Greenhouse

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A pressurized greenhouse can be used on Mars as part of an ISRU or Closed Loop Life Support System, or even as a source of recreation for the crew. A greenhouse, depending on its design can produce food and/or oxygen as well as provide a means of recycling waste products.

Thermal Design Considerations

Most models of greenhouse heat transfer suggest that small greenhouses will have difficulty maintaining stable temperatures under Martian conditions, especially for designs using ambient sunlight as a heat source. However, potential limits on available power, coupled with the ready availability of free sunlight sufficient for plant growth, make natural lighting an attractive option. Options for dealing with the relatively poor thermal performance of greenhouse designs using natural sunlight must be considered.

The first preference (and structurally simplest option) in the design of a Martian greenhouse is to design a single layer shelter to maintain the conditions necessary for plant growth. The greenhouse should provide a means of maintaining its internal thermal environment. This can be done by the power intensive method of using fuel fired or electric heaters. It can also be accomplished using natural sunlight to heat the greenhouse interior.

File:Thermosiphon Solar Heater.jpg
Schematic of a Thermosyphon Passive Solar Heater

With or without thermal mass to store the heat absorbed during the Martian day, a greenhouse exposed to the combination of feeble daytime sunlight and tremendous nighttime temperature drops expected on Mars would need more surface area than a simple cylindrical shape can provide in order to absorb enough heat. A Martian greenhouse using primarily solar heating must both have a great deal of thermal mass and have solar thermal collectors which extend beyond its growing area in order to trap enough heat to carry it through the Martian night without freezing solid.

Thermosyphon heaters using air or antifreeze as a working fluid can collect sufficent heat if large enough. Sunlight heats the internal partition, producing convective flow up the sun-heated front of the collector. At night, when the collector is too cold for its working fluid to rise back into the greenhouse system, the flow stops and heat is lost only from the collector, not the greenhouse thermal mass.

File:Earth Sheltered Greenhouse.jpg
Schematic of a soil-sheltered greenhouse using a deep cold sink to regulate temperatures

Thermal mass can be provided by water or rock which is thermally isolated from the surrounding Martian surface.

A second option for maintaining internal temperatures is to divert the cooling air as it chills. A cold sink can be used for this purpose.

Essentially, a cold sink is a deep, narrow well for the air at the bottom of the greenhouse. Air sinks as it cools, and can be kept away from the plants if allowed to pool in the cold sink. Placing the bulk of the greenhouse's thermal mass alongside the cold sink rather than alongside the plants will reheat the air convectively, preventing precipitous drops in greenhouse temperature as the cold sink is overwhelmed.

File:Hydroponically Grown Squash on a Trellis.jpg
Hydroponically grown squash supported by a combination of netting and an overhead trellis

Similarly, the plants can be grown in the elevated upper sections of the greenhouse, where warmer air accumulates.

File:Covered Crop in Greenhouse.jpg
Use of wire supported crop covers to deal with nighttime temperature fluctuations
File:Greenhouse Cold Frame.jpg
Glass front cold frames used within a larger greenhouse to deal with uniformly cold internal temperatures

Another, equally important option is to simply accept that a simple, single layer shelter is inadequate to Martian conditions given the demands of plant growth and low power supplies. In this case, the function of the greenhouse becomes to create livable internal conditions which are as stable as possible, but not necessarily optimal for plant growth. Favorable growing conditions can then be maintained by the use of internal structures, such as nighttime insulating blankets or cold frames contained within the greenhouse. Small volumes are easier to heat, cool and insulate than large ones given the same heat budget, especially if the surrounding conditions are kept more favorable by a larger surrounding greenhouse.

This approach also has the added benefit of conserving materials. Simple thermoplastics, such as polyethylene, make excellent construction materials and can be readily made using in situ resource utilization. Unfortunately, they have poor thermal characteristics for greenhouse windows, particularly with respect to radiative heat transfer. Better greenhouse materials, such as glass, are available, but harder to make on site. Since these will be in shorter supply, it makes sense to use them for smaller internal structures.

