Simple Designs for Efficient, Low Cost & Reliable Systems Steve Willey After a dozen issues of Home Power, you will have noticed the wide variety of systems. Folks are using wind, solar, hydro, & engines of all kinds and combinations to make electricity. Some use big batteries to store the power, some use smaller ones, some folks use several different sets of batteries at the same time. The system voltage may be from 12 to 130 VDC. Some folks use the stored power as direct current (DC)- right from the batteries. Others use an inverter to convert all the low voltage DC to 110 vac- just like downtown. And others wire their home for both 110 volts ac and low voltage DC. What the folks use the power for is even more varied. This diversity is to be expected from individualist, pioneer types. Some designs fit one set of opportunities and needs; different approaches are called for elsewhere. There are many home power design concepts that work and more or less meet our needs. The field narrows if we require the working system to also have the lowest possible cost, high reliability, simplicity and low maintenance. All of this at the same time. Here are some ways to do just that. REDUCING POWER CONSUMPTION COSTS LESS AND WORKS BETTER THAN OVERPOWERING THE REAL PROBLEM WITH BRUTE FORCE KILOWATTS. Example: Replacing one 60 watt bulb operated 6 hours a day with one PL-13 compact fluorescent lamp saves 282 watt- hours each day. That would be the same extra power available each day as adding one more 48 watt PV panel. The lamp change over costs about $35. An added panel costs over $300. Notice that this savings of 200 watt-hours is gained every day the lamp is used: on the sunny days when an extra panel would give the same, but the lamp change ALSO saves 200 watt hours on sunless days when an extra panel would do little or nothing. That is why conservation in design is MORE important than the energy source itself. The Choice: ac, DC, or Both One of the first choices is whether ac only, DC only, or a combination of the two will be used in the home. I use and recommend a combination of ac and DC together. Even though this means two separate sets of wiring, it usually meets your needs better while giving you significant cost savings and reliability gains. Alternating current of course, is appropriate for most regular 120 vac appliances, power tools, large screen TV's, microwave ovens... But not all. Telephone answering machines, alarms, chargers for cordless appliances or ni-cad batteries, two-way radios and radio-phones and electronic clocks and intercoms all need power on a FULL-TIME basis. All of these use minutely small amounts of DC power. When connected to ac, they use a few more watts, because they must convert it back to the DC which they actually use internally. If an inverter is operating 24 hours a day to continuously maintain a few watts of ac for any of these, it is forced to operate in its lowest efficiency range for a large part of the day. Look at the efficiency chart for the greatest inverter you can find. It will be 85 to 95% efficient from about 80 watts to over 3000 watts, but plunges to 50% efficiency or less anywhere below 80 watts. 50% efficiency means it wastes as much power as it uses. If you run just 35 watts full time you will use most of an extra kilowatt hour every day. For this you must buy at least three extra PV modules and you lose power you could use more productively. Most solar technicians can set up answering machines, alarms and ni-cad battery chargers to plug into 12 volts DC, where the power consumption is far less. Fans and lights are other items that are often used in low enough wattage to make inverters operate inefficiently. The very efficient Compact Fluorescents provide good light in 7 and 13 watt versions. Although available in efficient ac versions, three, four or more of these can be used together and still not add up to an efficient loading of a large inverter. DC wiring for the most used lights can save substantial energy each day for the rest of your life! Other classes of appliances are simply not available in energy saving models for ac, but the portable or DC versions are quite efficient. The most interesting examples are computers and refrigerators. The computer I am writing on is an IBM compatible "laptop", with hard disk. It is designed to be portable, although I never move it from my desk. It is built with "CMOS" circuitry, which uses far less power than ordinary integrated circuits. My meter shows it is using 9 watts right now and NOTHING can interrupt my power. Inverters will run nearly any computer, but the power used will be 10 to 20 times higher than a computer designed for DC power source. This will cost you a lot in dollars and inconvenience, if it is used many hours a day. Refrigerators generally require about 350 watts in conventional versions and run time is about 8 to 14 hours of each day. Sun Frost DC refrigerators run less than 50 watts DC and their run time is also 8 to 14 hours of each day. Part of this power savings is because the compressors are designed to use minimum power and part is because the Sun Frost refrigerator is so well insulated. Yes, inverters made today can handle a 350 watt standard ac refrigerator with ease. Just plug it in, add more solar modules and feed it the kilowatts. Let's say I add such a refrigerator to a home that already has 6 or 8 PV modules. Instead of adding 3 to 5 extra modules to handle the Sun Frost load, I would have to add 10 to 20 extra modules to handles the added inverter load. And bigger batteries too. You can add up the prices. Ideally, I like to provide nearly all outlets for ac, but provide one or more DC outlets in each room for the items discussed above, with heavier wiring to the refrigerator outlet. Then lighting circuits, with associated wall switches, are nearly all DC circuits. This requires dividing the wiring to two