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

Tim Wescott wrote:

John Popelish wrote:

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...

Very few switchers draw an instantaneously constant power from the
unregulated source. Almost all can draw an average constant power
(over the switching period). The difference means a lot when you
consider what the variations do to the total losses in the solar
cells. You missed my point, completely.

--
John Popelish

Tim Wescott wrote:

John Popelish wrote:

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...

Very few switchers draw an instantaneously constant power from the
unregulated source. Almost all can draw an average constant power
(over the switching period). The difference means a lot when you
consider what the variations do to the total losses in the solar
cells. You missed my point, completely.

--
John Popelish

On Tue, 13 Apr 2004 16:27:29 -0700, Tim Wescott
wrote:

John Popelish wrote:


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.

While there certainly are going to be stability issues, using a
switcher with say 50 % duty cycle will draw 0 A half of the time (i.e.
the PV cell is operating in the constant voltage mode) and 2 Iave the
other half of the time (i.e. the cell would operate in the constant
current mode) and never operate at the maximum power point (here
assumed to be at Iave).

Sufficient parallel capacitances and/or series inductances or some
push-pull arrangement will keep the current constantly at Iave and
thus at the maximum power point.

Paul

On Tue, 13 Apr 2004 16:27:29 -0700, Tim Wescott
wrote:

John Popelish wrote:


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.

While there certainly are going to be stability issues, using a
switcher with say 50 % duty cycle will draw 0 A half of the time (i.e.
the PV cell is operating in the constant voltage mode) and 2 Iave the
other half of the time (i.e. the cell would operate in the constant
current mode) and never operate at the maximum power point (here
assumed to be at Iave).

Sufficient parallel capacitances and/or series inductances or some
push-pull arrangement will keep the current constantly at Iave and
thus at the maximum power point.

Paul

Paul Keinanen wrote:
On Mon, 12 Apr 2004 13:02:38 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" wrote:


Joerg wrote:


Another option might be to use a different voltage panel,
whatever has a good price, and then use a small switcher to run
the cells at their optimum load.

Regards, Joerg.

Seems foolhardy to me, to use a boost circuit, and waste a lot of power.
Just put more PV cells in series to increase the voltage.


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


Anybody got any real data on this stuff.
I set out to build a constant power solar battery charger.
I was gonna just put a PIC to measure the voltage/current and
ratchet the switcher duty cycle up and down around peak power.

Went out in the yard at noon and plotted some curves. Yep,
there's a pronounced power peak right around 14V.
At lower intensities, the shape of the curve is the same, but
it moves sideways. Ok, my pulse width strategy should track that.
Cool.
Then I turned the panel ever so slightly away from the sun.
I was amazed at how dramatically things changed with just
a small angle. Looks like I'd gain WAY more watt-hours/day
by tracking the sun
than by anything else I could think of.

mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

Paul Keinanen wrote:
On Mon, 12 Apr 2004 13:02:38 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" wrote:


Joerg wrote:


Another option might be to use a different voltage panel,
whatever has a good price, and then use a small switcher to run
the cells at their optimum load.

Regards, Joerg.

Seems foolhardy to me, to use a boost circuit, and waste a lot of power.
Just put more PV cells in series to increase the voltage.


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


Anybody got any real data on this stuff.
I set out to build a constant power solar battery charger.
I was gonna just put a PIC to measure the voltage/current and
ratchet the switcher duty cycle up and down around peak power.

Went out in the yard at noon and plotted some curves. Yep,
there's a pronounced power peak right around 14V.
At lower intensities, the shape of the curve is the same, but
it moves sideways. Ok, my pulse width strategy should track that.
Cool.
Then I turned the panel ever so slightly away from the sun.
I was amazed at how dramatically things changed with just
a small angle. Looks like I'd gain WAY more watt-hours/day
by tracking the sun
than by anything else I could think of.

mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

mike wrote:
Anybody got any real data on this stuff.

There's no shortage of information about this. Useful
keywords are "insolation" and "solar insolation" (the word
"solar" is slightly redundant but it's commonly included).
In summer, you can expect a maximum of 1 kWatt per square
metre to reach the surface of the earth. The units most
commonly used are kW-Hour per square metre per day - I'll
call them Units here.

