Research Project Compares Two Solar Hot Water Systems

output in cold weather, which is basically usurped by your wood boiler. Given your desire to mount at a very low angle due to the surplus of energy from your boiler once fired up and snow concerns, I think I would have waited for the upper roof where more appropriate angles were available and perhaps a simpler more consistent load could be designed for your solar systems, better representing the loads required in a typical residential environment without the wood boiler. Under those conditions you would have a greater chance of becoming more enlightened as to the real differences between the two different technologies.

Also snow is major problem at this latitude...with standing seam roof it sheds quickly and violently..we have installed 3 sets of roof guards above the sytem to protect it that will also serve as staging for the upper roof install

i guess i would be concerned about mounting racks at angles steeper than the existing roof..a rack at a steeper angle might be more "inclined" to go for a ride with the snow

if anyone feels like contacting us direct to anser further questions or bounce ideas, email or call..we will make the testing data available as soon as possible

keeping trying new ways to create sustainAbility...

the system is designed to heat domestic hot during the summer because we have a Tarm wood boiler in the basement for the heating season

there are two pitches on structure that are south facing...this lower 4/12 pitch was chosen for this side by side because it was safer to install and we felt more confident in the piping route on the lower roof...in the future we hope to install side by side tests on  the upper (9/12) roof and gable end. the gable end idea is my favorite because of its ability to shed snow

Chill out Mr Fitch.  No need for name calling...politician indeed!

In truth I admit not knowing the exact angle for mounting evacuated tubes in NH.  Do you?  please supply a link with some support so I can become educated.

For flat panels I happen to know that small (<15 degrees) misaglinments will reduce performance by 3-4%; even a 25 degree misalignment would only reduce performance by ~10% compared to what would be optimal.  At least in terms of how much less sun will strike the panel (cosine falloff). 

Finally, one can optimize different things (summer production, winter production, average year-round production), how do you know which was their design goal?

Hi: A few degrees!!.. I hardly call 2 to 2-1/2 fold a few degrees. Evacs should be mounted at a min of 25 degs just to even function right... I suppose you feel that because you can not put diesel fuel in your standard ICE, that it is a failing of the ICE and not a mis-configuration issue.... and of course the real world is not optimal.. all the more reason to pursue correct design configs... It is over compromise to good engineering designs that have resulted in mediocre to poor system performance in the past for solar, which has only hurt the solar industry in the long run, in exchange for short term financial gain. If you want to indulge in compromise, be a politician....

Hi: Not at that mounting angle for DHW. The angle for the evacs as best I can tell from that photo is way to shallow for them. In NH, he should have mounted them at 60 to 65 Degs from the horiz. The flats would be better mounted at lat., I.E. approx 42 Degs. If you are to compare two different types of panels then each should be mounted and configured in a fashion that optimizes each system, otherwise your test is basically meaningless. There are allot of projects out there and allot of them have good intensions, but stop short of well thought out engineering... The conclusions they will arrive at will be not be representative of either systems real potential.

I have to applaud this effort. But there's a difference in the value to each type depending on the latitude. In Florida or Georgia the flat plates, being cheaper, have to be better value for money. Further North the evacuated tubes come into their own as their greater efficiency pays off delivering more hot water, particularly in Winter.

At least this is the common wisdom among solar hot water folks that I know. So while reasearch in NH may be useful for NH, it might be more interesting to compare them in different areas to validate how they compare across the country. 

Mounting solar thermal at the ideal angle is just that... an ideal.  Many "civilians" are repulsed by residential solar because so many installations have been UGLY!  These systems have to be palatable to the average home-owner, and installations that blend in are certainly worth losing a few percentage points of efficiency. 

I attended a week-long workshop at NC State Univ., and their data showed a surprisingly low penalty in annual BTU's collected when systems were mounted across a wide range of non-ideal azimuths and elevations.  The take-home message was clear: Mount panels parallel to the roof deck for aesthetically pleasing systems, and, if necessary, add an extra panel to offset the BTU penalty. 

One more thing occurred to me.  Heat from the Street was an article in the Economist about storing hot water in an acquifir for winter use and cool water in a side-by-side acquifir for cooling in summer. They plumbed pipe under an asphalt road, where water heated in summer and cooled in winter.   

If you plumbed so you could pump your wood-heated warm water to melt the snow, you could get your solar system going in winter as well.  Maybe this isn't practical, but I couldn't help mentioning it.  The other thing about this is you could control when and how much snow you melt.  If you collect rainwater for use, you could control your storage to what room you have in your storage containers.

Interesting thread.  Thanks for posting. 

you can visit my site, www.We AreSolar.com. On the contact page you will find my email address and on the special offer page you will find raw owner collected data on evacs in SDHW and SDHW and Radiant heating systems for 2005, 2006 and 2007. I did not direct you to this site for sales purposes. There is good info out there and if someone has some well thought out questions, that are specific in nature, I will take some time, that I really don’t have, to answer them. It just “kills” me to see some mush confusion when it really does not have to exist.   …..Bill

MIT generates its energy from its own natural gas plant, if I understand my son correctly, but they are doing research on solar under grants now. 

