DIY vacuum solar panel

[meow2222]

or go double glazed with T8 inside T12 and no vacuum. You get more protection against breakage that way, as one break still leaves the system insulated.

NT

[meow2222]

I'm not sure where you get that from. Flat panels harvest much more in full sun, whether youre comparing per area or per pound spent. Tubes only absorb over a percentage of their area, whereas flat panels absorb over nearly all the area. And comparing per pound spent there's no contest. The advantage of tubes is in extending the season by concentrating and minimising losses. Panels collect more, but tubes produce (less hot water) at higher temp, and its that higher temp thats valuable.

Surely a DIY panel is a panel built specially for the purpose, and which yields more depends entirely on the design used. If one were to generalise, DIY systems give much more percentage payback on average than commercial.

NT

[David Hansen]

On Thu, 28 Feb 2008 05:10:59 -0800 (PST) someone who may be [e-mail] wrote this:-

It's common knowledge, which people challenge at their peril.

Note in particular that reflectors can be placed in the gaps between tubes.

You may have the last word.

[Dave Liquorice]

Surely the important measurement is the delta T across the panel not the absolute T of the outflow?

It is that delta T which measures the amount of energy transfered to the circulating fluid and thus into the storage (less losses).

[MM]

Nevertheless, it would be an interesting exercise to do the comparison.

MM

[Bob Mannix]

Well, not that interesting! From the Honda published figures, a 3000W Honda generator (rated load 2800W) will run for 7.2h on a full tank at the rated load (most efficient). This equates to 20kWh. The tank holds 3.4gallons, or ~15l. At a conservative £1.00/l, this would cost £15, making the electricity generated 75p/kWh. Current peak rate electrcity prices are about 14p/kWh (cheap rate much less).

Petrol would have to stay the same price and electricity rise about 5-fold before it got interesting, and those aint going to happen together!

[Mark]

Now try it again with a Diesel gen

Red is currently 32p/litre for 2000litres

Untill the next Budget, when the EU shaft us again.

-

[Dave Liquorice]

Where, it'd almost be worth the drive to get some!

Kerosene is >40p/l in that sort of quantity and that doesn't include any duty. Red has 9.69p/l of duty on it, so I'd expect a price over 50p/l. Yep:

Recent bulk red prices of the last few days at the bottom of the page.

Duty on red goes up to 10.07p/l in 1st April 2008, 0.38p is hardly shafting. Road diesel goes up by 2p on the 1st April 2008 to 52.35p/l but I think these rises are down to our government not the EU.

[Andy Champ]

We really do need heat, not just energy.

10,000 litres of water per day at 10c above ambient ain't a whole lot of use for your bath. 200 litres at 50C above is 1% of the energy, but a whole lot more useful.

It occurs to me that using a solar system as a pre-heater, and fossil to bring it up to useful heat, might be a good compromise. Except of course you need two tanks...

Andy

[Dave Liquorice]

But the tank is colder than the panel outflow thus the tank becomes warmer. The flow to the panel also warms with the tank but it still has X amount of energy added to it thus still returns to the tank with that delta T, warming the tank further and so on and so on up to the point where increased losses, due to everything being hotter, balance the gain from the panel or things boil and/or vent...

But that isn't 10,000l as one lump it's a flow of lump of say 100l around the system 100 times each time dumping its delta T of energy into the 200l of storage. So assuming you can only transfer 50% of the delta T energy into the store and using the above volumes and a 10C delta T. One 100l cycle will raise the 200l store by 2.5C, say the store starts at 10C then

16 cycles later the store is at 50C. In theory, much simplified.

In this country you'll still need a conventional boiler but the amount that fires can be significantly reduced.

[Mark]

Eek that was the price I paid at the beginning of January

You're not a Boat owner then

-

[David Hansen]

On Thu, 28 Feb 2008 21:27:34 +0000 someone who may be Andy Champ wrote this:-

That was the thinking in Ye Olden Days of the 1970s. However, things have moved on somewhat. Twin coil cylinders and thermal stores can be heated by a variety of sources and, provided the controls are set up properly, not clash.

[Mary Fisher]

...

Why?

Mary

[Andy Champ]

Doesn't it get a bit less efficient (possibly 0%!) when you're pumping water at 47.5 into a panel on the roof?

Andy

[Andy Champ]

Twin coil? So there are two indirect coils? Doesn't that mean there isn't much room for hot water above the boiler fed (presumably hotter) coil?

Andy

[Andy Champ]

Where do you put the pre-heated water without another tank?

Care of Mr Legionella, we're advised to keep the hot over 60. I don't want to pump the 60 degree water onto my cold roof (OK warm, but not

*that* warm) so I use the roof panel to feed a preheating tank which on a good day will be up to... ISTR you said 44 a few weeks ago? ... so the fossil only has to heat from 44 to 60, a *big* saving over the normal 10-60 requirement.

Andy

[Dave Liquorice]

The losses are higher but you still get the same delta T (more or less).

Assuming the energy input to the panel and the flow through is constant then it doesn't matter what the inlet temp of the flow is. The water still collects the energy and thus the same delta T. True, the whole thing gets hotter so losses rise which ultimately limit the maximum temp that a given energy input can acheive.

[David Hansen]

On Fri, 29 Feb 2008 21:17:32 +0000 someone who may be Andy Champ wrote this:-

Two or three coils, depending on the number of heat sources. More than three becomes interesting.

gives some (external) photographs and a drawing.

If sized correctly there is enough room above the boiler coil.

This arrangement also has the advantage that solar can heat the whole cylinder and thus store more heat.

[David Hansen]

On Fri, 29 Feb 2008 21:20:45 +0000 someone who may be Andy Champ wrote this:-

In the case of the Solartwin system, water is pumped from the bottom of the cylinder where it is not at 60C.

[Mary Fisher]

Not if you have a pv powered pump.

Mary