. 

NASA Technical Memorandum 102 127 

Solar Poweryor the Lunar Night 

[NASA-TH-102127) SOLAR POUER FOB THE LUNAR N89-26799 

NIGHT INASA, Lewis Research Center) 19 p 

CSCL 03B 

Uaclas 

G3/91 0222717 

Geoffrey A. Landis 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

9th Biennial SSI/Princeton Conference on Space Manufacturing 

sponsored by Space Studies Institute 

Princeton, New Jersey, May 10-13, 1989 
SOLAR POWER FOR THE LUNAR NIGHT 

Geoffrey A. Landis* 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44 135 

Providing power over the 354 hour lunar night provides a considerable 

challenge to solar power concepts for a moonbase. The paper reviews 

concepts for providing night power for a solar powered moonbase. The 

categories of solutions considered are electrical storage, physical storage, 

transmitted power, and "innovative concepts". Electrical storage is the 

most well-developed option. Less developed electrical storage options are 

capacitors and superconducting inductors. Physical storage options 

include storage of potential energy and storage of energy in flywheels. 

Thermal storage has potentially high energy/weight, but problems of 

conduction and radiation losses during the night need to be addressed. 

Transmitted power considers use of microwave or laser beams to transmit 

power either from orbit or directly from the Earth. Finally, innovative 

concepts proposed include reflecting light from orbital mirrors, locating 

the moonbase at a lunar pole, converting reflected Earthlight, or moving 

the moonbase to follow the sun. 

*National Research Council-NASA Research Associate. 

1 
1. Introduction 

The possible options for the power source are solar (either photovoltaic 

or dynamic), or nuclear. There is interest in making a lunar base solar 

powered, due to the considerable political and technical barriers, real and 

perceived, to the use of nuclear power. A permanent lunar base is a 

considerable challenge for solar power technology, due to the necessity of 

providing fourteen days of power during the darkness of the lunar night. 

While some base systems can be shut down or run at reduced power during 

the night, other systems, such as running greenhouse lights, providing air 

recycling, etc., may even have increased power consumption during the 

night . 

For a typical moonbase design, the storage required for lunar night 

operation will be the major mass component of the electrical system. For 

conventional Ni-H batteries with 32 W-hr/kg (current technology), a 

lOOkW daytime power requirement and 50% night power, one estimate 

puts the mass of the batteries alone at over a million kilograms [1.21. In 

contrast, the photovoltaic panels themselves would be quite light: 500 kg 

for an APSA technology array with thin GaAs cells [I], and potentially even 

less for advanced thin-film technologies I3.41. Clearly, then, higher 

performance concepts for power during the lunar night would be desirable. 

Due to the high cost of delivering mass to the moon, the important 

engineering figure of merit for an energy storage system is the energy to 

weight ratio, or specific power, measured in watt-hours per kilogram 

(W-hr/kg). A similar consideration is applicable to beamed power and 

innovative power systems, where the effective stored energy is equal to the 

power times 354 hours. Other important figures are the ratio of charging 

energy to energy output during discharge, or energy efficiency, and the 

lifetime, both of which must be high; and the required maintenance, which 

should be low. Of considerably less importance is the capital cost, since 

the transportation cost is likely to dominate the total cost. 

Finally, for long-range use it is important that the system can be 

manufactured from materials which can be mined and refined from 

available lunar resources, since the long-term evolution of the lunar base is 

likely to be by a "bootstrap" process. It is widely accepted that solar cells 

can be manufactured on the moon, however, for a self-sufficient lunar base, 

it is important that the energy storage (or transmission) capability also 

have the possibility of being locally manufactured. 

This paper briefly surveys and discusses the possible options for night 

power. The paper is intended as a brief and perhaps superficial survey of 

the concepts proposed, and should not be taken for a comprehensive 

critical review, which has not to date been written. 

The concepts discussed can be divided into storage technologies and 

continuous power concepts. Storage includes electrical storage methods 

and physical storage methods, while continuous power concepts can be 

roughly categorized as transmitted power systems or "innovative 

concepts". 

2 
2. Electrical Storage 

Electrochemical Storage 

The existing state of the art in electric 1 storage for pacecraft is the 

nickel-hydrogck (Ni-H) cell. The specific ene; for the best cells 

currently on spacecraft is 32 W-hr/kg (Intelsat-VI) E, with 45 W-hr/kg in 

the prototype stage 151, and up to 75 W-hr/kg projected 161. Lithium and 

sodium-sulfur batteries, neither of which are currently in use, have the 

potential similar specific energy, up to a maximum of about 100-150 

W-hrs/kg 171, 

A potentially lower mass technology is the hydrogen/oxygen 

regenerative fuel cell. "Regenerative" indicates that water produced 

during the discharge is electrolyzed by the solar panels during the 

charging cycle. The technology is not fully developed. The highest mass 

element is the pressure tank required to store the reactant gasses. 

