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What power sources are viable on a 'long term' Venus lander?

An RTG relies on a temperature differential, and is not likely to be doable with a 457C 'cold side'.

Based on this answer there is not enough light for a solar panel to work.

As far as I know this leaves batteries, but NiMH will fail around 65C.

And hydrogen fuel cells come in many varieties, but at least one seems to be able to operate in the 500-1000C range.

Given that work is progressing on high temperature chips, a high temperature rover seems at least possible.

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  • $\begingroup$ If the Venus lander would not survive the temperature and pressure at the surface very long, there is no need for a power source suitable for longer operation. $\endgroup$ – Uwe Mar 22 '17 at 8:55
  • $\begingroup$ I have clarified that I was interested in the high temperature, long term options. Apologies for not including that to begin with. $\endgroup$ – OrangePeel52 Mar 22 '17 at 11:01
  • $\begingroup$ @Uwe I assume the point is that you'd have a lander that operates at ambient temperature. This would require some specialized technology, including a specialized power source, but there's no reason to consider it impossible - it would just likely be a huge investment, since most technology we build is designed to work around room temperatures and close to standard atmospheric pressure. E.g. you can't use lead solder, you probably want to avoid cavities disconnected from ambient pressure, and many other constraints. The conditions are harsh, but not that harsh. $\endgroup$ – Luaan Mar 22 '17 at 15:09
  • $\begingroup$ I wonder if it wouldn't be possible to exploit the in-situ chemical environment. Sulfiric acid could certainly make for a great electrolyte. $\endgroup$ – leftaroundabout Mar 22 '17 at 16:04
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    $\begingroup$ @PavelPetrman I've made the acronym a link in the question. $\endgroup$ – OrangePeel52 Mar 23 '17 at 11:26
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Thanks to @MarkAddler for his search suggestions

I've tried to balance length against completeness, and both lost here. However I have included enough material to try to be convincing that RTGs, singe-use storage batteries, and rechargable batteries for higher power events have all been investigated and solutions exist to provide at least the electrical power for a long-term lander or rover on Venus.

The fear about batteries comes from the idea of putting Consumer batteries on a Venus lander at 460C. Of course that's the wrong kind of battery to think about, and there are several kinds already demonstrated to work at these temperatures, some even in use in low volume on Earth.

RTGs work fine on Venus. In fact, the high density of the atmosphere ~67 kg/m^3, or 6.7% of liquid water; will cool the sink end of the RTG much much more effectively than the RTGs in space or on the Moon or Mars. With source (hot) end at 1200C, thermodynamic efficiency would be better than half of an RTG in space.


PAPER I. RTG ON VENUS doi:10.1016/j.actaastro.2006.12.031 paywalled

enter image description here

For the analysis case, we assumed thermoelectric converters similar to those used on Cassini [ref]. While the high temperature of waste-heat rejection to the Venus atmosphere reduces the theoretical Carnot efficiency of any thermal converter, the density of the atmosphere means that heat transfer is very efficient, and hence the required area of the convective radiators is small.

The assumed hot-side temperature (Th) is 1350 K, and the cold-side temperature (Tc) ejected to the radiator is 870 K. The calculated net thermal to electrical efficiency was 0.05 (5%). A GPHS heat input Qh of 594W was required to produce 30W of output electrical power. The total heat rejected is 564W (Table 2).

Three such units are required for 100W of electrical power...

Table 1: Power system trade-offs

Radioisotope power source

  • Demonstrated in space
  • Dynamic [14] or thermoelectric [refs] conversion approaches are possible
  • 460◦C is a higher heat rejection temperature than conventional dynamic conversion approaches Radioisotope was chosen as the baseline technology for the Venus rover

Microwave beamed power

  • Station in atmosphere produces solar power; power is transmitted to surface by microwaves [ref] Not demonstrated in Venus environment
  • Many technical questions need to be answered
  • Chosen as a backup approach—not analyzed in detail

Solar power

  • Solar power is difficult due to low light levels at surface [ref]
  • High temperature at surface makes photovoltaic conversion inefficient
  • Approach would require new technologies to be developed [ref]