It is desirable to place as little cultivated area as possible in the lower portion of the greenhouse. Plants grown on surfaces at lower elevations – such as the greenhouse floor - will be subject to temperatures colder than elsewhere in the greenhouse, regardless of the ventilation and thermal control systems used. The use of thermal mass for temperature control is also most effective if the bulk of the thermal mass is located low in the greenhouse. All plants should be grown on shelves or other structures elevated above the greenhouse floor, for the most efficient use of convective heat. Variations in temperature within the greenhouse can also be exploited to grow crops with different optimal temperature ranges.

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Spatial Design Considerations

File:Epcot Exhibit Hydroponic Greenhouse.jpg
An exhibition greenhouse, showing plants grown by various hydroponic methods, allowing varying use of greenhouse space for each
File:Hydroponically Grown Miniature Wheat.jpg
Miniature wheat grown on multi-tiered hydroponic arrays

Two important design considerations are the amount of growing space required in the greenhouse to fulfill its function for the crew and the internal volume restrictions necessary to its thermal performance.

Growing space is related to the size of the plants selected. However, because plants need to be illuminated to grow properly (illumination power being a function of area, not volume), and many plant species can be trained to configurations more dense than simply spreading them out on a single shelf (trellises, hanging containers, racks, etc.), it is apparent that the sufficiency of a growing space is more dependent on its cultivated area than on its cultivated volume. Because small volumes are generally easier to heat and maintain, it is desirable that the greenhouse be as volumetrically efficient as possible, in order to fit as much cultivated area into as small a volume as possible.

Thermal considerations and ventilation place limits on the useful cultivated volume of the greenhouse. The greenhouse air will sink as it cools overnight, causing it stratify. The cooler air will pool in the lowest reaches of the greenhouse and the warmer air will collect in the upper reaches of the greenhouse. Circulating fans can be used to counter this effect. Locating the bulk of the greenhouse thermal mass and/or active heating system to release heat into the lower portion of the greenhouse is also effective at reducing stratification. Other options include the use of internal structures to isolate the plants from the stratified air (cold sinks and/or cold boxes) and/or support them in the warmer air near the top of the greenhouse (racks, trellises, etc.).

Compost piles, storage, and other greenhouse components will also need space allotments within the greenhouse. These are not typically as temperature sensitive as crops, though, and can readily be located in the lower reaches of the greenhouse. Separate additions constructed without the degree of attention to controlling internal conditions that is necessary in the greenhouse solarium can also be employed for these components.

The growing area needed for each crop is determined by the productivity of the crop plants used. Each species of crop plant will require its own range of lighting, temperature, and volume for optimal growth. If multiple crop species are used (a likely necessity for a food producing greenhouse), multiple configurations will likely be required for each. Thus, any internal structures used within the greenhouse must be versatile enough to adapt to both multiple plant sizes and multiple positions within the greenhouse.

Lighting Design Considerations

Crop plants need light for growth. (Fungi crops, which are not plants, are a notable exception to this rule, and often grow best without light.) This light can be provided by either natural lighting or artificial lighting.

Natural lighting is provided by sunlight. It is what terrestrial plants are adapted to use. The spectrum of the sunlight on the Martian surface is not dramatically different from that of earth’s surface over the range used by plants (visible light). Surface sunlight on Mars tends to be dimmer than the average terrestrial sunlight, with a solar radiation constant of approximately 440 W/m2at the Martian equator on a clear day as opposed to 1000 W/m2 for the terrestrial equator. Not all plants on Earth grow at the equator, though. The intensity of sunlight reaching the Earth’s surface is attenuated at higher latitudes because the thickness of the atmosphere it passes through increases as the angle of the surface to the sunlight increases. As a rule, crop plants which are productive in greenhouses at Earth latitudes above 60 degrees north or south on Earth can also be productive in a Martian greenhouse, being capable of producing under sunlight as dim as that found on Mars. Although the Martian atmosphere is significantly thinner than that of Earth, it attenuates sunlight with latitude almost as rapidly as Earth’s due to its high dust content, limiting the use of solar lighting in Martian greenhouses latitudes less than 70 degrees without the use of artificial lighting. The lighting needs of individual species vary as well, making some species better adapted to lower light conditions than others regardless of the latitude of their native range on Earth.

