Learing curve: Trying to do the math...

Peterr · August 2009

Bear with me, trying to get educated here. The original post this refers to is DC fan & Panel for small dehydrator, learned that being on the grid is the way to go with this project but am very interested in solar and using this as a learning tool. The dehydrator is built and being 'softly' tested with a couple of tomatoes and basil for the last two days. Inside, the solar box, it is heating up to 175* but the lower trays (using a 5 tray Excalibur without its rear electrical) are not getting the same amount of heat, as the heat is to the top. So I will need a light fan to mix the air around, but I'm so temped to do it off grid, and using this as a learning tool to convince me that it's not necessary.

BTW... July is are highest use of electrical, reading this forum and taking conservation steps we halved it. Sit down.. from 601 kwh last year to 299 kwh this year.

eat ur heart out.

That's for the whole month.

Ok... 12v car fan is labeled .8amp, which would all I would need.
To get the watts you take .8 x 12V = 9.6 watts

To get the panel size, using a previous example of 17 volts (typical Vmp for solar panel) I get... .8 x 17v = ~13.6 watt panel
Requiring #2 Power Up 12v Panels (#1 5watt & #1 10 watt = $173.00)
Running just during sunlit hours
~~~~~~~~
Now running the fan 24 hours per day, Summer operation only, taking in consideration local obstruction (hill) and location (Northern Az) should get 7+ hours of sun per day.

9.6 watt fan * 24 hours per day * 1/0.77 sys eff (solar PV + controller) * 1/0.80 battery eff * 1/7 hours of sun = 54 watts of solar panels

Needing #1 Kyocera KC50T 54 Watt Solar Panel 12volt for $279.00

How am I doing so far?
Now comes the questions... How large a solar charger will I need. Previous example for 351 watts of solar panels used 30+ amp solar charge controller. How is this figured?

I don't have to use the standard of 3 days of no sun, because its a dehydrator, and if it rains or gets that cloudy around here we can always bring the Excalibur case back in, hook up the original back to it with its six screws and go on grid. So its not necessary for the need of ~6 times the daily load.

9.6 watts *24 = 231 Amp*Hour Battery bank @ 12volts.

Since this only for summer and battery would not be in use during the winter. (Unless put battery into the work truck RV battery location to keep it charged up and current, but it would have to be a 12v battery.)
What would be the best battery to go with. Perhaps as suggested in previous tread example, use #2 Trojan "cheap" flooded cell golf cart batteries, T105 225 Amp*Hour (6 volt) for $140 each. But there not 12v to keep up in old truck. If I figured the amp hour correctly, would #1 12v battery do?

Cost: $279.00 panel, + $?? charge controller, + ?? battery or batteries, + wire + fuses + mount + taxes (no shipping, just down the hill from Wind & Solar just need gas $)

Thanks in advance for any suggestions and corrections. p.

Cariboocoot · August 2009

Re: Learing curve: Trying to do the math...

If you just need mix the air around, maybe all you need is a 12 V "muffin fan" from a computer (easily and cheaply acquired) and a relatively small panel to run it directly.

niel · August 2009

Re: Learing curve: Trying to do the math...

i'm confused as to why the 24hr thing and battery. just run it from a 15w pv or something like it and for regulation you could use a sunsaver (not applicable to the mppt model) without the battery.

BB. · August 2009

Re: Learing curve: Trying to do the math...

You have the math correct... Although, 7 hours of sun may be a bit much to expect every day. You can check this link for "hours of sun" for various locations in the US based on collector orientation.

Typically, the battery is sized for 3 days of no sun and 50% maximum discharge--You could go with 1 day and 50% maximum discharge--or 2x your daily load... Need to monitor your system so that you don't take the battery below 50% state of charge often (and never below 20% SOC) to keep the battery healthy and have a long(er) life.

Charge controller rating is based (roughly) on Maximum Battery Voltage * Maximum Current.

P=Vmax*Imax

P=54 watts, Vmax=15 volts for 12 volt battery bank:

I=P/V=54W/15V= 3.6 amps maximum

So, roughly any 4-5 amp or larger controller will work fine (I skipped the details between PWM and MPPT type controllers--the above number will work fine for our use here).

I would still argue--do you need a stirring or ventilation fan for the 12 hours a day it is dark?

Or, perhaps you can use a simple thermostat that only turns on when the box temperature is > 70F (or whatever). Will save you from wasting power when it really is not doing much good.

-Bill

And congratulations on the conservation! It feels good--don't it!

Peterr · August 2009

Re: Learing curve: Trying to do the math...

If you just need mix the air around, maybe all you need is a 12 V "muffin fan" from a computer ...

Well, I'm using the Excalibur fan as the case study. I tried a smaller fan (4") and it just doesn't move enough air. The one used in the original example by Bill, is way to much.

i'm confused as to why the 24hr thing and battery

Well when the monsoons aren't here the humidity is low. Right now its at 45, and will continue to get lower. So, even without sun the dehydrator can continue to work, but not as efficient, and as you both are suggesting, perhaps dumb to do as there is really no need.

...for regulation you could use a sunsaver

If you didn't have the fan running 24hr, and just had it daylight hours, could you just have the PV panel direct to the fan, and not use the sunsaver?

So, roughly any 4-5 amp or larger controller will work fine

Ok.. I'll have to check that out. But what about the battery. I understand what your saying about typical, but what do you suggest for the battery?

...perhaps you can use a simple thermostat that only turns on when the box temperature is > 70F (or whatever).

That may be a good idea.. but still if you did that, why bother at all with the fan, controller and battery.

...conservation! It feels good--don't it!

Abbbsolutely...

BB. · August 2009

Re: Learing curve: Trying to do the math...

Regarding the battery... It depends... Buy a good quality but cheap flooded cell deep cycle battery--and if you kill it, you are not near as upset as if you get a good quality and 2x as expensive AGM battery (which can go down to 20% state of charge without damage).

The problem is that batteries are very difficult to read just "voltage" and determine their current state of charge when under load or charging conditions... A flooded cell, you can always use a hydrometer (messy, pain to use often). A sealed battery (like AGM)--you cannot even do that...

So, then you are left with using a battery monitor (really a cumulative, bi-directional Amp*Hour or Watt*Hour meter).

The small ones used by RC folks and the more expensive ones used by the off-grid solar folks (aka Battery Monitors). Some battery monitors even have an output that you can use to turn off the load or turn on an alarm when the bank has reached a depletion set point.

But--they are not cheap. Absolutely worth it for people with large battery banks... May be hard to justify with a small battery bank... But how else are you going to, with a quick glance, now how far down you have cycled the battery bank (when to shut down fan so as not to damage the battery bank)?

For the battery--Finding a good deep cycle battery near you will save on shipping... You can sometimes find sales on AGM's (older stock).

Sizing:

10 watts * 24 hours * 1/12 volts = 20 Amp*Hours per day...

Battery should probably be 2-6x that size (40-120 Amp*Hours at 12 volts).

-Bill

Learing curve: Trying to do the math...

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