# Can large capacitors allow power plants to run at full load and store excess electricity?

We know that demand for electricity is not constant. Power plants are not operated under full load conditions. What if power plants were operated under full load condition and excess electricity was stored in capacitors? I have read that capacitors of 10 kF capacitance are available. Could the use of such capacitors make it reasonable to operate power plants under full load at all times?

Capacitors are currently "rather too costly" for this purpose.
I've revised my estimated cost after some more research but you appear to be in the 'well over one hundred thousand dollars" range!

The dear way: If you were to assemble a 10kF 150 volt capacitor from available smaller capacitors now it would cost around 1 million dollars and store about 30 kWh of electricity - worth maybe 5 to 15 dollars retail depending where you are and a lot less wholesale.

Using 160 VDC rated parts, 1700 of these would cost about $2,000,000 (really), weigh about 9 tones and occupy about 17 cubic metres (!). You'd get a discount for quantity but overall they are not viable Datasheet here One of 1700: "Doing it yourself" is hardly cheaper.. In 1000 quantity these 3400 $$\mu$$F, 2.8fV cost a mere 53 dollars each. Each series string = 150/2.85 = 53 capacitors. Capacitance in series divides by number of caps so C per spring = 3400/53 = 64F. Number of strings = 10,000/64 = 156. Total caps = caps/string x strings = 53 x 156 = 8268 Cost = 8268 x 53 = 438,204 dollars That's better than one million plus above - but that's a lot of mounting to do AND balancing circuitry will be essential. 1 to 2 million starts to look almost good. Using smaller voltage high capacity units "off the shelf Alternatives include: "Flow batteries" using eg Vanadium oxide liquid in various oxidation levels. "Tanks" of liquid are pumped through a cell to "charge" and stored in another tank for subsequent discharge. Large trial systems exist. Not yet mainstream. May or may not "make it" commercially. Lithium Ion - as being used in Tesla cars and their Powerbank home storage technology. JUST becoming economic with careful timing of charge and discharge to buy cheap power and use it at peak cost periods. Large commercial installations exist in eg Germany - costs are about break even overall so far with advantages of continuity being a factor. Lithium Titanium (a LiIon variant) - coming - used in eg Suzuki LEAF along with traditional LiIon cells. Dearerthan than standard LiIon but immensely fast charge rates and can have 5,000 - 10,000 usage cycles with due care. Molten salt - used to store energy thermally and make power off peak - eg station at Guila-Bend near Phoenix. This is solar thermal heated but electric heating could be used. Pumped storage as John mentioned. Some use but not very common. Efficiency overall about 60%. UK has a large system used for grid leveling applications. Flywheels - investigated for many decades - good in theory but to get acceptable energy densities you need large masses )(tons) rotating at 10s of thousands of RPM. Mechanical failure is not pretty. So far many have tried but there are no significant working systems. Flywheel added comment - March 2020: There are some flywheel storage systems available used for peaking load control with about 10 kWh capacity per flywheel. Others exist, but that gives a feel. . • what happen if we use low voltage capacitors & connect it with step up transformer because low voltage capacitors are cheaper. – sanket Aug 4 '15 at 3:00 • @sanket Read the part in my answer starting at ' ..."Doing it yourself" is hardly cheaper...' and ending ' ... look almost good.' -> That is using many low voltage capacitors to do the same job. Using a step up converter makes it LESS efficient, not more. Each capacitor stores a fixed amount of energy and the "problem" is the cost per energy stored is currently too high regardless of the version of capacitor you use. No arrangement of them physically or in time sequence or in any available manner will increase their energy storage capability. Unfortunately. – Russell McMahon Aug 4 '15 at 5:19 • Where is the 150V is coming from? I believe, the voltage we can inject into the capacitor has a limit, stepping over that we will get un-economical loss of energy or the capacitor is harmed. We can inject energy into the system only until this limit. The capacity of the device does not mean too much until we don't know, how big voltage can we inject into it. – peterh Mar 15 '20 at 20:16 • @peterh-ReinstateMonica The 150 VDC was probably based on peak value of AC 110 V mains. 110 V sine wave has a peak value of 110 x 1.414 =~ 155 VDC. Under load about 150 VDC is a reasonable limit. HOWEVER - choose any voltage that seems appropriate and the$ result will be similar. – Russell McMahon Mar 15 '20 at 21:58
• @RussellMcMahon Ok, but the 110V (230V) exists only at the homes, large power networks use far higher voltages, and power plants use all of them different powers. Furthermore, such an energy storage system would use its own voltage. – peterh Mar 15 '20 at 22:14

Capacitors typically store power over short periods of time, seconds or minutes. This is too short to be useful for the electrical power grid.

Typically peak power demand occurs during the day, often mid-afternoon. Excess power is available at night. To be useful, then, the energy must be stored for many hours. There are many possible technologies for storing energy but few are economical.

The most practical energy storage approach at the moment is pumped water storage. Excess electricity is used to pump water up to a reservoir. When power demand is high, the gravitational energy released when the water flows back downhill is used to generate electricity.

For capacitors to compete for practical power grid energy storage, they will need to have lower cost and higher leakage resistance.

• I assume that "higher internal resistance" refers to leakage paths, not series resistance. – Dave Tweed Aug 3 '15 at 16:28
• @DaveTweed Yes, correct. Answer updated to remove that ambiguity. – John1024 Aug 3 '15 at 17:28
• yes capacitor stores electricity for shorter time but if i connect capacitors in a cycle so that discharging of one capaciter results into carging of other? can I do that? – sanket Aug 4 '15 at 3:04
• @sanket The power is still being lost at the same rate, as determined by the $RC$ decay time, even if the power loss is spread over several capacitors instead of just one. – John1024 Aug 4 '15 at 23:29
• Pumped power requires digging and many beton building. It works, but the price is surprisingly high, compared to other solutions. – peterh Mar 15 '20 at 20:18