EME 812
Utility Solar Power and Concentration

10.2 Key Metrics and Definitions for Energy Storage


Key Metrics and Definitions for Energy Storage

There are a few key technical parameters that are used to characterize a specific storage technology or system. Those characteristics will determine compatibility of the storage with a proposed application and will also have impact on its economic feasibility. Let us go through some definitions.

Storage Capacity

Capacity essentially means how much energy maximum you can store in the system. For example, if a battery is fully charged, how many watt-hours are put in there? If the water reservoir in the pumped hydro storage system is filled to capacity, how many watt-hours can be generated by releasing that water? Those amounts are determined by storage capacity.

Understandably, the capacity of any storage will increase with the system size. The more battery stacks are installed, the more electric energy can be put in for storage. The larger the water reservoir, the greater energy turnaround becomes possible. The system size should be matched with the load and specific application.

Storage capacity is typically measured in units of energy: kilowatt-hours (kWh), megawatt-hours (MWh), or megajoules (MJ). You will typically see capacities specified for a particular facility with storage or as total installed capacities within an area or a country.

Table 10.2 Examples of storage system capacity:
Portable scale A portable battery pack with a storage capacity of 450 Wh...
Utility scale One of the largest PV + storage projects in Texas – Upton 2 – has storage capacity of 42 MWh (which would be sufficient to power 1400 homes for 24 hours)
National scale The total installed capacity of energy storage is the US is around 1000 MWh

Sometimes you will see capacity of storage specified in units of power (watt and its multiples) and time (hours).

For example: 60 MW battery system with 4 hours of storage. What does it mean?

60 MW means that the system can generate electricity at the maximum power of 60 MW for 4 hours straight. That also means that the total amount of energy stored in the system is:

60 MW x 4 hours = 240 MWh

But it can also provide less power if needed. For example, if the load only requires 20 MW, the system can supply it for 12 hours. The total amount of stored energy is the same, but it is used more slowly:

20 MW x 12 hours = 240 MWh

So power and time ratings give us a little bit more information: we not only know how much energy is stored, but can also define at what maximum rate this energy can be potentially used.

Check Your Understanding Questions 1 & 2 (Multiple Choice)

Energy density

Energy density is often used to compare different energy storage technologies. This parameter relates the storage capacity to the size or the mass of the system, essentially showing how much energy (Wh) can be stored per unit cell, unit mass (kg), or unit volume (liter) of the material or device.

For example, energy densities for different types of batteries are listed in the table below [IES, 2011]:

Table 10.3 Energy Densities for Different Types of Batteries
Battery type Energy Density, Wh/liter
Lead-Acid battery 50-80
Li-ion battery 200-400
NiCd (nickel cadmium) battery 15-80
NiMH (nickel metal hydride) battery 80-200
NaS (sodium sulfur) battery 150-300
NaNiCl2 (sodium-nickel-chloride) battery 150-200
Zinc air battery 130-200
Vanadium redox flow battery 20-70
Hybrid flow battery 65

Of course, we are interested to store as much energy as possible while using as small and light device as possible for this purpose. From the table above we can conclude, for example, that a fully charged Lead-Acid battery will run out of charge much sooner than a fully charged Li-ion battery of the same mass/size.

Energy density is related to capacity and determines the duration of power generation. Also materials with higher energy density help make the power block more compact, which is useful in portable electronics and vehicle applications.

Just for comparison, the energy density of the pumped hydro storage is 0.2—2 Wh/kg, which is rather low and requires significant masses of water and large reservoir size to deliver utility scale power.

Check Your Understanding Question 3 (Multiple Choice)

Power density

Power density (measured in W/kg or W/liter) indicates how quickly a particular storage system can release power. Storage devices with higher power density can power bigger loads and appliances without going oversize. Imagine an electric vehicle accelerating from 0 to 60 MPH – which takes a lot of power. If you look at the table below, you will see why Li-ion battery remains the technology of choice for powering electric vehicles, even though some other battery types exhibit similar energy densities.

