Spray's cruising power - Part 2

 

I've occasionally wondered : If Spray's diesel engine ever died (not likely but you never know), rather than spend $20k for a replacement diesel, would I want to consider electric propulsion? Rip out the massive diesel engine and the huge fuel tanks and replace with a small electric motor and appropriate bank of batteries. Imagine the benefits :

 

- No smelly fuel or diesel exhaust. No chance of a fuel leak.

- Nearly silent and vibration free operation.

- No FNR transmission needed.

- Much less waste heat means probably no seawater heat exchanger is needed.

- Much less maintenance. No oil changes. No fuel filter changes.

- Smaller carbon footprint. Motor would be 90% efficient rather than 39% efficient (see last blog entry).

- Could recharge batteries from the sun? No fuel expense?

 

That's a lot of benefit! Is it for real? Let's look at the realities.

 

The toughest reality to face is the fact that we must store the energy we use to push Spray along. Diesel's biggest asset is how densely it stores chemical energy : 136 million joules per gallon! Even with its piddling 39% efficiency Spray's 240 gallon diesel capacity yields a cruising range of about 1,000 miles. Even though we usually fill up before the tanks are half empty, we go weeks without even checking the fuel level, knowing that only after 400 or 500 miles of cruising will we want to top off the tanks.

 

In the electrical world the storing of the needed energy is much tougher. The best solution (today) is the electrochemical battery, either the traditional lead-acid type or a newer lithium-ion type. Lets see how many batteries we would need. To help out the electric solution we'll only require a cruising range of 100 miles (3 days typical cruise or 2 days if we push it).

 

100 (nautical) miles at a speed of 6.5 knots. That's 100 / 6.5 = 15.4 hours of cruising. Recall that Spray needs 30 HP of engine output for cruising, and that equals 22.4 kW of power. Since our electic drive motor is 90% efficient it will need 22.4 / 0.90 = 25 kW of input power from the batteries. Multiply the 25 kW by the 15.4 hours (energy = power x time) and you yield a needed battery capacity of 385 kwh.

 

Well, not quite. Unfortunately, as a battery spits out amps (especially at high rates), it heats up, and that heat is wasted energy. Let's assume that our battery is 90% efficient while discharging, ie. that 90% of its stored energy comes out as usable juice and 10% is lost to heat. Now our needed battery capacity is 385 / 0.90 = 428 kwh.

 

How many 4D marine wet-cell (basic lead-acid) batteries do we need for our 428 kwh capacity? A typical such battery might have a rated capacity of 200 amp-hours, but the battery will live much longer if we only half-discharge it, using 100 amp-hours. Since electrical power in Watts is current x voltage we get 100 amp-hours x 12 volts = 1200 watt-hours = 1.2 kwh capacity per 4D battery. It'll take 428 / 1.2 = 357 of these bad boys to give the needed capacity. Each 4D battery weighs about 100 lb so we are looking at 35,700 lb of total battery weight.

 

Houston, we have a problem! Spray will need to tow a barge to hold the 357 heavy batteries needed to move her a mere 100 miles. Towing that barge will seriously impact our 6.5 knot cruising speed. Not only that but at around $200 each, we'll need to shell out over $70,000 to buy those batteries, and they won't last forever.

 

So wet-cell marine batteries are obviously NOT a viable solution. What about more modern battery technology? Check out Part 3 of this analysis to find out!

 

Posted with Blogsy