Hey all,

I had a conversation with a solar-company owner friend the other weekend about the most cost-effective way to do an above-garage studio at some point in our ownership of this house.  It'll be SIP or frame, depending on cost at the time, as passive-solar as we can make it within our lot constraints.  We talked about grid-tied PV, but the cost of the inverter is so high, and we've no space or money for 3kw of PV on our roof.  So this alternative emerged:

A non-grid-tied DC system!  As in outer rim country low-cost, cheap tech.

Why is this post in the radiant forum?  I'd like to use radiant for heat, since it's so darned comfy!  So are there any electric-resistance heating mats (like the ones you put under tiles) that run on DC current?  I know we can get lighting, etc that is DC, but the heat is the wildcard.  It'll be small, maybe 12wx12lx8h.

ideas?
charlie

ps-this project is WAY out, so this is more of a fact-finding mission. . .

Electric mats don't care if the power is AC or DC, just that the voltage is correct.

Are you sure that such a system is cost effective?

not necessarily. just weighing options at this point. Waiting for the snow to pile up to go plow it. Idle hands . . . you know the rest.

I'm hoping to build it with a pretty low heating load, so we can get away with a one or two-panel awning (for that passive-solar thing) and a small array of batteries wired for 12 or 24v. I can also get the PV parts pretty close to cost. Our buddy's company has a very generous "friends & family" program.

OK this is nuts.

State of the art commercial silicon solar cells convert ~20% of the solar radiation into DC electricity. The cheapest stuff per peak-watt out there gets about an 8-10% conversion ratio.

Solar thermal flat panels convert anywhere from 20-70% of the solar radiation into heat, depending on the difference in temperature between the outdoor air and the temp of the solar absorber (which is the average temp of the heating water loop.) If you're heating the floor with above-the-subfloor tubing (like the WarmBoard system, etc) your water temp requirements in a well-insulated room will be under 100F, and average mid-winter efficiencies will be in the 40-60% range

15% efficiency silicon solar panels are running $400/square meter retail, but let's say you find scratch & dent versions that fell off the back of your buddy's truck for $300/m, that's about the full retail price per square meter of solar-thermal panels with 3x the performance!

Not to mention it would take 3x the roof area!

Or that storing the afternoon sun for 3AM heat takes a large bank of batteries, at easily 10X the cost per stored kwh or btu thermal storage as heated water in tanks.

Even if you somehow managed to get the cost per unit-energy-collected down to parity with solar-thermal (which may require out & out theft) the cost of storing that much energy in batteries would be prohibitive (which is the attraction of grid-tied systems). You're talking literally a dozen Prius batteries to get the storage equivalent of a single 120-150 gallon tank.

And you're concerned about the cost of the INVERTER?

There are plenty of low-voltage radiant mat solutions out there, but methinks that's the least of the problems with your concept.

For far less money than designing in a PV or active solar thermal heating system you could built yourself a PassiveHouse, that needs no heating system, and control the indoor temp mostly by ventilation rates.

I'd thought about solar thermal, but where to put the tank . . . 

Obviously I haven't priced any componentry yet, so thanks for the info, Dana.

My largest goal would be to get the heating load DOWN.  As far as 3am heating, it'll likely be rare when we're up there late.  It's a studio/office/massage space.  Doesn't solve for days like this one though  . . .

just plinking here . . don't get testy!

Well done Dana!

Even thermal panels are of little practical use in cold climates as the loads are generally inverse to panel output (low sun, high heating load). Thus the solar "water heating" marketing. Here is MN I can make about half of the DHW for a family of four with solar. Most of that in the warm months. Now in NM, AZ, s.CA things are different.

I like your insulation idea and have been starting many of my retrofit ModCon jobs with generous closed-cell foam specs.

Have y'all had decent luck in MN (and other climes up nort') with evacuated tubes? They're independent of air temp, but I gather they don't "melt off" the way a flat panel would. I also gather that's not as big a problem as some would have you believe . . .

We're spoiled down here, with 300 days of full sun, but I would still probably opt for evac. tubes to cut the sq. footage.

