Question for Darrell the EV Nut

Darrell, I recently installed 3 PV attic fans and have been much impressed with their performance. I would like to maximize my power output for the July through September months. Any reccomendations or websites that can help out with this info. The only thing I found was to set my panel at my latitutude plus 10 to 15 degrees. this seems a little steep to me, any ideas??


BTW I have measured a 10 to 20 DF drop since adding these to my passive ventilation system and am now thinking of the radiant barrrier paint from Sherman Williams (have a 30% of coupon with no limit or expiration date).

Darrell the EV Nut is showering at the moment.
He asked me to give you these links and to bring him a teabag and a cup, (he likes to brew his tea in the shower; it saves energy, he says... )


http://search.altenergystore.com/search?p=Q&ts=custom&w=attic+fan

http://shop.altenergystore.com/educate_yourself.htm

Id suggest homepower.com

Off the top of my head, make sure no shade spots on the panels. Also they sell two different voltage regulators for use with solar panels. One keeps the voltage from going too high for use with recharging batteries. The other uses the amps to offset the voltage, you so you do not get voltage sage. I imagine this maybe more important for powering stuff like fans, lights to have constant voltage and varying amps than vice versa.

Couple of things:

1. You probably want your latitude minus 10-15 degrees (not plus). I.e., you want the panels pointing at the sun during the summer, not during the winter.

2. Linear Current Booster I believe is what is being talked about by Cobb. Basically, a DC motor is more of a current device, a solar panel is more of a voltage device. The current booster plays with the P=IV equation (P is constant, keep V high on panel--current is more or less constant, and you can drop V and increase I to the motor). See here for some info.

3. Not sure about the reflective paint, but there is a reflective aluminum material that is supposed to work well. Try this place for some information.

4. Big help is keeping your solar panels cool. For me, a hot day can reduce their output by 20% or more. Mounting panels flat to roof is good for wind proofing and strong mounts. Placing up in breeze produces more power.

-Bill

Tedtheled

your first link actually shows the type of fans that I got, except they were a little less expensive. I got that particulat type because the panel was adjustable and mounted on the dome and allowed air to circulate. they are direct powered only (no batteries or electronics except for a thermal switch set at 80 DF). this time of year they start turning as early as 7:30 am and run to after 7:00 pm. I have checked them on complete overcast days and they still run at low rpm for about 7 hours. What I like the best about them is that the motors have a warrenty for five years and the system is designed to be repaired. Remove a cotter pin, then the fan blade, disconnect the wires remove two nuts and the motor is out and can be replaced. Other attic fans I have had were not maintainable.

If it is lattitude minus 10 to 15 degrees then it looks like I am set. I installed these about 6 weeks ago. for one week befpre installation I took daily temps at 3pm that ranged from 105 to 130 DF. The week after the temp has been 97 to 118 DF. In the last week we have had ambient temps of 95 DF plus and the temp has maxed out at 122 DF. The max temp I measured last august was 145 DF so time will tell and the comparisons for power usage for this summer vs last summer since all other equipment is the same.

the best thing I like is these things are so quiet. Golfers stop to ask about them because they can not hear them but see shimmering heat venting from them.

BB said:

Couple of things:

1. You probably want your latitude minus 10-15 degrees (not plus). I.e., you want the panels pointing at the sun during the summer, not during the winter.

2. Linear Current Booster I believe is what is being talked about by Cobb. Basically, a DC motor is more of a current device, a solar panel is more of a voltage device. The current booster plays with the P=IV equation (P is constant, keep V high on panel--current is more or less constant, and you can drop V and increase I to the motor). See here for some info.

3. Not sure about the reflective paint, but there is a reflective aluminum material that is supposed to work well. Try this place for some information.

4. Big help is keeping your solar panels cool. For me, a hot day can reduce their output by 20% or more. Mounting panels flat to roof is good for wind proofing and strong mounts. Placing up in breeze produces more power.

-Bill

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the foil will have the best barrier rating and will cost the most my estimate for the best insulation/barrier will cost almost $4000 with installation. The e-barrier paint and installation will cost under $1100 and have a barrier rating of 62%. So now it comes down to a tradeoff. If I was building new I would get the foil barrier that is part of the plywood for roof decking and be done. The attractive thing with the paint is that you rent the equipment your self the cost drops to about $700 and four hours in your attic to spray it in.

aka "MPPT" for "Maximum Power Point Tracking" --

volts X amps = watts and you want the maximum watts, so the tracker sets the voltage just high enough to keep the current flowing through the batteries..then the excess voltage is converted into more amps (or watts.)

so for example if hooking up a panel directly to a battery (that is starting out at say 12.3 volts) produces 13.9 volts and 3 amps going into the battery (= 41.7 watts) then putting a mppt in the circuit would lower the voltage to about 12.8 volts and 3.8 amps (= 48.6 watts) -- a significant increase in current to the battery, usually around 20 to 30% -- and is cheaper than adding more solar panels..

