4 ms·

>Any good paints or fabrics or metamaterials that could be used as a tunnel liner that only allow water to wick in one direction (from tunnel internals to surro

>Any good paints or fabrics or metamaterials that could be used as a tunnel liner that only allow water to wick in one direction (from tunnel internals to surrounding soil)? Thermodynamically impossible without an input of energy. FWIW.

dredmorbius 9y ago
It turns out that most transportation tunnels do in fact have a significant input of energy. Ventillation to exhaust waste heat is a major concern. From discussions of net-zero housing in Alaska (Thorstein Chlupp), my understanding is that water tends to flow from hot to cold -- it will condense along a cold surface. A condenser + sump system where some engineering mechanism keeps the inner tunnel wall from remaining at equilibrium temperature with the interior, might work, though how cold that would need to be to keep humidity manageable would be a question. Routing the cold-side air ducts such that they waste heat to the subsurface and draw heat from the tunnel wall might be one approach. Further cooling through water (fresh, sea) is another possibility, though both present significant issues, scarcity of fresh in Los Angeles, and general nastiness of sea,
- Baeocystin 9y ago
  While you're right, in that heat is an issue in tunnels, harvesting low-temperature-delta energy is incredibly inefficient. It is always better to source the needed electricity from a generator that is specifically designed for such. (The only time this may be untrue is when dealing with very small things like remote sensors, where running a wire might actually be more expensive than harvesting the microwatts needed locally. But this is a fraction of a fraction of a percent of total system needs.)
  - dredmorbius 9y ago
    What I'm saying is that for the purposes of removing moisture, the temperature gradient itself is an energy flux which might be utilised. In some ways, this is the inverse of an evaporative cooler, where the incoming air is saturated with moisture to lower its temperature before being vented to a warm space to be cooled ("swamp coolers" are best suited to dry climates, and have humidity issues). Here, instead, cool (possibly chilled) air could be ducted through the warm space, with the intent of condensing moisture onto the ductwork (for drainage by other means) prior to being vented into the space. You're trading cooling capacity (condensing water warms the cold air supply) for humidity control. I'm not trying to imply that this is practical, only that a latent energy flux might exist.
bnegreve 9y ago
I'd like to know more about this. Check valves [1] do just this, and they work without energy input, what is the difference ? https://en.wikipedia.org/wiki/Check_valve https://en.wikipedia.org/wiki/Check_valve
- Baeocystin 9y ago
  Something that can discriminate without energy input is a form of Maxwell's Demon. If you are unfamiliar, it's a lot of fun to learn about. https://en.wikipedia.org/wiki/Maxwell%27s_demon https://en.wikipedia.org/wiki/Maxwell%27s_demon is a reasonable start. Check valves absolutely do consume energy. Something has to resist the spring forcing them closed, or they would never open. They are convenient in that their energy is sourced from the very thing they are controlling, but they are energy-consuming devices.
  - MacsHeadroom 9y ago
    Higher air pressure could provide the necessary energy. There are easier ways to get moisture out of a tunnel though.
    - Baeocystin 9y ago
      Sure, there's lots of ways to do so. My only point is that none of them are 'free'.

dredmorbius 9y ago

It turns out that most transportation tunnels do in fact have a significant input of energy. Ventillation to exhaust waste heat is a major concern. From discussions of net-zero housing in Alaska (Thorstein Chlupp), my understanding is that water tends to flow from hot to cold -- it will condense along a cold surface. A condenser + sump system where some engineering mechanism keeps the inner tunnel wall from remaining at equilibrium temperature with the interior, might work, though how cold that would need to be to keep humidity manageable would be a question. Routing the cold-side air ducts such that they waste heat to the subsurface and draw heat from the tunnel wall might be one approach. Further cooling through water (fresh, sea) is another possibility, though both present significant issues, scarcity of fresh in Los Angeles, and general nastiness of sea,

Baeocystin 9y ago
While you're right, in that heat is an issue in tunnels, harvesting low-temperature-delta energy is incredibly inefficient. It is always better to source the needed electricity from a generator that is specifically designed for such. (The only time this may be untrue is when dealing with very small things like remote sensors, where running a wire might actually be more expensive than harvesting the microwatts needed locally. But this is a fraction of a fraction of a percent of total system needs.)
- dredmorbius 9y ago
  What I'm saying is that for the purposes of removing moisture, the temperature gradient itself is an energy flux which might be utilised. In some ways, this is the inverse of an evaporative cooler, where the incoming air is saturated with moisture to lower its temperature before being vented to a warm space to be cooled ("swamp coolers" are best suited to dry climates, and have humidity issues). Here, instead, cool (possibly chilled) air could be ducted through the warm space, with the intent of condensing moisture onto the ductwork (for drainage by other means) prior to being vented into the space. You're trading cooling capacity (condensing water warms the cold air supply) for humidity control. I'm not trying to imply that this is practical, only that a latent energy flux might exist.

bnegreve 9y ago

I'd like to know more about this. Check valves [1] do just this, and they work without energy input, what is the difference ? https://en.wikipedia.org/wiki/Check_valve https://en.wikipedia.org/wiki/Check_valve

Baeocystin 9y ago
Something that can discriminate without energy input is a form of Maxwell's Demon. If you are unfamiliar, it's a lot of fun to learn about. https://en.wikipedia.org/wiki/Maxwell%27s_demon https://en.wikipedia.org/wiki/Maxwell%27s_demon is a reasonable start. Check valves absolutely do consume energy. Something has to resist the spring forcing them closed, or they would never open. They are convenient in that their energy is sourced from the very thing they are controlling, but they are energy-consuming devices.
- MacsHeadroom 9y ago
  Higher air pressure could provide the necessary energy. There are easier ways to get moisture out of a tunnel though.
  - Baeocystin 9y ago
    Sure, there's lots of ways to do so. My only point is that none of them are 'free'.