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

[BMurray]

Ran a test this time with a more accurate temperature data logger. Used an Elitech RC-51 themometer that is designed mostly for monitor temperatures on shipments of perishable goods. It plugs into a USB port and the companies free software allows you to change several parameters. I set the unit to sample the temperature every 5 minutes. It has a metal disk at one end that is the sensor and a USB plug at the other end under a clear plastic cover.



I ran a test at 24 volts with the heating element mat sandwiched between one inch of XPS foam board (R5) and two inches of concrete paver. I placed the Elitech sensor side down against the stone and started the data recorder. Ran the power for five hours, simulating a winter days solar cycle. As expected it took longer for the heater to get up to temperature due to the larger mass, 2 inches rather than the previous 1 inch paver. I think the heater could have gotten hotter, but I shut it down after the five hours.

Below is a graph from the temperature sensor. It automatically creates a PDF file showing a graph and the individual data readings. And/or you can use the companies free software to graph the data on your computer.



The initial spike is my error, I had the sensor in my pocket before I placed it on the stone. Next time I will let it come to room temperature before starting. The red line is at 5 hours when I turned off the DC power. Another error, I should have let the recording run longer. Every indication I had was that the top of the stone had returned to room temperature of 65F. I will run the test again.

NOTE if you ever want to do long term recording of temperature you can buy these Elitech RE-51 data logging thermometers on Amazon for $33.

I think I will try this again but let the recorder run longer. Also looking forward to trying some PCM instead if the stone mass.

Thoughts? Suggestions?

 

[newbostonconst]

Much of your "testing" can be done with just evaluating ideas using calculations of Resistance, Amps, Volts and capacity.....

 

[BMurray]

Much of your "testing" can be done with just evaluating ideas using calculations of Resistance, Amps, Volts and capacity.....

In most cases you would be correct. Problem is that the heating mat material output varies with its temperature. It is self-regulating. At 24v I've seen the wattage move from 8.4 at startup to 5.9 at what I think is max temperature. I hope to match the heat output to the melting point of a PCM.

Trying to figure out if it would be best to use a lower PCM melting point and keep the mat cooler using more watts, or pick a higher temp and melt the PCM using a lower wattage.

Also trying to figure out the optimum temperature to keep a crawl space so that the uninsulated floor surface above would be appx 73F. Not sure where to start with that one....

 

[Q-Dog]

In most cases you would be correct. Problem is that the heating mat material output varies with its temperature. It is self-regulating. At 24v I've seen the wattage move from 8.4 at startup to 5.9 at what I think is max temperature. I hope to match the heat output to the melting point of a PCM.

Trying to figure out if it would be best to use a lower PCM melting point and keep the mat cooler using more watts, or pick a higher temp and melt the PCM using a lower wattage.

Also trying to figure out the optimum temperature to keep a crawl space so that the uninsulated floor surface above would be appx 73F. Not sure where to start with that one....

The temperature of the uninsulated floor above your crawlspace is just as dependent on the temperature above the floor as below the floor. I live in Louisiana where my house is built about 16" off the ground on piers and has an uninsulated floor. The main complaint I have is the cold floors when outdoor temps are below about 45º F. We recently had temps down to 20ºF but that is really rare for us. My feet are still cold!

I have added hard foam insulation under my floor in a few places and put a thermometer on the insulated and uninsulated parts to compare. It only makes only a couple of degrees difference until the heat in the house comes on. The insulation reduces how fast heat inside the house goes through the floor, so the floor feels warmer even though the difference is only a couple of degrees. I want to finish the main living spaces but I have to wait for the dry season as right now I have standing water under the house.

I think you would use a lot less energy by insulating the crawlspace and heating the floor directly. I can't seal my crawlspace as the ground here is too wet and it would rot the bottom of the house, but we do put up wind blockers on the north side during the winter and it makes a huge difference in the floor temperature.

