Crazy? mirror scheme

[ericw]

I have succumbed to PV greed and have removed my 20 tube thermal system from the roof.

I'm now trying to find a simple way to re-use it. The only place I can easily put it without breaking planning rules or getting displeasure from SWMBO is to mount it vertically on a south-ish facing wall.

This position has a flat roof a few feet below it, so the idea is to have a roughly horizontal trackable mirror on this roof to reflect additional sunlight onto the tubes to try and regain some of the lost output.

Is it a crackpot scheme?

[Baz]

you live in a clean air bird free zone?

[rondurrans]

mirrors - solar thermal.......I'm sure there was a thread on here about this some time ago............

[desperate]

Makes a lot of sense to me, any half decent mirror will reflect fully 90% of the radiation hitting it even with a good layer of dust and bird splatter on it. The tricky bit will be a drive system to track the sun, but I bet there are quite a lot of old telescope equatorial mounts kicking about in peoples garages, that could be amortized (ok bodged) into usefullness. I guess you would need to run the drive at half sidereal rate to keep the beam on the panel, but a man of your calibre (should have been a human cannonball)........err I mean should be able to diddle the electrickery.
Another thought, try the local glass shop for busted mirrors, no reason for it to be all in one piece.

Desp

PS Eric, google up "Poncet platform" for lots more inspiration

[charlieb]

Eric, you've just proved what I thought before that the government's doing exactly the right thing by lowering the FIT rate for PV. (I appreciate you aknowledge this in your post, so I'm not saying anything new).
Has anyone done the sums on carbon savings from a m2 of solar thermal cf a m2 of PV?  It should be a pretty simple sum, as leccy is roughly 3-4x more carbon intensive than heat (broadly because they're both using gas but thermal generation is only about 30-40% efficient).

Great idea with the mirror, though. Can't do any harm and I can imagine it doing plenty of good. Why bother tracking though? Easier to just make the mirror huge and light up the eaves - would keep the sun off the flat roof which'd probably be good for the felt.  You can get huge perspex-backed mirror sheets cheapish online - my Mum recently got one for her studio.

[billi]

Has anyone done the sums on carbon savings from a m2 of solar thermal cf a m2 of PV?  It should be a pretty simple sum, as leccy is roughly 3-4x more carbon intensive than heat (broadly because they're both using gas but thermal generation is only about 30-40% efficient).

Have only found out  a few years back  (when PV was dear)  that i am better off  with a heatpump and PV compared to solar thermal , but that is just me off grid   and no FIT  and liking ST very much as well ,
 for a simple reason  i did this calculation  , cause on an very overcast Day   all the Solar thermal  installs   did stand still , while the PV was producing some watts  that could power a heatpump to transfer usfull heat to the house

But other way round could work as well  , ie  solar thermal as well produces some watts , but these watts are not allowed into the thermal store  cause not warm enough ,  so a heatpump could use those watts  as well

Perhaps  too much full moon  in my case     

Billi

[billt]

Unfortunately the sum isn't simple at all. PV output is proportional to incident radiation, solar thermal isn't. Evacuated tubes need a certain amount of incident radiation to make them work at all and then the energy extracted depends to some extent on the water temperature that is being heated.

A grid tied PV installation allows all the collected energy to be used. A significant amount of the solar thermal energy is likely to be wasted due to the impossibility of stopping heat loss from the storage and distribution system.

I log the output of both my PV and solar thermal systems. Since the middle of June, when the PV started, the solar thermal system has been producing about 1.7 times the energy of the PV system per square meter, but I'd guess that the actual usable output would be about equal. That implies that PV is considerably better at reduce emissions - disreagrding all the practical complications of a mixed source grid!

[skyewright]

Great idea with the mirror, though. Can't do any harm and I can imagine it doing plenty of good. Why bother tracking though? Easier to just make the mirror huge and light up the eaves

I'm pretty sure I've seen comments on BuildItSolar about systems that are vertical or near vertical seeing increased production when there is snow on the ground acting as a reflector(and sunshine too, of course)?

[dhaslam]

Where PV and  heat pumps lose out against solar thermal  is  in long term cost.    Because solar thermal is  fairly simple it  will run for a long period, just needing a pump eventually and possibly a replacement controller.  Heat pumps are very prone to gas leaks and expensive to repair.  Also the electrical side is  more complex and  likely to fail.   

[knighty]

i wouldn't bother with tracking... just lie some big mirrors on the flat roof below the panels.... maybe at a small angle so the water drains off them and the rain washes them?

previous threads about mirrors used very small mirrors and strange angles are reported a massive increase...


if the mirrors are big, it could make quite a difference.... mirrors are cheap enough to try it

[charlieb]

Thanks Billt. Just what I was after. Will re-think my rethink, to some extent.

[book_woorm]

Three thoughts for you

Firstly, a route that I'm about to embark on is a double tank system for solar thermal consisting of a pre heat tank and a main thermal store. The panels primarily feed the mains store as normal BUT when the panels are cooler than the store instead of shutting down the flow switches to the pre heat tank. Now the main tank gets up to say 70 and the pre heat gets up to 40 when you would not be taking anything from the panels. Your DHW comes into the property at say 10DegC and passes through both tanks. When you draw a sink or bath of water it passes first through the pre heat tank and then the main one, thus the heat comes half and half from each tank (provided thay are the same size  and have the same size heat exchangers). The SonnenKraft/resol controllers are capable of controling two tanks and or 2 sets of panels as standard. Yes you have the cost of a second store and a high temprature diverter valve but mu sums show that I should get 95% of the hot water requirements of a 5 bed/2bath/3 shower room house from this as opposed the the 40% that a normal system might give you.

Secondly: The reflection off of a tank of water infront of your proposed solar thermal position might have enough reflection. Ive seen this done somewhere before as an 'architectural feature' to get more light through a big glass wall thus getting more heat into the building in question.

Thirdly: I've a masive spreadsheet 10+Mbyte that will calculate the power take for PV and solar thermal for any azimuth and elevation of panel that uses 10 years worth of real Met office Insolation data, as opposed the a lot of the theroretical satilite interpreted data that is around. Let me know if if you want to play with it to compare what your tubes are getting now with what the might get in the new vertical position.

[StBarnabas]

Eric
mirror systems can work if designed properly, but if they are they would need to be large. Here's one I used to work on during my PhD



 I suspect you are thinking on a smaller scale?

[michiel]

I have 100x Navitron ET's on a flat roof.
These were initially fitted with the supplied tin reflective strips between them, which made sense.
After last winter, with tons of snow on and sliding down the tubes, half of these strips had come loose, bent, or blown away.
I wonder now if indeed a large mirror, angled parallel to the tubes, fixed to the roof, under the tubes would benefit??

Do mirrors reflect the correct wavelenght for the ET's?

[desperate]

Yes.Mirrors reflect pretty much all wavelengths of interest.

Desp