MPPT would be used, usually, for charging batteries. Yes, it can be used well in several types of solar setups:

1. If you have "12 volt" panels, they are typically generating 17-22 volts (or more) DC. A MPPT controller tries to optimize the power (I*V) coming from the panels. Inside, there is a DC to DC converter that, basically, behaves like the DC version of an AC Transformer.
2. Another area MPPT controller work well is for systems that have lots of panels and long wire runs.
The linear current booster would usually only be used for motor type loads (lights and electronic loads probably would not like being attached to this type of device).

-Bill

That's the first I've heard about the cold weather stipulation. My Seimens panels put out higher voltage than the battery can use even in hot weather..
(that's what 'controllers' are for, just about any panels will happily boil away your electrolyte once the batteries are full in any weather -- the controller shuts off or lowers the voltage once a predetermined voltage has been reached.. some controllers have the MPPT's built in, raising the price $100 to 300$ depending on how much power they will convert.

Obviously spending this much extra to boost the output of one or two panels 20% isn't worth it, but once you increase the output of say ten 75 watt panels 20% the device pays for itself by increasing the output by the equivalent of two more ($800+ worth) 75 watt panels..

Outback makes a good (somewhat pricey) one that can handle up to about 60 amps more or less..and many various voltage input and output levels..

The only efficiency graphs I have found for the Outback MX60 MPPT controller list connecting to a High(er) voltage set of arrays--not for the 12 volt nominal array:

MX60 PDF Manual (see page 33 for efficiencies)

The only efficiencies listed start with at least 12 Vdc panel voltage higher than the battery voltage (the Outback recommends a minimum of 12 volts Panel Voltage over Battery Bus voltage for the MX60).

If you read through the manual, they suggest, for example, that 6x 12Vdc (72 volt nominal array) panels in series (135vdc max working voltage, 150vdc never exceed voltage).

Except for:

The maximum input voltage for an MX60 should not exceed 150 Volts, which can happen if a 72-volt nominal array is used, in sub-zero temperatures.

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For example, just a standard Kyocera KC130GT panel with:

Vmp=17.6 Vdc (@70F)
Voltage Coefficient=-0.0821 V/°C=0.14778 V/°F
Imp=7.39 Adc
Panel "Rated STC" = 130 watts

Power at 70F:
P=IV=7.39A * 17.6V = 130 watts

Power at 110F (full sun warm day, no wind, panel temperature)
P=7.39A * (17.6V - 0.14778*(110-70)) = 7.39A * 11.6888V = 86.4 watts

Power at 25F (very cold day, sun behind clouds for a while):
P=7.39A * (17.6V - 0.14778*(25-70)) = 7.39A * 24.2501V = 179.2 watts

Notice the huge amount of power difference between 25F and 110F. And at 110F panel temperature (very real in my area), you cannot even direct connect the one KC 130 panel to a 12 volt battery and float charge it (13.6V required, less than 11.7 volts from panel--neglecting voltage drop in wiring/controller).

I don't know what Siemens panels you have, or how you have them wired... But, as above, hot weather kills the output of standard Silicon based solar panels. Looking up the specs. for a Siemens 50 and 100 watt panels; the numbers look to be almost identical to the examples above.

For the 100 Watt Siemens SR100 (12V):


Vmp=17.0 Vdc (@70F)
Voltage Coefficient=-0.079 V/°C=0.1422 V/°F
Imp=5.9 Adc
Panel "Rated STC" = 100 watts

Power at 110F (full sun warm day, no wind, panel temperature)
P=5.9A * (17.0V - 0.1422*(110-70)) = 5.9A * 11.312V = 66.75 watts

Again, 11.3 volts would seem to be under the 13.6 volts required to float charge a standard lead acid storage battery at nominal temperature.

I may have found some bad data regarding the solar panel ratings above (got it from the Xantrex website--nice consolidated listing of solar panel ratings in their sizing tool)... But I don't see how any "12 volt" nominal (really around 17-18 volts Vmp rated) panel can properly charge a 12 volt battery without being placed in series behind a "MPPT" type controller in any area that gets reasonably warm (I am excluding the battery voltage change with temperature at the moment to keep things sane in this discussion).

Certainly, the panel voltages will be higher in the mornings and cooler weather--but it does not appear to be possible to charge with a standard controller and 12 volt nominal panels in hot weather with noon-day sun.

Not saying that there is anything wrong with the controller(s) or panel(s)--but you should design the entire system according to physics of the problem.

Myself, I believe in MPPT controllers installed with series connected (and/or high voltage) solar panels for exactly the above reasons -- especially if you are trying to extract the maximum amount of solar energy (and aren't we all).

