The goal of this thesis has been to develop a concept exploration tool for zero-emission double ended ferries, that determines the feasibility of different energy systems, creates corresponding design parameters and helps decide between zero-emission energy types. The research goal was achieved in three steps. Firstly, knowledge was gained on double ended ferry design, double ended ferry operations, zero-emission energy systems and the early design stage. Research into zero-emission energy systems resulted in lithium-ion battery energy systems, hydrogen fuel cells energy systems and supercapacitor energy systems being identified as likely feasible energy systems for double ended ferries. In the second step, the knowledge was used to create the concept exploration model. This model consists of a set of algorithms that determines technical and economical parameters of double ended ferry concepts for each of the identified zero-emission energy types, for a provided transportation need. The model also does this for a diesel-electric energy system, that serves as a benchmark. Equal economical comparison of the concepts is aided by providing the net present value of the investment in the energy system, for each concept. In the third step, the model is transformed into the concept exploration tool, through implementation in the Python programming language. This step also includes obtaining results from the concept exploration tool. One use of the concept exploration tool is by shipyards, in their early design process. The tool can provide naval architects with insights into which zero-emission energy systems are possible for a provided transportation need, as well as the characteristics of the corresponding concept designs. A second use for the concept exploration tool is the analysis of large amounts of input. This can provide insights into how design input parameters influence the concept designs and the preferences for certain energy system types. This was done by performing a parameter variation study. The results of analysing the obtained data are expressed through 24 conclusions. These are divided into four sets, per party for which the conclusions are expected to be of the highest interest. These are Energy System Suppliers, Governments, Ferry Operators and Shipyards. The most important conclusion of this thesis is that zero-emission energy systems are technically feasible and can be cost competitive with diesel-electric energy systems. This to such an extent, that they should be seriously considered for every double ended ferry. Based on the net present value of the energy systems over the vessel lifetime, battery energy systems are most frequently cost-competitive with diesel-electric systems. Hydrogen energy systems are only competitive in highly specific cases and mostly too expensive to be cost competitive with diesel systems. Supercapacitor energy systems are cost competitive when upward of 30 daily trips are sailed, over short distances.