I agree with both of you, the demand side of the energy equation is ridiculously downplayed. Not sexy enough, I suppose. I catch myself falling into the sexy technology trap fairly often and have to snap myself out. Better to start with a zero demand structure than to start with a high demand structure and try to make it "net" out with expensive systems. Reality check!

charlie

I am all for cool technology, but using Photovoltaics to power resistance heat  is as a good friend would say "bout as usefull as tits on a bull"  They look good but don't do much.   

The cheapest solution for space heating is passive solar withhigh solar gain windows.  Heck you want light anyway.  The next cheapest solution is a solar airheater.  Why bother heating water and then circulating it, adding radient, etc.  Just heat the air in the first place and circulate that.   For that matter mount them vericaly and you don't ever have a snow build up problem.

Cheers,
Eric

Posted By treeguy303 on 19 Mar 2010 06:14 PM
I'd thought about solar thermal, but where to put the tank . . . 

Obviously I haven't priced any componentry yet, so thanks for the info, Dana.

My largest goal would be to get the heating load DOWN.  As far as 3am heating, it'll likely be rare when we're up there late.  It's a studio/office/massage space.  Doesn't solve for days like this one though  . . .

just plinking here . . don't get testy!

The key to getting the heating load down is massive insulation and minimizing unintended ventilation.  In a super-insulated structure, adding thermal mass as a modest amount of thermal storage you can then support a substantial portion of the remaining load with a modest amount of low-temp solar (either air panels or hydronic systems), taking on heat during the sunny days without overheating.   High-R structures suitable for solar start at about R25-R30 clear-wall R-values, and in most of the lower 48 climates with more than 5000heating degree-days, going PassiveHouse needs ~R35-R55 clear-wall R, and getting there usually involves foot-thick walls with painstaking care in design & construction to minimize thermal bridging through the insulating layer.  The PassiveHouse Institute has a proprietary software package for working out the design details to make it work, but there are other good thermal modeling packages out there as well.

Making solar heating work for 95-100% of the load is like trying to win with the deck stacked against you.  When it's coldest out and the need is highest the solar collectors are at their lowest efficiency.  Seasonally the weeks/months of highest need are also of the lowest solar availablity (fewer hours, of less intense sun.)  The only way to make it work so that the the size & cost of the collection array is within any sort of reason is by lowering both peak and average loads.  In new designs the cost of making R50 walls that need little more than some well-placed windows for solar gain to heat the place is usually less than what it takes to do it with R25 walls + an active solar thermal array.  As you lower the load with higher R, you also lower the temperature requirements of any solar heating system, which INCREASES it's collection efficiency:  When all else is equal, doubling the whole wall R-value cuts the average load roughly in half, but cuts the size of the solar array necessary to support that load by more than half since it can run more efficiently, at say 80-90F instead of 120-130F.  but with enough insulation the windows themselves are the low-temp collectors, and the thermal mass of what's inside the insulation becomes the storage.  You can buy a lot of insulation for the cost of 5-6 square meters of solar panel, 100 gallons of tank, pumps, and radiant floor (which is already a far cheaper solution than 15-18 square meters of photovoltaic, low-voltage radiant, and a barge-load of batteries, as previously discussed.)

Of the simplest ways of getting there,  building standard studwall structures, but doing it air-tight, (caulking/sealing the seams & penetrations of the sheathing before the interior insulation is installed) then adding increasingly thick amounts of foam to the exterior to bring it up to an arbitrarily high R usually works.  With 2x6" 24" o.c. framing & batts or cellulose  and an inch of foam over the sheathing as a thermal break against thermal bridging at the studs you end up with ~R20-21.  But an additional 2" (3" total) of foam brings it to over R30. Similarly on roof structures, an inch or two of sheet foam over the roof deck mitigates thermal bridging at the rafters, but going thicker adds another R4-6 per inch (depending on foam typ) of additional R.  But there are double-studwalls, Larsen Truss, and Mooney Wall solutions as well, if you prefer to spend it on more complex framing rather than foam.   Don't neglect floors & foundations either, or that will quickly dominate the heat loss once you're north of R30 walls & roof.

