For posterity, the conversation started here:

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8888&page=3#Comment_142350

Ok I've not been able to get much detail off the web so this is based a bit on reading between the lines.

I worked with a adsorbtion (not a spelling mistake) desiccant system some time ago (for a super fast tumble dryer). It came in the form of little pea sized beads. From memory there was a significant "loss" in terms of energy in to energy out, although we were using electricity to dry the stuff. I seem to remember calculating that our prototype used 3 times the energy of a standard dryer, so a storage efficiency of around 30%. Of course if you are using "free" solar energy to dry it it's less of an issue. I imagine that it would store it's energy well as long as it was kept air tight, but it would have to be properly air tight, as in hermetically sealed air tight, not building "air tight" (which just means not as leaky as a normal building).

I'd be a little worried about building it into the structure of a building, partly because of the air tightness thing and partly because of the difficulty of providing ducts, plus what happens when the bath overflows/roof leaks? This stuff can punt out serious (like +80C) heat if you get it properly wet.

I could see this process (possibly) working in a sealed package like a storage heater or water tank system, but not as a building component like insulation slabs.

Posted By: beelbeebubI worked with a adsorbtion (not a spelling mistake) desiccant system some time ago (for a super fast tumble dryer). It came in the form of little pea sized beads.

What is it exactly though, I tried some dried peas, they did not work, cat litter next.

Piet from http://www.xsorb.nl/ mentioned that the product has been used widely in industry for the last 15 years, to dry natural gas before pumping it through pipelines- http://hengyeusa.com/community/natural-gas-drying

The stuff we used was called zeolite, or possibly zeyolyte or similar. The 15 year timescale fits as it was cutting edge stuff when I was fiddling with it.

Again all this is from memory:

It was some type of ceramic normally in bead form as a pebble bed (basically a jar of beads) is one of the most efficent ways of getting good gas to solid contact. The trick was the ceramic beads with nano scale pores. (again going from my memory of a simplified explanation given to me by our chemist) the pores were about the size of a water molecule.

The water molecules effectively slotted into the pores, when they did this something about surface energies or something basically made the water have the thermal energy as if it were a solid. So from a gas the water would give up its latent heat of evaporation (from gas to liquid) then of liquification (from liquid to I). Hence even liquid water would get adsorbed and release heat. This gave great power densities when it came to taking a cool wet air stream and turning it into a hot dry one, hence the tumble dryer.

Different pore sizes would adsorb Different molecules, I think there was one set that would target oxygen, so could be used in oxygen generators, seem to remember we had one though it may have been membrane based.

I seem to remember discussions about casting it into a block with internal channels so it's format didn't have to be beads.

To put energy into the beads, You have to heat the beads up to place the latent heat back onto the water to take it back to the gas phase, the drive the gas out with an air flow. This process was the lossy part, though as i said with "free" solar this might not be a problem

If i were to speculate on the best way to use this Tec it would be as a thermal battey unit, so hot fluid (solar) would be used to heat the block up and whilst fans or natural convection drove out the water. When the time came to get the heat out water could be added, heating the block, whilst a coil containing the central heating water, pr mains water for she took the heat away to be used.

Although the energy density may be higher, I'm not sure if it's high enough to give a reasonable sized store, that is to say one that is interseasonal rather than a few days.

Sorry for the typos, doing this on a phone :-)

Sodium acetate.

One issue is that you have to get it to boiling water temps to recharge it, and getting incremental release of energy is hard.

Potentially something that gradually sucks up our unwanted excess winter humidity and turns it into heat at a sensible safe temperature is very good news. I suspect that I'd still need 10t of the stuff, but maybe it could help a little.

Rgds

Damon

Thanks beelbeebub for your background information on this stuff. I found this comment particularly interesting:

So from a gas the water would give up its latent heat of evaporation (from gas to liquid) then of liquification (from liquid to I).

The reason I did the calculation in the previous post was that the numbers initially seemed implausible to me and I thought it would turn out to be a near-perpetual motion machine. What I wasn't taking into account is water's hugely greater energy of evaporation than of fusion (liquefaction/freezing). At 100 °C the latent heat of evaporation is 2260 kJ/kg (equivalent to heating the water by 538 °C) and, as mentioned above, it's higher near 20 °C. On the other hand the latent heat of fusion (at 0 °C) is “only” 334 kJ/kg (equivalent to only an 80 °C rise). Not sure what the actual heat of fusion at 20 °C would be but I imagine it wouldn't be much different from 0 °C.

Anyway, assuming that both latent heats apply is interesting as it would mean that the amount of water which needs to be condensed to release the claimed amount of energy would be less, though not by a very large amount.

