|
The alternator is extremely inefficient. At 6000rpm engine speed the alternator coils are producing 55v. But we only require 14v so 41v is simply thrown away through heat in the regulator. At high rpm the alternator is 30% efficient. At idle it is 50%.
The rectifier is also rediculously inefficient. We produce voltage then pass it through 2 diodes of 0.7v drop. At idle, a 1.4v drop out of 15.4v is a 9.1% loss. At 100A (low rpm, high rotor current) our alternator is a 1.4 x 100 = 140w heater. We add this heat to bearing and fan friction then wonder why they burn out. At high rpm it's worse because the voltage regulator becomes a 41 x 100 = 4.1kw heater. This heat makes the copper coils less efficient so even more heat is created. Now you can understand why undersizing the alternator is a really silly idea. Luckily we spend very little time at 6000rpm so these rediculous losses are short lived. If we cruise at 2700rpm, our voltage is about 30v so regulator losses at 100A are around 1.5kw (2hp). Now add friction from the belt, fan and bearings. Now add rotor current. Now add rectifier losses. Are we starting to understand why we save 10% fuel with a alternator removal?
How do you kill an alternator? Have a flat battery that gets jump started, then run the headlights, demister and fan on high while doing high rpm.
To improve alternator efficiency, remove the rectifier and regulator and mount them externally. But now you need to keep them cool so add a fan. Let's reduce those losses. Use schottky diodes which have 0.2v drop which will save 50w at 100A. Much less heat = no fan. As for the regulator, get rid of it and instead use a buck converter. 100A at 14v is 1.4kw. At 55v, only 25A is required to get the same 1.4kw. That's right, by using a buck converter you can do 75% less work. I've ignored losses in both cases. Now we are able to undersize the alternator but might still burn it out at idle with high current (because 100A, no surplus volts to convert to amps). You can use an mppt controller of high current rating (eg Fangpusun 70A) which is rated to 150v. But I don't know if the mppt will struggle with changing rpm. At cruise it will be very effective. Or you can buy a 60A 35-60v buck converter but it's not going to work at lower rpm (that might be a good thing) because voltage is below 35v.
Want more efficient? Get rid of the lead battery and replace it with lithium iron phosphate which has a charge efficiency of 97%. Half the weight and 5x longer life. Go one better, double the capacity and charge it from the grid, no alternator
|