Functionality and setup of the algae based ISS experiment

46th International Conference on Environmental Systems ICES-2016-203 10-14 July 2016, Vienna, Austria

Functionality and setup of the algae based ISS experiment

PBR@LSR

Jens Bretschneider1, Stefan Belz2, Harald Helisch3, Gisela Detrell4, Jochen Keppler5, Stefanos Fasoulas6

University of Stuttgart, Stuttgart, Germany, DE70569

Norbert Henn7

German Aerospace Center (DLR), Bonn, Germany, DE53227

and

Peter Kern8

Airbus Defense and Space, Immenstaad, Germany, DE88090

Hybrid life support systems combining physicochemical and biological algae based

subsystems are in great interest for the midterm future of manned spaceflight e.g. extensive

ground missions on Moon and Mars. Whereas many possible systems have been theorized

and tested in laboratory conditions, no experiments on realistic system level have been

performed in space. The DLR technology experiment PBR@LSR (Photobioreactor at the

Life Support Rack, former name PBR@ACLS) is set to give a first technology and

performance demonstration on board the ISS in the Destiny module in 2018 by combining

an algae based photobioreactor with the carbon dioxide concentrator of ESA’s Advanced

Closed Loop System built by Airbus DS. This paper shows the design process of the ongoing

flight hardware development. The prototype design is detailed in conjunction with the

operational requirements, safety aspects and scientific needs. The connection to the carbon

dioxide concentrator enables the use of carbon dioxide from the cabin. The algae medium

loop consists of different components of specific functionalities (pumping, illumination,

nutrient supply, temperature control, etc.) to allow cultivation of the microalgae Chlorella

vulgaris. The progress of ground experiments is presented and the derived decisions for the

system design and setting parameters are explained. The paper concludes with an overview

of reached and open milestones to the flight design of PBR@LSR.

Nomenclature

DS = dry substance

PBR = photobioreactor

ECLSS = environmental control and life support system

p/c = physico-chemical

PPFD = photosynthetically active photon flux density

OD = optical density

µg = microgravity

1 PhD candidate, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected] 2 Head of working group, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected] 3 PhD candidate, Institute of Space Systems, Paffenwaldring 29, 70569 Stuttgart, [email protected] 4 Postdoc, Institute of Space Systems, Paffenwaldring 29, 70569 Stuttgart, [email protected] 5 PhD candidate, Institute of Space Systems, Paffenwaldring 29, 70569 Stuttgart, [email protected] 6 Head of Department, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected] 7 Space science engineer, Space Administration, Königswinterer Str. 522-524, 53227 Bonn, [email protected] 8 Senior expert life sciences, Claude-Dornier-Strasse, 88090 Immenstaad, [email protected]

https://de.wikipedia.org/wiki/Photosynthetically_Active_Photon_Flux_Density

International Conference on Environmental Systems

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I. Introduction

With the renewed interest in extending the capabilities of manned space flight beyond low earth orbit in the

coming decades, advances in many areas of space flight are required. The ECLSS is one of the subsystems requiring

the most improvements compared to current technology in use on board the ISS, if we want to achieve long-time,

long-distance, and safe missions. The main driver for all developments of the ECLSS are crew healthiness in

general, system reliability, operational robustness or the increased closure of material cycles and thus the decreased

need for resupply. Improvements in physio-chemical (p/c) ECLSS-technologies are an important part of this

development. With the high percentage of loop-closure for water and oxygen, p/c-systems are able to cover the bulk

of resupply need. However, only biological systems are able to achieve the last few, important percent of loop

closure by also recycling carbons, nitrates, phosphates, and organic compounds.

As on Earth, many species are candidates for biological components in an ECLSS. Vascular plants and

especially edible plants, e.g. crops, potatoes, salad and tomatoes, are the most common plants regarded for

utilization in manned space travel based on publications in the last decade. Algae and cyanobacteria species have

been considered as suitable alternatives since the early 1960s, also being used in ESA’s MELISSA pilot plant1. At

the Institute of Space Systems in Stuttgart, research on algae-based ECLSS has been performed since the early

2000s. Results so far have shown comprehensively, how algae systems perform in laboratories, how such systems

can be optimized, how algae-based ECLSS compare to systems based on vascular plants, and in which scenarios

biological systems theoretically are more efficient2-4. In addition, some experiments on actual algae cultures under

microgravity (µg) conditions were done on board the ISS5-7. However, an actual demonstration of a large-scale,

biological ECLSS component in µg conditions is still

missing.

