Energy based availability (EBA) measures the true impact of plant unavailability for variable power resources such as wind and solar farms. An hour of downtime at high irradiance costs more revenue than the same hour of downtime at low irradiance. Calculation of energy based availability requires the knowledge of amount of downtime and an accurate estimate of the energy that particular device would have produced, typically called, Reference Energy. Calculation of reference energy needs to be accurate to calculate EBA accurately.
Reference Energy: The estimate of energy that an inverter would have produced over given time period if it was continuously operating. If there is no fault during a given time period, Reference Energy is equal to Actual Production (or very close to it).
Energy Loss: Energy loss is the amount of lost energy because of downtime and is calculated based on Reference Energy that is calculated for every 10-minute period. If the inverter was not operating for the full 10-mins, the energy loss will be equal to reference energy for those 10-minutes. If the inverter operates for part of the 10-minutes, the energy loss is calculated based on the fraction of time that the inverter is not operating.
The Renewables Suite Platform uses a power curve method to calculate reference energy for solar asset types. This is done by looking at production vs irradiance over an extended period of time for each inverter to determine its power curve. This is then stored and used to calculate reference energy during time periods where inverters are not producing power or are derated.
Energy Based Availability (EBA): Contractual energy based availability can be calculated in two ways:
Reference Energy Method
EBA = 1 - (Contractual Energy Loss) / (Reference Energy)
Production
EBA = 1 - (Contractual Energy Loss) / (Actual Production + Contractual Energy Loss)
Typically, solar operation and maintenance contracts use Production based method for actual availability. The choice of the calculation method is determined based on the contractual requirements or customer preference during onboarding.
Typically downtime is classified as a responsibility of the Operator or owner. If downtime occurs because of weather (high wind, snow), grid (requirement by grid to stop or curtail inverters) or owner (customer stop, included maintenance hours) this is generally outside the scope of Contractual Energy Based Availability. This normally only accounts for energy loss during downtime that is responsibility of OEM/Operator. Technical Energy Based Availability accounts for all downtime or lost production events is described as
Technical EBA = 1 - Total Energy Loss/Reference Energy
The energy loss included here is irrespective of the fault being a manufacturer category or owner/weather/grid.
Data quality is an important metric for calculating energy loss for the downtime. There are many situations where all data signals may not be reporting with data compression, communication error, sensor error etc. being a few of them. Renewables Suite calculates data quality for every 10-min data duration and assign a 0 if data quality is poor and 1 if data quality is good. Energy based availability is reported as energy based availability weighted by data quality. For availability purposes, Renewables Suite uses the following definition of data quality.
Data quality is 0 if
-
Target Inverter is not reporting active power (NULL values) but other inverters are reporting data. This covers a case where inverter may be operating but not communicating.
-
Target Inverter is reporting only “Active Power” but no irradiance is available and it is not possible to calculate reference energy e.g. nearby inverters (neighbor algorithm) on the site are also not reporting.
-
The whole site has lost communication and none of the devices are reporting data for the particular time period.
If data quality is 0 for a 10-min time period, the time period is not used in calculation of aggregate energy based availability.