Research on greenhouses & assumptions
Research on greenhouses, the assumptions included, and assumptions not included
A greenhouse on Mars should not get down to 2 degrees Celsius, at which temperature the desired plants would not be efficiently producing high energy chemical reactions by photosynthesis. A constant 20 degree Celsius adds a slight economic burden upon the greenhouse growth system as compared to a constant 13 degree Celsius, but the difference is too small to counter increased productivity at 20 degrees. In practice a temperature varying a couple of degrees either way around 17 Celsius might be most economic for some crops. Testing variations and maximizing for food production per dollar can be done by mathematical models. A solar heated greenhouse may be more economic than and artificially lighted and heated underground greenhouse.
Some researchers investigating solar heated greenhouses on Mars have assumed a dome shaped greenhouse with 1/2 the solar light intensity available on Earth. This is a valuable set of assumptions to research but sunlight concentrated by mirrors and entering a greenhouse only through the roof while all walls are regolith insulated is another set of assumptions that could be modeled. With the flat roofed sunken greenhouse it would be possible to have a thermally insulating cover on the roof during the night and only open the roof to sunlight in that portion of the day during which light is strong enough to maintain operating temperature. The light in turn would directed horizontally by flat panel mirrors then directed vertically downward by cylindrically curved concentrating mirrors. This might provide more than adequate heat for temperature maintenance . The flat transparent roof would require strong squares of glass in a steel framework to form a triple or quadruple paned insulating window. In the quadruple pane case, the pressure in the three insulating spaces would be: outer layer, 2.5 psi; second layer, 5.0 psi; third layer, 7.5 psi; interior, ten psi. An 8 foot by 8 foot section of such a greenhouse roof would carry 42 tons of upward force from the contained atmosphere. This is well within the range of what can be carried by structural steel tensile members wrapped around the circumference of the buried cylindrical greenhouse. A flat roof top exposed to the Martian environment would provide easier maintenance by automated cleaning and polishing devices but a sky light that continues the cylindrical curvature of the outer wall of the greenhouse would be cheaper to build. Each 8 inch by 8 inch square of glass held in the steel framework would hold 160 pounds of force, also well within what can be achieved with current materials. To limit the damage from possible accidents 8 foot by 8 foot sections of greenhouse could be separated from each other and from living spaces by pressure bulkheads with access through a pressure lock set of double doors. The size of an isolated section might be considerably larger, depending upon what is the most economic way of dealing with perceived risks.
Other researchers claim that there is no material which can safely contain greenhouse atmosphere pressure while providing proper thermal insulation, puncture resistance and admitting sufficient light for crop growth. It would be interesting to know if this research was based upon assuming a dome shaped green house with 1/2 Earth standard solar radiance as the competition for their artificially lighted growth rooms. The actual situation on Mars makes the dome greenhouse more practical than one might think. The very low atmospheric pressure and low gravity combine to make convection between two walls with gas between much more difficult. So, a thick dead air space between an outer protective dome and an inner pressure retaining dome might provide all of the insulation needed for a greenhouse when the sun shines. This is an arrangement suggested by Robert Zubrin in which the outer dome also protects against UV radiation. At night and during dust storms a further insulating cover could be deployed to stop radiant heat loss.
Solar power on Mars can produce just as hot and just as large a spot of concentrated sunlight as solar power on Earth. The differences are that on Mars the mirror must be 1.5 times greater in aperture and 1.5 times farther from the focus; and the average interference from clouds on Earth is greater than the average interference from clouds and dust on Mars. There is no question that materials suitable for a solar powered greenhouse on Mars are available. The question is how much should the sunlight be concentrated before it is directed into the greenhouse through the skylight and diffused under the skylight to the desired intensity for crops. The desired level of concentration would have the energy density of the entering sunlight equal to the energy density of the heat loss at a minimum and somewhat greater than the passive heat loss at the maximum. Excess temperature would be reduced as needed by enhanced cooling. Pumped fluid would carry the excess heat outside to radiators. Pumping power can come from natural convection. Enhanced cooling and/or additional reflected sunlight could be used along with the dome greenhouse concept as needed.
All available options for greenhouse architecture should be considered before settling on one or two architectures to be fully developed. It may be best to set up a colony initially using nuclear power for artificially lighting growth rooms, because this would be the quickest and easiest method to set up. Later solar heated greenhouses could be developed and used, allowing the diversion of electrical power to competing uses as they are developed.