Photovoltaic & Space Environments Branch

Summary

Mars exploration will require power systems to operate on the Martian surface. One power system is a photovoltaic array. The properties of Mars relevant to photovoltaic array performance are surveyed. Several types of solar cells usable in a photovoltaic array are detailed and analyzed, and possible applications on Mars discussed. The applicability of photovoltaic technology to manufacturing methane propellant from Mars carbon dioxide is analyzed.

Introduction

Mars is a challenging environment for the use of solar power. In addition to the lower sunlight intensity compared to Earth orbital applications, the twelve-hour night (although not as severe as the moon`s 14 day darkness) requires that any solar power system include a large storage system. Wind, low temperature, sand, dust, and corrosive peroxide-rich soil, make the Martian surface an environment one could hardly call benign.

For any manned mission, a significant priority for a power system will be reliability and absence of dangerous failure modes. Due to the high price of transporting materials to Mars, an additional priority for a surface power system will be low weight. Photovoltaics provides low-cost power with high reliability and no moving parts. It has powered the space program since Vanguard, and there is every reason to believe it will play a major role in the coming exploration of Mars.

Sunlight on Mars

The solar intensity on the surface of Mars is considerably lower than that available in Earth orbit. This is primarily due to the greater distance of Mars from the sun. The average solar intensity at the orbit of Mars is 590 W/m2, compared with 1370 W/m2 in Earth orbit. The eccentricity of Mars` orbit results in a variation in intensity of about ±19% over the course of a year. Scattering and absorption of light by dust in the Martian atmosphere also decreases the sunlight available.

The availability of sunlight on Mars has been reviewed elsewhere [1,2,3] and will be only briefly discussed here.

As can be seen in figure 1, the sky of Mars has considerable dust content, which scatters the incident sunlight. This dust content varies considerably, depending on the dust storms. When the dust levels are high (typically near perihelion; around the northern hemisphere winter [4]) the optical depth of the atmosphere can be much greater than one [1]. At these times very little direct sunlight reaches the Martian surface, and nearly all of the light reaching the surface is diffuse scattering from the atmospheric dust. Despite the low amounts of direct insolation, however, the total light reaching the surface is appreciable. Figure 2 (from reference 1) shows the total sunlight availability (integrated over a day) for a horizontally oriented surface, showing the variation during the year. [Note that the time of year is expressed in units of areocentric longitude, the position of Mars in orbit around the sun in degrees].

season (areocentric longitude in degrees).

Like Earth, Mars has seasons due to its axial tilt. For locations off the equator, the number of hours of sunlight varies during the year. This has a considerable effect on the design of solar systems, particularly if they are to be designed for worst-case conditions, winter months far north or south of the equator. Figure 3 shows the number of hours of sunlight as a function of the time of year ("areocentric longitude") at various locations north of the equator. Above the Martian arctic and antarctic circles, Mars (like Earth) has no sunlight at all during the winter, and continuous sunlight during the summer.

Equator to 40 N
40 N to 80 N
Figure 3: Daylight Hours. Length of daylight in the northern hemisphere of Mars as a function of latitude and time of year (areocentric longitude). Top: equator to 40 degrees N; bottom: 40-80 degrees N. Daylight hours in the southern hemisphere the same, but with the seasons exactly reversed.

Solar Cells

There are three approaches to photovoltaic power. The conventional approach is the use of deployable high-efficiency flat plate arrays. Existing solar arrays used in space use either crystalline silicon (Si) or gallium arsenide (GaAs) solar cells.

Silicon is the most well developed solar cell technology, and has been used on all but a tiny fraction of space solar arrays. The conversion efficiency of standard-technology silicon cells currently flown is about 14.5% under standard space conditions ("Air Mass Zero," or "AM0") [5]. Note that for calculating operational power, the cell efficiency numbers must be adjusted for the array packing factor and for corrections to efficiency due to intensity and temperature.