Current technology uses steel pressure tanks, with a specific power on the 

order of 50 W-hr/kg. Next-generation technology will use composite 

(Kevlar) filament-wound tanks, with specific power on the order of 500 

W-hr/kg. Even with composites, the tanks still comprise nearly 60% of 

the system mass. Future technology may use cryogenic reactant storage, 

with up to 1500 W-hr/kg 181. The technology will require a cryogenic 

refrigeration plant to liquefy the reactant gasses and store them at 

cryogenic temperatures. 

Capacitive and Inductive Storage 

An alternative to electrochemical storage is energy storage in 

capacitors or inductors. In a capacitor, energy is stored as electrical 

charge on layers of metal separated by thin insulator film. Capacitors have 

lower specific energy than electrochemical storage, typically under 1 

W-hr/kg, although capacitors of up to 10 W-hr/kg are possible, and 20 

W-hr/kg is seen as a reasonable goal in the timeframe 2000+ 171. An 

attractive possibility for the long term could be capacitors manufactured 

using aluminum as the metal and Si02 or A1203 as the insulator, both 

refined from locally available materials. 

The amount of capacitance required would be very large. For example, 

at a storage voltage of 10 kV, 100 kW of storage for 354 hours would 

require over two thousand farads of capacitance, a very large figure by 

conventional electronic standards. 

In inductive storage the energy is stored as a magnetic field associated 

with a continuous current flow. To avoid resistive losses, an inductive 

storage system would necessarily have to be made using high critical field 

superconductors, and such storage is often referred to as "Superconducting Magnetic Energy Storage System". or SMES. Recent advances 

in high tem erature superconductors (HTS) make inductive storage more 

and Earthlight and need no refrigeration. The recently discovered class of 

high-temperature superconductors are composed of materials that, except 

for oxygen, are not generally found on the moon. However, HTS 

technology needs to be significantly advanced in terms of critical field, 

current densities, mechanical strength and stability, and the ability to 

make the materials in the form of wires. 

reasonable F I, since the inductors could possibly be shielded from sunlight 

3 
Typical inductive storage projects using conventional (NbsSn) superconducting technology have specific energy of about 0.5 W-hr/kg. For 

example, a recent design for a 14 kW-hr demonstration su erconducting 

inductive energy storage ring had a coil mass of 26 tons 8 However, a 

design study for a 5 GW-hr storage ring estimated a mass of 50,000 to 

270,000 tons (depending on the design) [lo], for a specific energy of 

18-100 W-hr/kg. This mass is for the NbaSn superconducting coil alone: 

additional elements such as mechanical supports are likely to reduce this 

value to only a small fraction of the coil-alone value. 

The ultimate limit to the specific energy of an inductor is set by the 

strength of materials, which must withstand the magnetic forces on the 

system. The upper limit is about 300 W-hrs/kg at structural failure, 

assuming that composite materials are used for the strengthening 

elements. Current storage systems do not approach this limit. 

Potential Energy Storage 

On Earth, the most common energy storage medium used by electrical 

utilities is Earth's gravitational field, where the storage method is to hold 

water behind a dam, running it through turbines when power is desired. 

Due to the absence of water on the moon, this is not a usable solution. A 

variation of this concept suitable for the moon would be to store and 

release energy by lifting and lowering lunar rocks, e.g., on cables 

suspended from a tower, raised and lowered by an electric winch. The 

advantage is that the storage medium, rocks, are easily available and need 

no processing. The problem is that it is difficult to store much energy this 

way--the moonls gravity is just too feeble. The storage capacity is about 1 

kW-hr for each 150 ton boulder lifted 30 meters. 

Energy storage in the form of compressed air has also been studied as a 

method of load-levelling for electrical utilities on Earth. These 

applications typically use an auxiliary combustion stage to heat the gas 

during the discharge phase [Ill. Storage is typically in natural caverns or 

mines. Energy storage without auxiliary combustion is not competitive on 

Earth [I2]. This is unlikely to be a useful storage system on the moon, since 

auxiliary combustion is not possible, the gas itself would have to be brought 

from the Earth, at least for the early use (for expansion, lunar generated 

oxygen may be an option), and leakproof natural lunar caverns are unlikely 

to be available at the moonbase site. 

4 
Thermal Energy Storage 

Thermal storage is being considered for the solar-dynamic power 

systems planned for use on the phase two version of space station 

"Freedom" [131. In this storage option, energy is stored in the form of 

heat, typically in a phase-change material. The heat storage itself is 

expected to have specific energy of -250 W-hr/kg [I*], although only a 

fraction of this will be accessible as electric power. Thermal storage is a 

much more likely option for space station applications, where the storage 

required is only 30 minutes, than for the moon, where the material must 

remain hot for fourteen days. This is a viable option if the primary power 

system for the lunar base is solar dynamic. Since the energy losses are 

likely to be dominated by radiation loss, a low-temperature system is more 

amenable to long period storage than a high-temperature system. 

