Reverse Thermosyphon

Hello,

I just had a Solar Hot water (thermosyphon) system installed. I tried to follow all rules guiding thermosyphon. I discovered that the water in the reservoir isn't getting hot.

Below are some specifics of the system:
- There are two collectors side by side.
-The outlet of the second collector is connected to the reservoir which is place about 600mm from the top of the collector.

Faults noticed:
- The inlet temperature the first collector (from the bottom of the reservoir)  is greater than the outlet of the first collector and this is greater than the inlet temperature into the second collector which is also greater than the outlet temperature into the reservoir.

I suspect reverse thermosyphon but I don't really know the cause.
Please, I need help.

Opeyemi.

Hi Opeyemi;   Welcome to Permies!
I think we will need some photo's to better understand your system.
Is this hot water for your home ? How big is the reservoir?  How long is the sun shining on your collector's?
If you just had this installed ,can you ask the installer why its not working properly?
Is this a widely used system in your area? Or is this a new company?
Maybe there are others with a similar system, that have had the same problem?

thomas rubino wrote:Hi Opeyemi;   Welcome to Permies!
I think we will need some photo's to better understand your system.
Is this hot water for your home ? How big is the reservoir?  How long is the sun shining on your collector's?
If you just had this installed ,can you ask the installer why its not working properly?
Is this a widely used system in your area? Or is this a new company?
Maybe there are others with a similar system, that have had the same problem?

Thank you so much for the reply.

Below are additional details:


Size of reservoir = 200 litres.
Suntime = About 9 hours.

I built it myself.

Hi Opeyemi;    First let me tell you that I have never built or lived with solar hot water.   That said, so you know I am not an expert.
The only system I had first hand experience with, was for heating an outdoor swimming pool in Canada.  The collector's were definitely higher than the pool.
I believe your collector's should be higher than your reservoir, to get a thermos syphon going.    

I have thermosiphon hot water, and the panels need to be below the tank.  But I don't think your panels should be flat like that,  on mine my panels are on a slope too and that way water also moves from the bottom to the top of the water panel so I think that the first thing you need is that the water needs to be able to rise from the bottom of the hot water panel to the top.  

There we have it !  Our resident expert!   Thank you Debi, I knew there would be a permi with solar hot water.

Also on mine the panels are in parallel not in series,  so water does not go thru one panel and then thru the other .  It is plumbed so that fresh water goes into the bottom of both panels and hot water out of the top of both panels,  so the inlet of both are connected together and the outlets of both are connected together ( not outlet of one connected to inlet of the other) That at least is how mine and others I have seen are done

Maybe you could try having the panels on one of the slops of that roof, maybe the morning sun slope, instead of balanced on the ridge and plumb the bottom of the tank to the bottom of the panels and plumb the top of the panels to the top of the tank.  So the waster thermosiphons thru the panels and tank.  Around here what I see is the the cold tap water then connected also to the bottom of the tank

thomas rubino wrote:Hi Opeyemi;    First let me tell you that I have never built or lived with solar hot water.   That said, so you know I am not an expert.
The only system I had first hand experience with, was for heating an outdoor swimming pool in Canada.  The collector's were definitely higher than the pool.
I believe your collector's should be higher than your reservoir, to get a thermos syphon going.    

Thank you very much Thomas.

Debi Baker wrote:Also on mine the panels are in parallel not in series,  so water does not go thru one panel and then thru the other .  It is plumbed so that fresh water goes into the bottom of both panels and hot water out of the top of both panels,  so the inlet of both are connected together and the outlets of both are connected together ( not outlet of one connected to inlet of the other) That at least is how mine and others I have seen are done

Maybe you could try having the panels on one of the slops of that roof, maybe the morning sun slope, instead of balanced on the ridge and plumb the bottom of the tank to the bottom of the panels and plumb the top of the panels to the top of the tank.  So the waster thermosiphons thru the panels and tank.  Around here what I see is the the cold tap water then connected also to the bottom of the tank

Thank you Debi.

Actually, the unit has been test - runned on the ground before placing on the roof and it worked fine.

From your suggestion, it must have been from the slope I didn't consider. While I test- runned  it on the ground I tilted it at an angle.

I will try this.

This has helped a lot.

Many thanks once again.

Couple things.

The collectors need at least a bit of pitch or elevation difference, outlet highest port or high corner.

