What size solar panel ?

[ScottMW]

I'm building a solar kiln. I think I've narrowed my fan search down to using 3- 8" 80watt radiator fans (max rpm 2200, max cfm 800 each).  I can't seem to find a simple answer as to the size solar panel or panels that I need. I do not plan to use a battery.  Has anyone here cracked the code on this? 

[doc henderson]

I do not know the answer, but with no battery, the dc motors will speed up and slow down depending on sunlight.  a battery may take some of the give and take out a little.  the fans may not kick on at first sunup and you can begin charging the battery.  if you need full rpm/cfms then you need a battery to keep fully charged.  I would think.  I have a little fan with a solar cell, and it is very dependent on orientation, clouds, and intensity of sunshine.  If you air dry first, then not as important to have lots of air in the beginning, like with green wood.  

[Sod saw]

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Hi Scott,  Good for you and your kiln.  Mine is two chambers tied together for a total length (combined) of about 35 feet long pieces.

I also use auto fans and solar panels that drive the fans directly with no battery or voltage regulator.  One chamber has 4 fans and 4 @ 100 watt panels.  The other chamber has 3 fans and 3 @ 100 watt panels.  

The 3 panels are wired together in parallel and one pair of oversized wires feed one fan then go a few feet further to the next fan and then go a few feet further . . . .  and so on.  In that chamber.

The 4 fan chamber is the same but with 4 panels wired in parallel.

When it gets light in the morning the fans Start out slow and as the brightness goes up, so does the fan speed.  In late afternoon things will slow down.

When you wire all these up, be sure to leave the solar panels in the dark (turn them over) until all the fans are wired.  If you only have one fan as the only load, that number of panels can over voltage your lone fan and damage the motor.   (Please don't ask me how I know this, it only took a couple of minuets.)

A battery is an un necessary expense as you want the fans off at night to give the wood a chance to relax to prevent case hardening.  

The rule of thumb as far as electrical loads and source?  This is true of house wiring and simple DC circuits too,  80 %

This means that if you have a 20 amp circuit breaker at home, that circuit should not see a load of any more than 16 amps.

If each of your fans is rated at 80 watts, each fan should have a source capable of 100 watts or more to drive that fan.

Lets go back up to my chamber with 4 fans.  Each of those fans is rated at 80 watts so there is 400 watts driving those 4 fans.




.

[YellowHammer]

You didn't specify the voltage, I assume 12V?

Anyway, a basic power equation P=I*V where P is power in Watts, I is current in Amps, and V is the Voltage

So for each fan, for a 12V system, it would be I=P/V or 80/12=6.7 Amps, which is the rating you would need to get max performance from each fan.  If you have 3 fans, you need 6.7 * 3 = 20.1 Amps, so that is the rating you'd need for a 12V DC pack of solar panels, which is quite a bit.  With DC motors, they will run on much less power, but to get max performace that is what you would need.  

[wannabyooper]

I built my solar kiln using the Virginia Tech plans back in 2010 using 2 Endless Breeze fans that are 12 volt with 3 speeds.  I won't get into all the technical data used to figure out the voltage needed to power these fans using a photovoltaic panel or solar panel as everyone calls them.  I use 1 Sunwise single cell (single cells joined together instead of multi crystalline) which is rated at 65 watts.  On a bright sunny day this solar panel can produce over 21 volts which could burn out your fans that are only rated at 12 volts.  Since they are 3 speeds, I leave the fan control on the 2nd speed because it has resistance and will not allow more voltage that can burn your motors out.  Yes the Fans will speed up and down with cloud cover, and not even work on rainy days but your wood will not dry with all the moisture in the air anyway.  At night the fans shut down giving the wood time to recoup some of the lost moisture on the surface to avoid checking.  Very efficient system if you allow time to dry.  Like I said my system has been operating since 2010 on a solar panel that has a 30 year lifespan.  I have replaced 1 of the Endless Breeze fans in 2021 but 11 years from this fan I can't complain.  I would post pictures but this site is very difficult to navigate let alone log in.

[ScottMW]

Thanks for all the responses!

[Sod saw]

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Hi Scott,  You are fortunate to have the responses from YellowHammer and Wannabyooper.

YellowHammer has a great ability to understand math and physics.  What is most appreciated by some of us, here, is his ability to teach, explain how it "all" goes together.  He should be teaching  local kids.

His math is correct (thanks to an older guy, Mr Ohm).  He assumed 12 volts for his math example.

Wannabyooper has made an important observation.  He pointed out in more detail than I had talked about above, stating that the voltage can go up to 21 volts in bright sun.

