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As an example, the Ingenuity helicopter on Mars charges its batteries each sunny day (which is every day except when it isn't) and discharges them substantially each cold night (which is all of them) to keep themselves from freezing damage and to keep other critical goodies from getting too cold. Solar powered Mars rovers and landers on Mars do the same, which is all of them except for MMRTG-equipped Curiosity and Perseverance that use heated fluid circulation.

A thermal battery is:

...in general something that can store thermal energy and while an electrical battery can serve this purpose, a pot of water, wax, or any other material that undergoes some kind phase change within a useful temperature range with a sufficient enthalpy of change can potentially be used as a thermal battery.

From Wikipedia's Thermal battery:

A thermal energy battery is a physical structure used for the purpose of storing and releasing thermal energy—see also thermal energy storage. Such a thermal battery (a.k.a. TBat) allows energy available at one time to be temporarily stored and then released at another time. The basic principles involved in a thermal battery occur at the atomic level of matter, with energy being added to or taken from either a solid mass or a liquid volume which causes the substance's temperature to change. Some thermal batteries also involve causing a substance to transition thermally through a phase transition which causes even more energy to be stored and released due to the delta enthalpy of fusion or delta enthalpy of vaporization.

But in that case it was called a "thermal capacitor" rather than "thermal battery" for some reason.

The nice things about a thermal battery include

  • You "charge it" with heat and there's sort-of five times as much heat in sunlight as there is electricity to be had from it; solar photovoltaics don't get much better than 20% when all real-world effects are taken into consideration (optical transmission & dust, de-rating for non-optimal temperature, power conversion and charging efficiency of the battery, etc.)
  • Phase changes like melting/freezing don't "wear out" a simple bulk material the way repeated charge/discharge cycles wear out an energy density optimized electrical battery will. You're not going to get "tired water".

Question: What would a solar thermal battery system look like for an aircraft on Mars? Roughly how much mass would it add? How much would it replace?

Remember that Ingenuity's battery's mass and energy capacity are driven by the need to both have a usefully long and satisfying flight and still have enough energy to stay warm overnight along with a good margin for safety.

Adding a thermal battery may substantially reduce the required energy capacity and therefore mass of the electrical battery, so adding the thermal battery is not all "doom and gloom."

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    $\begingroup$ Related SBIR by Masten Space Systems for their MOWS project: sbir.gov/node/1670587 . $\endgroup$
    – ChrisR
    Nov 23 '21 at 4:48
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    $\begingroup$ "Thermal battery" has multiple meanings (sadly), with the one I'm accustomed to being completely different from how you use it. See, for example, pubs.acs.org/doi/full/10.1021/acs.chemmater.1c01864 for a different usage. $\endgroup$
    – Jon Custer
    Nov 23 '21 at 16:55
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    $\begingroup$ I wonder whether it's just about mass and space. You still need electricity to fly the aircraft, so the PV panel stays. You need another solar panel to capture the heat. Then you will need a circulating special fluid, which means pipes. And those pipes must withstand the negative temperatures so that the frozen fluid won't burst them. Related: space.stackexchange.com/questions/51257/… $\endgroup$
    – Ng Ph
    Nov 23 '21 at 18:28
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    $\begingroup$ I said "go under" figuratively. What would be much simpler, much more efficient, much safer is: Ingenuity lands 100m from Perseverance; Perseverance approaches Ingenuity; with one of its arm equipped with a male plug, Perseverance connect Ingenuity to its electrical circuit. I understand this scheme won't make sense if Ingenuity is only a nice-to-have wrt Perseverance main mission (which seems to be the case, which also nullifies the "go under" approach). $\endgroup$
    – Ng Ph
    Nov 23 '21 at 22:37
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    $\begingroup$ @NgPh youtube.com/watch?v=uMM0lRfX6YI $\endgroup$
    – uhoh
    Nov 23 '21 at 22:39
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Question: What would a solar thermal battery system look like ?

Answer: It would look the same as Ingenuity !

enter image description here

Well, it might look a bit different since the PV panels could be smaller. Less power for night time heating would be needed. The battery capacity would be lower, but the reduced battery volume would be (at least partially) replaced by thermal battery volume.

If the four landing legs were replaced with heat pipes, and the central core of the battery with a phase-transition thermal battery, there would be very little change in the look of the helicopter.

Heat pipes transfer heat by using absorbed heat at one end for phase change of the working fluid from liquid to gas. Gas diffuses along the tube until it condenses at the other end, releasing the stored latent heat. The condensed working fluid returns by gravity. Heat pipes have no moving parts and work forever, or until they leak, whichever comes first. They operate over a large temperature range (which must be above the triple point of the working fluid). For instance, ammonia-filled heat pipes function well from 200K-350K https://llis.nasa.gov/lesson/698

Heat pipes are common on spacecraft (usually using ammonia as the working fluid). In our application, design is simplified because the spacecraft is operating in a gravity environment. Gravity makes it easy for heat pipes to act as a heat diode: heat is conducted “up”, but not “down” the pipe, preventing stored heat from being wasted back down to the feet.

The working fluid would be chosen to optimize for the temperature difference between daytime “foot” temperature and maximum tolerated battery temperature. Viking measured regolith temperatures from -107C to -17.2C. The triple point temperature of the working fluid must be below heat source temperature while heat is being transferred. There is no problem if the fluid freezes in the feet at night. Mars atmospheric temperatures may get up to 20*C in summer, but heat transfer from the tenuous atmosphere would be minimal compared to regolith.

Material choice for the heat pipe tubes would need some thought. Mechanical requirements for landing stress would need to be considered. Heat transfer at both ends of the heat pipe would be maximized by aluminum extrusions with internal fins, as is used in spacecraft heat pipes. The mid-section could be made of composite to minimize weight and thermal conductance.

There are many materials available for thermal battery latent heat storage. In a pinch, linseed oil (MP -24*C) might work but there is likely something more suitable commercially available https://www.dynalene.com/heat-transfer-fluids/?gclid=EAIaIQobChMIyO61la_n9AIVQ0FyCh3o7AO9EAEYASAAEgLj7vD_BwE

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