Hi: Thanks for the response. I am glad people are responding to this because it allows me to clear up some of the mis-conceptions out there about evacs.. In order:
"Also wouldn't increasing your flat plate area for less money be a better option?"
It depends. The main difference in flat VS evac is not so much about peak energy output but times when a flat produces no usable TEMPERATURE where an evac would. Notice I specifically said temp, not Btu's. You can have a system that acquires millions of Btu's of energy yet offers a ZERO useful amount of energy, because it never gets up to the contribution point which is 1 deg more then the input temp of the medium you are trying to add to. To put it another way, you could have a thousand flats on a cloudy day sitting in no-circ at 80 DegF and a solar tank at 85 DegF and there the system will sit all day. On the other side of town you have 10 evacs able to reach 140 DegF with the same solar insolation level and the same 85 DegF tank and the system will cycle on and off through out the day yielding some energy capture. So, none VS some. This is why the type of climate you have is important.
"Second you're looking at an evacuated tube, any guarantees how long that glass to metal seal will hold up?"
Only single wall glass evacs have glass to metal seals. Double wall designs which are less money have glass to glass welds so there are no expansion issues or issues relating to keeping different materials together over thermal extremes and years of time.
"Also if a good sized hail storm comes rolling through, I think I'd rather replace a piece of glass rather than my entire collector."
Just the opposite. Evacs you just replace the tubes effected which require no stopping of the system or opening up to the pressure environment. I would much rather replace a few glass tubes than a huge sheet of thick glass. You can break 50% of the tubes on an evac and you only replace 50% of the glass. You break a flat cover glass and say good buy to the whole thing, plus most flat cover glasses are not exactly sealed for easy field replacement, and usually require the frame to be disassembled.
"However in snow, the evacuated tubes aren't going to melt any accumulated snow off, either, a flat plate will."
This is a tough one. It depends on the snow type, outside temps and solar insolation level. If the outside temp and sun are both high enough to allow the snow to melt without the heat loss from underneath, the extra time will be more than made up from the increased efficiency of the evac. in the cold.. Evacs will run with 2" of snow on them, something which is quite amazing if you are used to seeing flats sit there in the same situation. The best cure I have found for heavy snow areas is a mounting angle of 60 to 70 Degs. Snow most of the time, especially dry snow in cold climates will fall off and accumulate less depth on smooth surfaces at that angle. My opinion on the whole snow thing is unless you live in a lake effect area that gets allot on constant heavy snow, the over all better efficiency will offset the snow issue.
"Evacuated tube collectors give you more btu output in the morning and evening, the flat plate gives more with direct sunlight, so unless the ambient temperature difference is quite great, then the two can perform similarly through out a days sunshine with the total btu's generated."
The use of the word "direct sunlight" I find a bit ambiguous since both types of collectors use diffuse and direct sun rays, so I will not comment.
"If a tracking system was used, a flat plate would definitely outperform, however this requires way too many additional engineering requirements to make it practical, which would all add to the cost."
Its funny you would say tracking since my evacs have been on an East - West tracker for years and I reap the benefits of such.. but I would definitely say it is a LABOR OF LOVE to do it and it is not worth the aggravation for the average person or maybe even ME if I had it to do over!!! LOL!!
.....Bill
