Using PV to power imbedded concrete slab 24v heating elements.

[BMurray]

Hello! New to this forum. Designing a medium sized off-grid home for Southwestern Colorado. Great sun exposure, lots of land, good budget.

Looking for you input.

I'm seeing if I can't come up with a combination of solar energy collection and passive storage during the day and heat release during the night. Goal is to limit "moving" / active parts. During the day 24v solar panels will directly power 24v heating elements embedded in the concrete slab. After dark the slab would radiate stored heat into the conditioned crawlspace and into the house. This would not be the primary heat source.

Or I might place the heating mats under a PCM (Phase Change Material) that melts during the day absorbing heat, then solidifies after dark releasing that heat. Wiki page on Phase Change Material. Example of the Infinite R PCM I am considering. Here is a 4 min video that describes the basics of PCM. This technology is getting close to the point where it is cheaper to store thermal energy than electrical energy. I'm talking to various manufactures right now to see what I can buy in small quantity.

So far I have started building a test bed: I bought 10 feet of STEP Warmfloor a Dr Meter DC Power Supply, and a thermometer with remote sensor. Have a variety of wire, connectors and fuses around the garage.

See attached PDF spec sheet for the 24v heating mat I am using. It has a unique feature in that the semi-conductive polymer becomes less conductive as it heats up. Manufacture says this results in a very stable and efficient heat output. Not sure, will test. Specs say heat output is 9 watts per foot at 32f/0c and decreases to less than 7 watts at 86f / 30c.

Lots of 120v and 240v heating matt solutions are available this is the only commercially available mat I could find that uses low voltage and that the manufacturer promotes for solar use. If anyone has seen other options let me know.

I would like to hear any input you all have.
Thanks!
Bruce

 

[Johncfii]

Very interesting potential way to store/utilize excess solar energy In cold months.
How much did that 10-foot sample of the material cost?

 

[BMurray]

The sample of the heating mat cost, with shipping, $157. Not sure what the costs are for larger amounts. Less I would suspect.

 

[BMurray]

Well I hooked things up yesterday for the first time. Ordered 10ft of the STEP Warmfloor but they sent me 11ft. Put some corrugated cardboard down on the garage floor then placed the heating mat directly on top of it. Connected it to the DC power supply. Set it to 24 volts. Power supply output said 3.54amps and 84.92 watts. Mat should run around 7.5 watts per running foot so that looks about right.

Room air temp was 63f, garage floor temp 61f. In the first 10 minutes the mat went to 72f then slowly increased to 74f two hours later when I stopped. I measured the temp using a gun style Etekcity Infrared Thermometer. This heating element is designed to be sandwiched between floor materials so I think exposed to the open air is not a good test.

Plan on placing it on top of a piece of foam board insulation with stone pavers on top for next test.

Any thoughts or suggestions? Am I doing this right?

 

[Freep]

I can't help but wonder if that ten foot sample could be cut into 1 or 2 foot strips and be used to keep LiFePo4 batteries warm.

 

[BMurray]

I suspect so. It doesn't have a thermostat, but they say it is self regulating nano technology. Here is a link to the website detailed description. I'm considering cutting one foot off of it so do test on a short piece then again on a longer section. Would be easier to encase a 1ft piece in concrete.

 

[Johncfii]

It seems like you are approaching this test in a reasonable way.

An interesting product. You mention having a good budget, but this stuff seems rather expensive for the wattage. To embed in concrete would leave it unserviceable in case of failure. An alternative might be to affix to the underside of a wood subfloor and cover with insulation board.

You mentioned a conditioned crawl space. What is that about? It seems like you would lose half of your heat to a non-living space?

Being a DIY’er, without a good budget, I‘m thinking of drums filled with water, heated with a common 24-volt, 600 to 1200 watt heating element. Hot water is useful in all seasons. Perhaps a water-filled wall? But that leaves you without dispersed heat.

 

[Diysolar123]

moving heat around is just called thermal energy storage...heat pumps hehe
You could just do some math and figure out how much concrete you have; this could make quite a difference as the thermal conductivity would probably vary based on the type/mix of concrete. there would be a limit as to how much energy the system could take and how long it would take to thermally radiate back out.. you could end up with a hot floor from 2pm to 8pm and then cold all night!

would that heating material handle the movement of concrete during the heat/cooling seasons?
infloor water lines to heat the floor are rather tough.

The easier way to store thermal energy for later use is a liquid. You can heat up well insulated storage tanks and pump it whenever you need it.
In addition, the concrete infloor tubing is a known and can take the liquid/concrete interaction for decades without worry.

oops I forgot, this is diy solar so the real answer is, clearly, ... you just need a large battery bank because everybody needs a large battery bank!!!

 

[HRTKD]

oops I forgot, this is diy solar so the real answer is, clearly, ... you just need a large battery bank because everybody needs a large battery bank!!!

More Power!!!

 

[BMurray]

It seems like you are approaching this test in a reasonable way.

