On Thu, 08 Feb 2007 21:31:26 +0000, Ian Malcolm
wrote:

Assuming the panels can be modelled as an ideal voltage source with a
(variable) series resistor (they cant but its a useful approximation)

Assume what you like...

Ian Malcolm wrote in
:

Assuming the panels can be modelled as an ideal voltage source

Geez. If boat technology gets any better, we'll be able to run a light
bulb off the panel, shining on the panel, and the panel will have so much
power boost there'll be a surplus to charge the batteries!



Larry
--
Democracy is when two wolves and a sheep vote on who's for dinner.
Liberty is when the sheep has his own gun.

On Feb 8, 12:36 pm, Charlie Morgan wrote:
On 8 Feb 2007 12:48:58 -0800, "b393capt" wrote:
....
Practical Sailor has a history of recommending and trashing products
on a fair basis, and is very reliable.

I would dispute that in very strong terms. Their testing methods are about as
unscientific as possible, and their "recommendations" often have nothing to do
with what product was better.
....

There are certainly some problems with PS. They are a small shop,
perhaps even a little inbred, and have limited human and financial
resources. I suspect their work is sometimes influenced by their need
to maintain some good will with the industry. Despite this, they
appear far less beholden to the industry than any other marine
publication I know. I'm convinced that they are making an effort to
be honest in their evaluations and are concerned about their
reputation. I read them. Still, your point about testing and rating
has some validity. For instance, I think their recent review of
winches was worse than useless. I've put thousands of hours onto
three of the brands of winches they reviewed and can attest that there
are huge differences in design, quality and maintainability between
them, and some have well known failure modes. Yet they missed all of
that and concluded that there was no significant difference between
the brands... So, I don't take their opinion as gospel, but I do
think that, on the whole, they do a better job of reviewing marine
gear than anyone else and their opinions deserve a sympathetic
reading.

-- Tom.

Larry wrote:

Ian Malcolm wrote in
:


Assuming the panels can be modelled as an ideal voltage source


Geez. If boat technology gets any better, we'll be able to run a light
bulb off the panel, shining on the panel, and the panel will have so much
power boost there'll be a surplus to charge the batteries!

Why not see if you can patent that idea?
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Anyway, if it was possible to get a net energy gain, you'd still be
dissipating half the energy in heating the panel so you'd burn it up for
sure if you tried to get out more than you put in.

This may in fact be a problem in real life in hot climates unless you
have very good air flow over the panels, as the MPPT controller will be
working them harder and they *will* be getting hotter. Will the
supplier stand behind their product if its connected to an advanced
controller? If they are on an elevated mount, it may well be worth
spraying the backs with the thinnest possible coat of matte black paint
to help dissipate the heat.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.

Get a controller that diverts voltage above the maximum charging voltage of
the battery to a resistive load. In my case this is the water heater with
12V and 120V heater elements. 100% of the power of the solar panels will
go to charging the panels up to that point. You can't produce more power
than the panels are generating.

I personally use a Morning Star TriStar controller to control both my wind
generator and solar panels and it works great.

-- Geoff

Geoff Schultz wrote:
Get a controller that diverts voltage above the maximum charging voltage of
the battery to a resistive load. In my case this is the water heater with
12V and 120V heater elements. 100% of the power of the solar panels will
go to charging the panels up to that point. You can't produce more power
than the panels are generating.

I personally use a Morning Star TriStar controller to control both my wind
generator and solar panels and it works great.

-- Geoff
So, are you saying that if the panels are putting out 17 volts and the
battery is only taking 14, then 3 volts are applied to the heater? I
don't think that's the way it works.

On Feb 8, 3:21 pm, Charlie Morgan wrote:
... You seem to agree that they have big problems, and then you ask that they
be given some undeserved "break". [Why?]... It's
junk and it's deceitful. They deserve no special consideration. ...

They have problems. However, I don't think they are deceitful at
all. They tell you just what they've tested and just how they tested
it and then they give their opinions about the results. Sometimes
their tests aren't very useful and sometimes it isn't all that clear
how their opinions follow from their testing, but I've never come away
feeling that they are doing anything less than their honest best.
Moreover, some of their tests are perfectly useful and even when they
are a little silly the product descriptions can be helpful in and of
themselves. Also, I am not aware of any other industry publication
that even tries to provide disinterested product reviews (though I
know of one that routinely trades endorsements in return for free
products). So, yes, I think they are worth a read, but I agree with
you that calling them "very reliable" is a stretch.

-- Tom.

I agree with Tom.
One need not take what PS concludes as Gospel, but they bring information
that can be helpful. More so than any other sailing publication I know of,
especially when it comes to new developments/products.
You can come to your own conclusions, based on what data they develop.
Certainly you don't think they falsly report their findings do you? What
kind of scientific methods would you want ? How much would you be willing to
pay for this ?
PS has time to look into and evaluate many more items/systems/whatever than
I do. They more often than not do this in a way that makes sense to me. They
provide useful information. Their readers provide useful information. And I
think that the knowledge of their readers, as appears from their letters,
would suggest there must be some value to the magazine, or they wouldn't
waste their time and money on it.
But, it's a free country.

"Charlie Morgan" wrote in message
...
On 8 Feb 2007 16:03:48 -0800, "
wrote:

On Feb 8, 12:36 pm, Charlie Morgan wrote:
On 8 Feb 2007 12:48:58 -0800, "b393capt"
wrote:
...
Practical Sailor has a history of recommending and trashing products
on a fair basis, and is very reliable.

I would dispute that in very strong terms. Their testing methods are
about as
unscientific as possible, and their "recommendations" often have nothing
to do
with what product was better.
...

