Wednesday, June 11, 2014

Let the wind blow or forecasting the wind power

I have mentioned recently, that the production of renewable energy sources is difficult to predict. Let's examine some of the challenges.

The difference between production and forecast can fluctuate wildly, sometimes exceeding +/-50% of the installed wind farm capacity.

Chart: differences between production and P50 forecast
(P50 forecast is the average expected energy production)

The distributions of differences between production and forecast have high kurtosis, and both tails are pretty long. 

Chart: Distribution of differences between production and P50 forecast
(P50 forecast is the average expected energy production)

It seems, assumption of the normal distribution of the production, customarily used in wind power forecasting, may not always be correct.

Chart: Assumed normal distribution of (long term) power production
(P50 forecast is the average expected energy production)

It shouldn't surprise then that production often exceeds both conservative P50 and aggressive P90 forecast levels.

Chart: Production (gray bars) vs. P05-P90 forecast band (red and green dotted lines)

On the other hand, production significantly lower than forecast is also not welcome. Since forecasts are the basis of the declared planned production levels on the Day-Ahead Market (DAM), all deviations require costly balancing.

The forecast spread, or the width of the forecast (difference between P90 and P05), which should reflect the forecast probability does not always add any value.

Chart: Forecast spread vs. production


Also, it doesn't help that in addition to the weather-related production forecast, one needs to deal with planned availability of a plant. The planned availability is connected among others with scheduled maintenance, but not everything always goes according to the plan.

As a result, a plant operator needs to deal with additional random variable. It sometimes happens that the plant stops producing long before the planned availability goes to zero, and starts producing, when the planned availability still equals zero.

Chart: Production (black line) vs. planned availability (grey area)

Weather is a factor that affects both production forecast and availability. While wind may be strong, suggesting high production, temperature may lead to operational problems that may not be fully reflected in planned availability. As a result, one may experience noticeable periodic disparity between forecast and production.

Chart: Average monthly difference between production and P50 forecast for different farms

Even with operational information, less than one year of data is too little to decide whether we face here a seasonal effect which should be adjusted for in the next years, or one time event.

To get better alignment between forecast and production the following directions seems promising:
  • use energy storage
  • develop better weather forecasting models
  • include farm operational data it into the production forecast models
  • utilize prices, and price forecasts, where adequate
For more about wind forecasting, see: https://pinboard.in/u:mjaniec/t:res/t:wind_forecasting/

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