5.5 Power Generation

We’ve motivated why the spacecraft bus needs to generate power, now how do we actually go about generating power? Spacecraft either use resources in space or bring energy sources with them into space. In this section, we’ll discuss the various ways to generate power in space and the relevant environmental parameters that affect the system design.

Solar Cells (or Photovoltaic/Photoelectric Cells)

Solar energy is by far the most popular energy source, dominating missions around the Earth and in close proximity to the Earth. “As of 2010, approximately 85% of all nanosatellite form factor spacecraft were equipped with solar panels and rechargeable batteries. Limitations to solar cell use include diminished efficacy in deep-space applications, no generation during eclipse periods, degradation over mission lifetime, high surface area, mass, and cost” [NASA]. Typically, solar energy is not the primary power source for spacecraft farther than Jupiter because 1) solar radiation is too weak, 2) current solar technology is not efficient enough, and 3) the solar arrays would be too massive [Wikipedia]. The intensity of sunlight is the driving factor in determining whether solar energy should be the primary power source for a spacecraft mission. The intensity of sunlight scales with distance squared from the sun. Juno’s mission to Jupiter has broken the record to become humanity’s most distant solar-powered emissary [NASA].

A major consideration in utilizing solar energy is the efficiency of solar cells in converting sunlight into electrical energy. The efficiency of solar cells depends on the ability of the technology in capturing the energy within the wide spectrum of electromagnetic radiation that is sunlight. Sunlight is a specific distribution of light, spanning wavelengths of primarily ultraviolet, visible, and infrared light. Although the sun does emit all frequencies of electromagnetic waves, except gamma rays, wavelengths shorter than ultraviolet and longer than infrared light have very low spectral irradiance; spectral irradiance is energy per surface area, which is what we ultimately care about. For those reasons, solar cells focus on capturing light energy through surface area and converting this light energy into electricity using the photovoltaic effect.

The photovoltaic effect is the “generation of voltage and electric current in a material upon exposure to light”, the combination of a physical and chemical phenomenon. Photons hit a semiconductor, are absorbed into the semiconductor material structure, and create in essence free electrons in the solar cell material. The positive charge is attracted to the p-type semiconductor and the negative charge to the n-type semiconductor. The difference in potential creates a current, or electricity!

^{How Solar cells work. The Anatomy of a Solar Cell by Save on Energy}

The material affects the ability of photons to be absorbed, the mobility of the electrons to move or transfer freely within the structure, conversion efficiency, and the ease of manufacturing. The selected material must match the spectral distribution of sunlight. Photon absorption may be improved by maximizing surface penetration, minimizing reflection, and reducing obstacles. Monocrystalline (single crystal) cells can be manufactured in a way that retains high purity or quality, which makes them more efficient and prolongs their useful lifetime. These cells appear black, are more energy-intensive to make, produce more waste, and thus are more expensive. Polycrystalline cells appear blue, are less energy-intensive to make, produce less waste, and thus are more common and inexpensive [EnergySage]. Finally, consider that solar cells surface and electrodes degrade over a mission lifetime, carrying a finite useful life and diminishing returns in later years. There are many different types of solar cells and we’ll review the most common choices for spacecraft.

When selecting solar cells for spacecraft, key metrics to evaluate selection are specific power (watts generated per solar array mass), stowed packing efficiency (deployed watts produced per stowed volume), and cost. Specific power incorporates solar cell efficiency and surface area but carries the evaluation one step further by incorporating mass. As mentioned before, solar cells degrade from usage but, in a space environment, additionally degrade from ionizing radiation as a function of “differential flux spectrum and total ionizing dose” [Wikipedia]. The effects of ionizing radiation may be mitigated for special glass coverings, reducing efficiency loss to 1% to 10% a year.

Solar cell technology progresses year by year, as they become more popular for terrestrial applications. In the solar cell industry, terrestrial applications mostly use single-junction cells (or a single pair of p-type and n-type electrodes), which usually carry less than 20% efficiency. Terrestrial applications take advantage of rather limitless surface area and the non-critical nature of power generation. Spacecraft are limited in surface area and power generation is mission-critical, so spacecraft designers prefer multi-junction solar cells with higher efficiency. Multi-junction incorporates “multiple layers of light-absorbing material that efficiently convert specific wavelength regions of the solar spectrum into energy, thereby using a wider spectrum of solar radiation” [NASA]. Theoretically, an infinite amount of layers could be stacked to achieve 86.6% efficiency [Green]. In an implementation, triple-junction cells balance high efficiency with cost.