Let's combine YH's math with Wanna's observation.  If you look at Ohm's Law, you will notice that as the voltage, in YH's circuit example, goes up, the current goes down. (Remember that the load  {  80 watts. } stays the same.)

When the sun is full brightness the fans are trying to take off and fly around the room, with that high speed.  High voltage and lower current.

When the clouds partly block the sun, the voltage will start to drop but the current will rise until the sky is dark when both the voltage and current will be zero.

Have you ever driven down the road and looked at the overhead electric wires going from the electric pole to a house?  Have you ever wondered why those wires appear larger in diameter than the wires at the top of the poles, even though the top wires feed whole neighborhoods full of houses?

The answer is the same as YellowHammer's example above.  Those wires at the top of the pole are thousands of volts higher than the ones that feed directly to most homes.  Therefore allowing fewer amps of current flowing thru those top wires allowing a smaller diameter (gauge) wire.

Most solar panels (cells) are rated and sold by watts.  Your automobile fans are also rated by watts.  I am not too sophisticated and prefer to keep things simple, therefore I used the example in post reply #2 above using watts.  It was my intention to leave out the confusing math associated with Ohms Law with all the varying voltages through out the day.



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[wkf94025]

I went a route that's probably not very interesting to you.  Following the Aug 2020 CZU fire, PG&E and County wouldn't let us reconnect to the grid for ~6 months, so that was all the impetus I needed to go solar for the first time.  $20k in components, $5k in ground mount frame materials and equipment rental, and viola whole house powered nicely off grid.  Then everything about the solar kilns are PV-powered, but using Lithium storage, DC -> AC inversion, and AC-based fans, DH, monitoring electronics, etc.  The one thing that really helps understand dynamic power needs are the Emporia WiFi-enabled smart outlets that allow me to see wattage consumed in real time of anything plugged in to them.  Again, not very helpful if you're purely DC->DC off grid, but fun science for watt geeks.

[Sod saw]

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wkf,  How much power do those wifi outlet circuits use while they are in stand by mode and not being actively used for monitoring?

Have you ever measured the wasted power from your batteries while each GFI outlet is waiting for something to be plugged into it?  Some off grid homes have all GFI outlets on a switch, instant on TV sets, and other "electronic" appliances also  on switches so that the circuitry within the respective outlet (appliance) will not waste battery power.  By not waisting battery power, the solar (or wind) part of the system may be reduced somewhat.

Do you find that you may wish to, not connect to the grid after investing in your own system?


.

[Ianab]

How much power do those wifi outlet circuits use while they are in stand by mode and not being actively used for monitoring?

From their web page the plugs use less than 3 watts.  The smaller the system I guess the more important that 3 watts becomes, but on a whole house system, it probably doesn't matter. If you are controlling a 1,000w load with it, it probably saves more power than it uses. A 300w panel to give some power and light at a remote cabin? Yeah, you don't want a 3w parasitic load, because then every w/hr counts.  

But you are right about the "parasitic load" of devices that are in "standby" Computers can be especially bad for that as they keep standby power to the system board, powering the network card and USB ports, waiting for a "wake up" signal. Each machine is only a few watts, but multiply that by the millions of computers.... 

[wkf94025]

As @Ianab  pointed out, the parasitic drain pales in comparison to the wattage of the device under control.  Also, there's the math that a smarter, better monitored and controlled kiln is less likely to destroy a valuable load, which blows all small-watt economics out of the water, right?

[wildtmpckjzg]

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Hi Scott,  Good for you and your kiln.  Mine is two chambers tied together for a total length (combined) of about 35 feet long pieces.

I also use auto fans and solar panels that drive the fans directly with no battery or voltage regulator.  One chamber has 4 fans and 4 @ 100 watt panels.  The other chamber has 3 fans and 3 @ 100 watt panels.  

The 3 panels are wired together in parallel and one pair of oversized wires feed one fan then go a few feet further to the next fan and then go a few feet further . . . .  and so on.  In that chamber.

The 4 fan chamber is the same but with 4 panels wired in parallel.

When it gets light in the morning the fans Start out slow and as the brightness goes up, so does the fan speed.  In late afternoon things will slow down.

When you wire all these up, be sure to leave the solar panels in the dark (turn them over) until all the fans are wired.  If you only have one fan as the only load, that number of panels can over voltage your lone fan and damage the motor.   (Please don't ask me how I know this, it only took a couple of minuets.)

A battery is an un necessary expense as you want the fans off at night to give the wood a chance to relax to prevent case hardening.  

The rule of thumb as far as electrical loads and source?  This is true of house wiring and simple DC circuits too,  80 %

This means that if you have a 20 amp circuit breaker at home, that circuit should not see a load of any more than 16 amps.

