Microalgae-based Photobioreactors for a Life Support

49th International Conference on Environmental Systems ICES-2019-92 7-11 July 2019, Boston, Massachusetts

Microalgae-based Photobioreactors for a Life Support

System of a Lunar Base

Johannes Martin1, Jochen Keppler 2, Gisela Detrell3, Harald Helisch4, Reinhold Ewald5 Stefanos Fasoulas6

Institute of Space Systems – University of Stuttgart, Stuttgart, D-70569 Germany

Life Support Systems (LSS) for a crewed lunar base differ from those for Space Stations

in Low Earth Orbit (LEO), mainly due to the increased distance, and therefore more

expensive resupply possibilities. Microalgae-based photobioreactors (PBR) can help reducing

the required resupply mass by closing material mass flows with the help of regenerative

elements. By means of photosynthesis, the microalgae use CO2, water, light energy and

nutrients from waste to provide oxygen and biomass for the astronauts. Moon based PBRs

have different requirements than PBRs for Earth bound applications. Criteria such as: long

term process stability and controllability, energy-efficiency, scalability and space-efficiency

have high priority. One way to influence microalgae growth phases and biomass composition

with non-intruding methods is to adjust the spectrum of the lightning system. This paper

presents the possibilities of matching the total emitted spectrum of a limited amount of

different LEDs with a desired spectrum. In an artificially LED-illuminated PBR, the LEDs

are the main source of heat. The possibility of reducing space between stacked PBR cells

without disturbing the thermal control of the reactors by using mirror systems to guide and

distribute the light will be investigated. Another important issue that has a great impact on

cultivation possibilities is the reactor chamber geometry. It is critical that nutrients spread

evenly throughout the algae medium. To realize that, many earthbound reactors rely on

gravity and air lift. The paper will also elaborate on lunar specific air lift chamber-designs

due to the lower gravity compared to Earth and evaluate other reactor concepts for a lunar

application.

Nomenclature

ESM = Equivalent System Mass

FPA = Flat Panel Airlift

IRS = Institute of Space Systems

ISS = International Space Station

LED = Light Emitting Diode

PAR = Photosynthetic Active Radiation

PSU = Power Supply Unit

h = Planck’s constant

c = Speed of light

λ = Wavelength

k = Boltzmann’s constant

1 PhD candidate, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected] 2 PhD candidate, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected] 3 Head of LSS research team, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected]

stuttgart.de 4 PhD candidate, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected] 5 Professor in Astronautics and Space Station, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart,

[email protected] 6 Institute Director, Institute of Space Systems, Pfaffenwaldring 29, 70569 Stuttgart, [email protected]

International Conference on Environmental Systems

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T = Temperature

A = Area

r = radius

D = Diameter

I = Intensity

ε = Absorption coefficient

DS = Dry mass substance

L = Length

CO2 = Carbon dioxide

H2O = Water

I. Introduction

xtensive fundamental knowledge and innovative technologies have been obtained as part of crewed spaceflight

programs. Fifty years ago, the first humans set foot on the Lunar surface and over twenty years ago, the first

component of the International Space Station (ISS) was launched. On the ISS today, astronauts are provided with

oxygen that is recovered on board, partially from the carbon dioxide they exhaled and greywater is recycled into

drinking water, using phyisico-chemical technologies. However, part of the oxygen and the water is still provided

from the Earth, as well as the entire food, which can only be produced by biological systems. Waste is mostly burnt

up in the atmosphere. The requirements of a LSS, and the way they can be met, are dependent on the mission target

and duration. The roadmaps of the major space agencies for the next decades suggest space exploration missions such

as permanent settlements on Moon or Mars.1 These kind of long duration and far distance missions will require a

different approach, with most resources produced in-situ. A combination of physio-chemical and biological

technologies is required for sustainable food production and waste management. For these kind of systems, cultivating

microalgae in a photobioreactor offers advantages over systems with higher plants. Microalgae have a higher harvest

index, higher biomass productivity and require less water. Earthbound photobioreactors (PBR) cannot be transferred

one to one on a Lunar application and must be adapted to the changed requirements and boundary conditions. An

outline of potential technologies and an investigation of the applicability of reactor concepts based on gravity are

needed. 2 The project dubbed “Algae on Moon and Mars ensure astronaut survival” carried out at the Institute of Space

Systems (IRS) at the University of Stuttgart offers an overview of some of the aspects that need to be considered. 2

This project was possible thanks to the seed funding from the Dubai Future Foundation through Guaana.com open

research platform. This paper summarizes the results of this project. The cultivation of microalgae in PBR is a complex

endeavor, coupling the effects of photosynthesis, light distribution and utilization, gas- and nutrient supply, and

mixing.3 Proper mixing is crucial because it is needed to reduce nutrient, pH and temperature gradients, to minimize

dead zones, which enable sedimentation and adhesion, and to promote that all cells are equally exposed to the light4.