Chemical (battery or fuel cell) storage

  • Requires high-temperature technology
  • Practical approach for short missions or low powers

enter image description here


PAPER II. BATTERIES ON VENUS DOI: 10.2514/1.41886 paywalled.

enter image description here


Thermal Batteries (single use):

Based on low temperature molten salt eutectics, these are use-once batteries that do not become active (or self-discharge) until they reach operating temperature and the electrolyte melts.

enter image description here


Sodium Sulphur Battery (rechargeable):

An alternate high-temperature battery technology is the sodium– sulfur battery [refs]. Sodium–sulfur batteries were initially developed as a high-specific-energy rechargeable battery system with a low self-discharge rate for electric vehicles. They are currently being demonstrated in electric utility applications [ref] to serve as an energy storage system to store energy for use during peak demand periods. Such batteries have demonstrated hundreds of charge/ discharge cycles with a low decrease in capacity.

[...]

Current sodium–sulfur batteries use a beta-alumina solid oxide electrolyte. This electrolyte also serves as a separator between the liquid sodium anode and the liquid sulfur cathode. At operating temperatures above about 300C, sodium ions are mobile in the solid-electrolyte material. Because of the relative impermeability of the solid electrolyte, the self-discharge rate of the NaS battery is extremely low. A schematic of the battery is shown in Fig. 2.

Because of the high specific energy, there has been some interest in the use of sodium sulfur batteries for space operation [refs], despite the high temperatures. A potential difficulty of the NaS battery technology is the fragility of the beta-alumina electrolyte. A demonstration test of the sodium–sulfur battery in space was done on space shuttle flight STS-87 in November 1997. This is shown in Fig. 3. The experiment lasted 10 days and showed that the NaS battery could be successfully qualified for space operation and operate in space conditions. (emphasis added)

enter image description here


Sodium Metal Chloride Battery:

Both the Na=NiCl2 and Na=FeCl2 chemistries have been demonstrated, although the NiCl2 chemistry is preferred because of the wider temperature range of operation, 200–400C [ref]. The schematic of the Na=NiCl2 battery is virtually identical to that of the NaS battery shown in Fig. 3, except that the Na anode is typically on the outside, whereas the NiCl2 cathode is in the center. The cell voltage Vo is slightly higher than that of the NaS battery. Specific power has been demonstrated up to 143 W h=kg.

The technology development status of this battery for terrestrial applications is high, with an experience base of thousands of batteries built and many demonstration projects with over 15 years of experience. However, terrestrial batteries typically operate at the lower end of the temperature range, typically 270C, although Pistoia [ref] reported that 450C operation of the Na=NiCl2 battery is possible. Recent work by the U.S. Department of Energy [ref] has tested single-cell Na–FeCl2 batteries at temperatures of 500 and 600C, with no failures after 7 h of operation at 500C, and Na–ZnCl2 for up to 50 h of operation at 425C. However, comparatively little development effort has been done on the higher temperature range of operation, because high-temperature operation is not of interest for terrestrial applications.


New Battery Technology: Lithium/Lithium Carbonate Battery

For the operation at Venus ambient temperature, a molten salt is used for an electrolyte. The obvious choices are either halide salts or carbonate salts.

The optimum electrolyte would be a molten carbonate. This makes the battery structure very much an analog to the molten-carbonate fuel cell, and much of the technology development for molten carbonate fuel cells will be directly applicable. This is a device structure that has undergone considerable technology development for terrestrial applications.

Although pure lithium carbonate, with a melting point of 723C, is not liquid at Venus temperature, a eutectic mixed carbonate is. The ternary eutectic Li0:44Na0:30K0:262CO3 has a melting point of 393C [refs], low enough to be liquid at Venus surface temperatures at all locations across the planet.