They had to angle collectors less than optimally because of the danger of wind carrying off the collectors.  I'm pretty sure this was true for the water panels as well as the PV panels.  The wind can be so fierce where the panels are mounted. 

MIT is pretty open about sharing the information.  It is likely you could contact them about this, and I believe they would like to get information from you as well.  It is good to have clearing houses for this sort of information so that what is best can go to scale as quickly as possible.  Their contacts around the world are phenomenal, and many there are motovated to share with less developed countries.   

Even in more northern climes, evacs are usually not a better choice for SDHW.  Their superior heat retention during the winter is generally offset by their higher cost and much smaller aperture during the rest of the year.  On a per square foot of roof space basis, and on a per dollar basis, flat plates will outperform evacs during the summer months.  For space heating it's a different story.

so far for four seasons energy output is approximately equivalent..the evac tubes do perform better spring fall while flats are stella summer performers, both panels have been inaccessible and snow bound for the last 3 months ( next year we will rig safe climbing access for snow removal) all of this information will be documented and available ..i agree that we do need more testing if the evacuated tubes do provide a technological superiority that will have to be demonstrated and i am leaning towards optimalization in this region through gable end installs...we would be happy to do the testing. 

Not much mention of cost. Even if the evacuated tubes are 10% or even 20% more efficient, if they cost twice as much , that efficiency is largely irrelevant as flat plate remains cheaper. Okay, there is morre space taken up, but it is seldom the case that there is not enough roof for solar panels - usually not enough money!

My system is 6 square metres of flat plate solar thermal (plus about 30 or PV but that is not relevant to this thread). Unfortunately in the UK, tube is largely pushed by the rip-off merchants using hard sell to sell at at twice or three times the realistic cost of the installed systems bringing the industry into some disrepute!

Ultimately, whether you buy tube or flat plate, shop around and as soon as you get the hard sell, walk away - you're going to be ripped off!   

Hi: Drain back is best if the “site” can accommodate the consistently downward slope for both the feed and return collector lines. If you can not, then closed loop glycol is the most typical and popular way left, with the associated heat exchangers. For efficiency reasons though, the fewer number of HX needed the better. Just in case you are not aware, there is a difference between drain down and drain back systems.

I am curious about the way the hot water is connected to the existing hot water tank and whether extra storage tanks and transfer coils are used to get the hot water where it is needed in the house. Also are the systems controlled by draindown thermostats and valves in this cold latitude. How many pumps are involved and what is their electrical consumption? Is there a single best way of handling the freezing problem?

Fitch says "The looks “thing” is such BS!! "

Perhaps, but having it look good can really boost margins! 

 I'm not sure why you rail against what consumers want...you must be a disgruntled engineer, you complain about poorly designed systems dragging down the industry and turn around and insult your customer's tastes...maybe after 30 years in the industry you could try selling people what they want rather than what you want.

Hi: Mine are the double wall as well, but pointing at the sun gives the best performance do to the reflectors in between and once you get to the side more than about 30 degs, the tubes will start shadowing each other. Yes, 75 deg is great for snow shed but that is the steepest you want to go. You want to be at least 15 degs off vertical for proper fluid return down the heat pipe. My Lehighton customer has had his array return as high as 235 DegF on my open loop drainback DPDB system....

Interesting discussion, not beeing well versed in solar, but beeing a retired HVAC professional, it seams to me the CVT system has to have significant added value. Recently I built a new home with oil fired radiant floor heat through out. About 5000 linear feet of PEC and it preofms well. I would like to supplement with solar and am interested in the experiance knowledgable individuals may have with system integration and sizing. The same boiler currently provides hot water via a domestic coil. It would seem a large factor for inegration for space heating would be the ability to store solar heat during peak production. Any ideas as to how much and most practicle method. I have 2800 sq. ft. of walk out basement, all radiant heat, seams that could be asigmificant source of storage, may be a little warmer than normal some times.

Thanks and keep up the intersting, objective debate. 

Interesting discussion, not beeing well versed in solar, but beeing a retired HVAC professional, it seams to me the CVT system has to have significant added value. Recently I built a new home with oil fired radiant floor heat through out. About 5000 linear feet of PEC and it preofms well. I would like to supplement with solar and am interested in the experiance knowledgable individuals may have with system integration and sizing. The same boiler currently provides hot water via a domestic coil. It would seem a large factor for inegration for space heating would be the ability to store solar heat during peak production. Any ideas as to how much and most practicle method. I have 2800 sq. ft. of walk out basement, all radiant heat, seams that could be asigmificant source of storage, may be a little warmer than normal some times.

Thanks and keep up the intersting, objective debate.