Future Developments of Space Photovoltaics and Its Use in Space-based Solar Power

Spacecraft in the Outer Solar System versus Space Craft in the Inner Solar System. Spacecraft operating in the inner Solar System usually rely on the use of photovoltaic solar panels to generate electricity from sunlight. In the outer solar system, where the sunlight is too weak to produce sufficient power, radioisotope thermoelectric generators (RTGs) are used as a power source. For example, the radioisotope Plutonium-238 is a very powerful alpha emitter which makes it suitable for usage in RTGs.

Future Developments of Space Photovoltaics

For future missions, it is desirable to reduce solar array mass, and to increase the power generated per unit area. This will reduce overall spacecraft mass, and may make the operation of solar-powered spacecraft feasible at larger distances from the sun. Solar array mass could be reduced with thin-film photovoltaic cells, flexible blanket substrates, and composite support structures. Flexible solar arrays are being investigated for use in space. The “Roll Out Solar Array” (also known as ROSA) was deployed on the International Space Station in July 2017.

Solar array efficiency could be improved by using new photovoltaic cell materials and solar concentrators that intensify the incident sunlight. Photovoltaic concentrator solar arrays for primary spacecraft power are devices which intensify the sunlight on the photovoltaics. This design uses a flat lens, called a Fresnel lens, which takes a large area of sunlight and concentrates it onto a smaller spot. The same principle is used to start fires with a magnifying glass on a sunny day. Solar concentrators put one of these lenses over every solar cell. This focuses light from the large concentrator area down to the smaller cell area. This allows the quantity of expensive solar cells to be reduced by the amount of concentration. Concentrators work best when there is a sole source of light and the concentrator can be pointed right at it. This is ideal in space, where the Sun is a single light source. Solar cells are the most expensive part of solar arrays, and arrays are often a very expensive part of the spacecraft. This technology may allow costs to be cut significantly due to the utilization of less material.

Space Photovoltaics Use in Space-based Solar Power

Space-based solar power (SBSP) is the concept of collecting solar power in outer space and distributing it to Earth. Space-based solar power is the energy of the future. It is generated via solar power satellites, otherwise known as “powersats”, and the energy is transmitted wirelessly to receiving stations on the Earth’s surface. There are both advantages and disadvantages associated with this means of power generation.

The SBSP concept is attractive because space has several major advantages over the Earth’s surface for the collection of solar power: It is always solar noon in space and full sun. This means solar power collection is virtually unaffected by the day and night cycles of the sun. On the earth’s surface, solar panels can only collect solar energy for a maximum of 9 hours per day and when there is cloud cover, this number is even lower. Collecting surfaces could receive much more intense sunlight, owing to the lack of obstructions such as atmospheric gasses, clouds, dust and other weather events. Consequently, the intensity in orbit is approximately 144% of the maximum attainable intensity on Earth’s surface.

A satellite could be illuminated over 99% of the time, and be in Earth’s shadow a maximum of only 72 minutes per night at the spring and fall equinoxes at local midnight. Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas earth surface solar panels currently collect power for an average of 29% of the day. Power could be relatively quickly redirected directly to areas that need it most. A collecting satellite could possibly direct power on demand to different surface locations based on geographical base load or peak load power needs. With very large-scale implementations, especially at lower altitudes, it potentially can reduce incoming solar radiation reaching earth’s surface. This would be desirable for counteracting the effects of global warming.

However, the SBSP concept also has several problems: the large cost of launching a satellite into space (can cost anywhere between $10 million and $400 million, depending on the vehicle used). A small launch vehicle such as the Pegasus XL rocket can lift 976 pounds (443 kilograms) into low-Earth orbit for about $13.5 million. The maintenance of an earth-based solar panel is relatively simple, but construction and maintenance on a solar panel in space would typically be done telerobotically. In addition to cost, astronauts working in GEO (geosynchronous Earth orbit) are exposed to unacceptably high radiation dangers and risk and cost about one thousand times more than the same task done telerobotically.

The space environment is hostile; panels suffer about 8 times the degradation they would on Earth (except at orbits that are protected by the magnetosphere). Space debris is a major hazard to large objects in space, and all large structures such as SBSP systems have been mentioned as potential sources of orbital debris. The enormous size and corresponding cost of the receiving station on the ground. Energy losses during several phases of conversion from “photon to electron to photon back to electron”, as Elon Musk (founder of SpaceX- an aerospace manufacturer and transporter) has stated.