Shipping batteries

Currently the only feasible power source for shipping is fossil fuels. I propose as an alternative, the “shipping battery”, which would take the form of a standard 20 ft shipping container storing  up to 0.25 TJ of energy. The maximum weight at any part of the discharge would be 25 tonnes, the maximum that can be lifted by normal container handling equipment.

Consider a real life example of the container ship, MS Eilbeck, launched in 2005. The specifications are:

Capacity 1620 TEU
Engine power 12640 kW
Fuel consumption 46 tonnes/day
Tonnage 16.324 Gt
Speed 20 knots

The diesel engine efficiency works out at 55% and it produces about one TJ /day,
The operator of a hypothetical battery powered version of MS Eilbeck will need to buy energy at the rate of one TJ for each days sailing. They do not care about the chemistry involved. If it comes in batteries, they are interested in taking them on board fully charged and off-loading them discharged. They are using it as a primary cell and will not care how they are recharged. They will be concerned about cost and weight and/or volume and may be interested in the  discharge efficiency and waste heat removal but not at all about recharge efficiency.

In the hypothetical battery powered version of MS Eilbeck the space and load of the engines and fuel could be given over to containers. Let us guess that this increases the capacity to 2000 TEU (twenty-foot-container equivalent units).The total battery weight will include the reactants, the cell structure, internal support structures, pumps to drive air through the cells and DC-DC convertors to achieve an output at a reasonable voltage and the container itself. If we assume that the reactants contribute half of the total weight, then the number of batteries needed is given in the table:

Chemistry: Li-air Mg-air Mg-air* Zn-air Li-ion Rechargeable
distance, nautical miles days Proportion of total load Destination
854 1.8 0.4% 0.5% 0.2% 1.7% 1.4% UK – Stockholm
3195 6.7 1.4% 2.0% 0.8% 6.5% 16.7% UK – New York
11300 23.5 5.1% 6.9% 2.7% 22.8% 59% UK – New Zealand

* with removal of discharged batteries

(note that the figures for the metal-air batteries are based on idealised chemistry involving only the active elements. Real chemistry will involve electrolytes, containment, catalysts, and compounds of the reactants that are needed but do not themselves store energy. Also that metal-air batteries get heavier as the metal is converted to its oxide. The figures in this table are based on the final weight)

Shipping batteries do not need to be rechargeable at sea, which means that the chemistry of the cell does not need not be reversible and can be optimised for efficient discharge.  Cells could operate at an elevated temperature and reasonably have a warming up time of a few hours. They will need a reasonable storage life but can have a short operating life after start of discharge.
At refuelling stops discharged batteries are unloaded and replaced by fully charged batteries using standard container handling equipment. Discharged batteries are dispatched to a reforming plant where the reactants are topped up and the reaction products removed. The cell components will have a limited life. Replacing cells after a relatively small number of cycles will not reflect on the efficiency at sea.
In the case of metal-air, the oxide would be sent to be reduced at a complex based on a renewable energy source, for example solar, wind, tidal, or wave. The intermittency of the source and the remoteness of the location will be of little concern and the complex could be in the middle of a desert or ocean. The only requirement is that the site of the reduction plant can be reached by transport for delivery of oxide and collection of metal.
The average output of a square kilometre of 30% efficient photovoltaic panels in desert regions near the equator is about 75 MW, or 6.5TJ/day. This would be enough to power six vessels the size of MS Eilbeck. A rough estimate suggests that about 100 km2 of photovoltaic panels would power the entire world shipping fleet. A similar calculation for wind power shows that six 7MW wind turbines working with a 30% capacity factor would be needed to power the MS Eilbeck and about 3600 would power the worlds shipping.
The development of this system would take some decades. Current lithium-air battery technology is nowhere near the energy density implied. The development of the infrastructure and ship propulsion will need a build up of investment on a time scale of decades.-
Magnesium-air is a strong contender, its energy density is not far short of Li-air, the technology has already been developed, Magnesium is abundant and non toxic (about 10% of sea-salt is MgCl) and it may be the least dangerous in a disaster. Also, for marine shipping there might the option of disposing of the MgO into the sea and achieve a better energy density.
Hydrogen – air has not been considered in this page, the main problem being storage. 0.25 TJ of energy could be stored  by 3.1 tonnes of H2. However, with current technology, the best storage density is 6% H2. by weight, ending up with over 50 tonnes/TJ. Cryogenic storage could give a much better energy density, but would work better on a larger scale.
Hydrogen power must be a strong contender for powering shipping but not in the form of shipping batteries.

Other uses:
• Railways. A high speed railway train consumes about 4 MW. If it were in motion for 70% of the time, this translates to 0.25 TJ/day or one battery per day. This application might need a higher rate of discharge than for shipping. It would however get rid of all that expensive overhead wire.
• Emergency back-up power supplies.
• Fairgrounds, instead of those noisy and smelly diesel generators.
• Transmission of energy from remote natural sources.
• Supply of power to remote communities.
• Air, at least for shorter journeys of a few hours, would be feasible, but the batteries might have to work at a much higher current density. The size would be smaller than a shipping container and the weight of the package and auxiliary equipment more constrained. For longer journeys it may be necessary to consider ejecting discharged batteries using an unmanned glider, programmed to land at a strategically placed depot.

Off-loading shipping batteries on-route

Added Feb 2022

As has been noted Mg-air shipping batteries get heavier during discharge which limits the amount of energy that can be loaded at the start of the voyage. Considerably better performance could be obtained if discharged batteries could be off-loaded during the voyage. An obvious way of doing this would be to have refuelling stops which could load new batteries but might just remove used ones. It would have limited container handling facilities and be optimised for fast turn round.

Another option might be to have a battery removal vessel which draws alongside and picks he spent batteries off with suitably agile container handling means. This option would require sufficiently calm waters.

The third option would be to have a supply of inflatable rafts designed to hold a spent battery and put it into the sea probably on a tow. An independent operator would then pick the raft and battery out of the sea.

I imagine the pickup vessel having a catamaran form and it would approach the raft from behind, catching it between the two hulls. The raft would be lifted out of the water and the battery taken out. The raft would remain attached to the tow cable, allowing the raft to be hauled in to be used again.

The deal would probably be that the battery would become the responsibility and property of the pick-up operator. This supposes that its value on the open market would be enough to cover the cost of the operation.

This solution would be more tolerant of rougher seas, but not too rough!

Note : a 20ft shipping container weighing 25 tonnes will have a density of 0.76 and so will float. The container would be sealed with all the air inlets and outlets cut off. The function of the raft is to provide fenders and suitable hydrodynamics as well as added boyancy in the right place.

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