Silicon solar cells with up to 20% conversion efficiency have been demonstrated in the laboratory. These cells are not yet space qualified and not currently available on the market.

Advantages of silicon cells are that large area cells are available (8 by 8 cm cells are being manufactured for Space Station Freedom), the array manufacturing technology is well developed, and the technology is well characterized for vibration, thermal-cycling, and other environmental loads of the space environment.

GaAs cells have higher efficiency than silicon cells. GaAs cells currently available on the market have an average conversion efficiency of 18.5%. Efficiency of 21.5% has been achieved in the laboratory. Gallium arsenide cells are smaller and more brittle than silicon cells, but the technology is being rapidly developed.

Gallium arsenide cells are currently heavier than silicon cells. Several technologies under development will make GaAs cells much lighter in weight [5]. The most well-developed of these technologies is CLEFT [6], where an extremely thin (5 micron) large-area cell is separated from a single-crystal substrate.

An important measure of power system performance is the specific power (power output per unit mass). Note that this is typically specified for Earth orbit conditions. Operation at the Mars surface will decrease the specific power by the ratio of Mars sun intensity to Earth orbit intensity.

Note that it is possible to measure specific power at the cell level, at the blanket level, at the array level, or at the power system level. Specific power at the cell level does not include array structure and is many times higher than array level specific power. At the blanket level, specific power includes the coverglass, interconnections, and the backing material, but not the array structure. This may be an appropriate figure of merit if a flexible or semi-flexible array is to be simply unrolled horizontally onto the Mars surface without support structure.

Specific power at the photovoltaic array level (including array structure) for the best arrays developed to date are shown in table 1.

For currently designed space power systems, the photovoltaic blanket weight is only about a quarter of the total power generation system mass (excluding storage). This is shown in table 2 for the space station Freedom solar array. The array structure and the power management and distribution (PMAD) system account for three-quarters of the power system mass. This provides a powerful incentive to develop new and more efficient PMAD systems and to design new array structures to take advantage of the ultralight blankets.

The second approach to photovoltaic arrays is to use a thin layer of photovoltaic material deposited onto a flexible substrate [7]. In the 1980`s considerable research was devoted to development and commercialization of thin-film photovoltaics for terrestrial power generation. Thin-film solar cells consist of thin (~1-5 µm) films of photovoltaic material deposited on a supporting substrate. This approach has lower conversion efficiencies, but due to the low amount of the active material used, has the potential for high specific power.

Efficiencies over ten percent have been achieved with three thin-film materials: amorphous silicon (a-Si), copper indium diselenide (CuInSe2), and cadmium telluride (CdTe). However, very little current research is aimed at depositing thin-film cells on lightweight substrates, since most of the applications being considered are terrestrial, where weight is not as critical. To enable their use in space, technology for deposition on extremely lightweight substrates will need to be developed. Thin film solar cells have not yet been demonstrated in space.

A conservative projection of an achievable thin-film solar cell blanket usable for space would be a 5% efficient thin-film cell fabricated on a 25 micron thick Kapton substrate. This yields a photovoltaic blanket specific power of 1.7 kW/kg. An optimistic projection might be a 15% thin-film cell on a 7 micron thick Kapton substrate, leading to a photovoltaic blanket specific power of 15 kW/kg. These numbers compare favorably to current state of the art spacecraft solar blankets.

Thin-film cells have other desirable features for space applications. In addition to low mass, thin-film photovoltaics are also projected to have considerably lower costs. Materials cost is reduced due to the small amount of materials required; the cost of labor and assembly is reduced by the fact that large-area, integrated assemblies are produced directly on the substrate sheet.

A final photovoltaic alternative is to use a concentrator system to focus light onto small, extremely high efficiency solar cells. This approach has been tested in space only on small-scale experiments. While conversion efficiencies of over 30% have been demonstrated using such concentrator systems and high-efficiency tandem solar cells, concentrator systems will not be practical for Mars surface power. The difficulty is that concentrator systems can focus only the direct ("beam") component of the light, and not the diffuse. Since a baseline power system must be sized to operate under worst-case conditions, where most of the incident sunlight is diffuse, concentrator systems can be ruled out.

