How Do We Pressurize a Greenhouse?

Dook · 2017-04-13 13:46:11

Mars atmospheric pressure is very low. If we build a greenhouse out of hard plastic panels, how do you then pressurize it?

Initially, I think a small oxygen bottle would be opened to give the inside some oxygen for the plants to use but that won't be enough to pressurize the greenhouse to 2-5 psi unless it's a small greenhouse.

Would a fan built in to the entry hatch be able to do it?

Would we have to use zeolite panels, set them out, let them absorb CO2, bring them inside and use mirrors in the daytime to heat them?

louis · 2017-04-13 16:44:03

I recall from previous discussions that plants can thrive in low pressure, CO2 rich environments...the figure of 20% of Earth pressure is floating in my mind.

Dook wrote:

Mars atmospheric pressure is very low. If we build a greenhouse out of hard plastic panels, how do you then pressurize it?
Initially, I think a small oxygen bottle would be opened to give the inside some oxygen for the plants to use but that won't be enough to pressurize the greenhouse to 2-5 psi unless it's a small greenhouse.
Would a fan built in to the entry hatch be able to do it?
Would we have to use zeolite panels, set them out, let them absorb CO2, bring them inside and use mirrors in the daytime to heat them?

Dook · 2017-04-13 17:43:26

louis wrote:

I recall from previous discussions that plants can thrive in low pressure, CO2 rich environments...the figure of 20% of Earth pressure is floating in my mind.

Dook wrote:
Mars atmospheric pressure is very low. If we build a greenhouse out of hard plastic panels, how do you then pressurize it?
Initially, I think a small oxygen bottle would be opened to give the inside some oxygen for the plants to use but that won't be enough to pressurize the greenhouse to 2-5 psi unless it's a small greenhouse.
Would a fan built in to the entry hatch be able to do it?
Would we have to use zeolite panels, set them out, let them absorb CO2, bring them inside and use mirrors in the daytime to heat them?

Plants do great in low pressure but you have to have pressure for water to be in liquid form. Otherwise it's either frozen as ice or vapor.

RobS · 2017-04-13 18:46:43

Plants need a certain amount of oxygen and a certain amount of carbon dioxide, and a certain amount of air pressure so the water doesn't evaporate out of them. The details probably vary from species to species, too. I'd make sure the atmosphere is breathable so people can do some of the gardening work and can just go in there and be with the plants; the greenery will be psychologically healthy. You can't use a fan to pressurize the greenhouse, but you just use a specially designed air pump. You probably will want to circulate the air of your habitat through the greenhouse because the plants will use up the carbon dioxide and produce oxygen. Also, if you have a big barrel full of moist ground with, say, grass or plants growing on top, and you blow the air up through the dirt, microorganisms in the soil will utilize and remove nitrogen oxides and various waste gasses (like fart gasses) that otherwise will accumulate in the habitat's air. A small pond and blowing air through the water will also remove some unwanted gasses and smells. So a greenhouse can play an important role in a closed life support system.

Dook · 2017-04-13 18:59:08

RobS wrote:

Plants need a certain amount of oxygen and a certain amount of carbon dioxide, and a certain amount of air pressure so the water doesn't evaporate out of them. The details probably vary from species to species, too. I'd make sure the atmosphere is breathable so people can do some of the gardening work and can just go in there and be with the plants; the greenery will be psychologically healthy. You can't use a fan to pressurize the greenhouse, but you just use a specially designed air pump. You probably will want to circulate the air of your habitat through the greenhouse because the plants will use up the carbon dioxide and produce oxygen. Also, if you have a big barrel full of moist ground with, say, grass or plants growing on top, and you blow the air up through the dirt, microorganisms in the soil will utilize and remove nitrogen oxides and various waste gasses (like fart gasses) that otherwise will accumulate in the habitat's air. A small pond and blowing air through the water will also remove some unwanted gasses and smells. So a greenhouse can play an important role in a closed life support system.

Is the "specially designed air pump" just a high speed fan that brings outside Martian CO2 in to your greenhouse?

Initially, I don't think the greenhouse atmosphere would be breathable unless you want to use a lot of the oxygen from your habitat.

Over time the plants would convert the CO2 to oxygen so, at some point, you could breathe it for a while until the oxygen level comes down. The greenhouse plants will not be able to provide enough oxygen for your settlement.