Insolation tables for the USA can be seen at:
http://www.suntrekenergy.com/sunhours.htm
These figures are somewhat suspect - the difference between
"high" and "low" seems too small (a maximum of 6 Units is
rather low), especially when compared with the following,
which contains some good maps:

http://www.wattsun.com/resources/ins...map_index.html

On this page, click on Flat Plate Collector, Single Axis
Tracker and Double Axis Tracker. The latter can produce up
to 14 Units in summer. The improvement when tracking the
sun's angle is very large. It pays to live in California.

I have seen a similar table somewhere for the UK, showing
that 5 Units is the best that can be expected, and maybe
less than 1 Unit in winter.

Bear in mind that the efficiency of Solar Cells is less than
20% in the very latest state-of-the-art devices, typically
10%, and maybe as low as 5% in reject/hobbyist cells.
Generating hot water directly from flat solar collectors is
probably more efficient, and certainly cheaper, but not much
use if it's electricity you want.

If, on a bad day, the cell voltage is less than the battery
voltage, you can still charge the battery. Look at:

http://www.elecdesign.com/Articles/A...262/6262.html#
This article appeared in Electronic Design, Sept 14 1998.

It describes a circuit for a Maximum-power-point-tracking
solar battery charger. The principle is simple: the
duty-ratio of a switch-mode power supply is continuously
modulated at about 50Hz. The change in output on each cycle
is used to determine whether a higher or lower duty-ratio
would increase the output power. A phase-sensitive detector
and feedback loop determines whether to increase or decrease
the average duty-ratio. It settles at the point of maximum
power.

As the article points out, it works for other energy sources
such as water-wheels and other devices where the shape of
the "energy curve" is not precisely known.

When used as a battery charger the voltage of the battery is
fairly constant, so "maximum power" means "maximum
current". At the solar cell end, we are working at maximum
power, although the voltage may vary. The "maximum power
transfer" condition is when 50% of the power goes to the
load, and 50% is dissipated in the cell. I don't know if
this is precisely true in a solar cell, but it certainly
implies considerable power dissipation in the cell, which
may shorten its life. On the other hand, a cell of 1 square
metre will have 1000 watts of solar power falling on it, and
may generate 100 watts of electrical power, of which we may
get 50 watts into our battery. The 50 watts dissipated in
the cell is much less than the 1000 watts from the sun - so
maybe it doesn't matter.

J.S.Blackburn,
London UK.

mike wrote:
Anybody got any real data on this stuff.

There's no shortage of information about this. Useful
keywords are "insolation" and "solar insolation" (the word
"solar" is slightly redundant but it's commonly included).
In summer, you can expect a maximum of 1 kWatt per square
metre to reach the surface of the earth. The units most
commonly used are kW-Hour per square metre per day - I'll
call them Units here.

Insolation tables for the USA can be seen at:
http://www.suntrekenergy.com/sunhours.htm
These figures are somewhat suspect - the difference between
"high" and "low" seems too small (a maximum of 6 Units is
rather low), especially when compared with the following,
which contains some good maps:

http://www.wattsun.com/resources/ins...map_index.html

On this page, click on Flat Plate Collector, Single Axis
Tracker and Double Axis Tracker. The latter can produce up
to 14 Units in summer. The improvement when tracking the
sun's angle is very large. It pays to live in California.

I have seen a similar table somewhere for the UK, showing
that 5 Units is the best that can be expected, and maybe
less than 1 Unit in winter.

Bear in mind that the efficiency of Solar Cells is less than
20% in the very latest state-of-the-art devices, typically
10%, and maybe as low as 5% in reject/hobbyist cells.
Generating hot water directly from flat solar collectors is
probably more efficient, and certainly cheaper, but not much
use if it's electricity you want.

If, on a bad day, the cell voltage is less than the battery
voltage, you can still charge the battery. Look at:

http://www.elecdesign.com/Articles/A...262/6262.html#
This article appeared in Electronic Design, Sept 14 1998.

It describes a circuit for a Maximum-power-point-tracking
solar battery charger. The principle is simple: the
duty-ratio of a switch-mode power supply is continuously
modulated at about 50Hz. The change in output on each cycle
is used to determine whether a higher or lower duty-ratio
would increase the output power. A phase-sensitive detector
and feedback loop determines whether to increase or decrease
the average duty-ratio. It settles at the point of maximum
power.

As the article points out, it works for other energy sources
such as water-wheels and other devices where the shape of
the "energy curve" is not precisely known.

