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bartman99
Registered Users
Posts:27
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| 03/12/2008 10:46 AM |
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Hi,
We live in central Virginia and want to install a solar HW system for DHW. We have gotten 2 quotes from dealer/installers in the area. One uses flat plat closed loop (glycol) system while the other uses an evacuated tube (closed loop glycol) system. The costs of the two systems is roughly equivalent.
The collectors will be roof mounted on a 5/12 roof oriented to magnetic south.
Are there any advantages or disadvantages to the different collector types with regard to efficiency, durability (meaning how long they will last) etc? From what I can gather by reading, the evacuated tube systems are a bit more efficient, but taking into account that we live a in pretty moderate climate, does it really matter?
Which would you chose?
BM99
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Topgas
Registered Users
Posts:64
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| 03/13/2008 9:01 PM |
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| I did flat plates because they were less complex and way cheaper. I like the idea of the vac tube panels though. The only thing I didn't like about them is you can't drainback vac tube panel like you can with flat plates. The single biggest problem with solar hot water is overheating. |
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bplwithu
Registered Users
Posts:3
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| 03/17/2008 5:31 PM |
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Hi,
* Evac tubes put out more heat during winter than flat panels but are comparitively less efficient than Flat-plate collectors during summer months. So, use Flat panels for DHW production only and use Evac. tubes for both DHW+winter heating.
* Durability wise no big difference I'd say. Both should last atleast 20+ years and should come with 5+ years of manu. warranty.
* In moderate winter regions, Flat plate collectors will perform better than the vacuum tubes.
* The tubes are slightly expensive than the flat panels and complex.
* OverHeating--> Evac. tube system will run into over heating problems during summer, so will need a heat dump as well. But if you are using all the solar heat produced(for pool,DHW etc), then over heating is not an issue. Using higher tilt angles (50-55 deg) will reduce oveheating during summer.
I'd say the $11K bid for the 5 AET panel system is a fair price. During summer, each AET panel will produce approx. 34,400 Btu/day or approx. 45 gal hotwater/day @ 140F. So, 5 panels will yield 45 x 5 = 225 gal HW daily during summer. (One person consumes 25 gal HW/day avg)
Let me know how the other bids are, feel free to post ur queries(more info, the better). Make sure the collectors used are SRCC rated - to qualify for Fed tax credits.
Best wishes
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Topgas
Registered Users
Posts:64
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| 03/18/2008 7:24 AM |
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| Just a few notes. The vacuum tubes should work better during winter but the people that have them that I know in the Northeast say they actually produce very little because they get iced up in bad weather and it takes forever for the ice and snow to melt on them. On the flat panels: 34K btu's per panel is a potential amount of energy that can be harvested but will rarely be seen. Think of it this way, your min. design water temp. would ideally be 100 degrees because below that you can't shower or do dishes etc. Say you have 400 gals. storage (what I have) and the max temps are 140 for a given day, multiply 400x 40 x 8.33= 133280 total btu gain divided by 6 panels (what I have) and you end up with 22K btu's per panel on an average real day, real usage. One other observation, during the fall/winter there's very few days of sun at all. Nov.- Feb. averaged 8-12 days of sun per month (that actually registered any gain on my system), March so far is averaging 50%, so you can see how things change with the seasons. My point is this, you have to be careful with theoretical and potential numbers when designing your system. I can easily achieve 34K as bplwithu says, it's just not the norm all season long. Total btus harvested during Dec. was 570K, Feb was 780K, March looks like a million and last year my best all season was 1.7 million in Aug. in Maine. Btw, I'm very happy with my system but don't plan on heating your house with it unless you have a very well designed setup like Radiantec did in the early 80's. |
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bplwithu
Registered Users
Posts:3
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| 03/18/2008 12:07 PM |
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Posted By Topgas on 03/18/2008 7:24 AM
Just a few notes. The vacuum tubes should work better during winter but the people that have them that I know in the Northeast say they actually produce very little because they get iced up in bad weather and it takes forever for the ice and snow to melt on them. On the flat panels: 34K btu's per panel is a potential amount of energy that can be harvested but will rarely be seen. Think of it this way, your min. design water temp. would ideally be 100 degrees because below that you can't shower or do dishes etc. Say you have 400 gals. storage (what I have) and the max temps are 140 for a given day, multiply 400x 40 x 8.33= 133280 total btu gain divided by 6 panels (what I have) and you end up with 22K btu's per panel on an average real day, real usage. One other observation, during the fall/winter there's very few days of sun at all. Nov.- Feb. averaged 8-12 days of sun per month (that actually registered any gain on my system), March so far is averaging 50%, so you can see how things change with the seasons. My point is this, you have to be careful with theoretical and potential numbers when designing your system. I can easily achieve 34K as bplwithu says, it's just not the norm all season long. Total btus harvested during Dec. was 570K, Feb was 780K, March looks like a million and last year my best all season was 1.7 million in Aug. in Maine. Btw, I'm very happy with my system but don't plan on heating your house with it unless you have a very well designed setup like Radiantec did in the early 80's.