RobertDyck · 2017-04-13 19:45:18

University of Guelph did low pressure experiments with plants. They found pressure as low as 10 kPa (100 mbar) was fine, but below that plants wilt, stop growing. Earth at sea level is 101.325 kPa (1013.25 mbar). They did they experiments with spinach, and one other plant whose name I forget. And they even simulated greenhouse pressure loss: they drop pressure to Mars ambient. Spinach completely wilted, stopped growing. They held it for an hour, then restored pressure. As soon as pressure came back, the plants perked back up, continued growing. They found a low pressure plants grow at the same rate, but require more water. The lower the pressure, the faster water transpires through their leaves. However, in a sealed pressurized greenhouse, that water just condenses on walls, drips back into the soil.

Remember, Mars surface pressure is about 0.7 kPa (7 mbar), but varies with weather, and varies with altitude.

Last edited by RobertDyck (2017-04-14 03:54:50)

elderflower · 2017-04-14 03:28:26

There may be a slight slip of a decimal point there, Robert.
I would think that crews would want to avoid the need to suit up to enter the greenhouse, so you would need a human survivable atmosphere, which might be supplemented with bottled oxygen, but does need a minimum total pressure.
You would also need to remove CO from the outside air before putting it into the greenhouse.

Last edited by elderflower (2017-04-14 03:30:45)

louis · 2017-04-14 04:39:03

CO is only 0.08% of the Mars atmosphere. Surely that's too small to have any damaging effects either for humans or plants... ?

elderflower wrote:

There may be a slight slip of a decimal point there, Robert.
I would think that crews would want to avoid the need to suit up to enter the greenhouse, so you would need a human survivable atmosphere, which might be supplemented with bottled oxygen, but does need a minimum total pressure.
You would also need to remove CO from the outside air before putting it into the greenhouse.

elderflower · 2017-04-14 05:32:45

Prolonged exposure to low levels of CO are still damaging due to the relatively high stability of carboxyhaemoglobin. Maybe there's a catalyst which will oxidise it at low temperature. High temperature catalysts which do this are commonplace in gasoline engine exhausts, but we should avoid the need to heat incoming gases to high temperatures to minimise energy use.

louis · 2017-04-14 06:57:54

With reference to plants:

"Carbon monoxide does not poison plants since it is rapidly oxidised to form carbon dioxide which is used for photosynthesis."

http://wgbis.ces.iisc.ernet.in/energy/H … vol321.htm

Hadn't realised it was quite so dangerous at relatively low concentrations for humans, but point taken.

But if we were talking of a low pressure mainly CO2 environment, humans would have to have artificial breathing apparatus in any case if they entered the growing area, so not an issue in that context.

elderflower wrote:

Prolonged exposure to low levels of CO are still damaging due to the relatively high stability of carboxyhaemoglobin. Maybe there's a catalyst which will oxidise it at low temperature. High temperature catalysts which do this are commonplace in gasoline engine exhausts, but we should avoid the need to heat incoming gases to high temperatures to minimise energy use.

RobertDyck · 2017-04-14 09:22:24

If you expect workers in a greenhouse to be able to breathe without a spacesuit, then this entire discussion is redundant. We discussed elsewhere how to produce air for a habitat, such as Analog(ue) air. As I pointed out there, to produce air for humans you could remove CO2 from Mars atmosphere, pressurize, and add oxygen. But if you do that, removing CO2 will concentrate everything else; that will concentrate CO so much that it's lethal. Never mind long term, since over 95% of Mars atmosphere is CO2, removing that will concentrate CO so much that it's immediately lethal. After 9/11, NASA developed a catalyst that can be used for building fires. It's rhodium over a substrate, and operates at +24°C. The catalyst combines CO with O2 to produce CO2. It's still CO2, but that's a lot less toxic. NASA intended it to be used in a breathing mask, it would be used just long enough to get out of a burning building. I suggested that for habitat air you do this in the same device that removes CO2, so the CO2 produced by this catalyst is removed as well.

This discussion is for a greenhouse. Do you want humans to be able to breathe in this? If so, then this discussion is moot, just do the same as habitat air. If not, then is CO moot or do you sill want to remove it for safety? You could use the same rhodium catalyst to combine CO with O2 to produce CO2.

