Operational Optimization With Look-Ahead
The calculation of optimization is a complex process. It is not carried out individually for each time step, but rather in several calculation runs for one period in the future (Look-ahead = time frame for calculation). To avoid reaching the limits of computer technology, the Look-ahead and the Result time frame must be set for the respective active Simulation node in the Simulation ribbon in the Operational optimization group (see following figure).
If, for example, an action must be performed at a certain point in time in order to save costs at a later point in time, the simulator is given a time frame for the calculation with the look-ahead. The look-ahead method is mainly used for storage loading and unloading strategies (e.g., E-Mobility) and Operational Side Conditions (e.g., starts per day, load change ramp). When using the Priority component, the Look-ahead must be set to 0.
With a Look-ahead of 24 hours, the optimization is calculated for a period of 24 hours in the future. This time period is considered simultaneously in the simulation.
From this calculated period, the results from the first part – the Result time frame – are transferred to the final result of the optimization. The calculation process then repeats for the following time periods until the entire time period is optimized. The Result time frame (time frame of result transfer) is marked as Transfer Results in the following figure. It serves as a step size for the optimization and should be smaller than the Look-ahead. By default, it has the value zero, then only the first time step is transferred as the result.
The Look-ahead determines the size of each optimization problem while the Result time frame controls the total number of optimization problems calculated.
The larger the Result time-frame, the shorter the calculation time.
The complete computing time depends on the model complexity and the computing capacity.
The following applies to the Look-ahead: as little as possible and as much as necessary!
The reference value for the Look-ahead of 24 h can be significantly exceeded with Gurobi.
The time frame can be specified in any time unit (default setting: hour) and does not have to correspond to the number of data points.
For example, with a resolution of the values in quarter hours and a time frame of four hours, a total of 17 time steps are coupled because the current time step is not included in the Look-ahead, and looking 4 hours into the future from the first time step results in 1 time step + 4 x 4 time steps (4 hours divided into 15-minute steps) = 17 time steps.
With an optimization with a total number of 10 days (240 time steps with one hour each) with a Look-ahead of 23 hours and a Result time frame of 11 hours, a total of 20 optimization runs with a preview of 24 time steps are carried out (actually only 19 optimization runs because the last optimization problem writes everything back because nothing follows afterwards).
The disadvantage of an optimization with look-ahead is the very long computing time with long time frames. The larger a storage or the period of time that is to be buffered with a storage, the larger the time frame must be selected. The optimization problem with 24 time steps Look-ahead is 25 times as big as without Look-ahead. The computing time increases generally by more than 24 times, it can easily rise from minutes to hours.
On the one hand, a large Look-ahead can significantly increase the simulation duration; but on the other hand, the larger the Look-ahead, the more costs can be saved because the optimization looks further into the future.
Please note the advice on Year-round coupled optimization without time series compression. Look-ahead and Result time frame are automatically set with year-round coupled optimization and structural optimization without time series compression. In these cases, they are grayed out in the ribbon and output as Utilized look-ahead and Utilized result time frame on the Simulation form Metrics → Optimization main stage.
