Wind farm energy yield and PPA shortfall
A 20 MW wind farm in Tamil Nadu uses five 4 MW turbines. The independent energy yield assessment gives a gross P50 annual energy production of 62 GWh at hub height. The assessment applies a wake loss of 8%, availability of 96%, electrical and transformer losses of 2%, and grid curtailment of 5%. The project company has signed a power purchase agreement (PPA) requiring delivery of 55 GWh/year, with a penalty if annual generation falls below that level. The lender's sensitivity case uses a P90 that is 12% below the gross P50. The site has monsoon seasonality and the grid operator has already curtailed neighbouring farms during low-demand nights. The asset manager must decide whether the PPA volume is prudent and which loss to attack first.
1. Calculate the net P50 annual energy production and the net capacity factor.
Read the worked answer
Apply the losses sequentially to the gross P50:
- After wake loss: 62 × 0.92 = 57.04 GWh
- After availability: 57.04 × 0.96 = 54.76 GWh
- After electrical losses: 54.76 × 0.98 = 53.66 GWh
- After curtailment: 53.66 × 0.95 = 50.98 GWh
Net P50 ≈ 51.0 GWh/year.
Rated annual generation = 20 MW × 8,760 h = 175,200 MWh = 175.2 GWh.
Net capacity factor = 50.98/175.2 = 29.1%.
This is a plausible capacity factor for a moderate Indian wind site, but it is below the 55 GWh PPA requirement even before P90 uncertainty is considered.
2. Compute the net P90 and judge whether the 55 GWh PPA volume is robust.
Read the worked answer
Gross P90 = 62 × (1 − 0.12) = 54.56 GWh.
Apply the same losses: 54.56 × 0.92 × 0.96 × 0.98 × 0.95 = 44.87 GWh.
Net P90 ≈ 44.9 GWh/year, which is about 18% below the 55 GWh PPA volume. Even the net P50 of 51.0 GWh is below 55 GWh. The PPA volume is therefore not robust: the expected case misses the target, and the lender's P90 case misses it by roughly 10 GWh/year.
The manager should not treat the PPA as achievable without additional capacity, a lower contracted volume, or a curtailment compensation clause. A penalty exposure at P90 is highly likely.
3. Which loss has the largest energy impact, and what mitigation would you prioritise?
Read the worked answer
Using the gross P50 as the reference, the losses are approximately:
- Wake: 62 × 0.08 = 4.96 GWh
- Curtailment: 57.04 × 0.96 × 0.98 × 0.05 ≈ 2.68 GWh
- Availability: about 2.38 GWh
- Electrical: about 1.10 GWh
Wake loss is the largest single item. It should be attacked first through layout optimisation, adequate spacing, wake steering, and possibly taller towers or lower-specific-power rotors that reduce wake sensitivity. However, curtailment is the second-largest and is outside the plant's control; it should be addressed commercially through a grid agreement, storage or a flexible PPA. Availability can be improved through condition monitoring and spare-parts strategy. Prioritising wake alone will not close a 10 GWh P90 gap, so the decision should combine layout review with contractual protection on curtailment.