Wes Johnston

Solar power

Here's a guy I've known for YEARS thru my ham radio hobby. He's definitely a think out the boxer. What he discovered a few years ago was that most switch mode supplies (ie your cell phone charger and your desktop pc power supply) are happy with 120-330v DC. I'm going to guess that newer LED 120v lights will also run on 120v dc. I can't say you should wire this to your breaker box in your house (think of the wall switches that are not suitable for DC arcing, and ceiling fans), but a lot of home things will run on 120 or 220v DC. If you do build a stack of batteries for 120v, do not simply tap off the bottom battery to get 12v... you'll reverse polarity that battery. If you need 12v from a 120v stack of batteries use a 12v power supply. Golf cart people do this all the time, they take 48v batteries and use a power supply to get 12v.

So instead of wiring your grid-tie array for 500 VDC to minimize wire loss, wire them in series parallel to about 250 to 330 VDC so that you can use this power directly if needed. Just about any system that has a nameplate showing it is good for 100 to 240 VAC will work on 150 to 330 VDC too! And almost all modern electronic systems come that way! See more details. But most appliances and lights are a problem. You CANNOT wire two CFL bulbs in series to equal 250 VDC, they are unstable and one will take all the voltage and burn out instantly. But you can series two incandescents of the same wattage, or you can special order special 220 volt CFL bulbs normaly sold in Europe. But you will pay 10 times their value. I'm looking for surplus LED christmas light strings! Large LED bulbs are ideal beacuse they run on 100 to 250 volts AC or DC but cost a king's ransom ($50 each).

Advantages of AC: Transformers allow us to step up voltage easily for long distance transmission without current losses. Arcs across switches extinguish within 8ms at 60hz.
Disadvantage of AC: Power losses in transformers. Peak to peak voltage is 2.8 times higher than the label says on AC. For example, your typical 120vRMS wire in your house actually runs 336v peak to peak. They call it 120, because if you average the power, it does the same work (energy) as 120v DC, so they call it 120v AC RMS.

Advantage of DC: Voltage is actually lower than AC for the same number of watts.
Disadvantage of DC: If it arcs, the only way to extinguish the arc is distance or killing the power at another point. At 120v, that distance is less than 1/8".

You can be killed by DC just as easily as AC, but AC due to it's higher P-P voltage is more likely to jump off the wire and bite you (in the context of those big high voltage lines that go from state to state). Really, the reason we went to AC was b/c more voltage means less amps for the same watts and super high voltage is not suitable for home use. So we step up and step down volts easily with transformers. The safety of the workers wasn't a factor - esp in the 19th century.

Nikola Tesla was the genius who realized the whole step up, step down thing. An amazing man! He's *the* reason we don't have a power plant in every neighborhood.

For *this* application DC is, IMHO, a suitably efficient means to get power from your remote solar cells. The emphasis here is on efficiency. To convert the solar cells' power to AC so you can have batteries near the cabin, then back to 12 or 24v DC, then back to AC to use appliances introduces more inefficiency. In my mind, better to convert once. Make a remote 120v DC stack of solar cells and use switch mode power supplies to get lower voltage DC.

By the way, if any of you open up a computer UPS power supply, how many batteries are inside? Many of the larger units run 120v worth of batteries. Most smaller ones use 60v batteries. The reason is to reduce the amount of current that has to be switched.

Just my thoughts.
Wes

Jose Romero wrote: Remember that AC power is right for transmitted over long distances. Remember to measure how much current AC going to transmit between 450 feet, so use the correct wire. I hope I was helpful.

You are correct in putting the batteries and inverter and such at the remote location and running 120vac to the house. Perhaps the original poster here should run 120vac from the remote location to his house and run a 2nd pair of wires to remotely turn the inverter on and off.

But just to be pedantic, what makes AC suitable for long distance is the jacked up voltage lets you use less current and therefore smaller wires and smaller voltage loss. A company called ABB is working on high voltage power supplies (like 400kv) to build a long distance transmission line that uses high voltage DC.

I'd like to weigh in on this subject. There is a guy I've known for 15 years that uses his prius' 220v battery to run everything. If you leave the car on, it'll crank up as needed to recharge the batteries. What he found that is relevant here is that switch mode power supplies rated for 110-240v will run on dc as well as AC. So you could stack your solar cells in series to get 220vDCand transmit that over a great distance with a switch mode power supply at the home end giving you 12vdc.

Anyway, his web site is http://www.aprs.org/APRS-SPHEV.html