Why use plutonium when sunlight is everywhere? Any proposals to use concentrated solar power instead of Pu238 as a heat source for TGs?

Question

Why use plutonium when sunlight is everywhere? The case for powering thermoelectric generators (TG) with concentrated solar rather than plutonium:

Spacecraft operating in the inner Solar System usually rely on the use of … photovoltaic (PV) solar panels to derive electricity from sunlight. Outside the orbit of Jupiter, solar radiation is too weak to produce sufficient power within current solar technology and spacecraft mass limitations, so radioisotope thermoelectric generators (RTGs) are instead used as a power source.

https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft

Solar flux falls with the square of the distance from the Sun. For space exploration in the inner solar system, solar flux is high enough for solar PV.

For more distant voyages (i.e.: beyond Jupiter), RTGs take over. For instance, LUCY has solar arrays for working at Jupiter distance, while Cassini used RTGs in its Saturn voyage.

An alternative to RGTs is to use solar concentrators paired with solar Thermoelectric Generators (TG)

TGs are solid state generators which convert heat directly to electricity with no moving parts.

https://en.wikipedia.org/wiki/Thermoelectric_generator

The "R" in RTGs is usually plutonium as a heat source. TGs will also work with sources of heat other than radioactive decay, such as solar. https://www.nrel.gov/csp/facility-hfsf.html#:~:text=The%20solar%20furnace%20can%20quickly,up%20to%203%2C000%C2%B0 .

Most current RTGs operate at over 1000C. Solar TGs would need a solar concentrator to produce temperatures in this range.

High performance TGs can attain efficiencies of over 30%, which is similar to the photovoltaic (PV) panels on the ISS. However, this is an apples-and-oranges comparison since TGs can use a larger portion of the solar spectrum than PV. And PV performance is actually reduced by the heat produced from the thermal part of the solar spectrum.

https://www.researchgate.net/publication/283471128_Spectrum_splitting_for_eicient_utilization_of_solar_radiation_a_novel_photovoltaic-thermoelectric_power_generation_system

PV cells use only about half of the light spectrum provided by the sun. The infrared part is not utilized to produce electricity. Instead, the infrared light heats up the PV cells and thereby decreases the efficiency of the cell.

A solar concentrator can be created with single layer of aluminized Mylar. Using an inflated structure, the mass per area of the reflector could be very low. Below is a picture of the 30m Echo satellite. The launch mass of the satellite was 180Kg, or 250 grams per square meter of cross sectional area. https://en.wikipedia.org/wiki/Project_Echo

As an example, the 1.1m long RTG generator used in the Galileo spacecraft (drawing below) had a TG with a surface area of about 0.16m^2. If the light from a 6m solar collector at Saturn’s orbital distance is focused on a 0.16m^2 TG target, the solar flux would be the same as planet Mercury where daytime temperatures are 800F (700K).

This implies that a solar TG with an 8m reflector at Saturn orbital distance would out-generate the RTG used in Galileo.

Plutonium costs about $9,600,000/Kg to produce. That is 75,000,000 clams per RTG. Cassini had 3 of them. https://www.forbes.com/sites/williampentland/2015/11/08/peak-plutonium-238-u-s-starts-making-nuclear-fuel-for-deep-space-missions/?sh=3be1744b53b4

Since RTGs have limitations due to longevity, cost and safety, have there been any proposals to use concentrated solar power as a heat source for TGs in exploration of the outer solar system?

More to read: What is the status of concentrated solar energy (CSE) in space exploration?

Answer 1

Smaller, More Robust, Proven

OP's question basically includes all the information needed to show that RTGs outperform solar concentration.

The 8m concentrator OP proposes would weigh ~12 kg, per the numbers given, which is already heavier than the ~8 kg plutonium core. Whatever is used to deploy the concentrator - the pressurized gas canister, the foam to fix it in place, etc. only adds more weight.

The RTG has basically no failure modes or moving parts - plutonium produces decay heat, and the TG turns that into electric power.

In contrast, a solar concentrator would need to be deployed, which has some risk of failure. Exposure to the space environment will likely degrade the reflective properties of the concentrator, and building a larger or more robust concentrator to account for that would add to the weight.

Any maneuvers that point the concentrator away from the Sun would cause a loss of power, so presumably you'd want to add an active pointing mechanism to keep the concentrator working at all times, which would add weight and additional failure modes.

Finally, RTGs are proven. There's no new development cost, the risks are well understood, and they just work. Even if the material cost of the concentrator was zero, how many missions would it take to pay off the development cost? How does that math change if even one mission fails completely when an unexpected failure mode kills all electrical power onboard?

Still Not Enough Power

OP states an 6m concentrator can produce a solar flux on the 0.16 square meter TG as similar to the flux as on Mercury. This NASA fact sheet gives Mercury a solar irradiance of ~9,000 W/m². That gives 1,440 watts of thermal energy on the TG, which is about a third of the power that the RTG in question. So since Cassini had three RTGs, the 6m concentrator might supply about 11% of the power required by Cassini.

So you'd need a rather large concentrator to make this work.