Direction of Water in Solar Thermal Collector

The temperature rise is a function of how long it takes the water to circulate. If you pump it downward, you are bucking the natural circulation due to thermal convection, so it would make more sense to pump it upward or let it circulate naturally.

Michael,

I would suggest you do some testing on your conical contraption. There are several affordable temperature measuring devices you can readily purchase from eBay.

Here is one that I recently purchased and it works great!

It has 4 inputs and it came with 4 K-Type probes that you could attach to the input and output and get instant readings. You could also attach the other 2 probes in other strategic areas of the SOLAR CONE or SCONE as I call it.

Sorry I wasn't able to stop by the solar store in Las Vegas to meet. I had some unexpected visitors and was unable to get away. Maybe another day.

Good luck.

Agreed. Difference too small to tell. if any.

I would think that you should run it upward. I am not familiar with a concho spiral, but if it is mounted like pictured with the point upward the air inside should be hotter at the top, this combined with the heat rise convection overall upward trend in the pipe makes me think it would be best pushing your water from bottom to top so it is hottest at the top.

Drew K

There won't be enough convection to make the water flow on it's own. It won't matter which way you pump it.

When I made my system I first tried a flat panel and there wasn't enough differential to get flow despite the water getting very hot... too much internal friction through the small bore pip, despite short big bore feed pipes.

IMO the design is over complicated to produce although it has fewer pipe bends than a simple zig zag on a flat plate. Ok it looks funky...

Del

If I recall correctly, when you built your collector you had two panels. One had more collector tubing than the other and you calculated which collector should be first in line to get the best performance. I don't recall the answer but it was a convincing argument.

I have to agree, with the small amount of change in density of the water as it heats and the shallow slope of the piping, convection would be very weak.

It depends on how you model it.

If you see it as a tube in a spiral for the flow restriction, you see for laminar flow there is very little resistance when compared to a straight pipe.

When you look at the column of water for the pressure differential you can see that there should be a temperature differential that will cause convection to move the water.

I think that as long as there is limited resistance outside this system there should be fluid flow due to heating.

It would be interesting to set it up with a bucket of water at the top with the inlet and outlet connected to determine what kind of flow it could make.

Drew K

Depends how you model it!!!????

First ... before I rant... apologies if I have misunderstood you post or taken the quote out of context

WHAAAAAAA!

Jeez you can model anything to show any result you like... it won't effect the reality... the convection force is so weak it ain't going to work. Maybe at the equator with 2" bore pipe and a 10 foot column you might see some movement.

Del

Dunno, I was thinking of net forces involved...perhaps too weak. I figure that when it expands, it has to go somewhere and all pressures being equal at the T=1 that when T=2 with heat rising it would have a net increase at the top.

I want to build one to find out now : )

Drew K

The heat transfer efficiency is much more important than flow direction here....next is cost to produce...

http://www.freehotwater.com/solar-thermal-101-collector-efficiency/

https://www.renewableenergyhub.co.uk/the-different-types-of-solar-thermal-panel-collectors.html

One would think that pumping the water up may tend to trap air at the top of the coil.

Agree with you on pumping really making any convection a moot consideration. As for air entrapment, a self purging valve at the highest spot will take care of that.

Releasing the entrained air at the top of the system is critical, otherwise over time it will become air-bound and no flow will occur. A solar thermal system is not much different than a hydronic baseboard heating system, water is not "pumped" through it, water is circulated through it.

That means there can be no gaps in the fluid throughout the entire system because the available head from the circulator is more than sufficient to overcome the resistance of the piping system (or it wouldn't move at all), but not enough to overcome the weight of the water column to the highest point in the system.

Very true...

I live in a four plex (2 units on the bottom and 2 on top) and we use hydronic heating, which is the majority of heating systems in northern climes (I am... like my tag line says... north of 60).

Our hydronic system has a special piping setup with an automatic air purge valve right after the boiler discharge to get the air out of the system. When that valve fails (every 3-4 years) it is not to long before the upper floor units do not "flow" the heating medium fluid properly.

TThe water won't cafe.

It does not matter.

As there is a slight incline, perhaps simple ball-check valves on segments of the coil would work.

Interesting, so the entire system is pressurized at mains pressure. But is the check valve necessary? Other than thermally motivated flow, what would make opening the supply outlet cause any difference to the closed loop of the collectors?

Drew K

I would say that the water will get hotter at the base of the spiral cone. This is because the surface area increases as you move away from the center. Solar energy is rated as Watts/Sq meter. The collection surface is greater at the base and therefore the energy available is greater at the base. Another more important point is that solar thermal collectors are more effective under direct irradiance and not as effective with diffused irradiance. Cloud cover plays an important part in determining the % of direct irradiance your solar thermal collector receives.

Welcome.

But, the volume of water and thermal mass inside the tubing is constant regardless of the position of the coil.

I say the water doesn't care.

What is the height of the cone and what is the diameter of the base. These data can be used to tell which way the water will syphon convect (no pump).

Nope. I dont think one can say that.

What difference does the height do for where the water goes and where it is warmest?

And whats with the diameter?

Please explain!

When the conch base is sloped at 27 degrees, what will be the the angle of the upmost conch slope wrt the horizontal? Yes, forget it. Will the pipe on the conch have an upward pointing tangent at all points on the conch? If not, there will be natural convecting only over individual 360 degree loops, and the top side of the sloped conch will be hotter than the bottom side. There will not be continuous convecting over the full length of the pipe. I suspect that, without doing the maths, the proposed 27 degree base angle will not allow natural convection thru the whole length, because the tangent along the pipe will vary from positive up to negative down. I would say that the pipe is moving up the conch slope at an angle much less than 27 degrees upwards. If the coil rises at an angle greater than 27 degrees on the conch, the coil will convect continuously over the length of the coil. The maths is just straight geometry of triangles, given the base diameter, height and the spacing between pipe loops. Thanks for questioning my point.

The math is unnecessary. First the OP never mentioned convection. Second, the air in a well sealed collector naturally stratifies.

For maximum heat gain the cool fluid should be admitted into the hottest zone in the collector and flow to the coolest; think in terms of a counterflow heat exchanger where the LMTD (Logarithmic Mean Temperature Difference) is usually higher than the AMT (Arithmetic Mean Temperature) that is achieved when the fluids travel in the same direction.

That being said, nearly all collector designs perform nearly the same under ideal conditions; i.e., full insolation, high noon, collector tilt equal to the azimuth of the sun, inlet collector fluid significantly cooler than the surface of the collector. The differences in system design and operation become more apparent in terms of energy harvest outside of the +/- 15º window either side of high noon.

Is this a sealed system? Most of the flat panel collectors I saw in Turkey had a drum-like container at the top of the panel that stored hot water that was piped into the house for secondary storage or direct usage. All of them had a recirculation system where the storage was refreshed during the day.

Drew K

Drew, post #18 has to do with what you are describing, I was using it to illustrate a check valve, what is lacking in the illustration, is the means for circulation and direction in the collector circuit. I will go away now......