Batteryless Solar Blower

There are actually two different problems here:
1. The output of the panel will be at a more or less constant voltage but variable current depending on the light level. The fan will try to draw current from the panel based only on the voltage and will collapse the voltage and not run at all when the panels are not producing at or near full output. The fan will not just slow down to draw the power available.
2. The panel short circuit current through the non-rotating motor windings in the morning and evening is more likely to damage the fan than any excess power from the panels. The fan will rotate at a maximum speed/power according to its design and will not try to draw more power from the panels just because it is available.

Both of these problems are solved by using what is called a linear current booster between the panels and the fan.
It will do some of the things that an MPPT circuit would do, without requiring batteries or other energy storage.

Going back to your original question, solar PV panels are not harmed in the least bit by being left unconnected (no load) or having a short circuit connected (infinite load) or anything in between. They will deliver a different combination of voltage and current in those two cases, but without any bad side effects.

Thanks for that answer!

I was just playing with a 12volt bilge blower I found in an old parts bin and was wondering how to run it battery-less with a PV panel... shopping linear current boosters now

Am I reading this correctly, if I short the + & - cables on a PV panel, it won't do damage?

That is correct.

Is the fan motor capable of running on variable voltage? If so you should be able to run it direct without problems. Fan life might be reduced from the extreme of low panel output times. I do have an automotive fan (one used to in the cooling system) that I pulled from a junkyard running solar direct through a snap switch to vent hot air from the attic. Been working for 2 years.

WWW

There are a number of solar roof fans that run directly off of a panel. I installed a couple to vent my attic. The fans didn't have a high rating in CFM but the panel that powered it was only 10 watts. They did do a nice job of keeping the attic temp lower but they were designed to run at different speeds depending on the amount of sunlight hitting the panel.

Just be aware the larger the fan motor the more power is needed to start and the higher chance that too low a voltage may cause a brown out and damage the motor. You have to know what voltage range the motor is rated to run without damaging it because as the sunlight changes so will the available voltage going to the motor.

Just keep in the back of your mind that what is really changing is the available current. You may see a lower voltage at the fan, but only because it is acting vaguely like a constant resistance and the voltage is dropping below V_MP to whatever voltage corresponds to the maximum panel current for that light level.

The problem associated with this is that for most of its useful operating range some DC motors are NOT acting as a constant resistance and may try to pull more current to keep the shaft turning instead of decreasing the current drain as the voltage drops.

For any given (non-electronically controlled) motor, two things will determine the current drain:

1. The effective series resistance, which can be measured with the shaft not turning.
2. The back EMF generated by the motor, which will be proportional to rotation speed and sometimes current, depending on the way the motor is wound.

Small DC fans tend to be designed so that the first factor is the more important in limiting current (inherently safe against being run with a jammed rotor.)
Larger DC fan motors tend to be designed so that the second factor is dominant, since that reduces the amount of power wasted in the resistance of windings. Such a motor may need protection from undercurrent if the voltage drops below the design limit.

If the motor is a brushless DC motor with an electronic switching circuit built in, then all bets are off, and you have to know how it was designed to behave.

Hi all, thanks for the responses. I finally got a day in the office, thanks to food poisoning. Here is the system I am thinking about...

Blower - 24VDC Lil Champ blower

It usually runs around 100W, 24V, 4 amp. Because of the high static pressure its blowing against.

Panel - Solar World, Sun Module SW 265 mono
Maximum power Pmax - 265 Wp
Open circuit voltage Voc - 38.1 V
Maximum power point voltage Vmpp - 31.9 V
Short circuit current Isc - 8.82 A
Maximum power point current Impp - 8.33 A

Linear Current Booster... I've never bought one of these before, what booster would you suggest?


Will it achieve my goals of protecting the blower from any volatge-current combo the panel could put out? Will the panel be under any stress?

Simple you do not have to worry about it, the motor regulates how much power it can use, not the panel.

Think about it for a second. You have a fan motor in your car. It uses 12 volts. You car battery along with alternator is capable of delivering 3000 to 5000 watts. Yet your fan motor only uses 100 watts or whatever it needs.

Inetdog, sorry to be pedantic and slightly off topic, but there are solar panels that are harmed by being left at Voc.
reference: http://midniteforum.com/index.php?to...12973#msg12973

For everyone else, don't worry about it... these panels are very toxic and not common.

--mapmaker

Good point!
These days what is available to consumers is all silicon based, so the generalization applies pretty well.
CdTe (Cadmium Telluride) is potentially toxic enough that I would not want to get anywhere near intact panels, let alone damaged panels, in a residential environment. However, I will say that I would be even more worried if I were associated with the plant where the cadmium raw material was handled than with the CdTe compound that actually is found in the panels.

Although I do remember when many of the shiny hand tools on the market were cadmium plated.
I believe that the Japanese word for cadmium poisoning is "itai itai" meaning "it hurts, it hurts"

????? indeed!
One more post like that and you are gone.
--Moderator

Yes there is not a problem with too much power from the panel, but rather too little power. At least thats what someone told me. If there is not enough power in the early morning light, it may cause the fan to stall and burn out. Is this true for my blower? 24VDC Lil Champ blower http://www.deltatsystems.com/specs/LIL_Champ.pdf

That is only true for induction AC motors operating on a hard source like utility. Ac motors will draw more current as the voltage goes lower which results in more heat dissipated in the windings. That cannot happen with solar PV because a solar panel is a current source. If the current supply is less than the motor demands the voltage of the panel collapses to support what ever current level the panel can supply. When that happens there is no more current available to burn anything up. In your application lower voltage = lower RPM

Scenario with a 30w DC fan motor/50w panel combo... is a linear current booster necessary?

Could the low current/low rpm condition damage the motor?

However in a few tests i've done with a dc blower hooked right to a panel, lower voltage meant the fan stalled. It would stall, then jump for a bit and start rotating, then stall again. is this not bad for the blower?