An alternative version of thermal storage is to use lunar rock as the 

storage medium. This drastically decreases the amount of material which 

has to be brought from Earth. In a low-temperature thermal system, an 

insulated pit containing lunar rocks would be heated to a storage 

temperature of -300" C by embedded heat pipes carrying solar energy 

during the lunar da , and this energy would be used to run a heat engine 

during the night In a high temperature system, the regolith could be 

heated to 1700'K (161 and thermal radiation from the hot rock used to 

illuminate a photovoltaic array optimized for IR conversion. Radiation not 

usable by the solar cells could be reflected back to the source. Eder [l61 

estimates that, neglecting losses, a volume of regolith 4-5 m in each 

dimension should suffice to provide 100 kW of night power. 

The difficulty of this system is insulating the rock bed against heat loss 

to the surrounding lunar soil. Higher temperature systems have 

considerably greater difficulties with both radiation and conduction losses, 

although the higher energy densities allow more compact storage and thus 

reduced surface area. This problem has not to date been analyzed in 

de th. Solar furnace designs for melting regolith are discussed by Lewis 

1700'K to 1200°K in roughly 1.5 days 1181. However, if this could be done 

without a large amount of required mass, the system weight could be 

considerably lower than that of other storage methods. 

Losses in thermal systems decrease as surface to volume ratio 

decreases and thus are less important as the system size increases. The 

cooling time of larger mass systems scales proportionately to the cube root 

of the mass. Thermal storage thus becomes increasingly attractive for 

larger base sizes. 

Since a manufacturing facility is likely to use many high-temperature 

processes such as magma electrolysis or glass manufacturing, the waste 

heat of the processing could also be used for the electrical power [179191. 

If thermal storage is to be considered, issues of conduction and 

radiation losses during the night must be examined in detail. Because of 

the long storage times, thermal storage is unlikely to be competitive using 

present day materials, but may be possible with improved materials. 

11 ? 1, who estimates that a 21 ton mass of regolith glass will cool from 

I. 
Flywheel Storage 

A final possibility is storage as kinetic energy by use of a flywheel. The 

best current technology flywheels have specific energy of about 20 

W-hrs/kg. A composite flywheel with specific energy of 120 W-hr/kg at 

failure has been demonstrated [20] (not counting support systems, bearings, 

motor/generator, etc.) Counting a factor of three for safety margin and 

50% additional mass for support systems, this comes to about 36 

W-hrs/kg. These values are considerably below the theoretical limits of 

advanced composite materials [21], about 300 W-hrs/kg at failure. 

A problem with flywheels for terrestrial storage is the requirement for 

vacuum. This is not a problem on the moon, where the vacuum is available. 

A second problem is that a flywheel must have a containment system, to 

prevent high-velocity fragments from causing injury in case of a 

catastrophic failure. On the moonbase, the flywheel can easily be placed 

below ground level, where this is also not a problem. 

It is likely glass fiber for a composite flywheel could be manufactured 

from available materials [191, albeit with ultimate strength less than that of 

advanced composite fibers such as Kevlar or graphite. Materials for the 

polymer matrix, however, is not likely to be available. If a metal matrix, 

such as titanium or aluminum, could be used, then flywheel storage is an 

attractive option for future storage based on locally manufactured, all-lunar 

material technology. Flywheels have losses due to residual friction, eddy currents, etc., 

which in some cases can be quite large. These losses would have to be 

reduced to below about 0.2% per hour for flywheels to be useful for the 

entire lunar night. 

4. Transmitted Power 

Power could be beamed to the moonbase in the form of an 

electromagnetic wave. Beamed power has been investigated in some detail 

for other applications, including satellite solar power systems (SSPS) [221. 

The main options for the beam are microwave or laser. 

Electromagnetic Beams 

The fundamental limit to the transmitter and receiver aperture sizes is 

set by the diffraction limit, 

r,rt= 0.61 d h (1) 

where r, is the aperture radius of the receiver rt the aperture radius of 

the transmitter, d the source to receiver distance,’ and h the wavelength. 

For a microwave beam, aperture size is the antenna radius: for a laser, 

it is the radius of the lens or reflector used to focus the beam. (The 

receiver radius is here defined as the first zero in the diffraction pattern: 

this contains 84% of the beam energy. If a larger fraction of the 

transmitted energy is to be captured, the receiver aperture must be larger 

than this value.) The minimum total area is when the transmitter and 

receiver are of equal size, and the area is proportional to the square of the 

wavelength used. Thus, it is important to use the shortest practical 

6 
wavelength (Le., the highest practical frequency). 

The power efficiency of a microwave beamed system is projected to be 

very good, both as the transmitter and at the receiver: typically over 85% 

can be expected, with efficiencies above 90% not unreasonable. Unlike 

laser systems, an additional receiving rectenna is required on the surface. 

