  Ports are striving for innovative technological solutions to cope with the
ever-increasing growth of transport, while at the same time improving their
environmental footprint. An emerging technology that has the potential to
substantially increase the efficiency of the multifaceted and interconnected
port processes is the digital twin. Although digital twins have been
successfully integrated in many industries, there is still a lack of
cross-domain understanding of what constitutes a digital twin. Furthermore, the
implementation of the digital twin in complex systems such as the port is still
in its infancy. This paper attempts to fill this research gap by conducting an
extensive cross-domain literature review of what constitutes a digital twin,
keeping in mind the extent to which the respective findings can be applied to
the port. It turns out that the digital twin of the port is most comparable to
complex systems such as smart cities and supply chains, both in terms of its
functional relevance as well as in terms of its requirements and
characteristics. The conducted literature review, considering the different
port processes and port characteristics, results in the identification of three
core requirements of a digital port twin, which are described in detail. These
include situational awareness, comprehensive data analytics capabilities for
intelligent decision making, and the provision of an interface to promote
multi-stakeholder governance and collaboration. Finally, specific operational
scenarios are proposed on how the port's digital twin can contribute to energy
savings by improving the use of port resources, facilities and operations.