II. Requirements on next generation Life Support Systems for a Lunar Base

A LSS in an astronautic mission is in charge of providing the

right conditions for the astronaut to survive, such as atmosphere,

water food and waste-management4. The requirements, and the way

they can be met are dependent on the mission target and duration. To

evaluate the degree of closure during the design process the

equivalent system mass (ESM) can be calculated6 7. The ESM takes

start mass, resupply demand, power demand and mission duration

into account. Fig. 1 shows the qualitative ESM of LSS with different

degrees of closure over the mission duration. The design driving

requirement is to minimize the ESM and the mission costs. For a

permanently crewed lunar base, mission duration as well as costs for

resupply suggest the evaluation of biological components.7

Figure 1. ESM over mission duration 7

E


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III. Photobioreactor geometries

A. Solubility rates with different gas inlets

The mixing in PBRs can be realized by aeration with CO2 enriched air or by mechanical agitation, e.g. pumps or

stirrers. However, it is important to avoid too extensive mixing of the microalgae cells, especially when they are

sensitive to shear stress. Studies, in which pumps have been used to mix the culture, have shown that shear forces

induced by the pump can cause severe cell damages that might eventually lead to the loss of the complete culture.

When aeration is used for mixture, microeddies can be formed. Liquid velocities greater than 1 m/s are assumed to

lead to microeddies of less than 50 µm size, which could potentially damage the cells3. Therefore, liquid velocities of

0.2 - 0.5 m/s were selected for this study. For microalgae cultivation process management, the ability to solve CO2 is

critical. For this study, the focus was set on principles of mixing. This is best done by air injection, if possible, since

pumps can cause severe cell damage. 8, 9 Two approaches of mixing will be investigated:

The effect of different gas inlets on the CO2 solubility

The adaption of the reactor geometry of air lift systems to lunar gravity levels.

There are two options of injecting gas into a moving liquid. Either parallel or vertically to the flow direction. If

injected vertically, a Venturi-Inlet can be realized, where the dynamic pressure of the liquid sucks the gas in.10 The

advantage of such a Venturi-system is, that the bubble size decreases with increasing flow velocity and is independent

from the gas rate11, whereas the bubble size of an airlift system with parallel inlet increases with increasing gas rates.12

The solubility rate is influenced by the surface area of the gas contact and the contact time 13. The rising velocity of

gas bubbles increases with increasing diameter12 and the surface to volume ratio decreases with increasing diameters,

as eq.1 shows. So the bubble size should be kept at a minimum, in order to maximize the solubility rates.

𝑆𝑢𝑟𝑓𝑎𝑐𝑒

𝑉𝑜𝑙𝑢𝑚𝑒=

𝜋⋅4⋅𝑟2

4

3𝜋⋅𝑟3

=3

𝑟 (1)

Solved CO2 in H2O can be measured through the pH

value, as in water it decreases when CO2 is solved.14 The

experiment was conducted with 700 ml of demineralized

H2O and a gas flow of 50 ml of CO2 per minute. For both

experiments, a liquid loop was run through 3mm tubes at

0.5 m/s. This is only necessary for the Venturi inlet, but was

also done in the experiment with the flow inlet to exclude

the effects the mixing might have. For both gas inlets, a 21G

needle was used. The pH value was measured with a Mettler

Toledo InPro 3250i sensor.

The conducted experiment shows that the pH value

decreases faster if the Venturi inlet is used. This can be

explained by higher CO2 solubility rates, explained with the

theory above. Fig. 2 shows the principal of the two inlets

and presents the data from the experiment. It shows, that, if

a high solubility rate at low gas rates is desired, a Venturi-

inlet gives better results and lets parameters such as bubble

size and flow velocity be adjusted independently which

gives more options for a system control. Figure 2. Experimental data Venturi and flow inlet

B. Reactor geometry for light utilization

Another purpose of proper mixing is the enhanced light utilization of the culture. Dense cultures with high biomass

output, as desired for space missions, are subject to severe self-shading effects of the cells. These effects are strongly

depending on the flow dynamics and hence the reactor geometry. As the light intensity follows the Lambert-Beer law,

decreasing exponentially from the illuminated surface to the center of the reactor, different illumination zones can be

found in PBR. Especially in insufficiently mixed reactors, highly illuminated zones exist in surface areas close to the

lighting source, whereas much larger dark zones are located towards the center of the reactor, since only few photons


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can penetrate deep into the dense culture. Both zones are disadvantageous for cell growth. Too high light intensities

might lead to photoinhibition,15 whereas lack of light might lead to photolimitation. The shuttling of the cells between

the highly illuminated and the darker zones alters the radiation field and can promote photosynthesis.16 This effect is

known as the flashing light effect. A regular shuttling between light and darkness with a frequency of about 1 Hz and

a light versus darkness residence time of 1:10 is recommended.4

The flashing light effect can be utilized

By geometry adaptions, making the flow take the single cell towards and away from the light

By adapting the illumination system to blink at the desired frequency.

