OK, so the topic is Trombe Walls not whether and how I should insulate externally. Lucky chap that I am I have a three story house in Italy with 3m ceilings, and balconies right around the SW and SE wall, large windows and doors on each wall and floor - not so lucky in that it needs a lot of work. The top 2 stories are solid stone 300mm thick 'Tuffa' (much softer than sandstone and full of tiny air holes but heavier than pumice) the bottom storey is more like 400-450 mm thick made of some hard stone like granite and is faced with rubble fill. Given that I decide not to insulate externally, I was looking at Trombe Walls - that is building a thin wood frame covering it with whatever is best (that isn't glass) such that there is a 20mm gap - and hey presto one Trombe wall, and I get a lot of winter sun it is just that it is really coooold. OK so I could look at holes top and bottom, solar powered fans, obviously making sure the sections are placed correctly to catch the most sun. But, and at last here's the point, with external shade temps in the 30s in the summer I do not need any more heat in the summer!!!! Also the walls have to be painted black or have foil stuck to it under the frames. So, since I would have access to the frames, what about making them removable. The problem is that the wall is then unacceptably ugly (which has 'consequences' in Italy). I thought about water soluble paint, but you just know it would stain. What about soluble glue and standard kitchen foil, sounds a right pain! What about fine chicken wire type mesh holding kitchen foil tight? Is there enough contact?

There is plenty of stuff on building Trombe walls, holes, dark surfaces, etc but nothing on the advisability, system for achieving or effectiveness of temporary Trombe walls. Any suggestions? Here is a very short article on a DIY Trombe Wall for the uninitiated:
http://www.builditsolar.com/Projects/SpaceHeating/nicksthrombe.htm

Oh and if anyone can translate the 'easy' sum the professor did so that I could understand it, see the relevance and do it on my house that would be very helpful! Very many thanks.

Yes, thanks Nick, I have read about Trombe walls and solar chimneys but my situation is a retro-fit, so pipes etc could be a significant issue and outside air temps are 5-10 degrees above my indoor temp normally but less sometimes, I know all about cooling rather than cooler air because otherwise we couldn't sleep at night (fan!) but it is contra-intuitive for me to brings hotter air into the house during the day - so maybe I need to do some more research and try and get my head around it. Oh - and I have a 11 year old, 2 dogs, 2 cats and a wife who are all more disciplined than I so if it meant some doors and windows open and some shut...no chance.

Welshboy, yes, my ''making it operate as a solar chimney in summer'' is exactly that - open low-down internal vents through the masonry wall, open vent in top of TTW and you've got a sun-powered through-flow.

Nick - at last, found it - the answer to your question ref foil, you know how it is - you read the 'what' and understand the 'why', at the time, but later you just remember the 'what' and can't find the why!!! The extract below is from here:
http://www.nrel.gov/docs/fy04osti/36277.pdf
"Applying a selective surface to a Trombe wall improves its performance by reducing the
amount of infrared energy radiated back through the glass. The selective surface consists
of a sheet of metal foil glued to the outside surface of the wall. It absorbs almost all the
radiation in the visible portion of the solar spectrum and emits very little in the infrared
range. High absorbency turns the light into heat at the wall's surface, and low emittance
prevents the heat from radiating back towards the glass."

Now then, where I am at is feeling jolly committed to seriously large sections of (removable) Trombe wall both on my SE and SW walls - I'll post a photo of the outside of the house so everyone can get an idea what I am on about. Now I need to do some expectation management - for my and my wife! Firstly, I like KISS lots so can anyone give me some idea of how important colour is. For example solar panels are best due south and at a particular angle but 90% as good around a huge variety of tilts and directions. Is it possible that whilst black is best, a light green or dark sand colour is actually not that bad. Not that bad in that I need to see some figures! If 'not bad' then a removable Trombe wall is just a wooden frame holding a bit of plastic slipped over a flat hook to hold it against the wall.

