Passive Indoor Dehumidification
Here is a paper to be published in the proceedings of the Emerging Practices Conference at Tongji, which describes this entire passive dehumidification project in more detail over 13 pages.
Shanghai (and many other places on earth) possesses a rather humid climate. A relative air humidity of 70-90% is typical, and situations above 95 are frequent, especially indoors. This humidity exasperates temperature for humans, both in winter and summer, and greatly affects people's comfort: In summer, the human body cannot cool down through perspiring (sweating), because air that is already saturated with water does not easily take on more humidity. (Our perspiration needs to evaporate into the air for us to cool down, because the evaporation process consumes heat energy from our bodies.) In winter, the cold temperatures are felt even colder, as humid clothes will evaporate the water, stealing energy from our bodies in the process.
Psychrometric Chart for Shanghai. The higher up the days (blue patches) are located, the more humid they are. The brighter the patches, the more number of days are in that patch. Both, in winter and summer, too many days in Shanghai are outside the human comfort zone (yellow frame)
This means that in our technology-oriented society and cities, we start artificial heating and cooling much earlier than we would in a dryer climate, because we feel uncomfortable (too hot, too cold) earlier in the year and over longer periods. As research has shown, air conditioning of indoor spaces is one of the biggest culprits for energy use and greenhouse gas emissions. Thus, if we could better regulate the indoor air humidity in climates such as Shanghai, we could reduce the use of air conditioners by approximately 3-4 months every year.
In order to lessen the high amount of water content in the air, conventional air conditioners typically cool the room air until its temperature is lowered to below its dew point, where the water vapor subsequently condenses and exits the air. This is a process with a heavy energy investment and flawed with several inefficiencies. We at BiDL are looking to rectify the current situation by asking nature for a mechanism that dehumidifies the air without any external input of energy. We believe if such a mechanism could be harnessed and scaled, huge reduction in energy costs for buildings would occur. And it would be healthier for humans and the environment.
Our goal at BiDL is to create a modular interior building material/surface that will adjust air humidity to an ideal state for human comfort (40-60%). We currently envisage a ceiling material, as the ceiling is usually the most available surface and out of human activity. Upper wall sections are also considered. Both these surface types seem ideal, as the warmest air (and thus the air that contains more absolute humidity) rises to the top of a space, because it is less dense and lighter. We hope to create a feature that can easily be integrated into conventional, contemporary interior design for office, commercial as well as residential uses. We also think we might have to not only absorb water from the air, but also lead it out of the room or reuse it, as often dehumidification would be necessary over several days, because when relative air humidity is constantly high, the pores might get full quickly.
We aim for a dehumidification system that is completely passive in its entire process and does not need any external energy whatsoever. We also aim for something that is affordable and can compete with other contemporary ceiling materials.
Here is a little ppt that offers a shorter, illustrated version of this whole page.
Progress Log: Updated 17/04/2015
We at BiDL commenced the quest to solve the problem of dehumidification by consulting AskNature for biological strategies that have already been documented. By searching "humidity" we stumbled upon the use of a liquid desiccant by the brown dog tick. We discovered that liquid desiccants are currently used in industry as improvements to air conditioning systems. While we did further investigate the use of this desiccant, we began to diverge from this path due to the fact that we wished for this system to operate without any energy, and energy would have been involved in keeping the desiccant cool (for optimal adsorption properties), re-strengthening it once it had become diluted with absorbed water and possibly also pumping it to keep it in cyclic motion (taking water - releasing water).
So, we again searched for water-collecting organisms, and found the page of the Spanish Moss, which has the remarkable ability to absorb water from the air through its many nano-sized pores. We consulted Prof. Guo GuangPu, plant biologist in Tongji University, to help us understand the biological mechanism, and what we garnered from this enlightening conversation was this: this moss automatically absorbs water through pores in a structure called a trichome due to differences in vapor pressure created by the pore geometry. We must make a generally important note here for benefit of all who, like us, were previously misled: while we have been taught in high school biology that diffusing species move from areas of high concentration to low concentration, we must be more specific. The 'concentration' in concentration gradient can actually be either concentration (i.e. molarity), pressure, temperature, etc. Fluids (the scientific umbrella term for liquids and gases) move from an area of high pressure to an area of low pressure. So, in our case, lots of gaseous water molecules enter the lower-pressure pore, and because there are many gas molecules in such a small space, condensation occurs (think of states of matter in terms of density - when gaseous molecules become more tightly packed, they become liquid, because all states of matter really describe is the qualitative measure of how free molecules are to move due to packing). Thus, the pores of the Spanish moss become filled with water without any external energy input. These pores become capillaries that transport water to where it needs to be (e.g. the moss to use it for biological processes). This process as a whole is called capillary condensation.