By local intensity changes of the illumination e.g. mirror a system creating differently illuminated areas

over the flow course

At the IRS, Chlorella vulgaris has been cultivated and

the flat panel airlift PBRs from the company Subitec®

have shown the best growth rates. This can be explained,

by the airlift principle, and the geometry that induces a

circular movement around 1 Hz, exposing the single

microalgae cell to the light source at intervals. This

enables using the described flashing light effect. Airlift

reactors are based on air bubbles rising and differences in

average densities of liquid / gas mixtures and pure liquids.

CO2 enriched air is pumped into the bottom of the PBR.

When a force field such gravity (or centrifugal force in a

rotating system) is present, the gas bubbles will rise up

due to their lower density. While rising upwards, the gas

bubbles follow the path that is determined by the

geometry. By doing that, they can induce a barrel swirl in

the mixers. This introduced liquid movement leads to a

mixture of the algae culture and a circular movement of

the single cell, enabling the utilization of the flashing light

effect. Once the gas leaves the liquid, the gas-free liquid

drops in the downer due to its higher density. Fig. 3 shows

the functioning principle of such reactors. However,

bubble size, rising speed and buoyancy are dependent on

gravity. This means, that cultivating microalgae with the

same reactor geometry and the same gas rates could lead

to very different results under lunar gravity.

Figure 3. Working principle of a flat panel airlift

PBR4

Due to the lower gravity level on the Moon, an airlift system is harder to realize. In this study, the focus was set

on designing a geometry that induces the barrel swirl under lunar gravity. Three different profile geometries shown in

fig.4 were designed and investigated with the computational fluid dynamics software tool ANSYS® Fluent, version

19.1. The flow is simplified as a two phase flow with water and air. A Eulerian two phase model is used. Turbulence

is modeled by a standard κ - ϵ model. The surface tension is modeled with standard wall functions. Since the flat panel

airlift configuration shall use a downer for the descending liquid, a liquid velocity will be present at the gas inlet and

therefore taken into account for the simulations, as well as the different gravity levels of Earth and Moon. Firstly, a

simulation under Earth gravity is conducted and the results are shown and discussed. The reason behind this simulation

is, that the results could be compared with experimental data to validate the simulation, if the different design were

built. Secondly the results with lunar gravity are shown and a suggestion for a PBR design is presented.


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Design A

Design B

Design C

Figure 4. Reactor designs for investigation

For all designs, a thickness of 100 mm was chosen. The maximal possible thickness is determined by the maximal

optical density, hence the desired amount of biomass.

Design A uses panels of quadratic shape. The separation of each panel is realized by a rectangular profile having

1/10 of height and 1/4 of width compared to the panel’s dimensions.

Design B is similar to design A but uses rounded corners to avoid stagnation regions in the corners. At the end of

each separation, the profile is still orthogonal to the general flow direction.

Design C has no horizontal walls are present. Here, the flow in the wall areas will always have a vertical

component.

The investigated parameters are:

Liquid flow velocities (preferably 0.2-0.5 m/s to avoid micro eddies)

Existing of barrel swirls (With a frequency of about 1 Hz)

Light to dark ratio of about 1:10

For the simulations with Earth gravity, a gas inlet speed of ½ the panel’s diameter per second and a liquid inlet

speed of ¼ of the panel’s diameter per second have been used.

Design A

Design B

Design C

Figure 5. Simulation results under Earth gravity

Fig. 5 shows the results of the conducted simulations. Design A shows a barrel swirl with a maximal velocity of

0.25 m/s and a rotation frequency of about 0.5 Hz. Assuming a one sided illumination and assuming that only

perpendicular movement of the cells to the light source contributes as light intake, a light to dark ratio between 1:8


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and 1:4 can be assumed. Dead water zones occur in the edges, caused by the discontinuity of the geometric shape.

Design B shows as well the barrel swirl and maximal velocities of 0.25 m/s but has a more uniform velocity profile.

