transmitting power via space tether

[MrUniverse]

Nasa funds the power beaming competitions to power the climbers up the future space tether, right. Why can't they just transmit the power through the cable itself? If need be they could send it positive one way and have a parrallel cable being negative, with an insulator in between, right? remember carbon nano tubes are conductive right? It seems like the obsticles to such a system (the van Allen Belts) could be easily overcome, right?

 

[neuvik]

I'd assume they don't want to have transmission lines in the tether because it would act as one giant resistor if brought to scale.

 

[aphh]

On nano scale we can only expect nano Amper currents and nano Watts. Not enough to power electrical appliances. Having said that, a conductor or a wire placed and travelling in the magnetic field of Earth should induce and generate significant currents.

 

[MeteorWayne]

Huh? Amper (sic) currents resulting in nanowatts? Do you know what the voltage would have to be for that statement to make any sense?

 

[kg]

Huh? Amper (sic) currents resulting in nanowatts? Do you know what the voltage would have to be for that statement to make any sense?

Nanosense?

 

[aphh]

Huh? Amper (sic) currents resulting in nanowatts? Do you know what the voltage would have to be for that statement to make any sense?

It clearly says in my text 'nano Amper' and 'nano Watts', meaning that both units correlate also in nano scale. Nothing fundamentally wrong there, except Ampere is actually spelled Ampere. Not Amper.

Having said that, measuring nanoamperes or nanowatts might turn out to be rather difficult. 1 nanoampere would be 1 / 1 000 000 000 amperes. Without truly high quality equipment any leak current in the circuit would dwarf your nanoampere current.

Article about very recent advances in nanotubes and thermoelectric currents: http://www.gizmag.com/carbon-nanotubes- ... ity/14442/

 

[neilsox]

I'm unsure why we prefer generalities to numbers. CNT likely conducts electricity no better than copper. Worse, we presently make most CNT in lengths of about one millimeter so a million splices are needed for each strand with a diameter of about 900 nanometers and a length of one kilometer. That is trillions of splices for a practical tether = unreliable as a conductor of electricity. Obviously, even worse for sending electricity 36,000 kilometers to GEO stationary orbit. Neil

 

[MrUniverse]

Thanks for all of your replies, guys. Sorry I haven't responded, but I've been a way from the computer a few days.
I don't know much about electricity. However I can deduce that nano-amperes and nano-watts are very tiny. Keep in mind that the supposed tether would be much thicker than a single nanotube, though.

Where did you hear that nanotube conductivity would be similar to that of copper? From Wikipedia:

In theory, metallic nanotubes can carry an electrical current density of 4 × 109 A/cm2 which is more than 1,000 times greater than metals such as copper.

Here's a thought exercise.
Assume:
a cable diameter of .5cm (half a centimeter) at the bottom (Earth end)
a cable able to carry an electrical current density 1,000 times that of copper
Also let's not worry about them being all interconnected and losing some power that way. Also forget about the Earth's magnetic field and the effects of the van Allen belts.
How much power would you have to start with to get a megawatt 20,000 kilometers?
I'm asking about 20,000 km instead of 36,000 because less and less power should be needed as the climber ascends.
Thanks for pondering these things with me.

 

[SteveCNC]

Correct me if I'm wrong and maybe this is old science (I have a lot of that floating around my noggen) but if a satellite were to hang a cable that hung far enough to touch the upper atmosphere it would create a static charge along the cable . I would think that would be a problem to using a conductive cable .

 

[neilsox]

Hi Steve: The promoters of the space elevator think this static charge will be a minor problem, but possibly they are over optimistic. They are also thinking the 90,000 kilometers of ribbon will be 4 million ohms. If so 4 million volts is needed to produce one amp of current flow.That is 4 megawatts, but 4 million volts is likely very impractical. Strength of the ribbon will likely be marginal, so we won't want to optimize for lowest resistance. Wikipedia did say theoretical, which could mean at one degree k = -272 degrees c. Generalities abound. Numbers are rarely given, and may be wrong. Neil

 

[SteveCNC]

too bad we can't somehow convert the static charge to usable electric power , then you would just have to use a rocket burst every so often to overcome the drag the cable would cause .

 

[neilsox]

We likely can convert the static electricity to power the climber on the tether, but there is not much energy = megawatthours that can be extracted in a cost effective manner, even after we learn how to do it. The voltage is high and the current can be high, but that drains the atmosphere or space close to the tether, and that space refills slowly, for the Dr. Brad Edwards type space elevator which is close to stationary near the anchor ship. Portions of a bolo = end over end rotating tether cut though space as fast as 15 kilometers per second, so a megawatt might be produced, during perhaps 1/3 of each rotation. We can build a demonstration bolo, but not even a pilot program has been funded, so there will be some surprises, when and if we build a conducting bolo tether. The closest experiment at the ISS, burned the tether possibly due to a brief surge of high energy. Neil

 

[MrUniverse]

Ok, I think that it would be beneficial to make the tether conductive up to about 100 km. That is where, according to wikipedia, the inner van Allen belt starts. I imagine a metallic, carbon nanotube tether being insulated by not metallic carbon nanotubes. Could someone speak to the effects of a conductive tether stretching through the van Allen belts and the effects that would be effected up it? That was the main cause of my first post. Does anyone know why, details appreciated, we would not be able to power our tether craft with power from the ground or harvested from the van Allen belts? If the tether stretched from geostationary orbit to the surface of Earth, then it would stretch through the inner and outer van Allen belts. What would be the effect on a conductive cable that stretches to geosynchronous orbit? :?:

 

[MrUniverse]

been reading about electrodynamic tethers. I came to the idea of using charge on a space elevator tether to support it against the magnetic field of the Earth. came the the realization that the north and south magnetic fields might neutralize around the equator. then started thinking about a tether anchored at the south pole, the tether charged to repel the Earth's magnetic field. hmmm... but the tether might twist due to Earth's rotation. anyone think this could be overcome, making this feasible?
Since tether would be attached to the south pole, the entire tether would be in sunlight most of the year, the vast majority of it's length in sunlight even in the southern hemisphere winter. If the tether supported it's self magnetically, it seems like it wouldn't have the stress on it of a cable that goes from the Earth to Geostationary orbit. You could attach solar panels up the tether and maybe use this power to support the tether and stabilize it somehow? you could also run power up from the ground or down to the ground depending on the situation. maybe computer controlled electromagnets along it for stabilization. any ideas??

 

[neilsox]

Maglev is a related analysis. It uses huge super conductive currents in a giant loop to produce enough thrust to lift off Earth's surface with the help of Earth's magnetic field and/or the solar wind in deep space. We can build a demonstration maglev which produces about as much thrust as a ion engine, so it can very slowly change the orbit in space of a space craft. At present medium temperature super conductors stop, if they produce strong magnetic fields, plus they are expensive and not very reliable. If we use copper or aluminum at 25 degrees c or there abouts, the power losses are high and the thrust is tiny. There is a very slight possibility that CNT will be superior to copper or aluminum, besides being stronger. Multiple propulsion systems typically are inferior to one system for a vehicle or craft as hybrid car buyers are learning = initial cost and repairs are more expensive, and performance is only so, so. Single conductors have some current flow in space and in the atmosphere = 1/2 kilometer radio towers have a few amps current flow, but as far as I know no one has successfully harnessed this energy. Neil