On Tue, 13 Apr 2004 09:18:00 +0300, Paul Keinanen
wrote:


The solar cell operates as a (badly) regulated power supply with
current limiting. At low load currents, the cell operates nearly as a
constant voltage source, but after a specific current (for a given
illumination) it operates nearly as a constant current source and
deliver approximately that current even into a short circuit.

The largest power from the cell (for a specific illumination) is
obtained at the point it switches from constant voltage to constant
current mode, in which both the voltage is quite close (within 30 %)
of both the maximum voltage (as measured at open circuit) and maximum
current (as measured at short circuit).

This maximum power point varies with illumination, but if the switcher
always loads the cell at this maximum power point, the largest
available energy at a specific time is extracted from the cell
independent of illumination.

Even if the losses in the maximum power point tracker is 10-20 %,
usually more energy can be obtained than running the module in some
non-optimal constant voltage or constant current mode.

Paul

I have seen elegant ckts where a simple switcher was used, regulating
the *input* voltage coming from the solar cell, keeping it in max
efficiency mode at all loads. This obviously only works with flexible
loads such as slow chargers or such.

--
- René

Watson A.Name - "Watt Sun, the Dark Remover" wrote:
My attitude is that rather than try to do this (and in the process lose
reliability), it's better to go supersize on the cells, add more area
and overall capacity to get you thru the cloudy days, and have a higher
capacity overall.

The argument usually goes that getting, say, 10-20% more power from a better
charge controller (one of these so-called 'maximum power point controllers')
can be cheaper (in additional expenditures) than getting 10-20% larger
panels. It's sometimes difficult to show, though, particularly on small
systems -- but MPPT controllers have been getting cheaper for awhile, now,
and I expect that eventually all but the cheapest/smallest will have this
functionality.


---Joel Kolstad

Watson A.Name - "Watt Sun, the Dark Remover" wrote:
My attitude is that rather than try to do this (and in the process lose
reliability), it's better to go supersize on the cells, add more area
and overall capacity to get you thru the cloudy days, and have a higher
capacity overall.

The argument usually goes that getting, say, 10-20% more power from a better
charge controller (one of these so-called 'maximum power point controllers')
can be cheaper (in additional expenditures) than getting 10-20% larger
panels. It's sometimes difficult to show, though, particularly on small
systems -- but MPPT controllers have been getting cheaper for awhile, now,
and I expect that eventually all but the cheapest/smallest will have this
functionality.


---Joel Kolstad

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.

Roy Lewallen, W7EL

René wrote:

I have seen elegant ckts where a simple switcher was used, regulating
the *input* voltage coming from the solar cell, keeping it in max
efficiency mode at all loads. This obviously only works with flexible
loads such as slow chargers or such.

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.

Roy Lewallen, W7EL

René wrote:

I have seen elegant ckts where a simple switcher was used, regulating
the *input* voltage coming from the solar cell, keeping it in max
efficiency mode at all loads. This obviously only works with flexible
loads such as slow chargers or such.

That is exactly the point, Joel. Upsizing 20% is several times more expensive
compared to providing an intelligent switcher to match to the illumination or to
adapt a panel voltage that isn't matching the storage devices. That situation
won't change unless there is a tremendous breakthrough in technology or serious
new government subsidies.

As to reliability, a switcher that is designed correctly and conservatively
should easily outlasts the cells. Even for small installations it is easy,
especially in view of the large variety of micro controllers that retail for a
few Dollars. One of these plus a few discretes and an inductor can do the trick.

Regards, Joerg.

That is exactly the point, Joel. Upsizing 20% is several times more expensive
compared to providing an intelligent switcher to match to the illumination or to
adapt a panel voltage that isn't matching the storage devices. That situation
won't change unless there is a tremendous breakthrough in technology or serious
new government subsidies.

As to reliability, a switcher that is designed correctly and conservatively
should easily outlasts the cells. Even for small installations it is easy,
especially in view of the large variety of micro controllers that retail for a
few Dollars. One of these plus a few discretes and an inductor can do the trick.

Regards, Joerg.

Roy Lewallen wrote:

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.

Just for efficiency reasons, I think you would want ot have enough
capacitance across the regulator input that the cell resistance drops
voltage only with respect ot the average output current, not the
switcher peak value. This can be a pretty big factor in the overall
efficiency. Using a switcher that has little ripple current on its
input (two phase boost, for instance) makes this much easier.

--
John Popelish

Roy Lewallen wrote:

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.

Just for efficiency reasons, I think you would want ot have enough
capacitance across the regulator input that the cell resistance drops
voltage only with respect ot the average output current, not the
switcher peak value. This can be a pretty big factor in the overall
efficiency. Using a switcher that has little ripple current on its
input (two phase boost, for instance) makes this much easier.

--
John Popelish

John Popelish wrote:

Roy Lewallen wrote:

In my limited experience, you have to be a little careful using a
switching, or even a series pass, regulator with a solar panel. Most are
designed to regulate voltage coming from a relatively stiff source, and
some become unstable when hooked to a high impedance source like a solar
panel. This can often be overcome by putting a big capacitor across the
panel, and it can of course be overcome by designing the regulator to
function properly with the high impedance source in the first place. And
quite a few regulators work just fine without modification. But it's
something to keep in mind when using a regulator designed for more
conventional applications.


Just for efficiency reasons, I think you would want ot have enough
capacitance across the regulator input that the cell resistance drops
voltage only with respect ot the average output current, not the
switcher peak value. This can be a pretty big factor in the overall
efficiency. Using a switcher that has little ripple current on its
input (two phase boost, for instance) makes this much easier.


That's not the point. Because a switcher tends to draw a constant power
from a load it's input impedance has a negative resistive component. If
you match this with a source that has a too-high impedance it'll be
_unstable_; a big capacitor would just slow it down in this case.

Presumably what you need is a controller that detects when the supply
voltage gets down to some threshold, then regulates the supply-side
current rather than the load-side voltage.

Come to think of it that'd be a fun thing to design...

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com