The experiment PBR@LSR targets this gap by

providing applied research on a complementary

biological life support element on board the ISS

during up to 6 months. An alga reactor represents the

biological component of the system. The experiment

is connected to the Advanced Closed Loop System

(ACLS) (see Figure 1), which itself is currently under

development at Airbus DS and planned for launch

onto the ISS in late 2017 to early 2018. PBR@LSR is

targeted to launch after the commissioning period of

ACLS in 2018 and installation is currently planned in

the destiny module. PBR@LSR shall demonstrate the

standalone performance of a PBR with the species

Chlorella vulgaris as component of a hybrid ECLSS.

Therefore, this technology verification experiment focuses on measuring the CO2 consumption, O2 production, and

biomass production of the algae over extended periods in the context of actual system boundaries and conditions on

board the ISS. With this data, it will be possible for the first time to evaluate performance over time, system

stability, reliability, and biological stability of a biological ECLSS.

This paper addresses some of the engineering challenges that have been studied during prototype design, ground

testing of the prototype, and flight design of the experiment hardware. After an initial overview of the experiment

hardware and a summary of the experiment methods, section IV highlights the design for the main subsystems and

testing of these subsystems. Lastly, exemplary data from on-ground operation of the complete experiment are

presented. This paper is accompanied by a second paper presented at ICES 2016, which focuses on the biological

experimentation done for PBR@LSR8.

II. General System Design

The flight design of the technology demonstration experiment PBR@LSR is currently in active development and

optimization. So far, prototypes have been designed and built to perform a number of biological and engineering

tests. The general setup of both the flight design and the prototypes is shown in Figure 2.

The central component of the reactor is the algae loop. A pump driven loop circulates algae-medium-solution

constantly through reactor chambers, tubing, sensors for pH and optical density, and a flow indicator. The loop is a

single containment for liquids. This setup is placed in a gas-tight experiment compartment. Semi-permeable

membranes realize gas exchange between the algae loop and the experiment compartment. Multiple sensors for gas

Figure 1. Photobioreactor in context of the existing

ACLS consumables flows.


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composition, pressure, and temperature are placed inside the gas compartment. Fans constantly circulate the gas

inside the compartment. Temperature regulation is ensured by heat exchangers and an external water-cooling loop.

Gas exchange with the environment is realized by multiple ways. Either intake gas is delivered either by ACLS or,

in case ACLS is non-operational, by a backup CO2 supply. If required, the compartment can additionally be vented

with cabin air. LEDs inside the experiment compartment provide lighting for the algae. Gas removal is done either

by an O2 absorber or by venting accumulated gas to the cabin.

When algae grow in the loop, the cell density increases, making it necessary to remove algae material from the

Figure 2. Schematic Setup of experiment PBR@LSR

Figure 3. Prototype setups for PBR@LSR. Left: early prototype; right: current prototype with flight design

dimension.


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experiment and to replace the algae-medium-solution with new nutrients and water at the same time. This exchange

can be scheduled periodically. Automation of this operation is not foreseen in this experiment. Instead, a support

equipment has been designed to have this performed by crewmembers during short periodical tasks. As the growth

rate of algae depends on many variables8, several control loops are used to control the development of the algae

culture inside the experiment preemptively.

The experiment will be accomodated into a standard mid-deck locker and is rack-mounted. The experiment

houses about 1 L of algae culture in two reactors. This volume can be scaled by adding more reactors and is only

limited by the capabilities of the pump. About 9 L of gas volume are contained in the experiment compartment.

Figure 3 shows the progression the system design took throughout different prototype stages. The earlier designs

were much bigger than the described values, which provided higher flexibility in the setup. In contrast, the current

prototyped design represents the flight design dimensions, but also requires specifically sourced components.

