Ports’ electricity bills depend on both their actual monthly electricity consumption and their peak electricity demand. They are billed for their peak demand within a calendar year for the next 12 months; as such, 25-30%[1]Iris, Çagatay, and J. Lam. “A Review of Energy Efficiency in Ports: Operational Strategies, Technologies and Energy Management Systems.” Renewable and Sustainable Energy Reviews, 2019. … Continue reading of their monthly electricity bill is attributed to peak power demand alone.
Often this peak consumption of electricity results from crane motions happening at the same time, since the power needed for cranes alone accounts for 71%[2]Parise, Giuseppe, et al. “Comprehensive Peak-Shaving Solutions for Port Cranes,” 2016. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7731941&tag=1. of the electricity usage at each pier of a port. Ship-to-shore (quay) cranes specifically are put under high load during times of day with peak traffic to get containers on or off ships as quickly as possible. “Peak shaving” thus refers to a collection of methods that limit the peak electricity consumption of the cranes in a port through spacing out operations strategically.
As a side effect, peak shaving helps to reduce congestion at different areas of the port and leads to a more stable energy demand at the terminal overall. This is important for container terminals in areas where grid infrastructure is lacking. In addition, reliability of the grid at the port is a current barrier to crane electrification, replacing diesel powered cranes with cranes run by electricity. Peak shaving makes investing in electric cranes and thus reducing greenhouse gas emissions at ports easier because it lessens the load on the grid.
Quay cranes are situated on the quay side of the port, which refers to the region of the port that services ships and transfers containers between the storage yard and the ships. Their operation consists of three basic motions: lifting the spreader (labeled in Fig. 1), lowering the spreader, and moving the spreader horizontally between the ship and the yard.
In the operation of quay cranes, the vertical motions (lifting/lowering the spreader either at the ship or at quayside) cause the most variation in power. The total energy consumption over time of a crane is thus heavily influenced by the lifting and lowering movements.
Because of this, one way to accomplish the goal of reducing peak energy demand is to implement rules that limit the number of cranes lifting simultaneously, as well as limiting the total amount of power drawn at any given moment by all quay cranes. A 2018 study by the University of Rotterdam implemented these two rules in a simulation for the Port of Rotterdam, a port with an annual throughput of 1.6 million TEUs and 8 quay cranes. By limiting the number of simultaneously lifting cranes to half of the total number of cranes and limiting the total power demand to 50% of its pre-peak shaving maximum value, the effect on waiting time is kept negligible while maximizing the reduction in peak power[3]Geerlings, Harry, Robert Heij, and Ron van Duin. “Opportunities for Peak Shaving the Energy Demand of Ship-to-Shore Quay Cranes at Container Terminals.” Journal of Shipping and Trade 3, no. 1 … Continue reading.
In practice, this means that if another crane needs to lift when half of the cranes are already simultaneously lifting, it has to wait until one finishes. Also, if a crane needs to perform an operation, it puts in a request with the projected amount of power needed, and if this amount of power would cause the total power demand of the cranes to exceed 50% of the pre-peak shaving maximum value, then the crane needs to wait. The monetary cost of implementing these rules is negligible. If cranes are already automated, then implementation of these rules can be achieved by modifying their programming; if cranes are not automated, crane operators can be instructed to modify their operation of the crane accordingly.
Admittedly, there are logistical challenges associated with implementing peak shaving rules for manually operated cranes. Crane operators would need a way to observe the number of cranes currently lifting/lowering and know when there would be an opening for their crane specifically to begin lifting/lowering next. This could potentially be addressed through an app accessible by all crane operators, such as the one currently offered by the Port of Boston to truckers to coordinate their trips.
The University of Rotterdam study found that implementing these peak shaving rules, with the aforementioned optimal limits on power demand and crane operations in place, would reduce peak energy consumption by 43% and lead to savings of about $311,000 a year at the Port of Rotterdam. For reference, applied to the Port of Boston, which has 7 quay cranes and in 2020 had a throughput of 268,418 TEUs, operation by these rules would save about $45,700 a year.[4] “Port of Boston FACT SHEET Connecting New England with the World,” n.d. https://www.massport.com/media/v0afqgoh/port-of-boston-fact-sheet-august-2021.pdf.