Dead zones are only located within the swirl. The frequency of 0.6 Hz of the swirls is higher, due to the uniformity of

the velocity profile. The light to dark ratio is about 1:4. The results of Design C are very similar to Design B. The

maximal occurring velocity and rotation frequency is with 0.275 m/s and 0.65 Hz a little higher. The Light to dark

ratio is about 1:4.

Taking the given constraints, all three designs are close to fulfill the requirements ideally. Design B and C have

an advantage over Design a due to having less dead zones. Therefore, for further investigations with lunar gravity, the

focus was set on Design B and C. Since the lunar gravity is only about 1/6 of Earth’s gravity, gas bubbles will rise

slower. To get similar results for the velocity and rotation frequency of the liquid phase, the gas and liquid inlet

velocities can be adjusted. However, if the velocities are increased too much, the flow cannot follow the panel’s

geometry smoothly. Numerous simulations were conducted. The best results are shown in Fig 6. and are being

presented.

Design B

Design C

Figure 6. Reactor designs for investigation

To realize a circular movement, the gas speed was 2x higher, and the liquid velocity 8x higher as for the ones used

in Earth’s gravity simulations. A sole adjustment of the inlet speeds, does not lead to as satisfactory result. Another

option is, to adjust the geometry dimensions in order to minimize dead zones. A focus was set on Design C. The Inlet

speeds were reset the conditions specified for the simulation under Earth’s gravity.

Fig. 7 shows the results of the geometry optimization loop. All lengths

are reduced by a factor of 3.33, leading to a panel dimension of

30x30cm. By reducing the panel’s geometry, the dead zones in the center

are reduced as well. There is a circular liquid movement in each of the

panels. However, this movement, with regard to evenly distributed

cycling velocities, is not as uniform as under Earth’s gravity. The

maximum liquid velocities occur at the entry of each panel and are about

0.2 m/s. There are small dead zones in the center of the panels, which

are surrounded by a bigger area of relatively low velocities slightly

above 0 m/s. Regarding the outer rotation cycle, the lowest velocities are

about 0.06 – 0.08 m/s. Assuming an average cycling velocity of about

0.1 m/s for the outer circle, a rotation frequency of about 0.8 Hz is

present and a light to dark ratio is approximately between 1:8 and 1:4.

Since the design in Fig. 7 meets the defined requirements, it has been

proven, that a flat panel airlift PBR with a geometric approach to use the

flickering light effect is a feasible option .


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Figure 7. Optimized reactor profile

C. Active illumination system based use of flickering light effect

Barrel swirl reactors show the best results for algae growth, but only for a small group of microalgae e.g.

C.vulgaris.17 To choose the fitting microalgae for the LSS is a task, that should not be limited by the reactor design.

Another option to use the flickering light effect would be to turn on and off the illumination at the desired frequency.

For this option to be feasible, the illumination intensity throughout the PBR should be at a constant level. A simulation

is conducted, to compare the different degrees of illumination of different reactor setups. The simulation was done

with Matlab R2018b, and is based on the Lambert Beer Law of absorption shown in eq. 2.18 The light intensity was

calculated by integrating all incoming rays analytically for each step.

log10 (𝐼0

𝐼) = εs ⋅ 𝐷𝑆 ⋅ 𝐿 (2)

For the simulation, a light flux of 400µmol/s m² was assumed, an illumination with light of 680 nm wavelength

and a biomass of 0.2312 kg/m³ C.vulgaris. This setup was chosen due to the available data on algae- and wavelength

specific absorption coefficients and due to previously obtained experience with light intensities.19–21 Three options

were evaluated as shown in fig. 8. A tubular reactor, illuminated from all sides, a FPA, illuminated from both sides

and a FPA, illuminated from one side. Fig. 9 shows the results of the simulation.

Figure 8. Analyzed reactor profiles Figure 9. Results of illumination simulation

For both, tubular and both-sided illuminated FPA, a use of the flickering light effect could be feasible with a

blinking illumination system. The advantage of such a system would be, that the frequency is controllable and that the

reactor profile is easy to clean since it does not have separations and dead water areas, where adhesions are more

likely. The advantage of tubular reactors is the high volume to surface ratio with illumination profiles comparable to

a FPA that is illuminated from both sides. Another potential advantage is that round profiles are easy to extrude22, and

would facilitate production with in-situ resources.

If the reactor design with the barrel swirl cannot be implemented due to microalgae specific requirements, it is still

possible to make use of the flickering light effect by controlling the light.