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  • $\begingroup$ The high temperature battery data looks good, but all other parts of the lander should be able to operate at 450 or 480 °C too. The electronics should work in a very wide temperature range from below zero up to 450 °C. But batteries working well at 450 °C are inactive at low temperatures. A combination of different battery technologies would be necessary to cover such a wide temperature range. The cells suitable for low temperatures would die and explode when temperature inside the lander is rising. But the lander needs continous electrical power during decent, landing and surface operation. $\endgroup$ – Uwe Mar 22 '17 at 14:46
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    $\begingroup$ @Uwe (aka Dr. No) your (n+1)th "it won't work" comment belongs on the question. This answer addresses only "Potential high temperature power sources for a Venus lander", i.e. the question asked by the OP. Nobody here said there were no other engineering considerations - your comment is superfluous. If you keep this up stack exchange is going to run out of ASCII! $\endgroup$ – uhoh Mar 22 '17 at 14:54
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    $\begingroup$ @Uwe OK let's try this - when I get home I'll ask a "what are the most challenging issues for an extended lifetime Venus lander?" type of question. Let's see if we can figure out what are the most difficult problems to solve! P.S. I was just PM'ed, SE won't run out of ASCII, they'll just get more from stackoverflow, those guys have infinite loops so they never run out. $\endgroup$ – uhoh Mar 22 '17 at 15:00
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    $\begingroup$ @Uwe: It would seem reasonable to have an entry/descent stage that would function at lower temperatures, dropping a lander that doesn't start to function until it warms up. (Which makes me wonder: given the density of Venus' atmosphere, would you even need a parachute?) $\endgroup$ – jamesqf Mar 22 '17 at 18:57
  • $\begingroup$ If the RTG has small radiators because of the thick atmosphere on Venus, how is it supposed to survive the trip to Venus? $\endgroup$ – Schlusstein Mar 22 '17 at 19:25
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An RTG certainly can and would work on Venus, since the hot side is about 1200 C. It just wouldn't be as efficient as it would be with a colder cold side. Search for papers by Geoff Landis on this.

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  • $\begingroup$ Thanks for the suggestion - let me know if you see any glaring errors or omissions. $\endgroup$ – uhoh Mar 22 '17 at 14:33
  • $\begingroup$ Why is the hot side restricted to 1200 C? Is it not practical to contain the Pu at higher temps? $\endgroup$ – Martin James Mar 23 '17 at 9:44
  • $\begingroup$ I suppose you could reduce radiators and increase insulation to increase the temperature, but at some point you have to worry about materials. $\endgroup$ – Mark Adler Mar 23 '17 at 15:22
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For most practical probes, batteries are the only real option. RTGs are for missions that last years, and Venus is an environment where "long duration" translates to "three hours".

For that sort of timeframe you don't need recharging, and keeping the inside of the probe cool can be as simple as insulation and maybe a small supply of ice.

If you do manage to engineer all the other components to survive long term, a wind turbine would be a good choice. Venus surface conditions are no more extreme than the inside of a common jet engine, so the engineering involved is fairly well known.

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    $\begingroup$ What sort of battery might be used? $\endgroup$ – Schlusstein Mar 21 '17 at 23:42
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    $\begingroup$ Yeah but the system can work longer with a battery that operates at higher temperatures. $\endgroup$ – Schlusstein Mar 22 '17 at 0:21
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    $\begingroup$ @uhoh There's a lot of work to be done before a hot lander is more than theoretically possible, but the batteries aren't for that scenario - given currently available technology and sufficient insulation they'll be empty before they melt along with the rest of the electronics. $\endgroup$ – Quentin Clarkson Mar 22 '17 at 4:34
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    $\begingroup$ @QuentinClarkson for which missions not yet budgeted/scheduled is there not a lot of work to be done? In planetary exploration, everything is hard. Do you mean there are problems with no obvious paths to solution? High bandgap semiconductors are a path for electronics - though I don't know if there are issues of unintended dopant mobility, probably that will be OK. Large amounts of data storage may be tougher, but that's addresable with a companion orbiter. $\endgroup$ – uhoh Mar 22 '17 at 11:50
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    $\begingroup$ "Venus surface conditions are no more extreme than the inside of a common jet engine" -- this is my favorite quote from the SE network this week. $\endgroup$ – Wayne Conrad Mar 23 '17 at 1:12

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