Characterizing Biological Closed-Loop Life Support Systems for Thermal Control and Revitalization of Spacecraft Cabin Environments

Algal photobioreactors have been researched as potential solutions to air revitalization in a spacecraft cabin environment by absorbing CO2 and producing O2 through photosynthesis. This photosynthesis, and consumption of produced biomass, theoretically provides a solution for carbon loop closure during long-duration spaceflight. Algal culturing techniques such as growth in water-based media, could offer additional ECLSS functionalities. Utilizing the algal media as a thermal transport media, paralleling the operation of the current ISS thermal control system, the algal culture could be used for active thermal control of the spacecraft environment. However, this imparts rapid, extreme thermal swings on algal cells not accustomed to that environment, potentially resulting in a reduction of air revitalization capabilities. This body of research investigated the ability to utilize an algal culture for simultaneous air revitalization and active thermal control of a spacecraft or surface habitat. Developing a multifunctional system, such as using an algal culture for simultaneous air revitalization and active thermal control, has the potential to save on mass, power, and volume by addressing multiple ECLSS functionalities with one volume of algae. The conducted literature reviews, and subsequent experimentation investigated an algal culture’s metabolic response to dynamic thermal and CO2 concentration environments, reflecting the transient environment of a spacecraft cabin, to understand the potential benefits and shortcomings of this type of integrated system. Reviewing failure causes and modes of utilizing an algal bioreactor for ECLSS purposes informed the experiments going forward, as well as potential bioreactor designs for spaceflight. This resulted in a review article publication in the Life Sciences in Space Research journal entitled, “Failure modes, causes, and effects of algal photobioreactors used to control a spacecraft environment”. The effect of dynamic thermal environments, reflective of temperature profiles experienced in the ISS thermal loop, on the CO2/O2 turnover of algae was characterized to provide a first-order assessment of combined system feasibility. Culture viability was provided through measurements of metabolic response in terms of oxygen production rate, cell growth rate, and photosynthetic yield. Isolation of the independent variable (environment temperature) was achieved by executing experiments in 12-well plates on a Peltier cooling system. The dynamic temperature cycles oscillated between +9C to +27C over a 14-minute cycle, reflecting exposure between LTL and cabin temperatures. Results indicated that algal cultures continued to be viable in these dynamic temperature environments. Replications of these conducted temperature cycling experiments were executed using Antarctic algal samples, comparing the extremophile’s metabolic response to that of the previously tested Chlorella. It was hypothesized that the cryophilic algae would be adapted to the temperature cycle range found in the LTL, due to the Antarctic environmental temperatures, causing increased CO2/O2 turnover rates. Results suggested that the Antarctic cultures had significantly higher oxygen production rates than that of the temperature-cycled Chlorella, however additional testing is required to increase the strength of these results. Findings of both experiments plan to be discussed in a manuscript for journal publication. Algal metabolic response to changes in CO2 concentrations of the culturing environment were also investigated. The previously presented metrics were used for viability assessments, as well as photosynthetic quotient, a ratio of produced O2 to consumed CO2. Step-change profiles in provided CO2 concentration were used to study culture response to the minimum and maximum respired CO2 concentrations reported in NASA’s Baseline Values and Assumptions document (BVAD). These step changes reflected transitions in crew activities (i.e. sleep to exercise and vice versa). A gas-sparged photobioreactor was built with commercially available, off-the-shelf (COTS) parts to isolate the effects of the independent variable (input CO2 concentration). While the culture sustained viability through the changes in CO2 concentration, there was not a significant change in photosynthetic quotient, suggesting adaptation of the culture to the dynamic environment (ex. increasing O2 production with increased CO2 consumption). This research also included developing a benchtop photobioreactor that could support testing of simultaneous air revitalization and thermal control capabilities while minimizing the inclusion of gravity-dependent designs. A nonporous, gas permeable liquid to gas membrane contactor was selected for non-bubbling transfer of CO2 and O2 to and from the algal medium, appropriate for microgravity conditions. It was hypothesized that this membrane module could also be used for heat transfer from the cabin air stream to the algal medium. The simultaneous heat and mass transfer capabilities of the contacting membrane was characterized, and empirical equations describing this simultaneous heat and mass transfer were established for future system modeling. A journal article was prepared presenting the experiment and the resulting equations and is currently being prepared for journal publication. A tubular photobioreactor, including the nonporous membrane, was built and utilized for system testing. The design of the reactor was studied through utilization during experiments to understand potential difficulties in operation and its ability to support an algal culture in a dynamic environment. Experiments simultaneously cycling temperature and input CO2 concentration were executed, the longest duration experiment lasted about 25 days. The tubular reactor successfully supported the algal culture. However, the culture was filtered out by and contained inside the membrane contactor, but this did not significantly affect the heat or mass transfer capabilities of the membrane. The experiments and their outcomes are currently in work to be published as a journal article. In conjunction with the lessons learned from operation of the benchtop photobioreactor, a literature review was conducted to execute a first-order comparison of the mass, power, and volume of current ISS ECLSS systems to that of an algal photobioreactor fulfilling those same functions. This comparison suggests that using a photobioreactor-only approach to address all ECLSS functions may not provide significant mass, power, or volume savings. However, utilizing a mix of bioregenerative (algal photobioreactor) and current ISS approaches to ECLSS requirements may still provide savings in mass and volume. This comparison was presented and published (“Feasibility of photobioreactor systems for use in multifunctional environmental control and life support system for spacecraft and habitat environments”) as part of conference proceedings for the 46th International Conference on Environmental Systems. Forward work from this research includes conducting replications of the previously conducted experiments to increase confidence in the results, using bioprospecting techniques to select algae currently growing in environments reflective of the spaceflight operational regimes or investigating blends of cultures (instead of monospecies experiments) to broaden operational ranges, and investigating variation of other input factors -such as irradiance to control culture metabolic rates. The experiments for this body of work were conducted over relatively short timescales (7-25 days) when compared to the potential of use on a long duration spaceflight (>90 days). Increasing culturing time may elucidate any long-term culturing issues or changes in the culture biome that may influence system operation. Conclusions from this work indicate that algae, specifically Chlorella, are viable in dynamic temperature and CO2 concentration environments. Adapting to the surrounding environment, the culture was able to measurably shift its metabolic processes to sustain itself through the duration of various experiments. While the implementation of photobioreactors into human spaceflight designs are just commencing, this research has indicated algal cultures can withstand the tested cabin environments and may still be sustainable for simultaneous air revitalization and thermal control of a spacecraft cabin environment.