direct system

Karl_Northwind

Articles, I didn't find this one till this spring.



<a href="http://www.cincosolar.com/newsroom/articles/aspe-article-solar-thermal-system-design">http://www.cincosolar.com/newsroom/articles/aspe-article-solar-thermal-system-design</a>

<a href="http://www.cincosolar.com/newsroom/articles/solar-thermal-system-design-part-2-of-3">http://www.cincosolar.com/newsroom/articles/solar-thermal-system-design-part-2-of-3</a>

<a href="http://www.cincosolar.com/newsroom/articles/solar-thermal-system-design-part-3-of-3">http://www.cincosolar.com/newsroom/articles/solar-thermal-system-design-part-3-of-3</a>



thoughts, particularly on article part 2, about using DHW only with evacuated tubes. 



I know what everyone that I've told about this setup says, but I'd like input from others around the country.



Karl

Kevin_in_Denver_2

How About a Direct Split Self-Freeze-Protected Thermosiphon?

Karl,



Wow! This strategy is my current obsession.



The seminal paper on what you are talking about is here: http://www.thermomax.com/Downloads/Reciculation.pdf



It proved that the heat lost by an evacuated tube (ET) collector loop is negligible at night. Therefore, recirculation freeze protection is back in play for cold climates. This is old news in Germany: http://www.ritter-xl-solar.com/en/technology/water/



I've tested this through two Denver winters and learned a few things, mainly that it does work!



For the ultimate in system simplicity and low cost, you can eliminate the pump and controller if the house is configured correctly (south facing wall) as in this video:

http://www.youtube.com/watch?v=Q5rKLsJl3cY



Other strategies can be employed that involve pumps if the collectors are higher than the tank.

Kevin_in_Denver_2

"Boiling it Down"

Here's the passage in the magazine articles that jumps out at me as "new stuff":



"(Heat pipe) evacuated tube collectors, by their very design, do not contain any water

in the absorber, and the absorber itself can freeze without any damage to the collector.

The water in the header piping is subject to freezing temperatures, but it is enclosed completely by insulation. While heat loss from a flat panel collector is significant in freezing conditions, heat loss from evacuated tube collectors is nearly zero in the exposed tubes and minimal (200–500 British thermal units per hour) in the header. This means that the engineer now has a third option besides glycol and drainback: circulation of water during freeze conditions, or a direct water system (see Figure 3).

Using direct water in the solar loop obviates all of the issues described above for glycol and drainback systems. The engineer must be concerned with only two issues:

scaling and ensuring circulation during freeze conditions. Both of these are easily addressed."



I wouldn't say "easily addressed", but here's some more of what I've learned:



1. Ritter, the German solar company that employs recirculation freeze protection in cold climates, is currently wrestling with some code officials and OG-300 requirements. The main issue is how big of a battery backup for the pump is required to prevent freezing? http://www.cpuc.ca.gov/NR/rdonlyres/22FFF2E8-20FD-4F50-BD46-B88921387972/0/LibreEnergyProtest.pdf



2. Ritter actually employs a brazed flat plate heat exchanger in their systems to prevent scaling in the collectors. Ironically, therefore, theirs is not a direct system. They use water in a conventional closed system for the collector loop, and rely on steamback for stagnation protection. (Which, surprisingly, was overlooked in the ASPE magazine article).



3. Ritter touts recirculation as also solving one more (rather rare) failure mode of glycol systems: On an extremely cold morning, a glycol collector loop will have a significant amount of sub-freezing fluid in it. When the collectors reach the proper temperature, the collector pump will kick on. The still-freezing glycol in the collector loop can instantly freeze the domestic side of a flat plate heat exchanger and and ruin it.



Don't dismiss Ritter (even though, yes, it's the candy bar company). They own 75% of the ET collector market in Germany and 35% in Europe. ETs are about 10% of the solar thermal market. They have begun to address the US market, but so far, only for commercial.

hot_rod

seems partial to ETs

I've never used a vacuum breaker on a drainback, don't see a need to? Also no need to have an open drainback system?



I'm not sold on a drainback evac tube either., potential for some serious stagnation temperatures, see pics.



With the Ritter system how often would the pump run to warm the tubes at night with the energy from the tank? What about up in the mountains where the temperatures were in the teens and lower.



To be fair you need to run a simulation for the area of installation at the design conditions. I doubt tubes out perform flat plates under all conditions, in all locations?



hr

Mark Eatherton

So...

They are depending upon two items (pump and controllers) both of which have a notorious record of failures to protect a $10,000 investment? Seems to me that we've been down this road before, and ended up in the same place. Total Disaster City.



