Solar hot water. Thermo syphon problems

[sparticuss]

This is a bit of a question for the experts.
I've bouth a couple of the olde style solar hot water panesl going as cheap as two mating canaries and also an old gas hot water service as a storage tank.

The tank is planned to go on the roof ridge with no pump and to relay on thermo syphon to circulate the water.

Now, being a prudent customer I've tried setting the thing up on the ground, to see if it will work.

And I'm having trouble getting the thermo syphon to move.

Tanks above the panel but I suspect that the panels are boiling the water and creating a steam pocket that the thermo syphon cant get past. This is just a trail run at air pressure, not mains pressure.

I can't bleed the tank properley in its present position due to the loaction of the inlet pipe and the bleed outlet.


But, before anybody else jumps on the steam pocket bandwagon, does anybody know of any other causes of thermo sypnon problems.


Better still does anybody know where I can buy a re circulaing pump that can run off a blue solar electric panel.? ie Only runs when the sun comes out. Perfect for solar hot water.

[Muddypause]

Well, I wouldn't claim to be an expert in solar systems, but I know a little about domestic hot water systems.

I'm wondering about what you call 'thermo syphon' - I'm guessing this is what I would call a gravity system - i.e. cold water is heavier than hot water, so hotter water goes up one pipe while colder water comes down another, giving you circulation (convection). This would normally heat water in an indirect cylinder (one with a heat exchanger in it, so that water in the primary circuit does not come into contact with that of the secondary circuit). If it was a boiler doing the heating, you would need as much height in the system as possible - at least one storey, preferably two. You also need to be fairly careful about the route the pipes take, keeping them as vertical as possible and without any downward bits on the rising pipe, for example. The heated water leaves from the top of the boiler (and presumably the solar heater, too), and enters the tank at the top - the cooler water leaves the tank at the bottom, and enters the boiler at the bottom, ready for re-heating.

I don't know how hot ozzy sun can get, but it takes quite a lot of energy to make water boil - despite its name, a boiler shouldn't boil the water, in spite of all that heat energy being put into it. Does a solar panel really capture that much energy, even in your hot climate? In any case, if your pipe routes are good, you should get convection before then. Maybe something that heats the water to a lesser degree than a boiler would not be able to create a tempeature gadient sufficient to result in good circulation, particularly if the height is not so much. A small pump of some sort may be the answer.

[sparticuss]

Thanks for that muddy.
And believe me, the Australian sun, even down here in the relative cold of Melbourne has more than enough biter to have given me skin cancers and to boil water.

I simply hook the garden tap up to ONE panel at a very slow dribble, (about the same as a slow pee) I get boiling water out the other end. I can stick a cup with a tea bag under it and make a cup of tea. And I've done so too. At that rate ONE panel can boil about 25 litres in a good day.

What you are talking about is what I call thermosyphoning but there's no two stories. Only two feet. I've been told to just make sure that the hot water pipe only ever runs up hill ( the roof) with no downward dips, and the cold water only ever runs downhill. with no upward rises.

Had another go today with things re arranged and the tank laid down almost flat and seemed to get better results. But , even if it's working, it will take a few days to boil the 175 litres in the tank.


Any more opinions anybody??????

[sparticuss]

Just one morre quick note muddy.

I know Londons reputation for smog but how clear are the skies in Scotland.
Clear sun, not hot sun, is what makes the solar hot water panels work. It can be zero degrees but if its still a clear day and a bright sun the water still comes out of the panels piping hot.

[Muddypause]

I'm sure that our northern contingent will correct me here, but statistically, some parts of Scotland get as many hours of sunshine per year as much of the rest of Britain, and is perhaps 25% less than the sunniest parts. Having said that, most of the highland region only gets about half of what the south of England gets.