Energy storage systems can be implemented to compound the benefits of peak shaving. They refer to systems of components like ultracapacitors and flywheels installed in conjunction with electric cranes in order to recover the energy from a crane’s lowering motion and allow it to draw from that energy when it is lifting. Specifically, ultracapacitors work by capturing electrical energy over time and storing it in strong electric fields so that very large amounts of energy can then be discharged when needed instead of relying on the grid to supply peaks. Flywheels work by using the gravitational potential energy released during lowering of the spreader to accelerate a rotor to very high rotational speeds. Then, when the energy is needed again, the rotor produces electricity through induction, as a generator would. These are analogous to how some cars can regenerate energy while braking to use when they are accelerating again, a mechanism offered by many automobile brands from Chevy to Nissan.
Energy storage systems thus even out the energy demand curve even more and reduce the total amount of energy that needs to be drawn from the grid, as seen in the following two figures. While the peak power demand in the usual energy demand curve of five RTG cranes reached over 10,000 kW, after implementing peak shaving rules the demand decreased to 6,000 kW, and after implementing energy storage systems on top of them the demand on the network decreased to 650 kW.
[6]Parise, G., L. Parise, A. Malerba, F.M. Pepe, A. Honorati, and P. Chavdarian. “Comprehensive Peak-Shaving Solutions for Port Cranes.” In 2016 IEEE Industry Applications Society Annual Meeting, … Continue reading
Installing energy storage systems incurs an upfront cost of about $200/kW[7]Parise, Giuseppe, et al. “Comprehensive Peak-Shaving Solutions for Port Cranes,” 2016. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7731941&tag=1. for both flywheels and ultracapacitors, which comes out to about $80,000 per crane for a crane with average power demand of 400 kW[8]Parise, Giuseppe, et al. “Comprehensive Peak-Shaving Solutions for Port Cranes,” 2016. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7731941&tag=1.. When this energy storage system is also implemented for each crane, peak energy demand is reduced by 74.3%[9]Parise, Giuseppe, et al. “Comprehensive Peak-Shaving Solutions for Port Cranes,” 2016. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7731941&tag=1.. For the Port of Boston this would lead to savings of about $79,000 a year, and breakeven after about seven years.
The main obvious disadvantage of peak shaving is that operations will take more time, an average of less than half a minute more per one hour of operations[10]Geerlings, Harry, Robert Heij, and Ron van Duin. “Opportunities for Peak Shaving the Energy Demand of Ship-to-Shore Quay Cranes at Container Terminals.” Journal of Shipping and Trade 3, no. 1 … Continue reading for fully automated cranes. This number would likely be higher for manually operated cranes. To make up for this and decrease resistance to the implementation of peak shaving, container ships can be compensated monetarily for the additional time spent in port.
Although this analysis focused on quay cranes specifically, peak shaving can also be applied to rubber tyre gantry (RTG) cranes, which perform operations in the yard rather than at the quay. Many ports still use diesel RTG cranes, but electrifying these cranes is desirable both for the purpose of reducing emissions and to reduce operating costs. Ports can save up to 30%[11]Alasali, Feras, Stephen Haben, Victor Becerra, and William Holderbaum. “A Peak Shaving Solution for Electrified RTG Cranes,” October 2017. … Continue reading on maintenance and repair costs for electric RTG cranes over diesel RTG cranes, and cut emissions by 25-70%[12]Alasali, Feras, Stephen Haben, Victor Becerra, and William Holderbaum. “A Peak Shaving Solution for Electrified RTG Cranes,” October 2017. … Continue reading. Apart from the initial investment, one large concern for ports in electrifying RTG cranes is the additional load it would put on the port’s energy grid. Peak shaving and energy storage systems can help mitigate these effects and make RTG crane electrification more feasible.
Peak Shaving Timeline
- 2030: All fully automated ports should implement peak shaving by this date.
- Peak shaving requires essentially no new infrastructure in ports which are already automated. Cranes will need to be reprogrammed to lift at specific times, and that’s it.
- A delay of several years is expected, especially for those ports which choose to implement energy storage systems. It will also take time for individual ports to plan their switch to peak shaving so as to minimize disruption to the supply chain.
- 2030 to 2040: Phase in peak shaving for non-automated ports. This will take longer than for automated ports because crane operators need to be trained.
- Ports which benefit most from peak shaving are those with high traffic or a less reliable electric grid.
- Because it stabilizes energy demand from a port, peak shaving makes port electrification more feasible, especially in areas with a less reliable electric grid. This makes peak shaving a key step many ports should take as they transition to electric-powered equipment. As such, implementation of peak shaving should happen at or ahead of the pace of overall port electrification.
References