A difficulty of microwave systems is that to form narrow beams the 

antennas need to be large due to diffraction effects. However, a microwave 

antenna can in principle be made from a very light metal mesh. Microwave 

transmission at 2.5 GHz (h =10 cm) has been demonstrated. 30 GHz (h = 1 

cm) has been identified as a target frequency for transmission. Shorter 

wavelengths yet would be desirable. 

Laser transmission is an attractive option because optical wavelengths 

are considerably shorter than microwave wavelengths, ' which reduces 

diffraction effects and allows narrower beam spread and consequently 

much smaller apertures. If the laser wavelength is selected properly, the 

receiver can be the same solar array used to provide daytime power. Laser 

power transmission is discussed by de Young et al., 1231, A difficulty is that 

the power efficiency of conventional lasers is typically not very high, e.g., 

about 10% for a Kr-F excimer laser. Free-electron lasers have potentially 

efficiencies as high as 65% [241 as well as high power and a wavelength 

range down to 50.2~. but are extremely massive, too massive to be used 

except for a surface-based system. De Young et al. recommended 

development of large arrays of diode lasers for power transmission. Since 

the maximum power of each individual diode laser is typically about one 

watt, an array consisting of a very large number of individual lasers would 

have to be used. Arrays of diode lasers have recently demonstrated power 

densities as high as 50 W/cm2 with total energy efficiency of 40% [251. 

An attractive alternative is the possibility of a laser directly powered by 

solar energy, which increases the effective efficiency by eliminating the 

intermediate step of conversion of solar energy into electricity [261-. 

The efficiency of conversion at the receiver is also not nearly as high as 

microwave conversion. The best solar cells can be expected to convert 

about 50% of the incident light into electricity at the optimum wavelength 

(energy just higher than the bandgap). For photovoltaic receivers the 

efficiency drops to zero for wavelengths much longer than the optimum. 

For wavelengths shorter than the optimum, the expected efficiency E is 

roughly 

where q (optimum) is about 50% for the best cells, and hcutoff is 

determined by the bandgap of the solar cell material, 

Thus, as long as all of the transmitted power falls on the receiving 

array, the solar cell material is preferably tailored to the laser wavelength, 

or vice versa. However, if the spot size is larger than the receiving array, it 

is optimal to decrease the wavelength to put more of the power on the 

7 
array. This can be from seen comparing equation (1) and equation (2). 

The beam area is proportional to rs2, and so the fraction of the beam which 

is intercepted by the array increases as h2, while the efficiency only 

decreases proportionately to h. 

Transmitted Power from Space 

A question of considerable importance to beamed power systems is the 

best location for the primary [transmitter) station. 

Placing the primary power station in low lunar orbit (LLO) minimizes 

transmission distance. Low-lunar orbits tend to be unstable for periods of 

more than about 100 days: this means that the orbit will have to be actively 

maintained. 

If a single primary power satellite is used, it will be in line-of-sight to 

the base for no more than half of the orbit. A LLO satellite will also be in 

the lunar shadow for a considerable fraction of its orbit. For a 1000 km 

orbital altitude, the orbital period is roughly four hours. Thus, several 

hours of energy storage will still be required at the base. Since providing 

several hours of storage when the satellite is on the other side of the moon 

is considerably easier than providing 354 hours of storage for the entire 

lunar night, this is still a major improvement. 

Alternatively, at least three primary power satellites are required if one 

is to be in line of sight of the base at any given time. 

Placing the primary power satellite at libration point L-1 (between 

Earth and Moon) requires transmission of power over a longer distance. 

The advantage is that only one satellite is needed, since L-1 is always in 

sight from near side of moon. The orbit is unstable for periods greater 

than about 50 days, but the corrections needed are small if the location is 

not allowed to drift very far from the equilibrium point. Occasional 

eclipses by the moon and the Earth will interrupt power for brief periods. 

Placing the primary power station on the lunar surface, at a location on 

the far side of the moon preferably exactly 180' away from the base, 

requires the use of one or more relay satellites to transmit the power. 

This allows a single photovoltaic array to be used, fxed on the surface. The 

requirement for relay satellites means that this arrangement is unlikely to 

be more efficient than producing the power directly at the satellites. 

An alternative to beaming power would be to transmit the power across 

the surface of the moon on a physical link from the second solar array on 

far side of moon. This would require roughly 5500 km of power lines; 

about distance from NY to San Francisco. While the link could be a 

conventional high-voltage lines, possibly made from locally-available 

aluminum or calcium, resistance losses would be high unless extremely 

high voltages were used or very large diameter wire was used. (For 

example, at 0.1 cm2 cross section and 100 kV, the resistance losses for a 

100 kW system are about 20'30, and the mass of wire required is about 

600,000 kg). If two ground power stations were used, each located 90" in 

longitude from the base, the transmission distance would be halved and 

the wire mass proportionately decreased. The required transmission line 

length could be greatly reduced if the base is not located at the equator. 

