Marc West

Hi Frank,

Your post puts me in mind of the Kachelofen (='Tile oven' - really a wood burning stove), where there's a combustion chamber and a very long exhaust path which winds its way through a lot of thermal mass and gives up most of its heat on the way. There's a rapid, high-temperature, very high-efficiency burn which puts a lot of heat into the thermal mass, and the heat in the thermal mass diffuses out comfortably for the next 10 hours or so. Anyway, one of the virtues of this kind of heater is the high temperature of the burn, which produces very efficient combustion (which continues along the length of the exhaust path) in which almost all that is produced is waste gases [and heat] - very little tars or other gooey substances. (You can get tars condensing in chimneys in systems with less efficient combustion, and they can cause trouble - I remember reading an account of tar dripping onto the congregation in churches with less efficient heaters, and they can catch fire - the tars, not the congregation, that is). The point of this paragraph? - I wonder whether extracting heat from the exhaust gases might cause more tar production etc. Just a thought...

You say: "If you just want to capture the energy from sunlight on a warm day, I would look for something that changes phase at room temperature, possibly at the minimum (20C). That way it is absorbing heat from the moment the wall gets hotter than is necessary,..."

Well, exactly, hence my comments about trying to devise a paraffin wax mixture that melts in the region of 22 degrees.

HOWEVER, I have since been wondering whether my approach is wrong. Do I really need a phase change material at all? If we are able to control the thickness of the wall, then can we calculate and choose just the right wall thickness so that it will absorb the sun's heat that falls on it and will undergo a slight temperature increase as it absorbs that heat and conducts some of it to its interior, and then as the sun goes, that stored heat is available to radiate out of the front again at a nice slow rate. (We will need to insulate the reverse side of the wall to prevent it 'leaking' out in that direction). By carefully choosing the wall thickness, can we contrive to get that 'slight temperature increase' so that the stored heat doesn't radiate out at too fast a rate, so we get that slow release of heat to the plants that we want. I suppose that this matter is at least partly governed by the thermal conductivity of the wall material - if that were really low then the surface of the wall would heat up more rapidly than we want, irrespective of the thickness and thermal mass of the wall. Any thoughts, anyone - do I really need phase change material after all? I've already said I'm not a chemist. I'm not a thermodynamicist either...

Hi Allen Lumley,

What an extraordinary and wonderful welcome! Thank you so much for that.

I'll let you know how I get on with my search for the right PCM material. In the meantime, if anyone has any thoughts about altering the melting temperature of paraffin waxes through mixing or dissolving or ...... then do please post!!

Hi all,

Firstly, to answer the original poster: there is an encapsulated PCM material called "Micronal" made by the German company BASF:
http://www.micronal.de/portal/basf/ien/dt.jsp?setCursor=1_290798
but I appreciate that that post dates from a couple of years ago...

I have a slightly different enquiry, and I wonder if anyone can help.

I co-own some land with a small community, and we wish to build a cobb wall on it, partly to provide support and warmth for trees like apricots and peaches. (We are in Cardiff, Wales, UK). Now, it has long seemed to me that incorporating PCM in the cobb structure would be a great way to extend the warm period at the end of a hot day. I've just read this thread and realise that paraffin wax in tins embedded in the wall may be just the thing. (Thanks to Frank R for his clear posts!)
The issue for me, then, is to find a paraffin wax that melts at the right temperature, which I guess should be in the region of 22 degrees C or so, for it to be most use for extending the warmth of the day for plants. (Anyone have a better guess?). Now, as I understand it, paraffin wax has a melting point of between 46 and 68 degrees C (variable because paraffin wax is not a single substance but a mixture of compounds [called 'alkanes'] of the form CnH(n+2), for example C25H52). So using 'ordinary' paraffin wax wouldn't be any use, because its melting point would be too high.

So how to get a lower melting point paraffin wax-like substance?

2 possibilities occur to me:
1) mix in or use some shorter chain alkanes. Ought to work fine, but looks like it will be a lot more costly than standard paraffin wax is;
2) Dissolve something into standard [inexpensive!] paraffin wax to lower the melting point, in rather the way that dissolving salt in water lowers its melting point (which is what we do to ice to make it melt when on driveways etc). Does anyone know what substances can be dissolved in paraffin wax to lower the melting point? Salt? Some other "more organic" substances that will dissolve better? I'd be happy to experiment with a thermometer and a saucepan of the stuff in a water bath, but I don't really understand how to calculate the likely effect on melting point of dissolving a certain quantity of material X into paraffin wax.

Not being a chemist, I don't really know where next to go with this. I'd appreciate any thoughts you guy may have on this. Or if someone knows of affordable off-the-shelf paraffin wax substances with melting point in the right range perhaps they could let me know.

Thanks in anticipation...

Marcus