As hard as I try, I seem to keep getting all the "do gooders" or perhaps "damage control freaks", so intent upon replacing and/or substituting the required environment that's already seriously hot, with one where it's much cooler (Earth like) as well as where it's relatively easy as to invoke a thermal differential. Well, it's certainly possible to achieve a thermal differential on Venus, as long as that's starting in at 650K, as otherwise, to press that open environment downward is going to take a rather serious amount of energy, which by the way does happen to exist as in CO2 pressure and temperature differentials/km, however, implementing that sort of application essentially circumvents the fundamental need as for applying anything thermopile related.
The idea and/or pretext of applying a substantial thermopile is obviously that of creating reliable electrical energy where the alternatives are either unavailable or simply not sufficiently portable. Thus I've stipulated upon utilizing the nighttime (elevated) environment as currently being 650K (maximum 700K), as for energizing something one would haul about as external (back pack) to a thermal insulated jump suit, like of the R-100 capability which could certainly utilize a little surplus electrical energy, that which in turn could power some form of CO2 compressor, thus offering an effective solution for some serious thermal heat exchanging. I do not have the exact figures, although that occupied R-100 jump suit might demand 1 kw in order to dump or extract the extra 200 watts being generated by it's occupant as well as for rejecting the 1% influx of the toasty ambient environment, perhaps even as for operating a small electro-processor of CO2-->CO/O2.
Obviously, for those sufficiently evolved (acclimated), the amount of thermal rejection might be fairly minimal if their exoskeletal already incorporated a sufficient number of N2 or H2 cell layers, like less than 10% of what it's going to take to cool off our buns. So, as for accommodating that extra energy allotment, I thought having a number of Alumel/Chromel (alternate: Chromel-Constantan) cold junction stacks, those of sufficient cross sectional area might actually do this trick quite nicely, since the thermal energy required as to powering any such "warm fusion" devices is going to be absolutely free, very clean and pretty damn reliable.
However, as within the intended ambient application there's little or no significant natural differential aspects (no shade, no bright/dark sides, nearly all conduction thermal issues), just of the continuous heat pumping fusion offered by relatively pure CO2 (in other words; damn little if any O2). This is where I keep thinking of this cold junction principal as "warm fusion", where alloys such as Alumel/Chromel offer a reliable flow of electrons at a relatively low impedance, thus a 15 mV junction when thermally maintained at 650K can sustain a relatively substantial load per cross sectional area.
Actually, without adding a combustion phase, the thermal range I'm looking at is not as such initially accommodating any conventional differential, just the toasty 650K to 700K as being ambient and relatively consistent. The added considerations (where thermal differentials could coexist) are from what a little CO/O2 can generate and/or from what most any good old xenon ignition byproduct of electro-mechanically compressing and/or electrically exciting that CO2 in xenon illumination has to offer, as in either case we'll be exceeding 1000K in no time flat and, that's capable of a good 350K differential (1450K pushes the potential differential to 800K), at which point if there were to be thermal differential equivalents of Bismuth Telluride (Bi2Te3) that will survive those higher thermal conditions, then obviously that certainly is worth considering as for whatever heat transferring principals, even though compressed CO2 seems to offer an overwhelming amount of thermal transfer coefficient/watt/kg, perhaps many times that of anything solid state and, obviously not being thermally critical nor the least bit complex to implement. A mere 2:1 CO2 compressor is serious thermal energy.
Here's one typical CO2 energy extraction cycle: 75 bar @650K, compressed 2:1 for thermal heat exchanging as well as for obtaining a thermal differential of perhaps 300K, then further utilizing that heat exchanging rejection exhaust of 950K to effectively pump additional electrons from a number of thermopiles (1800 junction type K at 27 mv/junction or 1075 type E at 46 mv/junction), thus creating the necessary electrical energy that's somewhat proportional to the demand, as going into powering up that R-100 insulated jump suit. The obvious reason this is nearly perpetual energy is because of all those cold alloy junctions that have recently become toasty warm junctions (freely heated from the ambient Venus environment) pumping out electrons long before that CO2 compressor comes on line, then obviously hotter yet and thus more electrons after the CO2 compressor is affecting the required heat transfer.
Unlike the greater voltage generating potentials of Bismuth Telluride (Bi2Te3) = 1.2+ volts or the Antimony Telluride (Sb2Te3) = 250 mv, where each have been typically evaluated at the thermal differential of 30ºC, however neither are suited to the environment of 650+K, so they're obviously not solutions but utter disasters, where as the Alumel/Chromel (K) is seriously robust as well as right at home, all the way to 1450K and, type E (incorporating Constantan and providing 65% additional voltage/junction) as a thermopile is good for 1150K. Either K or E formats have become my idea of "warm fusion" energy generators (as no apparent cold side or differential is involved) and, on Venus that's going to be more likely defined as being a "hot fusion" thermopile power generation, that which is being sustained by the absolutely free ambient environment (something obviously far better then nuclear, offering loads of BTUs at essentially no radiation and no stinking half life considerations, as there's no apparent end in sight).
It seems like a true (open minded) scientist could and should have realized upon this as an opportunity to explore the "positive" aspects of what all that nifty heat has to offer, instead they nearly all seem to devote their talented energies as to defeating whatever and whenever they can, obviously establishing and maintaining good employment candidates as for NASA/NSA/DoD agendas (hidden or otherwise).
As a reminder, at 1000K; Type K is pushing 29 mv/junction and the type E is pumping out 49 mv/junction and unlike Bismuth Telluride, in either case the impedance is damn low and the environment is well within spec. I believe type K offers somewhat lower impedance (@12.7 mm diameter; 3.8^-6 ohm/mm), thereby relatively good electron delivery potential per kg. Although, as for fewer junctions, type E formula is certainly the better candidate.
Obviously you can't keep pulling electrons out of alloys without something degrading over time. The greater cross sectional junction area (for example 12.7 mm diameter) along with the near void of any O2 should however greatly improve upon alloy purity as well as retarding aging affects of oxidisation. Consumption of Alumel/Chromel is perhaps known, although it may not become a significant issue, unless you start running out of those alloys (seems unlikely that a truly hot planet would ever run themselves out of thermally tolerant alloys, as that's nearly as stupid as Earth blatantly using up all the oil and gas reserves without a backup plan, which by the way, the rich nations have such a plan [it's called plutonium, of whatever for power generation and the remainder as for vaporising any potential intrusion into their energy independent nation] and, those poor nations simply don't have a freaking chance and, as such will soon have to wither and die because of that fact).