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Economical Essential Solar Electric

Post in 'The Green Room' started by jebatty, Aug 4, 2012.

  1. jebatty

    jebatty Minister of Fire

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    I've been trying to find a way to achieve essential solar electric that is feasible, provides a sense of accomplishment, and doesn't cost $thousands. A focus on lighting seems to be very possible, and while it may not achieve the biggest savings on the electric bill, it does highlight the possibilities in a very "visible" way.

    The first babystep was the simple purchase of two rechargeable solar light bulb devices from Nokero, the N200 and N220. Both recharge from sunlight during the day (have to hang these outside), and the N200 then provides under the counter lighting for early morning coffee making, while the N220 provides ceiling lighting for use of the computer. For me these demonstrated that simple solar lighting is feasible, and also made clear the benefits of turning a light out when not needed, as leaving these "on" when not needed runs down the battery charge.

    The next step is beginning to become clear. I bought at close-out prices 11 - 7 watt LED bulbs, about 40 watt incandescent and 13 watt CFL equivalent, with medium bases that fit standard light fixtures. These were $5.00 each. They are daylight white vs warm white of the CFL's they replaced. I didn't tell my wife I did this and she hasn't noticed the change. LED's are well suited to solar electric.

    The next step I think can be a conversion to full solar for most of our basic lighting, which is quite possible with LED bulbs. Fortunately, our breaker panel has several circuits that are or can be converted to lighting only. A fairly small solar panel system with batteries can be wired to feed these circuits directly with 12vdc, and the lighting can be 12vdc LED, or with an inverter the 120vac circuits can be used.

    Still thinking this through. Have others gone this route? Better ideas?

    Question for the gurus: the 120vac LED's have a PF of about 0.7 (7 watts and 10va). Do 12vdc LED bulbs operate at a higher efficiency? Are there other 120vac LED's that operate closer to PF=1?

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  2. woodgeek

    woodgeek Minister of Fire

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    When I had a 40 Amp-hour 12V lead acid battery left over from another project, I decided to set up a 'hobby' off-grid PV system for fun and to amuse my kids.

    My system was the above mentioned battery ($100 would get you one twice this large), a plastic 'battery box' ($10), 20A automotive fuse (few bucks), a small solar charge controller ($20-30), and a 20W silicon PV panel (cost me $60 delivered two years ago). I popped the panel on my roof edge, zip tying it the gutters (no roof penetration), and wired the controller and battery to the panel leaving the whole thing outside. I then ran a 12V line inside my house through my wall mounted kitchen exhaust fan, ended it in a 'car charger receptacle' and voila....regulated 12V solar power 24/7 in my house. Add a car USB charger, and we can run our phones, Ipad, 120V inverter, and my laptop on it.

    The battery stores ~400 Watt.hours, and the system 'harvests' 50-80 Watt.hours/sunny day (depending on the season). Works out to maybe 15 kWh/yr, or about $2 worth of grid electricty/yr, retail. Not really a money maker....a 'hobby' system. Might be handy in an extended power outage, but I already have a cheapo HF genny.

    Since panels got cheaper, I just bought a 60W panel to upgrade the system by 3x.

    I think you could run a bunch of LED lights using a system made using a couple hundred $$ in parts. The cost per kWh will likely be over $1/kWh, so it will not save money. The environmental impact of the lead-acid battery (5 yr lifetime) prob makes the system not as green as other choices.

    IMO, the PF thing is a non-issue, but the losses in the inverter can be significant, esp on standby. RVer websites will be a great resource for info and hardware.
  3. peakbagger

    peakbagger Minister of Fire

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    You really shouldnt mix voltages and AC and DC in a breaker panel. I expect it violates several sections of the electrical code. I am not a code expert but at a minimum you would have two sources feeding one panel. If a fireman pulled the meter to kill the power in the house, he would not know about the 12 volt circuit and could potentially get zapped. I can legally do this with a grid tied PV system as the gird tie inverter goes dead if the utility is not there, plus I have to have a large red label on the circuit panel warning of two feeds.
  4. woodgeek

    woodgeek Minister of Fire

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    Indeed, I would keep the 12V circuits totally separate, and fused at the battery.
  5. jebatty

    jebatty Minister of Fire

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    That's exactly what I would do. Because the lighting circuits are separate, it would be easy to pull the wiring from the main panel and have a separate 12vdc supply/box.
  6. jharkin

    jharkin Minister of Fire

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    Power factor is an AC only concept, so i dont believe you can use that to compare to a DC led efficiency wise.

    LEDs generally run on DC only so the 120v AC bulbs have a rectifier and voltage converter, likely costing some efficiency loss. It stands to reason that direct driven 12v dc leds could be more efficient.

    I think there is reaseach ongoing into LEDs that can run directly on AC but i dont know if anything has been commercialized yet.
  7. Thomas Anderson

    Thomas Anderson Member

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    DC has no power factor, which is one of the things that makes it more efficient. All of the energy is always 100% available. The other is that if you're producing energy via solar, it's already DC, so converting it to AC has significant losses. An LED is a diode, which means that it only allows electricity to pass in one direction. You can't have a single AC LED without losing half the power, but you could build a multi-LED bridge rectifier to get nearly 100% out of it.

    I built a small off-grid solar demo system recently which compares an AC vs DC LED bulb side-by-side. Both bulbs were rated at 4W nominal. The DC bulb used about 3.5W, the AC bulb used 12W as measured at the batteries. So clearly, DC is preferable where it makes sense.

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