Last edited by RobertDyck (2017-04-14 09:49:21)

elderflower · 2017-04-14 09:42:46

I would expect that people would want to get in the greenhouse with the plants, without a suit. That means a certain total pressure and a certain partial pressure of Oxygen. I forget the actual numbers. The plants will grow better for a fairly high partial pressure of CO2 but humans don't, so I would envisage a simple breathing mask, maybe like you get on an airliner, to allow the people to spend some time amongst the plants. Then the total pressure in the greenhouse would need to be equivalent to maybe 5 or 6 thousand metres elevation on earth -so about 50 kPa. This avoids pressurising and depressurising to get in and out of the hab which would likely be at a similar pressure. There would be a different composition between the two, so you still need an airlock. The airlock has to stand full pressure difference to Mars in case of loss of pressurisation and it becomes a refuge for humans in case they are caught in the greenhouse when the pressure fails.

Dook · 2017-04-14 09:44:58

RobertDyck wrote:

If you expect workers in a greenhouse to be able to breathe without a spacesuit, then this entire discussion is redundant. We discussed elsewhere how to produce air for a habitat, such as Analog(ue) air. As I pointed out there, to produce air for humans you could remove CO2 from Mars atmosphere, pressurize, and add oxygen. But if you do that, removing CO2 will concentrate everything else; that will concentrate CO so much that it's lethal. Never mind long term, since over 95% of Mars atmosphere is CO2, removing that will concentrate CO so much that it's immediately lethal. After 9/11, NASA developed a catalyst that can be used for building fires. It's rhodium over a substrate, and operates at +24°C. The catalyst combines CO with O2 to produce CO2. It's still CO2, but that's a lot less toxic. NASA intended it to be used in a breathing mask, it would be used just long enough to get out of a burning building. I suggested that for habitat air you do this in the same device that removes CO2, so the CO2 produced by this catalyst is removed as well.
This discussion is for a greenhouse. Do you humans to be able to breathe in this? If so, then this discussion is moot, just do the same as habitat air. If not, then is CO moot or do you sill want to remove it for safety? You could use the same rhodium catalyst to combine CO with O2 to produce CO2.

Having a breathable atmosphere in your greenhouse would definately be preferred but it's not required and would take too much effort to keep the atmosphere breathable. Also, plants do better with high CO2 levels.

How do you pressurize the greenhouse?

Dook · 2017-04-14 09:46:26

elderflower wrote:

I would expect that people would want to get in the greenhouse with the plants, without a suit. That means a certain total pressure and a certain partial pressure of Oxygen. I forget the actual numbers. The plants will grow better for a fairly high partial pressure of CO2 but humans don't, so I would envisage a simple breathing mask, maybe like you get on an airliner, to allow the people to spend some time amongst the plants. Then the total pressure in the greenhouse would need to be equivalent to maybe 5 or 6 thousand metres elevation on earth -so about 50 kPa. This avoids pressurising and depressurising to get in and out of the hab which would likely be at a similar pressure. There would be a different composition between the two, so you still need an airlock. The airlock has to stand full pressure difference to Mars in case of loss of pressurisation and it becomes a refuge for humans in case they are caught in the greenhouse when the pressure fails.

How do you pressurize the greenhouse?

Oldfart1939 · 2017-04-14 09:59:09

Only one way possible: make certain all means of ingress and egress are equipped with gasket type seals and then from the outside, use a compressor. Just like blowing up a balloon. Pressurize with the desired mixture of gasses; this necessitates an airlock between any lower pressure compartments (such as outside and 7 mbar pressure).

elderflower · 2017-04-14 13:47:26

maybe the question should be :How do we make a greenhouse to withstand the internal pressure and not be too delicate?

Oldfart1939 · 2017-04-14 14:18:17

That's a great engineering materials question. We've had some indicate that glass is the answer, but it's brittle and once cracked, increasingly subject to catastrophic failure.

I've suggested Polycarbonate plastic, known as Lexan. It's very tough stuff and highly transparent. It's used for making the canopies for fighter jet aircraft, and the clear visors of the ISS crew spacesuits. My idea is to bring the necessary molding equipment for standardized modular panels of the stuff, then manufacture the plastic in situ on Mars. I recall that Zubrin's company, Pioneer Astronautics did some research on manufacturing some of the chemical components from Martian CO2, Bis-phenol (BPA) in particular. If we can find a source of Chlorine on Mars (and if we find briney water, there we have it!), the other component is Phosgene is within reach. On Earth, approximately 1 Billion KG of bis-Phenol is produced annually.

The canopy of the F-22 fighter jet is made from optical grade polycarbonate, the largest single injection molded part yet made. It's also used to make the windows for drive up bank tellers, and when laminated, is sometimes known as "bullet proof glass."

Quoting Wikipedia: "Polycarbonate is a durable material. Although it has high impact resistance, it has low scratch resistance. Unlike most thermoplastics, it can undergo large deformations without breaking or cracking."

