Did you know that solar radiation is the primary source of power for most satellites and space stations orbiting our planet? Although most of us routinely use satellite-derived products and services in our daily lives, we rarely think about how these spacecraft are powered. The answer is very simple: solar panels, and lots of them!

Although nuclear power and fuel cells were also used in space exploration in the past, today, solar panels are the main source of power in space. In fact, solar cells were initially developed to power spacecraft, decades before their mainstream adoption on Earth. In the 1950s, Bell Laboratories in the US produced the first solar cells for a specific application: powering the Vanguard I satellite. The simple act of covering the satellite with a few solar cells increased its expected lifetime and kept it beaming back signals to Earth for a full seven-year period. This was just the beginning: today, solar panels are a standard feature in many space missions, from Earth orbit to the surface of Mars.

The use of solar power in space highlights some of the best things about this renewable energy source.

Solar is scalable

Solar panels powering a small "cubesat" (source: Montana State University)

Solar panels powering a small "cubesat" (source: Montana State University)

From a tiny 10-cm mini satellite to the enormous International Space Station (ISS), solar panels can power a variety of spacecraft. At one end of the spectrum, solar cells covering the sides of a small satellite can generate about 4 watts of power. At the other end of the spectrum, 8 large solar arrays provide a total of 84 kilowatts of power to the ISS. Although adding more solar panels to a spacecraft definitely increases the power output, there is a trade-off: the larger the structure, the higher the "atmospheric drag". It turns out that the tiny traces of atmospheric gases can cause a spacecraft to slow down by applying pressure on the solar panels. Thus, larger solar panels increase the fuel consumption needed to keep the spacecraft in its proper orbit. No wonder it took NASA engineers years to optimize the design of the solar panels on the ISS!

Solar panels on the ISS (source: NASA)

Solar panels on the ISS (source: NASA)

Efficiency of the solar cells is a key driver of performance

As we discussed in a previous article, the efficiency of the solar panels is an important factor that determines the output of your solar power system. The average efficiency of a standard solar panel is about 13%. In contrast, solar cells produced for space applications boast some of the highest levels: around 30%. This simply means that space-grade solar cells can harvest more than twice as much electricity as their terrestrial counterparts. How? By using "triple-junction" technology. Let's hope that the cost of this technology will continue to fall, and more and more terrestrial applications will follow.

Solar works best when there are no clouds (or atmosphere!)

Earth's atmosphere gets progressively thinner as the altitude increases. As we highlighted in an earlier article, atmospheric absorption acts as a filter and decreases the amount of solar radiation that reaches the ground. Another atmospheric phenomenon, clouds, further decreases the amount of solar radiation that reaches solar panels. In space, there are almost no atmospheric gases or cloud cover. Thus, solar panels have direct access to the full spectrum of the sun's rays. Although the lack of atmosphere definitely increases the output of solar cells, there is one drawback: the panels have to be pointed directly towards the sun to produce any power. Since there is no atmosphere, there is also no diffuse radiation in space.

What can we learn from solar power applications in space?

There are a few important lessons that decades of solar power applications in space have taught us. First, system sizing is a very important task and it is critical to the success of your solar investment. Make sure that the proposed design of your solar panels is a good match to the conditions on your roof. Remember, bigger is not always better as we've seen in the case of the International Space Station. Secondly, you need to understand the environmental factors that determine the amount of solar radiation available for your solar panels. There are other factors on Earth, in addition to atmospheric events, that have a big impact on your solar panels (check out this article for more details). Finally, higher efficiency for your panels is always desirable, but there is generally an additional cost. You can use Sunmetrix Discover and compare the benefit of increased efficiency with the cost of purchasing more expensive panels.