NASA's Juno spacecraft is a study in low power equipment improved solar-cell efficiency and array-structure design.
The Juno spacecraft, launched from Earth in 2011, has an amazing solar array that has powered the dauntless spacecraft on its five-year journey into space and even now as it settles into an 18-month orbit of our solar system’s largest Planet, Jupiter. A photovoltaic solution was chosen by NASA due to a shortage of plutonium coupled with improved solar cell efficiency and array structure design.
The almost 19,000 cells on Juno are outfitted on three arrays the length of a big rig tractor trailer. In Earth orbit, the solar panels would generate 14kW of electricity, but when Juno settles into its orbit around Jupiter they will only output a weak 400W. It is fortunate that the scientific instrumentation and on-board computer are very energy-efficient.
The Sun’s rays reach distant Jupiter
Being this far from the Sun only allows a meagre solar intensity of only 3.4 to 4.1% of what the Earth receives. To make things more difficult in powering Juno, the solar arrays operate at extreme low temperatures.
Let’s do a bit of analysis using the Stefan-Boltzmann Law to analyse the conversion of solar radiation to heat. We can calculate the flat-plate equilibrium temperature (Teq) using the Stefan-Boltzmann equation:
where α is the solar absorptivity, I the solar intensity, ε the front and back thermal emissivity (assuming two-sided emission), and σ the Stefan-Boltzmann constant, 5.67 x 10-8W/m2K4.
Solar intensity drops off as the square of the distance from the Sun. If we look at Table 1, we can see the solar intensity at Jupiter, along with the calculated equilibrium temperature, for an assumed absorptivity and emissivity that is typical of current generation high-efficiency solar cells.
Table 1: The solar intensity at Jupiter, and the equilibrium flat-plate temperature of a solar array.
The low solar intensity, which reduces the array’s voltage output produced, gives way to poor conversion efficiency and means that Juno’s solar array needs to have thirty times the area of an array in Earth orbit to produce the same power. Now, along with low intensity add low temperature to get the effect known as LILT (Low Intensity Low Temperature) which greatly compounds the problem of powering the spacecraft. Screening solar cells for Juno, that show the least degradation in performance due to LILT, helps in a small way to optimise solar conversion.
Back in 2008, production State-of-the-Art Triple Junction solar cells were measured under LILT conditions as part of a study to investigate the feasibility of PV for missions to the outer planets. The testing indicated that these cells could be used for missions out to Uranus. In 2008, triple junction solar cells operated with close to 28% efficiency with new cells nearing 30%. Work was underway to improve this efficiency to 30% and also thin the cells to reduce weight. Experimental cells with Inverted Metamorphic (IMM) technology had exceeded 32% efficiency and were lighter still.
Powering Juno’s on-board instruments
Juno’s mission power needs are relatively modest, its science instruments require full power for only about six hours out of each 11-day orbit (during the period near closest approach to the planet). The mission design voids any eclipses by Jupiter, minimises damaging radiation exposure and allows all science measurements to be taken with the solar panels facing the sun, making solar power a perfect fit for Juno.
Figure 1: In this image we see the vast array of instrumentation that needs to be powered on Juno. (Image courtesy of NASA/JPL-Caltech)
As we venture beyond Jupiter and even beyond our Solar System, there will be a point where solar power may no longer be an option. We want to minimise Radioisotope Thermoelectric Generator (RTG) power in order to not contaminate the purity of space and other planets and heavenly bodies. Solar energy will surely advance in the future, but will it be enough to propel us further into the unknown as explorers seeking what is out there? At a minimum, Solar Energy improvements propelled by NASA research for space will certainly benefit Earth.