Advice needed with new Thermosyphon Low Profile hot water system

[paulshea57]

IMG_1081.JPGIMG_1082.JPG Location: Kempsey, New South Wales, Australia. Latitude: 31 degrees south

I was hoping someone can help me with my recently setup, low profile, thermosyphon hot water system. The system is made up of two Chromagen solar collectors about 12 years old that appeared to be working very well in the testing stage. The tank is an almost new Rheem 315 litre. All seems to be working well, with very hot water and a certain amount of storage, except I don’t seem to be getting the storage I was hoping for. We a have quite a few trees shading the house, hence the PV panels and solar hot water system set up about 15 metres from the house. On a good day I can get sun on the panels from about 10am until about 4pm. A friend of mine has a professionally installed forced circulation system with a 250 litre tank. He says that himself and his wife can get about four days use with full cloud. Most of the time it is just myself and my wife and we only get 1 to 1.5 days with full cloud. When we have continuous sunny days, we are really enjoying our free hot water and the reduced gas bill!!

So, a few questions

Does my setup look to be ok, and does a low profile system have inferior efficiency compared to a system with tank fully above the collectors?
As I don’t have full sun all day, could that be the reason that my storage doesn’t seem to be right.
Hope you can help

Regards
Paul

[Bala]

I have a standard Edwards solar system. 2 Panels 300l tank horizontal above the panels in Far North Qld. Even with the tank full of boiling water I will only get a couple of days if there is cloud. It just depends how you use it. Multiple long hot showers a day uses a lot.

You may have some issues with panel angle or the water reversing flow and cooling on cold nights but JPM is the man to help you there.

[paulshea57]

Thanks for your info, that gives me more of an idea on usage and storage. I have the collector panels angled at 31 degrees, the same as latitude, which I believe is the optimum angle for PV panels and solar hot water collector panels. I have a check valve on the cold water pipe in to tank, but not on the cold water pipe in to the collector panels. I have rectified a slight low spot on the cold water pipe from tank to collector panels after reading a post by JPM

[J.P.M.]

Short suggestions:
1.) Increase the height difference so that the bottom of the tank is above the top of the collectors.
2.) Insulate the inlet and outlet lines with closed cell foam of ~ 25 mm thickness and wrap the insulation in aluminum (NOT duct) tape so that UV doesn't destroy the insulation is a year or two.
3.) Move the return line from the collectors to the top of the tank. You want to utilize as much of the vertical distance from the bottom to the top of the system as you can.

If Lucman is still around and reading this, he may have more/better advice.

Without some numbers and particulars on what you expect for hot water production, it's hard to say what the system is doing or if it has a problem. However, I'm pretty sure it will produce more hot water if the lines to/from the tank are insulated and the tank elevation is raised so that the bottom of the tank is above the level of the top of the collectors - the more the better, but at least 0.5m or so. The way it is now, that (relatively negative) height difference will result in some reverse thermosiphoning that will cool the water in the tank at night or when the sun isn't shining.

If the water is consistently not warm enough on sunny days, it may be that the collectors are not seeing enough sunshine to heat 300 l of water. Solution(s), more collectors or a smaller tank. a third (and less costly) solution: Use less hot water by shorter showers or low flow shower heads/other devices or both. My guess is that the collectors, being 15 yrs. old, may have some fouling in the tubes or more likely and also, the solder between the tubes and also absorber sheet may have failed in most places and sent efficiency to hell.

One other way to prevent that reverse thermosiphoning - although not as preferable as increasing the height difference - is to use a thermostatic valve of the type similar to an automotive cooling system thermostat - and note, NOT a regular check valve as the driving force from fluid density differences in the fluid is nowhere near great enough to open a regular check valve.

What drives the flow in thermosiphon systems is the density difference between hot and cold water (or other fluids) acting through a height difference between high and low points in the system - and to a point, the greater the elevation the better. The current tank elevation will mean the hot water in the tank will flow slower than it would if the elevation were higher, but that's not necessarily a bad or fatal thing with thermosiphon flow systems. What may be a problem however, is that the way you have it now, flow will pretty much stop or be greatly reduce when the hot water returning to the tank stratifies down to about the level equal to top of the collectors. At that point, the flow will be greatly reduced, the collectors will simply get hotter, losing heat that would otherwise go into the water but can't because the water is not flowing.

