Pipe Dream

Boot Key Harbor

Many larger boats used on-board diesel generators to charge their batteries.  Those things are very expensive, a maintenance nightmare, and worst of all, extremely noisy.  Being shut up in a boat together with a running generator sounds like a minor hell.   Despite that, a trawler owner in Vero told me that he runs his diesel generator 6.5 hours per day.  Another trawler moored near us in Boot Key Harbor seems to run his 12 or more hours per day.  Ay ay ay.

I could get on a soap box and sermonize about conserving power, but I won't.  At times in the past when our batteries were in poor condition, I had to run our generator as much as four hours per day.  Recently, with good batteries and sufficient solar panels, I think we run the generator as little as 3 hours per week.

The problem is not the energy use per se, or the generator per se, but rather the physics of lead acid batteries that can accept their final charge only very slowly. My pipe dream, and no doubt the pipe dream of those trawler owners, is for a device that would allow us to capture and store all the energy the engines can make for only a short running time each day.   Let me analyze the dream from an electrical engineer's perspective.

Our daily electric energy use is on the order of 50 amp-hours per day.  At an average of 12.5 volts, that amounts to 625 watt hours per day.  Let's say 1 kWh per day to be generous.  My diesel engine is rated at 37.5 hp which is 28 kw.  Therefore, one might think that I could run my engine for 1/28 hours (2 minutes) per day to make all my energy.  But I can't do that for several reasons, the main one being that the batteries can't accept the charge that quickly.

My pipe dream is for a device about the size, shape, weight and cost of a car battery that could accept charge at a rate of 28 kw for up to 2 minutes, and then trickle the energy out to charge my lead-acid batteries at an average rate of 167 watts for 6 hours.   The obvious choice for doing that would be a capacitor.  There is no theoretical ceiling on the rate that capacitors can accept charge.  Using clever electronics design (which I have not done yet) they could discharge that energy at a rate ideally suited for lead-acid batteries. The big question is how many capacitors would be needed?

1 kWh = 3,600,000 watt-seconds.   3,600,000 watt-seconds @ 14 volts = 257,000 ampere-seconds = 257,000 coulombs of charge.  257000 coulombs @ 2.5 volts or roughly 100,000 farads of capacitance.  (I use 2.5 v instead of 14 v because that is the maximum voltage rating of ultracapacitors.  Several capacitors in series would be needed to make 14 volts.)

When I was an electrical engineering student in college, we measured capacitors in micro-farads. Today we have super-capacitors and even ultra-capacitors measured in farads.  So the technology of making capacitors has improved by a factor of one million.

I went online to checks prices and specifications.  Twenty WIMA 5000F capacitors would do the job with a total of 19.5 kilo-farads, for a a volume of 0.7 cubic feet (about the size of two large car batteries), and a weight of 500 pounds, and a price of about $57,000.

Bottom line: the performance and prices of capacitors need to improve another factor of 10 to 100 before the pipe dream becomes reasonable.

Caveat:  My engineering skills are rusty, and I don't have much confidence in those calculations.  If any of you are able to double check them for me, I'll be grateful.  The main reference is here.
 
p.s. Put this in perspective with some of the more idiotic renewable energy advocates who think that they can use capacitors to store the energy of solar wind farms in quantities large enough to power entire states and countries.   That would take a capacitor somewhat larger than the moon.