For example, if the base is located at 60"N and the transmission lines run 

8 
across the north pole, the total length can be reduced by a factor of three. 

This has an additional advantage that the lines could probably be 

"tapped" at points along the length to run remote experiments, as well as 

to serve as charging stations for electrically powered exploration vehicles. 

Use of superconductors for the lines would eliminate the resistance 

losses and allow lower wire cross section. These will need to be kept cool; 

this could be done by shielding them from the ground and from direct 

sunlight (also possibly from reflected light from the Earth). 

Alternatively, the link could be fiber-optic light-guides (made from 

locally available silica) which direct a laser beam. 

In any case, this option is likely only for an advanced moonbase. 

Beamed Power from Earth 

Finally, power could be beamed directly from stations on the Earth. 

The advantage of this is that electric power is cheap on Earth 

(-5#/kW-hr), and there is no need to loft a large solar array or power 

beaming equipment into cislunar space. For the following example I will 

assume power transmission by laser. 

Consider a baseline system with a wavelength h =1p= 1.10-6 m. This is 

the approximate wavelength range for a Nd:YAG laser (1.06 p), or a GaAs 

laser diode array, and is near the optimum wavelength for conversion for a 

Si solar cell. d(Earth-Moon) is 3.8.108 m, and the lens diameter is 2 

meters. A two meter diameter lens (or mirror) is very large by telescope 

standards (for example, the Hubble Space Telescope is a 2.4 meter 

diameter mirror). However, the lens need not be telescope quality. The 

lens could be a fresnel lens, or, since it need only function at a single 

wavelength, a holographic optical element. 

For diffraction limited beam spread, the spot size is 230 meters; the 

illuminated area 170,000 m2. For the array specified at 100 kw and a solar 

conversion efficiency of 18.5% efficiency, the array area is 400 m2, and so 

the array intercepts only 0.25% of the beamed power. The required beam 

power would be 85 MW. 

This can be reduced by decreasing the laser wavelength to 0.5~ and 

increasing the solar cell bandgap from l.leV (Si), to 2.0 eV (GUS alloy). 

This is about the widest bandgap that will still give good solar conversion 

efficiency for daytime power (also, wavelengths below about 0.3p begin to 

be significantly absorbed in the atmosphere.) The array is then oversized 

by a factor of two over the size required for daytime power, and stationary 

reflectors are used to intercept an additional factor of 4. Since the Earth 

is nearly stationary in the Lunar sky, these reflectors do not require 

tracking, and need be no more than thin reflective sheets, of plastic. The 

array will now intercept 8% of the beam. If 50% power at night is now 

assumed, the required beam power is 2.2 MW. Further reductions in 

power could be achieved by the use of a larger area of thin reflectors, as 

discussed in the next section. 

Note that the total system requires twice as many lasers as are actually 

in use, since at any given time half will be on the wrong side of the Earth. 

Such a system would utilize many lasers from different sites--presumably 

desert areas and mountaintops--so that laser failure will not interrupt 

9 
power. The required 4.4 MW could be provided, for example, by 56 

eighty-kw lasers (twenty-eight of which are running at any given time). 

Such power levels are high compared to those achieved by current 

technology CW visible light lasers, but in the range likely to be reasonably 

achievable for future high-power lasers. It is many orders of magnitude 

higher power than currently achieved by diode lasers. Problems of 

tracking, reliability, and atmospheric turbulence remain to be addressed. 

The intensity of the beam at the site is considerably less than solar 

intensity, and thus would not present a hazard to base personnel unless 

they look directly into the beam. This hazard could be removed if the solar 

array is located in an area which is kept off limits to astronauts during the 

night, or if the suit visors and the windows of the base are designed to 

incorporate a rejection filter at the proper wavelength. 

5. Innovative Concepts 

Solettas 

One proposal has been to use "Solettas", or orbital mirrors, to reflect 

sunlight to the surface solar panels 1271. The fundamental limit to soletta 

illumination is the minimum spot size ds at the receiving array. This is 

fixed by the angular diameter of the sun and the orbital altitude: 

d, = ha + dm (4) 

where h is the slant range between the orbiting mirror and the ground 

spot, a is the angular diameter of the sun (about .01 radian), and dm is the 

mirror diameter. The spot size can be decreased by lowering the orbital 

altitude, but this means that the fraction of time that any particular mirror 

can view the receiving array also decreases, thus increasing the required 

number of mirrors in orbit. When the mirror size dm is less than ha, the 

illuminated spot size is constant and the illumination intensity decreases 

with mirror area. Except for very large systems, the spot size is much 

larger than the solar array, and thus the array intercepts only a small 

fraction of the energy reflected by the mirror. The mirror size needed is 

thus independent of the array size. Consequently, the concept is most 

suited for very large power requirements. 

An initial design discussed by Criswell 1271, as shown in figure 1, is 

calculated for 800 kW of night power. Four mirrors are assumed, each 40 

km2 in area. The illumination level is 11% of the daytime illumination. 

