> If you assume 170 wh/kg

I've wondered if thermal storage might be superior. Lower round trip efficiency if one uses resistive heating, but I think that would be ok.

Lithium hydride, heated to melting, stores something like 4 MJ/kg, more than 6x the specific energy of your assumption there (admittedly with loss on discharge due to the losses in the turbine.) If that is too expensive, graphite is another possibility, at even higher temperature.

▲

SigmundA 3 days ago | parent [-]

Round trip efficiency would be very poor, looks like thermal storage is around 75% efficient for the heat then the heat engine (turbine) maxes out around 45% so maybe round trip 33% efficiency if you lucky.

So that gives you around twice the wh/kg but you must keep the heat energy for the entire voyage which is constantly being lost once the onboard storage is heated up. Not sure what that look like I imagine it would be difficult to keep lithium hydride at 680C very efficiently or safely in an ocean going vessel for any length of time.

▲

pfdietz 3 days ago | parent [-]

> looks like thermal storage is around 75% efficient for the hea

That seems unreasonably low. Thermal losses can be made arbitrarily low with insulation, and this is fairly large scale, so insulation can be thick and volume per surface area can be kept low.

▲

SigmundA 3 days ago | parent | next [-]

Would love to see any data you have, what kind of storage tank on a ship could keep 680C lithium hydride insulated without significant losses for say a 30 day voyage?

▲

pfdietz 3 days ago | parent [-]

We were talking about a 180 hour battery (7.5 days), not a 30 day battery.

But back of the envelope: 2.7 GWh of heat in LiH at 4 MJ/kg and 820 kg/m^3 is a sphere 9m in radius. If we put a 1m layer of microporous silica insulation (conductivity ~0.03 W/mK) this gives a thermal time constant of 300 days.

	▲	SigmundA 2 days ago \| parent [-]
		Sorry I was considering a standard container ship voyage at 30 days. A 9m sphere of 1m thick Aerogel will probably cost above $500k just for the material. Also lithium hydride itself reacts violently with water, not sure if you want to have a 9m radius 5 million pound sphere of it on a water vessel, this is the "safely" part.

▲

pfdietz 3 days ago | parent | prev [-]

I mean, can be kept high.