systems, not necessarily twice as much wiring. The Choice: Battery Voltage When autos changed from 6 volts to 12 volts, it would have been better if they had gone to 24 volts instead. They established a very solid standard of 12 volts without foreseeing the future of high powered automotive stereo and motor homes with all their electrical loads. Nevertheless, we have today a very well established standard of 12 volts. I suppose the most practical standard for homes someday might be 150 volt batteries. Alot easier to convert to high power 120 volt ac (which has 150+ volt peaks) than any other voltage. But that doesn't fit most small and growing solar electric homes because it requires lots of PVs and lots of batteries and a large inverter- all right at the start. And, since it's not yet a standard, few inverters are available without voltage step-up for home scale power. Common choices are 12, 24, 36, or 48 volt and a few 32 volt from the original windmill era. I advocate 12 volts for the typical remote home unless there is a very good reason for a different choice. Most DC applications: lights, answering machines, auto and RV accessories, as well as remote home products like DC ceiling fans are most available in 12 volt. Portable computers and video cameras have 12 volt power cords now. PV equipment such as charge controllers and inverters are often more easily available at lower prices in 12 volt. Other voltages are available if there is a real need, usually 24 volts. Some 32, 36, and 48 volt and 115 volt DC systems are in use. You will find that 24 volt inverters cost more per watt. Several 48 volt inverters have been discontinued because small sales and technical problems did not justify further development. 24 volt systems have half the current flow, which means smaller DC wires can be used. Some lights are available in 24 volt, usually at a higher price and less variety. If a system is all ac, no DC used directly, than a 24 volt battery system may be an advantage. PV or hydro transmission can cover twice the distance with the same wire size. If a cottage industry calls for motorized tools to run many hours a day, 24 and higher volt motors are less in demand, lower cost on the surplus market and higher horsepower. These are a few good reasons to use higher battery voltage. But IF THESE GOOD REASONS DO NOT APPLY TO YOU, consider that in the typical remote home, a 12 volt system will offer more opportunities to save money and headaches over the years. Limits To The Pursuit Of Efficiency? There are other tricks available to the resourceful inventors and tinkerers, such as converting washer motors to DC. This does save some energy, but is not a path for everyone. If the budget is very tight and the washer is used a lot, such savings can be important. For most, the inverter is the easiest way to power a washer. Inverter Tips 1) A 1200 watt or larger inverter with high surge capability can run most clothes washers. The starting surge is the real test, sometimes the motor won't start and it will overheat. This is usually cured by adding a motor start capacitor if the washer doesn't already have one (most newer ones don't). This device costs from $4 (solar dealer) to $10 (washer servicemen) and is easy to install. Unplug the washer. Find the diagram of the washer's wiring, locate the START wire of the motor by color, cut it and attach the cut ends to the two connections on the capacitor. If that frightens you, the washer serviceman can do it. (Kenmore washers with only 2 or 3 wires to the motor don't take capacitors). 2) Wiring an inverter to a home that also uses an ac generator should be done so that power from both CANNOT be connected to the wiring at the same time. The simplest way is to bring the generator power in on a separate line direct to just one outlet, next to the inverter. The house wiring is fed from a fuse box or breaker box. The power TO this box is fed through a permanently attached "line cord", just as if the whole house were a giant appliance with a cord to plug in. Its plug matches the outlet from the generator, OR the outlet on the inverter, but of course cannot be accidentally plugged into both at once. If the inverter is a "standby" model, its power cord plugs into the generator outlet and the house line cord plugs into the inverter outlet. The inverter switches everything automatically, you never move the plugs. If the inverter should need service, you can remove it and during its absence simply plug the house into the generator outlet directly. This arrangement absolutely requires cords and outlets rated to carry the full power capability of your of your house breaker box. Keep hot and ground polarities correct on all plugs and outlets used. 3) Small neon lights plugged in around the house will let you see at a glance whether your automatic inverter is running or idling or shut off. These orange neon night-lights use so little power that 5 of them in my home will neither start up the inverter nor hold it on. Trace inverters are quite clear in their indication, the nights flicker when idling and glow steady orange when running. Heart inverters show two levels of brightness for idle or running. One of these in the bedroom has stopped me from forgetting to turn off the computer printer several times. Another accessory is the clamp on ammeters used for testing auto starters and alternators. These can be clamped onto the inverter's battery cable to get a reading of current and a confirmation of standby charger operation. They are not numerically accurate, but do help a lot. We give both meters and neons free with each standby inverter installation. If your solar dealer does not have them, they are available in drug stores and auto parts stores and both together will cost you about $15. Steve Willey can be reached at Backwoods Solar Electric, 8530 Rapid Lightning Creek Road, Sandpoint, ID 83864 or call 208-263-4290.