Blasphemy..... Solar power.

[LowGear]

As a sidewalk superintendent it would seem Drain-back is what -35 anything would be the best answer.  And the same would go for too high temperatures or just turn the darn thing off in the warm season.

Ahhha.  The system freezes before the sun comes up and can't be pumped?

Casey

[carlb23]

Carlb,



Carl, how do you dump your heat in summer.?
My plan is to tarp cover the panels. or...push the heat to an external radiator mounted on the outside wall.

Veggie

With the angle of the shed and parabolic trough they are most efficient at winter solstice so they are much less efficient in the summer.   the controllers have high limit settings and will turn off water flow to the parabolic reflector and shed wall when temps reach the high limit.  Sitting stagnate will not hurt the parabolic trough.  In the summer i usually just turn off the linear actuator and leave it sit facing east and it will only add a little amount in the very early day.   The shed wall will provide all of the hot water needed in the summer without the use of the parabolic trough.  Again with the angle it isn't all that efficient in the summer     

[veggie]

BruceM,

Sorry I missed your comment above regarding Drainback systems and very cold climates.
I have read several sources that recommend against it if the snowfall is high in the region and temps stay below -15C for extended periods.

One is in my book "Solar Water Heating" by Bob Ramlow (A Mother Earth News Publication)

Here is another example..
http://shop.latitude51solar.ca/Closed-Loop-versus-Drain-Back-s/97.htm
See the very last paragraph.

The reasoning seems to be that if the pump were to stay on (circulating) due to a switch or controller failure, then the coolant would freeze in the collector during the night and and possibly split the piping.

The issue I have with this reasoning in that there could be additional safeguards put into the system such as using "DowFrost" glycol or propelene glycol mixed in a ratio to give -40 degF protection.
Also, a flow detection device could be added to test if flow is occurring when incoming liquid is cooler than the interior.
(These additions do add some complexity to an otherwise simple system.)

For simplicity, I do prefer the drainback type system and I may still pursue it using some additional safety features.

PS: I scored a 50 gallon solar hot water tank !
It has two internal coils for heat exchange and also a heating element for emergency backup which can be powered by generator or wind turbine or PV panels.
It should make a good heat reservoir.

cheers,
Veggie

[veggie]

For anyone interested, here is a good tutorial on solar H2O drainback systems.
Commonly confused with solar H2O "draindown" systems. (There is a difference)

Drain back = GOOD
Drain down = BAD

Helpful design and installation tips for drainback systems:
http://www.uvm.edu/~gflomenh/CDAE106/readings/sdhw86.pdf

...and why drain down systems are problematic (are recommended against)
http://www.sunnyhotwater.com/draindown.html

Terminology is important in this instance because I found several sites on the web that were erroneously calling a drainback system by the frowned upon name of draindown system.  

cheers,
Veggie

[BruceM]

Dear Veggie,
The article you referenced was written by an idiot who forgot to learn about the topic he was writing about.  This is a common human problem, and affects most journalists severely.

There is NO VALVE in drainback systems, and they are fine for cold climates.  If the pump fails, the system drains back to the storage tank, period.  

Here's what the idiot wrote:

"A drain back system has critical weakness in cold climates. It relies on a mechanical valve to open and close the system to drain itself. Any mechanical item is prone to failure and a failure of a drain back system in freezing climates would result in nearly total loos as the collector and piping would be completely destroyed by the expanding ice. For this reason we feel the risk of damage is too great to overcome the small advantages of a drain back system and as such we do not offer any type of drain back solar water heating systems."

Drainback systems are well time and service life proven in cold climates.

Antifreeze is a problem because of cost and ongoing replacement cost.  When the panel stagnates, the antifreeze is ruined from overtemperature. It becomes acid and the pipes corrode faster.  It has to be replaced periodically, something humans do poorly.  Glycol systems really need a heat dump to not stagnate, but if the pump breaks down, oops, stagnation cooks the glycol.

Drainback systems require a bigger pump for starting since the water must be lifted the entire head from tank surface to top of panel, and at a flow rate high enough to keep the sloping return line full.  This can be solved with a multispeed pump, or a second pump on a timer for 10 minutes of starting only.  My system uses the 12 V, 20 watt Laing D5 Vario pump, and I added a little circuit to slow it down after 10 minutes.  It just barely had the flow needed at my 8 feet of head, I had to add a ball valve restriction on the return line.  