Table 10.4 Energy and Power Densities for Different Types of Batteries
Battery type Energy Density, Wh/liter Power Density, W/liter
Lead-Acid battery 50-80 90-700
Li-ion battery 200-400 1300-10000
NiCd (nickel cadmium) battery 15-80 75-700
NiMH (nickel metal hydride) battery 80-200 500-3000
NaS (sodium sulfur) battery 150-300 120-160
NaNiCl2 (sodium-nickel-chloride) battery 150-200 250-270
Zinc air battery 130-200 50-100
Vanadium redox flow battery 20-70 .05-2
Hybrid flow battery 65 1-25
Power Density w/ liter vs Energy Density, Wh/liter
Figure 10.2 Classification of energy storage systems by energy and power density. Key to abbreviations is provided below.
Credit: Mark Fedkin © Penn State based on the data from IES, 2011

Key to abbreviations to Figure 10.2

CAES – Compressed Air Energy Storage
DLS – Double Layer Capacitor
FES – Flywheel Energy Storage
H2 – Hydrogen storage
LA – Lead Acid Battery
Li-ion – Li ion Battery
Me-air – Metal Air Battery
NaNiCl – Sodium Nickel Chloride Battery
NaS – Sodium Sulfur Battery
NiCd – Nickel Cadmium Vented Battery
NiMH – Nickel Metal Hydride Battery
PHS – Pumped Hydro Storage
RFB – Redox Flow Battery
SMES – Superconducting Magnetic Energy Storage
SNG – Synthetic Natural Gas

The technologies located in the lower left corner of the diagram (low energy density and low power density) take significant amount of space and material to enable the storage conversion and are mostly suitable for very large scale projects. Systems such as PHS and CAES also rely on the availability of specific landscape and geological features to accommodate the storage reservoirs.

The technologies located in the upper right corner of the diagram are most coveted for portable and efficient power supply, such as electric vehicles. These compact systems can carry a significant amount of energy and release it quickly on demand.

The technologies in the upper left corner are special devices that can be used in quick response electronics. These systems store small amounts of energy (and therefore charging can be fast), but are able to provide high power by releasing energy within short period of time.

Finally, the technologies in the lower right corner are characterized by slow charge and discharge, but the advantage is the total high amount of energy they are able to store, providing longer duration of energy supply.

Check Your Understanding Questions 4 & 5 (Multiple Choice)

Storage efficiency

The main function of any storage device is to uptake and release power on demand. In case of a battery, for example, it would be electrochemical charge/discharge cycle; in case of pumped hydro storage, this process involves pumping water into the elevated reservoir and later releasing the flow through the turbine. Both charge and discharge processes include one or more energy conversions (Figure 10.3). In the figure, each arrow indicates the energy conversion from one form to another.

PV electric entering a battery labeled Chemical and exiting to the grid electric. The battery is underscored by charging and discharging
Figure 10.3 Main types of energy conversions in battery. 
Credit Mark Fedkin © Penn State
PV electric entering a battery labeled Chemical and exiting to the grid electric. The battery is underscored by charging and discharging
Figure 10.4 Pumped Hydro Storage systems
Credit Mark Fedkin © Penn State

Regardless the number of transformations, the energy comes to its initial electric form, which is finally ready to be dispatched into the grid. This is the charge-discharge cycle, the "round trip". 

In each conversion, energy is partially lost from the cycle and dissipated into the surroundings, and the efficiency of conversion at every step accounts for those losses. 

Efficiencies of all energy conversion steps in this cycle are combined in the metric called round-trip efficiency, which essentially indicates the percentage of energy delivered by the storage system compared to the energy initially supplied to the storage system. The obvious goal is to minimize the conversion losses and thus maximize the overall storage efficiency.

Here are some round-trip efficiencies of various energy storage systems:

Table 10.5 Round-Trip Efficiencies of Various Energy Storage Systems
Storage system Round-trip efficiency, %
Lead-Acid battery 75-90
Li-ion battery 85-98
Pumped hydro storage 70-80
Compressed air energy storage 41-75
Flywheel 80-90
Hydrogen 34-44
Double layer capacitors 85-98
Vanadium redox flow battery 60-75

These numbers mean the following. For example, out of 1 MWh of energy spent to pump water up to the hydro storage, only 0.7-0.8 MWh will be available to use after the water is released to run the turbine and generator to produce electric power. The other 0.2-0.3 MWh of energy will be converted into non-useful forms of energy and “lost” from the cycle. Some of the energy losses occur in the auxiliary devices used in the energy storage process, very often in the form of waste heat. Furthermore, energy losses may be linked to the mechanical or material losses: for example, leaks and evaporation of water from pumped storage, air leaks in CAES, chemical degradation and incomplete reactions in batteries.

Check Your Understanding Questions 6 & 7 (Multiple Choice)