don't you need at least two fans and two chambers of heat storage material?
"in... thermal regenerators, the hot and cold fluids pass through the same channels in the packing, alternately, both fluids washing the same surface area. In recuperators, the hot and cold fluids pass simultaneously through different but adjacent channels." https://www.thermopedia.com/content/1087/
While they are indeed simpler, my understanding is that they don't really perform well.
i'm surprised there's no humidity recovery; i'd think that, with a regenerator, humidity recovery is unavoidable even if you don't want it. maybe you're talking about cases where humidity or ΔT is so low that you don't get any condensation? https://news.ycombinator.com/item?id=38746150 points out that a little desiccant can solve that problem
Too early to see energy benefits yet, but one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting. I should probably finish that hood sensor (looks at sensor sitting on desk).
In trickle/night mode the rated performance is "up to 90%" (at 1.2W and 15m³/h in unspecified test conditions). These ones require a 165mm diameter hole, so the heat exchanger cross section is ~2.7 times the 100mm version, and presumably more efficient.
I also installed (refurb, old house) a recuperative MHVR upstairs. This type has two fans for separated permanent intake and exhaust channels. The alternative in my country is knocking 7000mm² holes in the walls (aka passive ventilation). I have to meet various building requirements as a refurb to qualify for energy upgrade grants.
> one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting
Do these standalone units recover heat only (HRV) or heat and humidity (ERV)?
The house also has new external wall insulation and other upgrades, so it's rather more airtight than before (though not formally tested just yet). Otherwise they run in synchronised cycle mode for heat recovery and air movement.
I'm not well versed in all the subtleties of theory and current technology, but I don't think regenerative and true ERV go together. If condensation forms in the unit on the exhaust cycle I'd guess does contribute a little to the recovered heat?
These units are BSK Zephyrs, with one heat mass component (ceramic honeycomb) and one fan inline in a telescopic duct. To prevent condensation issues they are fitted with a 2° slope outwards, and a humidity sensor with settable thresholds to purge high humidity internal air at full rate (equivalent to about 1.2 air changes/h for me).
However if you want maximal efficiency you want to add some zeolite 3A adsorbent, which you can sort of do yourself I am pretty sure. You just put some through a coffee grinder, put the regenerator in a garbage bag with it and shake it to get the powder inside the regenerator. It's not idea but it'll certainly help. I tried this with a thermal wheel of comparable size with silica gel and measured the result with AHT10 sensors and it worked pretty good actually.
Zeolite lasts longer but you need more. Obviously you gotta try not to get the powder everywhere by getting excess off etc.
Once the thermal mass of the "batch" of air is relatively small compared to the thermal mass of the regenerator, adding more thermal mass doesn't help.
The reason you can't go to town with high surface area is viscous drag. The fans make a lot of noise when they have to work at high speed, which they have to do to produce higher pressures. Fan blade noise increases in proportion to the eigth power of blade velocity. So really really fast. So if you make the holes in the regenerator smaller and the walls thinner to increase surface area, you don't get enough flow.
The usual units really hobble at the best of times. If you calculate how much energy (which increases linearly with flow*efficiency) they save a year, they usually actually cost more in amortized total cost of ownership than they save.
The best thing is to redesign both the fan and regenerator, that's what I did.
There is a nice fan that's produced by a german company that I'm trying to get samples of but they are super slow.
obviously this structure is not feasible to manufacture by conventional processes, but you can 3-d print it
cf. https://dercuano.github.io/notes/capillary-heat-exchanger.ht...
it seems crazy to me that people would use alumina as a regenerative medium for house ventilation; it's a pain to get into the shape you want, which makes it expensive in practice even though it's an abundant natural mineral. it does have an immense virtue as a regenerative medium, which is that it can withstand like 1750°, but if part of your house is over 1000° you have bigger problems to worry about than the efficiency of your hvac system. if you're trying to maximize the thermal mass of a stable solid per dollar, something like unfired clay is probably optimal, though it expands and contracts with humidity; lime mortar (quicklime and sand) avoids that problem. those are low thermal mass per kg; to optimize that, you'd probably want polypropylene instead (or paraffin wax, but it might melt)
unfired clay has significant moisture-adsorbent ability too, which is why it expands and contracts with humidity. muriate of lime is commonly used as both a (deliquescent) desiccant and a flocculant for clay colloids used as ceramic glazes; i'm not sure to what extent clay with muriate of lime in it can function as a solid desiccant
but maybe i'm missing some basic knowledge here, so plausibly these are stupid ideas; wikipedia is no substitute for experience
The ideal space-filling "core" of a rete mirabile recuperator is (I argue) a checkerboard, with black and white squares flowing in the opposite direction.† This makes all adjacent channels opposite polarity (unlike a honeycomb) for maximum heat transfer, and also reduces internal drag and manufacturing complexity (unlike triangles). Mass manufacturing can be done via extrusion, or by gluing together 90° corrugated sheets. If the core consumes 80% of the total system volume this reduces 3D printing cost.
Trees typically branch 1-to-3, which is probably optimal because evolution. For an 54x54 grid that means 1458 endpoints, or 6.6 branchings. That's one 2-to-1 branching (common on the first "trunk" branch of a tree) followed by six 3-to-1 branchings, for seven total branchings. This is a typical value for a real biological tree, so we're probably moving in the right direction re:biomimicry.
In real biological trees the fluid flow slows as the branches get smaller, so it's probably good to emulate that.
† arguably a honeycomb shell-and-tube is more efficient geometrically, but it's non-symmetric and (more important) the manufacturing and rigidity/robustness is questionable
Aside: paraffin wax is used in actuators, e.g. automatic green house window openers, on account of its impressive expansion. The MVHR I have also uses it for automatic "summer" bypass of the heat recovery path.
They're simply a small fan that reverses every few minutes and a heat storage material. They also have wireless ones that coordinate the fan direction between them.