Making Comparisons
A quick look at the units and comparators commonly used in this space
The analyst firm, Lazard, has established what many regard as the industry standard metric - the Levelized Cost of Storage (LCOS). Its purpose is to give an even-handed metric of the full lifetime of costs for a particular storage technology, expressed in $ per megawatt hour. LCOS is very useful when comparing technologies for a similar use case and physical context.
Like most of these metrics, LCOS has its limitations. A broader comparison must look beyond LCOS and take account of multiple factors: space constraints, round-trip efficiency, self-discharge rates, performance degradation & useful lifetime, constructability & maintainability, topologies required, component materials & supply chain security, spare parts availability, recyclability & sustainability. In some cases a round trip efficiency that compares poorly on power services could be acceptable if paired with useful heat recovery in the specific application. A good fit to the context and use-case is essential.
We take a quick look in this post at the units and comparators commonly used.
(Caveat - all of the following worked examples are for illustrative purposes only & assume zero losses, linear performance curves etc.)
The basic measures of power & energy are the Watt (W) & the Joule respectively. The Watt-hour (Wh) is more commonly used than the Joule. One is the aggregate of one joule per second for the duration of an hour - 3,600 joules in total.
Domestic batteries are rated in kWh (kilowatt hours), with MWh (megawatt hours) more commonly used at grid-scale. This is largely down to practicality. It would be correct to describe grid energy in joules, but as each MWh represents 3.6 billion joules, you quickly run up a lot of digits at grid-scale.
The difference between voltage, power rating and capacity is also worth noting.
Take a hypothetical 48V battery, with 40kWh capacity and a 12kW power rating (as might be deployed in a small commercial application). This battery is hard-wired to deliver 48V at a maximum of output of 12kW. If the battery were charged to 100% capacity (40kWh), three hours operation at full rated output would discharge the battery down to 4 kWh (or 10% of its full charge).
Energy Density describes the units of available energy per mass. The term comes with a caveat and is often misused. Energy Density in batteries is measured in Watt-hours per kilogram (Wh/kg). It has a different definition in flammable fluids (hydrocarbons or hydrogen), where it describes energy in a given mass or volume of the fuel - typically released through combustion.
Comparing an electrical battery with a hydrocarbon fuelled generation set must include the full package of equipment required to deliver electrical power. The battery is often ready to operate "out of the box" whereas the hydrocarbon needs an engine, a generator, storage, pipes, valves, fuel pump, controls etc. before it can function on an equivalent basis.
Energy to Power Ratio is less often used but is an important measure when looking to match a storage technology to a particular use case. It is a measure of the number of hours of discharge at full rated power and is calculated by dividing the energy capacity by the rated power.
In our example above, the Energy to Power Ratio would be calculated as [40kWh / 12kW] or 3.33 hours. This battery could serve intraday imbalances adequately, but would not work in a long-duration use-case where 150 hours of energy at varying power were required (think of an off-grid wind turbine connected to a microgrid arrangement).
Round Trip Efficiency ("RTE") describes how much energy is retained when a battery goes through a full cycle of charge & discharge. The higher the RTE, the less energy is lost in the storage process.
Heat generated must be accounted for in the energy balance, as must any energy to power moving parts (e.g. pumps and valves in a flow battery).
RTE is calculated by expressing the total energy returned by the battery in the discharge cycle as a percentage of the total energy drawn by the battery in the charge cycle.
Let's amend our earlier example & say the battery had drawn 42kWh in the charge cycle before delivering 35kWh in the discharge cycle, its RTE is [35kWh / 42kWh] or 83%.
Other metrics/considerations:
- Charging Rate - the speed at which the battery can be charged
- Cycle life - the number of discharge cycles a storage method can withstand before capacity is impaired
- Depth of Discharge ("DoD" as applies to batteries) - the percentage of the battery's capacity that is discharged during each cycle. Variable DoD can impact the battery's performance and cycle life
- Minimum Voltage - Discharging below this voltage can impair the battery's performance and cycle life
- Operating Temperature - the range of temperatures at which a storage technology can function properly (this requires particular attention with batteries where thermal runaway can lead to fire hazard)
- Self-discharge - the rate at which a battery loses its charge over time when not in use
- Thermal Runaway - caused by a chain reaction in a cell where more heat is produced than can be dissipated potentially causing a release of flammable gas.
A fully loaded equivalence must take account of the full build cost. For example, Li-ion batteries have been prone to thermal runaway with some disastrous consequences2. In the case of a Li-ion deployment for grid-services, cooling and fire-suppression services with additional redundancy (as might be warranted in certain instances) would add to the build costs. Large scale pumped hydro results in ecology damage from flooded valleys or uplands, land loss or transformation, social costs in the form of community dislocation.
In conclusion … comparing the spectrum of storage technologies across the full range of use cases is an involved exercise, there is no simple metric.