Once you're sufficiently insulated for your climate, heating the space then becomes a matter of minimizing/managing glazed area, and controlling ventilation rates, with at worst a 1kW space heater to bring it up to temp quickly after a night where it was -25F at 3AM.   Whether you're there at 3AM or not there's still a  3AM heat load, a real thermal loss, creating a BTU deficit that needs filling to bring space up to a usable/comfortable temp.

Once again, Dana, thanks for a thorough and informative reply.

Again, I tend to be a bit of a tech head, so sometimes the reality check is in order. Must remember that people survived plenty well in Mesa Verde with an understanding of the sun and rock, even though they didn't likely refer to it as "thermal mass." I forget that we'll be at a distinct advantage when it comes time to construct the studio. We're playing so much catch-up with the house, I suppose I fall into that habit. Might think about an itty-bitty woodburner as the space heat as well. Yes, I realize that's absolutely overkill, but it's a free heat source for us, and looks & feels so nice for massage clients she might see up there.

Now thermal mass for a second-story addition. Tile? How much does one need for effectiveness?

Morso wood stove.

Posted By treeguy303 on 22 Mar 2010 09:44 PM
Once again, Dana, thanks for a thorough and informative reply.

Again, I tend to be a bit of a tech head, so sometimes the reality check is in order. Must remember that people survived plenty well in Mesa Verde with an understanding of the sun and rock, even though they didn't likely refer to it as "thermal mass." I forget that we'll be at a distinct advantage when it comes time to construct the studio. We're playing so much catch-up with the house, I suppose I fall into that habit. Might think about an itty-bitty woodburner as the space heat as well. Yes, I realize that's absolutely overkill, but it's a free heat source for us, and looks & feels so nice for massage clients she might see up there.

Now thermal mass for a second-story addition. Tile? How much does one need for effectiveness?

Depends on how much heat you intend to buffer with it, a function of what your coldest-night heat loss is from that space and how cold you want ot let it get.  Pound for pound you get a lot more thermal mass out of water than stone/tile/masonry- do you think (unheated) aquariums would work in your studio space, for a little atmosphere?

Some rough numbers to work with:

A 3" concrete slab floor gives you ~36lbs for every square foot of floor area.  Since concrete has specific heat of ~0.2 BTU/lb/degree-F, you get about 7btu/square foot per degree-F.  Say you have a 300 square foot studio, the whole floor gives you 2100BTU/square foot/degree-F.  If you've insulated the hell out of it and gotten the heat loss down to 2000BTU/hr when it's wicked cold out, you lose about a degree per hour once the fire's out in the stove, so say in a 10 hour overnight you'll lose about 10F.  If you left the place at a toasty 74F for the massage victims, in the AM it'll be in the mid sixties, and stoking your mini-stove would make it comfortable before the room actually got up to temp.  But if you're at 2000F/hr loss you're losing 4000BTU/hr when it's wicked-cold out it'll be in the mid-50s in the AM and you'll need a warm-it-up hour. 

It's enough thermal mass to keep something like solar thermal-air-panels from roasting you out of there quickly, but not so much that it takes forever to come up to temp.

With water at 8.34lbs/gallon, and a specific heat of 1BTU/lb/degree-F, 250 gallons of aquariums would be the equivalent thermal mass of a 3" slab floor in the same space.

With either/both you have to do the engineering of what it take to keep it from collapsing the first-floor. Concrete has 5 times the mass of the water for the equivalent thermal storage (5 tons to 1 in the above example), but could be part of the structure rather than mere dead-weight. 

I hate aquariums. It's a personal thing. Bad experience.

Food for thought here, for sure. How to incorporate water without dealing with an aquarium. . . We saw a solar decathalon house a while back with water-filled pillars as a design element. Wasn't sure how I felt aesthetically, but it's a neat idea. I'd planned on a tile floor, but a 3" lift would be pretty interesting. Maybe interior stucco for the walls and a brick hearth, just to get a little extra mass in there.

Maybe some kind of water "screen" flanking that fancy morso stove (aquariums without fish, essentially). Pretty neat looking stoves on that site, and I like that they make one "certified" for 800 sq. ft. Since those numbers are generally arrived at by overfiring the heck out of the stove, I'd feel pretty good about it in a space the size of the one we'll be building.

Thanks guys!
charlie