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Space, the final frontier, for solar
In this blog we will be asking the following questions: Solar in space. What panels are used? How are they used? How are they tested and what are some future applications for PV as a world power source.
Satellites, space and solar panels
Why is solar technology used in space?
- Solar technology can be a political football on the ground
- In space, it encounters little opposition
- The power alternatives in space are politically difficult
The options are radioisotope power systems, or RPS.
RPS converts heat generated by the natural decay of the radioactive isotope plutonium-238 into electricity.
Why is solar technology used in space?
- Batteries have a habit of running low on power,
- An expensive spacecraft turns into space junk
- Few countries on want a spacecraft powered with plutonium-238
Also Governments are not keen on radioactive elements falling to earth!
What is the preferred method?
Solar paired with batteries is the preferred way to power satellites and the space station uses nickel-hydrogen batteries to support its solar panels.
Spirit, another Mars rover, also uses batteries paired with solar.
What kind of solar cells are used in space?
There are two types of solar cells used: silicon cells and multi-junction cells made of gallium arsenide and similar materials.
The International Space station uses:
- Silicon cells covered by thin glass to avoid degradation from radiation
- This makes up the 16 arrays up there
- They are the largest representation of solar in space, occupying enough area to cover most of an American football field
Gallium arsenide, the other solar
- Multi-junction cells made of gallium arsenide and similar materials resist degradation better than silicon
- They are the most efficient cells currently made
- Energy conversion efficiencies up to 34%
- “Junction” refers to the number of light-absorbing layers in the cell
- Common in space today are three-junction cells, but four and six are on the way
Why do solar cells degrade in space?
There are 4 sources of radiations: the Earth's radiation belts (also called Van Allen belts), galactic cosmic rays (GCR), solar wind and solar flares
The Van Allen belts and the solar wind contain mostly protons and electrons, while GCR are in majority very high energy protons, alpha particles and heavier ions
Solar panels will experience efficiency degradation over time as a result of these types of radiation, but the degradation rate will depend strongly on the solar cell technology and on the location of the spacecraft.
How do they combat this degradation?
With borosilicate glass panel coverings, this may be between 5-10% efficiency loss per year
Other glass coverings, such as fused silica and lead glasses, may reduce this efficiency loss to less than 1% per year
The degradation rate is a function of the differential flux spectrum and the total ionizing dose
In other words, radiation!
What are some of the uses of solar in space?
Solar panels on spacecraft supply power for two main uses:
- Power to run the sensors, active heating, cooling and telemetry.
- Power for spacecraft propulsion – electric propulsion, sometimes called solar-electric propulsion
The Vanguard 1 satellite, launched by the US in 1958 largely because of the influence of Dr. Hans Ziegler and the satellite was powered by silicon solar cells with ≈10% conversion efficiency.
Where does Photovoltaic research take place?
- NASA Glenn Research Center in Cleveland
- The campus houses a center for photovoltaic research used in space
- Altogether, more than 100 buildings occupy 350 acres
- Facilities include wind tunnels, drop towers and vacuum chambers
They use their own semiconductor cell-making machine called a metal-organic vapor phase epitaxy reactor
How are the cells tested?
They can attach the cells to one of the high-altitude planes and fly it high above the clouds where the atmosphere is thin.
They measure production and extrapolate for its effectiveness in space and follow-up experiments are then completed using a solar simulator.
The lab’s solar simulators are used to recreate the light seen in space and consist of a dark box attached to a set of powerful light bulbs.
How do the panels orientate towards the sun?
As the spacecraft moves, the solar panels can be pivoted and stay in the direct path of the light rays no matter how the spacecraft is pointed.
A tracking mechanism is often incorporated into the solar arrays to keep the array pointed towards the sun. Sometimes, satellite operators purposefully orient the solar panels to "off point," or out of direct alignment from the Sun.
This happens if the batteries are completely charged and the amount of electricity needed is lower than the amount of electricity made.
Off-pointing is also sometimes used on the International Space Station for orbital drag reduction.
What are some future uses?
China was planning to build the world's first solar power station to be positioned in Earth's orbit and because the sun always shines in space, an orbital solar power station is seen as an inexhaustible source of clean energy.
Above the Earth, there's no day and night cycle and no clouds or weather or anything else that might obstruct the sun's rays, so a constant power source is available.
The photovoltaic array is composed of a lightweight, deployable structure made of many smaller "solar satellites" that could easily connect together in space to form a much larger array and "harvest sunlight."
Equally, this approach also makes assembly, maintenance and repair considerably easier.
This completed array would orbit about 35,000 km above the Earth and "beam" the energy back down to the surface.
The photovoltaic array converts the sunlight into electricity, which in turn is converted into RF electrical power (microwaves).
They are beamed wirelessly to ground-based receivers.
These would take the form of giant wire nets measuring up to 6.5 km across that could be installed across deserts or farmland or even over lakes.
This is such an interesting subject and, effectively, the sky's the limit. In addition to helping power spacecraft the idea of space mounted arrays harvesting a sun producing energy 24/7 is brilliant and I look forward eagerly to reading more in this space.
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