A battery electric cargo submarine: a techno-economic assessment
*Christophe Pochari, Bodega Bay, CA
Abstract: An electric submarine powered by lithium-ion batteries with a cruising speed of under 8 knots is proposed for ultra-low cost emission free shipping. This concept, if developed according to these design criteria, would be able to lower the cost of shipping compared to conventional container ships. We believe this concept could revolutionize shipping. The relatively small size of the submarine allows it to avoid large ports, reducing congestion, and increase turnaround time, allowing the shipper to delivery directly to virtually any calm shoreline.
Due to the absence of resistance generated by the stern and bow waves, a submarine can sometimes be a more efficient marine vehicle than a conventional vessel, especially in rough oceans, as there exists a substantial parasitic load engendered by waves crashing in the incoming direction. Additionally, submarines are safer due to the immunity from storms, swells and rogue waves. Furthermore, current velocity diminishes rapidly with depth, so if currents are in the incoming direction, there is an additionally reduced parasitic load on the vessel. The risk of cargo losses are also minimized, as container ships routinely lose valuable cargo at sea.
The resistance at 6.2 knots is estimated to be 0.3 lbf/ft2 of wetted area, at 8.4 knots, it’s estimated to be 0.5 lbf/ft2. This means a submarine capable of carrying 950 cbm of cargo only needs a paltry 90 hp at 6.2 knots. The propulsive efficiency of large low vessel ship propellers is in the order of 25-30 lbf/shp. With 28 lbf/hp being a realistic estimate for this size submarine.
Henrik Carlberg at NTNU (Norway) estimated that a commercial submarine designed for oil and gas applications would use 165 kW at 6.2 knots with a wetted area of 1920 m2.
The submarine design featured here has the same wetted area, with net hull volume minus ballast tanks and battery volume of 2300 cbm.
This 2300 cbm cargo submarine would take 1000 hours to traverse 7000 miles and consume 215,000 kwh along the way, costing $6450 at $0.03/kWh, with a 3000 cycle Li-Ion cycle life OPEX of $3.4/cbm at $110/kW battery prices (2170 Panasonic), or $7800 per trip. The total cost per cbm would be $12.4/cbm ($930/40 ft container equivalent) including an empty trip back. Excluding the round trip, assuming goods are transported the other direction, the cost per 40 ft container equivalent would be $465, far below the pre-Covid price of $1500 (Freightos Baltic index) for existing bunker fuel powered mega-container ships. For such a small vessel, using non-hydrocarbon fuel, it is remarkable the cost is close to massive highly optimized container ships.
Construction costs for the submarine have been estimated at $5,000,000, with the steel structural materials costing $1,100,000 at a price of $800/ton. With a lifetime of 40 years, hourly CAPEX and depreciation is minimal.
The submarine would be unmanned, saving on crew costs, and would require no AIP as the use of a manned crew and air-breathing combustion propulsion is eliminated
Maximum displacement: 3,890,000 kg
Structure weight: 700,000 kg
Cargo volume: 2300 cbm (186 kg/m3 cargo density avg)
Wetted area: 1990 m2
Ballast volume: 530 m3: 543,000 kg
Battery at 200 wh/kg and 500 wh/liter (rectangular): 268,000 kWh (80% depletion): 1,340,000 kg: 536 m3
Total loaded weight: 2,850,000
Front and rear weights: 700,000 kg steel plates
Motor power: 270 hp
Length: 72.9 m
Diameter: 9.2 m
The Images below are sourced from Henrik Carlberg’s thesis on a commercial submarine for oil and gas. Skin friction estimates were corroborated with Martin Renilson’s estimate of 59,000 newtons for a wetted area of 1400 m2 at a speed of 9.7 knots. To adjust for our lower speed, a ratio of approximately 2.2 is found from the CFD analysis of Moonesun et al and Putra et al from 6 to 9 knots. Propulsive efficiency is calculated using the formula below.