Ground Mounting Solar Evacuated Tubes

[callmeburton]

Seeking resources on how best to do this, rules to follow, what to expect, etc. Like how many panels to put in a string, considerations for types of glycol and its ratings, considerations for what to do when you have stored all you can store / use, suggested materials, etc. There are a ton of PV resources out there but not too many detailed SHW resources so far as I can tell.

Details:
We have a about 150-170 tubes if I recall, 5 complete panels and half of another if I recall ... Originally these were used to heat someone's house and we plan on using them for Domestic Hot Water (DHW) + hydronic radiant heating (HRH). Based on my WUFI calculations we should only need 2.5-3 cord of wood to pull this off but I will believe it when I see it. We are not going to mount these on the roof because we get a lot of snow so I was thinking of going with a ground mount. Whole south side of the building is glazed and depending on the time of year it is also shaded.

I found exactly one website, which I wont link because they sell stuff, who had an example of how to do this. They were using coper from the array back to the structure with insulation / sand bedding / and frost protected depths.

We install several extra 4" pipes going out the south side of our building along with our 24" earth tubes so getting the plumbing back into the house from the south side of our property likely wont be an issue. The earth tubes have insulation on top of them so the ground temperature can penetrate the tube but not the frost.

For excess storage originally I was going to install at minimal a 1k gal insulated unpressurized tank to help us pull through dark days / weeks ... the largest tank I can fit is something close to 9k but at that point it might be better to get a 1.5kgal indoor wood boiler and run the solar through that as we gain the backup of the wood as well. Wife would also like it if we can use this setup to heat up a hot tub, we did plan a space for one in our solarium space.

Thoughts? This likely won't even happen this year as I am still building the house so we have time to think about it. Depending what is suggested though we might have to change some of our plumbing plans outside the building with the external pipes we have in excess.

 

[callmeburton]

Will reply to my own thread as I find information.

Solar Hot Water Rules of Thumb
Load Est: 20gal / person / day (seems high vs the way we use hot water)
Collector Sizing: 1sqft per gal hot water needed
Storage: 1(cold areas) - 2 (warmer areas) gal for each sqft of collector

This means given I know I have at least 5 arrays my current unassembled system would be:
Rough array size: ((75"x90")/144)x5 = 234 sqft
Max gallons / day & Max Storage: 234 ..

if we assume this is for the longest day of the year then the shortest day should see ~140 where I live ... for HRH this would be enough for ~58kBtu of heating / day (I think ... if we assume 140-90F useable range for this purpose)

Looking at SRCC ratings ( https://secure.solar-rating.org/Account/CompanySearch.aspx? ) for a similar sized panel. For Cat D we are looking at 35.2kBtu/day (high radiation day), and 11.5kBtu/day (low radiation day) ... so for my situation that means 57.5kBtu/day likely in winter ... if I am reading this right. I think that could bring a hot water tank of ~140gal from 90 to 140F in a day without loads on the system.

Lots of good stuff found here:

An example of a "modern" (it is older now) take on annualized thermal heat storage into high thermal mass (HTM) buildings. Our home has HTM and I thought about setting up something like this as well but it seemed overkill given the WUFI modeling we did. The main difference vs our building is our foundation is also insulated and isolated from the ground.

 

[SolarRich]

Interesting way to insulate the basement from the earth. Too bad all houses aren't made this way.

We have 110 glass tubes with 120 gallons of storage for DHW. It works well. There are a few days in the winter when the backup electric instant hot water heater turns on. For many years we had no backup heater. Tubes are ground mount.

 

[callmeburton]

Interesting way to insulate the basement from the earth. Too bad all houses aren't made this way.

I found a place which was conducting "experiments" on different types of passive solar building and it seemed their go to was 12" of foam on the ground, 24" of sand over that which has tons of 4" drainage pipe for air tubes in it ... they would run the air out of attached greenhouse like spaces or something better to capture the heat from summer to use through winter.

We have 110 glass tubes with 120 gallons of storage for DHW. It works well. There are a few days in the winter when the backup electric instant hot water heater turns on. For many years we had no backup heater. Tubes are ground mount.

What SQFT, # of occupants, and how many heating days do you have? Do yo use it only for DHW or for heating as well?

From what I was reading the other day a large thermal storage tank is likely something I am going to want to setup to help carry through days of no sun. I also thought I could technically do a "drain back" system if the tank is right next to the panels then I would pump through the panels to the house and back to the tank as a way to avoid glycol ...

