Vadim Ye Rygalov

Water cycle and its management for plant habitats at reduced pressures.

Experimental and mathematical models were developed for describing and testing temperature and humidity parameters for plant production in bioregenerative life support systems. A factor was included for analyzing systems operating at low (10-101.3 kPa) pressure to reduce gas leakage and structural mass (e.g., inflatable greenhouses for space application). The expected close relationship between temperature and relative humidity was observed, along with the importance of heat exchanger coil temperature and air circulation rate. The presence of plants in closed habitats results in increased water flux through the system. Changes in pressure affect gas diffusion rates and surface boundary layers, and change convective transfer capabilities and water evaporation rates. A consistent observation from studies with plants at reduced pressures is increased evapotranspiration rates, even at constant vapor pressure deficits. This suggests that plant water status is a critical factor for managing low-pressure production systems. The approach suggested should help space mission planners design artificial environments in closed habitats.

Low Pressure Greenhouse Concepts for Mars: Atmospheric Composition

The main principles of artificial atmospheric design for a Martian Greenhouse (MG) are described based on: 1. Cost-effective approach to MG realization; 2. Using in situ resources (e.g. CO2, O2, water); 3. Controlled greenhouse gas exchange by using independent pump in and pump out technologies. We show by mathematical modeling and numerical estimates based on reasonable assumptions that this approach for Martian deployable greenhouse (DG) implementation could be viable. A scenario of MG realization (in terms of plant biomass/photosynthesis, atmospheric composition, and time) is developed. A list is given of technologies (natural water collection, MG inflation, oxygen collection and storage, etc.) that are used in the design. The conclusions we reached are: 1. Initial stocks of oxygen and water probably would be required to initiate plant germination and growth; 2. Active control of MG ventilation could provide proper atmospheric composition for each period of plant growth; 3. MG operation based on simplest technological solutions could provide for oxygen accumulation for people arriving on Mars. There is a reasonable prospect of achieving cost effectiveness during a single 600-day mission. A short description of future development of a Mars Greenhouse-project is presented. INTRODUCTION: GREENHOUSE ATMOSPHERIC COMPOSITION BASED ON THE NATURAL RESOURCES OF THE PLANET Terrestrial plants need a balanced composition of atmospheric gases to be maintained within certain limits for growth (Wheeler et al., 2000). Carbon dioxide and oxygen are the basic gases used in the processes of photosynthesis and respiration: CO2 + H2O ⇔ CH2O + O2. During plant growth, carbon dioxide is used in greater amounts than oxygen, but oxygen is particularly critical during germination and for respiration at dark period. Presence of some trace organic gases in a closed volume (ethylene and etc.) is also important for plant growth regulation (Devlin, 1975; Chernigovsky, 1975; Lisovsky, 1979). The average composition of the Earth’s atmosphere compared to the atmosphere of Mars is presented in Table. 1 (APPENDIX, III). From the analysis of this table the following conclusions could be formulated: There are considerable differences in atmospheric composition between Mars and Earth; Carbon dioxide is presented in the Martian atmosphere in significant amounts; Oxygen and water are not common in the Martian atmosphere; The existing Martian atmospheric composition is probably not suitable for Earth’s plants growth; (other environmental parameters: temperatures, high UV – radiation, toxic gases and dust, etc., may also be a problem) Growing plants on Mars will require artificial growth conditions inside a closed environment. An analytical model can be used to simplify analysis of the minimal greenhouse atmosphere for plant growth and development. This minimal, most cost effective, atmosphere (Rygalov et al., 2001) should include CO2, O2, and water vapor at sufficient concentrations to support vigorous plant growth. We need to determine by experimentation the cost of maintaining particular conditions, and what effect different conditions would have on plant productivity. This atmosphere could probably be maintained by controlled gas exchange between the greenhouse and outside Martian air during all period of plants growth. However, due to the low concentration of oxygen and water vapor, other sources of these commodities might be more cost effective. The rate of gas exchange should be regulated to account for plant growth (photosynthesis rate changes). It is expected that any gas mixture dense enough to support plants will be adequate to maintain the mechanical pressure needed to keep the soft Deployable Greenhouse (DG) covering inflated. If the most cost effective pressure is found to be near Earth-normal pressure, there might be a problem in designing a cover that is both strong enough and transparent. Both composition of the atmosphere & DG mechanical design are closely connected through the desired exchange of air inside a greenhouse and with outside atmosphere. So, this becomes the task of Artificial Atmosphere Design (AAD). This design issue can be solved in different ways. The cheapest and most cost effective design approach is probably using of indigenous Martian resources (carbon dioxide, oxygen, water, etc.), and the lowest pressure that is suitable for vigorous plant growth. COMBINING ELEMENTS OF ARTIFICIAL ATMOSPHERE DESIGN The basic description of the atmospheric constituents dynamic inside enclosure could be presented as: Velocity of changes in atmospheric concentration (or partial pressure) of the gas = Initial atmosphere + Input from