Is it more ecological to take a diesel ferry or drive an EV the longer route?

Question

I'm trying to figure out my daily dilemma: whether it's less polluting to take a diesel ferry that hops me over the river (roughly 160 m) or to drive my electric vehicle (EV) on a 15 km detour to take the nearest bridge?

Ferry

• Ships consume a ton of fuel.
• On the other hand this ferry travels no matter if I'm there or not.
• However this ferry will carry +2 tons of cargo (car+me) which will consume more fuel.

Electric Vehicle

• An EV doesn't directly consume gas but indirectly it does consume some fuel and that can be easily approximated to gas consumption.
• Odd thing is that driving the EV around costs only about 1/5 compared to taking the ferry.
• EV is charged from the power grid. Our energy mix is 55% nuclear, 40% fossil, 5% renewable. However, nuclear plants have a fixed capacity therefore it's sad but fair to assume that excess energy is coming from coal+natural gas peak plants. BTW I also have solar panels on my roof which could flip it to solar but since I mostly charge my car at night -> let's go with a mix of coal+gas as the worst case scenario.

Is there an easy way how I can calculate the amount of extra diesel-fuel the ferry will consume when 2 tons of extra load are added on board?

Time savings are not a factor; environmental gains are more important to me. I'm interested in the overall environmental impact (CO₂ + toxic air pollution both local and global). And I'm looking for the most accurate answer possible.

Motivational image:

Answer 1

Take the ferry!

The short ferry trip likely results in lower emissions of CO₂ compared to the long detour, if the EV is charged from a grid that uses fossil fuels (coal/gas).

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I think there are actually three different change scenarios worth considering here.

1. Marginal: You change your route, but nobody else does. (This is looking at the problem from an individual "carbon footprint" perspective).
2. Group: a significant number of people all change their route.
3. System: the ferry is shut down entirely.

Marginal Scenario (one more car on the ferry)

From my reading I've learned there are several factors that significantly impact fuel usage and emissions on passenger/vehicle ferry transports (eg. trip frequency, speed, prevailing winds, vessel size, fuel type) but number of vehicles on board is not a significant factor. (disclosure: I am not a marine engineer.)

Although I think your assumption about ship's fuel use (below) is intuitively correct, it's such a small factor that it doesn't make a significant difference and cannot be reasonably measured.

However this ferry will carry +2 tons of cargo (car+me) which will consume more fuel.

Although it's tempting to use a measure of fuel use like litres/passenger-km to estimate the impact of a single trip, the reality is that the actual carbon emissions and air pollution don't really change in any significant measurable way whether you board the ship or not.

Therefore, you can assume approximately zero emissions for the diesel ferry. On the other hand, natural gas peaking plants are designed to scale power output up or down based on power demand from the grid, so using electricity to charge your EV likely does result in a very slight marginal increase to carbon emissions and air pollution.

Winner: Diesel Ferry

Group Scenario (many more cars on the ferry)

Vessel speed and number of return trips have a very significant impact on fuel usage. If many more people decide to take the ferry, the ferry may need to perform more round trips and/or operate at a higher sailing speed. (Although sailing speed probably doesn't matter much for a very short-haul trip like the river crossing you're asking about.)

If the ferry needs to make an additional round trip to handle increased demand, then we can distribute the fuel usage across the group using a measure like litres/passenger-km. Luckily there's a study we can refer to: Comparison of Ferry Boat and Highway Bridge Energy Use.

Examinations of three of the systems found that the passenger-MPG of the ferries ranged from 2.61 to 14.00 (1.11 to 5.95 km/L) [0.90 L/km to 0.16 L/km]

On a per-passenger basis, your 0.16 km river crossing would use between 0.0256 - 0.14 litres of diesel fuel. Diesel engines produce 2.7 kg of CO₂ per litre of diesel fuel consumed.¹ So a single trip by diesel ferry would result in around 0.07 - 0.38 kg of CO₂ emissions.

Electric passenger vehicles typically use around 20 kWh/100 km, so your 15 km drive would use around 3 kWh. Natural gas peaking plants emit 0.91 pounds2 (0.41 kg) of CO₂ per kWh produced, so the 15 km drive would result in 1.23 kg CO₂ emissions.

Winner: Diesel Ferry

System

[Maybe I'll add this in the future. This answer is already quite long and I have to stop for now.]