Over the past ten years Garrad Hassan has assessed hundreds of operating wind farms worldwide to help verify wind farm energy production predictions. These assessments have led to a significant evaluation of availability - the percentage of time that wind turbines are available to work, whether there is wind or not.

A wind energy assessment default position Wind energy assessments typically precede technical due diligence efforts. Therefore, a default position must be established to provide a reasonable energy estimate – pending a subsequent engineering due diligence review at a later time, which range from availabilities in the low 90s to the warranted level over the ramp-up period.

The final financial projections for any specific project are then subject to the independent technical due diligence process to ascertain the appropriate turbine availability ramp-up and long term parameters to be used for the final energy predictions.

Availability ramp-up Based on the evaluation of wind farm availability data, Garrad Hassan has concluded that the ramp-up period required to reach a consistent long term availability level in most wind farms outside the United States is in the range of one to three years, depending on the availability warranted level. However, the ramp-up period for United States wind farms is approximately three to four years. The recent demise of availability warranties in the United States market has exacerbated this longer ramp-up period.

Availability and wind speeds There has been an argument put forth by some in the industry that failures and downtime occur more frequently during periods of higher wind speeds. In other words, one per cent of downtime of a wind farm results in a loss of greater than one per cent of energy production.

Garrad Hassan has assessed availability versus wind speed for 43 wind farms using ten minute data. The wind farms included represent geographically diverse locations, varied turbine type and wind farm size as well as varied climatic conditions.

The results conclude that the availability is relatively constant between 7 and 14 metres per second (m/s) – the range in which most energy is generated. Above 15 m/s availability does decrease. This could be due to high load faults, such as vibration alarms, pitch mechanism malfunctions, generator overheating, grid capacity constraints, as well as longer repair times due to access restrictions. At wind speeds below 7 m/s availability is also lower. This is primarily due to down-time scheduled for calm periods.

The assessment results show that where grid constraints do not cause curtailment, one per cent of downtime is roughly equal to one per cent of energy loss. For wind farms subject to grid curtailment, one per cent of downtime equates with approximately 1.3 per cent of energy loss.

Availability and turbine size The trends from Garrad Hassan’s historical wind farm data show that, once teething issues have been resolved – usually in the first year or two of operation – availability is relatively insensitive to both turbine size and wind farm size. During the first year, larger turbines have shown lower availability than smaller turbines, mainly due to the greater complexity of newer and larger units.

Availability and age How does availability vary with wind farm age? Figure 1 shows that availability ramps up over time as teething problems are resolved.

On average, wind farms show availability of approximately 93 per cent in the first quarter of operation. This rises to almost 96 per cent at the end of the second year. Beyond two years, the availability hovers around 97 per cent, with the availability increasing after year six. The increased scatter in the last couple of years is primarily due to the smaller number of farms in the database.

References 1] Garrad Hassan, “Why is America’s Availability Lower than Europe’s?”, AWEA Wind Power Asset Management Workshop in San Diego, January 2008. 2] Garrad Hassan, “Validation of Energy Predictions by Comparison to Actual Performance,” AWEA Windpower 2008 Conference, June 2008. 3] Garrad Hassan, “Understanding Availability Trends of Operating Wind Farms,” AWEA Windpower 2008 Conference, June 2008. All Garrad Hassan reference papers are available from www.garradhassan.com