This study presents a novel approach to compare various alternative energy carrier and power system options with the Ships Power and Energy Concept (SPEC) exploration tool. We first introduce the baseline vessel and introduce the various fuels and technologies considered. We consider marine diesel oil as a baseline and alternative energy carriers hydrogen, methanol or ammonia and batteries. We review the fuels, their current and future availability and their impact on the environment. Moreover, we review the power system technologies, considering diesel generators running on marine diesel oil, methanol, ammonia or dimethyl ether, fuel cells running on hydrogen or methanol and batteries as the only power supply, recharged when ashore. Furthermore, we review power system designs with the combinations of fuel and power supply identified above and will consider: the mass and volume of the power system configurations and energy storage, fuel or batteries; the estimated capital and operational expenditure; technology readiness level; logistic availability of the fuel; and the estimated yearly CO2 emissions. Electrical propulsion with electrical power supply from internal combustion engines running on methanol appears a mature and cost-effective candidate to achieve the reduction target of 70% reduction in CO2 emission and its related dependancy on fossil fuels, with a 10% increase in capital cost and double fuel cost.