Evacuated Tube Solar addition to Radiant Floor: is Thermal Heat Tank necessary?

[rkymtnoffgrd]

Hello all, first post, I have searched, and although Ive found lots of similar great info, Im still questioning if I should purse this without a thermal storage tank. A little backgound: Im self-building a passive solar, south faceing, super insulated cabin at 10,000 feet in the rockies. Im roughing in the embedded radiant heat plumbing this month. My location is Southern Colorado, heating days only, average of 5.5-6 peak sun hours per day. Im curious if the addition of an evacuated solar array would provide a significant assit to justify the immediate pursuit. But assume no thermal storage tank... Here are the details. 1000sqft frost protected 4" concrete slab with 4"HD EPS subslab & 8" perimerter XPS. 2x 300' 1/2" pex loops embedded at 12"-14" spacing. Estimating .75GPM flow through each loop for a 1.5 GPM total. Estimating a 20degree delta once the slab is warm. 18KW Electric instantous water htr is the boiler. (because eventual netmetered solar w/ backup is going in) So, on the surface, it appears, that if I could assit the wtr htr during the day with 5-10 degrees at 1.5gpm it seems like a worthwhile pursuit. im envisioning the glycol mix would cycle through an addition loop through the tubes when sunny prior to going into the self modulating wtr htr. The slab would be the only thermal strorage. Things are never as simple as I assume.

 

[OzSolar]

Great first post!

I would offer that in my experience in Missouri and surrounding states that solar space heating is challenging. Short cloudy days and long cold nights present serious challenges that no amount of storage can completely solve.

That being said, since you are already planning on another source of heat, my opinion is that you should put whatever solar BTU's you can directly into the slab and be content with that as the cost to benefit ratio of storage is not favorable.

 

[rkymtnoffgrd]

Thanks Oz, I think so as well! I wonder how many tubes would be necessary for a 10 degree rise at 1.5 gpm on a sunny (but fridgid) day? I would think just plugging 5 hours of that additional heat into the slab would be an investment with a short payback? It sounds logical, but real word experience is what I seek.

 

[Hedges]

Try some math based on spec of the tubes and specific heat of water.
But my SWAG is that 90 gallons/hour will not get much warmth. How long would that take if you heated a tea kettle 180 times on the stove?

Vendor specs probably quote gallons and temperature rise. Then adjust for sun's intensity in winter.

 

[OzSolar]

Thanks Oz, I think so as well! I wonder how many tubes would be necessary for a 10 degree rise at 1.5 gpm on a sunny (but fridgid) day? I would think just plugging 5 hours of that additional heat into the slab would be an investment with a short payback? It sounds logical, but real word experience is what I seek.

You're welcome but I wasn't much help.

Delta T x 500 x GPM = BTU's

10 x 500 x 1.5 = 7,500 BTU's

Once upon time they used to rate SHW collectors in BTU's per day/and or BTU's per hour but's it's been while since I've dug deep into SHW so that's from a shaky memory. See if you can find those specs and play around with numnbers. Derate whatever you find by 50% to be safe and see if still makes sense.

 

[SolarRich]

We have 110 evacuated glass tube collector. It's just enough to heat 120 gallons of water for domestic hot water use in the winter. When the 120 gallons are up to max temp the hot water goes to a radiator in the air handler. This air handler sends the air into ducts in the 12" thick insulated concrete slab.
Our house is solar heated and uses the slab as the energy storage. But that's another topic...

The glass tube collector generates about 5,500kwh of power each year. Energy production is calculated by a program that uses temperature rise and is subject to error.
I saw a house that was totally heated by glass tubes. Both the shop and house. The owner was a plumber. He had perhaps 300-400 tubes. I don't recall. This is NJ.

 

[rkymtnoffgrd]

Wow, ok... thanks for the specific details. I will do the math, and this is really good info... I always go down these rabbit holes... is it just me or is the return on investmest always greater by just adding more diy pv and going all-electric? Even inefficient electric? It seems much simpler In a netmetered year averaged kw scenario to simply add more panels. Later... my goals will be self sufficiency.... I dream of the totally off grid passive and active systems that could keep this place comfortable without feeding the woodstove... but that pursuit comes later. Got to get it built right and tight first. I will put the 3/4 pex in attic and stubbed into the utility closet, but likey spend the dollars on pv vs tubes. I think I have my direction. Thanks.

 

[Hedges]

Yes, net-metering giving 100% efficient "energy" storage for 6 months to a year before use, taking advantage of long summer days for heat during long winter nights, is best. But PG&E don't like that.

20% efficient PV vs. 80% evacuated tubes.

Flat plate vs. evacuated tube solar hot water collectors

Flat plate vs evacuated tube collector systems for solar hot water collectors - which is better? Find out in this article from Energy Matters.

www.energymatters.com.au

Evacuated tube and slab does sound interesting; please publish your analysis.

I have a building on a slab, half on grade or excavated below, half above with apparently gravel (not dirt?) fill. I've thought about drilling holes in block wall and shoving in a pipe-in-a-pipe as heat exchanger. Whether for solar-thermal or time-shifting electric heat.

