Energy storage is one of the hottest topics in the energy world. SolarCity’s partnership with Tesla to provide solar-charged battery systems, the California PUC’s mandate of 1.3 GW of energy storage by 2024, and energy storage plants entering into PJM’s ancillary services markets are just some of the many examples we hear about every day.
While the headlines are clear, discussion of the details often gets murky. This is the first in a series of three posts, in which we explain the applications of grid electrical energy storage. Later this week, in Energy Storage 102, we will discuss the technologies and value chain in more detail. Next week, we will examine the detailed economics of commercial behind-the-meter storage in Energy Storage 201.
Power and Energy
To understand the energy storage landscape, it is first important to draw a clear line between power and energy. Power, usually measured in kilowatts (kW) or megawatts (MW), is the load that a storage system or generator can serve at any instant in time. Energy, measured in kilowatt-hours (kWh) or megawatt-hours (MWh), is the amount of power that can flow over time.
This is where terms in the energy industry can get confusing. When discussing the capacity of a power plant people mean the amount of power it can generate (MW). When discussing the capacity of a storage system, however, they mean the volume of energy it can store (MWh).
Response Time and Discharge Time
Response time is the time it takes for a system to provide energy at its full rated power. Discharge time is the amount of time a storage technology can maintain its output. A one MW battery that has a discharge time of five hours can provide five MWh of energy.
Depth of Discharge (DOD)
Depth of discharge is the percentage of capacity discharged. Deep discharges (>50% DOD) shorten the lives of some batteries, while others operate best this way.
Energy Storage Customer Types
Five major groups may be customers of grid-tied energy storage:
- Commercial users (including government and non-profits)
- Residential users
- Independent Power Producers (IPPs)
Some of these have a number of applications for storage.