Personally, I don't think its a good idea to once again try and go down this road, because really, nothing has changed since we went down this road before. Controllers still freak out or get fired and fail, and pumps, especially in high temperature potable applications fail. Then when pipes freeze and break, you have city water pressure causing water damage from the TOP of the house, all the way to the basement, destroying everything in between. Hate to be a curmudgeon to the application of solar, but in my professional opinion, I think its not a good idea.



HR, man, that's some bad looking heat damage photos you've got there. Almost looks like it could have ignited combustibles if it had come into contact. Looks like the copper has been annealed, which takes considerable heat.



ME

Kevin_in_Denver_2

Draindown vs glycol vs drainback vs recirculation

It all comes down to a pretty simple failure mode analysis problem.



All four strategies have several failure modes. Consider this analogy: Gas boilers have many complex failure modes but through the years all of them have been solved. And all the while costs have come down. Their current failure rate is negligible.



By comparison, ET collectors, with freeze tolerant collector loop piping and redundant controls is a dead easy problem for good design engineers.



Installed system cost is currently the biggest problem for the SDHW market. Since recirculation is the cheapest system design, it starts becoming obvious that whoever can drive down its failure rate may have a winner. Ritter and Resol already claim to have done it, and continue to gain market share due to cost.



Not a trivial topic, the potential US market is 252 billion

Karl_Northwind

direct system

I tend to agree with all the statements above, except the ones that say it's a good idea to have that setup. 

if the spec'ing company is going to maintain the system, sell heat, or have a long term (10 year? 20 year?) warranty against problems, with some sort of performance guarantee, maybe it could fly.  unfortunately, as stated above, all failure modes are costly, some potentially disasterous. the other thing not dealt with in the article is the appropriateness of this setup for various climates.  the author is in southern/central texas.  there, this may be an entirely appropriate situation.  not in plenty of other applications. 



would the installer care if they had to cover the operating costs of the system over the years, where the system demanded lots of energy to prevent freezing? 



it seems like the article set up a lot of straw men, just to tell us how they've solved all of the problems that the rest of us have never had, only happen in extremely rare cases, or are the result of "sub-optimal" design in the first place. 



I especially like the bit about replacing glycol every 1-3 years.  hogwash.  I've seen glycol 30 years old that tests out.  I have glycol that is 7 years old in my system up north, and it's still fine.



has any one else here seen the flatplate HX freeze up before?  I know of one case, but I'd like to hear of others. 



Kevin: I need to read that article in depth, but the range of situations in north america is as wide as it gets, ST design is in many ways a regional speciality.  what I'd think about installing in denver is not what I'd consider installing in Wi, Mn, or Ny. 



HR, that's a spectacular failure.  thermomax?  they're the only ET's I have experience with, and that looks like them.



K

Mark Eatherton

Freezing heat exchangers..

Karl, I had a red headed step child of a solar system that I inherited the maintenance on many years ago. This system was on a public senior housing building that was 12 stories tall.



It had a 1,000 gallon open, poly butylene storage tank in the basement mechanical room of the building, and in the top of the inside of the tank were immersed DHW preheat coils. The water from this atmosppheric tank was pumped through the tube side of a large shell and tube heat exchanger that was mounted on top of the atmospheric tank.There was a base mounted pump that pumped the tank water through the shell and tube heat exchanger. The shell side of the heat exchanger was a glycol loop that went up the 12 stories to the roof to the FP solar collectors. There was a swing check on the solar loop only. It kept freezing and breaking the tubes on the inside of the heat exchanger. It had me going crazy trying to figure out how it could freeze. I was thinking about the possibility of it reverse thermo siphoning, but the check valves should have prevented that possibility.



In a last ditch effort, I decided to spend the night in the mechanical room (this was LONG before we had the data loggers we have today) so I could see exactly what was happening. Much to my surprise, I determined that the systems was not revers thermo siphoning, but was FORWARD thermo siphoning, I found that the original installers had installed the return from the heat exchanger on the atmospheric side of the tank in such a manner that the return was actually submerged below the flood level of the tank, so that the heat exchanger was constantly filled with tank water, and during peak insolation days, at night, the whole shebang would forward thermo siphon at night, and glycol below freezing was coming back to the heat exchanger from the array, and freezing and bursting the tubes.



As a simple fix, I drilled a 1/4" hole into the copper return line coming back from the water side of the HXer, and ran 1/4" cooper tube and put it back into the tank above the flood level of the tank, so that when the tank pump shut down, all of the tank water from the heat exchanger would drain back down into the atmospheric tank, eliminating the heat source/thermal bridge that was causing the forward thermo siphoning on the glycol portion of the loop.



That was one of those "Whooda thunk" moments... In the case of a FP HXer, if both pumps are on at the same time, I find it hard to believe that it would be possible to freeze and break the heat exchanger, but after seeing this one, I guess anything is possible.