III. Experiment Methods

A. Cultivation parameters and culture preparations

The experiments performed for PBR@LSR are performed with Chlorella vulgaris strain SAG 211-12 obtained

2005 from the Culture Collection of Algae Goettingen. The culture has since been cultivated in the Institute of Space

Systems facilities in Stuttgart, mostly in flat-panel-airlift-reactors provided by Subitec and partly in smaller, in-house

flat-plate reactors. The culture is nonaxenic, but is free of other alga strains or any major contaminants. Inoculums

for the prototype experiments described in this paper are taken from these long time cultures during exponential

growth phase of the culture. Preparation for inoculation in the experiment include dilution with growth media to the

desired initial biomass concentration of experiments, visual inspection and measures for homogenization of the

inoculum. The growth media is a diluted seawater nitrogen medium at pH 7. Macronutrients are used as phosphate

and nitrogen source and the concentrations are regularly monitored and adjusted for NH4+ at 100 - 500 mg/L and for

PO43- at 50 - 200 mg/L. The cultivation is performed at 25 - 29°C.8

B. Optical density and biomass concentration

The optical density (OD) is one of the major quantitative measurements used to determine the condition and

development of an algae culture. For a high density culture like the ones used in these experiments, direct

measurement is not possible as OD measurement is only defined for clear liquids with reliable results in a range of

OD = 0 - 1. High concentrated samples are diluted with de-ionized water at 1:10 or 1:100 and the derived, measured

values are multiplied by this factor again to reach the high, virtual ODs above 5. The optical density can be

measured at different wavelengths. For liquids containing chlorophyll, particularly light at λ = 680 nm or λ = 750

nm is used9. For values in this paper, indices indicated the used wavelength. OD values have been determined with a

Hach DR 2800 spectrophotometer.

Closely correlated with the optical density is the biomass dry substance (DS) of the alga-suspension. This

parameter is of actual interest for later applications. However, as it is more laborious to measure then the OD, the

OD and dry-biomass concentration of our culture have been correlated over multiple months and a wide range of

densities, allowing an indirect measure of the DS through the OD. For dry-mass determination, cellular material was

dried at 105 °C for 24 h and the dry mass was weighted. Eq. 1 and 2 present the correlation for both OD750 and

OD680 with a coefficient of determination R² = 0,986.

DS = 0.2886 [g/L] x OD750 (1)

DS = 0.2312 [g/L] x OD680 (2)

C. Culture vitality

The health status of an algae culture is usually determined by macroscopic factors. Suspension color, smell, dry

weight and gas turnover rates are often good indicators for status of a culture and the imminent future development.

However, these indicators are unsuitable for the detection of sudden changes in the status of the culture and

problematic signs can take multiple days to show up in macroscopic behavior until it is too late for counter

measures. The easiest way to circumvent this delay is a microscopic inspection of the culture. At magnifications of

400 to 1000, single cell morphology can be monitored. As many problems do not involve morphologic changes, an

additional colorization was used to monitor culture health.


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Cellular staining during ground tests was performed by using Eosin Y disodium salt solution (10 mg/ml)10. Eosin

Y is able to penetrate the porous cytoplasm membranes of damaged or dead cells. The intracellular machinery of

dead cells is not able to degrade or remove the dye resulting in a selective red staining. Living cells remain unstained

or green. Before counting, cells were diluted to a standardized concentration of DS = 0.3 g/L. Total cell counts

(TCC) as well as living cell counts (LCC) were determined by using a Thoma hemocytometer. The vitality ratio is

calculated according to Eq. 3.

Vitality = LCC / TCC (3)

IV. Subsystem Design and Testing

A. Reactor chamber

The reactor chamber forms the core of the experiment setup. It combines the algae culture, lighting, gas transfer

and active mixing into one component. A wide range of reactor designs has been developed over the years for on

ground operations. Simple and cheap designs as open-pond-reactors or raceway-ponds are used widely in farms.

More advanced designs like pipe-reactors and flat-panel-airlifts are in active development and achieve significantly

higher cultivation efficiencies11. All of these designs have major problems with combinations of operational safety,

biohazard, contamination control, automation, gravity dependency, scalability, and cost when transferred into

microgravity. The Institute of Space Systems has researched many designs for a µg-compatible reactor design over

the last years3, 4 and uses a meandering, flat-bed reactor developed by Airbus DS for other cell cultivation

applications.