And I have considered hard the solar chimney idea but have found several references that say, despite such effects, Trombe walls effectiveness (cost saving) have to include the additional costs of summer cooling and Ii do not have effective shading of the walls in the summer so they already radiate lots of heat in the evenings. So first choice - a compromise colour.

Second choice is the foil thing, but if I understand the techy stuff right, the effect will be very sensitive to any gap between the foil and the wall so can some tight fine gauge chicken wire do the job? What about a spray of that temporary photo-mount fixer that comes in cans. Surely little old me is not going to be ground breaking here. Any suggestions/encouragement welcome!

Finally, the numbers, in my initial post was a link to some Mad Professor 'simple' looking calculations. I am just not up to understanding them, and whilst I am just naive enough to trust a mad professor I would be happier if I could extrapolate at least some ball park figures, otherwise perhaps i should accept the 'generally good feeling' I get from the MANY figures in the link in this post.

I hope someone can help, please

Gotanewlife: As I understand it you are thinking of just having a passive panel in front of the wall with no airflow between the house and the panel. Is that right?

Have you considered that at night the effect of the glazing will be to only slightly reduce the wall's heat losses by radiation but to significantly reduce the wall's uptake of warmth from the air?

Firstly, thanks everyone for the inputs - this is not one of those dreamy what ifs but a real problem I really will be doing something about so I am grateful for all your help. And, if people want, I'll document/picture/datalog and report back.

Howdytom, I think that anything that has been used in a solar panel will work - so, no I don't think plastic will work.

EdDavis, yes and no - I am thinking about A LOT of wall something like 100m2 so holes in the walls for each or even many panels is not something I will undertake lightly. First, not ventilated, then, once I have proven the principle in practice, improve. When it comes to it I will want to understand whether I get 'more' heat with ventilation or just a more reactive effect, which might not be something I want - which might mean modelling which is way, way beyond me! Not sure I quite get where you're heading with "at night the effect of the glazing will be to only slightly reduce the wall's heat losses by radiation but to significantly reduce the wall's uptake of warmth from the air", what warmth in the air at night? The air is cold outside at night so no warmth to uptake and the air trapped between the glazing and the wall will cool almost immediately. What I am envisaging is that the wall will get really quite warm during the day and then radiate that heat inside and, sadly, outside - more inside than outside. And I envisage that even if that heat does not actually add much heat as such it will at least have the effect of turning the wall into a unit which does not leak heat from inside ie insulated - which is a weird way of looking at it but is the logic that brought me her in the first place.

Nick, there really isn't so much out there on Trombe walls is there, because I recognised all your quotes! But if you followed on e the links from Wilkipedia you get:
http://www.redrok.com/concept.htm#absorptivity - and I quote:
"Classically flat black paint, Parsons black, was used on solar collectors. While it is cheap to apply it is not necessarily the best material to use. The black paint has an absorption factor of .98. This means that 98% of the available solar energy is converted into heat in the collector. The collector gets hot. Some of the heat is emitted as radiation. In this case 98% of the possible black body energy is radiated. Look at it this way. If the emissivity factor is lower we get to keep more of our hard earned energy. The best we can do is to look for materials that have high absorption and low emissivity."

It then gives a table of absorption and emissivity and critically the ratio, here are 3 examples:

Solec Solkote Solar selective surface paint Absorb=0.88-0.94 Emissivity=0.28-0.49 RATIO 2.36
Snow, Fine particles fresh Absorb=0.13 Emissivity=0.82 RATIO 0.16
Pyramil Black on Beryllium Copper Absorb=0.92 Emissivity=0.72 RATIO 1.28

But for aluminium foil Absorb=0.15 Emissivity= 0.05 Ratio=3.00
To which is added the note: “Aluminium foil gets very hot because of this high ratio”.

So, it might not absorb much heat but it emits virtually none of what it does absorb.