The optical grades are even more transparent to visible light than glass.

RobertDyck · 2017-04-14 15:00:13

Glass can be strong. And it's easy to make. Plastics, on the other hand, require a lot of power. Most chemical reactions to make plastic require hydrogen, which is made by electrolysis of water. The reaction usually produces water, so you continually have to run water through electrolysis. That's what consumes most energy.

Glass can be strong. How often has a window of your house broken? Other than a baseball batted into your window, has a window ever broken? Windows of high-rise apartment buildings and office buildings are glass. I think it's Ok. And glass is hard; harder than plastic. Regular window glass hardness is 5.5 on the Mohs scale. Something harder can scratch it, but not anything softer. Plagioclase is 6 to 6.5, augite 5.5 to 6, olivine 6.5 to 7. Since Mars soil is composed of these, ideal is to make windows harder. That would prevent scratches or crazing due to sand-blasting during Mars dust storms. Tempered glass is 7 on the Mohs scale, so it would be a good idea to use tempered glass windows on Mars. Polycarbonate "shot" is 2 to 3 on the Mohs scale.

You don't want a multi-month Mars dust storm leaving your windows looking like this...
2013-01-13+15.52.17.jpg

SpaceNut · 2017-04-14 16:11:18

Make it in a windshield safety glass...also make it multi-pane such that if the outer layer fails that the inner one is most likely to still be intact....

Another way to pretect the atmosphere with in the greenhouse is to draw it down once a shirt sleeved man leaves the area to some value that still promote plant growth and when its time to enter reinitaillize the repressurization to occur before entering.

Oldfart1939 · 2017-04-14 18:38:50

Robert-

The ideal way to do it would be a double pane set of windows; polycarbonate inner, auto safety glass outer. With a greater safety margin and some reduction of heat losses.

elderflower · 2017-04-15 02:55:51

Glass making isn't that simple. You have to find a source of pure silica as mineral contaminants will discolour it. You have to get lime and soda, similarly uncontaminated. You have to heat it strongly in a furnace, then you have to cast, blow or spin it to get it flat and thin. Modern glass for windows is made by flowing the molten glass onto a bath of liquid tin.
If you bring your greenhouse from earth lighter weight plastic will be favoured to restrict mass.

SpaceNut · 2017-04-16 18:18:37

I was thinking about the volume of air and making it breatable within the greenhouse and wondered if using the compressor parts and moxie to make it so that we can bring it up to what we would want for internal content would be as low as we can go for energy useage. If we can send down a preloaded clear inflateable unit then wait for it to pressurize for use for the next mars cycle....I think we have something which can be landed by current space x Red Dragon capsule.

louis · 2017-04-17 07:44:34

Or you could make your greenhouse a redhouse - cut and cover the space you need for artificially lit plant growth.

Certainly I think that makes sense for early colonisation. You can have several layers within a 2 metre high facility (whereas with the greenhouse, you basically only get one layer). Yes, it's high energy consumption - but energy is one thing we can easily provide at an early stage on Mars either with PV or nuclear power. The plant trays could easily be manufactured on Mars. LED lighting is pretty lightweight.

RobertDyck · 2017-04-17 12:34:02

At the Mars Society convention in 2004 in Chicago, I gave a presentation about processing aluminum ore to produce aluminum metal. I thought I had made a great wonderful invention, but since then I discovered there's a company in Sweden already doing it. I have re-invented the wheel! Well, the good news is it works. Bauxite is produced by a tropical rain forest, there never was a tropical rain forest on Mars, so you won't ever find bauxite on Mars. But this other process works with anorthite or bytownite, two types of plagioclase feldspar. These are igneous minerals, and have been confirmed on Mars. My PowerPoint with notes is available here.

The reason I mention this is a major byproduct is silica gel. That's the stuff packed in little paper bags with electronics. When it's first produced, it will be saturated with water. Calcinate to dry it; that means heat it hotter than the boiling temperature of water, but not hot enough to melt it. That will drive off water, creating silica gel desiccant. Or calcinate to completion to produce silica, also known as silicon dioxide. That can be melted to produce glass. So if you can't find any white sand to make glass, then the byproduct of producing aluminum can be used to make glass.

GW Johnson · 2017-04-19 10:10:55

Plants require both oxygen and carbon dioxide. The carbon dioxide inside any greenhouse experiment or structure (regardless of how built), can be compressed from the Martian atmosphere. The oxygen must come from some sort of pre-pressurized supply. Without that oxygen, they cannot grow.