Figure 4 and Figure 5 show the reactor design for PBR@LSR. Each reactor is a sandwich of a reactor core, a

semi-permeable membrane, membrane holders and LED panels. The reactor is based on a pipe-reactor design as the

alga-suspension is pumped through an extensive, narrow, meandering path and looped constantly. Commonly, the

problem pipe reactors face is the constant enrichment of the suspension with O2, which is produced by the algae and

can not escape the loop until dedicated gas exchangers are reached, leading to an oversaturation of the liquid loop

with O2 and creating a less favorable, bi-phasic environment for the alga. The chosen design eliminates this

problem. Large parts of the reactors surface a covered by membranes which allows a constant removal of produced

O2 and resupply with CO2 as long a partial pressure difference is maintained between the inside of the reactor loop

and the gas volume of the experiment. The selected membrane shows a negligible water loss . The membrane

surfaces are are optically transparent, with a

transmission of >98% in the illumination spectrum.

A major challenge with the reactor is the

formation of biofilms and general adhesion of cells on

the walls of the reactor. The formation of visible

layers is critical for two reasons. First, cells that are

adhered to the reactor are not circulated in the loop

anymore. This is not a problem for the supply of

nutrients and gases to these cells, but it is for

measuring optical density and biomass. The derived

calculations for turnover and the operational decisions

on harvesting algae from the loop are negatively

impacted by these immobile cells. Secondly, adhering

is observed most intense towards light sources. The

forming films block light from entering into bulk of

the culture and overexpose the immobile cells. These

sticking effects are hard to counter reliably. Tests on

mechanical removal of the films by continuous

shacking, scraping with added particles in the algae

suspension and polishing of surface areas were not

successful. A great reduction of adhering cells was

observable when the level of provided light is finely

tuned to the current optical density. Nearly no biofilm

was observable for a culture at DS = 2 – 3 g/L and a

photosynthetically active photon flux density of

PPFD=300 µmol/m²s. Even if biofilm can not be

Figure 4. Design of reactor chamber in cross-section.

Figure 5. Design of reactor chamber in frontal view.

https://de.wikipedia.org/wiki/Photosynthetically_Active_Photon_Flux_Density


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avoided completely, the constant pumping of the

culture ensures a homogenization of parameters

in most regards. The constant flow equalizes

inconsistencies in cell density, lighting intensity,

temperature distribution, dissolved gas

concentrations and gas transfer.

B. Pump and liquid loop

A pump is required to drive the alga medium

loop. Any pump introduces mechanical stress

onto the alga cells by either shear forces,

pressure swings or vibrations. Damaging effects

of these mechanical stresses on algae as

Chlorella vulgaris are well documented12, 13. For

initial testing, multiple pumps representing

different concepts were chosen. A Watson-

Marlow peristaltic pump has been tested and was

previously used for short-duration pumping of

algae cells without noticeable, negative effects.

Further, a membrane pump by KNF was tested

because one of its advertised use-cases is the

damage-free pumping of human blood cells.

Lastly, a gear pump from Ullermann KFT was

tested.

All three pumps were investigated with a

fixed test setup. A suspension of algae cells and

nutrient solution with a fixed starting optical

density is circulated in a small sample loop of 10

ml for up to 54 h, equivalent to a comparable

amount of passes of each alga cell through the

pump in the actual 6 months PBR@LSR

experiment. Over the run time of the experiment,

small samples have been taken from the loop and

analyzed for their cell vitality with Eosin-Y. This

vitality is normalized to the vitality of alga cells

in an identical setup without a pump, leaving

only the damaging influence of the pumps as

variable. The progression of vitality over time at

a fixed pump speed of 45 ml/min for all three

pumps is shown in Figure 6. It is prominent that

the peristaltic pump performs superior in

comparison to the other two concepts.

Especially, the gear pump shows a very weak

performance. The gear pump leaves many cells

with destroyed cell wall as shown in Figure

9Figure 8 and is therefore completely inadequate

for usage with Chlorella vulgaris.

The peristaltic pump performed best overall,

was chosen for implementation in PBR@LSR

and performed an additional test. The described

test was repeated for different pump speeds.

Results for the correlation between cell vitality

and pump speed are shown in Figure 8. The

pump speed range for PBR@LSR was limited to

Figure 8. Remaining vitality of sample cultures (10 ml)

with peristaltic pumps at varying speeds.14

Figure 9. Microscopic view of culture after pumping with

gear pump (54 hours at 45 ml/min) and staining with

Eosin-Y.14

vital cell

non-vital,

intact cell

Figure 6. Remaining vitality of sample cultures (10 ml)

with pumps running at 45 ml/min.14


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50 – 200 ml/min to avoid the increased cell degradation seen above 200 ml/min.