The mirrors were assumed to be fabricated from light-weight solar-sail 

technology, with a total mass of 1 million kilograms. 

Finally, it should be noted that the soletta concept requires 

exceptionally good mirror surface accuracy and pointing accuracy at 

comparatively high slew rates. A quarter degree of pointing error will 

result in the illuminated spot missing the surface array; a similar amount of 

ripple in the surface will defocus the spot. These surface and pointing 

tolerances are considerably higher than those needed for solar sail 

technologies. 

10 
Conversion of Earthlight 

The Earth is nearly fxed in the lunar sky. This raises the intriguing 

possibility of utilizing the solar array to convert sunlight reflected from the 

Earth [28l. 

The albedo of the Earth is 0.36 (k.06, depending on cloud cover). Even 

when full, the Earth is 10,000 times less bright than the sun. At half 

phase, which is the worst case for a base located at the center of the near 

side of the moon, the Earth is 20,000 times less bright than the sun. 

Therefore, to produce full power at sunset and sunrise, a solar array would 

have to be 20,000 times larger than the one required for daytime power. 

If 30% power is acceptable as the nighttime average, the array need be 

only 4,000 times larger than the daytime array. 

This could conceivably be done using mirror concentrators. A 

minimum mirror need be no more than a flat sheet of very thin reflective 

plastic. One micron thick aluminized Kapton has a mass of 1.4 gr/m2, 600 

times lighter per unit area than the 300 W/kg solar array assumed. Thus, a 

minimum mass 4000x concentrator could weigh as little as about 7 times 

the mass of the solar array itself. This is still a considerable problem: the 

system would require 4,000 sheets of Kapton, each one carefully aligned, 

for each panel of the array. The mass does not include the additional area 

required for cosine theta loss and reflectance losses, and additional mass 

for the structure to support it. Also, the array will likely also require 

tracking: the Earth doesn't move much, but it does move'some. In short, 

this solution is unlikely to be practical. 

Lunar Polar Base 

It is possible that locating a lunar facility at one of the poles of the 

moon could alleviate the problem of the lunar night, since the axis tilt of 

the moon is so low that placing the arrays on a relatively short tower (or 

conveniently placed mountain) could allow them to be constantly 

illuminated [29.301. This is shown in schematic in figure 2. 

A problem with the polar location for a lunar base is that, like the 

Earth, the polar regions of the moon are subject to six months of darkness 

during the hemisphere's "winter". This could make exploration and work 

outside very difficult. Even during the "summer," the sun angle remains 

very low (within 1.5O of the horizon). This means that inky black shadows 

will cover almost all of the surface, making exploration (and even walking 

around!) very tricky. 

Sun-following Moonbase 

In the sun-following moonbase concept, the lunar base constantly 

moves around the moon to stay continually in sun [3l]. The rotational 

velocity at the lunar equator is 16.6 km/hr (10 MPH). If the moonbase is 

sited 60" N, where the local sun angle is a comfortable 30" off horizon, the 

required average velocity is only 8.3 km/hr (5 MPH): and even less if the 

path chosen is across the pole. Actual moonbase speed will be 10 MPH 

during 12-hr "drive" shift and 0 MPH during 12-hr "work shift. Figure 3 

shows a version of such a mobile moonbase using design concepts familiar 

from other space habitat structures. 

Conceptually this is an extreme solution to a simple problem, but as 

well as providing continual solar power, it does have other advantages: it 

11 
eliminates the 354-hr dark period when outside exploration is difficult or 

impossible, thus effectively doubling the working hours of the staff; and the 

base is not "stuck" in one spot, but continually samples new territory. 

The path should be maintained near the sunrise terminator, to give as 

close to 14 day of "slack" as possible for repairs. Since the moonbase 

would consist of many independently mobile modules, no single failure 

would be critical. Any one unit could be evacuated if necessary and 

repaired on the next cycle. 

An alternative version would be to have two separate.lunar bases on 

opposite sides of the moon, with the crew transferred from one to the 

other on a two-week cycle. This has the disadvantage that the entire 

moonbase must be doubled. 

6. Conclusions 

A constant supply of electrical power is important to human occupancy 

on the moon, and one such supply is solar energy. The major difficulty in a 

solar-powered moonbase consists in providing steady power over the long 

and dark lunar night. At the moment, the promising solution for a 

near-term moonbase appears to be the use of regenerative fuel-cells with 

cryogenic storage, a technology which is not yet fully developed but is 

unlikely to have any fundamental technical difficulties in development. 

Nevertheless, a wide variety of other concepts for solar night power have 

been proposed, which are summarized in table 1. Not all have been 

examined in depth. All have some apparent drawbacks: many will only be 

useful for a large, "evolved" moonbase. 

There is certainly room yet for clever new ideas. 

