I don't understand. We only have one tank. The water heated by the solar panel is stored in the tank. It's not an indirect system.
That's wasteful.
It varies from day to day.
Quite.
Mary
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16 years ago
DIY vacuum solar panel
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16 years ago
On a day like today, which is probably a fairly average day - bright but not much sun, temp. about 10 - what's the equilibrium temperature of the panel?
Mary,
The PV pump will only mean I don't waste electricity running the pump. It won't help if the panel is doing no heating.
Andy
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16 years ago
Ah I see. But how long before the finned coils scale up?
Gotcha. It'd work for me, I have a full height airing cupboard, and it's a better than 2 cylinders.
I'm still far from convinced it's cost effective at current prices. (And the best thing I could do for the environment would be in transport, not heating)
Andy
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16 years ago
If the panel isn't heating the p[u,p won't be running, I don't understand your point.
Mary
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16 years ago
Do both. And more.
Mary
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16 years ago
No mystery there Peter, the Nuclear Fusion Device was our nearest star and provided the energy to grow plants and forests 100 million years ago. What's left of them now is a collection useful fuels, one of which is eminently suitable to be burnt in a 30Kw boiler to do the washing up for three people via a 120 litre hot water cylinder and leave enough hot water for a "Deep bath" the following morning.
Peter, those two "Don't do numbers".
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16 years ago
Fecj knows I haven't got one and it would vary from system to system as pipe lengths insulation levels etc would all be different.
BTW air temp is not particulary relevant apart for affecting the loss rate. Solar panels can produce useful energy is sub zero temperatures, they absorb solar radiation not heat from the air.
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16 years ago
If I have hot water in my cylinder, and there's not much sun, I may lose heat by pumping the hot water out onto the roof.
Andy
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16 years ago
It is, but the delta T varies hugely according to stagnation temp, and its stagnation temp thats the Big Issue with flat panels. With a flat panel, you can get 80% or more efficiency with a cold cylinder, but as the cylinder warms up the efficiency always falls gradually... all the way to zero. When cylinder temp = stagnation temp, efficiency is 0%, and for flat plate panels:
- in bright summer sun stagnation temp roughly = target hw temp
- in winter, stagnation temp is far below target hw temp
This is why absolute temp delivering ability, or stagnation temp, is so important.
If stagnation temp were a non-issue, no-one would use vac tubes, since they perform worse than flat plates in every other respect.
NT
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16 years ago
On Sat, 01 Mar 2008 23:57:50 +0000 someone who may be Andy Champ wrote this:-
The answer to this question for the Solartwin has already been given several times
"Q: During the winter months when most of the hot water will be generated by the boiler, what happens if there is enough sun to set the pump in action. At which time there is not enough thermal energy to heat up the water sufficiently so that when it is returned to the hot water tank it reduces the temperature there?
"A: The pump runs only on solar energy in the form of electricity. It has no temperature sensor, only a high pressure bypass in case the panel or its pipes are frozen. In response to your question: First - best not to have the hot water system on all day since this is wasteful anyway and does not allow for optimum solar performance. Most boilers have separate timers for this, but not all. Ideally time the boiler to add heat to the domestic hot water after 4pm. Second, even in winter some hot water is made by Solartwin, not all by the boiler as you say. Third - the panel is well insulated and so will still raise the temperature of water going into it since it collects heat from the sun and not the air. Fourth - at 100% sun and a water input temperature of 50C and air temperature of freezing our mathematical model (based on extensive tests at Napier University) suggests that the water will still leave the panel at least 10C hotter than when it went in. Fifth - if they really want to put cold water in under these circumstances they can connect a second cylinder behind the first and draw water off it! This will also allow for more summer hot storage and is a neat solution for people with AGAs and Rayburns."
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16 years ago
On Sat, 01 Mar 2008 23:40:38 +0000 (GMT) someone who may be "Dave Liquorice" wrote this:-
It also depends on the control system. In systems with more complicated controls than the Solartwin nothing would happen until the panel has been warmed up to a suitable temperature above the storage. Once the pump has been started, what happens next depends on the sun and the control system.
People in this newsgroup would argue against such a statement if a few of us made it. Let's see what they do with you having made it.
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16 years ago
On Sat, 01 Mar 2008 16:05:39 +0000 someone who may be Andy Champ wrote this:-
That depends on your local water quality. If this is bad then there is a lot to be said for a thermal store with plate heat exchangers (AKA a heat bank). The latter are the things which will scale up and they can be replaced without replacing the store.
In terms of simple payback period at current fuel priced it is a long term investment. However, compared to other things people do, for example a new kitchen or new double glazing....
As Mary has said, you could do things in both areas. You could do things in other areas too.
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16 years ago
What's "stagnation temp"?
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16 years ago
Why should you do that?
Mary
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16 years ago
To stop it freezing?
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16 years ago
Can you explain "stagnation temp", all I get in my mind is static pool of black smelly water, which doesn't fit with a circulating system with an energy input.
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16 years ago
On Sun, 02 Mar 2008 11:16:46 +0000 someone who may be The Natural Philosopher wrote this:-
Why?
The Solartwin heats the domestic hot water directly. If it freezes then it freezes, the panel tolerates this without problems. When the sun comes out next it will soon unfreeze the panel.
With systems which heat the domestic hot water indirectly the solar circuit can be provided with a suitable amount of anti-freeze. There are also drain-down systems. Even if people haven't done either of these, the pump will generally run for not many minutes even on the coldest nights.
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16 years ago
The piping isn't affected by freezing, it's a flexible silicone material, perhaps the poster you're replying to didn't understand that.
But I still don't understand how hot water could be pumped onto the roof ...
Mary
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16 years ago
IOW in some cases yes, you will lose heat. However what you gain far outweighs what you lose annually
NT
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16 years ago
Stagnation temp is the temp the panel will eventually reach with no circulation. Flat panels stagnate at around 60C or a bit above, so by the time your HW has reached 60 the panels are running at almost zero efficiency. And in winter when stagnation temp might only be 35 or 40C, youre getting little thats any use from them. Stagnation temp is the big issue with flat panels.
This is why flat panel performance can be greatly improved if you can add a little concentration, eg by reflectors. This raises stagnation temp, and shifts panel operation into significantly improved efficiency. Hence on top of extensions can be a good place to site such panels, as one can then use wall above and roof under to reflect a bit more sun onto the panel.
NT