For many of us, the most tangible form of energy storage we regularly encounter comes in the form of the batteries in our laptops, cell phones, and other scattered portable electronics.  However, energy storage can also play a critical role in the electric grid, in transportation, or even in heating buildings. The demands of these applications vary widely and in turn require fundamentally different kinds of energy storage, such as thermal storage in molten salts or potential energy stored in pumped hydroelectric systems.

In the next few posts we will look at a number of different energy storage technologies and applications.  We will begin here by discussing the basic tradeoffs affecting energy storage and the metrics used to evaluate them.

Batteries, ultracapacitors, and other storage devices have all been designed with characteristics to fit the specific requirements of certain applications. We find lithium-ion batteries in our computers because they have high energy density (measured in energy per unit mass or unit volume) and can be cycled many times before failing, but they are too expensive to deploy for grid storage where energy density is far less important.  There are trade-offs to using each kind of storage mechanism, which we’ve broken down here so we can understand what metrics should be used to evaluate each device.

• Energy vs. Power

Energy refers to the total amount of work that is required to do some action, and is frequently measured in joules or watt-hours. Power is a measure of how quickly this energy is used, and can be given in watts (equivalent to joules/second).  For applications like electric cars, we would like to maximize both of these features.  The more energy stored, the longer we can travel before having to recharge the battery.  We also want the ability to increase our power, however, when we are accelerating onto a freeway or up a hill.  Unfortunately, there’s a hitch: the faster you draw energy from a battery (i.e. the higher power) the less efficient it is and therefore the less total energy you can get out.  This phenomena occurs because the amount of power lost to internal resistance is given by:

where:

P = power lost as heat

I = current

R = internal resistance.

When you step on the pedal of an electric car, the amount of current being drawn from the battery increases, and therefore the power loss also increases. Furthermore, high powers tend to degrade batteries faster than low powers.

Energy storage devices all exhibit this trade-off between energy and power. Ultracapacitors, for example, can deliver more power than batteries, but store much less total energy.  We refer to these attributes as high power density and low energy density (see next section).

The energy and power capabilities of different devices are typically shown in what’s called a Ragone plot, which illustrates energy density as a function of power density [see plot].  The curves show that for a given device, the faster you take energy out, the less you get.  However, by plotting multiple devices on the same plot, you can see which ones are better suited for high power or high energy applications.

A Ragone plot illustrates the trade-off between energy and power density for different storage technologies. (Source: G. Rincón-Mora, Power Management Design Line, 2006)

• Size and weight

The size and weight of a battery are also important parameters in energy storage.  You could get a phone battery which lasted twice as long as your current one, but it might be twice the size and twice as heavy.  The amount of physical space a battery takes up to deliver a certain amount of energy or power, called volumetric density, is an important design element for portable electronics and cars. Similarly, specific energy (or how much energy is stored per unit mass) is critical in these applications – no one wants to carry around a five-pound phone, and heavy batteries drag down electric cars and reduce their overall efficiency.

However, if you’re storing energy for the grid, size and weight don’t matter as much.  Large stationary applications can therefore employ very different kinds of energy storage than required by portable devices.  Large flow batteries, for example, have only about a tenth of the energy density of lithium-ion batteries.

• Capacity, cycle life and storage life:

When people complain that their cell phone has “died” the battery is rarely actually dead and has instead just been fully discharged.  The amount of time your battery lasts on a single charge depends on the capacity of the battery, referred to either in amp-hours or watt-hours.  After an initial set of cycles (during which capacity can actually increase for certain battery chemistries), batteries experience capacity fade and can store less charge on each cycle.

Battery manufacturers usually consider the cycle life of the battery to be the number of times you can charge and discharge a battery until it has dropped to 80% of its rated capacity. Sometimes, battery lifespan can also refer to the shelf life of batteries.  Even without cycling, batteries can lose their stored charge over time due to self-discharge and can also experience capacity fade, so batteries in backup power supplies need to be recharged or replaced periodically.

• Efficiency

Energy efficiency in this context refers to how much energy a storage device can deliver compared to the amount of energy that went to charge it.  The energy efficiency of a lithium-ion battery is typically over 90%, but lead-acid batteries may be only 75% efficient. As mentioned above, the faster you use the energy in a battery (i.e. the higher power the application) the lower the efficiency because more energy is lost as heat.  The plot below shows both the energy efficiency and the cycle life of various energy storage devices.

Energy efficiency and cycle life for various energy storage devices (Source: EPRI)

• Price

Lead-acid battery bank used for a stand-alone solar system at a school in Fondwa, Haiti

The decision to use one type of storage over another often comes down to price. A lithium-ion battery is much more energy dense and can be cycled many more times than a lead-acid battery, but it is also much more expensive.  While this may be a necessary investment for portable electronics, a stationary backup battery can be heavy and doesn’t need to be cycled much so a lead-acid battery will probably be much more cost-effective.  The cost of energy storage devices is usually heavily dominated by capital costs, but during the course of use operations and maintenance costs can be important as well.

As you can see, there’s quite an assortment of options! Optimizing across these tradeoffs allows for the selection of the best energy storage device whether it be your computer or your car.  In the next posts we will dig deeper and discuss different energy storage technologies in depth. Now that we know these important metrics, we will see how they each measure up.