Solar Radiant Heat for Airplane Hangar

trentp

Hello, I just stumbled upon this forum and have found it super helpful although I seem to be struggling to find a system similar to what I am trying to build, or at least with any clear details.

I recently built a shop/airplane hangar at my house just north of Reno, NV (about 300 days of sunshine a year). Its 3100sf and I did put insulation as well as 11 different runs of 1/2pex in the slab. I am now at the point that I want to supply heat to the floor. For this system I am looking to use solely for heating and not for DHW.

I am planning on using solar hot water collectors as the primary heat source, which I would like to have as the sole heat source for a while (I am on a bit of a tight budget after going so over budget on the building itself). So my hopes are to use solar heat to make it comfortable enough to work in (above 55 degrees would be ideal) but I don't need it to be fully climate controlled and understand on cloudy days it will get cold. I will have an additional input to my manifold to support a boiler if I find that I need it in the future but with the cost of propane out here I am hoping to get as much as possible from solar.

My question is if there is a simple solution for what I am doing that doesn't involve any large expensive heat exchangers and tanks? Is it feasible to just circulate fluid out of the floor, through the collectors and directly back to the floor during the day when the sun is up? I recognize a basic system like this will not have a good way of bleeding off heat so on warmer than average winter days it may get very warm in the building but I figure I can always just open the large door on the front and let the cooler air from outside come in.

I can't seem to find any systems like this anywhere so that may be my first clue that I am missing something so I would appreciate any input you may have.

Again, I apologize if this was covered already in another thread that I missed!

Larry Weingarten

Hi, I'll take a shot at it! It seems you could have a simple system without complexity, as your slab could be the storage tank. A thing to consider is using "inefficient" solar collectors so that high temperatures are not a problem. Using glazed or unglazed pool collectors would be a step in this direction. I recently built a collector using 3/4" poly pipe and twinwall polycarbonate glazing over it, with no added insulation. This collector cannot get over 170F and is not hurt by freezing. I would add a differential control to know when it makes sense to add heat to the slab. Also, depending on what boiler you might add, there could be concerns about oxygen in the system damaging iron.

I should add a question. Did you try for any direct gain, as in having sunlight directly hit darkened concrete?

Yours, Larry

Teemok

With no storage system heating drops off with solar loss. The slab can hold lots of btu's and the space cooling is delayed some amount of time. There will also be a delay in feeling heat after the sun starts heating panels. It can be a game of how hot can you tolerate at the end of the day so it's not too cold in the mornings. How cold it gets during the no sun cycle and how delayed the heat up cycle is depends on slab thickness, tubing spacing, tubing depth, panel btu output, envelope heat loss, etc. The ratio of collector power to the structures heat loss will determine on average if you overheat or under heat. On a day of low btu input it will get cold and may take a long while to recover. Rational sized storage only covers X hours of structure heat loss. It can make the space consistently more comfortable in the hours outside of sun shine. Storage helps to prevent over heating the space and spreads the days heat harvest of the day. The heat in the tanks can be used to extend heating into the evening/night or can be saved over night to give a free heat shot in the dark of mornings.

Jamie Hall

Aircraft hangers are an almost ideal space for direct, passive, solar heating. I did one, years ago, for a maintenance hangar at an airport in New Hampshire. A specially designed glazed roof with a south facing slope allowed the sun in -- on a sunny day -- which shone on the concrete floor of the hanger. The floor was deep enough so that it stored enough heat to keep the temperature reasonable during the day -- and kept the building warm enough for three or four days without sun. No moving parts. No water (airplanes and water don't get on well).

hot_rod

Assuming you do not have any or much passive, here are some solar thermal numbers.

First knowing the load would be helpful. Assume 15 btu/ ft to maintain 60°, X 3100 46,500 BTU/hr.
Reasonable winter temperatures in Reno, mid 30's oct- Feb.

The solar collector output is all about the ambient temperature around them.

This graph shows 3 types of collectors and the formula for calculating efficiency.

Assume the temperature to the collector from the building is 95F- 35F ambient. Collector supplying about 100- 105°F

Divided by the radiation, 200 is a realistic winter time radiation number, summer may see up to 300.

So Ti- Ta. 95°- 35° divided by 200 =.3

On the bottom axis, find .3 run straight up to the collector slope. So a glazed flat plate would run about 45% efficient on that winter day. Unglazed collector, about 0%, sorry it just cannot overcome that 35 ambient.

At the SRCC website you can pull down collector rating. Here is a typical 40 sq ft SunEarth spec.

Use cat. D for heating, so somewhere between 5- 15,000 BTU per DAY (6 hours) from a 4X10 collector

You need 46,500 BTU per HOUR. So about 800 sq.ft of collector gets you close. 20 collectors at around $800.- 1000. For maybe 6 hours of a design day load. You still have 18 hours of no sun heating to cover per day.

I've spent a lifetime trying to make solar thermal pencil out for heating. Even with wholesale cost on copper fin, glass and aluminum and making them yourself it doesn't pencil out.

The solar contractors I know shoot for maybe a 30% SF solar fraction. Cover 30% of the heating load. But the installed cost still puts it in the 1% ers price range.

When incentives were in place covering up to 50% of installed cost, ROI was still 15- 20 years out on a even a basic basic 2 panel DHW system.