If each of your fans is rated at flexible solar panels, each fan should have a source capable of 100 watts or more to drive that fan.

Lets go back up to my chamber with 4 fans.  Each of those fans is rated at 80 watts so there is 400 watts driving those 4 fans.




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Designing a solar-powered system for your kiln requires considering the power consumption of the fans and matching it with the solar panel's capacity to generate electricity. Without using a battery, the solar panels must directly power the fans during sunlight hours. Here are some steps to estimate the size of the solar panels you may need: Calculate total power consumption: First, calculate the total power consumption of the three 8" radiator fans. Each fan has a maximum power consumption of 80 watts. So, the total power consumption of all three fans will be 3 * 80 watts = 240 watts. Determine daily operating hours: Decide how many hours per day you plan to operate the fans. For example, if you run them for 6 hours per day, this would be your daily energy consumption in watt-hours (Wh). Account for efficiency and losses: Solar panels are rated under ideal conditions, and actual output may vary based on factors like temperature and shading. To account for these losses, you can use a derating factor (usually between 0.7 and 0.9). Multiply the daily energy consumption by the derating factor to get the total daily solar panel output needed. Calculate panel capacity: Divide the total daily solar panel output by the peak sun hours for your location. Peak sun hours represent the number of hours per day when the solar panel will receive its maximum output. This value depends on your location and can be found from solar maps or local weather data. Choose solar panels: Based on the calculated panel capacity, select solar panels that can provide the required output. Solar panels are typically rated in watts, and their capacity will depend on factors like size and efficiency. Consider panel orientation and tilt: To optimize energy production, make sure to orient the solar panels towards the sun (south-facing in the Northern Hemisphere) and adjust the tilt angle according to your latitude. Keep in mind that the actual performance of the solar panel system may vary due to weather conditions, seasonal changes, and panel aging. It's a good idea to leave some margin when sizing the system to ensure it meets your kiln's needs even during suboptimal conditions. While these steps provide a general guideline, it's crucial to consult with a solar panel installer or an expert to get a more accurate and site-specific assessment of your solar kiln setup. They can help you determine the best panel size and configuration based on your location, kiln usage pattern, and local solar conditions.

[KcMatt]

Just a heads up: if those fans are from Amazon save yourself the trouble and don't buy them.  I've gone through well over a dozen of them in like 6 months. 

[peakbagger]

I have couple of solar standalone solar power systems around the house. I use whatever panels I have laying around. Going too big on a panel is not typically a problem, a fan on any DC load is going to pull whatever power it needs, no more no less. The one issue to be aware of is many used panels out there are from rooftop arrays and are designed to for higher voltage operation. It is important to buy 12 volt nominal panels. 

[Ianab]

Some are ~18V output, and designed to feed into a charge controller that regulates the current depending on the battery voltage. That's so you can get some power into a 12v battery even with poor sunlight, and the charge controller will stop the battery cooking on a sunny day. 

If you are doing a straight panel to fan connection, then you want ~12V peak. I also wonder what a low voltage does to the fan motors. Not enough to spin them, but "some" current will still be flowing. Hopefully not enough to cause damage, but I'm not 100% sure about that. A simple control circuit that cuts the power to the fans when the light level drops too low maybe?

[peakbagger]

Panels put out a range of voltage and amperage. As long as the internal insulation in the fan can hold up to higher voltage it just will pull lower amperage. I do agree that charging a battery requires a charge controller or a higher voltage panel can overcharge the battery but for a direct drive setup like fans, no need unless the fan is very low quality. Do note standard charge controllers are Pulse Width Modulated (PWM) they do not regulate the voltage coming out of the panel to the battery, the controller just turns the connection to the panel on and off rapidly to maintain a set charge voltage. The panel has a "sweet spot" of voltage and current that varies with load and temperature that may not match the battery voltage but its cheap and easy way to protect a battery. More expensive Maximum Power Point Tracking (MPPT) controllers operate the panels to be at the sweet spot all the time and convert the power produced to match the battery voltage. The net result is more power out of the panel but increased up front cost. 

[Broncman]

The larger solar panels such as the 100 watt panels from Renogy output about 21 volts in full sun. That's why they use a charge controller.  You can wire your fans in series, 2 at a time and then you have a voltage divider network so each fan only sees half the voltage.  The Renogy panels are 24.3 volts open circuit, 20.3 optimal. Even though they rate them as 12 volt panels. Most 12 volt battery chargers fast charge at a higher voltage  than 12 .

If you are worried about over voting your fans, wire 2 in series. They will last longer, but run a little slower.