An interesting product. You mention having a good budget, but this stuff seems rather expensive for the wattage. To embed in concrete would leave it unserviceable in case of failure. An alternative might be to affix to the underside of a wood subfloor and cover with insulation board.

You mentioned a conditioned crawl space. What is that about? It seems like you would lose half of your heat to a non-living space?

Being a DIY’er, without a good budget, I‘m thinking of drums filled with water, heated with a common 24-volt, 600 to 1200 watt heating element. Hot water is useful in all seasons. Perhaps a water-filled wall? But that leaves you without dispersed heat.

Johncfii, Thanks for the input. Good point, at $13.30 per foot, and 7.5 watts per foot, that would be appx $1.77 per watt. Regarding maintenance, placing the heating element into concrete would be no different than hydronic water heating. A very common practice. The product is rated for in slab installations.

Your right placing the elements on the bottom of the subfloor would more directly heat the interior floors. But my thought to get an even heat into the home I would need lots of heating elements. Placing them in the crawl space would diffuse the heat into a larger space and then up into the house. And I'm not using this as a primary heat source. I just want to create a mass below the house that would be warmer than an unheated slab.

I have been thinking about using water as the storage mass. And your idea of using a low voltage heating element would work. I guess each drum would need some kind of thermostat and expansion tank. I would also need to install a drain into the crawl space floor for emergencies. It may be a more cost effective solution than mine.

Conditioned or highly insulated crawl spaces are common in net-zero homes. Four inches of closed cell foam below the slab and sidewalls isn't uncommon. Biggest problem I've heard is cold water isn't that cold anymore. But it brings more of the mass of the structure inside the conditioned envelope.

 

[BMurray]

I can't help but wonder if that ten foot sample could be cut into 1 or 2 foot strips and be used to keep LiFePo4 batteries warm.

I'm going to cut off a 1ft piece and do some testing with it. Will post info here. Depending on your battery environment I would think all you would need to add is a Winter/Summer - On/Off switch. So far is doesn't seem to get very hot.

 

[BMurray]

moving heat around is just called thermal energy storage...heat pumps hehe
You could just do some math and figure out how much concrete you have; this could make quite a difference as the thermal conductivity would probably vary based on the type/mix of concrete. there would be a limit as to how much energy the system could take and how long it would take to thermally radiate back out.. you could end up with a hot floor from 2pm to 8pm and then cold all night!

would that heating material handle the movement of concrete during the heat/cooling seasons?
infloor water lines to heat the floor are rather tough.

The easier way to store thermal energy for later use is a liquid. You can heat up well insulated storage tanks and pump it whenever you need it.
In addition, the concrete infloor tubing is a known and can take the liquid/concrete interaction for decades without worry.

oops I forgot, this is diy solar so the real answer is, clearly, ... you just need a large battery bank because everybody needs a large battery bank!!!

Yes, at some point when I figure out the heat output I can calculate the time it would take for the heat to move through the concrete. I have also read that different kinds cement have different thermal mass properties.

The PCM Phase Change Material I am trying to get melts at a specific point. Lets say 72f, while I heat the PCM it will radiate heat till it gets to 72f then stops radiating while it melts. The melting process consumes/stores a very large amount of heat energy. When melted it begins to radiate heat again. When it cools, and hits 72f again, it will radiate all that stored energy as it changes from liquid to solid. Moving back and forth between 71f and 73f stores then releases huge amounts of energy. Something water cannot do. But I think water might be the more cost effective solution. We will see.

If I can match the output sweet spot of the heating elements with the melting point of a PCM I think the combination would be highly efficient.
Thanks

 

[BMurray]

More Power!!!

Yes Scotty! But one of my goals is to slim it down. PV panel directly to a heating element, no controllers, no storage, no inverters. Only a switch and fuse. The heating mat self regulates. During the summer I would probably route the PV power over to a hot water preheat tank that would sit in front of an on demand propane water heater.

 

[newbostonconst]

They tried phase change in drywall and it was cost prohibitive.

I would heat water and pump it through cheap tubes in the concrete. That way you are keeping you cost down.

If you cost out a hot water heater with the approximate 6kWh of energy it can store in water you will see that the cost of batteries is cheaper. You get batteries at under a $100 per kWh now.

Then you might see that battery storage is where it is at and then using a Air Source Heat Pump like a Mini Split to heat your place as needed. Minisplits with a Seer of greater then 20 are more efficient then Natural Gas now.

GSHP likely have a COP of greater then 4, so you put in one unit of energy and get out 4 units of energy.

Heating a large area of concrete is expensive and the heat loss around it is great with out infinite insulation.

Keep thinking....we all need to be more efficient.

 

[BMurray]

Still testing this idea of heating a mass with PV with no controllers, batteries or inverters. 24v PV directly to the 24v heating element.



Cut a piece of 12"x12" XPS insulation board as a base. Have placed a wired temperature sensor into the surface. Placed some foil around it to expand the sensors footprint.



Cut a one foot long piece of the 24v heating mat. This gives me appx 1 sq foot of element.



