Solar powered radiant (hydronic) heat

[mechjackt]

I want to put down PEX-AL-PEX tubing when they pour my basement slab and connect it to solar collectors on the roof, using a glycol solution as a heat transfer medium.

I'd like to find calculations for the amount of heat I can generate from the panels and store in and retrieve from the concrete slab.  

For cloudy days I plan on using a conventional propane fired water heater as a backup.  Comments on that?

Many thanks!

[wdyasq]

I've done them in the past and it is simply a matter of setting up spread sheets, putting in all the variables and getting GOOD numbers. There are lots of things to consider. That is also the reason good engineers get paid well for that service and others use canned programs meant for selling and give industries a bad reputation.

Proper design will keep a house warm for days without need for backup heat. Proper shutters, good sealing of doors and windows and proper insulation are important.

It is all relatively simple. BUT, the whole system needs to be considered. One cannot economically stick RE stuff on a building not properly designed and expect it to work. Do your homework, learn about heat transfer, learn about insulation and make the design as simple as possible and it will probably work. Get it complicated, put lots of controls and having it where it takes a full manual to service is not a recipe for a successful instillation.

Remember with each energy transfer there are losses. An efficient gas furnace is more efficient than a gas/water/air transfer. If you decide on a water/transfer system you may also want to consider a storage tank in addition to your slab.  

Ron

[altosack]

Hello mechjackt,

Yes, I've done these calculations before (as have others, I'm sure). One problem is that you don't have any numbers to calculate from in your post.

In general, you can collect (reflecting about 50% efficiency) 150 Btu/ft2 per sun hour that you receive on the collectors. For example, with (3) 4x8' collectors and 3.5 sun hours per day, you could collect and store 150Btu/ft2-hr * (3) * (4x8)ft2 * 3.5hr = 50 kBtu per day.

A concrete slab stores about 20 Btu/ft3-F, meaning that if you feed 20 Btu into it, it would raise one cubic foot of concrete 1 degree F. If you have (for example) a 20x50' slab 6" thick, this is 500 ft3, so 50 kBtu would raise it by 50 kBtu / (20Btu/ft3-F) / 500ft3 = 5F. A useful number from this is 10kBtu/F, or each 10,000 Btu raises the temperature 1 degree F.

If your slab is really well insulated, you could raise it up to about 100F (before it burned your feet too much and lost heat to the air too fast), so assuming you started at 70 F, it could store 300 kBtu. This is enough to heat most houses for at least a day.

There are a few conceptual glossings-over from the above, but I hope this gives you the stuff so you can do calculations for your slab/collectors yourself. If you want metric, the useful numbers are: 500W-h/sun-hour/day for your collector and for concrete, 400W-h/m3-C.

Comments on the propane-fired water heater is that propane is a relatively expensive way of space heating; for hot water uses, it's OK. If you live somewhere you can't get natural gas, you can usually get wood, which is much cheaper per Btu, and it heats you twice (at least).

Best Regards,

Dave

[Nando]

Adding to Ron comments:

For those cloudy days, use a larger hot tank, in reality two or three interconnected via bottom tubing and heating one or two of the tanks, leaving the last one as the cool return tank to reduce the hot/cold mixing.

Use of aluminum thermal barrier on the external walls, the ceiling and possible above the rafters, this extra expense will reduce the infrared energy by about 92 to 95 % which is the equivalent to more than 24 inches of rock wool insulation.

http://www.radiantbarrier.net/

http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html

http://www.lpcorp.com/radiantbarrier/radiantbarrier.aspx

http://www.radiantbarrier.com/

http://southface.org/web/resources&services/publications/factsheets/14radiantbarriers.pdf

Google " radiant energy " to find more places if interested, saving about 20 % in energy is quite achievable.

Nando

[willib]

altosack

Been reading your calculations, would you recommend a system where the collectors store the heat in a tank , and use direct glycol to air transfer , through a radiator type setup?

or is the concrete slab idea worth the effort?

[RP]

Bear in mind that glycol to air really needs fairly hot water to work well.  Radiant floor only needs about 85-90°F.  That is an advantage for a solar system.

[Countryboy]

You aren't going to be able to store much heat in a concrete slab.  It will act like a dump load.  You won't be able to retrieve any heat from the slab later to heat water or anything.

The solar collectors will collect heat, and that heat will get dumped in the floor of the basement.  You will need some sort of pump system on the glycol solution to circulate it.

I believe 13 inches of concrete has the insulating effect of one inch of wood.

If you want to store and retrieve heat, use your lawn.  Dirt is an excellent insulation and storage medium.  Run some geothermal lines through your lawn.  The lawn will act like a solar collecter, store the heat energy in the ground, and you can retrieve that heat as needed from the ground.

[GaryGary]

Hi,

Some rough calculations:

On a sunny day, about 1600 BTU fall on each sqft of properly oriented collector.  How much of that you actually collect and get into the slab depends on the collector design and the outside temperature -- 50% is a fair rule of thumb.

