## Energy efficiency

The energy efficiency of OPS relative to ship-board generation needs to take into account all the various forms of energy transfer and transformation losses along the transfer route. In addition, the thermal efficiency of a land-based power plant versus ship-board systems needs to be taken into account.

In general, it is estimated that transmission losses from a land-based power plant to the ship will be around 10 to 25% depending on the supply transmission network (i.e. an average value of about 17.5%). This means that from the energy efficiency and CO2 reduction points of view, the land-based power plant needs to be generating less CO2 by at least 10 to 25% compared to ship-board generation. As indicated above, this will vary from case to case and needs specific studies for various ports.

On the other hand, the case in favour of OPS is the operating condition of auxiliary diesel engines while at berth. This should be borne in mind while at berth, since the auxiliary engines normally work at a part load of about 40 to 50%. Under this loading condition, the engine efficiency is lower than the optimum value and the emissions are higher.

There is occasionally discussion on future low-carbon electricity that could be supplied to ports (or generated by a port itself) for supply to ships. There are a number of solutions, such as the use of greener energy in ports. As an example, there have been cases where LNG-based power plants are advocated for port-side power generation. Such cases yield a significant reduction in both CO2 and pollutants and get rid of transmission losses over the grid. The drawback is port self-generation that is not the core expertise of shipping ports. For ports deciding on self-generation, there is a case to help the grid when they have excess electricity and thereby impose less overall load on the grid (MariEMS 2017).