So, now understanding the mechanism better, we consulted a materials scientist who has actually developed a clay material that both dehumidifies and rehumidifies the air using similar process. He obtained this idea from the many traditional earth and clay dwellings of China, which have always been rather comfortable to live in. Where we differ in purpose is that this clay material has pores to contain the water (not move it away), and release it back into the space when the relative air humidity drops. However, we want the material or system's pores to extend to capillaries that contain and then transport it (ideally only using gravity or capillary forces etc.) for other use (therefore, our system would be used for dehumidification only). This could be done by making the porous structure out of an organic material, i.e. we could use cellulose, which makes up the cell wall of the trichome cells that do the absorbing. Further research needs to be done to decide if cellulose is the appropriate material for this function, but because nature does it this way, we are optimistic. We obtained the method behind obtaining what size the pores need to be in order for capillary condensation to occur - it will be somewhere in the range of approximately 2 to 50 nanometers.
More recently, we have been working with Professor Cheng Yi-Heng, who thinks that
"Theoretically it should be possible to create a hydrophilic capillary with a hydrophobic sink on cellulose base."
Prof. Cheng also knows that researchers in Taiwan are experimenting with recycled PET as well as PP nano-structures with hydrophobic and/or Hydrophilic properties.
In an email on April 17, 2015, his colleague Professor Chen from Taipei Medical University (TMU) explains:
" All these can be put together as a nanoporous microtube array membrane a.k.a. MTAM. [...] We did successfully prepare cellulose acetate MTAM and are characterizing its fine structure... These MTAMs siphon liquid quite easily.... Personally I think there is possibility that our MTAM can be put to test for this application."
- ---Prof. Chien-Chung Chen
- We know we want to use capillary condensation with pores to take the water out of the air
- We know how to dimension the diameter of the pores
- We think that using a natural material like cellulose would be ideal
- We know that cellulose has been used in cancer medical research and applications for materials that absorb substances through capillary condensation
While the problem of extracting water from the air is crucial, we are currently also looking for skilled professionals that would be willing to help us in simulating the movement of water molecules in architectural space. We hope that the following applications would be suitable to investigate our processes:
- AutoDesk CFD
- OpenFoam Wiki
If you are able to support us with computational fluid dynamics (CFD) or similar simulation in human environments, please do not hesitate to get in contact with us.
And so, the next step is a big one - to find an material engineer and/or industrial partner who can help us design the pore network of this material and produce the material in quantities to make a viable prototype for experimenting and testing.
The next questions that we currently see before us are:
- How long should the pores be? on which length does condensation happen?
- How do we create/build these nano-scaled pores? What material(s) and process? (Can we grow them?)
- How do we make the water move in the pores once it has liquified? How does the moss move the water? (can we take advantage of hydrophobic/hydrophilic surfaces, capillary forces, gravity,...?)
- What layout/arrangement should we create to lead water away? How large is one module, how do we connect modules?
- Where should we lead the water to? just down the drain? For plant irrigation? Into the toilet cistern? For janitor and cleaning purposes?
Here is a list of all the websites and sources that I used up to this point for the biomimetic dehumidification project:
- Description of how typical air conditioning systems work and how liquid desiccants work: http://www.ailr.com/liquid_desiccant_tutorial.htm
- AskNature page of the brown dog tick’s dehumidification strategy (uses a liquid desiccant): http://www.asknature.org/strategy/f777a0792ad3bc95a91c2b42ae61bab4#.U47eBPmSxU8
- AskNature page of the Spanish moss’ dehumidification strategy (uses nanopores): http://www.asknature.org/strategy/f550ac5152d7af443a9abb9d6b220c74
- Explanation of capillary condensation: http://en.wikipedia.org/wiki/Capillary_condensation
- The equation behind how to dimension the pores so capillary condensation can occur: http://en.wikipedia.org/wiki/Kelvin_equation
- Article about Rice University grad student who created a water-absorbing scaffold using carbon nanotubes (mechanism is also capillary condensation): http://news.rice.edu/2014/06/11/nanotube-forests-drink-water-from-arid-air-2/
- Description of the plant trichome (the part of the plant where the water absorption occurs): http://www.daviddarling.info/encyclopedia/T/trichome.html
- Explanation of some plant biology relevant to water storage and use: http://plantsinaction.science.uq.edu.au/edition1/?q=content/15-4-3-epiphytes
- Deeper explanation of the Spanish moss and its trichome: http://www.tandfonline.com/doi/pdf/10.1080/00087114.1988.10797853
- Discussion of the Spanish moss with relation to capillary condensation (the equations seen in this source are also on the capillary condensation Wikipedia page): http://books.google.co.jp/books?id=PtVt6XYV0kkC&pg=PA154&lpg=PA154&dq=de+santo+et+al+1976+absorb+water&source=bl&ots=W0DlOEHQ0l&sig=RCmBTR3INe8EcYGPHOuqszbNn50&hl=en&sa=X&ei=jZSeU-C6Kcn_8QWJioLoDg&redir_esc=y#v=onepage&q=de%20santo%20et%20al%201976%20absorb%20water&f=false
- Explanation of water vapor absorption of the Spanish moss (the previous paper merely hypothesized that water vapor absorption occurred, and this paper says that it is true): [Page 11 of] http://ican.csme.utah.edu/wp-content/uploads/2012/02/Martin-1994.pdf
[collected: Morgan Pagliasso]
Links about Namib Desert Beetle:
If you have any suggestions, idea and/or would like to collaborate on this project, please email BiDL. We warmly welcome any input!