I can see that it must be right that a sunny day, even in sub zero temperatures, will still make a solar panel work. But don't forget also that the further from the equator you go, the greater the curvature of the earth means that the sun's rays have more atmosphere to penetrate, and that will absorb more of its energy before it gets to the panel. So it will take longer to heat the same amount of water.

Also, in defence of London (actually I hate the place), although the quality of the air will propbably not be very good compared to most of Scotland, the smogs are a thing of the first half of the last century, when things got so bad you literally could not see 6 feet in front of you. Controls on factory and house chimneys eventually put an end to those pea-soupers.

[ina]

No idea what the statistics are - but yes, we do have clear skies up here. The wind sees to that...

Unfortunately, the sunshine hours are rather short in winter (although it's lovely out there at the moment - had a few snowshowers, a strong northerly blowing, and the sun is out!). So even if the sun is clear in winter, solar anything is not really an option. In summer, however, we do have longer hours of sun!

Ina

[sparticuss]

Cloudy skiess the past few days. Even so the system got comfortably warm.
Clear warm day today and the whole 175 litres came up to about the simmer.

There must have been a bubble in the system the first time preventing the thermosyphon. Can't think of anything else.

In any case the problems pretty much over.

Now I just need to figure how to get 100kgs of hot water tank up onto my roof.

[sparticuss]

No idea what the statistics are - but yes, we do have clear skies up here. The wind sees to that...

Unfortunately, the sunshine hours are rather short in winter (although it's lovely out there at the moment - had a few snowshowers, a strong northerly blowing, and the sun is out!). So even if the sun is clear in winter, solar anything is not really an option. In summer, however, we do have longer hours of sun!

Ina

wind eh??? Whats the market like in domestic size wind generators up there??

[sparticuss]

These solar panels are meant to have a frost protection vlave attached.
When the temperature gets below about 3 deg C the valve opens a bit of a dribble from the hot water in the storage tank and into the panels stopping ice frm forming in then and destroying the panels.

These valves are $180 to buy. A bit pricey.
Does anybody know how to make them.???

[Muddypause]

Is it an indirect circuit (ie. is the water that runs through the solar panels a closed circuit, and separate from the water that comes out of the tap)? If so, wouldn't anti-freeze be an option?

[Martin]

I think you're learning first-hand why most modern systems are a bit more complicated! The problem with simple flat panels is that they are as good at radiating the heat as collecting it, so it's better to have a pump which cuts in when needed. Again, most modern systems heat the water indirectly - you have a thermal store tank, and the water from the panels circulates through a heating coil in the tank, allowing you to add something like 40% antifreeze to the small amount of water circulating through the panel and coil. For your present system, it may well be that you haven't allowed enough "rise" between the panel and tank - the tank needs to be well above the panel! 8)

[sparticuss]

Is it an indirect circuit (ie. is the water that runs through the solar panels a closed circuit, and separate from the water that comes out of the tap)? If so, wouldn't anti-freeze be an option?

Afraid not Muddy. It's direct. The water that goes through those panels is the water that comes out of the shower.

The problem with the closed circuits is that the tank can't be just and adapted gas hot water tank. It needs specialised fittings to hold the heat exchanger coil.

I've been chatting to a few places and they are saying that it's probably nearly as effective to just manually drain the panels for two months over winter.

[Martin]

we can source special "immersion heater" replacement coils - they aren't as efficient as a proper solar coil in a pukka tank, but they will do the job, and don't cost an arm and a leg!(£105 plus carriage) 8)
That way, you could have very simple loop that only circulates your "heat transfer" liquid (water plus 40% antifreeze). The easiest way to have a loop like this is to have a simple pressurised circuit - (pressure loop kit, about £60 plus your ordinary plumbing parts).
Best of luck! 8)
just noticed you're in OZ - there must be some suppliers of similar equipment down under!

[Muddypause]

I'm wondering if a simple TRV (Thermostatic Radiator Valve) could be used. Maybe fit it on a separate shunt circuit, so that it doesn't interfere with normal operation.