Some resource-utilization applications require high-temperature processing. For these applications it may be desirable to concentrate sun to high intensities using mirrors or lens concentrators. This will allow a system to use the sunlight directly as heat, rather than converting the sunlight first into electricity using solar cells. A concentration system can make use of only the light coming directly from the sun (the direct, or "beam" component of the insolation). As discussed in previous work [1-3], the direct component of the insolation is about 50% of the total sunlight during the (northern hemisphere) spring and summer, but can drop to 10% or less of the total available light during periods of highly dust-laden air, particularly during the period around perihelion (northern hemisphere winter), when global dust storms can occur. An application requiring a concentrator to produce process heat must thus be designed for operation during relatively clear periods only. This implies that the mission be scheduled to avoid perihelion, and that the processing system must be tolerant of the possibility of intermittent shut-down if dusty atmospheric conditions arise.

A concentrator system will also have to be tolerant of dust loading of the mirror or lens surfaces.

Environment

In addition to the solar insolation and the fraction of direct and diffuse sunlight, other environmental factors of importance to photovoltaic system operation on the Mars surface are the temperature and the wind.

The Martian surface temperature varies from a minimum of 130¡ K to as high as 300¡ K, with a mean of 215¡ K. Air temperatures were measured by Viking at a height of 1.6 meters above the surface over a (Martian) year of measurement, including both local and global dust storms. Peak daytime temperatures varied from about 170¡ K at the Viking-2 site during a global storm, to almost 250¡ K near the summer solstice [2].

Photovoltaic cell performance increases with decreasing temperatures, with peak efficiency occurring at 150-200¡ K; at lower temperatures the efficiency decreases. The temperature coefficient of efficiency depends on the material, and in general increases as the material bandgap decreases [8]. Thus, low bandgap materials such as silicon and CuInSe2, which have high coefficients, increase in performance rapidly at the low temperatures to be found at Mars.

Wind was measured by the Viking landers. Average wind speed at site VL-2 was about 2 m/sec [4], with winds of over 17 m/sec observed less than 1% of the time. The atmospheric density varies considerably with season and temperature; at typical atmospheric conditions, dynamic pressure at the maximum likely wind velocity of 30 m/sec will be 10 nt/m2. This is a small pressure loading by terrestrial standards, however, Martian solar arrays will have to be somewhat more robust than arrays for the moon or deep space.

A final aspect of the Mars environment not well characterized is the soil. One interpretation of the Viking lander life science experiments is that the Martian soil contains large amounts of peroxides and superoxides [4]. This hypothesis needs to be confirmed by direct chemical analysis of the soil. If it is confirmed, it will be important that solar arrays be built using materials not subject to attack by oxidants.

Missions

The first use of power systems will be to run the unmanned rovers and scouting missions needed to investigate the geology (or `areology`) of Mars and scout out a location for the manned Mars base. Such a vehicle will not require much energy storage for the night, since it will not typically be operated in darkness. At dusk, the rover points its solar arrays at the eastern horizon and "goes to sleep," waking up again when the sun rises high enough to power the vehicle.

Under contract to NASA, B.D. Hibbs has done a design study on a small Mars rover with a high-efficiency photovoltaic array mounted to the body of the vehicle [9]. While the total power is decreased by using a fixed array rather than one which tracks the sun, this is more than made up for by the added robustness of the array. Depending on the technology chosen, the array required can be as small as seven square meters. Power requirements can be quite modest--only 275 watts average power (less than a moderate-size toaster), with a night-time `sleep` requirement of only 100 watts. Peak power capability of up to 2000 watts are required, though, for difficult tasks such as climbing over large boulders.

Figure 4 shows another approach to a rover design, where larger arrays are designed to be retractable during high-wind periods or when the rover is moving.

