### Results for limiting number of lifting quay cranes

If the number of lifting STS cranes is reduced, the peak demand decreases, as shown in Fig. 6. What is striking is that the handling time does not increase in the same proportion. A reduction to four lifting cranes leads to an extra handling time of 0.37% (i.e. less than half a minute per hour). The handling time is not impacted that much because of the fact that the maximum peak demand with eight cranes (around 19,000 kW) occurs only briefly. As shown by Fig. 7, for a peak demand of 19,000 kW, an energy demand of more than 9000 kW occurs only 1.1% of the time. Most of the time, the energy demand is lower than 9000 kW.

As can be concluded from Fig. 6, restricting the number of simultaneously lifting quay cranes has a positive influence on reducing peak demand. If one looks at the impact on cost savings on the one hand and handling time on the other hand, one can see the most cost-effective scenario (i.e. yearly savings per extra second handling time) and the total cost reduction against a particular extra handling time.

The optimal cost-effective implementation is to reduce the number of lifting quay cranes to six (eight-crane terminal) or five (six-crane terminal) as can be seen in Fig. 8. In these cases, the savings per extra second handling time are higher than for other scenarios.

If one looks at the total yearly savings against an extra handling time of less than 1.0%, the number of quay cranes can be limited even more. In the case of an eight-crane terminal, this could result in a reduction to four lifting cranes. This saves €195,000, which is 39% of the peak-related costs. For a six-crane terminal, this would result in a reduction to three lifting cranes, which saves €155,000 (reducing total peak demand costs by 38%).

### Results for limiting maximum energy demand

To limit the maximum energy demand per second, the relation between the maximum allowed energy demand and handling time (see Fig. 9) is comparable to the situation where the number of simultaneously lifting STS cranes is limited. The maximum energy demand can be reduced by almost 50% (from 19,000 kW to 9000 kW), while the handling time increases by 0.1%. Only by restricting the energy demand too much (to less than 6000 kW) does the handling time increase by 3–45%.

Restricting the maximum allowed energy demand has a positive influence on reducing terminals’ peak demand. The influence on handling time is only minimal when the allowed energy demand is reduced by approximately less than 50%, whereas it enables terminals to reduce their peak-related energy costs hugely.

When the total savings per year are divided by the extra handling time needed to handle all containers on a yearly basis, the savings per second are obtained, as shown in Fig. 10. For the eight-crane terminal, there is a clear optimum if the maximum demand is reduced to 14,000 kW. In this case, the savings are €141,000 per year, while the handling time is 0.01% faster. This results in a negative saving^{Footnote 1} of €197.10 per second. Restricting the energy demand to 17,000 kW, 18,000 kW, or 19,000 kW also results in a negative saving. However, in these cases, the total savings are only €12,000 to €61,000. Restricting the energy demand to 13,000 kW gives a saving of €251.53 per second, absolutely the highest saving seen.

For the six-crane terminal, the highest (negative) saving per second is obtained by reducing the maximum energy demand to 12,000 kW (−€180.27 per second) or 13,000 kW (−€115.86 per second), saving the terminal, respectively €80,000 or €53,000 per year. The highest positive value (meaning a saving against extra handling time) is achieved by reducing the energy demand to 11,000 kW. This saves €79.40 per second against an extra handling time of 0.01% and a total cost saving of €107,000.

### Analyses of results

The outcomes presented in subsections 5.1 and 5.2 show a clear result: it is possible to reduce peak energy demand, saving up to €250,000 per year against a little extra handling time (in some scenarios without extra handling time). The bigger the restriction of the rules of operation is, the larger the effect on the operations, as visualized by the number of temporarily delayed containers. However, since more and more container terminals are operating automatically, this can be integrated in the terminals’ software. Delay of containers should not be a problem if one of the rules of operation is implemented.

Regarding total cost savings, the optimal solution is to reduce the maximum energy demand per second by 50% of the original highest observed energy demand. By doing this, €160,000 to €249,000 (40–48% of peak-related costs) can be saved annually. The impact on terminal operations is small, as the extra handling time is only 0.1% and the number of temporarily delayed containers is 2.2–3.7%. The savings per second are €15.53 to €20.12.

If the savings per second extra handling time are considered to be more important than the total annual savings (for example to compensate container carriers, see next section), the maximum energy demand can be reduced by 30–35% (13,000 kW/s–14,000 kW/s) for an eight-crane terminal or by 5–15% for a six-crane terminal (12,000 kW/s–13000 kW/s). Because the handling time is hardly affected, the extra savings per second are very high, especially because some of these scenarios showed a small quicker handling time against a restricted energy demand.

If the number of lifting quay cranes is reduced by 50%, the peak-related costs are reduced by approximately 40% (saving up to €195,000 per year). The extra handling time is only 0.37–0.44% (less than half a minute per hour handling time) against a saving of €2.96–€5.92 per second extra handling time. By reducing the number of simultaneously lifting quay cranes by less than 50%, the total savings are less, but the savings per second do not increase. By reducing the number of lifting quay cranes by more than 50%, peak energy demand decreases even further, but the handling time increases drastically. The optimal solution would therefore be to reduce the number of simultaneously lifting quay cranes by 50%.