Towards a better tinyhouse

Inventing to freedom?

Ultra cheap energy recovery ventilator $20?

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The mini homestead. Unfortunately this place would be very difficult to heat and cool, a quick back of the envelope indicates about 3.5 kg ($7, $210 a month) of propane a day (at 0 deg out and 25 deg in) and more than 35 kg of ice (at 22 in and 28 out for 8 hours a day) each day (osb has a u value of about 1, figure 70 m^2 wall area, use the heat of combustion for propane and latent heat of fusion for water). Much more if it is badly sealed. You might be happy in this instance with some insulation and no ventilation, but I got me thinking: How could you make a really cheap energy recovery (heat exchanging) ventilator? That could allow you to build a cheap passive house.

Okay, it could use regenerators, which are used in units like this , but you don’t need the desiccant. And you don’t need it to move. The desiccant can improve efficiency (without it the air going out during cold weather would usually be at 100% relative humidity, whereas the air coming in is at a lower humidity, so you loose some water)

Instead, you just use a regular fixed matrix regenerator. Actually 2, because you need to have air flowing into the building at the same time that air is flowing out.
Then you need to reverse the air flow periodically.
Then you need some sort of motive force to push the air through the regenerators, wind would do nicely and cheaply here. To estimate how much wind you would need, you can think about the regenerators having an “equivalent free area”(EFA), if it has an EFA of x many square centimeters, then the same amount of air will flow through it as if you had a hole in the wall with that area, x. You can make the beds shallow and wide,o so that would not impede the airflow much. Theoretically it doesn’t affect the thermal characteristics, it is only the thermal mass of regenerator material that matters. And even then, that just determines how often you have to reverse the flow through the regen (it gets “full” of heat faster).

This method recovers not just the sensible heat, but a lot of the latent heat too. Water condenses on the heat exchanger material on the way out, then evaporates again when air is drawn back. I’m pretty sure the only practical way to get the latent heat back is to re-evaporate the water into the incoming air stream. You don’t get all the water back, because the air exiting the regen on the way out of the building will be at it’s dew point (so 100% RH), but no lower. As it’s going out, it cools down to the dew point, water starts to condense out (and accumulate on the regen media), and during this process the rh can’t go below 100%.

The regen media can be aby fibrous stuff that doesn’t emit smell into the air, fiberglass (maybe taked from and insulated duct so you know it is odor free) cotton, polyester clothing, polypropylene wool, etc. so it’s pretty cheap.

Then you need to harness the wind. You can see by one of the diagrams that this is being done just by orienting the intake and outtakes of the system so wind gets blown through.

Then the hard part is periodically reversing the airflow through the regens, let’s call them the left one and the right one.

I think a simple wind powered mechanism would do fine. The one shown in the diagram works like this: You have an “air switch”. This is basically 4 air valves (the seals don’t have to be very good or anything) connected together so they all switch at once. In the schematic vies shown in the figure, it goes like this: valve a and d are turned on, allowing air to pass, air flows into the building through the left regen and out through the right. Then, c and b open, and for a brief period air just sort of short circuits,going straight from the incoming air line to the outgoing air line – this serves no purpose, of course, it’s just a result of the way the air switch works while it is transitioning. Then, a and d close. Now, air is going out through the left regen and in through the right. Perfect.

Now you need a way to actuate the switch, so I suggest this:
you have a 6 bladed turbine (for lack of a better term), which is located between the air switch and the regens, one side of it is exposed to the air stream going to the left regen, and the other side to the air stream going to the right regen, these streams area going in the opposite direction, so the turbine turns.

As it turns, it winds up some fishing line or string. When it has wound it all up, the string becomes taught, and pulls, causing the valve to switch. The turbine has to have some momentum, so you might need to add some weights, because during the switching process there is no air flow to power it (because air short circuits briefly). Then, after the airflow has been reversed, the turbine starts turning in the opposite direction. Repeat.

Because the turbine rotation keeps reversing, you don’t need any bearings, either, you can just suspend it with some string. It gets twisted and untwisted a bit…

In between the air switch and the turbine you can imagine you need to do some air ducting to connect the output of the switch to the input of the turbine section, just connect the outputs of a and b together, and c and d.

So as you can see, it’s all pretty cheap… and would give you pretty good efficiency, too, there are some companies selling rotary (but no desiccant, I think) heat exchangers that claim 95% efficiency.

Hey, I bet this could have applications in the developing world, for homeless shelters, and other really cheap shelters. Combine it with some uncovered foam insulation on the inside of the wall of the mini homestead, and some great stuff for sealing all the cracks, and the whole place might not need any propane at all most of the time. It would pay for itself in only a few months.


Written by gregor

September 12, 2010 at 12:20

Posted in Uncategorized

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