 

[Llaves]

Wow, ok... thanks for the specific details. I will do the math, and this is really good info... I always go down these rabbit holes... is it just me or is the return on investmest always greater by just adding more diy pv and going all-electric? Even inefficient electric? It seems much simpler In a netmetered year averaged kw scenario to simply add more panels. Later... my goals will be self sufficiency.... I dream of the totally off grid passive and active systems that could keep this place comfortable without feeding the woodstove... but that pursuit comes later. Got to get it built right and tight first. I will put the 3/4 pex in attic and stubbed into the utility closet, but likey spend the dollars on pv vs tubes. I think I have my direction. Thanks.

I think you've made the right choice. Evacuated tube is tops out at about 40% efficient and requires you have plumbing running from your roof to your slab (and possibly a tank). I don't know about you, but the first thing that comes to mind is leaks. PV at ~20% plus a heat pump nets out well ahead of the 40% of the evacuated tubes and doesn't put water over your ceilings. Of course, 10K elevation in the southern Rockies of Colorado means some pretty cold weather, so you'd really want a ground coupled heat pump. If you haven't poured the slab, replace that ½" PEX with ⅝" or even ¾" for much better heat transfer and lower flow resistance (esp. if you're going to put glycol in the system, which you should.) Get a good concrete crew that will tool the expansion joints (as opposed to saw cut them) so that you can locate the PEX close to the surface. Even better would be to pour a 4" slab directly on the ground, add your insulation, then pour and inch or two of light-weight concrete on top of the insulation to give you a more responsive floor.

 

[SolarRich]

Today I would use PV instead of the solar hot water. This is based on cost. PV is much cheaper than SHW today. When I put my system in long ago it was the same cost per kwh.
With that said - my 110 tube SHW system has performed great. One of the tubes mysteriously broke a couple years ago. I had bought replacement tubes when I put the system in.
But I'm sure there are times when the SHW would be a better choice.

 

[OzSolar]

If you haven't poured the slab, replace that ½" PEX with ⅝" or even ¾" for much better heat transfer

Many years ago I attended several training classes put on by PEX and radiant floor manufacturers. This topic came up several times and ~my take away~ from what the factory engineers explained was that the heat transfer was more about a high ratio of surface area vs. volume. EG: You'll get better heat transfer with 1/2" pex. I wish I could find something to support that now but I can't.

 

[Zwy]

Larger tubing leads to warmer areas at the start of the loop. It's due to the velocity of the fluid. Larger tubing, less velocity due to higher volume of tubing. Smaller tubing, higher velocity.

The key to any radiant heat is loop length. Loops on a manifold should be close to the same length without sharp bends. Second is thermal mass, thicker slabs take longer to heat but the heat will be more even. My shop slab is 7 inches thick with 2 inches foam under it. 1/2" pex on the bottom. When a overhead door is opened to bring something in, recovery is just a few minutes due to the mass of concrete.

 

[rkymtnoffgrd]

I think you've made the right choice. Evacuated tube is tops out at about 40% efficient and requires you have plumbing running from your roof to your slab (and possibly a tank). I don't know about you, but the first thing that comes to mind is leaks. PV at ~20% plus a heat pump nets out well ahead of the 40% of the evacuated tubes and doesn't put water over your ceilings. Of course, 10K elevation in the southern Rockies of Colorado means some pretty cold weather, so you'd really want a ground coupled heat pump. If you haven't poured the slab, replace that ½" PEX with ⅝" or even ¾" for much better heat transfer and lower flow resistance (esp. if you're going to put glycol in the system, which you should.) Get a good concrete crew that will tool the expansion joints (as opposed to saw cut them) so that you can locate the PEX close to the surface. Even better would be to pour a 4" slab directly on the ground, add your insulation, then pour and inch or two of light-weight concrete on top of the insulation to give you a more responsive floor.

It is very cold here, It wont get above freezing in any real way for the next 3 months. The slab is constructed, the house is closed in. I've done almost all if it myself, mainly because I didnt trust the contractors would care about or understand the details. I spent a great deal of my time to put the build on an insulated slab (as opposed to piers or stemwall) hammering through rock on the back, ensuring the compaction in the front, detailing the insultation, spending the time getting it the way I wanted it. I embedded 1/2" pex due to the fact most of the limited research seemed to verify best results with installs of 1/2" pex loops, 250' - 300' in length, throttled to .25gmp per 100ft, with 4" thick slabs and utilizing grundfos three speed circulators with adjustable manifolds that have loop flow meters. (I can provide the rough calculations if it is helpful for my slab) These kind of 1/2" manifolds are also readily available. Becasue my build is superinsulated and south facing, I went with 2x 300' foot loops at wider 14" spacing instead of 3 x 300' loops at 12". Im hoping the sun will help. There was a time I thought I would heat with the new generation of super-efficient, low-ambient, air-sourced, heat-pumps. But the winter rates drop to .076 KWh at 4-11pm and 7-10 am. This changed my thought process and I began thinking that I would be best served with a passive solar inslulated slab with a radiant loop I could charge during the cheap nite hours and gain some solar during the day.