Harvest all summer coast all winter? Maybe. This apartment building in Switzerland does it. Got $$ and room for about 50,000 gallons of insulated storage and 3000 sq ft of collector?

Teemok

I wonder how PV and heat pump would pencil out? 7kw pv and a 4ton about 5+ hours.? Then your panels are harvesting all year. The colder the more efficient. Incentives and credits.

hot_rod

Www.dsireusa.org lists all incentives available.

Rich_L

How about PV solar panels with an air to water heat pump? Then you'd also have the option to add AC with a chilled water fan coil, if you like.
We built our house in Iowa 17 years ago with radiant floor heat and a ground source (geo) heat pump. Four years ago we added solar panels and our electric bills went from $2500 - $3000/ yr to $150. Makes for a pretty quick payback on the panels.
You'd have more up-front costs for a system like this but long term, it could give you what you're looking for - a comfortable building that would be very inexpensive to heat (and cool).

Jamie Hall

When you have the opportunity, as you do in an airplane hangar, to go 100 percent passive with a collection efficiency of 100%, why on earth would you go PV with a maximum efficiency even with a heat pump of 60% plus at least five figures worth of expensive parts and pipe?

I simply don't understand.

Bluntly, it's just plain dumb.

Sure, put up some PV and batteries for your lights and maybe enough for some power equipment, but for the heat? No. Just... no. Sorry.

hot_rod

Jamie Hall said:

When you have the opportunity, as you do in an airplane hangar, to go 100 percent passive with a collection efficiency of 100%, why on earth would you go PV with a maximum efficiency even with a heat pump of 60% plus at least five figures worth of expensive parts and pipe?

I simply don't understand.

Bluntly, it's just plain dumb.

Sure, put up some PV and batteries for your lights and maybe enough for some power equipment, but for the heat? No. Just... no. Sorry.

It sounds like the building is already constructed, so a passive opportunity may be already gone?
It sounds like he is looking for a retro fit idea.

bhowden

I was scornful of PV when direct heat collectors are much more efficient but the cost of solar kept going down and when I ran the numbers I was astonished to find it was fairly close even a few years ago (even better for pv now). When I looked at just how much I would have to oversize the system for winter solar which would leave me venting heat in the summer I gave up on direct gain collectors. If your utility will buy power from you in the summer and let you draw in the winter PV becomes a no brainer. At the very least, run the numbers before you give up on it.

Brian

Jamie Hall

I didn't mean direct gain collectors -- I meant a properly engineered passive solar system. No moving parts. I've designed and installed a few... and they work just fine, summer and winter.

But... they aren't sexy, and they aren't suited for a retrofit, so... oh well.

Teemok

Jamie of course you are right. Passive has it's problems and some can to be avoided. It is simple, durable and cheaper.
Collection efficiency really matters when area and solar exposure is limited and panels are expensive. Who cares if the panels are only 19% efficient? The unused 81% costs nothing. Investment to btu's provided is a valid metric. PV to air to water HP provides a level of control most passive designs can never offer. Power clocked in one season or time can be used in another. Investment costs turns me off as well but I don't have poor people asking me to solve these kinds of problems. So much of what humans do is just plain dumb. Why draw the line with heat pumps?

trentp

Hey Everyone, thanks for chiming in! I should have filled in more info, the building is already done so changing the design for passive solar isn’t really an option however I do have a few good sized windows facing the south that definitely help warm it up a bit. I don’t want to rule out the idea of adding a bunch of PV panels in the future but for right now I’m just looking to add some heat to the slab since I already put in the time and money to the underslab portion. I do feel like the building seems to hold heat well and that’s probably due to the insulation in the building as well as the rigid foam I have under and around the foundation.

I think I am going to continue down the path of solar thermal and will probably add a storage tank as that seems like it’s of value. I do understand that it’s not going to be enough on the coldest months of the year but should still be better than nothing and I’ll likely add a boiler to supplement when I feel it’s needed and when I am willing to burn some propane.

fentonc

I like @Larry Weingarten 's suggestion - what about just using some 'pool heater'-type black coiled plastic tubing, and have it run when the sun is out? I think the slab will provide more thermal storage than any reasonable buffer tank, and you've already paid for it.

hot_rod

Un glazed solar thermal has a very small operating window. On a 35 degree day, 75 degree from the collector you are around 1% efficiency. You would need an awful lot of collector and roof area to even put a dent in a heating load.

When ambient and collector temperature are within a few degrees, expect 90% plus efficiency. But as the ambient around the collector drops, efficiency drops quickly, see the steep performance profile, the black line on the graph.

You really want glazed plate or tube collectors for cold weather operation.

Larry Weingarten

Hi, This is the 10' by 12' collector I recently built. The polycarbonate glazing does a good job of allowing heat capture even on cool days. Systems like this cover 90% of the DHW load where I am at a latitude of 36 degrees.


Yours, Larry

hot_rod

Larry Weingarten said:

Hi, This is the 10' by 12' collector I recently built. The polycarbonate glazing does a good job of allowing heat capture even on cool days. Systems like this cover 90% of the DHW load where I am at a latitude of 36 degrees.


Yours, Larry

Have you had any issues with the polycarbonate warping and moving when it gets hot? I could never keep the edges sealed with poly glazing?

Larry Weingarten

Hi @hot_rod , Good question. What I did was to use the L flashing to capture the edges, but it still allows for some movement. So, no problem yet 😉

Yours, Larry