Natural lighting can be admitted through windows, light pipes, or other transparent openings in the greenhouse wall, with windows being the simplest method because they don’t require any sort of concentrator. The collector and aperture sizes must be chosen to allow for light lost to absorption by the window materials. Diffuse lighting is best for plant growth, and can be provided by introducing translucent glazes on windows and other apertures. Typical Martian sunlight tends to be more diffuse than that of Earth due to the high dust content of the Martian air, making it slightly more advantageous for plants than might be expected from its relative intensity alone.

Artificial lighting can also be used to grow plants. The spectra of most artificial light sources do not match the spectrum of sunlight, though, and thus are not optimal spectra for most plants. Thus, most artificial light sources need to use several different types of artificial light in combination in order to assure optimum plant growth. Plants require light in the blue visible spectrum to conduct photosynthesis and manufacture carbohydrates, as well as light in the red and far red visible spectrum to regulate their metabolism. Blue light allows the plant to grow. Red light allows it to decide when to do so. Most plants which grow, reproduce, or have some other aspect of their life cycle timed to a seasonal basis regulate their life cycle according to environmental signals given by the red light portion of the spectrum. Some leaf and root crops can be grown using only artificial lighting with a distinctly blue peak spectrum, but almost all flowering plants require a source of red spectrum light. Red spectrum light can also be used to manipulate plant growing seasons by timing environmental signals at the convenience of the grower.

Combining fluorescent light bulbs (which have a broad but predominantly blue spectrum) with incandescent light bulbs (which have a broad but predominantly red spectrum) will provide artificial lighting adequate for most plants. This arrangement also provides for independent control of the red light spectrum. Incandescent light bulbs have efficiencies less than a third those of fluorescent lights, and thus put out more heat and less light for the same power. Fortunately, plants tend to require less red spectrum intensity than blue spectrum intensity. The typical ratio of fluorescent lighting power to incandescent lighting power needed for most plants tends to be around 4 to 1, with small incandescent bulbs in the 10W to 20W range being adequate for many setups.

The power requirements for artificial lighting are typically in the range of 200W to 300W electrical power per square meter, depending on the plant species. This makes natural lighting far more energy efficient.

Many plants also require periods of darkness for their metabolic processes, preferably total. Natural lighting provides this on a daily basis, and artificial lighting can do so as well. The productivity of some crops benefits by extending illumination times beyond the natural day length, and a combination of natural and artificial lighting can be used to accomplish this, with heat losses from artificial lighting providing additional nighttime heat.

Structural Design Considerations

The structure of the greenhouse is highly dependent on its size and preferred light source. Larger pressurized structures need thicker walls and tend to be heavier. This design requirement will compete with the need for thin windows in a solar illuminated greenhouse, and with the need for thin walls in an inflatable greenhouse.

The windows of a solar illuminated greenhouse will tend to be the weakest portions of the structure due to the lack of transparent materials with both high tensile strength and reflectance at infrared wavelengths. Wire reinforcement is possible, and external reinforcing frames can reduce stresses considerably, but tends to introduce opaque material into the window structure, reducing its light transmission. It is possible to construct windows of sufficient thickness to withstand the pressures involved, but this also reduces light transmission. The design of greenhouse windows will be a tradeoff between light transmission, infrared reflectance, and structural strength which is likely best resolved with the use of composites rather than by constructing the windows of a single material.

The greenhouse size is also determined by the plants to be used, the planting schedule, and yield requirements.

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Plants for the Greenhouse

The dominant agriculture of terrestrial regions with latitudes greater than 60 degrees and mountainous regions should be examined for desirable plants and practices for Martian agriculture.