Hi Topgas,
* I totally agree that 34K is not the constant daily output but the maximum rated o/p during hot summer days(as per SRCC/FSEC). But I used the max btu output value only to discuss the OVER-HEATING problem during summer - It's upto to his solar company or him to figure-out if he'll be using (34k x 5)btu to avoid over-heating. Thanks for pointing out though. Another option to tackle over-heating is by using an external heat-dump with a fan-coil arrangement. To get a good idea of the monthly btu outputs for a solar HW system, RetSCREEN software may be used - fairly accurate.
(Topgas, if you want to give me info. like approx. HW used/day, geo. location, Name and # of panels and last 12 months of your btu outputs, I'd love to compare it with Retscreen values)
* Totally disagree on the 'ice-stagnation' issues with vacuum tubes. Usually for high winter yield, the tubes are installed atleat >40deg which should help the snow slide down easily. I have personally seen ice melt away fairly quickly on many vac. systems. Their tube system should perform much better during winter compared to flat-plates (unless the system was not properly designed).
Also, most people have unrealistic expectations from their systems - they expect the solar systems to provide uninterrruped heat like their boilers all year along. Hope I made some sense........
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Topgas
Registered Users
Posts:64
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| 03/19/2008 7:04 AM |
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bplwithu,
Good insight with the vacuum tube panels, I hope to do a comparison in the future. I'll post my data when I get a chance (if I remember..). We had a great day a few days ago, 151K btu's. Here's just a few spec's: AET 4x10 panels (6), 400 gals. of water in standard tanks (Home Depot and Sepco's), 15 gal. drainback tanks, 55 degrees angle in a Southern Maine (Arundel, 04046) 43.42 I think, three panels per 200 gal.,separate systems so later I can do side by side comparison with the vacuum tube panels. We love our hot water.....three of us, my daughter takes (timed) 45 min showers and my wife and I 15 min, I still can't believe my daughter has any skin left !! Am I correct to guess around 80 gallon HW/day with low flow shower head and rest of the house standard? I did the numbers with heat loss in the tanks and it looked like 100-120 gals if I remember correctly. I suspect there are some static losses in there I'm not taking into account. |
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bplwithu
Registered Users
Posts:3
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| 03/19/2008 5:52 PM |
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TOPGAS: Thanks for the details....I'll post the RetSCREEN solar BTU output values for the 6 AET collectors soon. As per ASHRAE, each person in US uses 25-30 gal domestic HW daily. So, your family must be using around 70-100 gal HW/day - your calculations are close enough. It is interesting to note that people in Europe use only 10-15 gal HW/day for household activities :)
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Topgas
Registered Users
Posts:64
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| 03/21/2008 6:54 AM |
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| 2007 Btu's/mo. April= 1,405,000 May=1,530,000 June=1,263,000 July=1,281,000 Aug.=1,577,380, Sept=1,498,700 Oct.=1,024,548 Nov.=742,000 Dec.=561,000 Jan.=769,000 Feb.=769,000 March =1400,000. 400gal. storage, drainback, 55 degree angle on panels. A tree slightly effects the input late Oct. to beginning of March, I'd guess for a total of 1,000,000 btu's over the winter. Six AET panels off south by 8 degrees (west). |
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deneb
Registered Users
Posts:7
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| 03/30/2008 2:17 PM |
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sunny day here - 101.8 F right now from (1) collector to 80 gal SEPCO, super low dewpoint outside.
Bartman99.
I do not find info in your original message to give cost per sq.ft. of collector, or the square footage of each system being evaluated. The bottom line is the cost per BTU delivered to your storage, preferably including info on total watts of pumping power / heat exchangers etc.
History has shown that a drainback system GETS the most solar input into storage. Extracting heat later through heat exchangers can be done with minimal power (or none). As has been said above, overtemperature leads to stagnating or broken systems and the need to monitor daily at the end of summer days with high outside temps and clear sky. Drainback systems conserve pumping power in overtemp situations as they just sit there without any fluid being boiled in the collectors, stored very hot temps awaiting the time clock on your washing machine.
I'm glad you brought up this very important discussion as there are masses of people out there that feel that evacuated systems are the fanciest -best- for all situations. Keeping systems simple and easy to troubleshoot is, in my mind, the only route to follow as we have wasted the last 30 years without adopting the alternative route.
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Topgas
Registered Users
Posts:64
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| 04/23/2008 8:28 PM |
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| I showed a guy that's in the solar business my system. He's got a 30 tube setup with 80 gals, his temps were 120 degrees yesterday. He damn near fell over when he checked mine out and I was up in the 150 degree range with three flats and 200 gal.s.(X 2) He said his average temps in the winter are 75 degrees, mine run 90-110. Tom Lane was right, I'm glad I did what he recommended in his book. |
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