 

[SolarPrep]

Interesting test you have going. My observation: the idea would seem to have some applications such as for keeping batteries warm. It would be much more cost effective to heat water, and pump it where needed. Pumps are very cost effective, and economical. You are going to heat water in your house anyway, so put in a storage tank, and have at it. (Like our friend in Finland said) A typical insulated tank size is 80 gallons, so just taller than a normal water heater.

Solar hot water is much more efficient overall, and easy to work with. If you are going to be building, look into using a wall mounted, direct vent heater, a tank, etc. There are combo units like HTP that do all of that in one unit, but I prefer separates for the same reason as not having an all in one PV unit.

 

[efficientPV]

Higher voltage is your friend. You don't want to loose power just getting to the element. Avoid high heat densities which will damage the element. Remember, when your self regulating temp kicks in you will loose power from the panels. Matching panels to the heater resistance is important. Depending on typical local weather, increasing resistance above ideal can result in more power on the days you need it. Panels are current sources. In direct connect when a panels current drops to half its maximum rating, the power drops to only 1/4. That may be a typical day. Solar calculators can give you a potential daily output. Drop that to 1/2 for direct connect. Get real data before you actually commit to a resistance. Switching in multiple elements is a much better way to go.

 

[740GLE]

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.

American Solartechnics, LLC. - About Us

American Solartechnics in Searsport manufactures top quality products.

www.americansolartechnics.com

In New Hapmshire we have an 800 gallon tank paired with a 40kwh gasification wood boiler for our heating and hot water needs for the winter months. 4 cords of wood and about 100 gallons of heating oil/year keeps our house nice and toasty winter months.

Been thinking about adding solar thermal but a 40 gallon hybrid hot water heater was the cheapest option.

 

[schmism]

Below is a graph from the temperature sensor. It automatically creates a PDF file showing a graph and the individual data readings. And/or you can use the companies free software to graph the data on your computer.

View attachment 42059

Were is the ambient temp control data? ^^ means nothing if the ambient temp went up 6 deg at the same time.

 

[Mike 604]

Have you done a Heat loss calculation on the house ?
This will let you know how much heat you will need in watts per sq ft or btu's, no guess work.
Here we build with hydronic heating in floor/ concrete on top of a minimum R12 insulation.
We use 6x6 steel mesh to fasten the tubing to do the layout.
But in some areas there is no natural gas , so in these areas we use electric in floor heating but not an electric mat.
We use electric heating cable that is made for use in a concrete slab installation. Works great as it can be installed as per
heat design layout made by heat loss engineer.
Messy pic of my installation of my off grid home.

 

[upnorthandpersonal]

Here we build with hydronic heating in floor/ concrete on top

Same here:

 

[740GLE]

Any insulation under the slab? Why no basements?

 

[upnorthandpersonal]

Any insulation under the slab? Why no basements?

Absolutely tons of insulation below, inside and outside the foundation. The 'slab' is actually a floating floor and not structural for the building, insulated from the actual foundation. The foundations go around the building. Like this:



Below the concrete floor with embedded radiant heating there is first a ton of insulation, followed by anti-capillary and draining to keep any moisture away at any times.

No basements here since we're on pure bedrock - just not cost effective except for things like larger buildings/apartment buildings where they need to blast for days to make a hole...

 

[newbostonconst]

I have done 2 houses with heated floors in Michigan. 1/2" Pex one foot spacing imbedded in 1-1/2" of concrete. The first house was R19 walls and R30 ceiling. Floors were toasty warm in winter.

New house is R30 walls and R80 ceiling. This house is very efficient so floors don't need to put out that much heat to heat the house so they aren't that warm, almost a waste to put the effort in doing it.

Both houses had heated floors on every level. So the floor heat goes up and down. So first house I had to turn off the upstairs floor because it was to warm for sleeping. New ranch house I had to turn off the basement because it was to hot from the upper floor heating down.

In new house I do run well water through the floors to cool them in summer when I water the grass. I had one summer where I only had to run the AC like 5 days but this year it was just slightly to warm and ran it about half the time in /July/Aug...

I did a heat loss calculation on the last basement floor and it was over a seven years to pay back 4 inches of foam so I only put in 2 inches.

