though the bucket o' concrete weighs more it can only store less than half the amount of heat as water in the same amount of space.

[pierhogunn]

yeah, this "litle" heat resivoir idea has been put to bed. and thanks for helping me cement that last nail in it's coffin

[jackellis]

Dan,

If its any comfort, lots of people are working on something similar, but for a different purpose. One of the very real practical problems with producing electricity in large amounts using solar panels is what happens when a cloud suddenly covers the solar field. Output drops rapidly, which creates problems balancing production and consumption on the required instantaneous basis. Some of the new solar technologies use concentrators to heat water and make steam to drive a conventional turbine. Some of these technologies include storage so that solar energy captured during the day can be used in the evening. Finding a cheap, reliably way to store captured heat has become pretty important with all of the focus on using the sun, wind, ocean and hot rocks for electricity instead of burning coal or gas.

[LCHIEN]

Shortly showed that concrete is a poor choice to store energy. IN addition to its poor capacity to store heat, I think it also has low conductivity which means its not even easy to get that heat in and out.

[pierhogunn]

and to top it off, if you put meaninful amounts of heat into it, unless you use refractory cement, you will destroy the block cause the water that stays in the concrete will turn to steam...

my other option was using lead ( poured into a steel container filled with about 100' of coiled copper tube) but the nanny state has declared lead a toxic waste product, and the best source for free lead ( or very cheap lead) now can't give it away, but has to pay a hazardous materials company to haul it off...

yay