8

D. Passive illumination system based use of flickering light effect

A third option, that can be considered is a mirror system as shown in fig. 10. The blue line represents the reactor

surface, orange and yellow the mirror setup. The green line represents the main axis of the light source. The advantages

of such a system are:

The total system thickness is reduced, making the setup easily stackable

The distance of the light source, that is also the main heat source, to the reactor is increased and easier

accessible for cooling,

The surface light intensity can be adjusted by the mirror angles and the light source main axis

The flickering light effect can be exploited by algae movement through light profile without adhesion

burdened separations and with a continuously operated light source

To prove the feasibility of such a

system, a simulation is set up with

Matlab R2018b, that calculates the total

light intensity at the reactor surface,

with a given mirror system, a given

radiation profile of the light source and

the main axis of the light source. The

FPA beeing 2m tall and having a liquid

velocity of 0.5 m/s. The green line in the

lower plot in Fig. 10 shows the total

light intensity at the upper half of the

reactor. The resulting intensity was

calculated for each mirror pathway

individually. The individual results are

plotted as well. The green line shows the

sum of all intensities and was

normalized on the maximal intensity.

The results of the lower half would be

mirrored. The calculated intensity is

relative, and can be adjusted by the total

intensity of the illumination system.

Assuming a setup where everything

below 0.4 would be considered dark,

and everything above as illuminated, a

microalgae cell, moving through the

reactor, would experience a 1:3 light to

dark ratio. This simulation shows that a

mirror based use of the flickering light

effect is feasible as well.

Figure 10. Setup and results of the mirror system simulation


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IV. Evaluation of natural light versus artificial light

E. Impact of landing site on source of energy and light

For microalgae based PBRs as components of a biological LSS the main energy consumption origins in the

illumination system, hence the power source and illumination method is relevant for determining the ESM. The chosen

landing and base site on the Lunar surface has a major impact on the chosen power source. Day and night cycles on

the Moon are equally long and lasting about 14-Earth-days each.23 Illumination of a PBR with sunlight would not be

feasible. However, since the rotation axis of the Moon is not tilted as the Earth’s axis and the Lunar surface is covered

with craters, areas of eternal light and complete darkness can be found near the Lunar poles. 24 Fig. 11 shows areas of

eternal darkness at the poles and fig. 12 enhances the scenario at the north pole.

Figure 11. Lunar Polar regions (88-90°) showing

zero illumination (dark blue) and high

illumination (light blue) 25

Figure 12. Illumination map of the Lunar north pole,

within 30 km (1-1.5°)26

These areas are rare, yet valuable and interesting as base sites, because within the dark areas, lots of rare resources,

such as frozen water, have accumulated over time27 and the illuminated areas offer constant solar power. With spots

of eternal light near the base, the option of using the sunlight directly to illuminate the PBR exists.

F. Evaluation scenario and criteria

A pure naturally illuminated PBR-scenario is only feasible with area of total illumination. These areas are limited,

making the required area to ensure a constant illumination at a desired intensity the criteria for comparison. Fig. 13

shows the two scenarios that will be compared. The scenario on the left collects sunlight into an optical fiber that leads

the light of the desired wavelength to the PBR within the radiation shield. The scenario on the right converts the

sunlight to electrical energy that powers LEDs with the desired wavelength. For each scenario the necessary area per

Watt illumination power in photosynthetic active radiation (PAR) is calculated. Occurring losses and degrees of

efficiency are being discussed and evaluated for each system.

Solar illumination Artificial illumination

Photosynthesis Wavelength exploit Solar cell wavelength exploit

Fiber entrance Battery & PSU efficiency

Optical wire LED


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Figure 13. Illumination map of the Lunar north pole, within 30 km

Without the atmosphere of the Earth, the Sun has an

irradiation strength of 1367 W/m² at Earth-distance and

can be modeled as black body radiation at 5778 K with the

Planck equation shown in eq.3.28, 29 The radiation that is

photosynthetic active is defined as 400 – 700 nm.30

𝐸(𝜆, 𝑇) =2ℎ𝑐2

𝜆5⋅

1

𝑒(ℎ𝑐𝜆𝑘𝑇

)−1

(3)

∫2ℎ⋅𝑐2

𝜆5⋅

1

𝑒(ℎ⋅𝑐𝜆⋅𝑘⋅𝑇

)−1

𝑑𝜆700𝑛𝑚400𝑛𝑚

∫2ℎ⋅𝑐2

𝜆5⋅

1

𝑒(ℎ⋅𝑐𝜆⋅𝑘⋅𝑇

)−1

𝑑𝜆∞0

= 0.36 (4)

Fig.14 shows the black body radiation model and

marks the area, relevant for photosynthesis. Eq.4

calculates the ratio of total radiation power to PAR power.