[LowGear]

One of the upsides of Hawaii - no minus temperatures.  OK, not even sub 60 temperatures.  I'm about half way there on a simple circulating system controlled by a differential thermostat.  Breaking through the roof is the next step.

Casey

[BruceM]

By the way, the article Veggie listed for drainback systems shows a system which is much more complicated that mine.  A single tank drainback system only has one insulated storage tank, and only one pump. The top of the tank must be below the bottom of the panel and allow for slightly downward sloped plumbing from panel bottom to top of tank, and the pump MUST be mounted below the water level of the tank so that it stays primed.  I would avoid a complex system like the one shown in the article; each tank and pump has a service life and must be maintained.  These tanks with heat exchangers built in are very spendy. I say NO to anything that is not a SINGLE TANK drainback system.  

I dug through the floor of my shop building to get the tank below grade so I could have the simpler single tank system.  My 800 gallon tank is EPDM lined, with 3" polyisocyanurate foam board lining the hole.  The lid is two piece with 6" of foam, epdm covered underneath.  It's sturdy enough to allow me to use the space on top of it.  The water level is about a foot above the floor, which lets my pump be at floor level.  

For my neighbor's system, we are putting the storage tank outside, with the lid at ground level.  When we get to that next year, I'll post some pictures.

Here's a link to a short video that explains why I like single tank drainback systems: 
https://www.youtube.com/watch?v=VlAfTYTA0Z0

I am using the storage tank water directly, with no heat exchanger for heating my house with in floor heat. I do this by modifying an air bleed valve to vent via silicone tube to a vacuum tank; the air bleed then works fine since it thinks it's still in a pressurized system.  A copper pipe heat exchanger preheats my domestic hot water. I shut down the backup propane water heaters all year, except for Dec-Feb.

[carlb23]

i am also using a single tank drain back system with both a flat panel collector and a parabolic trough feeding the same tank.  I use two pumps one for each collector and two controllers one for each collector.  there are no valves involved if there is any type of failure the water just drains back to the tank.   It can not freeze up gravity wont let it.  Once the pumps stop the collectors are empty in a minute or less leaving no time to freeze.
I control the flow by regulating the dc voltage going to the pump with a simple adjustable voltage regulator. 

[LowGear]

So how hot do your tanks get?

[BruceM]

My tank is limited to 140F by my pump controller settings.  The epdm rubber tank liner is rated to 170F but then I'd have to add an antiscald /temperature control mixer valve, so I decided to go the path of utter simplicity and longer liner life by reducing the max temperature to 140F.  My tanks stays at the max when I'm not heating the house.  In the winter the tank varies between 85F and 120F; the house heating load and cloudy days keep it working hard to catch up.

Flat panels are easy and fairly cheap to build, but they do not produce in cloudy or overcast days, ONLY in full sun. The evacuated tube type units fair better, though they may need variable flow rate to slow down enough for useful temperature gain on hazy days.  It's a storage tank size and cost trade off that you'd have to consider based on your own climate.  I prefer the simplicity of the flat panels, myself.

[carlb23]

I only use mine for domestic hot water so i have the controllers set for 130 degrees

[LowGear]

So you stop circulating water when the tank reaches 130 or 140 degrees respectively?  You'd also have a differential thermostat that comes one when the collector is a couple of degrees above the tank temperature?

Casey

[BruceM]

You got it right, Casey.  My controller has temperature sensors at top and bottom of my tank, and one on the copper pipe coming out of panel. (Inside the panel on the backside.)  When that's at the set degrees above the bottom of the tank, the pump goes on high and starts filling up the panel.  If after running water through, it cools off below the bottom of the tank, it stops.  They difference between start and stop is called hysterisis, to keep the pump from short cycling.  

When the tank temperature is maxed out, the pump stops and the panel drains down and stagnates (sits baking in the sun) dry.  A stagnating panel gets to 350F, so you must choose materials and glazing with care.  In Hawaii, I think I would use nothing but tempered glass for glazing, since your sun is always overhead and your panel must be horizontal as well.  Here I can tilt my panel about 53 degrees and in the summer my panel is shaded except for a few hours in the afternoon at an oblique angle, so I can get away with polycarbonate glazing.  I have the most stagnation time in March,April, and Sept, Oct.

[carlb23]

my controller has three sensors one at the top of the storage tank one at the bottom of the storage tank and one at the collector.  It measures the hottest part of the tank the coldest part of the tank and the temp of the collector.   you can set what the differential temp is as well as max high temp and a number of other things.

[LowGear]

What angle do you set your panels?