I have seen on these forums a proposal to run 5 psia carbon dioxide for the atmosphere, and let people wear an oxygen mask when working inside. That lacks the oxygen required by the plants. It also runs afoul of decompression time for the people, depending upon the nature of the atmosphere in their living spaces (whatever those are).

One of the questions being discussed explicitly in this thread is how to compress local Martian “air” to a usable pressure inside a structure such as a greenhouse. You can do it with a compressor, but this device will not resemble any compressor you might be familiar with.

The first thing to look at is the pressure ratio required: that sets the number of stages. Assuming Martian ambient is 7 mbar (0.10 psia), if we want 5 psia inside our structure or container, the pressure ratio is 50. That is beyond the capacity of any practical dynamic machines, whether axial or centrifugal. You are looking at a multi-stage positive displacement device, most likely a piston type.

As for “putting a fan in the doorway” and expecting that to work, that notion is patently ridiculous. Typical fan pressure ratio maxes out near 1.1. Such a thing might (or might not) get you to 7.7 mbar from 7.0 mbar. Some 41 stages of fans, all producing 1.1 pressure ratio, theoretically get you to pressure ratio 50. Practically, they cannot, because the chained fan stages simply will not all maintain 1.1 ratio. Hundreds of stages is not a practical device to build.

So, as a positive displacement device, that’s three stages. The first stage will have much larger dimensions than you are used to seeing, because it is processing “air” that is virtually vacuum in its density. Succeeding stages will not be as exaggeratedly-large, but still much larger than we are used to seeing in Earthly designs.

So, the compression machinery is going to be very large and therefore heavy. It will process little compressed massflow because of the low density. Power required depends upon inlet density, processed massflow, and to a lesser extent, pressure ratio. The power required to process thin gas is just lower. We will see much smaller electric motor sizes, turning much larger machinery, to produce modest mass flows at modest pressures. They will NOT look like Earthly compressors.

This sizing and design problem gets ridiculous very quickly on Mars. Look at the Martian equivalent of a shop air compressor, with a tank pressure of 125 psig (125.1 psia on Mars), and a regulated output of 85-100 psig (85.1 to 100.1 psia on Mars). You have to compress 0.1 psia up to the tank pressure. That ratio is 1251, which is way beyond the capability of even a 4-stage machine. It’s completely off the empirical chart (see below).

For compressor sizing purposes, the Earthly guidelines that are well accepted are as follows:
Max P-ratio…….no.of stages
10-11……………..1
35-36……………..2
175-180…………3
350-360…………4

On Mars, about 35-36 psig is all you can expect of a 4-stage design adapted to Mars ambient.

The kind of piston compression machines we are talking about here, have high geometric compression ratios in every cylinder. The piston face-to-cylinderhead dimension is almost nothing. There is no room for poppet or sleeve valves, quite unlike engine design. That is why nearly all these machines use thin reed valves. Reed valve fatigue failure is the cause of rebuilds. Those reeds require wrought spring steel in sheet form, die-cut and ground to shape. Both material tensile strength and ductility (elongation capability) must be high indeed, and that’s a difficult metallurgical tradeoff.

These reed valve parts simply cannot yet be made by 3-D printing. Most such printers that really are capable of printing real metal produce porous parts of substantially-reduced density, which resemble sintered parts. Such parts are both weak and brittle. So far, there are only a very few machines that produce parts of full density. These are the ones that have made rocket engine parts. The material has strength like a wrought part, but they are still quite brittle. You must make them thicker and heavier to compensate.

Just some practical real-world things to think about.

GW

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#1 2017-04-13 13:46:11

How Do We Pressurize a Greenhouse?

#2 2017-04-13 16:44:03

Re: How Do We Pressurize a Greenhouse?

#3 2017-04-13 17:43:26

Re: How Do We Pressurize a Greenhouse?

#4 2017-04-13 18:46:43

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#5 2017-04-13 18:59:08

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#6 2017-04-13 19:45:18

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#7 2017-04-14 03:28:26

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#8 2017-04-14 04:39:03

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#9 2017-04-14 05:32:45

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#10 2017-04-14 06:57:54

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#11 2017-04-14 09:22:24

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#12 2017-04-14 09:42:46

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#13 2017-04-14 09:44:58

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#14 2017-04-14 09:46:26

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#15 2017-04-14 09:59:09

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#18 2017-04-14 15:00:13

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#19 2017-04-14 16:11:18

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#20 2017-04-14 18:38:50

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#23 2017-04-17 07:44:34

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