Rarely Exploited Evacuated Tube Advantage

Kevin_in_Denver_2

Most of us know all the issues in the Flat Plate vs. Evacuated Tube debate.



This photo is a Brain Teaser.   Who can figure out what I'm trying to accomplish?

hot_rod

a metal pan

to collect the glass shards when they break to keep from injuring someone below?

bill_105

I got it!

Those metal sheets are to reflect to the backside of the panel. Kind of a AC/DC sort of thing:)

Kevin_in_Denver_2

How to collect sunlight from BEHIND a solar panel

Very funny, HR. Actually you might be serious. Denver's softball size hail would destroy any evac. tube collector. It's 25 years between storms though.



Bill, you're right! ding ding ding!



I haven't seen any studies on this, but in far northern latitudes, you could be collecting 24 hrs/day in the summer, almost doubling your output. At Denver's latitude, I get 10-15% more heat into the tank this time of year with the reflectors.

hot_rod

steep mounting

also limits the summer gain. Some would argue that tubes with curved absorbers never have the entire absorber in direct sunlight. This is why some brands add reflectors inside the tube to reflect some energy to the back side of the absorbers.



Are you seeing any ash from all the fires around you? That would change the performance on thermal and PV arrays.



hr

Kevin_in_Denver_2

steep angle hurts in summer, yes.

That's the main reason for the reflectors. As you can see in the picture, the reflectors blast the back of the panel after 3pm.



The collector tilt is about 70 degrees, and the roof is about 20 degrees





No ash in Denver, some smoke.

Mark Eatherton

Yeah but...

Most of the evac tube collectors I've seen only have the selective surface on one side of the absorber, no?



Would that be called a fixed non focusing hocus pocusing ejaculated tube corrector?



In Heeney, we use the snow. The incident angle of the Winter sun, along with a high degree array setting (about 48 degrees) gets you not only direct radiation, but also albedo radiation, thereby compounding the amount of solar gain. I am anxious to see how well it works this Winter.



This Summer, I am seeing something I didn't think I'd see. Forward thermo siphoning THROUGH a spring loaded check valve, and I only have 8' of vertical rise between the storage tank and the array. Wasn't quite sure what I was seeing from my data logging, so I went down at 2 AM and put hands on (and eyes on via infrared camera) and figured it out... Guess I am going to have to put another spring check in the circuit to cut it out. Its actually working to my advantage right now, keeping the array from stagnating. It loses quite a bit of the daily gain at night back through the array, without using any parasitic power.



Instead of a spring check, I might install a zone valve, so I can control the uncorking of the solar Genie whenever I think its right ;-)



ME

hot_rod

is the check clean?

the smallest particle under the seat will allow them to leak, solder, copper shavings, etc.



But I agree the even spring checks can be over powered. Do you have checks on both sides? and a way to bypass them if you need to drain down the collectors.

Kevin_in_Denver_2

Sun Maxx and Sun Task have 360 degree collection

This photo shows the parts in the type of panel I use:



More recently, the aluminum "heat transfer fins" touch the black glass absorber for the full circumference. Therefore the back of the panel works as well as the front and there is no orientational requirement for the tube assembly.

Kevin_in_Denver_2

For lack of a better term, I give you the brownstar heat trap valve

Mark,



I found this simple heat trap valve in tanks made by American. It's the first valve I've seen that prevents thermosiphoning in either direction, while allowing flow in either direction. Slits in the rubber cause the star shape. I couldn't find it in their literature.



Spring checks can be rebuilt with stronger springs. Most hardware stores have a spring assortment for $6.

Mark Eatherton

Bummer...

One of the advantages of a single sided evac tube, was that during peak insolation, and minimum usage (think Summer Vacation) you use to be able to roll the tubes 180 degrees so that they would not absorb solar energy. Guess you can't do that any more...



ME

Mark Eatherton

The Czech was clean.....

The Russian was dirty ;-)



At the time it was charged, I know for a fact that the check was in good shape because I inadvertently hooked my charging pump up backwards and couldn't get any flow. It is entirely possible that something may have gotten into the seat.



In addition to the check, there is also a flow meter that should give additional resistance to forward thermo siphoning...



I was talking to Der Heatmeister about this, and he and I have concluded that we really don't know all we need to know about thermo siphoning, but that the use of simple thermo siphon traps (regardless of their depth) do not work as advertised in every situation.



I will over come... Even if it requires me to do a zone valve, or additional ck valve.



Interestingly enough, at the end of the daily solar gain cycle, everything looks fine. Then about 20 minutes after cessation of the solar loop pump, the loop starts forward thermo siphoning.



Thanks for the input.



ME

Kevin_in_Denver_2

Water gets buoyant at 40F

This can't be your problem in Heeney... but thermosiphons get weird at 40F and below

hot_rod

weird

science that thermosiphon concept. I suspect many indirect tank over-heat because of un-checked flows. I agree, even 3 or 4 foot deep thermo drops seem to "leak'



This company makes checks for many of the pump manufacturers I think. They have sizes that will fit inside brass unions for example. You can also order them with higher "pop" pressure springs. I'd use the black, high temperature style for solar.



www.neoperl.com



This company will send 3 samples, if you want to try a different spring tension. They always have interesting booths at the trade show with a glass case full of different colored checks and aerators, very knowledgable staff also.



Caleffi offers a 3/4 and1" sweat brass union with a check inside. Commonly sold to help stop solar system thermosiphoning.



If you want a motorized valve consider a motorized ball valve style zone valve, they don't consume power once they are fully open or fully closed.



hr