 

[SolarRich]

If you can do drainback that will save alot of work and no need for anti freeze. Drainback with glass tubes has been proven to work very well over the years. For most of the life of the system here there's been 4 people. 2500 sqft but I don't know how sqft relates to DHW? After the (2) 60 gallon tanks are up to temp the hot anti freeze gets sent to a radiator in the air handler of the heating system - if the house heat is enabled by the thermostat. This really only helps during fall and spring when there's enough sun to heat the DHW and have some leftover for the house.
How do I find how many heating days? Is there a chart somewhere or do I count how many days the heating system was running/year?

The concrete slab and basement walls in our house is fully insulated from the ground and acts as heat storage for the house. But that's another topic.

 

[callmeburton]

I don't know how sqft relates to DHW?

thought maybe you were using it for HRH as well

 

[Hedges]

I was thinking 120 gallon likely too small to store heat for the house, but with enough collector area and loops through slab, the storage is the slab.

 

[callmeburton]

The concrete slab and basement walls in our house is fully insulated from the ground and acts as heat storage for the house. But that's another topic.

we have 150 cuyd of insulated concrete in our new build as well ... a lot of it exposed to sunlight in winter.

 

[callmeburton]

but with enough collector area and loops through slab, the storage is the slab.

The question then is how much slab : collector area would be required to not overheat during summer?

We have a 7" slab (average thickness, were aiming for 6), 50-55cuyd if I recall ... some is inside a sun room and some in living space; hydro 9" OC. Originally I was going to run the HRH pump in isolation during the summer to even out the slab temp and then likely reverse the flow direction in winter to push from N to S to warm the living space first. (its a double envelope home)

 

[Warpspeed]

The only solar/hydronic heating system I have ever worked with was over forty years ago, but there were a few basic truths learned.
That had a 50,000 gallon storage tank three stories high and the whole project was government funded.

When its really cold outside, there is always very little sun, so RELIABLE solar heating throughout winter becomes an oxymoron.

You can heat up a vast thermal water storage tank to boiling point (in summer) easily enough.
Each pass through the heating load decreases the water temperature significantly. You get to circulate your entire storage volume about twice, then the storage temperature falls so low the system becomes inoperable. Forget about stratification in the main storage.
Starting up a circulating pump will cause enough turbulence to completely mix the water in the storage.
The concept of drawing off hot water at the top, and returning cooler water lower down, only works in theory, not in practice.

While its possible to fit something like an electric or gas fired boiler into the hydronic heating loop, data logging will show that it would have been much more efficient to just add that energy directly to the space being heated. The hydronic system parasitic losses can be relatively high, even with very careful design.

It gets even worse for domestic hot water heating, because the required temperatures are much higher than for space heating.

With unlimited resources, of course its possible to make something that works.
But the overall economics of the whole exercise will make complete nonsense.

Many have tried, and many lavishly funded government projects along similar lines have been spectacular failures.
In my experience, once the abundant government money stops flowing, the final long suffering user quickly rips out and scraps the system and replaces it with something like multiple modern high performance heat pumps.

I wish you the best of luck.

 

[callmeburton]

The concept of drawing off hot water at the top, and returning cooler water lower down, only works in theory, not in practice.

Typically you need some really big tanks for that whole process to work and often a stratification filter for returning water if I recall correctly, or a system where the water never leaves the tank (just coils to transfer at different stratification levels). The Germans have been doing it for a while but there is no way we are spending 30k euros on a tank like that >__< https://www.sonnenhaus-institut.de/solarheizung/solaranlage-heizkonzept.html Their tanks do have backup wood fireplaces with a coil from the firebox to the tank ... from what I can tell they "work" and the site above does have some good data points to back it up.

The 150 solar evacuated tubes we have came out of a house which was using it for their heating, I don't know what type of storage solution they were using or heating solution ... but I doubt the house was as tight as ours is going to be when done as ours is new construction and their place was at least 30 years old if i recall correctly. We are likely going to install a wood boiler feeding into the large water storage tank as a backup though because we have a lot of land and wood we can sustainably harvest.

I wish you the best of luck.

