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.
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
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.
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)
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.
