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Old 03-15-09, 09:13 PM   #1
Bob McGovern
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Default Off grid living powerpoint

Not sure where this belongs, but here's a link to the powerpoint slides that were part of a University of Wyoming Enrichment Programs class I taught this past week. It hits pretty hard on the decisions involved with actually designing an off grid house: less on the Why and more on the How, which might have lost a few people. I don't reckon it's my job to sell this lifestyle to anybody; just to offer our experiences, what works and what doesn't.

Cheers!

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Old 03-16-09, 07:38 AM   #2
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Great outline Bob. I like that you give a no hassle basic setup along with its costs. Conservation really does look like it pays off!
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Old 03-16-09, 09:16 AM   #3
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Nice powerpoint Bob. You make some good points in there...especially the part about who's going to fix it...in January. That kind of turned on a light in my head. I was reading it and when I got to that I was like...huh...I better know what the heck I'm doing if I do stuff like that. And who knows how much it's going to cost to have someone come in and repair it.

There sure is more to things then I had originally thought. That's why I like this site, it opens up my eyes to quite a few things.
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Old 03-16-09, 02:57 PM   #4
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Does the content come off as too negative or discouraging? I was trying to show that designing an offgrid system involves difficult binds & tradeoffs; does it seem forbidding? I got a sense some attendees thought it was buzz-kill.
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Old 03-16-09, 03:24 PM   #5
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Well, it is kind of a buzz-kill, but that is reality. If there was super cheap renewable energy we'd all have it already. I thought it was very well balanced. The visuals were well laid out, and the small bits of humor are good.

BTW, what is an MPPT charge controller?
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Old 03-16-09, 04:15 PM   #6
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I wouldn't say forbidding, I would say realistic... the person looking at installing an off grid type setup needs to know it's a lifestyle, not a toy. A farmer has to take care of the cows or they don't produce, and it's no different here. Great presentation by the way. Thanks for sharing.
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Old 03-16-09, 04:22 PM   #7
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Quote:
Originally Posted by Daox View Post
BTW, what is an MPPT charge controller?
This. Multi Point Power Tracking. You know how PV panels make waaay over faceplate voltage, especially in cold weather? My 24V panels make 35-38VDC some days, at the controller. A 'dumb' charge controller like the Trace C60 just lops off the excess voltage. But an MPPT controller holds the PV voltage just a little above the battery bank (or grid-tie inverter) demand; the surplus voltage it converts to amperage, resulting in 15-30% additional Ah storage.

If you have 10 panels, MPPT is like adding two or three additional panels. A good MPPT charger is about $400 more than a 'dumb' one. Blue Sky and Outback are the big names in MPPT; Morningstar just released a smaller, more affordable unit; and Midnite Solar's 'Classic' is gonna be the schnitz when it's finished.

PV uses 'Buck' MPPT, where the voltage is chopped down. There is also 'Boost' MPPT, which raises voltage while sacrificing current. The Bergey XL1 wind turbine has a Boost controller, so in winds below 14 mph it's still putting amps in the batteries, albeit reduced amps.
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Old 03-17-09, 11:42 AM   #8
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I'm a little confused. I always figured that all solar charge controllers were mainly just dc to dc converters with some battery charge logic slapped on them. In that case, they'd all convert extra volts into more amperage, unless the batteries were already at full capacity. Now I guess I don't understand how the cheap ones work.
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Old 03-17-09, 06:13 PM   #9
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Quote:
Originally Posted by Daox View Post
I'm a little confused. I always figured that all solar charge controllers were mainly just dc to dc converters with some battery charge logic slapped on them. In that case, they'd all convert extra volts into more amperage, unless the batteries were already at full capacity. Now I guess I don't understand how the cheap ones work.
Cheap ones just suppress the voltage by resistive loading or by a form of pulse-width modulation -- turning the panels off and on rapidly to create a 'phantom' voltage lower than the actual. In absorption mode, PWM controllers allow the voltage to climb but suppress amperage. Only MPPT controllers act as what amounts to variable DC transformers and increase current to the batteries as they reduce panel volts. Not entirely sure how they up the amps; may be something like a built-in inverter, transformer circuit, then a rectifier? It's fairly efficient, anyhow -- around 90% for buck MPPT; much lower for boost, which requires capacitor banks.

PWM is often touted as a high-tech charging feature, and certainly it does have the benefit of shaking sulfate of the plates, but really it's a cop-out. Modulating voltage is difficult for solid-state digital electronics; but they know from clocks, right? They can open and close a circuit a certain number of times per minute, doing more OFF and less ON as the batteries approach charge. Interestingly, MPPT has been around for about 60 years -- IIRC, it originally used a couple of magnetos spinning in opposition for a smooth analog transformation.

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