Solar Thermal panel siting / design considerations.

Merrick

Do I have the cart before the horse?
The details:
I'm here at 7000' in Western Colorado. Off grid with a propane boiler with 130 gallon storage tank (two internal coils) Upper one for the boiler loop. Lower one for the future solar radiant contribution loop.
Originally I had figured on having a Drainback system with no glycol for two Panels. This would supply 100% of DHW for 3 to 5 months. (minor contribution during the winter heating months.
Fast forward.... After consulting with a solar installer for our area. 5 - 4x5 flat plate panels were purchased with the expectations that a Drainback system would still be viable for the original siting location the "south eave"
Well after using a solar pathfinder it turns out this location will not provide adequate solar due to afternoon shading.
Five panels (south eave)
Dec, Jan,Nov (2.75hrs), Feb/oct (3.0 hrs) March/sept (4.0 hrs), April/aug (4.25 hrs),
May/July/June (13+ hrs)

The north eave offers much better solar.
Five Panels (north eave)
Dec (6.5 hrs), Jan/Nov (7.25 hrs), Feb/Oct (8.0) hrs),March/Sept (9.75 hrs), April/Aug (12.5 hrs)
May/July/June (13+ hrs)


But now, It seems that the drainback is not as viable due to insufficient space in my mechanical room for an additional "drainback tank" as well as elevation constraints by moving the panels to the north eave location.

Now glycol is being proposed in a non-drainback system. My concern is having too many panels in the summer. Overheating of Glycol?
Recommendation by consultant / installer is just to cover the panels in the summer to control overheating etc. Thoughts?

My proposal to still use the South eave location by mounting fewer panels on the roof and a few on a vertical south wall got shot down due to complexity of doing this. I thought it could offset concerns of glycol degradation. Thoughts?

Rick

hot_rod

I like drain back, it is the system I have the most experience with. But it requires sloped collectors and piping, a high head circ. At high altitudes they struggle to maintain the siphon much over 25' of lift. The drain back tank doesn't need to be at the mechanical room. Actually the higher up the better, lower lift for the pump, an attic for example.

One other option, I have not tried but I know of installers putting in steam back systems, for 15 years or more now.

I took the Schuco Solar course years ago on the concept. The Viessmann collectors work well for steam back. They are serpentine so very small amount of fluid to steam. But they also have a header so they could be drainback.

Viessmann and others that offer steam back use a specific high temperature German glycol, Tyfocor brand.

More on steam back here.
https://www.renewableenergyworld.com/solar/steamback-shows-promise-for-solar-water-overheating/

Them pathfinder is a good, simple way to look at different orientations.
While latitude plus 5° is an ideal year around pitch, putting them flush on a roof pitch is not a huge penalty. Add 1 more collector to make up the difference.

100 sq ft of collector will not add much to you heating load in cold months. It should get you a good solar fraction for DHW.
Ideally a separate tank form the solar, having a boiler loop inn the tank will lower the amount of solar you can harvest.

Idronics 3 takes you through performance, or efficiencies of DT at various conditions.

Covering them is not an ideal method to regulate gain, IMO.

Jon_blaney

Got space for a ground based setup?

Merrick

Ground base could be another option. But with longer runs to mechanical room.

Dump load during the summer could be another avenue to use excess heat from the 5 panels. Hot tub for example.

hot_rod

How many people in the family? If you have 100 sq ft of collector and 130 gallon tank, you may not need to dump a lot, run the tank to 150- 160 and the collectors will have load
Most solar controllers have cool down function, running the pump at night to pull the tank down to 100f or so. Also collector cooling function that allows the tank to overshoot the temperature setting.

It’s all about balancing the load and array output. The array output changes se one by second as you know, so it not an exact science.

Merrick

2 people. Thank you for additional insight with respect to dump loads, controllers and the reminder that it is not an exact science.

hot_rod

Sounds like mechanical space is at a premium. The key to solar thermal is storage, heat tanks during the day, sip off overnight. If in fact you have 100 sq ft of panels, I don't think winter months will be an issue, plenty of load between DHW and heat.
I also think one of the Resol controllers will handle your summer over production without needing a dump load. Here is the manual, and the pages that show the "cooling" functions.

\https://www.resol.de/Produktdokumente/48005962_DeltaSol_BS4_V2.monen.pdf


If ever you need to replace the wood stove, in Europe you see these small "parlor boilers" nice looking wood stoves with a small external water jacket option for DHW or radiant loads. Some have a small oven up top also. With wood fired, you cover night time loads or cloudy day conditions, cooking, heat and hot water.

So many nice options out there. The ISH show in Germany has thousands of unique wood and pellet stoves, oven, boilers from across the globe. Scandinavian countries build a lot of wood burning products. Austria, Switzerland, Germany, etc. still a lot of solid fuel burning in rural areas.

