Solar Panels - Vacuum Vs Flat Panels? Which do you prefer?

I work with both and have no axe to grind but the following are what I see as the deciding factors;

Efficiency - On a hot sunny day, flatplates are marginally more efficient than tubes. On cloudy, cold or windy days, tubes will outperform flatplates because of their inherant insulation. To be honest, on hot sunny days, both systems will probably give you more hot water than you need, so tubes are more useful by working on the marginal days. Tubes also extend the working season quite a bit.

If your house is very well insulated and your central heating is only running for a few months, that would push you towards tubes. If the central heating is off, your hot water becomes more expensive. Thus tubes, by extending the season, will save you more money.

Aesthetics - Flatplates are less conspicuous. Some people just don't like the sci-fi look of tubes.

Costs - usually as you say properly installed flatplates are more expensive. However, if you are putting them in at the time of slating, it is less costly. Retrofitting flatplates is more expensive because you have to rip out slates etc.

In roof -vs- On roof - This is an aside to your question, but I would avoid on-roof installation. The wind loading on a surface mounted flatplate is unpredictable and there is a lot more buffeting. On roof is often an easier option for retrofit but I have seen it done on new roofs which I personally think is plain daft. Tubes are always on-roof, but that is OK because they don't suffer the sort of wind load that flatplates do.

Durability - Flatplates have a life expectancy of 40 to 45 years, Tubes will need to be replaced after about 20 years as they will lose their vacuum.

In relation to this, there are two very different types of tube - flasks where there is a twin-wall vacuum flask, usually made in China. This is as efficient as other tubes, but the real benefit is that the tubes are far cheaper and easier to replace. Replacement flasks are in standard sizes from a huge number of companies, and in bulk should cost about €5 each. Tubes are a single layer of glass with the heat pipe and innards welded in. Replacing these is far more expensive - about the same price as replacing the whole panel.

Tubes or flasks are more easily broken by stray footballs etc., though to be honest, on the roof this doesn't happen as much as you might think it would. Again, replacing the tubes is cheap enough if it is a bungalow, but if you have to put up a tower etc., to get up there, and you have football nuts in the family, it may be a factor. Of course if a slitter goes through a flatplate, that'll cost yer!

Working Temperatures - If you are on holidays, or the cylinder is up to temperature, you may need a heat dump circuit to get rid of surplus heat. Without one, the system goes into "stagnation" - the pump just shuts down and the water in the panel boils into the expansion vessel. I always recommend a heat dump circuit in either flatplates or tubes. Most installers don't bother with this. However in relation to your question, normally the peak temperature for a flatplate is about 190 degrees whereas the peak temperature for a tube system is a lot higher. The pipe insulation in close proximity to the panel is usually only rated to 160 so you will get damage if the system shuts down regularly, more so in a vacuum system. As I say, I recommend a heat dump every time because either way, stagnation damages the anti-freeze and shortens the life of panel and pipework insulation, but it is more of an imperative in a vacuum system.

That's my tu'pence worth... Q

selfbuildache wrote: »

How do we know we're getting the cheaper option?

There are two fairly standard generic size in China. One is 47mm outside diameter and 1.5m long, and the other is 58mm outside diameter and 1.8m (or sometimes 1.5m or 2.1m long).

Any of these should be available as and when needed and at reasonable prices. The flask just slides off the heat pipe or U tube or whatever heat extraction system is used, and the new one can be slid up in its place. Q