36% of the solar radiation are in wavelength, relevant for

photosynthesis.

Figure 14. Solar irradiation as black body

radiation over wavelength

Figure 15. Area of densest fiber packing and effect of fiber cladding

To get the light from the point of collection to the PBR, optical wires are needed. Optical wires consist of a core

and a cladding. The core has a higher refractive index than the cladding. The size of this acceptance cone is a function

of the refractive index difference between the fiber's core and cladding. The cladding is necessary to protect the core

and ensure that the angle of reflection is constant. Fig. 15 illustrates the densest packing of fibers on the left side and


11

the principal of light transmission on the right side. The theoretical efficiency of the optical wire entrance depends on

the ratio of core thickness to cladding and the densest packaging of circles.

Aeff =𝐴𝐼𝑛𝑛𝑒𝑟

𝐴𝐻𝑒𝑥=

1

4⋅𝜋⋅𝐷𝑖𝑛

2

1

2 √3 𝐷𝑜𝑢𝑡

2 =𝜋

2 √3⋅ (

𝐷𝑖𝑛

𝐷𝑜𝑢𝑡)2

≈ 0,907… ⋅ (𝐷𝑖𝑛

𝐷𝑜𝑢𝑡)2

(5)

Eq. 5 calculates the ratio of transmitting fiber area to total area of a fiber bundle. The ratio of core to cladding,

with regards to minimize the cladding thickness, is taken from literature. A ratio of 1/1.25 is suggested. 31 This leaves

the entrance efficiency at 58%. Losses within the optical fiber can be estimated with 1dB per km32 and would play a

major roll, if the fiber is long. But for fiberlength < 1km, these losses are < 10%. Other losses, e.g at the collector

mirror or the fiber exit were researched but are neglected since they are very small, compared to the influence of a

potential change in or 𝐷𝑖𝑛/𝐷𝑜𝑢𝑡 or an adaption of eq.5 that takes the absorption spectrum of a specific microalgae into

account.

The solar cell based system converts the Sun’s energy to electrical energy and uses LEDs with specific wavelength

to illuminate the reactor. Solar cells can use a broader section of the radiation spectrum and can convert up to 46% of

the solar energy to electrical energy in the lab, and 40% in serial production33. LEDs reach an efficiency of up to 0.85

and can be produced with various specific wavelength relevant for photosynthesis34, 35. The combined efficiency of

battery charging and Power Supply Unit (PSU) is estimated with 0.936.

Solar illumination Artificial illumination

PhotosyntheticWavelength exploit 0.36 Solar cell wavelength exploit 0.4

Fiber entrance 0.58 Battery & PSU efficiency 0.9

Optical wire 0.9 LED 0.85

TOTAL EFFICIENCY 0.188 TOTAL EFFICIENCY 0.306

257 W/m² 418 W/m²

The comparison of these scenarios shows, that artificial LED illumination powered by solar cells require

significantly less area than directly illuminated systems. Hence, regarding the illumination system, artificial

illumination is investigated, independently form the chosen power source.


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V. Energetic optimization

Different microalgae can absorb different

wavelengths of light based upon the types and ratio

of pigments that they exhibit.37 Fig. 16 gives an

overview over some photosynthetic active

pigments. The choice of microalgae, its adaption

over time and the purity of the culture determine

the absorption spectrum. If the microalgae that will

be used in the LSS is known, the illumination

system can be adapted to its needs. Choosing a

specific microalgae requires detailed knowledge of

the mission requirements. Factors such as growth

rates, culture long term stability, oxygen

production rates, nitrate and phosphate absorption

rates and others have to be considered. For this

investigation, the microalgae C.vulgaris was used

as prototype, due to the experience and data

acquired at the IRS.19 To reduce energy

consumption, illuminating the migroalgae at

wavelength where its pigments are most active is

suggested. A red and blue illumination is state of

the art and has been demonstrated before. To

further optimize this potential, a program was

developed, that pics out a defined amount of LEDs

with radiation spectra from a data bank and fits a

defined “goal-spectrum” as close as possible. Fig.

17 shows the result of the algorithm when 9 LEDs

are picked and the absorption spectrum of

C.vulgaris38 is approached. By creating an

illumination system with LEDs of several

wavelengths, the radiation spectrum and intensity

would be adaptable, throughout the cultivation

process, giving a tool for non-intrusive system

control.

