Adding a solar array to primary/secondary with storage system for space heating

[Mushroom Man]

I considered posting this to the green room but because it involves the P/S system, I thought it was appropriate for this forum.

This is really a question of whether it is a practical idea. I've read forums on solar space heating and the temperatures attained from solar arrays seem as though they'd be worth the effort. What I imagine is a large array with a controller that turns on a circulator from the heat exchanger only when temps exceed that of the primary loop. The wood/biomass would raise the temperature in storage to practical heating levels but from a higher starting temperature. If a large number of btus can be attained passively, then why not?

Note: this would be a DIY project as purchased solar arrays are too expensive for my wallet. The array would use glycol, hence the heat exchanger.

 

[Jeff S]

I have been going to tie in some homemade solar panels into to my boiler system in a similar fashion for the last 3 yrs that would supply DHW and a little space heating during the shoulder season but could never justify the cost.I wanted to cover the whole south facing roof (approx 25 x 13) of the building my boiler is in,but even the cost of homemade panels adds up.Now I think I would like to add maybe a couple 4 X 10 panels to provide for DHW in the summer time.I all ready burning in the winter to provide space and DHW so maybe a couple of panels would provide enough not to fire the boiler in the summer, and any heat from really sunny days could be put into my storage system to help buffer out the cloudy ones.

 

[woodsmaster]

I'm definitly not an expert on solar. From what I understand the water type solar isn't very usefull in the winter. Many
folks use it for DHW in the summer time and drain it before the winter. With the antifreez you loose some effiency in transferring heat. You would also have to be able to adjust your panels tilt to take advantage of the low winter sun. I plan on building some
homemade panels one of these years for DHW in the summer time.

 

[Hydronics]

I would agree; in the Northeast here, the cost/benefit of solar heating is a tough sell.
However, domestic hot water in summer is the biggest solar bang for the buck. I'm working on putting mine together, I got some used flat panels. I'm going to set it up as a thermosiphon system.

 

[huffdawg]

Checkout www.builditsolar.com there are some good home made designs on there . I think you can make a half decent one for around $1000

 

[jimbom]

Checkout www.builditsolar.com there are some good home made designs on there . I think you can make a half decent one for around $1000

+1 Excellent site with real world guidance and examples.

 

[peakbagger]

Flat plates only raise the water temp about 80 deg F over the outside temp. Therefore when its 20 degrees outside the water returning is going to be 100 deg F which is too low for most folks to heat with. You could do radiant heat but there arent that many Btus coming into the collector in the winter. Few panels are mounted at the optimum winter angle (30 degrees from vertical for my area) Its your latitude minus 15 degrees for other spots and not many roofs have that pitch. I have mine at optimum and they are rather ugly but they are not really visible to the neighbors.

There are some people who use evacuated tube collectors for heating as they heat the water up to much higher temps ( I think they get at least 150 degrees above ambient but I dont mess with them). Unfortunately they are less efficient per square foot of collector so you need more square feet for the equivalent BTU output and they cost more. Evacuated tubes require specialized manufacturing equipment so you are not going to be building them at home. There are some makes of tubes that appear to have quality and durability issues so be careful if you find a good deal on used evac tubes.

 

[in hot water]

Solar hydronic heating is an option. As mentioned it works best with low temperature distribution systems. If your heating system can operate and cover the load with temperatures below 120F you are in there.

The efficiency of a collector depends on the inlet temperature Ti and the ambient air temperature around the collector Ta. With that info you can plot the performance, or efficiency of various collectors. This graph shows 3 types of collectors, with performance data from the SRCC.

At 20F ambient, supplying 105F temperature to the load evac tube and flat plate collectors perform about the same. As ambient drops below 20F notice the evac tubes perform better. Basically evac tubes have better insulation, double walled glass and/ or vacuum so they lose less temperature to the ambient. Think of it as a single pane window compared to a double pane storm window. So they are not really more efficient at collecting energy, they just maintain the loss to the ambient better. It's just copper inside glass.

Notice also how a basic un-glazed pool collector approach 90% efficiency. That point on the graph is when the ambient temperature is near the same as the fluid going up to the collector. For example a 80F day and 78F water going to the collector.

But see how the pool collector efficiency drops as ambient temperature (lower axis) drops. So with proper operating conditions a thermal collector can operate around 40- 50% efficient. Much much better than PV collectors at 15% efficiency

Notice also that flat plate collectors out-perform evac tubes under operating condition aboove 20F. There here are a few options for piping solar into a hydronic system. Dump it into storage, or pre-heat the return from a radiant system before it goes to the storage or boiler to be heated. That is the common Euro method, done with a simple 3 way zone valve.