There are certainly some problems with PS. They are a small shop,
perhaps even a little inbred, and have limited human and financial
resources. I suspect their work is sometimes influenced by their need
to maintain some good will with the industry. Despite this, they
appear far less beholden to the industry than any other marine
publication I know. I'm convinced that they are making an effort to
be honest in their evaluations and are concerned about their
reputation. I read them. Still, your point about testing and rating
has some validity. For instance, I think their recent review of
winches was worse than useless. I've put thousands of hours onto
three of the brands of winches they reviewed and can attest that there
are huge differences in design, quality and maintainability between
them, and some have well known failure modes. Yet they missed all of
that and concluded that there was no significant difference between
the brands... So, I don't take their opinion as gospel, but I do
think that, on the whole, they do a better job of reviewing marine
gear than anyone else and their opinions deserve a sympathetic
reading.

-- Tom.

Why? You seem to agree that they have big problems, and then you ask that
they
be given some undeserved "break".

That makes as much sense as their reviews where they praise one product in
the
story and then rate another lesser product better in the final tally.

They do not use even basic journalism or scientific standards and methods.
It's
junk and it's deceitful. They deserve no special consideration.

CWM

Jeff wrote in
:

Geoff Schultz wrote:
Get a controller that diverts voltage above the maximum charging
voltage of the battery to a resistive load. In my case this is the
water heater with 12V and 120V heater elements. 100% of the power of
the solar panels will go to charging the panels up to that point.
You can't produce more power than the panels are generating.

I personally use a Morning Star TriStar controller to control both my
wind generator and solar panels and it works great.

-- Geoff
So, are you saying that if the panels are putting out 17 volts and the
battery is only taking 14, then 3 volts are applied to the heater? I
don't think that's the way it works.

That's exactly what I'm saying. It's called Diversion Mode and on the
controller you set the maximum voltage which is allowed to be applied to
the batteries. Anything above that is diverted to the load.

The only time that this occurs is when the batteries are fully charged.
The vast majority of the time the charging load of the batteries drops the
output of the solar cells to a voltage less than the maximum allowable
voltage and thus nothing is diverted.

For details please see the manual:
http://www.mrsolar.com/pdf/morningstar/TS_Manual.pdf


-- Geoff

Geoff Schultz wrote in
:

You can't produce more power
than the panels are generating.


Ok, here's a little electricity for everyone....

This is the schematic of a solar panel:


--[cell]---[cell resistance]--

Now, what this controller with the variable inverter load in it is doing
is simplified into this circuit:

|--[cell]---[cell resistance]---|
| |
| |
|-[controller input resistance]-|

In any series circuit like this where the SOURCE has a series resistance
(impedance in AC and RF circuits), Load POWER is "optimized" to a peak
level whenever load impedance (resistance) EQUALS source impedance
(source resistance). Moving away from this optimum power peak in either
direction of load impedance creates LESS POWER in the load, in this case
the controllable inverter's panel input. If the inverter's loading of
the panel can be made to electronically change its load resistance on the
panel to the same resistance as the panel's own built-in resistance,
which is quite substantial, Yes, the power generated will be greater than
if the panel were simply connected through a blocking diode to the
batteries. Very ingenious.

This is NOT magic. It's math. The source voltage is 19VDC, that's what
the cell puts out due to the number of cells in series, a function of
silicon junction voltage. Just for demonstration of this effect, let's
say the panel cell resistance (which varies, by the way with cell
temperature quite a bit) is 10 ohms. Two simple electric calculations
are needed:
Power = current squared times resistance.
Current = Voltage divided by resistance.

Let's check 3 points...varying the load resistance...to see what power
develops in the load. Let's check a load R of 10 ohms, same as source,
for a reference. Total resistance is 20 ohms. 19V/20 ohms = .95A
circuit current. Now square that (.9025) x the 10 ohm load resistance =
9.025 watts "output" for the inverter to send to the battery, minus its
losses, of course, in conversion.

OK, now let's change the load resistance to 8 ohms (lower). Total
resistance is 18 ohms. 19V/18 ohms = 1.055A circuit current. Now square
1.055A (1.113) times 8 ohms = 8.9 watts...POWER OUT DROPPED OFF from our
peak.

OK, now let's change the load resistance to 12 ohms (higher). Total
resistance is 22 ohms. 19V/22 ohms = .8636A circuit current. Now square
..8636 (.7458) times 12 ohms = 8.94 watts...POWER OUT DROPS OFF THIS WAY
TOO!

So, if the panels resistance right now equals the controller's input
resistance, right now, the maximum power output peak of the panel will be
realized. This can be accomplished with a little electronic trickery in
IC regulators, probably custom made for this purpose, so that at any
panel resistance, the IC can sense and adjust the controller's load
resistance on the panel to maximize panel output. With switching power
supplies now in the 99% efficient range, this is very feasable.

By the way, this is the exact same reason we strive to make a radio with
an output impedance of 52 ohms connected to coaxial cable with an
impedance of 52 ohms connected at the top of the mast to a 52 ohm
antenna....because it creates maximum power transfer from the transmitter
to the antenna. (The radio is actually designed to match certain types
of antenna's natural impedances, not the other way around.) It even
works the other way on receive....so we use a 52 ohm input receiver, too.

This completes today's electrical lecture. Please read pages 324 through
468 in your textbook and complete the workbook section 3-6 to hand in by
tomorrow's class. The workbook answers are 40% of your grade in this
course. (God, they all looked like they could kill me when I used to
tell 'em this just before the bell rang.)....(c;

Larry
--
I just can't stand it when I don't know why something does what it does!

I had to learn how our ship's steam turbine plant worked, to the dismay
of the ratings in the engine room, even though I was an Electronics Tech.
If you need power, give me a little time to fire the boilers and get the
pressure up and we'll go!