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Desiccant as heat concentrator?  RSS feed

 
Connor Macreno
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I know there are materials that release water when heated and produce heat when water is added. I understand that it normally requires higher heat to dry/regenerate the materials than the heat released when the materials are saturated, but the heat required can be reduced by lowering the air pressure.

I'm wondering, if you lower the pressure while heating the material, then raise the pressure before saturating, can you extract heat at a higher temperature than the heat it was regenerated at?

I know the total energy out will be less than the energy in. Iirc the best you can achieve with zeolite is 75% (or less) of the heat applied. I'm thinking that heating it slowly at low pressure and then saturating it quickly at high pressure could produce higher heat for a shorter duration.
 
John Wolfram
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Connor Macreno wrote:I'm wondering, if you lower the pressure while heating the material, then raise the pressure before saturating, can you extract heat at a higher temperature than the heat it was regenerated at?

Connor, I believe you've just described the compressor for an air conditioning/heat pump unit.
 
Connor Macreno
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I don't want a system where the compressor has to run continuously while heat is being extracted.
I'm thinking of zeolite in a pressure vessel, where the pressure is adjusted down before the heat is stored and back up before it's extracted. Like a solar zeolite heat pump, but upping the pressure and extracting the heat at a higher temperature than the solar collector originally provided; possibly even using a lower temperatures heat source (I'm thinking compost heat) to regenerate the desiccant.
I'm not sure how low the temperature can be for regenerating zeolite and at what pressure, or what other desiccant material may work better for lower regeneration temperatures.
 
Terry Ruth
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Connor I thought Zeolite performed best in a vacuum closed loop system. What grade of Zeolite are you referring to or can you source? Post some material properties and a system design so we can look at it. Do you have CAD capability?

I found some aluminum powder zeolite mfgs close to me I going to develop an open looped enthalpy plaster, perhaps a ERV(Energy Recovery Ventilation) systems in my homes using it as a desiccant, moisture and heat recovery x-changer if someone has not already beat me to it and I can purchase cheaper than I can R&D. I think it would also act as an air filtration like a hepa at the x-changer. Open loop I don't see dry air delta pressures as a big factor, more wet bulb vs dry, vapor pressures, humidity deltas. I'm early in my R&D but think it would make an excellent aggregate in my binder I develop. I need to call and talk to the mfg engineers and get some properties to design to. Lots of different grades and varying material properties out there, both natural and synthetic. The only way to do this is CAD defining a conceptual system design, properties, phase changes, then develop, test, back calibrate the design model. I been down this road many times.....Flomaster, WUFI, CATIA, SolidFLO, are some mechanical CFD system design tools I use. Without running the simulations we are "just saying"! Nothing! If you want to post some CFD inputs and outputs and a system design drawing I be ecstatic to analyze the data with you.
 
Connor Macreno
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"I thought Zeolite performed best in a vacuum closed loop system."
I thought the advantage of the vacuum was that it allows the zeolite to be regenerated at a lower temperature. That only helps for half the problem I'm trying to overcome.

"What grade of Zeolite are you referring to or can you source? Post some material properties and a system design so we can look at it."
I don't even know if zeolite is the best choice. I don't have enough material properties data.

"Lots of different grades and varying material properties out there, both natural and synthetic."
Yep, but so far I haven't had much luck finding useful information on the different materials.

Right now I have a general idea and lots of questions; nothing close to a design yet.
 
Terry Ruth
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Connor, I was just yanking your chain a little last night over some Permieshine

My understanding of the vacuum is to minimize losses and create a systems design with air and the desiccant in a closed loop that gets closer to an adiabatic process. Where are you getting a 25+% loss from without having a design? One thing good about zeolite is will not break down.

What ever material you are pondering I suggest looking at it's availability and cost before wasting to much time. Then get some mechanical and physical properties to design to. It will next to impossible to design closed loop without the proper design and instrumentation tools I mentioned above. Open loop plasters can be tested with some meters.
 
Connor Macreno
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"Where are you getting a 25+% loss from without having a design?"
When I had read up on zeolite heat pumps and heat storage I read that the efficiencies achieved with zeolite heat pumps were 10 to 60% and the highest efficiency that could be achieved (with the types of zeolite they were using in the zeolite heat storage system I was reading about) was 75%. My understanding is there are inherent loses converting thermal energy to chemical and back (probably less than the 75% before other loses,but I was accepting this as a reasonable best case).
 
Terry Ruth
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I'd have to see what you were reading. I'm pretty sure you are referring to mechanical losses from pumps, heat exchangers, etc. I did not see that drastic of property differences in zeolite grades. In realty enthalpy sees some small looses but, all the math I have seen assumes none or adiabatic. Certain molecular sieve materials like zeolite, clay, lime plasters produce heat, are net positive or 100%+ efficient in the presence of moist air. Work is created by phase change to for example, cool a room by heat of evaporation-opposite being heat of condensation, or latent heats. I found zeolite interesting due to the speed of adsorption or how fast and efficient it cools/evaporates, coverts moisture to ice . When a heat source is applied to wet zeolite such as radiant from the sun, it's desorption rate is much slower as it condenses back to it's surrounding's for humidity buffering and, slowly released that heat at night. Until heat is applied there are no condensation losses. I believe that is function of the heat source, the greater it is the faster the desorption rate.

We use the term "thermal mass" which is limiting. I prefer "Hygrothermal Mass" for those reasons where materials act as HVAC with no or little internal losses and no external mechanical devices. Not to say that mechanical devices are not needed in certain designs. The ERV is one I mentioned that would require a fan to blow air over an xchanger and they are 80+% efficient. The ERV that uses a molecular sieve (hygrothermal mass) is more efficient than the HRV(thermal mass) that does not. The ERV improves efficiency as moisture and heat levels increase. There are no other building materials that performs that well, most break down in their presence.

With that said, I do not think dry air pressure is going to change the efficiency of the internal process, I could however be wrong, heat and moisture will, especially renewable passive. I do believe there are ways to minimize losses depending on the mechanical design.
 
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