If you are a do it yourselfers, and look for deals on equipment then the solar component can pay off. Shop around for scratch and dents or take off collectors. To hire the work done would could thousands of dollars, energy is still to cheap in this country for solar thermal heating to make economic sense. There is a big push on in Europe for high solar fraction systems, with 8 bucks a gallon for gas, and heating fuel running at least double what we pay, the numbers, with some govt incentives start to look better.

hr

 

[Mushroom Man]

Flat plates only raise the water temp about 80 deg F over the outside temp. Therefore when its 20 degrees outside the water returning is going to be 100 deg F which is too low for most folks to heat with. You could do radiant heat but there arent that many Btus coming into the collector in the winter. Few panels are mounted at the optimum winter angle (30 degrees from vertical for my area) Its your latitude minus 15 degrees for other spots and not many roofs have that pitch. I have mine at optimum and they are rather ugly but they are not really visible to the neighbors.

There are some people who use evacuated tube collectors for heating as they heat the water up to much higher temps ( I think they get at least 150 degrees above ambient but I dont mess with them). Unfortunately they are less efficient per square foot of collector so you need more square feet for the equivalent BTU output and they cost more. Evacuated tubes require specialized manufacturing equipment so you are not going to be building them at home. There are some makes of tubes that appear to have quality and durability issues so be careful if you find a good deal on used evac tubes.

 

[Mushroom Man]

I have been researching evacuated tube technology with a view to feeding the heat into my primary loop. The last set of stubs before return to the boiler seems appropriate.

One credible user has indicated that he needs 20 tubes per 200 sq. ft. of space heating. In my case I would need at least 240 tubes, a major capital outlay even at DIY prices.

So, I started looking to other (cheaper) types of collectors including flat plate and Modified Trickle Down (MTD). These produce lower temperature water in winter but produce larger volumes of warm water for the same footprint. As contributors here have noted, the water temperatures have to be appropriate for the emitters and higher than storage temps (in my case 170 max - 122 min). If I could get 150 degree water from solar, I could passively raise my tank temps but only with sufficient volumes of water. The cheaper systems are likely only producing 80 degrees in winter.


BUT, what if I used cheaper larger volume panels as pre-heaters for evacuated tube collectors thereby producing larger volumes of 150 plus degree water at a more reasonable cost. I could use a HX and a controller to ensure that the loop would only operate when temps were above the primary loop temp and would use a non-freeze liquid on the collector side.

One weakness of evacuated tube collectors is snow and frost. It doesn't melt off so the tubes often they often produce less heat overall than flat plates. If the tubes were mounted inside a single glass pane greenhouse, this weakness might be avoided. I think it would work fine. Evac tube installations are often mounted at less than optimal angles to avoid overheating issues. I would not de-tune to avoid overheating, rather I'd build it for very high temperatures.

I read somewhere that very high temps (300 degrees) can be achieved inside evacuated tubes using heat pipe technology; so I thought that water as the carrier of the heat might be a limiting factor. Indeed, I saw a video of water boiling by evacuated tubes so I think they can reach at least 212 degrees. I am still trying to gauge the temperatures that might be practically achieved with evacuated tubes using heat pipe technology.

Heat pipes use a phase change (liquid to gas) to create heat. The phase change can occur at low temperatures. Various liquids can be used but one with a low boiling point is best. Acetone has been recommended. A stub or bulb on the heat tube interfaces with the heat exchange fluid or at least with the heat exchange manifold of the panel. Each tube can be self contained. Don't think of it as fluid running through copper tubes inside the glass tubes.

If hydraulic oil were used as the interface transportation fluid instead of water, it would carry more heat, because it would not boil or break down. Additionally it would not freeze.

I thought plumbing the boiler was a big project. This might be bigger ! I haven't started the project yet; the concept needs to work before any prototyping.

Your thoughts are appreciated.

 

[pybyr]

google copper cricket solar for an intriguing concept/ approach

 

[steam man]

I have a 120 evacuated tube system here in northerm Maine. They crank out heat no matter the outside temperatures and don't necessarily need bright sunlight. They also work half buried in the snow. I selected evacuated tubes to get higher temps. Before my overheat loop got put in I left for two days thinking the weather was going to be bad and I had enough storage. Came home and found my tank at 180 deg and the collectors shutdown at 275 deg. For now after my domestic water I use a modine heater that dumps the heat to the basement. In the summer I have a pool to dump heat to. I would say you would be better off using solar with a low temp radiant system to be efficient, even if it means keeping the high temp loops separate. I plan on adding the radiant loops when I can get to it.

 

[Hunderliggur]

Latitude + 15* is the winter design angle, Latitude + 21* is the max winter angle on December 21st. Depending on your roof pitch you may find that just flat mounting (simpler, cheaper) would work fine for three season (not winter) but you are planning on burning wood in winter anyway. I plan on adding some builditsolar.com flat plate collectors (double wall polycarbonate glazing, insulated box) with a in tank HX, glycol loop (it freezes here). My target roof is an 8:12 pitch (34*) and my latitude is 41. I plan on flat mounting for looks and primary 3 season performance. It is all trade offs. If you have less than optimum orientation you can add collector area. Just work out your numbers. Even in winter I will get some added benefit. We had two 4x8 panels on an old cottage mounted at most at 26* and we generated all the hot water for two people, laundry, dishes all year long (except for those two weeks one SUMMER when we had NO sun). We had NO backup for the solar panels, we did not have the AC panel (the cottage had a 30A service), no natural gas.