So now we need someone good at physics. Is the light reflected from foil lost? Does the Ratio fo 3.0 for foil mean it is better for this application than black paint with a ratio of 1-1.3. What i know is that the air in the gap will get hot and, clearly, I now know that the foil will get very hot independantly of the hot air - might we then get good conduction of that heat to the wall? Anyway, it is not simple and I think that the NREL meant what they said. On the otherhand I don't see many foil covered solar panels out there!

Finally - the point of this thread is I need to make a removable Trombe wall, I am learning a lot but no Ureaka moment yet!

A constraining factor to your proposed solution is the thickness of the wall. I think I read somewhere about 10 inches being an optimum. Heat travels through the wall at approximately an inch an hour so thick walls are an obvious no no.
I suspect you have 2ft thick walls or more.

I haven't worked out how to quote nicely yet so.

Ed Davies, you said "OK, not really a Trombe wall then"...er no actually it is exactly a Trombe wall. As said in my first post "Here is a very short article on a DIY Trombe Wall for the uninitiated:
http://www.builditsolar.com/Projects/SpaceHeating/nicksthrombe.htm" but if you don't consider that rigorous enough then have a quick browse through: http://www.nrel.gov/docs/fy04osti/36277.pdf , which I also cited; I am sure it will persuade you.

You also said "I think it's better to have a low thermal mass collector then store the heat produced somewhere ", isn't that called a solar panel?

You also said "By trapping a layer of air against a part of your wall I think you will increase the rate at which it cools", Ed I guess you just have not quite understood the principles here, try swapping the word 'body' with the word 'wall' in your comment..........and imagine lots of people trying to keep warm by wandering around nude.

Welshboy, you are correct about the speed that heat travels through stone. As I mentioned, I will not be using my (thicker) ground floor wall. My 30cm wall is the perfect depth. (though it would ideally be made from a denser stone).

To make my wall REMOVABLE I need to either accept a lower efficiency through not having a black wall, in which case I need to know how much energy I am losing (ie I still get all the sun's infra-red (how much) but I lose a proportion of the visible light (how much?)) in order to decide if it is a sensible compromise OR I need a solution to make the concrete surface a high absorption/low emissivity that is cheap and REMOVABLE.

Posted By: GotanewlifeYou also said "I think it's better to have a low thermal mass collector then store the heat produced somewhere ", isn't that called a solar panel?

Yes, it's called a solar panel and I think it's better. Apart from the odd anecdote (e.g., the Brighton Earthship, noted above) I can't back this up with very specific data. However, there are a number of people who argue for low thermal mass solar collectors and they do seem to make a lot of sense to me. Probably the foremost of these is Laren Corie: see, for example, his article on page 9 of the January 2005 Energy Self-Sufficiency Newsletter:

http://www.rebelwolf.com/essn/ESSN-Jan2005.pdf

Ludite, wow what an idea! For the heat to transfer efficiently to the wall there would have to be good contact I think and of course the material would have to be low cost and have v poor insulating propeties - which set me thinking about HOWDYTOM's mylar (think 'foil' helium ballons - except it's plastic) sugestion above. Its strong, v heat resistant, low thermal mass, could be black on external side and silvered on inside to reflect all those infr-red heat waves. Hmmmmm. supplier for more advice methinks.

Meanwhile, EdDavies I guess you (like me) have not heard of the Barra system http://en.wikipedia.org/wiki/Barra_system, I think it is what you mean, given that mounting solar panels vertically over my whole wall would not be cost effective! But I don't like it for lots of reasons, asthetics, cost, size etc, nevertheless, it just shows other people have had the same thoughts as you. Anyway we have now had 3 days of brilliant sun, we ate lunch outside (yet 10 degrees actual air temp), and then as soon as the sun tuched the horizon we dashed indoors, shut all the doors and windows, pulled the heavy curtains, put the stuffa on and lit the fire!