Additionally to the pump selection, the tubing, connectors and the meanders in the reactor chamber are sized to

ensure a laminar flow throughout the complete loop, greatly reducing mechanical stress on the alga cells. A

maximum Reynolds-Number Re = 400 is reached, staying clearly below the laminar cut off at Re < 2300.14

C. LED

Light drives the photosynthesis process performed by the algae in the reactor and is one of the limiting factors

for biological life support system performance. In comparison to higher plants, algae are more flexible regarding

their lighting source. A dedicated day-night-cycle is not required and the culture can be operated in a near steady

state. Still, an oversaturation with photons must be avoided as photo-oxidation can occur at high light intensities15.

As with higher plants, chlorophyll is the chemical component allowing for conversion between light energy and

chemical energy16. Chlorophyll a and b are the major chlorophyll components in Chlorella vulgaris, which is

reflected in the alga’s absorption spectrum. Absorption peaks occur at about 435 nm, 475 nm, and 676 nm in the

blue and red range of light. The exact spectra can vary noticeably by strain and the culture is additionally able to

adapt its ratio of Chlorophyll a to b based on the used lighting source17. The absorption spectrum shown in Figure 10

is data from the culture in Stuttgart, based on Chlorella vulgaris strain SAG-211-12 after multiple years of

cultivation. The spectrum is consistent with other publications18.

Extensive research on different light

setups for algae including Chlorella

vulgaris has been done19-21. Derived from

this work, two LED-based light setups

were tested for PBR@LSR. In multiple

tests, white lighting and red-blue lighting

was compared. The respective spectra of

these LED are also shown in Figure 10.

Main driver for the decision between

these two implementation were the power

consumption at give photosynthetically

active photon flux density (PPFD) and

growth experiments performed with

Chlorella vulgaris. The growth charts of

multiple cultures under varying lighting

conditions are shown in Figure 11. Growth

of algae cells was demonstrated with both

light setups with no significant differences

in growth speed. Both lighting experiments

were performed at an initial PPFD of 100

µmol/m²s, which was increased to

150 µmol/m²s after DS = 1.15 g/L was

reached to compensate for the higher

absorption potential of a denser alga

culture. At PPFD = 150 µmol/m²s, the

lighting for 4 panels of 200 mm x 200 mm

each require 11 W power for white lighting

and 10 W for the red-blue lighting. At

PPFD = 200 µmol/m²s, this raises to

14.8 W and 12 W respectively. Both

lighting solutions are equally suitable for

PBR@LSR. The red-blue combination was

chosen in the end, as it exerts a slightly

lower thermal load and allows for a higher

flexibility during experiments. The red and

blue LEDs are controlled individually,

making the distribution of blue and red light an additional degree of freedom for experiment parameters.22

vital cell

non-vital,

intact cell

Figure 11. Culture growth comparison between white lighting

and red-blue lighting in in-house PBR.22

Figure 10. Absorption spectrum of C. vulgaris (at DS = 1 g/L)

compared to LED-lighting options.22


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D. Gas Handling

Supplying CO2 to the alga cells and removing excess O2 is crucial to ensure continuous operation of the PBR.

This gas exchange challenge requires a solution able to handle the multiphase-flow of liquid and gas under

gravitational loads varying from 1 g during ground tests to µg on ISS. As previously described, the chosen solution

is a semi-permeable membrane.

Figure 12 gives a compact overview of the multiple steps required to use the CO2 from ACLS with the culture

volume and return the O2. In the context of this experiment, produced O2 is vented into the cabin.

Near pure CO2 is supplied by ACLS. PBR@LSR is intended to operate during periods of long continuous

operation of ACLS. In case that the CO2-supply from the ACLS is interrupted, the gas will be taken from an

redundant back-up storage of CO2 will be provided for PBR@LSR to ensure an uninterrupted operation of the

experiment. The gas-control systems

keep the CO2 in the range of 6-10 %vol

CO2 and the O2 concentration at 10-20

%vol.

From the experiment

compartment, the semi-permeable

membrane ensures an exchange with

the dissolved gases inside the reactor

as long as a trans-membrane pressure

difference is maintained. The used

Sarsted membrane was surveyed for

gas transfer capabilities as shown in

Figure 13. The data show that O2

transfer rate is about one magnitude

slower compared to CO2. The

achieved transfer rates are sufficient

for the PBR@LSR concept24.

