Getting Your Solar Feet Wet: the Hermit PV Power Box Chris Greacen The difference between no electricity and a little bit of electricity is huge. If your needs are small, do-it-yourself solar electricity becomes simple, inexpensive, and portable! The PV Hermit Power Box provides electricity for lights at night, a small stereo system or radio for communications, and/or a computer. I designed and built it to power a Macintosh SE computer, hard drive, and printer I used in college to write my senior thesis. It served its purpose well, providing the needed power throughout a cloudy Portland, Oregon academic year Ð and especially fun, running the computer out on the lawn on sunny days. Figure 1 Hermit PV Powerbox schematic Building the Hermit PV Power Box The system was not planned from the beginning Ð I found myself accumulating renewable energy components, and one summer afternoon realized I had all the parts and just needed to assemble them in a box. The heart of the system is a 48 Watt Kyocera J48 panel which charges a circa 1970 SAFT sintered-plate, nickel-cadmium 12 Volt 40 Ampere-hour battery. This battery was originally used to start the turbines on a DC 9 airplane. These cells won't leak if tipped at an angle, and they came securely packed in a stainless steel box. I got the battery from a battery recycler for a college project to turn my bicycle into an electric vehicle. The electric bike worked Ð but it was not a speed demon, nor very maneuverable. I drove it in a fourth of July parade, and that was about it. These sintered-plate nicad batteries packs are not readily available. If you can get ahold of one, count yourself lucky. Keeping safety and portability in mind, a reasonable alternative is sealed lead acid gel cells. If you use lead acid gel cells, use at least an 80 Ampere-hour pack to avoid excessive deep cycling. A 25 Ampere short circuit PV regulator (see Homebrew in this issue) prevents the panel from overcharging the battery by short circuiting the panel when the voltage climbs above a threshold level (set at 15.0 Volts). A homemade bar graph voltmeter (Home Power #11, p. 26) shows the battery's state of charge. I fabricated the voltmeter as a summer project, years previous, at Home Power headquarters, so I had it on hand when I started putting together the Hermit PV power box. If you're starting from scratch, I'd recommend buying a multimeter to monitor the voltage Ð you'll want to use one to put together a system like this. A PowerStarª POW200, 200 Watt inverter converts the 12 to 15 Volt DC current to 110 vac to run the computer or anything else I want to plug in. If I were to do the system over again, I would install a bigger inverter able to put out 700 Watts or more. The usefulness of a tiny portable system like the Hermit PV Power Box increases if it can power intermittent big power users like Skilª saws. All parts except the PV panel are contained in a cube 13 inches on a side and weighing 45 lbs. This makes the whole system portable, and suitable for demonstrations. A note about regulators Be careful that the voltage does not exceed 15 VDC while the inverter is operating. If you're careful to disconnect the panel when you're not using the system during several days of sunshine, then you can do without the regulator entirely. Everyone makes mistakes though Ð better to be safe than sorry. I learned this the hard way recently by accidentally leaving the regulator turned off and the inverter on and unloaded during a sunny day. The boys at PowerStar are serious when they say don't run the POW200 inverter at over 15 Volts! Flying the Hermit Power Box The Macintosh draws about 30 watts, and the hard drive about 10 watts. Under full sun the panel puts out about 50 Watts, so on a sunny day the computer can run indefinitely. When the computer is not running, the panel charges the battery for night time use. A fully charged battery powers the computer and hard drive for about six hours. When my college thesis was finished, I lent the Hermit PV box to a friend to power lights and a radio in his remote cabin. I found I had very few instructions to give him. In day to day operation, the system takes care of itself. The regulator prevents overcharging. When I'm using the computer during sequential cloudy days, I'm careful not to run it when the voltage drops below 11.5 Volts . At 10.5 Volts the inverter shuts itself off. Inverter shut off is no great loss if you're just using lights and a radio. I opened up the battery and cleaned the tops of the cells with a damp rag after a year and a half of operation. The electrolyte contains potassium hydroxide which reacts with CO2 in the air to form potassium carbonate, a non-corrosive white powder. I added distilled water to a couple of cells, but didn't really need to. Then I completely discharged the cells through a resistor, and charged them up again. A complete discharge reduces the memory effect for which sintered plate nickel cadmium cells are renowned. Now I look forward to years of more service. Conclusion Small systems like this are especially rewarding Ð they provide the little bit of very useful power for lights and communications at a remote site. With a bigger inverter, they provide for intermittent use of power tools. Small systems do this without power lines, and without the high cost and tons of batteries. They're portable, and the circuitry is simpler than that of large systems. The secret is keeping needs small from the beginning. If you eventually plan to build a larger home system, building one of these will get you comfortable working with batteries, low voltage wiring, and balance of components. Make your mistakes on a small scale first. So if you haven't already... get yer feet wet now! Access Author: Chris Greacen, c/o Home Power Magazine, POB 130, Hornbrook, CA 96044 Nicad sintered plate cells: Utility Free has sintered plate NiCd cells for $4.99 per Ampere-hour at 12 Volts ($200 for a 40 Ampere-hour 12 V pack (ten cells) - stainless steel case not included), Utility Free, POB 228, Basalt, CO 81621 ¥ 1-800-766-5550