We have geo and natural gas. Natural gas is 20% cheaper to heat with. Both houses were like this for running cost. I use the Geo more AC when needed. I have used a tank hot water heater in both houses to heat the water.

 

[Kameljoe21]

In most cases you would be correct. Problem is that the heating mat material output varies with its temperature. It is self-regulating. At 24v I've seen the wattage move from 8.4 at startup to 5.9 at what I think is max temperature. I hope to match the heat output to the melting point of a PCM.

Trying to figure out if it would be best to use a lower PCM melting point and keep the mat cooler using more watts, or pick a higher temp and melt the PCM using a lower wattage.

Also trying to figure out the optimum temperature to keep a crawl space so that the uninsulated floor surface above would be appx 73F. Not sure where to start with that one....

Bruce did you ever come up with anything?
Clearly some people can not read which is why they all keep pushing these "other' Ideas.
For those who did not read. The house is going to be empty most of the time and this solar to in floor heating is going to be used to keep the house mostly warm when its unattended. The goal is to build a very simple system that can run off a ground mount solar array.
I have been looking at doing something like this for a while. Once EG4 came out with the solar mini split which is a good option. For my triple plex I would need three 2 ton units at 6k along with 6k in solar. ( could I build a 6k 500v solar array and just feed all 3 systems off one array. I might need to build my array a bit bigger to cover any low light conditions.) The only thing that using a mini sucks is moving air between the 2 other enclosed rooms and this may have to be done with in wall fans or a air circulation system in the attic.
I have been looking at maybe using some heat tape which can be bought for as low as 1 dollar per foot and is around 5 watts per foot using DC voltage from 12 to 48v. I have also read other people connecting DC directly to old in floor heating and it working> Since I am going to be doing new construction in the next couple of years I would like to find some good options today that will work. Cheap in floor heating which will be a insulated floating slab as the finished floor using used solar panels in an array of 15kwh or greater for 3000 square feet and placing wires 1 foot apart and running all wires to 3 main locations and then to one central location for the switch to turn off the array and connect the power. So far I have only seen heat tape wire listed at 48v which is good. I would like to just find something that works at 100 to 500v. Which would be far better. I do not want to regulate the tempature at all. If the house it too hot in the winter then just crack a window for a bit. The whole point is to have that slab warm to release heat at night so the mini splits do not have to be on the grid power. I am calulating the life of the building at 50 years which is what I think I have left in my life it all goes well. If it takes 15 years to pay for its self or take 20 years, really does not matter. If there was a long power outage I would still have heat and cooling. While I may not have anything else unless I install a solar and battery system. I still want to avoid anything that requires the use of batteries if at all possible.

 

[BMurray]

Hi, Thanks for the note. Due to Covid not much has happened. Costs for getting water to my property outside of Durango have risen. But I still plan on doing something... someday...
With the continued decrease in battery storage costs I think the best way to approach my project is to feed the system with 120v from a battery powered inverter and use the manufactures transformer to feed the low voltage mat rather than trying to feed 24v directly from a solar panel to the heating mat.
The way I figure it is that I will be installing a solar power off-grid systems for the sumer occupied house power. Might as well use it for this heating project during the winter. Also any system "loss" that would happen going from solar panel to battery voltage, say 48V, to inverter 120V then back down 24V to feed the heating mat would all be losses that would result in heat. Which is what I want from this heating system that is intended to keep the mass of the house above freezing during the off season.
I am also now considering in slab hydronic heating system. Using daytime solar electric to heat a larger then normal fluid storage tank. And again since the house wouldn't be occupied I could diverty the limited winter solar power for this purpose alone. Future owners could expand the panel array if they wanted a four season home.
This heat storage tank would have a propane backup and would also be part of the domestic hot water system.
I understand you concerns about using batteries in the system. A solar panel direct to device setup sounds so simple and elegant. But with battery improvements, cost reductions and increased concerns of grid reliablity. I think you should reconsider having batteries in the system.
I've had a remote weather station located on the property for the past 15 months. It transmits data every 10 minutes via the cell network. I been able to see the dramatic drop in solar radiation that occures during the winter. I've also seen that it is rare that I get days that are constantly below freezing. At some point during most winter days the temps get above 32F. Even though my weather station doesn't measure snow fall I can see a nearby YouTube live webcam of Lake Nighthorse that shows me snow accumulation in my area. Right now my panels would be under a lot of snow. Two sided panels would help, and a steap angle would help with this.
Good Luck!

