Solar Heating - Some Main Questions

[John Nolan]

Hello All,

These questions apply, generally, and are not just for DIY.

There seem to be 2 main types - Flat panel and Tubular.

1. What are the Pros and Cons for the 2 types ?

1. Has either an advantage in terms of a. Effective power for a given roof area ? b. Effective power for a given active area ? c. Cost of panel for a given power.

I am thinking about the fact that some tube systems have considerable gaps between tubes. Evidently, this is to reduce likelihood of lift off in high winds, but obviously useful roof area is lost.

1. It seems that there are 3 different methods of mounting. a. In roof - mainly for flat panels. b. On a roof mount, fixed to rafters, and possibly standing off the roof. c. Slung from straps, passing under slates or tiles and fixed to rafters. Panels resting on roof.

It would appear that the Pros and Cons for these 3 are :-

Cost Danger of Wind lift Variable Angle 3.a. Highest Least Fixed 3.b Intermediate Perhaps highest Variable but more secure than 3.b 3.c Lowest Perhaps intermediate Fixed

If any of you have experience, or know some good websites where I can obtain such information, I should be grateful.

John N.

[David Hansen]

On Wed, 22 Apr 2009 18:08:26 BST someone who may be John Nolan wrote this:-

It may be an advantage, but is not the reason for doing it. Anyway, the space between the tubes is often taken up with reflectors, so the wind loading is similar to a flat panel.

One of the advantages of vacuum tubes is that to some extent they track the sun as it moves around the sky. Closing the gaps would reduce this.

You left out mounting on the wall, which can be either done vertically or at an angle using some sort of strut to make the bottom of the panel stand out further from the wall.

[adrian]

Flat panel is cheaper, tubular is more efficient (warms up faster, gives higher temperature, can be self-limiting if system 'stalls')

Way too difficult a question! - see simple answer above

More likely to be mechanical constraints of the manifold that connects the hot end of the tubes ..? - and the need for space between the tubes so you can hold safely them while fitting the tubes into the manifolds...

Don't lose sleep over it - manufacturers have designed their mounting systems so that panles don't blow away - the choice will depend on your own situation (e.g. 'built-in' is only really cost-effective in a new-build or roof-replacement)

CAT is worth a look

as they're not actually trying to sell you a system, they shouldn't be giving you any hype

And before anybody chimes in to say 'waste of time' - we had a solar tube system at the last house and it covered the majority of our dhw needs from May to September.... sadly we didn't stay there long enough to get the payback - but it's great to have a shower or bath in solar-heated water

Adrian

[Andy Dingley]

Go read the Navitron website.

Several sub-divisions of tubular too. Flowing water and heat pipe being the main ones.

Tubular heat pipes win hands-down (as they're now affordable) _except_ if you have intermittent cloud and a really high-end flat-plate collector. This can rise to operating temperature more quickly than a heat pipe tube (in most cases, but only for the best of flat-plates).

These aren't the most important question.

What really matters (like wind power), isn't the panel's ultimate performance in ideal conditions but rather how badly it falls off in non-ideal conditions and thus how well it performs over an average year. Remember that the "Spanish laundry" is the easy problem, we're looking at how to save money through a UK winter heating period and that's about coping with poor sun.

A second question, nearly but not quite as important, is the outlet temperature. This is why heat pipe tubes are now doing so well.

IMHO, the best UK solar system is evacuated heatpipe tubes running a low-temperature underfloor heating system, with some degree of heat storage. This isn't the best performing system, but it's the one with the best payback against capital investment, and also reasonable performance. Space-heating with radiators works too, but underfloor is a better fit to available temperature and availability.

Note that I exclude water heating and would just go for space heating. Water heating is hard to do, needs a higher temperature, and there just isn't as much fuel spending on it to make a saving against. You _can_ do it, but I don't think you'll see a good payback for it. works

It isn't "lost", it's traded for a wider acceptance angle. Anything works OK at noon, it's getting useful coverage through the day that fills out the efficiency curve.

[meow2222]

?title=Solar_Thermal_Collector_PlacementNT

[John Nolan]

Thank you all for your replies, so far :-)

I will digest them all and act.

In article , Andy Dingley wrote: om>IMHO, the best UK solar system is evacuated heatpipe tubes running a om>low-temperature underfloor heating system, with some degree of heat om>storage. This isn't the best performing system, but it's the one with om>the best payback against capital investment, and also reasonable om>performance. Space-heating with radiators works too, but underfloor is om>a better fit to available temperature and availability.

Thank you for that. Yes, we are going for underfloor heating, also, possibly with a heat pump. It is all going to be expensive and we are using a substantial part of our savings. But I don't want to be over a Russian barrel. After the credit crunch, I'm sure energy prices will saor, also inflation, since the Bank of E. if printing money. I hope I'm wrong.

I am, actually, a design engineer. We have a very large water store and hope to use as much off peak electircity as poss.

om>

om>Note that I exclude water heating and would just go for space heating. om>Water heating is hard to do, needs a higher temperature, and there om>just isn't as much fuel spending on it to make a saving against. You om>_can_ do it, but I don't think you'll see a good payback for it. om>works

I'm surprised you say that. In the Summer, solar hot water saves intermittent use of boilers or immersion heaters. I have done a Net Present Value on guessed future fuel bills and I guess payback may well be shorter than many of us think.

For your interest, I have the possibility of buying several panels each consisting of 10 tubes 1.5 m. x 47 mm. for 190 uk pounds each, per panel. These are in the Chepstow area. I wanted to see whether or not this sounds like very good value. They have a 10 years guarantee.

I calculated that, this works out at 270 uk pounds per sq. m. of active area. I'm trying to work out how this compares with other tubes. It is hard work to get information on sizes. And, anyway, I imagine that there are differences in perfromance per active area, even among tubes.

John N.

[meow2222]

If the flat panel is well insulated, eg double glazed, then in summer it will beat the tubes easily. But in winter it wont. But most output hapens in summer. But more can be used in winter. :)

tubes, no contest. Tubes are both concentrated and vacuum insulated.

flat panel's much cheaper, but its not very hot in winter.

There are many factors, its not simple. If I were building a HW system today I'd opt for heavily featured panels. Add reflectors to give some concentration, and add either 2ndary glazing, maybe in plastic film, or an external bubble that collects warm air as well as acting as

2ndary insulation. These features change a low cost flat panel's characteristics to be closer to those of tubes by raising the stagnation temp.

You can hybridise the 2 too. A flat panel type outer casing, but inside it runs black pipes with shallow trough reflectors, somewhat like a 2'x2' office ceiling light. The plus of these is mostly tube- like performance at much lower cost. And lower cost means better ROI. It also means you can have more active area for less cost, giving more energy return too.

But at the end of the day, if youre looking at winter space heat I'd generally use heated air panels, not massively increase the cost by using a wet system.

NT