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 efficiencyIn 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 Li-S Na-S
distance, nautical miles days Proportion of total load Destination
854 1.8 0.4% 0.8% 0.8% 1.7% UK – Stockholm
3195 6.7 1.4% 3.1% 3.0% 6.2% UK – New York
11300 23.5 5.1% 11.1% 10.6% 21.8% UK – New Zealand

(note that these figures 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)

Shipping batteries do not need to be rechargeable at sea, which means that the chemistry of the cell 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 Lithium-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 intermittancy 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 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.
Even though NaS is far more advanced, there is major problem with sulphide based batteries. We are looking at a future where there will be millions of batteries in circulation. Accidents are inevitable and any battery has a lot of stored energy. A Lithium-air battery would only burn at a rate limited by ingress of air. A metal-sulphur battery has all the oxidant in-situ and would burn like a massive firework. Also damaged sulphur based batteries in contact with water could react to release large amounts of very toxic hydrogen sulphide.
Magnesium-air has been left in the running even though the technology may be even further away because it may be the least dangerous in a disaster. Also, for marine shipping there maybe the option of disposing of the MgO into the sea and achieve the best energy density.
Hydrogen – air has not been considered, the main problem is 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.

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