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The NASA CELSS (Controlled Ecological Life Support System),is a project to use hydroponics for space travel and space colonies. Hydroponics supplies oxygen from plants, produces fresh food and purifies water. The hydroponics also scrubs the air of excess carbon dioxide, keeping the human space travelers alive.
The NASA CELSS unit is a full life support system to grow wheat in extraterrestrial colonies. There are several CELSS units, and many universities and research centers are particpating in the program. The CELSS provide a life support system to grow plants, and currently grow wheat 24 hours a day. One cubic meter of actively growing wheat provides enough oxygen for one person. Over a 60 day cycle the cubic meter produces 6 kilos of wheat, enough for 16-20 bowls of cereal. So a CELSS produces 1/3 a bowl of cereal a day. The growth chamber is approximately a square meter in length and width, and 2.3 meters high. The growth chamber is divided into three separate areas. 1) The roof portion includes eight lights, four 400 watt high pressure sodium and four 400 watt metal halide. Sunbrella's over the lights reflect light back down and use cold water to reduce light temperatures. The lights, which remain on 24 hours a day, produce 1300 micromoles of photons per m2 per second. 2) Wheat is densely packed (2000 plants/meter) in 4 trays each about 15" x 22" x 8" deep. The black plastic trays have a sloping bottom with a center over flow valve. Nutrient water is piped into the tray with perforated 1" PVC pipe. Above the trays there are support structures of coated expanded metal screen and on that is a 1" layer of rockwool planted with wheat. The overflow valve is adjustable from 5 to 8" and the height changes as the wheat grows. The roots grow through the rockwool and grid plate to the water below. 3) The bottom layer includes a return water tank which is 18" below the nutrient tanks above. The water drops through the 18" and gains enough air to keep the oxygen level of the water at 8 ppm. This is sufficient to keep enough oxygen in the water so no air pumps are needed. There are two external water pumps that supply nutrient water to the 4 plant tanks above. Each 1/8 horsepower TM 90 Little Giant pump has a filter in front of the water line. The Dayton Electric filters take out stray root matter and rock wool. Each pump line is attached to an in-line flow meter that monitors a flow set at 20 l/ m2 min. The flow meters used are Fisher Signet 3-8500. They are set to alarm the computer system when the flow drops below a certain rate (about 10 l/m2min). When the flow drops to that set point the computer shuts down the entire CELSS unit, lights pumps and circulation of air.
The CELSS computer control uses sensors to "sense" if plants can live in the environment. There are three separate nutrient tanks with automatic feeds. Computer Control System CELSS has an elaborate computer control system. The control system keeps the growth chamber at perfect growing conditions for the wheat. It is operated on a IBM PC using Paragon software. There are two external computer boards that keep track of the environment with input and output to the computer. There are several sensors in the chamber that record data about environment, and then send this information back to the computer. The computer then operates several mechanical devices based on the information it has as input from the sensors. The sensor information is provided to the input board, and plug and play type modules are used on that board to do the signal processing to get that environmental information into the computer. The input board includes analog and digital input. Analog input is a continuous signal which is changed to digital information that the computer can read. The conversion in the NASA CELSS uses 12 bits so the information can be converted to a scale from 0 to 4095. This allows for a strong signal definition
Gases The gases in the chamber, CO2 and O2 are monitored by the control system. The CO2 monitor is sensitive to a change of plus or minus 5 ppm CO2 for an optimum 1000 ppm. The control by MKS allows for CO2 to be added to the chamber when needed. The O2 sensor that monitors excess oxygen in the chamber is a TM-1B manufactured by Amtec. The gas monitoring allows for the growth chamber to maintain an optimum CO2 for growth. Normal atmospheric CO2 is about 350 ppm and there is significant scientific data that plant growth can be increased with increased CO2 to 1000 ppm. After 1000 ppm the growth rate does not seem to improve much and at about 3000 it gets a bit uncomfortable for people. Nutrients There are two sensors in the return nutrient water tank. One measure electroconductivity and the other measures pH. The pH sensor is a Cole-Palmer's general purpose epoxy covered sensor. It is in the nutrient liquid and then sends back a voltage signal based on the nutrient pH. When ammonia is used in the nutrient solution it the pH increases and when nitrate is used it goes down. Since the CELSS system is only using nitrate then pH only has to be adjusted to go back down. Ph adjustment is supplied by an injection pump and either sulfuric acid (H2SO4) or phosphoric acid (H3PO4 ) is used to reduce pH. A conductivity sensor in the nutrient tank measures the relative level of dissolved solids in the solution. It measures the conductivity of the solution or its ability to conduct a current. It doesn't give additional information on specific nutrients that might be missing, just overall ions in solution. Different crops have optimum growth under different levels of concentration. To receive optimum growth it is important to introduce additional nutrients to the nutrient tank as they are taken up by the plant. A continuous feed of nutrients should produce better growth than a system that allows for nutrient concentration to vary in the feeding cycle. Conductivity changes with temperature so the measurements should be taken at 25 degrees C or 77 degrees F. When conductivity registers that nutrient in needed, the computer control system activates the pumps on two concentrated nutrient tanks. The nutrient tank has three attached tanks which include water, concentrated calcium nitrate and the concentrated nutrient Hydrosol by Peters. When electroconductivity registers a reduction in nutrient concentration, the nutrient flow meters allow for extra nutrient to be added to the nutrient tank. Temperature and Humidity
The environmental control manages water content of the air, by extracting excess water when the humidity gets too high and putting water back when the humidity gets too low. The temperature of the air and the canopy are measured and sent to the computer program. The recirculation fans continue throughout the operation of the CELSS and the air is both cooled and water is removed by a condensing coil. If too much water has been removed it is added back in a humidity producer.
Revised: 1 May 2016 |
