When we first started OutSail our goal was to find a way to move a ship with zero emissions, cost-effectively, while matching fossil-fuel performance.

Three are three parts to the above goal:

  1. Move a ship with zero emissions. This is essential to limit the impact of climate change.
  2. Do so cost-effectively. This is difficult, but there’s a great lever: Fossil-fuels are relatively expensive.
  3. Do so while matching fossil-fuel performance. Combustion engines are powerful, and fossil fuels are energy dense. As a result, fossil-fueled ships move very quickly for long distances.

Moving a ship requires energy. Moving a ship with zero emissions requires clean energy. Clean energy can come from many sources (Solar, Wind, Nuclear, Hydroelectric, Geothermal…) , but in order to move a ship, it must be stored in advance, or captured on the vessel itself.

In this article, let’s examine some commonly touted technologies for storing and capturing clean energy to see why they alone can’t decarbonize the maritime world.

Our Test Ship: 

A relatively small standard size for container ships is the ‘Panamax’ size vessel. In round numbers, this vessel carries 5,000 twenty-foot shipping containers, and at 20 knots burns roughly 1200 metric tons of bunker fuel to cross the Pacific ocean. Bunker fuel has an energy density of roughly 40 Megajoules/kg, meaning that moving the vessel required 48,000,000 megajoules (MJ) of energy. This is enough energy to power roughly a California home for over two thousand years.

Of course, not all of this energy directly moves the ship. In the best case, more than half the energy in the fuel is wasted by the ship’s engines as heat, and more than a quarter of the energy output from the engines goes to the hydrodynamic inefficiency of the ship’s propeller itself.

Let’s see what it would take to move our test ship across the pacific with some commonly-proposed decarbonization technologies.

Batteries: Too large, heavy, and expensive.

Batteries are devices that convert stored chemical energy into electricity by means of an electrochemical reaction. A purely battery-powered ship would store electricity purchased while it is docked, then use that energy while under way to drive a propeller via an electric motor.

Batteries have the big advantage of not requiring any combustion engine and therefore having no tailpipe emissions. Unfortunately, even if we assume that batteries can be charged at the dock with clean energy, a big assumption, they have two big disadvantages: cost and energy density.

Batteries and electric motors don’t waste nearly as much energy as an internal combustion engine does. To show how far batteries are from working, we’ll assume that ours are perfectly efficient. This means that we need to store roughly 24,000,000 MJ in our ship’s batteries to cross the Pacific once.

Let’s use Tesla’s ‘Megapack’ battery units as a basis for comparison. Each unit weighs 38 metric tons, costs $1.58 Million, is roughly the size of a single 20-foot container, and stores ~14,100 MJ (3916 kWh).

In order to get our 24,000,000 MJ, we need 1,700 megapacks, or 64,600 tonnes of battery.

We can’t fit this on the ship! Our ship can carry roughly 52,000 tonnes of cargo, fuel, and water. In order to use purely batteries, we have to replace our entire cargo by weight, and then some! We also have to replace 1,700 of our 5,000 containers with batteries, losing over a third of our cargo volume.

Things get worse when we examine the cost of the batteries. The batteries to power our vessel will cost $2.6Bn. Even at the peak of the early 2020s explosion in container orders, the price of a brand new Panamax vessel was $65.5 Million. For one set of batteries, an operator could purchase 40 new vessels.

Purely battery-electric ships might be great for short voyages, but for the transoceanic needs of much of today’s fleet, they simply won’t work.

E-Fuels: Inefficient and Expensive

Let’s be totally clear: E-fuels such as Ammonia, e-Methanol, and Hydrogen, are electricity storage solutions just like batteries. A battery stores electricity using a reversible electrochemical reaction. Non-biological alternative fuels are manufactured using an electrochemical or thermochemical reaction, which uses electricity, and burned (or put in a fuel cell) to generate power. Manufacturing an alternative fuel will always take more electricity than would be released burning that fuel.


Hydrogen is a flammable, volatile gas. There are significant cost issues with hydrogen, but let’s cover those when we talk about methanol and focus on the elephant in the room: storage.

Hydrogen has an extremely high specific energy, with roughly 142 MJ stored per kg of hydrogen. Hydrogen fuel cells are very roughly as efficient as combustion engines, so let’s use our required 48,000,000 MJ from our first, fossil-fuel based analysis. We’ll need 338,000 kg of hydrogen.

Hydrogen gas at standard temperature and pressure (freezing temperature and sea level) only has a density of roughly .1 grams per liter. In order to store the tonnes of hydrogen that we’ll need to power our ship, we’ll have to compress it.

How much can we compress it? When we compress a gas like hydrogen, we cause potential safety issues. If a high-pressure tank fails, the gas inside will vent out in a high-energy jet or even cause the tank to explode; even before the hydrogen inside ignites.

Welding gasses, even flammable ones, are commonly shipped at 2000-2600 pounds per square inch (PSI). Let’s use 2600 PSI as our benchmark. How much volume does it take to store 338 tons of hydrogen at 2600 PSI and 20 C?