7. References 

[l] D.J. Brinker and D.J. Flood, "Advanced Photovoltaic Power System 

Technology for Lunar Base Applications," paper LBS-88- 139, Lunar Bases 

and Space Activities in the 21st Century Symposium, Houston, TX, April 

[2] G.A. Landis, S.G. Bailey H.J. Curtis and D.J. Flood, "Photovoltaic 

Power for a Lunar Base," to be presented at 40th IAF Congress, Malaga, 

Spain, Oct. 1989. 

[3] G.A. Landis, S.G. Bailey, and D.J. Flood, "Advances in Thin-Film Solar 

Cells for Lightweight Space Photovoltaic Power," Space Power, VoZ. 8 # 1-2, 

[4] K. Zweibel, The Potential of CuInSe2 and CdTe for Space Photovoltaic Applications," 2 1 st Intersociety Energy Canversion Engineering 

Conference, VoZ. 3, 97- 102, ASME 1988. 

[5] E. Levy Jr., "U.S. Air Force Design Nickel-Hydrogen Cells," paper 

869348, Intersociety Energy Conversion Engineering Conference 1986, 

[6] L. Miller, "Large Capacity Ni-Hz Battery Cells," paper 869365, 21st 

Intersociety Energy Conversion Engineering Conference, 1 986, VoZ. 3, 

5-7, 1988. 

3 1-5 1 (1989). 

VOZ. 3, 1537- 1540. 

12 
1595- 1597. 

[7] K.A. Faymon and S.J. Rudnick, "High Temperature Superconducting 

Magnetic Energy Storage for Future NASA Missions," paper 889416, 23rd 

Intersociety Energy Conversion Engineering Conference, 1 988, Vol. 2, 

[8] L.L. Kohout, "Cryogenic Reactant Storage for Lunar Base 

Regenerative Fuel Cells," NASA TM 101980: to appear in Space Power, 

Vol. 8 (1989). 

[9] M. Masuda and T. Shintomi, "Engineering Testing Plant for Superconducting Magnetic Energy Storage," paper 889505, 23rd Intersociety 

Energy Conversion Engineering Conference, 1988, Vol. 2, 54 1-546. 

[ 101 T. Nakano, K. Hayakawa, and M. Shimizu, "Design Study of a 5 GWh 

SMES," paper 889506, 23rd Intersociety Energy Conversion Engineering 

Conference, 1988, Vol. 2, 547-55 1. 

[ll] R. Pollak, "Status of First U.S. CAES Plant," EPRI Journal, Dec. 

[ 121 R. Pollak, personal communication. 

[13] D.E. Lacy, C. Coles Hamilton and A. Juhaz, "Selection of High 

Temperature Thermal Energy Systems for Advanced Solar Dynamic Space 

Power Systems," paper 879223, 22nd Intersociety Energy Conversion 

Engineering Conference, 1987. 

[14] T.K. Stovall and R.V. Armilli, "Transient Thermal Analysis of 3 Fast 

Charging Latent Heat Storage Configurations for a Space Based Power 

System," paper 889508, 23rd Intersociety Energy Conversion Engineering 

Conference, 1988, Vol. 2, 17 1 - 177. 

[ 151 J. Friefeld, Rockedyne Division, Rockwell International, private 

communication. 

[ 161 D. Eder, "Lunar Base Logistics," Space Manufacturing 6, Faughnan 

and Maryniak, eds., AUW, 1987, 299-301. 

[ 171 W.C. Lewis, "Towards a Lunar Glass Fiber-Metal Composite: 

Production," presented at Lunar Bases and Space Activities in the 21st 

Century Symposium, Houston, TX, April 5-7, 1988. 

[18] W.C. Lewis, letter dated 22 May 1989. 

[ 19) P. Ho and L.E. Sobon, "Extraterrestrial Fiberglass Production Using 

Solar Energy," Space Resources and Space Settlements, NASA SP-428, 

[20] M. Olszewski and D.U. O'Kain, "Advances in Flywheel Technology 

for Space Power Applications," paper 869414, 21 st Intersociety Energy 

Conversion Engineering Conference, 1986, Vol. 3, 1823- 1828. 

[21] J.A. Kirk and D.K. Anand, "Overview of a Flywheel Stack Energy 

Storage System," paper 889067, 23rd Intersociety Energy Conversion 

Engineering Conference, 1988, Vol. 2, 25-30. 

(221 P.E. Glaser, "Solar Power from Satellites," Physics Today, Feb. 

[23] R.J. De Young, et al., Comparison of Electrically Driven Lasers for 

[24] H.P. Freund and R.K. Parker, "Free Electron Lasers," Scientgic 

[25] reported at 1988 Joint Conference on Lasers and Electro-Optics 

[26] E.J. Conway, "Solar Pumped Laser Technology Options for Space 

51 1-514. 

1988, 49-52. 

225-232, 1979. 

1977, 30-38. 

Space Power Transmission, NASA Technical Memorandum 4045, 1988. 