Many of the recommended crops for gardening at these latitudes tend to be cold tolerant leaf and root crops, which can not only survive but produce adequate yield despite exposure to freezing temperatures. Examples of cold tolerant leaf crops include cabbage, spinach, and swiss chard. Examples of cold tolerant root crops include potatoes, turnips, carrots, and parsnips. Fruit crops tend to not be as widely cultivated at these latitudes, with a few important exceptions being strawberries, cold-tolerant cucubrits (e.g., pumpkins), currants, and fruiting body crops such as broccoli.

The limiting factor in choice of crops at high latitudes is more commonly temperature rather than ambient light. With the use of greenhouses and/or artificial heat, the repertoire of potential crops can be expanded. Tomatoes, various beans, citrus crops, and other fruit crops tend to require higher temperature than root crops. However, when this is provided they often yield as well regardless of latitude. Growing these crops will likely require unpressurized internal enclosures within the greenhouse to trap additional heat.

Selection of varieties adapted to the growing conditions can also improve yield and/or hardiness of crops. For example, most varieties of wheat do not perform well at higher latitudes, due to the reduced temperature and lower light. Barley and other cold hardy grain crops tend to give better harvests at high latitudes. However, winter wheat varieties are available with the necessary cold tolerance to produce similar yields under these conditions.

Hybrids often have superior yield, hardiness or other attributes in comparison to non-hybrid varieties (so-called “heirloom” varieties). However, if the hybrid in question cannot be cultivated asexually by grafting, rooting, self-pollination or some method other than breeding, then use of the hybrid variety in greenhouse agriculture will require additional seed mass and growing area for production of the parent varieties (the varieties bred to create the hybrid seed) – both of which will be at a premium for a Martian greenhouse. For plants germinated from seed, well adapted heirloom varieties are generally superior to hybrid, saving both launch mass and growing space.

Food Production

A suitably sized greenhouse can be used to provide up to 90% of the food mass for a manned mars base. With the use of high yield gardening techniques, this can be accomplished with a growing area of 6m2 (65 ft2) per crew member. This figure for growing area per crew member includes a 100% margin to allow for fluctuating and/or reduced production vs. consumption. However, it also includes several important assumptions.

First, this figure requires sufficient lead time to produce crops. 16 weeks, minimum, is required to produce sufficient food to feed a person for an additional 16 weeks, with an average consumption rate of 1 kg/man-day. And capability to prepare and store the food produced (freezing, drying, canning, etc. with necessary storage space) must be available. Otherwise, the bulk of the food produced will go to waste.

Second, growing conditions must be optimal for the cultivated crops and relatively constant over time. This may be a difficult assumption to meet, as conditions will vary with the Martian seasons with or without reliance on solar illumination, and both lighting and average humidity will not be as high as is typical on Earth. Optimal humidity may prove difficult to provide in a greenhouse where the boiling point of water and carrying capacity of air both fall with the internal pressure, and temperature fluctuations will likely remain significant all year long. This may limit the choice of crops for each season and/or increase power requirements.

Third, a staggered crop rotation must be maintained indefinitely. This does not amount to a “no seasons” requirement, but does require the entire growing area to remain in cultivation for the entire Martian year. It also demands that no section of the greenhouse growing area be allowed to sit idle, even during the Martian winter. This may require seasonal variation in the crew’s diet.

A staggered crop rotation is possible if soil nutrients are replaced continually or immediately before cultivation (as happens in a hydroponic system or a flower bed). It takes advantage of the fact that not all plants have the same growth period from cultivation to harvest. By planting a new crop in the same area immediately after harvest, the space does not sit idle. This improves production efficiency. Choice of crops in rotation, as well as staggered plantings of the same crop in adjacent spaces, can reduce times between harvest to only a few days and allow near continuous harvest of some plants. There is no “harvest time” with staggered crop rotation, only time to harvest particular plots. This method tends to be more labor intensive than conventional agriculture, on average, but produced more yield for a given growing area.

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