I do appreciate the luck for sure .. this whole solar passive house I am building is a large experiment ... I ran all the calculations assuming it is a single envelope house to make sure we knew the worst case scenario. This included a 1000 gal storage tank at 160F in the modeling as well as our evacuated tubes specs. I believe we backed this up in the model with a heat pump for the DHW side of things as it doesn't allow for wood boilers. The site still produces a surplus of energy with only one of the two arrays we planned to put in for PV so we will see :D

It gets even worse for domestic hot water heating, because the required temperatures are much higher than for space heating.

If I recall 180gal insulated tank of water at 180F can provide up to 224gals 110F at 3.25gal/min before the tank drops to 110F itself. I think our HRH needs like 90F for it to be functional but I would have to double check. We put in HRH as a "backup" if you will as I don't see our place needing much heat during the day during winter so long as the sun is out, and it is often is here. The slab will likely be uneven though and the HRH on a closed loop to help with this "problem" and to store the days heat for the night. Other people I have talked to who have built solar passive houses do something similar as their sun rooms get a ton of heat in the winter and buffer seasons.

 

[OzSolar]

I've been around this solar stuff in many forms for more than 30 years. In fact my earliest exposure to solar was for to try get dozens of active solar space heating systems working again. Cut to the end of the story... what I learned was no amount of storage, etc regardless of what sort of marketing magic had been sprinkled on it can get away from the fact that there's no way to escape that when your load is at its peak the resource is at its least. I'd be so happy to be proven wrong.

I designed and built my own passive solar house 20 years ago. It's amazing. Regardless of how cold it is we need no supplemental heat as long as it's sunny. We even have enough mass to coast for days of cloudy weather but it's still not perfect.

 

[Warpspeed]

but I doubt the house was as tight as ours is going to be when done as ours is new construction.

You will need plenty of ventilation, just for breathable air. Smells and humidity from kitchen, bathrooms and toilet need to be considered too.
I spent two years living in the Antarctic, and am rather familiar with problems of very close living in a very cold climate.
You might consider a rotary heat exchanger.
Stale air passes through a slowly rotating drum, where the warm air heats up a very fine honeycomb structure. Cold outside air then passes through the drum in the opposite direction, picking up a proportion of the wasted heat. But regardless, you will not be able to live in a tightly sealed building.

 

[callmeburton]

You will need plenty of ventilation, just for breathable air. Smells and humidity from kitchen, bathrooms and toilet need to be considered too.

Its a double envelope home that has two massive earth tubes in the Thermal Buffer Zone ... which is where our HRV will pull new air from to condition the living space air. The air changes per hour will be good for living.

You might consider a rotary heat exchanger.

I have seen those ... pretty cool, first time I have ever heard someone reference them though as they are typically used in commercial applications and I thought kind of rare :D

But regardless, you will not be able to live in a tightly sealed building.

I concur.

I designed and built my own passive solar house 20 years ago. It's amazing. Regardless of how cold it is we need no supplemental heat as long as it's sunny. We even have enough mass to coast for days of cloudy weather but it's still not perfect.

Would love to hear about which strategy you deployed. They never are perfect, we are not going for perfect we are going for mostly functional then falling back on wood for heating when it is needed since we have tons of it.

That said ... if you turned off all auxiliary heat what would be your lowest temp during the cloudy cold weather days, if it is above freezing I call that a win in our climate. If it is above freezing, and closer to 55F that is livable in our climate. We have friends locally who go all winter with the heating system set to 55F. We keep our home at 55F and only add extra heat to the room we are in most via our wood stove at our current 1880's place.

 

[Roy Robbins]

You might want to look into parabolic troughs for your tubes to get more heat per square inch and will allow for faster and higher temperatures. Use a GOOD heat exchanger. You can make some simple ones to test what I'm saying, you will be surprised Also use a SAFE heat transfer fluid. One problem to be aware of is "thermal syphoning" at night. Since heat rises, everything you gained during the day can disappear overnight. I've seen where the solar panels were quite warm at night because of it.

 

[callmeburton]

You might want to look into parabolic troughs for your tubes to get more heat per square inch and will allow for faster and higher temperatures.

the tubes we already own have their own manifolds they plug into ... unless I go out and buy or make new manifolds I can't see doing that right now but might try something with the extra tubes I have

Also use a SAFE heat transfer fluid.

If I don't come up with some crazy drainback system I will be sure to.

One problem to be aware of is "thermal syphoning" at night. Since heat rises, everything you gained during the day can disappear overnight. I've seen where the solar panels were quite warm at night because of it

the whole thing is going to be at ground level, likely lower than our slabs height so I am not sure this applies but good tip.