American, Vol. 260, #4, Apr. 1989, 84-89. 

(CLEO 1988). 

13 
Power Transmission," paper 869423, 21 st Intersociety Energy Conversion 

Engineering Conference, 1 986, Vol. 3, 1862- 1868. 

[27] D.R. Criswell, "The Initial Lunar Supply Base", Space Resources 

and Space Settlements, NASA SP-428, 218-224, 1979. 

(281 G.A. Landis, unpublished 

[29] J. Green, "The Lunar Polar Base - A Viable Alternative to L-5", 

paper AAS 78-191, The Future United States Space Program, Proceedings 

of the 25th AAS Anniversary Conference, Vol. 38, Part 1, Advances in the 

Astronautical Sciences, 385-425 (1979). 

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and Space Activities of the 21st Century, W.W. Mendell, ed., Lunar and 

Planetary Institute, Houston 77-84 (1985). 

[31] G.A. Landis, "A Proposal for a Sun-Following Moonbase," submitted 

to J. British Interplanetary Soc. (1989). 

14 
Table 1 

Approaches to solar power for the lunar night 

Storage Options 

Electrical Storage 

method Speci_Fc Power (W- hr/ kg) 

present anticipated 

Batteries 32 150 

Fuel Cells 50 1500 

Capacitors 10 20 

Inductors 0.5 100 

(impracticall physical storage Compressed Gas 

Thermal storage - 250* 

Lunar thermal storage 10000~ 

Flywheel Storage 20 35 

Gravity Storage - <1 

*heat 

theat. will depend on insuhtion requirement 

Continuous Power Concepts 

Transmitted Power 

method comme nts 

Microwave beam 

Laser beam 

Transmission lines 

Ground-based laser 

Primary station in LLO or at L-1 

Primary station in LLO or at L-1 

Prima y station on Lunar farside 

Primary station on Earth 

Innovative Concepts 

Soletta 

Earthlight Conversion 

Lunar Polar Base 

Sun-Following Moonbase Not atfi)ced location 

Orbital mirrors; practical only for large systems 

Requires very large collectors 

Low sun angle at base 

15 
Figure 1: the "Soletta" solution: 

Four mirrors in high-inclination, low altitude orbit 

provide 10% of one sun illumination to solar arrays 

orbital altitude = 500 km periapsis, 1050 km apoapsis 

8.7 km diameter mirrors at 6 grams/sq. m 

240 tons each; 1000 tons total 

minimum spot size 34 km 

illuminated area 1000 square kilometers 

Mountain of perpetual light" 

R 

Figure 2. Lunar Polar Base (Schematic) 

by placing the solar array atop a high enough mountaintop close to 

the lunar pole, it will always be in sunlight! 

16 
Figure 3: Sun-following moonbase 

moonbase moves continuously around the moon 

to stay in sunlight moonbase speed: 

10 MPH during 12-hr "drive" shift 

0 MPH during 12-hr "work" shift 

ORIGINAL PAGE 

BUCK AN0 WHITE PI-IOTOGRAPH 

17 
National Aeronautics and %ace Administration 

1. Report No. 

NASA TM-102127 

Report Documentation Page 

2. Government Accession No. 3. Recipient’s Catalog No. 

4. Title and Subtitle 

Solar Power for the Lunar Night 

7. Author(@ 

Geoffrey A. Landis 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-49 13 

10. Work Unit No. 

4 506-41-11 

12. Sponsoring Agency Name and Address 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

17. Key Words (Suggested by Author($) 

Moon Base; Photovoltaics; 

Energy storage; Solar cells; 

Solar power 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 9 1 

14. Sponsoring Agency Code 

9. Security Classif. (of this report) 

Unclassified 

I 

15. Supplementary Notes 

Prepared for the 9th Biennial SSI/Princeton Conference on Space Manufacturing sponsored by the Space Studies 

Institute. Geoffrey A. Landis, National Research Council -NASA Research Associate. 

20. Security Classif. (of this page) 21. No of pages 22. Price* 

Unclassified 18 A03 

16. Abstract 

Providing power over the 354 hour lunar night provides a considerable challenge to solar power concepts for a 

moonbase. The paper reviews concepts for providing night power for a solar powered moonbase. The categories 

of solutions considered are electrical storage, physical storage, transmitted power, and “innovative concepts”. 

Electrical storage is the most well-developed option. Less developed electrical storage options are capacitors and 

superconducting inductors. Physical storage options include storage of potential energy and storage of energy in 

flywheels. Thermal storage has potentially high energy/weight, but problems of conduction and radiation losses 

during the night need to be addressed. Transmitted power considers use of microwave or laser beams to transmit 

power either from orbit or directly from the Earth. Finally, innovative concepts proposed include reflecting light 

from orbital mirrors, locating the moonbase at a lunar pole, converting reflected Earthlight, or moving the 

moonbase to follow the sun.