Future Ship Propulsion

Ship propulsion poses one of the more troubling post-carbon problems. It should be noted that ships were once powered by wind energy, sometimes supplemented by oars rowed by slaves. This form of propulsion was very unsatisfactory and renewable energy was replaced by fossil fuel derived energy during the 19th century. There were a variety of motives for phasing out renewable energy.

* Sailing ships required large crews
* Winds were unreliable, making schedules impossible to keep
* Ship size was limited by the limitations of wind power
* Sailing ships were more vulnerable to loss during storms than powered ships
* Wind powered ships had more limited speed
* Wind powered ships were less useful for shipping perishable agricultural products
* Long range travel by wind powered ships was uncomfortable and could take months

Given these factors, it is unlikely that wind energy will successfully replace fossil fuel power in ocean commerce. Solar energy also seems impractical as a means of powering ships. There appears, at present, no serious effort to develop a renewables powered solution to the post carbon shipping problem. The United States Navy is examining expanding the use of nuclear power in its fleet (a tip of the hat to Rod Adams is in order). The Congressional Budget Office has examined the cost effectiveness of conventional nuclear power by the United States Navy. The Navy has fewer costs constraints imposed on it, than commercial shippers do. Thus if something is too costly for the Navy, it is totally impractical for commercial shippers.

The current Naval objection to building nuclear powered surface ships focuses on reactor costs. Conventional nuclear reactors are too expensive.

Estimates of the relative costs of using nuclear power versus conventional fuels for ships depend in large part on the projected path of oil prices, which determine how much the Navy must pay for fuel in the future. The initial costs for building and fueling a nuclear-powered ship are greater than those for building a conventionally powered ship. However, once the Navy has acquired a nuclear ship, it incurs no further costs for fuel. If oil prices rose substantially in the future, the estimated savings in fuel costs from using nuclear power over a ship's lifetime could offset the higher initial costs to procure the ship. In recent years, oil prices have shown considerable volatility; for example, the average price of all crude oil delivered to U.S. refiners peaked at about $130 per barrel in June and July 2008, then declined substantially, and has risen significantly again, to more than $100 per barrel in March of this year.

CBO regularly projects oil prices for 10-year periods as part of the macroeconomic forecast that underlies the baseline budget projections that the agency publishes each year. In its January 2011 macroeconomic projections, CBO estimated that oil prices would average $86 per barrel in 2011 and over the next decade would grow at an average rate of about 1 percentage point per year above the rate of general inflation, reaching $95 per barrel (in 2011 dollars) by 2021. After 2021, CBO assumes, the price will continue to grow at a rate of 1 percentage point above inflation, reaching $114 per barrel (in 2011 dollars) by 2040. If oil prices followed that trajectory, total life-cycle costs for a nuclear fleet would be 19 percent higher than those for a conventional fleet, in CBO's estimation. Specifically, total life-cycle costs would be 19 percent higher for a fleet of nuclear destroyers, 4 percent higher for a fleet of nuclear LH(X) amphibious assault ships, and 33 percent higher for a fleet of nuclear LSD(X) amphibious dock landing ships.

The CBO's cost estimate may be flawed by an overly optimist estimate of the cost of oil, but this is enough to establish that it will be expensive to build a future nuclear powered Navy. We can conclude from this that the cost of conventional nuclear power will impact sea based commerce, and may make the cost of conventional nuclear power impractical.

There are several possible alternative nuclear options which hold the possibility of lowering nuclear costs. These include liquid metal cooled fast reactors, Molten Salt Reactors, Molten Salt cooled solid fuel reactors, and pebble bed reactors. There is some history of liquid metal reactor use for naval purposes. It is not at all clear that liquid metal nuclear technology offers a cost advantage when compared to conventional reactors. In addition the history of liquid metal fast reactor use at sea, suggests reliability problems. In addition there are safety questions about fast reactors. Finally, fast reactors require large inventories of fissionable materials. Large inventories of fissionable materials may limit the number of ships that might be equipped with fast reactors. Thus fast metal cooled reactors may not be the best choice for commercial shipping motive power.

A second alternative nuclear option would involve the use of pebble bed nuclear technology. Gas cooled Pebble Bed Reactors are considered highly safe. However, gas cooled Pebble Bed Reactors have a large core. Large cores increase reactor manufacturing costs. In addition large cores occupy space that could be occupied by cargo or passengers. Liquid salt cooled Pebble Bed Reactors can have far mor compact cores, and manufacturing them would seem to potentially cost less than gas cooled PBRs. Liquid salt cooled PBRs can be refueled with few problems, and require small inventories of fissionable materials compared to fast reactors.

Like Pebble Bed Reactors, Molten Salt Reactors are very safe. They are simple and compact, lowering manufacturing costs. The principal difference between the MSR and the molten salt cooled PBR is that the fuel is disolved in the coolant salt of MSRs, while in molten salt cooled PBRs he fuel in embedded in graphite pebbles. It is easier and probably less expensive to process fission products out of a carrier salt than out of graphite pebbles. Graphite in the MSR core lowers fissionable inventory requirements as it does in PBRs. Thus it would appear that molten salt cooled reactors offer a potentially economical solution to the post-carbon ship propulsion problem.