-Bill

:shrug:

I guess it's a miracle then that I have been living off-grid with a bunch of 12 volt batteries and 12 volt solar panels for 11 years now..

I can't tell you how my panels operate at 25F or 110F for that matter, but I can tell you they never have failed to provide a charge w/o a mppt..

I checked my Outback brochure; the MX-60 provides 60 amps for a battery bank from 12 to 60 volts, and accepts solar panel input voltage up to 125 volts.
If you read carefully you'll figure it out. They even have wiring suggestions if you're using 12 volts (nominal) input.

The voltage output of solar panels depends on the number cells linked up, I'm not certain but I think you get about 1/2 volt per cell..too few cells and you get poor hot weather performance, and poor low light performance. The siemens panels are excellent in both these respects, I believe they have 36 cells so provide current at a bit over 18 volts in the sun..

Saying a 12 volt panel can't charge a 12 volt panel w/o a mppt device is silly, I do it all the time and so do thousands (if not millions) of other people..maybe your statistics prove more correct at extreme temps, but so far here in Southern California, I haven't encountered the problem..

I think the big advantage of higher voltage to a MPPT is not loosing any extra power in the wiring run or just because the input voltage it higher then the battery voltage. For example in a 12v in to 12v battery you can often get 10-15% more power running a Outback MX-60 MPPT controller.

But Ted your correct, that is why 12v panels put out 18-22v in sun, it gives you the room in case the panels get hot and drop the output voltage, your still above 13.4v in almost all cases. Ted what do you absorb up to? Often in to sun you would be lucky to hit 15v, although for most that isn't a problem. Heck with AGM's you only need 14.4 or 14.5

Brock, my controller shuts off the incoming current at about 14.6-7 volts.
It has done that only a couple of times in the summer when I wasn't living at home, so not draining the batteries..

on a daily basis, if it's a sunny day, the bank goes up to about 14 volts with my 'normal' loads -- that translates to about 80% full -- trying to keep them any fuller than that gets difficult; it's takes more energy to put that last 20% in than it takes to bring the batts from 60% to 80%.. (besides, above 14 volts the batts fizz more and lose electrolyte) and you end up not being able to use much of the power you're generating-- if it has been a foggy day and the batteries are below 60% in the morning, and it looks like the sun won't be coming out again any time soon, I run the generator,.. and when I run the genny I take advantage of the extra power available and run the well pump too, to fill the storage tank..

of course from time to time I do the equalization process and overcharge them to 15.6 volts for a while usually I use the gen. for this -- but may be able to do it this summer with just the sun, if the sun ever comes back out --
right now the fog ('marine layer') is protecting this area from what is reported as triple digit heat inland.. :sweat:

Then, from the Outback manual link, I don't know why they say:

The MX60 is a buck type converter and cannot boost the output current when the PV array peak power point voltage is at or below the battery voltage as may happen on hot days in 24VDC PV and a 24VDC battery system or a 48VDC PV and a 48VDC battery system.

Click to expand...

--I presume that this warning also applies to a 12 volt PV and 12 volt battery system too…

Again, I am not saying it won't charge ever--just on very hot, windless days--during noon-early afternoon, it is possible for mid-day temperatures of a, for example, nominal 12 volt solar array to output less than 14 volts or so... Which, for the controllers I am aware (PWM, On/OFF, and MPPT) of (I am not a solar expert), is not enough to fully charge a 12 volt battery... A buck type regulator like the Outback MX60 would normally take 1-2 volts of headroom to operate (i.e., a 13.6 volt battery bus would require 14.6 to 15.6 from the solar panels to move current through the MX60).

In the debug section on trying to figure out why the MX60 would not start:

Is the PV voltage greater than the battery voltage by at least 2 volts?

The PV voltage has to be at least 2 volts greater than the battery voltage for the initial wakeup.

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And from Appendix F:


And a little further down:

Sizing the nominal array voltage higher than the nominal battery voltage will ensure that the Maximum Power Point is always above the battery voltage. The Maximum Power Point of an array will decrease as the panels warm up, thus lowering the output of the array. The MX60 charge controller will not be able to boost the output if the Maximum Power Point of the array is at or lower than the battery voltage.

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You can play with the Xantrex link and see what numbers you find (power, hi/low voltages/etc.) for a whole host of different panels. So far, I have not really seen any panels out there that violate the basic physics of silicon solar panels.

Again, not saying your system does not work---If it's wired for 12 volt nominal panels in parallel (instead of a minimum series PV panel "Nominal Voltage Rating" of 24 VDC like the manual recommends for a 12 volt battery bank), you may be getting less than optimal hot weather performance from an MX60 or other MPPT controller.

On the other hand, running too high of panel voltage through a non-MPPT controller is just a waste of Solar Panel potential energy and money.

-Bill