Another, slightly separate point:

That Wikipedia Barra article makes reference to use of double or triple wall polycarbonate. The idea being, presumably, to reduce the heat loss.

I'm not convinced this is worthwhile as one advantage of a low thermal mass indirect heating collector is that the heat loss when the sun is not shining becomes pretty much irrelevant (as there's very little heat in the collector to be lost). Therefore it could well make sense to use single layer glazing as the extra heat loss during operation may be outweighed by the extra solar energy let in - particularly when the sun is not shining straight on (at 90°) to the surface.

A commonly used trick is put the absorber plate at an angle so that the cold air introduced at the bottom of the collector is in front of the absorber but gets warmed as it moves though the absorber and comes out the top of the collector from behind the absorber. This way most of the warmed air is kept out of direct contact with the glazing, thereby reducing heat losses.

Using a single layer of glazing could also contribute to making the collector lighter and thinner and therefore making it easier to demount for storage at times of year when it's not needed.

dear Ed Davies. I guess if you'd thought about it a while ago, It probably doesn't work in practise. I get so many ideas at once that I find it very hard to follow a single thought through to completion - apparently that's what happens to women when we have babies. . . . . . .

Tried what? Making babies

How about this???

Instead of a wall, a solid 'floor'. One you can either slide out or 'pivots', a bit like a revolving door. Like a moving turntable patio?????

The large thermal mass is pushed outside - somehow - during the day, where it soaks up the heat. It is then returned inside the house at night time, to radiate the heat back inside the thermal envelope of the building. . . . . . . All that's needed is a solution to moving a large flat bit of thermal mass every 12 hours. . . . . . .can't be that hard, apparently the ancient egyptians had it worked out AGES ago.

On Nov 11th I wrote:

Yes, but the reason that the air is cold at night is that it's been cooled by the ground and other solid things. The solid things have been cooled by radiating to space, or at least to higher up in the atmosphere. The rate of cooling of the surfaces of the solid things is reduced by the heat the surfaces gain from the insides and from the air. By trapping a layer of air against a part of your wall I think you will increase the rate at which it cools and therefore the rate at which it transfers heat from its insides.

In the spirit of experimentation that Tony has so nobly introduced I decided to test this. I tried yesterday (Wednesday) and failed for reasons which are of sufficient interest to be worth a separate write up.

I have had in my garage for over 10 years now the original tailgate for my Citroën BX which fell off when the plastic round the hinges failed. Anything kept long enough comes in handy but I had been meaning to break this up and take it to the tip. I chopped it up on Wednesday afternoon and extracted the glass.

For the experiment I lent the glass up against a block retaining wall in my garden held in place with four candles (actually a spade handle and a fork handle) and "sealed" round the edges by judicious stuffing with plastic bin bags. Today I did this at about 1630, just as it was getting dark.

Around 2115 I put my IR thermometer outside to cool down, did the washing up, then went outside and measured the radiant temperature of the wall to the side of where the glass was, at the same height as the middle of the glass and in the middle of a block. I did this a couple of times and also measured various other things to make sure the thermometer was giving sensible and stable readings. I then removed the glass and immediately measured the temperature of the middle of the block in the middle of the glass. I flipped backwards and forwards between the two measurement points to make sure the readings were consistent.

The temperatures were 2 °C for the exposed wall and 0 °C for the part which had been covered.

I then went inside for about a quarter of an hour leaving the thermometer outside to stay cool. I then measured the temperatures again at the same spots (to make sure there wasn't a difference in, say, the earth behind the wall). They both read 3 °C which was slightly surprising but the sky had clouded over a bit in the mean time.

I don't remember the exact sky radiant temperature when I was first measuring but it was in the -3x °C range (maybe -36 °C but I'm really not sure) whereas for the "control" measurements it was -29 °C so slight warming of the wall surface was not completely unreasonable.

A few more tests like this would help but it does seem to support my assertion that glazing a wall will tend to cause it to have a lower temperature in the evening.