Figure 16. relative absorbance of different pigments39

Figure 17. Absorption spectrum of C.vulgaris and

different LEDs with intensities determined by the profile

matching algorithm

VI. Conclusion

Future human space exploration missions will require a life support system that relies as little as possible on

resupply from Earth. Further closing loops by in situ food production and oxygen regeneration is only possible with

biological components. Microalgae as biological component gives advantages such as high biomass production rates,

high harvest indices and low water consumption. This paper investigated different approaches to microalgae

cultivation under Lunar conditions. An experiment on gas inlets was conducted. A Venturi-inlet is suggested at a

Lunar application. Several approaches to make use of the flickering light effect were analyzed, and shown to be

feasible. An analysis on using natural light versus artificial light was carried out with the result that artificial light is

more practicable, even at spots of eternal light. That being shown, an analysis to reduce energy consumption has been

carried out and an absorption matching illumination was suggested.


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Acknowledgements

This project received seed funding from the Dubai Future Foundation through Guaana.com open research platform.

References

1. “The Global Exploration Roadmap: January 2018,” International space Exploration coordination Group.

2. G. Detrell, J. Martin, J. Keppler, H. Helisch, “Algae on Moon and Mars ensure astronaut survival,” 2019.

3. Posten, C., “Design principles of photo-bioreactors for cultivation of microalgae,” Eng. Life Sci., V. 9, No. 3,

2009, pp. 165–177.

4. Degen, J., Uebele, A., Retze, A., Schmid-Staiger, U., and Trösch, W., “A novel airlift photobioreactor with

baffles for improved light utilization through the flashing light effect,” Journal of biotechnology, V. 92, No. 2,

2001, pp. 89–94.

5. Messerschmid, E., and Bertrand, R., “Space stations: Systems and utilization,” Springer, Berlin, 1999, 566 pp.

6. Levri, J., Fisher, J., and Jones, H., “Advanced Life Support Equivalent System Mass Guidlines,” 2003.

7. Hanford, A. J., and Ewert, M.K., “Advanced Life Support Baseline Values and Assumptions Document,”

NASA/CR-2004-208941, 2004.

8. Vandanjon, L., Rossignol, N., Jaouen, P., Robert, J. M., and Qumneur, F., “Effects of shear on two microalgae

species. Contribution of pumps and valves in tangential flow filtration systems,” Biotechnol. Bioeng., V. 63,

No. 1, 1999, pp. 1–9.

9. Keppler, J., “Entwurf, Aufbau und Test des Algen Medium Loops für das Experiment PBR@ACLS,” Master

Thesis, University of Stuttgart, Institut of Space Systems, Germany, 2015.

10. Reader-Harris, M., “Orifice Plates and Venturi Tubes,” Springer International Publishing, Cham, s.l., 2015, 192

pp.

11. Wiraputra, I. G. P. A. E., Edikresnha, D., Munir, M. M., and Khairurrijal, “Generation of Submicron Bubbles

using Venturi Tube Method,” J. Phys.: Conf. Ser., V. 739, 2016, p. 12058.

12. Gaudlitz, D., “Numerische Untersuchung des Aufstiegsverhaltens von Gasblasen in Flüssigkeiten,”

Dissertation, technical University Munich, Institut for aerodynamic, Munich, 2008.

13. Najafpour, G. D., “Biochemical engineering and biotechnology,” 1. ed., [Nachdr.], Elsevier, Amsterdam, 2008,

421 pp.

14. Zosel, J., Oelßner, W., Decker, M., Gerlach, G., and Guth, U., “The measurement of dissolved and gaseous

carbon dioxide concentration,” Meas. Sci. Technol., V. 22, No. 7, 2011, p. 72001.

15. Powles, S., “Photoinhibition of Photosynthesis Induced by Visible Light,” Annual Review of Plant Physiology

and Plant Molecular Biology, V. 35, No. 1, 1984, pp. 15–44.

16. Phillips, J. N., and Myers, J., “Growth Rate of Chlorella in Flashing Light. 1,” Plant Physiology, V. 29, No. 2,

1954, pp. 152–161.

17. Xu, L., Weathers, P. J., Xiong, X.-R., and Liu, C.-Z., “Microalgal bioreactors: Challenges and opportunities,”

Eng. Life Sci., V. 9, No. 3, 2009, pp. 178–189.

18. Lee, C.-G., “Calculation of light penetration depth in photobioreactors,” Biotechnol. Bioprocess Eng., V. 4, No.

1, 1999, pp. 78–81.

19. Helisch, H., Keppler, J., Bretschneider, J., Belz, S., and Fasoulas, S., “Preparatory ground-based experiments

on cultivation of Chlorella vulgaris for the ISS experiment PBR@LSR,” 46th International Conference on

Environmental Systems, 2016.

