CELSS – Controlling humidity in a closed life support system or Terrarium. Dew Point (DP) Condensation Trigger – Using Thermo-Electric Devices to Help Control Relative Humidity (RH).
Controlling humidity in a closed system involves crossing the current tank temperature over the Dew Point line. This is the trick to control RH in a closed system.
The correct RH needs to be available for the chosen plants to live in a CELSS. Some plants do not absorb the majority of their water supply through roots, but through leaves; while others only absorb water through their roots. It is important to have a PID (Positional, Integral and Differential) control loop for controlling Relative Humidity (RH) meeting the requirements of these plants. Case and point, plants growing in a Chaparral in California do not absorb the majority of their required moisture through roots, but through leafs. Yes, leafs. About 15 inches of rain is available to Chaparral plants through fog. This is how these plants evolved to survive next to the ocean. This is why RH needs to be controlled.
Another reason for controlling RH is rain. This is how rain is created. When running an CELSS system, rain is produced through condensation, but the majority of water is gathered by plants through a planned water table inside the CELSS. But for now, it is important to understand controlling RH in a small space where we don’t lose water.
The relative Humidity is the amount of water present (or evaporated/contained in air at a certain temperature) in air. Relative Humidity is in units of % (percent) . When the Relative Humidity, or RH, reaches 100%; that is when vapor in the air condenses and makes water. The Dew Point is a temperature where there is 100% RH and makes water (or water condensates). This temperature is a theoretical number and is calculated from the RH and the current temperature. Air pressure is also a factor, but it is not considered in this test results. The entire system is enclosed in a pressure tight system up to 5 KPa. The Dew Point, or DP, equation is listed below:
Dew Point (C degrees) = Temperature – (100 –RH)/5
This equation is an approximation of the Log rhythmic version used in the CELSS. Below is a graph of Dew Point vs. temperature.
If you change the temperature in a set volume of air, the molecules expand if warming and condense if cooling. When the air is cooled, the molecules come closer together and water density is higher. This means air pushes water into a tighter area. When the temperature goes down, the RH goes up. When the air temperature goes up, the RH goes down.
In a closed system, there needs to be a mechanism where you can change the RH on command. In this case, changing the chamber temperature to cross over into the DP does not condense water from air. As the chamber temperature is reduced, the DP also goes down. We can take our queue from nature. Cool air from the ocean goes over the land. That sudden/abrupt temperature change causes fog. In the CELSS there needs to be a device that abruptly changes the air temperature over a small gradient. In this case I’m using a thermo-electric device. Pictured below. When DC current is placed over the semi-conductor, one side turns cold and the other side heats up. The cool side of the device is placed in the ceiling of the CELSS chamber while the hot side is vented to the outside. When operational, water in the air condenses on the extension below and water drips to the bottom. From the graph, you can see a point where the thermo electric device is turned on and left on for 8 hours. At the end of the day, a small puddle of water is left at the bottom of the tank. When the device is turned off, you can see the humidity starting to increase. The RH starts at 85% and lowers almost to 20%. You can see the DP also decreasing; however, the air temperature stays relativity the same.
The control system in this CELSS needs to turn off and on the thermo-electric device to control the RH. This device is connected to a SSR which is coupled to a bit that can be written by the controlling PIC 18F4550. Below is a test to determine what duty cycle is best of the thermo-electric device. In this case the device is turned on and off every 15 minutes. You can see the movement of RH in the saw tooth wave. The system never freezes up. When designing the RH control system, it may be save to keep the thermo-electric device on long enough to get to the desired RH. Once the RH is reached, a histolysis of about 5 to 10 % can keep the RH in a working range.
All water that is collected on the thermo-electric device drips back into the system and is reused. Nothing is lost.
The change in air pressure can change the amount of water in air. By increasing the air presser by 5 KPa, the RH can go 20 to 30 percent. Further study is needed.
Below is a list of pictures of what was found in the CELSS system after the experiment/test. The first 5 pictures are images going through the depth of field of the microscope. I’m have a bit of a dilemma. Is this fungi or algae? If this algae, one would like it be green colored. The whole tank is tested in the dark. Light never entered into the tank. This means algae grew, but never got light for photosynthesis. If this is fungi, what is it eating? CO2, H2O or what?
The tank is completely sealed. Most of the water I put into the tank is still there. I lost about 10 ml. I attribute that to bacteria taking water and consuming it. In the next few tests, I need to monitor CO2 production.
The last two pictures at 100X.