The collectors need to have parallel risers. I cannot tell for sure, bit it looks like a serpentine waterway.
If so, it will not thermosyphon. It creates a series of traps.

Thermosyphon works best with a realatively large waterway, especially the manifold. Many small risers feed the manifolds and from that point it needs low restriction. At all points really but flow rate in the risers is so low they effectively have low resistance.

Keep all plumbing pitched or vertical when possible. Think chimney draft.

Long sweep elbows or bendable/flexible tube really help lower restriction and should be as few as will get it there.
That applies to systems with active circulation as well.

Cold feed to collector is the lowest corner, hot return to storage highest corner and tank above collector.

Since 2004.

Both Debi and Frank are right on. There are only two items I didn't notice in their posts:
-for convection flow to work you need atleast 18 inches or 45.72cm height difference (which you will get when you angle the collectors);
-many hot water systems using convection flow will not work if there is an air pocket trapped in the pipes/collectors connected to your tank.

Good luck!

Orin Raichart wrote:Both Debi and Frank are right on. There are only two items I didn't notice in their posts:
-for convection flow to work you need atleast 18 inches or 45.72cm height difference (which you will get when you angle the collectors);
-many hot water systems using convection flow will not work if there is an air pocket trapped in the pipes/collectors connected to your tank.

Good luck!

I agree I dont see a thermosiphon here. its natural convection flow. thermosiphon works by alternately heating and cooling a vessel full of air. this advice is correct except that height difference does no determine angle of inclination. Ideally your angle should follow the sun but not less done 30 degrees.

julian Gerona wrote:

Orin Raichart wrote:Both Debi and Frank are right on. There are only two items I didn't notice in their posts:
-for convection flow to work you need atleast 18 inches or 45.72cm height difference (which you will get when you angle the collectors);
-many hot water systems using convection flow will not work if there is an air pocket trapped in the pipes/collectors connected to your tank.

Good luck!

I agree I dont see a thermosiphon here. its natural convection flow. thermosiphon works by alternately heating and cooling a vessel full of air. this advice is correct except that height difference does no determine angle of inclination. Ideally your angle should follow the sun but not less done 30 degrees.

I dont think Orin meant that determined angle, only that it would get the height difference from a tilt.

In my mind there is that difference and it doesnt matter. The floor of the storage tank located higher than the collector output is the key to a tank full of heated water and at a decent rate. It could be done with a tank floor at collector input height and a tall tank, but storage capacity would suffer, collector efficiency should drop.
Then again the difference could be small enough to do anyway or i could be off and you wouldnt loose much as long as the tank floor is no lower than the collector input.

It needs a check valve a swing check horizontal if reverse flow is an issue and generally anyway. The collector could be insulated but difference in temp is power, flow if its arranged appropriately and in forward and reverse.

Sites vary and delta t is a double edge.

The best thing for the installation is likely going to be a high head circulator (pv drive) a tilt evaluation and some plumbing routing possible. I cant see whats going on with the circuits. In that case everything can be close and may not require the tower unless its for something else. Brings the question of heat exchanger/no HE for dhw.

If you want to thermosyphon, i would re work the collectors, assuming they are diy or source parallel manifold collectors. It needs risers and manifolds.

Imagine a ladder on its side, except the rungs are 6-10 feet long and the beams 3-4 feet. When fluid is circulated and fed and returned from diagonally opposed corners it will equalize flow through the risers and other parallel collectors of the same dimensions.

The same thing is done in electronics, batteries, etc.

We tilt to face toward the sun and rotate on that plane for thermosyphon and drain back placing the outlet at the highest corner of the collector/row.

one thing I notice from the pic. are the seemingly horizontal pipes. You cant do that on a natural convection circuit. The driving force for flow is the differential weight of hot and cold fluid. Cold is heavier and will tend to go down and displace the hotter and lighter fluid which will then go up. The simple rule is that the hot zone should be able to rise and the cold zone should be able to drop. Thus the pipes leading to and from the collectors and tank must always be on an inclined position. I mean all the pipes including the manifolds. From tank, water enters the lowest part of collector and from highest point of collector water enters the top of the tank but not higher than the water level.

There is another thing. Inclined piping system will make the circuit self purging. Once the system is filled with water the air in the system will automatically rise to the top where it can be bleed to a purging valve. This is also the reason I dont recommend any check valve. A check valve from the bottom of storage tank to bottom of collector is sure to mess with air purging. If the system is properly plumb there wont be any need for check valve in any part of the system.