How well would this work as a solar collector?

[ISeeDeadBTUs]

I have been reading a lot of stuff on Solar. I am committed to retaining the last tank of oil I bought for many years. I think I have decided to go with a twin coiled 160-gal DHW tank. I need to figure out if that could be efficiently used as an HX between my wood Hydronic and my oil boiler. But that's not important today. When I left the house this AM it was 52*, so even though spring's not here quite yet, I need to get a move on it!

A lot has been bantered about as to whether vac-tube collectors are worth it for summer DHW production in the NorthEast. And I got to wondering this AM if an efficient collector could be made thusly:

Flat box with reflective Alum insulation on the back.
A coil made from black iron.
Fill the box with black sand.
Insulate the box.

 

[solarguy]

Evacuated tubes are worth a dam. The perform better than a flat plate on cloudy days but annually, flat plates do win by a thin margin. The biggest drawback to tubes is they do not heat up so they will shed snow or ice. I'd rethink the double coil tank imho. Not only because of cost but also there will be conflicts with temp sensors. You're better off having a single coil tank for your solar system & feeding that loaded tank into a fossil fueled back up tank.

 

[ISeeDeadBTUs]

Tell me more . . .

 

[solarguy]

correction, vac tubes do not heat up therefore they will NOT shed ice or snow.

What do ya want to know?

 

[solarguy]

most the tubes are held in place on the manifold by a c clip & a retaining ring, fairly easy to replace

 

[mikeyny]

This is the post I put up about a month ago. I am not sure if you want to build one like this or not but this works great for me.

https://www.hearth.com/talk/threads/14262/

 

[Nofossil]

There's lots of different ways to skin this particular cat, but being an inveterate cheapskate I start looking for the simplest / least expensive and work my way up from there.

If you have plenty of space, then the added cost of high efficiency probably doesn't make sense. 80 square feet of low efficiency at $2.00 a square foot might deliver a lot more heat than 30 square feet of ultra-high efficiency at $100 a square foot.

If you heat with wood, then you probably don't care too much about performance at low temperatures and low sun angles. I only use my solar from mid April through mid-October. Wood solves the rest of the year.

If you can set it up to thermosiphon, then you don't need pumps, sensors, and controls.

Plan on lots of surface area if you can do it. Optimize BTU/$ rather than BTU/sf.

 

[ISeeDeadBTUs]

I don't see thermosiphon as an option.

I 'should' buy a tank anyway as a 'buffer tank' for the GW (as mentioned in another thread, my belief is that the naturally aspirated, refractory mass units do better without storage than the induced units do without storage) and I 'think' the 160 gal tank would be sized right for wood/solar.

I certainly hope your $2 vs $100 unit values were literary hyperbole!! :grrr:

Looks to me like I get 12 Btu/hr with piping/pumps and tank for 4k. Absolutly correct, this would be for DHW production only. For two humans, that is basically a tank of oil. So say $1000. That's 4 years, worst case scenario . . . 2 year payback if you consider the tank is more of a wood-fired add-on.

 

[solarguy]

Jimbo,

If you're going to do a single tank for both applications, your should plan on only using 1 system at a time. Although I see many diagrams showing twin coils in a tank hooked up to solar, the truth is a solar system & wood fired product run at 2 different temperatures. In the winter time when you're firing the wood boiler, the solar system very rarely will get above the tank temperature & when it does it's for a very short period of time. So the benefit of having solar hooked up to the same tank during the winter & expecting it to load the tank w/ thermal energy is minimal at best.

If your solar system is only going to be used in the summer you may want to consider a one coil tank. You could put in some valves & bypass that would divert water from the solar system in summertime use, to the GW for winter use. This is assuming you're not using a closed glycol loop for the solar system. If you're using water than you'll want to provide provisions to ensure you drain the solar system before the frost is on the pumpkins.

 

[ISeeDeadBTUs]

The bottom coil would be for wood/solar (only one at a time). The top coil would be for oil. The oil boiler could heat the water if for some reason no wood nor solar input. In the winter, the top coil would be the HX from the 'buffer' tank to the oil boiler. Not sure how to do that calculation to be sure it would work. I don't like my existing 20-plate HX, but at least I know it works 8-/

 

[Nofossil]

My solar panels cost less that $2 per square foot installed. The evacuated tube systems are a bit more than that ;-)

I started with a 4' x 20' inground pool heater at $140. Since I already had a storage tank and could set up for thermosiphon, the additional cost was just the fittings and plastic pipe to connect it. This unit delivered about 50,000 BTU per day on a good day - not quite enough.