So, 200 sqft of collector might get you (200 sf)(1600 BTU/sf)(0.5 efic)= 160000 BTU on a sunny day.  This is about equivalent to 2 gallons of propane burned in an efficient furnace.

But, not all days are sunny.  On a fully cloudy day, you will collect nothing.  On partly cloudy days, it just depends on how cloudy, but it can be pretty good.

To know how much heat you need, you need to do a heat loss calculation for the house.  

Here is a link to my heat loss calculator -- its pretty easy to use:

http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

If you plug in a typical outside temperature for the "Design Temperature", it will give the heat loss for that temperature.  You can then compare this to the gain for various size collectors.  

You will find that for solar heating, it really pays to insulate and seal the envelope well to make the solar heat go further.  Good windows also help, as does night insulation on the windows.

To the degree possible, you should also use the solar passive heating ground rules to design your structure to take advantage of free solar heat through south facing windows.

http://www.builditsolar.com/Projects/SolarHomes/guidesps.htm

In using a concrete slab to store heat:

• Be sure to insulate under it -- don't let anyone talk you into the thin bubble insulation.
  
• You don't really "retrieve" heat from the slab.  The slab directly heats the living space above it.   If the living space is not above the slab, then I would use a water tank to store heat rather than a slab.  
  
  

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Storage

• Since you live over the slab, you have to keep its temperature swing fairly small.  You can't heat it up to 140F like you would a water tank, or you will be living in a sauna.   This lower temperature swing means you need more mass to store a given amount of heat.
  
• Heat stored in the slab is:  (Thi - Tlow)(Specific Heat)(Weight)
  
  

For concrete,

specific heat is about 0.2 BTU/lb-F  (meaning that heating up 1 lb of concrete one deg F stores 0.2 BTU)

Weight is about 130 lb/ft^3

So, a 1000 sqft slab that is 0.5 ft thick, and that you run between 70F and 80F would store:   (80F-70F)(1000 ft^2)(0.5 ft)(130 lb/ft^3) (0.2BTU/lb-F) = 130000 BTU

One rough rule of thumb is that you might want about enough storage to store one sunny days worth of output from your collectors.  This would allow you to keep the house warm through the night following a sunny day, and perhaps a ways into a next cloudy day.  

A water tank to store the same 130K BTU listed above might be about 260 gallons of water.  This assumes the water goes through a temperature range of 140F down to 80F.

Heat stored in a 260 gal water tank:  (

 (140F - 80F)(260 gallons)(8.3 lb/gal)(1 BTU/lb-F) = 130000 BTU

You get away with less water for storage because it can be worked through a larger temperature range, and because it stores 5 times a much heat per lb than concrete.

Here are some links to systems somewhat like you want to build that might be helpful:

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Active

The very first one by Marsden is very much like what you want to do and gives a lot of detail.

The 2nd one is the system on my house.  It keeps the cost down by using collectors that are mostly homemade, and uses a homemade (plywood lined with EPDM) heat storage tank.

A good description of a house that uses a radiant floor slab (plus water under the slab) for heat storage:

http://www.builditsolar.com/Experimental/MearsSolarHeatedHome.pdf

"The Radiant Floor Company" site that is listed has some useful design information.

Hope that helps -- Gary

[CmeBREW]

I don't have any fancy numbers -- but what I have is from hard experience. I like the idea of slab efficiency alot. I just built a simple house on a slab and realized a few things too late. In the winter months, without ANY heating system running, my slab of 6" concrete keeps the rooms at a constant 50 degrees (F) when it is 20-30(F) degrees outside. (I don't heat anything but my bedroom,,,unless its a special occation...)  But the problem is, when I am heating the rooms with electric heating (or any source), that cold 50 degree slab keeps putting out that cold air like a giant air-conditioner - since the ground underneith the slab is that cold. I wish I had spent the money on some kind of a big, thick plastic (or rubber) liner to be put UNDERNEITH the 6" thick layer of small round gravel, and then that 6" of gravel would have been like an 'insulation' layer instead of a conductor of so much cold. Then, supposedly, the slab could heat up better with the heating above, and thus much better efficiency. Also i GREATLY regret not using 6" thick walls and insulation! (used cheaper,standard 4")  Also, don't use that fiberglass insulation with the paper on one side. It leaves air gaps in the walls all over the place! (I don't care what they say--I know--I just done it!)I feel cold air blasting in every electrical outlet and wall switch. (And I tried to be very careful when i done the insulating) -That also likens itself to a small air conditioner running, especially when it's windy out. And the opposite is true too-- in the summer the slab keeps the room remarkably cool up until about August 1st. (for me in Ohio)

Now I get to think about my mistake every time the electric heater comes on. Anyway, this is what I am 'leaning toward' now. Personally, I think ALOT can be done with a slab if its done smart. -Good luck.