All
I was disappointed to learn that regen would not "completely solve all problems" ;-) But as I read the last post I wondered, has anyone tried a different ratio for regen sort of a two speed trans, one for propulsion and one for regeneration. Wouldn't be that hard to fab up, but would it work? I'm inclined to think not, in that there is only so much force being exerted against the prop by the water flow.
You could get the motor to turn faster with a small resistance or the motor to turn slower with large resistance, which would produce more power?
Thoughts?
Glenn
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--- In electricboats@..., Sally Reuther <smreuther@...> wrote:
Eric,
Thanks for adding this clear explanation concerning regen. I've found the
people who are asking us about regen under sail seem to think of this as
"free power" and that it will solve all of the charging problems. What we
know is that it's a trade off and you really need to make some choices if
you want to have regen work for you at all. Most folks want to be able to
motor as fast as possible so they make sure the gearing on their electric
systems is such that they can get optimum propulsion. If this is 5 knots on
a boat that can only sail (because of displacement ratios) 6.5 knots, then
the window for regen is that 1.5 knots of speed. The regen does not kick in
until you get past the point where your motor is set to motor at top speed.
I'm a sailor and I know that being able to sail at my boats hull speed for
any length of time is dependent on many factors and usually doesn't happen -
especially if I am only out sailing for the day and tacking a lot. To be
able to make regen work, the top motoring speed needs to be set lower, and
you need to be able to sail for a sustained period of time at that higher,
constant speed to get anything back into the system. Even then, as you
pointed out, the numbers are not huge. I don't understand the math involved
- I'm the artist and idea person, but I do understand the basic theory as I
explained it above. Regen is a fantastic idea, it's just not the solution
to our charging problems we would love to have.
Sally
On Mon, Nov 29, 2010 at 6:52 PM, Eric <ewdysar@...> wrote:
Larry,
As the most visible "regen naysayer" here, the link that you provided
completely supports my previous premise.
According to the Whoosh site "... And since we average 5-6 kts, that's over
100 amp/hrs/day ¨C for us quite possibly 150 amp/hrs/day ¨C that is available
for running the radar at night, making water with offshore water quality,
feeding the SSB radio, and keeping the iPod charged..." So what does this
mean? They get 100-150Ah at 12V in 24 hours, that's 1200-1800Wh in 24 hours
or 50-75W at 5-6kts. They do say that the harvested water power is
sufficicient for house loads, a statement that I wholeheartedly agree with.
My statement is (and always has been) that regen at speeds around 5kts is
not a viable source of charging for a traction battery bank. The speed is
important because most displacement auxilliary sailboats between 27-32 feet
(optimal for a 5kW electric drive) have a hard time hitting sustained runs
above 6 kts. For this size boat, average sailing speeds of about 5.5kts is
more realistic.
So let's say that you use 4kWh of energy out of your 10kWh battery bank,
that's about 10-12nm at 4kts in most of our boats. How far would you have to
sail using the system described by Whoosh to bring your batteries back to
full charge, assuming that you are using no electrical power in the mean
time? Generating 75W at 6kts, that's 12.5Wh/nm. So 4kWh will take 320nm of
sailing at 6kts and will take over 53 hours. Slow down to 5kts under sail
and the recharge distance is up to 400nm and will take 80 hours. If you have
a 12V house load of only 2A, the recharge times and distances double.
Ignoring house loads, the ratio of drive distance (at 4kts) to regen
distance (at 5kts) is about 1 to 40. Motor one mile at 4kts (15 minutes) and
you need to sail 40 miles at 5kts (8 hours) to get the charge back. If you
use a radio, autopilot or lights, the ratio gets worse.
So regen works, but is not as effective at recharging your traction
batteries as some drive vendors would like you to believe.
Fair winds,
Eric
Marina del Rey
--- In electricboats@... <electricboats%40yahoogroups.com>,
"Larry Doyle" <ldoyle@> wrote:
Here's to those NaySayers that say regen doesn't work. Here's some boats
that are doing it using towed gens as well as the free wheeling the prop/dc
motor in regen mode.
www.svsarah.com/Whoosh/WhooshPacificPrepTowGenerator.html
