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Why is energy storage difficult?

Posted by Alex Barrett on 27 September 2016 at 12:25 pm

Our ability to generate renewable energy is increasing all of the time. It provides a great, green supplement to conventional power generation technologies. However if renewables are to one day stand on their own then there is an important issue that needs to be considered; Energy Storage.

Solar panels are not effective at night. Wind turbines need certain weather conditions to perform at peak capacity, and there is no guarantee that this will occur when power is needed the most. Only hydroelectric generators are reliable enough to provide a consistent “base load” of clean energy, and be quickly “ramped” up and down to match electricity supply to demand.

If we want the intermittent sources to produce all of our power then we need a way to store their output when it is high, so that we have energy to supplement our supply at times when wind and solar are not generating enough. There are numerous storage technologies, but implementing them on the scale of our electricity demand is problematic.

So why is energy storage difficult?

We often think of electricity as though it were a substance. After all we generate it in power stations, and send it around the world through wires. This seems similar to how we distribute water, we pump it into the pipe at one end and it comes out at the other. This suggests that like water we should be able to put electricity into a store of some sort, and keep it for use later on.

Unfortunately this isn’t the case. Electric current is not a substance, but rather a process, specifically the flow of an electric charge through a conductive material.

Instead of thinking of electricity as a flow of water through a pipe, think of it more like the movement of a bicycle chain. The chain is there whether we turn the pedals or not. But it doesn’t move until we apply a force to it. It is the process of the chain moving that turns the wheels of the bike, and causes it to move.

Electricity works in much the same way. A wire consists of conductive material with lots of charged particles, called electrons, within it. When we start a generator and “produce” electricity we are actually causing these electrons to flow through the wire, forming a current. This current can be used to do work at any point around the circuit, so long as the generator keeps using energy to keep the charged particles moving.

This means that electricity is effectively used as soon as it is produced. We can transmit it over long distances by building longer circuits, connected by miles of power cables. But we aren’t actually sending anything from one end of the wire to the other, we’re just causing electrons to start moving at every point along it.

This means that the only way to store the energy that drives an electric circuit is to convert it into a different form. There are many ways to do this, ranging from batteries to flywheels. They are all based on the same principle; Find a process through which you can put energy into a system, but which allows us to keep it there with little loss. This energy can then be removed from this system again, and used to generate another electric current.

One of the most intuitive ways to do this is also one of the best. We can use energy to lift a heavy object, leave it in a high place, and later recover much of the lifting energy by dropping it again. One of the best ways to do this is with pumped hydroelectricity stations.

We can generate an electric current using the energy in flowing water to turn a turbine. All we need is a large enough volume of water, flowing through a sufficient drop. Gravity accelerates the water, and we convert that gravitational energy into work, by turning a water wheel. We store energy in this system by doing the reverse, pumping water uphill and storing it in a large reservoir. Later we can use this water to generate electricity again.

Converting energy is never 100% efficient. So we never get the same amount of energy out that we put in.  Typically a pumped hydroelectric station has an efficiency of around 70-85%. Approximately 99% of the world’s large scale energy storage is currently provided by pumped hydro stations.

Whether we are storing gravitational energy in a hydro station, chemical energy in a battery, or kinetic energy in a flywheel the principle is the same. You use electricity to put energy into a system when it is readily available, then later you use that energy to generate electricity again. Numerous different storage technologies are in development, and a combination of them may one day allow us to use 100% renewable power.

References

Green economy post: Overview of different storage technologies

Hydro World: Pumped Hydroelectric Storage

Image Credit: Lightning Strike, John Fowler via Flickr

 

More information about Energy Storage on YouGen.

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