V. Experiment Performance

during ground testing

Throughout the prototype phase of

PBR@LSR multiple demonstration runs of the complete experiment have been performed. These runs mostly were

aimed at testing of the biological performance of the culture and of different settings or components. Experiment

durations varied between a few days up to 2 months. In summation, the prototypes have cultivated algae for multiple

months. For this paper, one experiment-run demonstrating long time cultivation is shown as example in Figure 15.

Figure 12. Flow chart of gas handling system.

Figure 13. Measured transfer rates for CO2 and O2 through Sarsted

membrane at varying pressure differences.


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The behavior of gas concentration

during the course of the experiment is

the most important experiment data, in

addition to the optical density and dry

biomass. During the 7 days shown in

Figure 15, the OD750 stayed constant

with additional biomass forming

visible biofilms. The CO2 is actively

regulated between 7 – 9 %vol creating

the typical saw-tooth pattern. The step

drops that are prominent in the O2

curve are indicators for a flushing of

the experiment compartment with N2,

which is necessary when visual

inspections of the reactors are done.

The logarithmical flattening of the O2

curve can be explained by the gas

transfer capabilities of the membrane

as described earlier. Higher O2

concentration in the gas phase cause a

higher dissolved concentration,

negatively affecting the algae cells.

Temperature was kept nearly constant

between 25 – 26°C. From these data,

the overall gas turnover rate of the

experiment can be calculated as shown

in Figure 14. Over seven days, 8.04 L

(14.34 g) of CO2 were consumed

whereas 7.96 L (10.28 g) O2 were

produced. In agreement with these

findings, an uptake of macronutrients

could be measured. Microscopic

inspections of the culture were

performed regularly. At the end of the

experiment, no visual damage of the

algae could be found and bacteria

growth was minimal.

Comparative results were achieved

in a continuous two-month cultivation

run. A maximum dry substance growth

of 0.5 g/L/d was measured. The

respiratory quotient fluctuated between

0.5 and 0.9.

VI. Conclusion and Outlook

The ongoing development of PBR@LSR has reached a point where long-term cultivation for Chlorella vulgaris

can be demonstrated. The technical system is capable of sustaining a healthy culture and experiment parameters can

be adapted for further optimization of system performance. The four central components – the reactor, pump choice,

lighting design, and gas supply approach – have been tested and proven functional in their current implementation.

Multiple prototype setups with different control strategies have been build and operated. Over the course of 16

growth experiments, the combination of reactor design and lighting solution has proven to be especially important

for a successful algae cultivation. The current solution of LED lighting with PPFD < 300 µmol/m²/s combined with

a shallow reactor and a fast, laminar flow of algae-medium-suspension has proven optimal.

The connection between PBR@LSR and ACLS has been simulated, but functionality still needs to be proven

experimentally. For the flight design of the experiment, some additional constraints still have to be investigated

Figure 15. Experiment Data during 7 days of prototype

experiments.

Figure 14. Summation of consumed and produced gas amounts

over 7 days of prototype experiments.


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compared to the current prototype status; e.g. thermal balancing in rack mounted position, vibration resistance and

safety compliance. In addition, experiment runs of up to six months duration are planned to prove the long-term

stability of the setup.

Acknowledgments

The authors would like to thank the German Aerospace Center for the financial support in the frame of the

technology experiment development and Airbus Defence and Space for close cooperation and financial support.

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Lebenserhaltungssysteme“, Diploma Thesis, University Stuttgart, IRS-13-S101, 2013.

22Yesil, A., “Entwurf, Aufbau und Test einer LED-Beleuchtungseinheit für das Experiment PBR@ACLS“, Bachelor Thesis,

University Stuttgart, IRS-15-S-078, 2015.

23Binnig, M., “Design, Assembly and Test of a CO2 supply for the experiment PBR@ACLS”, Master Thesis, University

Stuttgart, IRS-16-S-001, 2016.

24Rieger, R.,”Design, Assembly and Test assembly for an absorber unit for the experiment PBR@ACLS”, Bachelor Thesis,

University Stuttgart, IRS-15-S-065, 2015.

Documents

Functionality and setup of the algae based ISS experiment