20. Blair, M. F., Kokabian, B., and Gude, V. G., “Light and growth medium effect on Chlorella vulgaris biomass

production,” Journal of Environmental Chemical Engineering, V. 2, No. 1, 2014, pp. 665–674.

21. Filali, R., Tebbani, S., Dumur, D., and Diop, S., “Estimation of Chlorella vulgaris growth rate in a continuous

photobioreactor,” Proceedings of the 18th World CongressThe International Federation of Automatic Control,

2011, pp. 6230–6235.

22. Rauwendaal, C., and Gramann, P. J., “Polymer extrusion,” 5. ed., Hanser, Munich, 2014, 934 pp.

23. Ward, W. R., “Past orientation of the lunar spin axis,” Science (New York, N.Y.), V. 189, No. 4200, 1975, pp.

377–379.

24. Elvis, M., Milligan, T., and Krolikowski, A., “The peaks of eternal light: A near-term property issue on the

moon,” Space Policy, V. 38, 2016, pp. 30–38.

25. Noda, H., “Illumination conditions at the lunar pole regions by KAGUYA laser altimeter,” Jurnal of

Geophysical Research, 2008.


14

26. Bussey, D. B. J., Fristad, K. E., Schenk, P. M., Robinson, M. S., and Spudis, P. D., “Planetary science: constant

illumination at the lunar north pole,” Nature, V. 434, No. 7035, 2005, p. 842.

27. Feldman, W., Maurice, S., Lawrence, D., Little, R., and Lawson, S., “Evidence for water ice near the lunar

poles,” Journal of Goephysical Research, Vol 106, 2001, pp. 23231–23251.

28. Quiang, F., “Radiation (Solar),” Science, pp. 1859–1863.

29. Planck, M., “Ueber das Gesetz der Energieverteilung im Normalspectrum,” Ann. Phys., V. 309, No. 3, 1901,

pp. 553–563.

30. Asar, G., Myneni, R., and Kanemasu, E., “Estimation of plant canopy attributes from spectral reflectance

measurments,” 1989.

31. Takenaga, K., Arakawa, Y., Sasaki, Y., Tanigawa, S., Matsuo, S., Saitoh, K., and Koshiba, M., “A large

effective area multi-core fiber with an optimized cladding thickness,” Optics express, V. 19, No. 26, 2011,

B543-50.

32. Horiguchi, M., and Osanai, H., “Spectral losses of low-OH-content optical fibres,” Electron. Lett., V. 12, No.

12, 1976, p. 310.

33. Wojtczuk, S., Chiu, P., Zhang, X., Derkacs, D., Harris, C., Pulver, D., and Timmons, M.,

“InGaP/GaAs/InGaAs 41% concentrator cells using bi-facial epigrowth,” 35th IEEE Photovoltaic Specialists

Conference (PVSC), 2010: 20 - 25 June 2010, Honolulu, Hawaii ; conference proceedings, IEEE, Piscataway,

NJ, 2010, pp. 1259–1264.

34. Narukawa, Y., Sano, M., Sakamoto, T., Yamada, T., and Mukai, T., “Successful fabrication of white light

emitting diodes by using extremely high external quantum efficiency blue chips,” phys. stat. sol. (a), V. 205,

No. 5, 2008, pp. 1081–1085.

35. Yeh, N., and Chung, J.-P., “High-brightness LEDs—Energy efficient lighting sources and their potential in

indoor plant cultivation,” Renewable and Sustainable Energy Reviews, V. 13, No. 8, 2009, pp. 2175–2180.

36. Schulz, D., ed., “Nachhaltige Energieversorgung und Integration von Speichern: Tagungsband zur NEIS 2015,”

Springer Vieweg, Wiesbaden, 2015, 217 pp.

37. Rungrat, T., Awlia, M., Brown, T., Cheng, R., Sirault, X., Fajkus, J., Trtilek, M., Furbank, B., Badger, M.,

Tester, M., Pogson, B. J., Borevitz, J. O., and Wilson, P., “Using Phenomic Analysis of Photosynthetic

Function for Abiotic Stress Response Gene Discovery,” The arabidopsis book, V. 14, 2016, e0185.

38. Ley, A. C., and Mauzerall, D. C., “Absolute absorption cross-sections for Photosystem II and the minimum

quantum requirement for photosynthesis in Chlorella vulgaris,” Biochimica et Biophysica Acta (BBA) -

Bioenergetics, V. 680, No. 1, 1982, pp. 95–106.

39. Huber, M., “Lighting for Algae Cultures,” 2019, https://algaeresearchsupply.com/pages/lighting-for-algae-

cultures.

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Microalgae-based Photobioreactors for a Life Support