A friend donated some defective glazed panels - simple aluminum boxes with copper panels inside. I repaired and added those and doubled the output.

Writeup and performance data is here.

 

[ISeeDeadBTUs]

Ramblings . . .


I didn't see where you said the size on the three panels . . .you said the pool heater is 80[]

You also seem to indicate a 50kBtu gain was available under best case conditions, plus daily usage. Any idea what a 'typical' daily gain was?

Help me with my math 50k / 8 (# per gal) / 880 gal = 7* change, yes?

So, if your collector area is, say 200[], you get BCS 250 BTU/ day / [] ??? Did I do that right?

The Vac tubes I have been looking at are ~27[] with a max of 6.8 kBtu; thats ~251 Btu per [] (am I missing something, or is the max rating a per hour rating???)

So let's say my Solar gain is 120% of yours (a bit farther south, Vac tubes), and the max heat I can 'use' is 160 gal X (180* - 50*) X 8 = 166,400 BTU (Heating whole tank of well water to 180*)

Right now, in the spring and fall, I can heat the 50 gal DHW to 145* with the GW in the evening, and that is enough for two people for one day if the time between water heating and showers is kept to 8 hours or less. Seems like a better insulated 160 gal would go like 2.5 days. They claim 7.5kBtu / 24-hours standby losses.

So, If every three days I need to go from 50 to 180*(assuming one out of three days I get max production)

170k Btu / 1.2(efficiency factor) /250 (Btu/hr) / (4 hrs) = ~140 []????

Tell me what I did wrong here!!

 

[steam man]

Hey guys-I had to look through my files awhile to find this about evacuated tube verses flat plate collectors. Check it out. (broken link removed) I am not entirely convinced by one study. I may put a combination just to do my own analysis.

 

[leaddog]

Hey guys-I had to look through my files awhile to find this about evacuated tube verses flat plate collectors. Check it out. (broken link removed) I am not entirely convinced by one study. I may put a combination just to do my own analysis.

The reason the tubes don't do as well is because they frost up or are covered with snow. Why couldn't you enclose them in a glass panel . It would then melt the frost and snow just like the flat plate????????????
leaddog

 

[Bob Rohr]

you need to define the working parameters to truly determine solar panel efficiency.

Actually a flat EPDM un-glazed, un-insulated swimming pool collector can out perform a high dollar evac tube. Under the right conditions!

That might be a pool collector where the water temperature is at or below ambient. Little if any loss to from the collector to the air, a huge wetted surface, and no glass for the energy to pass through.

The info you need is, where it's located, and what you are asking it to do. Better put... the ambient air temperature surrounding the panel and the return temperature to the collector.

Then get the performance ratings of the collectors from the SRCC website.

For lower temperature systems, radiant under 100F for example the flat panels would be a better choice. performance and cost wise.

if you need high temperatures, evac tubes may be a better choice. Both my flat panels and evac tubes will run on slightly over cast days.

hr

 

[cbrodsky]

Hey guys-I had to look through my files awhile to find this about evacuated tube verses flat plate collectors. Check it out. (broken link removed) I am not entirely convinced by one study. I may put a combination just to do my own analysis.

Interesting - my solar installer included the same article in our quote package

I was originally pushing vacuum tubes as my preference, but his experience is that they inevitably don't hold up as well long-term - and even getting one tube failing in 10 will start working against you to the point that you've given up the potential gain. They're priced such that they have to perform at a much higher level and there is less margin for error. He's seen them damaged more than once by people trying to clear snow off them as well.

I think they have their place if you need a high density array but if you have room, the flat plates seem a bit more bulletproof and common to find/service so I went with those. Even if I was building a house I'd never intend to sell, I'd go with the commercial collectors - they are amazingly effective and worth every penny given the out of control oil prices we're seeing. Doesn't hurt that the government pays for half of them either.

-Colin