Sol_Brother

My experience installing and servicing solar thermal in North Carolina is that maintaining a pressurized glycol system with significantly too many collectors for summer load with is a continual headache and common cause of corrosion and failures. This includes ones designed as and set up for steamback (and yes, this can only possibly work with a serpentine collector with a tiny volume of fluid to boil, and even then it illustrates terrible design to intentionally stress a system) AND with controls that would attempt to de-heat the storage tank when collectors cool down at night.

Three problems with that controller "recooling" plan: 1. this mode can kick in only several hours after you need it and depends on a good amount of hot water being used every day, so overheating and boiling glycol is still likely under some conditions (not even considering grid power failures),
2. in our humid summer nights that sometimes hover above 70° there is not much opportunity for nightsky radiant cooling (of course, not a problem at your location), and
3. the better your collector is the worse it will be at radiating. You may have noticed engineers have gone to considerable effort to prevent exactly that.

Covering collectors works but is a clumsy solution. We have a long back-and-forth swing season where you might want all collectors exposed one day and most covered soon after.

A dump load is a viable option but it has to be something that can absorb well beyond the summer output (i.e., a shaded swimming pool or shop/garage radiant floor) or that you can heat as much as you need. (So a hot tub is not really a candidate as there is a definite limit to how hot you can safely push it and its plastic piping -- but could serve as the primary dump load if there is something cooler to turn to as the secondary, and you don't mind the added complexity). And you would want guaranteed electricity to keep your pump running or be there to drain glycol in case of grid failure.

For most homeowners, if drainback is not an option my advice would be to install AT MOST the collectors to serve 110% of domestic hot water load at your time of highest collector output, use a controller with recooling and holiday functions enabled, and make sure your valves are set up so that you can completely drain the collectors for service (no traps in the pipes, collectors sloped ever so slightly to drain to the inlet corner), and with an adequate expansion tank. If you are going to be away for long enough that the tank might overheat, cover the collectors even with this smaller array.

I wish we all had your roof access, by the way.

Tom

hot_rod

I have never tried to see how low you can pull temperature with night sky re-radiation. In theory the panel sees upper atmosphere temperature on a clear night. At 20,000 on an 80° day about 9°f.
With enough time you could pull the tank down considerably.
Friends in Colorado mountains tell me of freezing water collectors on 40° clear sky, ambient nights.

I've had the most trouble free systems when using properly installed plain water drain backs.

JakeCK

hot_rod said:

I have never tried to see how low you can pull temperature with night sky re-radiation. In theory the panel sees upper atmosphere temperature on a clear night. At 20,000 on an 80° day about 9°f.
With enough time you could pull the tank down considerably.
Friends in Colorado mountains tell me of freezing water collectors on 40° clear sky, ambient nights.

I've had the most trouble free systems when using properly installed plain water drain backs.

Have you ever looked up into the clear cold dark sky with a flir camera in the middle of January? If the temperatures it "sees" are accurate that 9F is outright balmy.

JakeCK

Merrick

"cool" Image.

Derheatmeister

Crestone ?...A T-sol program will shed some light on your questions before you get into "Hot water".

Merrick

Crawford, Colorado

Derheatmeister

Ahh near Hotchkiss. We are in the Breckenridge Area... I am a proponent of Drainback vs. Glycol filled systems...
As you allready indicated the proper tilt may reduce Boiling your glycol in the Summer months.
A good solar contractor will be able to run a simulation program with all given parameters such as your location,Tank size, Thermal Collector Data,Piping to and from the collectors,usage...This program will create a report on hourly temperatures of your tank and take the guesswork out of the equation..We use T-Sol by Valentin.
Maybe the manufacturer of your collectors can assit you with this.
Hope this helps.

hot_rod

The T-Sol simulation allows you to simulate the exact system you are considering. Enter your DHW load, and any heating loads, temperatures, etc. Then manipulate the size, position of the array to see the output and solar fraction. It can also run the economics of the system.

It takes some time and experience with the T-Sol program. Hire @Derheatmeister to run a SIM for you on the system you are considering.

Overall Colorado is a very good state for solar. Specific location info at NASA enter your Lat. and Long. And info at NREL. Some examples.

It usually comes down to either how much SF you want, or more often how much $$ you want to spend on gathering "free" solar energy

Derheatmeister

hot_rod said:

The T-Sol simulation allows you to simulate the exact system you are considering. Enter your DHW load, and any heating loads, temperatures, etc. Then manipulate the size, position of the array to see the output and solar fraction. It can also run the economics of the system.

It takes some time and experience with the T-Sol program. Hire @Derheatmeister to run a SIM for you on the system you are considering.

Overall Colorado is a very good state for solar. Specific location info at NASA enter your Lat. and Long. And info at NREL. Some examples.

It usually comes down to either how much SF you want, or more often how much $$ you want to spend on gathering "free" solar energy

We only do in house local designs for Ourself/Existing Customers ..(Not for hire)
Thank you for suggesting to hire us ...
Nowadays we try not to travel to far outside of Summit County other than once a year to service systems in Crestone that we installed 15 years ago .

Low Energy systems in Denver also has the T-sol programm and they may be up for hire. https://tanklesswaterheaters.com/