An alternative approach to investigating large areas of the Mars surface, and particularly for scouting for long distances and over rough--but interesting--terrain, is to use an airborne rover. A solar-powered airplane designed for Mars by Tony Colozza at NASA Glenn Research Center [10], shown in figure 5, is designed to be deployed directly into the air and fly continuously, night and day. Unlike the rover, the airplane must have continuous power over the night. Solar panels on the wing, tail, and fuselage charge regenerative fuel cells to allow the plane to stay in the air under cruise power for night flight. With a design criterion of 100 kg of payload and assuming a 25% efficient solar panel, the aircraft had a total mass of 438 kg and a wingspan of 120 meters.

For manned missions, two applications are power for life support and operations and power for resource processing (i.e., propellant manufacture). Once on the surface, the first priority will be to provide power. An example power system is discussed by McKissock, Kohout and Schmitz [11]. Their power cycle was to provide 40kW continuous power during the day, and a reduced power level of 20 kW during the night. For the night storage capability, they assumed that the hydrogen/oxygen regenerative fuel cell (RFC) has been developed to technology readiness. The RFC assumed pressurized gas storage. Cryogenic storage of reactants was determined to require too much equipment overhead to justify the slight advantages in storage volume. Round-trip efficiency for the storage system was 61.5%.

The power system is shown in figure 6. The flexible array is visible deployed on the ground behind the lander, while the regenerative fuel cell unit, with spherical pressure tanks for the hydrogen, oxygen, and water reactants, is visible in the foreground. A roll-out thermal radiator provides thermal management for the system.

Sample Case: PV Blanket for the "Mars Direct" Scenario Power

In-situ propellant generation on Mars is an option for drastically reducing the cost of Mars expeditions, which could be used for both manned missions and for unmanned precursor missions. One scenario for propellant generation on Mars (proposed by Robert Zubrin and David Baker [12,13]) is Mars Direct, which proposes to generate 107 tons of methane/oxygen propellant from 5.7 tons of hydrogen brought from Earth plus CO2 from the Martian atmosphere.

The following analysis shows how the energy requirements for this processing could be met with a surface photovoltaic array.

The process sequences is:

The hydrogen is then recycled to produce additional methane. Additional oxygen is produced by thermal decomposition of carbon dioxide [12] or by pyrolysis of methane back to hydrogen and carbon followed by reactions (1) and (2) repeated [13].

The baseline Zubrin/Baker scenario requires a 100 kW reactor running for 24 hr/day for 155 days for the propellant production. The energy required for manufacturing 107 tons of propellant on Mars is 370 MW-hrs of electrical energy. For the solar-powered case, it is desired to produce the required amount of propellant in half a Mars year, chosen to avoid the global dust storm season.

The baseline case studied used the following parameters:

• site = Viking 1 lander site
• propellant production completed in half a Mars year
• season = northern hemisphere summer and spring
• insolation available = 3 kW-hr/m2 per day
• fixed array (no tracking)
• solar array mass = 0.9 kg/m2 (APSA array technology)
• solar array efficiency = 20% under Mars conditions
• storage system: none (propellant production during day only)

The solar cell efficiency of 20% is about the efficiency of currently available GaAs cells operating at the temperatures expected at Mars. The solar array specific mass of 0.9 kg/m2 is roughly the target mass of a high-efficiency solar array designed for GEO [5]. Mars arrays will have a different mass, since the structure will be different from GEO, but in the absence of an array designed for Mars, this will serve as an order of magnitude estimate.

The global insolation on a horizontal (non-tracking) surface averages slightly over 3 kW-hr/m2 per day. Running the system for half a Mars year results in a total insolation of 1000 kW-hr/m2. No mass allocation for energy storage is assumed.