Here is the current setup: DC power supply. Two indoor/outdoor thermometers, not scientific quality but they will do. Appx 1 ft square of the 24v heating element sandwiched between two pieces of 1 inch thick XPS insulation board. (note: max operating temp for the Pink XPS is 165F.)

So I fired this thing up and here are some of my results. Doing this in my garage in Southern California with an air temp in the low 60's.

AT 24 VOLTS. Starting out it was using 8.5 watts, element was 59.3F. At 30 minutes, 6.2 watts and 105.8F. At 1 hour, 6.0 watts and 113.1F. At 3 hours, 6.0 watts and 118.4F. At 5 hours, 6.0 watts and 120.0F.

The self regulating feature of this heating mat was evident until it hit around 110F then the wattage stabilized at 6.0.

AT 12 VOLTS. Started out 2.16 watts with the element at 61.8F. 30 minutes, 2.04 watts at 76.6F. 3 hours it stabilized at 1.9 watts and 83.8F. 8 hours still 1.9 watts it was 85.6F. 12 hours still 1.9 watts, element was 86.5F. The heating element increased only 0.4F during the last 3 hours of the test.

Next test is a inch concrete paving brick on top of the element. To get closer to a real world application I plan on running the heater for only 5 hours to replicate a PV input. I have a data recording thermometer I plan on placing on the concrete paver to see what happens to the movement of heat through the stone.

Goal is to see how a PCM, Phase Change Material, on top of the heating element reacts. Matching the output of the heater to the melting point of the PCM. PCM would have an advantage over other types of physical mass in that it releases a large amount of energy at a specific temperature. A water or stone mass just increases and decreases its heat output over a wide range.

Question - What real world voltage variation can I expect from a 12v or 24v solar panel? This application would be for cold weather use with the PV panels aligned for optimal winter exposure.

Question - This would most likely be a ground mount PV installation. What kinds of hardware would I need to add to my system if I wanted a higher volt feed from the panels to cover the wire length? Don't want to assume, but I figured wiring the panels in series then a stepdown transformer to 24v?

Any thoughts or suggestions for additional tests? Thanks.

 

[schmism]

AT 12 VOLTS. Started out 2.16 watts with the element at 61.8F. 30 minutes, 2.04 watts at 76.6F. 3 hours it stabilized at 1.9 watts and 83.8F. 8 hours still 1.9 watts it was 85.6F. 12 hours still 1.9 watts, element was 86.5F. The heating element increased only 0.4F during the last 3 hours of the test.

Any thoughts or suggestions for additional tests? Thanks.

are you testing at room temp and not at 20 or 30 deg F ambient? (slab temp?)

 

[BMurray]

are you testing at room temp and not at 20 or 30 deg F ambient? (slab temp?)

Yes. I'm testing this at "California Winter" room temperature, which is in the low 60's in my garage. This would be installed at a larger scale in a crawl space under an off-grid home in the Colorado Rockies. But that crawl space will be constructed as a "conditioned" space with a heavily insulated footings, stem wall and slab. So the slab will be well above ground temp.

Heating ducts will be in the crawl space as well as vents I can control to keep the crawl space warm. It is intended to be a Near Net Zero home. I want to close off those vents and use this supplemental / experimental PV low voltage heating solution to keep the crawl space warm and to raise the temperature of the interior floors by about 5F above interior thermostat.

During much of the winter the home will unoccupied so I don't need to keep the place very warm. The house will have central propane heat.

Thanks for the question.
Bruce

 

[upnorthandpersonal]

So, not in California (obviously), but over here in cold Finland I have a 3000L water tank that services both hot water needs like showering and the underfloor hydronic floor heating. 3000L is about equivalent to 300kWh storage. Not only is excess solar going in, it also can be combined with an air to water heat pump giving you 4kWh of heat per kWh of electricity. Of course, in this climate there is also a wood burner coupled to it...

Both the concrete floor (floating, insulated) and the tank act as a thermal battery this way, and it's very effective and efficient.

 

[BMurray]

So, not in California (obviously), but over here in cold Finland I have a 3000L water tank that services both hot water needs like showering and the underfloor hydronic floor heating. 3000L is about equivalent to 300kWh storage. Not only is excess solar going in, it also can be combined with an air to water heat pump giving you 4kWh of heat per kWh of electricity. Of course, in this climate there is also a wood burner coupled to it...

Both the floor and the tank act as a thermal battery this way, and it's very effective and efficient.

Nice setup. How big is a 3000L tank? What temperature? Are you using the same water to heat the floors as your domestic hot water or do you have an isolated fluid loop?

Your approach seems like a logical step it my PCM storage idea doesn't work out.

Thanks

 

[upnorthandpersonal]

It's about 2.4 meter high with a diameter of 1.5 meter. This includes the insulation around it. Normally temperature goes up to 90C, but I tend to keep it at 85C. The water in the tank never leaves the tank. It's a battery and you get the energy our through heat exchange coils. The floor heating is one such circuit, and its water is mixed with glycol (antifreeze).