The projected surface area of the panels exposed to the Sun also affects generation and varies with the solar incidence angle, the cosine of the angle between the panel and the Sun [NASA]. As seen in the figure above the projected surface area for a tilted solar cell, a, is smaller than the projected surface area for the solar cell that is more directly facing the sun, A. The efficiency of a solar collecting device thus depends on the orientation of the solar cell relative to the sun, . The scaled intensity of solar flux is given by the following equation:

Where is the scaled intensity, is the full intensity of the sun from direct exposure, and is the incidence angle. The total solar irradiance around Earth, , is 1360.8 ± 0.5 [NASA].

^{ISS Solar Array Deployment. Video by Bruno The Questionable.}

To mitigate the inefficiency of solar cells not directly facing the sun, solar arrays may be deployed (for the larger surface area) and articulated to point more directly at the sun. For example, on the ISS, the huge solar arrays were deployed with an extending truss structure, pulling the end of the solar array out. The ISS uses gimbals to track the position of the sun by continually rotating the panels to face the sun, as seen in the figure. The RAVAN CubeSat shows the deployment and articulation of its solar panels.

For deployed solar panels and a spacecraft with attitude control, you can assume direct pointing toward the sun and no loss to efficiency. For solar panels that are rigidly attached to the faces of a cube, you can point the entire spacecraft body to face the sun and maximize the sunlight hitting the solar panels. The optimal incidence angle to point at the sun depends on how many faces you can point toward the sun. If there are two panels adjacent that can simultaneously face the sun, the optimal incidence angle from each solar panel face is 45 degrees. Instead of 1 panel facing the sun directly, 2 panels facing the sun at 45 degrees will achieve 1.41 times the amount of power of a single panel. If there are three panels all touching the same corner, which can point toward the sun, the optimal incidence angle from each solar panel face is 54.7 degrees, as seen in the figure. Three panels partially pointing to the sun achieve 1.73 times the amount of power of a single head-on panel, which is the configuration of the Artemis CubeSat kit.

To size the solar array surface area, you must collect the following information:

• Required power to generate based on your power budget,
  • The efficiency of the solar cells you’ve chosen,
  • Solar irradiance at your distance from the sun,
  • Incidence angle across all solar arrays,
• The formula to calculate the surface area of the array’s surface from your power generation requirement is:

Artemis Kit Specific

Solar Thermal Power Systems

Another way to utilize solar energy in space is to convert solar energy into heat. Mirrors and lenses concentrate sunlight into high-temperature collectors, through a technique called concentrated solar power. High-temperature collectors can then be used in conjunction with various thermodynamic cycle engines to generate electricity. In a thermodynamic cycle, a working fluid (liquid or gas) converts heat into useful work through pressure and temperature differences. Well-known thermodynamic cycles are the Otto cycle, used in spark-ignition piston engines of cars, and the Rankine cycle used in steam engines of trains. Spacecraft typically do not bring combustible or consumable fuel to incite internal or external combustion, instead, a sensical choice for a spacecraft solar thermal engine is the Baker or Carnot heat engine.

A Carnot heat engine transfers “energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work” [Wikipedia]. This cycle is special because the cycle may be reversed and the whole system may be contained in a closed system. The fluid that runs through the engine can be any substance capable of expansion. The total possible work is equal to the difference in the heat put into and heat that is taken out of the system. This material is covered in a thermodynamic course so I won’t expound further, as these heat engines are not commonly used in spacecraft.

These solar thermal power systems can be advantageous over solar cells, with higher areal efficiencies and lower procurement costs. The disadvantages are that there are moving parts that wear down over time and require maintenance.

Nuclear Power

Radioisotope Thermoelectric Generators (RTGs) are the most common power generator for spacecraft missions past Jupiter and the Martian planetary rovers: the Apollo missions to the moon, the Viking missions to Mars, and the Pioneer, Voyager, Ulysses, Galileo, Cassini, and New Horizons mission to Pluto and the Kuiper Belt all used RTGs [NASAfacts]. RTGs are nuclear reactors that do not rely on the space environment; all components and physical phenomena are contained in the technology built on the ground. These tiny nuclear reactors generate electricity from the heat of radioactive decay, typically from the element Plutonium-238. These energy sources constantly decay, emit heat, and degrade over time. A sample of technical specifications used for the Mars 2020 Rover, Perseverance, is seen in the table below. These energy generators do not miniaturize well, as there are many reactor components in a complex configuration to convert nuclear energy into electrical energy. 238Pu can also be toxic to humans if improperly contained and exposed directly in large doses. This highly radioactive element is also strictly controlled by the Department of Energy for which immense paperwork is required to obtain significant amounts for a spacecraft mission. These reasons motivate the use of RTGs for large unmanned spacecraft or robotic missions.