The solar array area required for propellant processing is 1850 m2, equivalent to an array 43 meters square. The PV blanket mass is 1.67 metric tons, which excludes the structure to hold the array in place, the mass of a robot or remotely-operated unit to deploy the array, and the mass of power conditioning, management and distribution equipment (PMAD). If the PMAD and ancillary equipment weight equals the PV blanket weight, the total required mass will be 3.33 metric tons.

This is comparable to the baseline power system considered by Zubrin and Baker, which used a 100 kW nuclear reactor and had a total mass of 3.96 metric tons.

Conclusions

Solar cells provide one option for providing power on the surface of Mars, both for unmanned precursor missions and for a manned base. Several types of photovoltaic technologies are available. Analyses of the applicability of solar power to Mars shows that photovoltaic power should be feasible, however, many questions remain to be answered, and development work on the solar cells, the array technology, and the technology for providing electrical storage for night operation, will need to be done.

References

1. J. Appelbaum and G.A. Landis, "Solar Radiation for Mars Power Systems," European Space Power Conference, 2-6 Sept. 1991, Florence, Italy; proceedings published as volume ESA SP-320.

2. G.A. Landis and J. Appelbaum, "Design Considerations for Mars PV Power Systems," 21st IEEE Photovoltaic Specialists Conf., May 1990, Vol. 2, 1263-1270.

3. J. Appelbaum and D. Flood, "Solar Radiation on Mars," Solar Energy Vol. 45 No. 6, 353-363 (1990). Also available as NASA Technical Memorandum 102299 (1989).

4. D. Kaplan, Environment of Mars 1988, NASA Technical Memorandum 100470 (1988).

5. P.M. Stella and D.J. Flood, "Photovoltaic Options for Solar Electric Propulsion," NASA Technical Memorandum 103284 (1990).

6. J.C.C. Fan, C.O. Bozler and R.W. McClelland, "Thin Film Gallium Arsenide Solar Cells," Proc. 15th IEEE Photovoltaic Specialists Conf., 666-672 (1981).

7. G.A. Landis and A.F. Hepp, "Thin-Film Photovoltaics: Status and Applications to Space Power," European Space Power Conference, 2-6 Sept. 1991, Florence, Italy; proceedings published as volume ESA SP-320. An earlier version of this paper is published in Space Power, Vol. 8, No. 1-2, 31-50 (1989).

8. I. Weinberg, C.K. Swartz, and R.E. Hart, "Radiation and Temperature Effects in Gallium Arsenide, Indium Phosphide, and Silicon Solar Cells," NASA Technical Memorandum 89870 (1989).

9. B.D. Hibbs, "Mars Rover Feasability Study, Final Report," AeroVironment, Inc., Report AV-FR-89/7011 (Oct. 1989).

10. A.J. Colozza, "Preliminary Design of a Long-Endurance Mars Aircraft", NASA Contractor Report 185243 (AIAA 90-2000), April 1990.

11. B. I. McKissock, L.L. Kohout and P.C. Schmitz, "A Solar Power System for an Early Mars Expedition", NASA Technical Memorandum 103219, , American Institute of Chemical Engineers Summer National Meeting, August 19-23, 1990.

12. D. Baker and R.M. Zubrin, "Mars Direct: Combining Near-Term Technologies to Achieve a Two-launch Manned Mars Mission," J. Brit. Interplanetary Soc. Vol. 43, 519-523 (1990).

13. R.M. Zubrin, D.A. Baker, and O. Gwynne, "Mars Direct: A Simple, Robust, and Cost Effective Architecture for the Space Exploration Initiative," AIAA Paper 91-0326 (1991).

Table 1: Specific Power of Solar Arrays (Earth Orbit Solar Intensity)

• 66 W/kg

Best Currently Built Array: Advanced Photovoltaic Solar Array (APSA)

• 130 W/kg

Best Array Combining Existing Technology: APSA with 20% CLEFT GaAs cells

• 300 W/kg

Table 2. Space Station Freedom Photovoltaic Power System Mass Breakdown per module

(28 kW power produced; 18.75 kW av. user power)