 

[Llaves]

So many decisions to make, often with a distressing lack of data. Sounds like you've made a lot of good choices - hope it all works out. I've been down that road - just be ready for surprises that you couldn't anticipate, but it eventually all gets worked out. Normally I'd say to look at the Grunfos Alpha-1 pumps, but with only two loops you don't have multiple zones to worry about.

 

[Warpspeed]

Think of it like this...
If you have 1,000 square feet of concrete slab, and maybe 1,000 square feet of solar thermal collectors, it should work about the same as having the sun strike your thermal slab directly.
Now your super insulated home will be able to trap that heat fairly well, much better than say a greenhouse, but the overall principle is the same.

Oz solar is quite right, this needs a fairly high water flow, or you will have problems of very high temperature differentials. The collectors will run hot, and there will be temperature differences between different areas of slab. The higher the flow, the better it will work.

It may still be quite difficult to maintain comfort levels in your home, you will still have a lot of heat loss from necessary ventilation and opening of outside doors, etc. Success will depend upon what range of temperature swings you can live with.

Its not going to be a toasty constant 70 degrees throughout the day and night no matter how many solar collectors you have.
Its always going to be very cold first thing in the morning, and gain heat very slowly throughout the whole day.

Its a great idea, but you will still need some extra heat from combustion of some kind, or better still a heat pump,or maybe a combination.
That extra combustion heat should be available to heat up the air very quickly when needed, not the slab.
You need instant heat first thing in the morning, for maybe at least an hour, and the slab is not going to be able to do that, no matter how much power you pour into it.

 

[Zwy]

It is very cold here, It wont get above freezing in any real way for the next 3 months. The slab is constructed, the house is closed in. I've done almost all if it myself, mainly because I didnt trust the contractors would care about or understand the details. I spent a great deal of my time to put the build on an insulated slab (as opposed to piers or stemwall) hammering through rock on the back, ensuring the compaction in the front, detailing the insultation, spending the time getting it the way I wanted it. I embedded 1/2" pex due to the fact most of the limited research seemed to verify best results with installs of 1/2" pex loops, 250' - 300' in length, throttled to .25gmp per 100ft, with 4" thick slabs and utilizing grundfos three speed circulators with adjustable manifolds that have loop flow meters. (I can provide the rough calculations if it is helpful for my slab) These kind of 1/2" manifolds are also readily available. Becasue my build is superinsulated and south facing, I went with 2x 300' foot loops at wider 14" spacing instead of 3 x 300' loops at 12". Im hoping the sun will help. There was a time I thought I would heat with the new generation of super-efficient, low-ambient, air-sourced, heat-pumps. But the winter rates drop to .076 KWh at 4-11pm and 7-10 am. This changed my thought process and I began thinking that I would be best served with a passive solar inslulated slab with a radiant loop I could charge during the cheap nite hours and gain some solar during the day.

Here is my LP fired system for my shop. https://diysolarforum.com/threads/radiant-in-floor-heating-system.51401/post-653492

The boiler shown is getting replaced soon, the heat exchanger has an internal water leak and the circulation pump in the boiler failed this morning, luckily I had a spare. New boiler is here, I've rounded up the fittings I need, just need to heat the slab up for a few days to about 75F or higher before I replace the boiler as it will be a day long event. New boiler has fittings in different locations than the current boiler. It's been in 12 years, was 2 years old when I purchased it off a fella putting in a ground source heat pump.

Can't remember where I purchased the flow meter manifolds, they crack over time and leak. The company sent me all new ones, I just replace them as they fail. Circulation pumps are run on high speed, I think the heat is more even. I installed the new boiler pump around noon today and the boiler is still running full bore, it's 6:30 pm here and -4F.

A friend of mine uses an electric boiler, he gets a cheap winter rate for it, about 1/2 the usual rate around here. If I was to do it over again, I might consider a ground source heat pump with a large array and a heat storage water tank. I have no idea how many Kwh his system takes. If you plan on powering with just solar generation, I'd look at a large array, super insulated storage tank heated with a ground source heat pump; the heat pump run off the array during daylight mainly. Would take a good sized battery bank, the storage tank would reduce the size of battery bank. And some type of backup unit for sunless days with extreme cold weather.

 

[yaq]

If it's not too late, I would forget all heating except for a small stove and insulate even more to reach the values of a passive house ~50cm of insulation in total (it's calculated!) and forget the VMC "gas factory" but don't forget triple glazing and bay windows + Trombe wall! it works...
Another trick, instead of pex which will require de-sludging, use corrugated stainless steel pipes which are a bit more expensive but you will have wider pitches so less loops and no maintenance at all!

Experience has taught me that building a house that requires a heating system (apart from a small point heater) is an absolute no-no when it comes to costs!

Finally, if an architect suppervises a project requiring a heating installation, he really has big gaps, both thermal and financial!