Gurps Spaceships: Rechargeable Fuel Cells

[austink]

Any ideas on how I could represent a tech setting that possessed advanced energy storage technology (energy banks, batteries, rechargeable fuel cells, etc.) and limited energy generation technology (reactors). For example, Blake's 7 (http://en.wikipedia.org/wiki/Blake's_7) had ships that ran off of massive energy banks that could be recharged by a reactor or solar panels. The Liberator (http://www.btinternet.com/~blakes.se...liberator.html) had seven energy banks.

These energy banks would work like fuel cells except that:

1. no "fuel" is involved,
2. they can be recharged by spare power points from reactors or solar panels.

[Peter Knutsen]

I don't know, but it has long been obvious to me that short-occupancy spacecraft, such as Space Fighters, might do well to use batteries instead of reactors. They'd be recharged by their carriers or other mother ships, whenever they dock, then when topped up with reaction mass and ammo, and the pilot replaced, they fly out again.

The simple solution would be to say that a Battery is like a Power Plant of a given TL, except it provides more Power Points, but can do so only for a distinctly limited period of time, whereas a real power plant can go on and on for years, decades or even centuries (depending on TL).

The big questions are what that endurance should be, how many more power points a Battery should provide compared to the best power plant of the same TL, what a Battery should cost, and whether it is actually all that useful given the constraints of the GURPS spaceships system.


Still, if a workable solution can be found, you can have various combat smallcraft, space fighters and scout fighters, who utilize lots of energy dependant (!) modules, while requiring only a single 5% mass module to power it all - for a short period of time.

[thtraveller]

Originally Posted by Peter Knutsen

I don't know, but it has long been obvious to me that short-occupancy spacecraft, such as Space Fighters, might do well to use batteries instead of reactors. They'd be recharged by their carriers or other mother ships, whenever they dock, then when topped up with reaction mass and ammo, and the pilot replaced, they fly out again.

There are a few G:Spaceships designs around on this board that use fuel cells for small craft. But they require fuel. I guess rechargeable batteries didn't make the cut for spaceships.

The simple solution would be to say that a Battery is like a Power Plant of a given TL, except it provides more Power Points, but can do so only for a distinctly limited period of time, whereas a real power plant can go on and on for years, decades or even centuries (depending on TL).

That was my thinking too. Choose a reactor, probably a superscience one - as very long duration fuel cells are superscience anyway - and just give it whatever duration you need.

Though there is the issue that Gurps Solar Panels aren't going to charge long duration cells up that quickly.

[Diomedes]

Originally Posted by austink

These energy banks would work like fuel cells except that:

1. no "fuel" is involved,
2. they can be recharged by spare power points from reactors or solar panels.

A thought:

The banks are essentially batteries or capacitors, and have two relevant statistics:

• How much energy they can hold.
• How fast they can charge or discharge.

The latter is however much you need to run whatever equipment you're powering, and should be greater than your total power output (otherwise you could just use the reactors directly).

An example: Say that a SM+5 fusion reactor produces 2 kW, but your hyperdrive requires 3kW to maintain operation. Clearly, your reactor can't run the drive. But if you add a battery with a capacity of 30,000 kJ and a rate of 3kW, you can spend 15,000 seconds (4 hours and 10 minutes) to completely charge the battery. (30,000 / 2)
The battery can run the drive for 10,000 (30,000 / 3) seconds before being depleted if the reactor isn't powering the drive as well. If it is, then the drain on the battery is (Power Requirements - Reactor Output = 3-2)=1 kW. Now you can run your drive for 8 hours and 20 minutes.
Energy capacity and discharge rate are both likely to scale with mass.

[Peter Knutsen]

Originally Posted by thtraveller

That was my thinking too. Choose a reactor, probably a superscience one - as very long duration fuel cells are superscience anyway - and just give it whatever duration you need.

Though there is the issue that Gurps Solar Panels aren't going to charge long duration cells up that quickly.

Keep in mind, I want to put the Battery module in a low-SM "fighter type craft" and the power plant (solar panels or some kind of reactor) on a high-SM carrior or "mother ship" type craft.

[malloyd]

Originally Posted by austink

These energy banks would work like fuel cells except that:

1. no "fuel" is involved,
2. they can be recharged by spare power points from reactors or solar panels.

So why not use a fuel cell and call it a battery? Assume it can run in reverse (which is often true of fuel cells, this is usually called regenerative rather than rechargable) and that its recharged when you've run as many power point-hours into it as you've pulled out.
The only difference between a fuel cell and a battery is that a battery has its fuel packaged in the same container as the reaction surface. Since that's what Spaceship fuel cells do (you don't have to take a separate fuel tank), I'm not sure what else you need. Are you looking for more power points but shorter durations than the 3/6/12/24 hours default of the fuel cells? Seems fair enough to just say that's free too, as long as the product of power points x hours stays the same. Yeah technically the cell weight changes too, but most of the package is fuel, so its probably below the system resolution as long as we aren't talking about durations of seconds or less.
If you are looking to convert power points between different sized ships, its the same ratio as most everything else x3 for 1 SM difference, x10 for 2. So a system that provides 1 power point for 12 hours takes 12/30 hours = 24 minute to recharge from a ship 3 SM bigger dedicating 1 power point to the effort.

[Peter Knutsen]

Capacitors, able to provide power to large systems (especially spinal mounts) but only for a very few shots before they're discharged, might also be of interest, in military spaceships. Especially small, fighter-type ships.

[austink]

What about something like this:

Energy Bank (TL 8^)
This power plant is a high-efficiency energy storage device - basically a large battery. Each energy bank lasts for 2 hours (TL 8), 4 hours (TL 9), 8 hours (TL 10), or 16 hours (TL 11) and provides one Power Point, two Power Points for half the listed duration, or four power points for a quarter of the listed duration (TL 9+).

SM Cost ($):
+5 150k
+6 500k
+7 1.5M
+8 5M
+9 15M
+10 50M
+11 150M
+12 500M
+13 1.5B
+14 5B
+15 15B

With this setup, energy banks are more space-effective up to TL 10 (antimatter reactors catch up to power output capabilities), and more cost-effective all the way along. Of course the trade off is that the power is available for only a short period of time.

[Peter Knutsen]

Energy banks are also safer than antimatter reactors!

[Anaraxes]

That's just what the lobby for Big Energy Banks wants you to think. My uncle knows a guy that invented an overthruster that got 200 ly to the gram of antimatter with less energy discharge than your average deflector dish, but it got bought and suppressed so the big companies could keep their profits.