Take the easy and smart way out way out: Raise the tank about 2 m or more, insulate the lines and move the collector return to the top of the tank.
BTW, do NOT lower the angle of the collectors. They appear about right for your latitude.

Welcome to the neighborhood.

[Mike90250]

Additionally to raising the tank and insulating the lines, enlarge the lines, go to 1" tubing (rated for hot water under pressure) The force in thermosphon is very low, and the larger the lines, the less restriction. You want as much natural flow as possible
I have 2 thermosiphon systems, one for my wood burning heater. and the other on my antique generator, The wood stove heat loop is 1" stainless at household pressure
The generator system uses 2" hose, and conventional automotive thermostat at 195F. You would want a thermostat for about 90F, so that flow is happening as soon as the sun warms things up

[paulshea57]

Thank you JPM for your very comprehensive reply. You have answered all the questions I was going to ask, like the height difference of tank and collectors and the reasons why raising the tank will make a difference and the science behind it. I can think about adding another collector and the other suggestions you made, but first will raise the tank, which I think will make quite a difference.

I think the collectors are ok, as when I first tested them, without a tank, I just used cold water gravity feed to the bottom inlet and a tap on the top outlet. There was a very good flow and the water heated up very quickly. Not sure if that is a good enough test though.

[LucMan]

Here is what I see.
Use only 3/4" ID minimum copper piping for your safety!
The bottom of the tank needs to be above the top of the collectors, give yourself enough room to easily make your connections.
The line from the top of the collector needs to be connected to the bottom of the tank. Maintain a minimum of 1/4" per foot pipe slope up to the lowest connection.
The top water connection of the storage tank needs to connect to the bottom of the collector.
These change in connections will solve all your issues and allow your water to naturally circulate through your storage tank.
Cheers

[paulshea57]

Thank you for your reply Lucman. I do now understand that I need to change my system so the bottom of the tank must be above the collectors. I don’t quite understand the change in piping and was hoping you could explain. To change line from top of collectors to bottom of tank would mean the hot water would enter where the water is cold and riser to the top. To change the line from top of the tank to the bottom of the collectors would be feeding the hottest water back in to the collectors, which doesn’t seem right to me.

[J.P.M.]

The line from the top of the collector needs to be connected to the bottom of the tank.
The top water connection of the storage tank needs to connect to the bottom of the collector.
These change in connections will solve all your issues and allow your water to naturally circulate through your storage tank.
Cheers

You sure about that ?

I suppose doing it that way will work, but it sure ain't common. Besides decreasing the total system thermosiphon head by destroying or greatly reducing any thermal gradient in the tank water, with the resultant total system density head reduction also tending to work against the collector thermal stratification head.

Piping the system as you suggest will also increase the collector operating temps. by putting the warmest tank water (such as it may exist in any tank water thermal stratification remaining by the sort of obverse way you suggest plumbing the system) into the collector.

Also, that loss of tank stratification will have the practical effect of a likely delay in availability of water of a usable (hotter) temp. until later in the day. Because such an arrangement will reduce the total system thermosiphon head some - which provides the potential energy that pumps the water - it'll also slow down the flow rate which will further increase the plate temps. some and in so doing decrease the collector thermal efficiency still more thereby reducing the amount of heat available to increase the water temp.

Thinking about it, I can't remember seeing a working thermosiphon DIY DHW system plumbed that way. Commercially available thermosiphon systems commonly/ usually/often have horizontal tanks, for practical and/or aesthetic reasons, and so don't need to have much if any consideration for tank stratification or penalties associated with losing potential tank stratification brought on by reverse plumbing scenarios, but I'd note that collector returns in such systems don't seem to plumb to the lower elevations in the tanks.

All my textbooks on thermosiphon systems (as well as my experience) have the flow outlet from the top (hot) side of collector going to the top of the tank, with the lower tank connection plumbed back to the bottom of the collector, that line serving as the collector inlet.

As for increasing line sizes, because at the very low flow rates at which most every thermosiphon DHW system operates, the decrease in pressure drop made possible by increasing line sizes from 3/4" to, say,1" is pretty miniscule. Given the low flows, and so low head losses that already exist in thermosiphon systems, I'd also wager that the added head loss incurred by adding the 4 ea. required fittings for the 3/4" to 1" transitions at collector and tank inlets/outlets might use up most if not all any advantage of increasing piping diameters.