Roughly 28,500,000 liters. A single 20-foot container is 24,000 liters; we will need to reduce our cargo capacity by roughly 20% (1188 containers) to store all our hydrogen. This is economically impractical, to say nothing of the mass and cost of the required high-pressure vessels.

There is another way of compressing hydrogen: liquifying it. By reducing the temperature enough, we can get hydrogen to be a liquid and keep it inside tanks at roughly one-fifth the above volume. This is common practice in rocketry.

Unfortunately, hydrogen boils at -259.2°C . Metals exposed to this extreme cold become brittle, and the hydrogen can leak through the tiny pores in most welds. Even if we insulate our tanks as in rocketry, between 1 and 5 percent of the hydrogen will boil off every day as heat makes its way through the insulation. These problems massively drive up the cost of tanks and handling equipment, and make liquid hydrogen impractical for applications outside of aerospace.


A better way to store hydrogen is to combine it with carbon to make a hydrocarbon fuel. The most popular of these is methanol. 99.8% of today’s methanol is made by steam reforming-methane and is a fossil fuel. In the future, we may be able to synthesize “e-Methanol” as a green fuel by combining carbon dioxide and hydrogen.

This sounds great, but there are efficiency issues which drive the cost up. In order to make green methanol, we need to first make green hydrogen. How does this work?

We take the cheapest green electricity we can get and use it to split water into hydrogen and oxygen. Unfortunately, this process is inefficient, wasting at least 20% of the electricity put in, and requires significant capital to build electrolyser plants.

An in-depth study estimated that in the best case, green methanol would cost roughly $1000 per ton. At a first pass, this is 50% more than the ~$650 per ton that shippers pay for bunker fuel as of the time of writing. Unfortunately, there’s another catch – methanol is half as energy dense as bunker fuel. For the same amount of energy ,we’ll need to burn twice as much! This means switching to methanol will, in the best-case-scenario, drive fuel costs up by a factor of three. There must be a better way.


Ammonia is another neat way of storing hydrogen – combine 3 hydrogen molecules with one nitrogen atom to produce a fuel that isn’t a hydrocarbon but can be burned much like one. Green Ammonia is manufactured by combining green hydrogen with nitrogen captured from the air, and costs are driven by the same economics as above, with an added complication: Ammonia is both corrosive and toxic. Exposure to high concentrations of ammonia in the air can cause immediate burning of the eyes, nose , and throat. Enough exposure can cause lung damage and eventually death. Safely dealing with ammonia is possible but will be expensive, likely driving the end cost to be significantly higher than that of Methanol.

Biofuels: Another expensive option.

Biofuels are another interesting proposed solution for decarbonizing shipping. Unlike the electrically derived fuels discussed above, biofuels are produced by adding alcohol to a vegetable oil and reacting it with a catalyst.

Unfortunately, the required plant mass makes biofuels expensive with prices linked to commodity markets. Currently, Biodiesel, which is roughly as energetically dense as bunker C, runs ~$1800/ton; nearly three times the price.

Predicted climate-driven agricultural issues into the 2030s and 2040s will drive the price of biofuels up, not down. These are another solution which sounds great but are economically limited.

Solar: The energy isn’t there.

Why batteries, solar, and new fuels alone can’t be cost effective 2

Many companies are selling solar-powered yachts or touting so-called ‘solar sails. Could solar alone power our vessel?

Solar energy is energy collected from the sun’s rays. The most common (and easiest to install on a ship) way of collecting solar energy is a photovoltaic panel. Photovoltaic panels use semiconductors to absorb energy from the sun and generate electricity.

Our ship is 294 meters long with a 32 meter beam. Let’s assume for a moment that we had a perfect way of covering the whole top of the ship in perfect solar panels. How much energy can we collect from the sun on our transpacific voyage?

We have 9,408 square meters of collection area on our ship. Ideally, we would install tilted panels which face the sun, but our ship turns! Because we don’t know which direction to tilt our panels, let’s assume for a moment that they’re flat. Modern solar panels can generate around 200 watts of power per square meter in the best-case conditions; roughly enough power to light two lightbulbs. To show how far solar is from working, I’ll assume that we have perfectly efficient panels which can generate 1000 watts per square meter. This gives our whole ship 9000 kw of power generation potential assuming that it’s 100% sunny all the time with the sun directly overhead.

Unfortunately, night, clouds, and changing sun angles mean that we’ll never see 9000 kilowatts of average power. In the Pacific Ocean, we’ll be lucky to see 1800 kilowatts of average power from our solar panels.

Our ship’s engines use roughly 30,000 kilowatts to move the vessel at 20 knots! We would need 17 ships worth of perfectly efficient solar panels (85 with the limits of current technology) to power us across the Pacific. Solar panels may help us run the air conditioning for our crew, but they won’t move the vessel on their own.

The promise of wind:

The wind is the only source of energy which can cleanly and cost-effectively move our vessel across the ocean. In our next blog post, we’ll examine how exactly this can work.

Until next time,