 |
 |
Contact Us: 3225A Richards Lane Santa Fe, NM 87507 (505) 424 -1112 info@positiveenergysolar.com |
|
Backup electricity for utility outages: a very difficult situation. Many people living in conventional homes with utility power have asked us about a solar electric system to provide power and water during an extended utility outage. Others have asked us about using a portable gasoline- or propane-fueled generator as a backup power solution. While the answer is "yes, we can do that for you," the solution is usually not simple. A photovoltaic (solar electric, or PV) power system that can provide abundant electric power for an "off-grid" home--that is, a home designed and built to use an independent electric power system, without connection to utility power--will seldom run a conventional home. This article will look at the uniqueness of this problem and offer some sensible recommendations. The Difficulties of Retrofitting a Home
Few conventional homes are this efficient. Here's one way to look at the difference: A typical off-grid PV power system sized to meet the needs of a family might cost $15,000. Supplied by the utility company, the same electricity would be less than ten dollars a month, plus base charge and taxes! Few utility customers have bills this small, because few have done the load shifting and high-efficiency improvements necessary to live comfortably on the amount of electricity supplied by an independent PV power system. In the winter when the nights are long and the days are short, a storm that brings a week of cloudy weather means that little solar electric power will be available to recharge the batteries. People who live off-grid typically use a combination of energy conservation, battery storage, and a backup generator/charger to weather these seasonal periods. The only other alternative is to stop using electricity and wait until the PV array can catch up, which can take several days. In short, if your emergency electrical needs are small and you have other ways to stay warm, cook food, and supply and heat water, a small backup power system may work for you. Otherwise, a solar electric power system by itself is not a good solution for a conventional home. Using a Generator At first glance, a portable generator seems like a solution to most of the problems we have described. Most portable generators can run a deep well pump. The larger ones will just about run a conventional home, especially on an emergency basis. And they're relatively cheap. So what are the problems with generators? Portable gasoline-fueled generators are built primarily for job site construction power and occasional use. They are not intended to run a home. Gasoline is a highly volatile fuel, which can explode if not stored properly; most people will store gasoline in portable cans. How many days' or weeks' fuel supply is realistic or even possible? Gasoline begins to "turn sour" after about two months' storage, unless stabilized with an additive. Will the generator run 18-24 hours per day, to keep a furnace and refrigerator running, and run a well pump that starts automatically? Will it be shut off two or three times a day for thirty minutes, to allow the engine to cool down before refilling the fuel tank? If not, an explosive fire from a gasoline spill is a real danger. A few hours' run time per day may be more realistic, in which case generator power is really no power except when the generator is running. There are other inherent problems as well with portable generators. The more hours a generator runs, the more it is prone to breakdown. Parts, repair and service are issues which will have to be addressed frequently during the life of a generator. Unless rewired by an electrician, a jumble of extension cords and do-it-yourself appliance and well pump wiring will be an electrical safety hazard as well. A large, high quality, propane- or natural gas-fueled generator that is built for "prime" (continuous duty) or "standby" power, rather than construction use, and is permanently installed and interconnected with the home wiring, will solve most of these problems. Propane fuel can be stored and piped to the generator with relative safety, although the total runtime is still determined by the size of the fuel tank, if tank refills are not available. Breakdowns, while not unavoidable, will be greatly reduced. However, two problems remain. In a backup power application a generator will run anywhere from no load to overload. Running a generator to supply power to a single light bulb or the furnace fan makes for highly inefficient operation, and rapidly depletes the fuel supply. Overloading a generator risks damage to both the generator and anything plugged into it. Second, generators last longest when the two legs of 120 volts AC each that make up their 240 volt AC output are "in balance"; that is, when the load is the same on each leg. A generator running out of balance works against itself, generating heat and shortening its life. Only 240 volt loads are inherently in balance; otherwise, the generator may well always be running in an imbalanced state. Our Preferred Backup Power Solutions
In selecting a backup power system, the first decision you must make is how long a utility disruption to prepare for. A system to supply three hours of backup power will be very different from one designed to last three days; three weeks' backup is met differently than three months'; and one to provide power for three years will be altogether different still! The second decision is to determine what you need to survive a utility outage? First consider lights, heat, water, communication and food storage. Is that blow dryer really necessary? Know your needs, know your wants, and know the difference! We recommend either of two alternatives to secure backup electric power. Each takes a different approach, and each has its own benefits and limitations.
Solar Micro Systems The first approach is a small, independent PV/battery power system for minimal critical loads, with clearly understood limitations on its capacity. This system may be as basic as 12 volt DC power for specialized lights and electronics, such as radio, television, and cellular telephone. It may include a small inverter to make 120 volt AC power for particular needs. It won't run a home, a refrigerator, or an AC well pump. Benefits
Limitations
The second approach is a much more powerful backup power system, tied into the home's AC wiring. While neither an independent PV power system or a generator works well as a backup power system by itself, a combination system offers the best features of each. It will provide the greatest reliability, some inherent redundancy (meaning that a failure of part of the system doesn't necessarily mean a complete loss of power), and the greatest fuel efficiency, while still providing electric power to run most of a conventional house. The system operates like this: A battery bank powers the home, with a pair of inverters (in some cases, a single inverter and transformer) providing clean, silent AC power on demand. When the batteries are depleted, a high-quality, propane- or natural gas-fueled, standby power generator provides power to recharge the batteries quickly at maximum efficiency. When the batteries are full, the generator is shut off. A PV array can be added as an option at any time to reduce generator dependency and increase long-term reliability. In addition, any efficiency improvements, such as compact fluorescent light bulbs, can significantly reduce generator runtime and the frequency of recharge cycles. Benefits
Limitations
Emergency Backup Power Systems |
Off-grid homes are designed and built with solar electricity in mind. Lighting, appliances, electronics, and wiring are all selected and installed to make each watt-hour of electricity do as much work as possible. (A watt-hour is a basic unit of measure of electric power; a thousand of these make up a kilowatt-hour; eight cents on your electric bill). Tasks such as cooking, clothes drying, water heating, space (home) heating are shifted to natural gas, propane, wood, or solar heat. Done well, the result is a home that is bright, warm, and comfortable while using a small fraction (typically 10-20%) of the electricity of a typical grid-tied home.
Whole House Systems