To Paulshea57: Raise the tank elevation relative to the collector elevation. In doing so I'd keep the connections in the same order, but I'd spend more effort on avoiding sharp changes/transitions in piping direction by using long radius elbows and soft (bendable) copper pipe.

And, while I was at it, I'd make sure the piping has no local high spots that can trap air (that may evolve from the water as it's heated).
A (local) high spot bubble anywhere in any line will stop the flow dead. That may also be part of the problem.

And for the love of the Almighty, insulate the lines after you get rid of that braided hose, and replace it with real tubing.

And then insulate all the lines.

[paulshea57]

Thanks for your post JPM. I’m glad you cleared that up. I did a fair bit of research before starting and did think I had the piping in the right places. That green pipe is actually 3/4” copper pipe with a green protective coating. The two cold water lines are a 3/4” blue line poly pipe. I intend to replace the poly pipe with 3/4” copper pipe. Thanks again for your help

[LucMan]

First let me say that I have never seen or worked on a system like this before. Mostly I have installed pumped flat plate drain back systems. They always work and with minimal maintenance, . Thermosyphon systems are not a good fit for NY state! I have seen some thermosyphon systems with horizontal tanks but never with vertical storage.
My reasoning for changing the connections is based on old gravity HW heating systems, but even they had radiators that had all connections on the horizontal. Hot water rises cold water falls.
The temperature in the top of the collector is going to be the warmest so it will naturally rise into the bottom of the tank and mix with the cooler water in the tank hopefully creating flow.
The water at the top of the tank will then naturally flow back to the bottom of the collector.
An alternate and possibly superior connection would be to connect at the center of the tank through one of the electric element fittings back to the collector.
I can't see the complete volume of water in the tank getting hot with the existing piping. The hot water enters from the top of the collector to the top of the tank where it will stratify and inhibit circulation. That's my thinking.
Please let me know what ends up working best, or at all.

[J.P.M.]

First let me say that I have never seen or worked on a system like this before. Mostly I have installed pumped flat plate drain back systems. They always work and with minimal maintenance, . Thermosyphon systems are not a good fit for NY state! I have seen some thermosyphon systems with horizontal tanks but never with vertical storage.
My reasoning for changing the connections is based on old gravity HW heating systems, but even they had radiators that had all connections on the horizontal. Hot water rises cold water falls.
The temperature in the top of the collector is going to be the warmest so it will naturally rise into the bottom of the tank and mix with the cooler water in the tank hopefully creating flow.
The water at the top of the tank will then naturally flow back to the bottom of the collector.
An alternate and possibly superior connection would be to connect at the center of the tank through one of the electric element fittings back to the collector.
I can't see the complete volume of water in the tank getting hot with the existing piping. The hot water enters from the top of the collector to the top of the tank where it will stratify and inhibit circulation. That's my thinking.
Please let me know what ends up working best, or at all.

LucMan: Thermosiphon water heaters have been around a long time. One of the earliest successfully marketed of this type was something called the "Day and Night" in 1909 in/around Los Angeles. They were downright common there until natural gas became more available. Later, between the late 1950's and early 1970's, 2 Australian companies, Solahart and Beasley Industries marketed 10's of thousands of what now look familiar as thermosiphon water heaters on roofs in that country. Until the 6 day war in 1967, about the only way the Israeli working class had any hot water in their homes was due to thermosiphon water heaters. The 6 day war ended the solar water heater boom in Israel when the captured some territory in the Sanai that had big oil fields. Every one of those designs are piped as I describe.

I'm not busting your balls but you're logic/thinking - as you explain it - is wrong. The gory details of why are too long for this forum.

Consult any decent solar energy text for explanation of why your logic is in error. I'd respectfully suggest a look at pp. 431-432 of: Lunde, Peter J, "Solar Thermal Engineering", 1980, John Wiley and Sons, ISBN # 0-471-0385-6. He has one of the best 2 page explanations of how/why thermosiphon systems work.

FWIW, the idea I mentioned of tank stratification adding to/enhancing the gravity head enabled by the thermal gradient of the collectors in a thermosiphon system was one of my contributions to solar thermal engineering done as a graduate student when I took a ringer course in solar energy engineering and knew more about the course title and subject than the instructor knew. He claimed any thermal gradient/stratification in the tank didn't affect the thermally induced system head and so had no influence on system performance. Why it does is what I did a paper on for the course. The gory details are too long to reiterate here. That was done in NYS BTW.