When used as a battery charger the voltage of the battery is
fairly constant, so "maximum power" means "maximum
current". At the solar cell end, we are working at maximum
power, although the voltage may vary. The "maximum power
transfer" condition is when 50% of the power goes to the
load, and 50% is dissipated in the cell. I don't know if
this is precisely true in a solar cell, but it certainly
implies considerable power dissipation in the cell, which
may shorten its life. On the other hand, a cell of 1 square
metre will have 1000 watts of solar power falling on it, and
may generate 100 watts of electrical power, of which we may
get 50 watts into our battery. The 50 watts dissipated in
the cell is much less than the 1000 watts from the sun - so
maybe it doesn't matter.

J.S.Blackburn,
London UK.

Hi J.S.;

"J.S.Blackburn" wrote:

mike wrote:
Anybody got any real data on this stuff.

There's no shortage of information about this. Useful
keywords are "insolation" and "solar insolation" (the word
"solar" is slightly redundant but it's commonly included).
In summer, you can expect a maximum of 1 kWatt per square
metre to reach the surface of the earth.

This is miss leading.
While there are places, nearer to the equator, that
can have 1KW/m^2 at noon this is not the norm.

The rule of thumb is 1KW/m^2 normal to the sun
not flat on the ground.
Or about 100W/ft^2.
This is a tilted surface directly facing the sun.

The units most commonly used are
kW-Hour per square metre per day -
I'll call them Units here.

Insolation tables for the USA can be seen at:
http://www.suntrekenergy.com/sunhours.htm
These figures are somewhat suspect - the difference between
"high" and "low" seems too small (a maximum of 6 Units is
rather low), especially when compared with the following,
which contains some good maps:

http://www.wattsun.com/resources/ins...map_index.html

On this page, click on Flat Plate Collector, Single Axis
Tracker and Double Axis Tracker. The latter can produce up
to 14 Units in summer. The improvement when tracking the
sun's angle is very large. It pays to live in California.

I have seen a similar table somewhere for the UK, showing
that 5 Units is the best that can be expected, and maybe
less than 1 Unit in winter.

Bear in mind that the efficiency of Solar Cells is less than
20% in the very latest state-of-the-art devices, typically
10%, and maybe as low as 5% in reject/hobbyist cells.
Generating hot water directly from flat solar collectors is
probably more efficient, and certainly cheaper, but not much
use if it's electricity you want.

If, on a bad day, the cell voltage is less than the battery
voltage, you can still charge the battery. Look at:

http://www.elecdesign.com/Articles/A...262/6262.html#
This article appeared in Electronic Design, Sept 14 1998.

It describes a circuit for a Maximum-power-point-tracking
solar battery charger. The principle is simple: the
duty-ratio of a switch-mode power supply is continuously
modulated at about 50Hz. The change in output on each cycle
is used to determine whether a higher or lower duty-ratio
would increase the output power. A phase-sensitive detector
and feedback loop determines whether to increase or decrease
the average duty-ratio. It settles at the point of maximum
power.

As the article points out, it works for other energy sources
such as water-wheels and other devices where the shape of
the "energy curve" is not precisely known.

When used as a battery charger the voltage of the battery is
fairly constant, so "maximum power" means "maximum
current". At the solar cell end, we are working at maximum
power, although the voltage may vary. The "maximum power
transfer" condition is when 50% of the power goes to the
load, and 50% is dissipated in the cell. I don't know if
this is precisely true in a solar cell, but it certainly
implies considerable power dissipation in the cell, which
may shorten its life. On the other hand, a cell of 1 square
metre will have 1000 watts of solar power falling on it, and
may generate 100 watts of electrical power, of which we may
get 50 watts into our battery. The 50 watts dissipated in
the cell is much less than the 1000 watts from the sun - so
maybe it doesn't matter.

J.S.Blackburn,
London UK.

Duane

--
Home of the $35 Solar Tracker Receiver
http://www.redrok.com/electron.htm#led3X[*]
Powered by \ \ \ //|
Thermonuclear Solar Energy from the Sun / |
Energy (the SUN) \ \ \ / / |
Red Rock Energy \ \ / / |
Duane C. Johnson Designer \ \ / \ / |
1825 Florence St Heliostat,Control,& Mounts |
White Bear Lake, Minnesota === \ / \ |
USA 55110-3364 === \ |
(651)426-4766 use Courier New Font \ |
(my email: address) \ |
http://www.redrok.com (Web site) ===