Solar heat without water storage, slab as storage?

Your cousin is flat wrong thinking concrete stores more energy than water.

Is he planning to run this during the winter? Does Wisconsin get 10 times the sun as Michigan? Because I can tell you solar wouldn't provide much heat on this side of the lake between November and March.

And last....concrete is more of an emitter than a storage medium. If he heats it "during the day" it's also going to give off heat "during the day". There won't be much of a delay. But again, this is assuming he gets any real sunshine...

Do you know how much he paid for the solar? I was thinking of doing this as well to see if it would help heat my dog kennel. We get a fair amount of sun in the winter here.

Concrete (and most other materials except water) store about .2 btu/lb so 60,000 lbs (rough guess?) of concrete will store about the same amount of heat as 12,000 lbs of water, so yes the concrete theoretically could store a million btu if he got it up to about 125 degrees (which ain't gonna happen with those tubes). Of course the water can be stored in an insulated tank and saved for later use, but that's irrelevant for this application, which I think is a perfect use for slab storage. What could possibly be gained by adding a tank, pumps and controls? unless he was going to use a heat source other than the slab that could be turned on only when the garage was in use.

I'm not a fan of evacuated tubes so I have no idea what kind of output he'll get, flat plate would be a much better choice for heating a slab to 45. In any case I doubt the slab will have any trouble handleing the output from solar in a garage, and if there is any insulation the warmth will last through the night just fine. It won't be balmy, but it will really help keep things comfortable and dry.

Look here for the answer, water is the way to go. Not just for capacity, but also for stratification and ease of transfer.

http://www.engineeringtoolbox.com/sensible-heat-storage-d_1217.html and http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

Good luck.

What's the goal here? to store heat or to heat the building?

If you ask an engineer you'll get an engineer's answer.

I think the real issue here is if the panels will provide adequate output in winter. I doubt he'll have excess output to store. Bear in mind that this is a garage so even 10* or 15* swings in room temp night to day aren't a deal breaker. One advantage of evac tubes is that they'll provide higher temp. output which is beneficial in winter. I've read that they don't shed snow well whereas a flat panel will allow the snow to slide off.
If he already has them; I quess he'll find out...

benjamin said:

What's the goal here? to store heat or to heat the building?

If you ask an engineer you'll get an engineer's answer.

Click to expand...

I hear you and agree, I am no engineer by any means. Just giving a hint for the answer. I think the problem is not in the winter, it is in the summer. Does he want to be able to use the floor to walk on? When he gets more heat than he needs, that floor becomes mighty hot. I would say hot water storage would make it flexible, add a pool or hottub and drainback for safety.

Henk.

If you store the energy in a tank of water you could provide DHW also. Here is a chart that shows the heat capacity of various materials. It was used to show the difference between forced air and hydronic heat. But you can see how sand and concrete stack up against water.

Also an output sheet for a common 30 tube array of evac tubes. On the right hand side notice some performance under different solar conditions and different operating temperatures. Category C or D and clear or mildly cloudy day would be close for that area. So expect between 23 - 40,000 BTU per DAY. Per day not hour!

So with 3 - 30 tube arrays maybe 100- 120,000 btu/ day.

2300 square foot of shop with a reasonable load at design of 30 btu/ sq. ft = @ 70,000 btu/ hr load. So on design days you would be able to provide about 1-1/2 hour or so worth of energy, with 90 tubes.

Granted you will see many days above design conditions and certainly some milder days where those 90 tubes will cover the load.

Of course you just need to supply temperature above the slab and ambient temperature in the shop. By driving those collectors at 80 or 90F supply you increase the efficiency. Really no need to drive them at high operating temperatures if 80 or 90 will add energy to the slab. That is what those different categories on the data sheet are showing you.

hr

Here's my solar shed.The collector is based on Gary from http://www.builditsolar.com/ design.Copper tubing with alluminum flashing.Collector area is about 5ft. by 28 ft. Shed is 20ft by 28ft. so not a big building. The collector feeds a 200 gal. storage tank(also Garys design).The tank also has a copper HX that the boiler feeds. The floor has 2 loops of pex in it.
I keep the temp in the building at 50F to 55F. I estimate after 2 winters now that the solar covers approx. 50% of the total heat load so I'm happy with it.
I think if you used the concrete for storage it might not work as good because after 3 or 4 days of no sun the slab and building would cool down so far that you would have a hard time regaining decent temps again.But even with water storage you need an alternate heat source.So---I don't know.
I do know that my building keeps up temp with solar only from the start of the heating season till the end of november,after that it requires imput from the boiler.Of coarse it depends on how cold outside temps are too and how much sun you get ,so lots of variables.

I hate to keep beating the same dead horse, but the heat load determines what a solar system will do.
Superinsulate and a solar system can contribute a large fraction in the winter.

The key factor with a slab is that the slab has to be really well insulated. People who are working with low energy homes are installing
6" under slabs and at the perimeter. This is not necessarily a simple detail for a building. The edge insulation is extremely critical and has to be
intact for the life of the building.

Heat load determines what a slab and/or tank can do.

I agree about the insulation. I put 2" under the slab and around the perimeter.Probably could of used more like Tom said.
Just for the heck of it I put a temp sensor under the slab and foam about 6ft in from the edge and approx. 6"or8" under the foam.At the beginning of the heating season it was about 70* and it has dropped steadily. Now it shows about 54*. I suppose even the ground temp affects your heat load as the winter goes on.

1. it doesn't matter what it cost now, see #3

2. adding water storage will do nothing to reduce lp usage, the solar is so small in comparison to the storage and loss that it will almost never be wasted by overheating the space.

3.more ideas
1 hot water makes the most sense and is the only possibility of reasonable payback because of year round demand instead of "paying full price for season tickets that are only good for the first preseason game" which is what using solar solely for heat is.

2 more fun and the array should have some excess to spare

3 heating a geothermal field with evacuated tubes is kinda like plowing with a prius. you could do it but why?