ME

Karl_Northwind

freezing heat exchanger

there was one in Wi on a huge system, that had hundreds of feet of outside pipe 2-4" dia. 

it was built as a pressurized system, and one day at -20 F, the pumps turned on, and the several hundred gallons of -20 glycol in the supply (hot) pipe came thru the HX and flash froze the 50 F water coming thru it. 

system was rebuilt as a drainback (with 1000 gallons of drainback tank)

and has been running without a hitch as far as I know.



the best practices manual here in Wi recommends in situations where there is a lot of outside piping, that there be a motorized valve in the solar loop that remains in bypass mode till the glycol gets to 50F before directing it thru the HX.



I've had to fix forward thermosyphoning systems installed by others (they told the customer it "wasnt' a big deal" and refused to fix it.



I use the caleffi spring checks in a number of applications.  just don't sweat them assembled.



K

Kevin_in_Denver_2

Cycling at night

Here's a couple plots of my collector return temperature.



On pretty cold nights (15F), the pump cycles eight times.

Karl_Northwind

meh

15 F?  that's a really nice winter night in the great white north,  I will again stress that what works in Co. isn't necessarily something one would want to recommend to just anyone anywhere. 



  I'd like to see some actual BTU monitoring (say just monitor temps at 4 points on the tank) and with no tank draw after a mild sunny day, see what the tank temp drops to overnight.  just to see what the actual BTU loss out of the tank is for a single array on a "normal" colorado night.  you appear to have the capability, I'd love to hear back what the results are while there are still some cold nights this spring.



also, let's keep in mind what the failure mode of that setup is/could be.  Failure mode is a huge consideration when one possible failure mode requires the replacement of the entire collector.

Kevin_in_Denver_2

Hasn't been very cold since mid Feb.

When the tank is at 160F, I lose 10F overnight without any pump cycling.



When the pump cycles, the increased heat loss is under 4%.

Karl_Northwind

numbers

we need some more details:  number of tubes, feet of piping exposed to outside, insulated or not?



is that tank top temperature? or an average of the gradient from top to bottom?

was that last night at very mild temperatures?



thanks

karl

Kevin_in_Denver_2

Getting some cool nights this week

Karl,



Most of my data so far was taken with our random daily usage thrown in the mix.



That makes it tough to tease out much meaningful info.



So I've isolated the solar system (zero load, only ambient and recirculation losses) for the next few days. We should then be able to compare a hot tank with and without recirc. losses. I think that's our big question.



My collector loop uses high pressure EPDM hose insulated with 3/4" armaflex. More info & photos to come.



I'm presenting these results at the WREF on May 14: http://ases.org/conference/

"FORUM -

Advancements in RE Technology: Radically Reducing the Cost of Solar Water Heaters - 5/14/2012 4:15pm - 5:30pm"

Karl_Northwind

thanks

I'm looking forward to seeing them.  I would love to get to the ases show, but it doesn't look likely.

"Cost" is a really tricky topic.  at cost of efficiency? disposable equipment? Cheaper systems at what cost?



K

Kevin_in_Denver_2

The Articles Ignore Steamback

Part 2 of the above articles talk about stagnation and overtemperature protection in a closed system or direct system. They don't mention the best and simplest method, which is steamback: http://www.heatinghelp.com/forum-thread/133064/Steamback-System-Design

Kevin_in_Denver_2

dp

Karl_Northwind

or

or the fact that the problems they repeatedly cite as major problems with the other ST technologies have mostly been addressed by competent design and detail oriented installation.

no different from other hydronic technologies.



steam back is one over heat option. I haven't used that, but we certainly use drainback and heat dumps which are both pretty easy and foolproof.

the other thing to consider is that ET's don't have nearly the track record of flat plate panels, and higher end flat plates have nearly the same performance numbers of the ET's, and all but the cheapest ET's are quite a bit more cost per SF than good flatplates, and cheap china made flatplates are quite a bit less, in quantity.



do I need to mention that the author of the article is the CEO of a company the distributes ET collectors? 

didn't think so.



K

Kevin_in_Denver_2

Recirculation heat loss data

Attached are plots from the nights of Apr 17-18 and Apr 24-25. On the night of the 24th, I put the pump on a timer which cycled the collector pump for 3 minutes on the hour starting at 10pm to 7am. There is zero load. Apr 17-18 is just standby losses, with no pump cycling until 6am where it cycled once since the collector hit 40F.



With the tank at 140-160, the tank will lose around 0.8F per hour strictly standby losses.

With the tank at 160F, and a three minute collector pump cycle per hour, the losses increase to about 1.8F per hour.



The overnight temperature swing was about the same both nights, from 65F down to 40F

Kevin_in_Denver_2

Recirculation heat loss data

dp

Karl_Northwind

bummer

too bad it didn't get cold.

in theory, you shouldn't have needed any recirc, as the temp never got near freezing.

anyone got a walk in freezer we can borrow?

Karl