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One of the most difficult problems facing spacecraft engineers is how to power a craft once it is beyond the Earth's atmosphere. Fuel is incredibly costly to transport due to its weight, and if a long journey is scheduled it becomes incredibly impractical to take that amount of fuel into the atmosphere.
This makes it necessary to find an alternative means of providing electricity to a craft. Depending on the nature of the mission, there are two systems that are generally used. If the mission is to the outer solar system and beyond, then a radioisotope thermoelectric generator may be preferred. A radioactive element, such as plutonium 238, decays and provides heat. This heats one side of the thermoelectric generator, with the other side at the near absolute temperature of space, which creates a temperature difference, generating electricity. These systems can last for an incredibly long time, but it would be disastrous for a crew if they went wrong. Missions that are only traveling as far as Jupiter, therefore use photovoltaics, or solar panels, as they are less catastrophic if they go wrong, and as long as they are close enough to the sun, can provide power indefinitely.
The primary use of photovoltaics is to provide power for the other systems within the craft including sensors, active heating, cooling, and telemetry. They can also provide power for movement, sometimes called solar-electric propulsion, but this is usually seen on smaller craft due to being an energy-intensive process.
Solar panels were first introduced onto spacecraft with the launch of the Vanguard 1 satellite in 1958. This ensured longer lifetime and helped to reduce the load. Since then they have been used on most major space missions, with the most famous spacecraft to extensively use photovoltaics being the International Space Station. The iconic shape of the ISS features large solar arrays at either side of the crew quarters in the center, covering over 2,500 square meters of space. This area is enough to generate around 84 to 120 kilowatts of power, and powers the entire space station. These panels operate at around 12% efficiency, and although cutting edge when they were first installed, improvements are being made in solar technology all the time. For example, the ones currently being used to power the Mars Curiosity rover twice are over twice as efficient as that at around 26% efficiency, and there are photovoltaic materials being developed in the lab that are 44.5%.
There are some disadvantages to using photovoltaics for power. The first is that the amount of power produced is dependent on the size of the array: larger vehicles need larger arrays, and this drives costs up. Cost is also a significant factor on the ground. Although solar power is getting cheaper all the time, this only really applies to those systems that are being used for mass power generation on Earth. Spacecraft use cutting-edge technology which is never cheap. Finally, it becomes more difficult for solar arrays to operate the further away they are from the sun. This restricts their use to applications that operate within the radius of the orbit of Jupiter.
These are the problems that future engineers will be looking to solve. Aside from the obvious desire to continue the efficiency of the panels, there are several technologies being researched for the continued use of photovoltaics on spacecraft. The first is the concept of concentrator solar arrays that use a flat lens, called a Fresnel lens, to focus a large area of sunlight onto a much smaller area of thepanel. This has the effect of massively boosting the amount of photons hitting the panel, allowing the panel to achieve the same power output of a much larger one that doesn't have the concentrating lens; this means fewer solar cells need to be produced. As the cells are often the most expensive part of the array, which are a very expensive part of the spacecraft, it massively reduces costs as less material is needed to be produced and transported.
The second strategy involves more outside-the-box thinking. Why bother sending up the panels in the first place when you just transport the raw materials and build them in space. This approach is not just restricted to photovoltaics; in 2014 a printhead faceplate was 3D printed on the international space station to become the first object made in space. This in-situ construction of components in space is the next step in being able to produce entire systems in orbit, some of the first of which will be solar arrays.
Additional Reading
- https://arxiv.org/ftp/arxiv/papers/1709/1709.01787.pdf
- https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft
- https://www.nasa.gov/pdf/163008main_SESE_TeachersGuide__Part_dc3.pdf
- https://www.weforum.org/agenda/2017/08/this-is-the-most-efficient-solar-panel-ever-made
- https://www.nasa.gov/content/open-for-business-3-d-printer-creates-first-object-in-space-on-international-space-station
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