# New idea about generating electricity

1. Jul 3, 2010

### trekjunky

A month ago or so, an idea popped into my head. Here it is in a nutshell: If you send a space device or ship in an orbit that takes it to do gravity assists over and over again to increase its velocity to generate lots of kinetic energy. If you do that, how much kinetic energy would be created? If we converted that kinetic energy to say electricity, would it be worth it?

All the ways of converting kintetic energy on Earth that I can think of have the property of the material with the kinetic energy has to touch, push, or pull against another material to convert the kinetic energy to electricity.

Coffeekraken's (from Atheist Nexus) idea of a turbine has that property. He didn't mind my changing his idea a little bit of the turbine: What if that turbine was on a satellite with a net to temporarily catch and then release the orbiting body thereby sending it on another orbital assist orbit? I think that would be one good way to convert the energy, but I would ask the question again: Is it worth it?

Thanks!!!!

2. Jul 3, 2010

### ThomasEdison

I remember that experiment where nasa tied a long conductor to a satellite and let it dangle to see how much current would be generated as it drifted through Earth's magnetic field. From what I understand it was too much ampacity for the conductor to handle and the tether broke. I'm not sure whether that is considered a failure (because it is too much power) or a success because it was too much power.

I can't find a link sorry. It was on TV and on youtube I just don't know how to search for it.

3. Jul 3, 2010

### trekjunky

Try http://www.nasa.gov/mission_pages/station/science/experiments/List.html

4. Jul 4, 2010

### Filip Larsen

Welcome to PF!

The short answer, I would say, is that this almost certainly is not a feasible way to produce electrical power. There is a lot that speaks against it.

Gravitational assist is a kind of "poor mans" rocket that mostly is used by necessity on interplanetary probes because it is much cheaper than building and launching a kick stage that can deliver the probe directly to the target planet on a much faster trajectory. However, even when using gravitational assist the launch cost is still huge. If you had a very large in-space supply of mass (like an asteroid) that you could send on a sling-shot around a few planet by only giving it a very small nudge then at least you may be able to get price per mass low enough.

You would probably also need to have the "system" work in a cyclic manner as not to be a one-shot operation. This implies you should be able to slow the mass down when it returns to Earth in such a way that you gain electrical power from it and at the same time the mass will end up on a trajectory that will make it do a new trip. How would you collect (part of) the kinetic energy considering the mass will enter with, say 15+ km/s and has to leave with 11+ km/s? And how will you control the trajectory of the mass along the way (probes need to do correction burns once in a while to fine-tune the sling-shot trajectory)?.

Lastly, I would say that pretty much all existing and near-future energy production systems appears to be far more feasible and capable of higher energy output in much more stable and simpler way than such a gravitational assist system. In fact, I am not able to think of a single advantage such a scheme would have. Can anyone?

5. Jul 4, 2010

### Filip Larsen

You probably mean the Space Tether Experiment, http://www-istp.gsfc.nasa.gov/Education/wtether.html

6. Jul 4, 2010

### Antiphon

If you made electricity this way for long enough, you might extract enough energy to shift the orbit of the bodies you use. Of all the possible side effects of various energy sources this one could have the gravest of consequences.

7. Jul 4, 2010

### trekjunky

TrekJunky's response: For some reason, in my gut I think that the great velocities would create great amounts of energy depending on the efficiency of conversion. Using the simple formula of F=ma, let's assume the craft is 500Kg and the captured velocity is 4km/s, we get 2,000 kg/km/s. That represents the total amount of available power from each passing craft that could be converted and say the efficiency is 80%, we get 1,600 kg/km/s. So what does 1,600 kg/km/s tramslate into electrical units? I don't know, but maybe someone out there does. Also, these numbers above have no basis in reality, so maybe someone else could come up with more realistic numbers at the same time?

I am very curious about this and would appreciate any help that you folks could give me. Thanks and thanks for the welcome to PF!!!!

8. Jul 5, 2010

### Filip Larsen

I'm sorry to say, but you're gut is wrong on this. Let us run some numbers.

Assume we with 100% efficiency can convert 3,6 km/s of a 3600 kg probe into electrical power and that we have inserted as many such probes as it takes in order for one to pass Earth every hour. This means on average 1 kg will pass Earth every second, resulting in a power production of 1 MW.