Of the pages I looked at this seemed one of the more useful:

http://www.polylam.com/desiccant_performance_data.htm

Posted By: beelbeebub: “A note on the RH of the heated air, it's very dry, too dry for use as interior air I would say.”

Yes, the Xsorb site proposes that the heated dry air go through a heat exchanger to heat incoming somewhat moister air (hence the interest in the Viking House fine wire heat exchangers):

http://new.xsorb.nl/?page_id=18

Still, I wonder if it wouldn't be a good idea to feed at least some of the warm dry air straight into the house. Probably depends on the temperature and humidity of the outside air.

Posted By: beelbeebub: “also the drying temps are above 100C”

Possibly - won't the water come out as soon as the RH is low enough and there's energy available to shift it? You've got all summer. But again you can use a (the same) heat exchanger to recover some of that heat as they show further down the same page - though I'd think you'd have to make sure your exchanger is ready for a good deal of condensation. You'd probably want to give it a good clean between the end of the drying season and the beginning of the heating season. Still, that shouldn't be a big problem.

ST: yes, basically. Actually in the dry state it's just at ambient temperature, give or take short term conduction.

The actual “stored heat” is in the water vapour in the moist air, of course. The dry material is just a potential to release that heat. This does point out, of course, that if the inlet air to the house is very dry then this system will not transfer net heat into the house during the heating season as all the heat released by adsorption in the cat litter will have come from evaporation within the house. The key is that the air entering the heat exchanger from the outside world must have more energy (enthalpy????) because of the water vapour it contains than the cold dry air which is chucked back out.

VH: I suspect that that won't be a problem in a British Isles heating season for a while!

I still fancy blending the dry air emerging from that stuff with my house air to get its RH down to sensible levels in winter. We've been up around 70%+ RH; getting and keeping the house to under 50% RH would be very good news and would work in conjunction with the MHRV to keep RH and temperature reasonable.

Rgds

Damon

Posted By: DamonHDwould work in conjunction with the MHRV to keep RH and temperature reasonable

Was my thinking, but with my ultra cheap home made jobby.

Could be used to help heat one room, say a living room or office, rather than assume it will do the whole house.

Where can I get a tin of it?

('there is Physics and the rest is stamp collecting' Rutherford)

Posted By: Viking Houseso please don't shoot the messenger!

Sorry, I wasn't having a go at you! Just a bit p**ed off by their website.

Posted By: Ed DaviesYou do need enough moisture.

There's quite a bit of material published about passivhaus becoming too dry during the winter. Because the incoming air is quite dry (i.e. 98% RH but not many molecules). That's one reason for reducing the ventilation rate and/or humidifying the air, which wouldn't be good in this case. It does need some serious checking to see if it all adds up in this scenario.

What worries me about the claims is that using adsorption as a heat store is an area of ongoing research. If any of the materials that have been in commercial use for fifteen years had obvious potential, I expect they would already be in use for this purpose. And xsorb's US patent wouldn't be as vague as it is. I really would like to see the UK one.

Zeolites are fascinating materials, they have so many uses. One thing I'd woory about in this usage would be poisoning. If you're blowing exhaust air from the dwelling through the zeolite, as is shown in their diagrams, then you have to be concerned about everything else in the air as well as water vapour. Obviously bits of dust or pollen will block pores, so you need some fine filters, but also other molecules than water might like to gather in the pores - VOCs, oils, smoke etc. So perhaps additional filters are needed as mentioned in Ed's link. Filters mean maintenance and any remaining poisoning may limit the useful lifetime of the zeolite. So again, solid technical data required.

On the other hand, phase-change materials were in this position for a long time but do now seem to becoming developed products, so anything is possible.

Posted By: djhAnd xsorb's US patent wouldn't be as vague as it is. I really would like to see the UK one.

I called him (Piet) again today, he texted back that his Dad's gone into hospital with heart problems. So it may be a few days before we get any more info.

Posted By: djhObviously bits of dust or pollen will block pores, so you need some fine filters, but also other molecules than water might like to gather in the pores - VOCs, oils, smoke etc. So perhaps additional filters are needed as mentioned in Ed's link. Filters mean maintenance and any remaining poisoning may limit the useful lifetime of the zeolite.

Its used over and over again to dry natural gas so I'm not sure how big an issue this could be!

Posted By: tonyIn this world there is never something for nothing, I cant see it guys, sorry

Isn't the idea more 'at least something' back during the winter from 'something' (i.e. some 'more than can use be used at the time' solar thermal kWh) put in during the summer?