outside (provided by controlled pumping-in process) Losses to outside (provided by controlled pumping-out process or regulated venting, and unregulated leakage) + Influence of system’s closure (conversion or modification by plants and microbes, physical sorption, chemical reactions, etc.). Assumptions: an elevated pressure is required for mechanical stabilization of DG, so greenhouse ventilation is realized by forced pumping-in and pumping-out or controlled passive venting and unavoidable leakage; plants inside the greenhouse take in carbon dioxide and release oxygen; relative humidity is maintained by evapotranspiration and condensation; neutral gases go through the DG without any modification; however, they may need to be purged at intervals if they result in excessive pressure. Partial and total pressure changes are due to the following: • gas pumped in • gas removed • photosynthesis and respiration • leakage • transpiration This qualitative description is represented in the following mathematical description: dPi/dt = (F/V)*Pio + fiX/V + ((KLG)/V)*(Pis – Pi) – (KCGC/V)*(Pi – Pic) – (L/V)*(Pi – Pio) – (R/V)*Pi ; where: Pi = partial pressure of certain gas inside greenhouse; Pio = partial pressure of certain gas in outside atmosphere; Pis = saturated partial pressure of the gas at system temperature (included in water vapor dynamic description only; Rygalov et al., 2002); Pic = saturated partial pressure of the gas at cooling unit temperature (included in water vapor dynamic description only; Rygalov et al., 2002); F = outside air flow rate into greenhouse; R = inside air flow rate out from greenhouse (or in other words forced leaks); L = total free leaks flow rate; (Note: parameters F, R and L characterize the regime of greenhouse ventilation); V = total volume of greenhouse; X = plants biomass; fi = specific rate of gas conversion in photosynthetic processes (can be positive, for example for O2, or negative, for example for CO2); G = area of the gas evaporation (included in vapor dynamic description only; Rygalov et al., 2002); GC= area of gas condensation (included in vapor dynamic description only; Rygalov et al., 2002); KL = specific rate of gas evaporation (included in vapor dynamic description only; Rygalov et al., 2002); KC = specific rate of gas condensation (included in vapor dynamic description only; Rygalov et al., 2002); t = time. This mathematical description is not specified for certain physical unites. The total dynamic pressure in the system is equal to the sum of the individual dynamic pressures for all gases in the system. It is assumed in the equation presented that DG operation is developing with a constant system temperature. Any changes in temperature could be taken into account through changes of numerical values parameters of the model, but wide temperature swings are generally detrimental to plants. The mathematical model presented includes the most general processes determining gaseous dynamic of the greenhouse (Henderson et al., 1997; Johnson, 1999). Any other components of DG atmospheric composition dynamic (trace contaminants, or some specific sources and sinks of gases) could be easily introduced in further specific development of this model. For calculation of DG atmospheric composition this mathematical approach is applied to each constituent gas, including: 1. CO2; 2. O2; 3. H2O; 4. Neutral gases; 5. Ethylene and trace gases (neglected in the approach developed in the paper); 6. Total pressure. The solution of the differential equation for each atmospheric gas was obtained by standard well known methods, and is presented in the (APPENDIX, I). We are going to use the steady state part of this solution only in further consideration. EXPERIMENTAL REALIZATION Some of the equation parameters that characterize a greenhouse enclosure can be more easily measured experimentally (for example: KL, KC, F, L, R) than calculated directly. For these experimental measurements a Martian Greenhouse prototype was developed. The prototype, shown in Fig. 1, has a hemispheric Lexan cover installed on a stainless steel base. The prototype had a diameter of ~ 1 m, and a total volume of ~ 0.417 m. We envisage the top being made of soft transparent materials for operational use. Soft materials are planned to take a test in later works. The prototype includes the following internal environmental controls: a cold coil with an external chilled water source; resistance heating elements; an air circulation fan; A pump with rate ~ 5 L/min air under 101.3 kPa is provided for gas exchange between the system and the outside air of Thermotron. This system allows us to realize a closed water cycle and provides atmospheric gases from outside. Fig. 1. Martian Greenhouse Prototype (MGP). (In the center of the Dome there is system climate control tower that includes a coolant coil and a water distribution unit.) This MG prototype was designed to be installed inside another vacuum chamber that allows us to imitate Martian surface conditions. The vacuum chamber (Thermotron, Fig.2) has the following internal dimensions: 1.22 m-width, 1.22 m-height, and 1.62 mdepth. It provides environmental control that ranges from terrestrial to Martian conditions: temperature ~ -72 to +177 C: pressure ~ 0.1 kPa ( ~ 1 mm Hg)

Design Parameters for Mars Deployable Greenhouses

Concepts for landing missions on Mars often include greenhouse structures for plant production. The types of structures proposed vary from small automatically deployed structures for research purposes to larger structures that would be used for food production. Present plans are that greenhouses on Mars will be operated at internal pressures as low as 0.1 to 0.2 Earth atmospheres. Low internal pressures permit the use of structures with lower mass, but complicate the heat and mass transfer processes involved in maintaining a suitable environment for plant growth and raise questions about the requirements of plants for growth at low pressures.