Storing 200TJ of Electricity for rapid discharge

Question

Here's the problem I'm having.

I need a solution for the following situation. I have a sci-fi starship drive that needs 140TJ of energy every 6 hours. I have a Fusion Reactor that can produce 200TJ in that time period.

The problem is the Reactors produces 10GW... so I need to store this charge somewhere for when it is used. That means I need a Battery or a suggested "Burst Circuit"... I can't figure out which is the better method of storage.

I can't figure out how a burst circuit would work as it seems like a 200TJ charge going around in a loop would burn awfully hot and destroy the circuit rapidly and also the rapid drop in Energy then reheated would damage it too.

The problem with the Battery solution is that I don't how big the battery would be, and I'm pretty sure a battery can't discharge such high energy at once.

So which is the better method or is there a better one? Can the Burst Circuit be done with current tech? How big would the battery be?

Answer 1

Storing 200TJ of Electricity for rapid discharge

Batteries store energy, and so do capacitors.

https://en.wikipedia.org/wiki/Capacitor#Energy_stored_in_a_capacitor

Conventional capacitors provide less than 360 joules per kilogram of specific energy, whereas a conventional alkaline battery has a density of 590 kJ/kg. There is an intermediate solution: Supercapacitors, which can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.

Thus, you need a set of supercapacitors to store the charge, and then discharge it when needed.

I can't figure out how a burst circuit would work as it seems like a 200TJ charge going around in a loop would burn awfully hot and destroy the circuit rapidly and also the rapid drop in Energy then reheated would damage it too.

Without using the term, you're describing a short circuit, and they're always bad. You never want that.

But you still need the supercapacitors to discharge rapidly without burning up the wires. Thus, you need superconductors.

https://en.wikipedia.org/wiki/Superconductivity

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic flux fields occurring in certain materials, called superconductors, when cooled below a characteristic critical temperature.

Thus, no heating nor destruction of your infrastructure.

But since maintaining temperatires below 30K is... difficult, you'll need high-temperature superconductors.

https://en.wikipedia.org/wiki/High-temperature_superconductivity

Whereas "ordinary" or metallic superconductors usually have transition temperatures (temperatures below which they are superconductive) below 30 K (−243.2 °C), and must be cooled using liquid helium in order to achieve superconductivity, HTS have been observed with transition temperatures as high as 138 K (−135 °C), and can be cooled to superconductivity using liquid nitrogen.

While 138K is still damned cold, it's doable. Presumably, though, since you have fusion reactors and star ships, you'll have also developed true high-temp superconductors.

Everything is in place now:

1. Fusion reactor is connected to
2. high-temp superconductors (big, fancy wires) which input electricity to
3. a huge array of supercapacitors. When that 200TJ are needed,
4. electricity flows through high-temp superconductors on the "output" side of the supercapacitors,
5. straight into your sci-fi starship drive.

Simple, really.

Answer 2

From your comments, "there is no set discharge period that I have." This basically means we can't provide a solution. It's the electrical engineering equivalent of "I want a vehicle that moves fast, but I don't really care how fast." The best vehicle choice varies greatly from tricycles to SR-71's and space shuttles.

I can point out that, as a general rule, the slower you allow the discharge to be, the more convenient your technology is. Things that can discharge in a millisecond tend to store less energy per kg (specific energy) and store less energy per cubic meter (energy density).

Three technologies that might be on your list (energy density numbers from this wikipedia page):

• Film Capacitors - These are fast discharge (microseconds to nanoseconds). However, they are very poor when it comes to energy density. There's literally orders of magnitudes difference between different film capacitors, but they are somewhere around 10J/kg
• Large Electrolytic Capacitors - These are slower discharges because they can heat up and boil their electrolyte. There's, once again, literally orders of magnitude differences between different products, but if you stick to microsecond to millisecond discharges, you're probably fine. Energy density is somewhere around 200J/kg
• Supercapacitors - Once again, slower discharge, but more efficient. The specific power of a supercapacitor is lower than that of an electrolyte capacitor, so it can't discharge as fast. However, it's energy density jumps to 10-40kJ/kg
• Batteries - Battries discharge even slower, but jump to 170kJ/kg (lead acid). The highest battery on my list is a lithium metal battery at 1.8MJ/kg

Now what you should note here is that all of these numbers are small. Even using the most dense energy storage on the list, batteries, you're talking 70 million kg of batteries. That's roughly the mass of a Nimitz class aircraft carrier.

You didn't specify how big your spacecraft was.

You could try storing your energy in a superconducting loop. If you did that, there'd be no resistance. However, there is always a finite chance of any one section of the loop becoming resistive. The more energy you put in the loop, the higher the risk. If any one section becomes resistive, it quickly heats up the nearby area and a "quench" occurs, where all of the energy is dissipated immediately. This is something they deal with in particle accelerators. As I have heard it described, they have to work a balance. Quench too many times in a day, and you don't get enough work done. Play it safe, use low energies, and don't have any quenches, and the results aren't interesting enough to warrant the huge cost of the collider. They have to find the right balance.

Sadly, I do not have numbers for predicting how much energy you could reasonably store in one of these loops. I do, however, have stats for the LHC. The circuits powering the magnets in the LHC have about 10GJ of energy in them (a small fraction of what you need, but useful none the less). When a quench occurs (typically due to something stray in the beam, but sometimes its due to random effects in the magnets), it takes about 2 minutes to dump all of the energy into a steel block -- it heats 8 tons of steel about 300 degrees in those two minutes.