Where/when can the discussion being followed up below be found? There seems to be nothing whatever in the ALUG 'main' archives with "solar" in the Subject, all the way back to Jan 2009! (Apart from the one below, of course).
Interesting topic. Ted.
On 20-Feb-10 02:36:29, Wayne Stallwood wrote:
Jenny Hopkins wrote:
Confused about solar. Tony reckons the inox have mirrors on the outside that need servicing, but Wayne showed me that the solaressence ones trhat he has have a newer version that have them on the inside.
Reflector details vary, but it's all terribly minor details. All you should care about is overall cost vs. power output. Is tony suggesting that you need to get up on the roof every 10 years and wipe down the reflectors? Probably a good idea. Mine are still v.shiny after 2 yrs. A wipe in another few years might be an idea.
Internal reflectors must be smaller, by definition, so collect less sun. Refelectors of any design make no more than 10% difference to output. At most sun angles they are hidden behind the tubes.
Is this from wookey ? I agree with everything he is saying there..the reflectors help with severely off axis sunlight and try and bounce some back onto the tubes...this helps me a tiny bit I think in the afternoons during the autum/spring where the sun's angle is such that as of about 3PM it is hitting the tubes more from the side than above. Yes my CPC mirrors are internal to the tubes and therefore smaller and less efficient than the ones that sit under the tubes with the advantage that they can't get dirty...but I can't imagine external CPC mirrors need frequent cleaning as has been claimed..and the tubes themselves will only get mucky at the same rate as a flat panel.
The flat panel systems were shown to be the basis of less efficient systems (when a whole years savings were taken into consideration) in a DTI report done in 2001...since then Evacuated Tube panels have only improved (and as far as I am aware there have been no further developments in flat panel tech)
As I understand it the issue with panels is two-fold...one there is a big surface area to radiate collected heat before it can be drawn down to the tank..so on cold but sunny days you are losing some potential.
Also as soon as the light output is a bit too far off axis you get significant reflection losses from the flat collection area..whereas naturally with a tube, part of the surface is on axis at all time sun is hitting that area of roof. Generally everything I have read says that in a climate that can be cold but sunny or where there is frequent cloud cover you will see better results from a ET setup..which is why most modern installations in this country seem to be using that technology. This becomes even more important if you are considering a dual setup where it is going to be assisting heating as well as hot water as of course you want whatever you can get from the panel on the colder days.
The other issue with FP collectors as I mentioned on IRC is that although FP technology is now 30 years old and well proven..if you do get a problem with a panel then the only option is to replace the whole unit which is both expensive and cumbersome..requiring a system drain and re-pressurisation afterwards (and probably lifting tackle/roof platforms if the repair was going to be done with health and safety in mind as they are very heavy). My tubes are indirect and can be replaced cheaply, individually, and without touching the solar circuit. Also a single damaged tube will not compromise the operation of the rest of that panel. The one downside here is that ET panels are relatively new technology and haven't been on anyones roof for more than 10 years or so..so long term life predictions are just that..predictions. Whereas I know there are people around with 25 year old flat panels that are still working.
There are two (admittedly looking quite old) FP installations near me and both have a damaged panel now..one is just clouded so I assume a weather seal has gone somewhere and the other has what looks like it might have been some nutcase with a catapult because the FP is visibly cracked. Both have remained like that for at least a year, presumably because repair would be expensive. A single tube for my system costs about the same as a meal out somewhere and I could change it myself.
All that said, It's one of those things where there is a lot of information out there..a lot of it is conflicting and each installer will have their own opinions, it's like buying a car or something. So maybe you *are* getting too wrapped up in all these technical details. The end story is as Wookey said cost vs power and remember you are tapping a free energy source so some losses here and there aren't the end of the world. As long as the system overall is making a net saving and the cost vs power ratio means it stands a chance of paying for itself at some point in the future then don't loose too much sleep.
Ta Wayne
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