 

[Mike 604]

Hey BMurray
The Mylar heating mat that you show in original post reminds me of a product that we used back
in the 80s and 90s on two projects.
Product was called Flexwatt, it was used in ceiling radiant heat applications.
Years later it had to be disconnected due to fire issues. Clients weren't happy.

Recently one of my clients asked if we could help his right hand person with a flooring issue at their newly renovated townhouse.
HW floating floor on a plywood subfloor and entrance was on a concrete slab.
The Heat mats must have overheated and charred (charcoal) the floating floor and the plywood subfloor.
Original contractor "left town" but were able to trace the product to a local wholesaler.
Heating system installer came back as did the HW installer.
"Never seen this before" was what they both said.
Personally I wouldn't install a mylar film in concrete.
An electric radiant system using cables would be fine as long as the floor is reinforced with mesh or rebar.
My 2 cents.

 

[Bluedog225]

I wonder what would happen if you hooked up and array of panels to a big 7/8” piece of rebar embedded in the slab. Dead short wired to handle the load. It would generate heat, right? Size the array to hit a target of 100 F or so on bright days. Concrete would soak up the heat. Crazy idea

 

[Kameljoe21]

I wonder what would happen if you hooked up and array of panels to a big 7/8” piece of rebar embedded in the slab. Dead short wired to handle the load. It would generate heat, right? Size the array to hit a target of 100 F or so on bright days. Concrete would soak up the heat. Crazy idea

Yes that would work. You can google more about in floor electric heating. I was reading a thread from uk maybe where they just install wire cable and use AC to heat up the loop. The resisient of the wire dictates how much energy it will consume. The person who was writing about it or another person was talking about using those wires with DC to provide some sort of heat during the day while it may not provide all the heat it could reduce the cost of overall heat.
I am looking in to some sort of cable and maybe a way to do it as well. While people clearly do not understand the goal. My goal is to heat of the slab enough that it can keep the inside warm enough at night. I do not mind putting up a huge arrray for these types of heating ideas. I want to steer clear of anything water related because it requires pumps that require replacement and water/glycol requires maintained.
The EG4 mini splits are one source of heat and cooling while I want to have in floor slab heat as well which could cut the need for night time heat. We are still young enough that we can live with out a warm night. Once we get older we will need lots of heat to keep our frail body warm.
I have been looking in to the 110v wire as well because I think that it can be used as DC wire as well. I am going to do some calling around sometime and check with the specs as I have learned that many things that work by resistant work both with ac and dc. They just use labels to change the specs.
Also the EG4 units would be really nice if they offered more than one head options which would really make it better for more than one room. Other than installing a whole house recirculator fan system. Mutli head units allow for temp change through out the home.

 

[timselectric]

The resistance wire itself will work fine on DC. You just have to make sure that any thermostat or other controls are rated for DC. And try to match the voltage as close as possible, without going over.

 

[Kameljoe21]

The resistance wire itself will work fine on DC. You just have to make sure that any thermostat or other controls are rated for DC. And try to match the voltage as close as possible, without going over.

Yeah this would be a direct solar connection with a shut off. Of some of the wire I have looked at which is rated for about 5 watts at 48v (which I think that all the wires are the same, yet I could be wrong?) Yet they require short is runs. Which means that I would need to have a lot of connections over a wide area. I really need to do a lot more research because I want a system that requires zero input and a summer shut off and also requires no real controller. I think that I can get away without needing a controller and if one is needed then I can just install a DC rated one with in the working voltage. Yet voltage is going to need to be high as panels voltage is quite high already. I am not going to buy low voltage panels and spend a massive amount in combiner boxes.