Linwood wrote: ↑
Mon Nov 05, 2018 10:16 am
What about a technology that converts heat into electricity directly?
As already noted, that defies thermodynamics. What can
be done is to convert a heat difference
into useful energy. A fusion reaction, in the absence of technological magic, takes place slowly at solar core temperature (circa 15 million K*), generating about the same amount of energy per unit volume as a compost heap. In a thermonuclear reaction (fusion bomb, inertial laser fusion, nova** star), the reaction takes place at a much higher temperature (over 100 million K) and much more rapid pace.
Given that the Traveller technology tree has gravitics and fusion arriving together, I'm going to assume that a Traveller fusion reactor works kind of like inertial laser fusion, except with a pulse of gravitic energy collapsing the hydrogen pellet to thermonuclear temperature. It spends a lot of energy to generate the gravitic pulse, collapsing the pellet to microscopic scale and heating it to a few hundred million K, where fusion takes place, releasing an excess of heat. The fusion products (including un-fused hydrogen) expand in the reaction vessel, cooling tothe point that the combination of magnetic (and possibly gravitic) containment keep the liner from eroding, probably around 6000 K. That heat powers some sort of generator, possibly a steam turbine, maybe a magnetohydrodynamic generator, maybe a peltier generator, etc.
But to convert the heat to another form of energy, such as electricity, there has to be a differential between that 6000 K and the heat sink. The blackbody temperature of space is under 3 K, so there's plenty of temperature difference. But unlike a terrestrial heat engine (such as a similar fusion reactor in a nautical ship), the only way to exploit the 6000 K difference is radiators. It's a lot easier to exploit the slightly narrower difference between that 6000 K reaction vessel and ~300 K seawater, through a heat exchanger.
So in space, we're back to radiators -- which, by the way, work adequately for the peltier-based radioisotope thermal generators used in real-world space probes sent to the outer solar system, where solar panels generate too little power.
One possible substitute to radiators might be dumping energy into cryogenic hydrogen and dumping it into space at middle temperature. But that may consume too much hydrogen. (Then again, maybe that's why a jump drive requires so much liquid hydrogen.) The fusion energy content of hydrogen is so large that a GURPS Traveller
power plant, which is refueled only during annual maintenance, is viable. But if heat is dumped to space in heated hydrogen, maybe the Mongoose and classic Traveller
fuel usage rates mean that the "fuel" is really mostly heat-dump coolant.
If we assume that waste heat is dumped in vented hydrogen, how much would be needed?
This means that there could be several types of fusion power plants:
- Planetary fusion plants, cooled by sea water, river water, or giant air-cooling towers.
- Space radiator fusion plants, which dump heat through giant space radiators, usable on interplanetary ships where solar panels are inadequate.
- Coolant-carrying fusion plants, which dump waste heat into coolant they carry aboard, and vent into space, for ship that don't have space for radiators.
Does anyone know how much coolant would be necessary to dump heat into space by venting coolant? Or, for that matter, how large radiators would need to be?
* Degrees Kelvin don't use the ° symbol, unlike °C and °F.
** A nova star is a thermonuclear explosion that affects the outer layers of the star; a supernova goes all the way to the core.