Intermittent Energy Source - Terminology

Terminology

Several key terms are useful for understanding the issue of intermittent power sources. These terms are not standardized, and variations may be used. Most of these terms also apply to traditional power plants.

• Intermittency can mean the extent to which a power source is unintentionally stopped or unavailable, but intermittency is frequently used as though it were synonymous with variability.
• Variability is the extent to which a power source may exhibit undesired or uncontrolled changes in output.
• Dispatchability or maneuverability is the ability of a given power source to increase and/or decrease output quickly on demand. The concept is distinct from intermittency; maneuverability is one of several ways grid operators match output (supply) to system demand.
• Nominal or nameplate capacity, or maximum effect refers to the normal maximum output of a generating source. This is the most common number used and is typically expressed in megawatts (MW).
• Capacity factor, average capacity factor, or load factor is the average expected output of a generator, usually over an annual period. Expressed as a percentage of the nameplate capacity or in decimal form (e.g. 30% or 0.30).
• Capacity credit: generally, the amount of output from a power source that may be statistically relied upon, expressed as a percentage.
• Penetration in this context is generally used to refer to the amount of energy generated as a percentage of annual consumption.
• Firm capacity the amount of power that can be guaranteed to be provided as base power
• Non-firm capacity the amount of power above the firm capacity that is usually to be sold at higher price on the spot market