# Can we store ac power like dc?

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1. Jan 24, 2015

### Dilip Y

I simply wanted to know, whether we can store AC power like DC?

2. Jan 25, 2015

### Mike_In_Plano

If you're referring to storing it in a battery or capacitors, no.
However, we can use motors that also function like generators to store relatively small amounts of energy in a flywheel and extract it should there be a power failure. This is used on some generators allowing time for the engine to start.

There has been much consideration for compressing vast volumes of air using electric motors and then recovering the energy using generators (which may again be the same part). These machines could run/deliver AC.

We can always convert AC to DC and DC to AC, though the latter becomes more complicated. Thus, you can store energy in batteries using a rectifier (AC-DC) and then return the energy using an inverter (DC-AC).

There are difficulties with storing any great amount of electricity in that you lose some energy in the process and for things like pumps and compressors that loss can be very large (i.e. more than half) of what you're attempting to store. In addition, amounts of electricity that a home commonly use through a day require a large investment in equipment. i.e. $30,000 to$40,000 to store a day's worth of energy.

3. Jan 25, 2015

### Simon Bridge

Welcome to PF;
It's not clear what you are asking - I'll explain:

In general you cannot store power - physically, power is the rate of change of energy while a stored object does not change, that's what the word "store" implies in this context.

We can store energy, and the energy carried by an AC electric current can, indeed, be stored.
It is stored whenever it is asked to do mechanical work: for example, driving a motor to lift a weight.

We store energy by converting it into a store-able form.

But you may be asking - "can we get AC electricity to charge a battery or a capacitor?" - and we can: if we use a rectifier first.

4. Jan 25, 2015

### zoki85

Generally, storing a decent amount of electrical energy is a difficult problem. It is easier and technically more feasible to store it in a DC form.

5. Jan 25, 2015

### Svein

This question has suddenly become more important when electric companies started using windmills. Wind is seldom constant, so if the windmills produce AC, the frequency will vary with the speed of the windmill and cannot therefore be added directly into the power grid which has a fixed frequency. One solution is to convert to DC and use the DC to create AC with the correct frequency and phase. My take on it was to use the power from the windmills in conjunction with a hydroelectric plant - use the windmill power to pump some of the water from the downside of the hydroelectric plant up to the input dam - to be used again. In this case we are storing the inconvenient AC from the windmills as potential energy in the water.

6. Jan 27, 2015

### cabraham

With ac induction generators, the frequency does not change with rotor speed, hence induction generators are commonly employed for windmill generators connected to the grid. The slip frequency between the rotor and stator changes with wind speed and rotor speed, but the frequency generated stays constant. If you search using the key words "induction generator frequency" you should get some detailed info. BR.

Claude

7. Jan 27, 2015

### dlgoff

Exactly. And pumped storage is a good way.

8. Jan 28, 2015

### Simon Bridge

It is certainly one way - whether it is a "good" way depends on the circumstances... which are not present in the original question, which has been answered.

9. Jan 28, 2015

### Svein

I do not doubt that, but back when I was working as a consultant for ABB, I put the question to the local R&D director. His answer: "They use DC".

Another thing about windmills - They seem to have poor longtime reliability. In all windmill parks I have seen, there are several just standing still - and the neighboring windmills are spinning. I think Siemens blamed the gearboxes - anybody have better information?