Now lets look at what we need in order to get that 1 MW. First, ignoring the problem with changing planetary geometry, assume each probe can do a cycle in one year. This means we need around 31,6 million kg in orbit. Launching that from Earth assuming current launch costs of 10000 $/kg we need way more than 316 billion dollar to get that mass into orbit. So lets assume we find an asteroid out there and that we can grab the needed mass from it for 2 billion dollars. For the operational costs, we will assume the life time of the system is 30 years and that operation can be sustained with an average of 10 man with 1 being in space, all covered with a 100 million dollar monthly budget. Putting it all together, the total energy production cost ends up being around 140000$/MWh. Note, that this is using hugely unrealistic and over-optimistic figures. I would not be surprised if the production cost would go up at least a factor of 100 or 1000 if a more realistic figures are used. And yet we haven't even included the cost for a long range of complicated technical challenges that needs to be solved.

Now, please compare this 140000 $/MWh cost to the "normal" production costs of around 100-200$/MWh[1] and tell me if you still think your gut is on the right track.

[1] http://en.wikipedia.org/wiki/Relative_cost_of_electricity_generated_by_different_sources

PS: In order to make it easier for people to quote your posts in the future, please place your own responses outside any quote-blocks from others. Inside a post you can manually insert additional QUOTE tags if you need to break up a big quote block in order to place you answers right after the relevant text.

9. Jul 5, 2010

### trekjunky

Thank you very much Fillip. I am a details oriented person and your response was perfect and greatly appreciated.

P.S. Thanks for the Quotes info. I will remember that for the future.

10. Jul 9, 2010

### sophiecentaur

Whatever the numbers you are quoting, there is a fundamental problem, I think. You use energy to put the system in place. Whilst generating current, there is a little, nuisance, thing called Lenz's law which tells you that there is a force 'opposing its cause'. This will use power (force times speed) which will slow the orbiting system down and it will fall to Earth eventually. The only energy you can get out of a system like this will be from energy you have put in to establish the orbit.

If you were to use the Energy from something that is huge and already orbiting the Earth then there would be plenty of energy available. For instance, the energy from the tides is readily available and the Moon just wouldn't notice the fact that we may be making use of a tiny fraction of it. The total rotational energy is already being lost through tidal action - very, very slowly..

11. Jul 9, 2010

### Filip Larsen

The idea was to put the mass on a gravity assist trajectory around one or more planets and then back to earth so that it will arrive with more kinetic energy than when it left. While it sure seems like an extremely infeasible way to produce energy, it should at least be theoretical possible. Not that this says very much since more or less all physical processes involves exchange of energy that, in theory, can be harvested however infeasible it may be.

12. Jul 9, 2010

### sophiecentaur

I am not sure that gravity assist will necessarily give you energy for any old route. It relies on what is, effectively, a hyperbolic orbit around a planet the vehicle is 'catching up'. The resulting trajectory , after things have settled down, will have a velocity with the planet's velocity added to it (some momentum has been transferred). That's great for giving you some extra speed but it has to be in the right direction - probably out of any path that would coincide with the Earth's orbit.
You would have to involve at least one other planet to turn the vehicle's orbit into a very eccentric one which might then be planned to come very close to Earth at some later time.
Perhaps achievable "in theory" but 'in practice' it might be a tall navigational order and involve a huge number of years' delay before we could collect on our investment.

13. Aug 13, 2010

### brainstorm

What about a satellite that picks up gravity assist from the moon in an orbit that circles both the moon and Earth? Could the satellite gain a certain amount of velocity and then exchange that velocity for energy by passing Earth at a low enough altitude for the atmosphere to drive a turbine of sails arranged radially around the generator? Could the trajectory be stabilized in such a way that the amount of energy the satellite loses to friction with the sales/blades hitting the atmosphere leaves it with enough residual velocity to make it back to the moon for another gravity assist? This sounds like a perpetual motion machine, though, so there's probably a catch that makes it impossible. I just can't see what that catch is yet exactly.

14. Aug 14, 2010

### DaleSwanson

It's not a perpetual motion machine. The source of the energy is the KE and GPE of the Moon and Earth. As stated above, if this was done on a large enough scale it would alter the orbits of whatever bodies it was getting its energy from.

15. Aug 14, 2010

### Filip Larsen

If you want to utilize the orbital energy of the moon for power production on Earth you should probably notice, that it is great many orders of magnitude easier and cheaper to simply tap into tidal energy from the moon. In fact, such power production plants are already in operation today. Search for "tidal power production" for more background information on this.