Monday December 12, 2011

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Comparing Types of Backup Power Adapted From:  Wiring Your Digital Home For Dummies

Power outages can happen anywhere and anytime. Include your power-outage exposure risk as you plan your home, considering both the likelihood of an outage and the loss you will experience during an extended outage. This will help you decide how much backup you need, as well as the best type for you.

Uninterruptible power supplies

The smallest backup power supply normally used in homes is an uninterruptible power supply (UPS). A UPS is basically a battery unit that charges itself when you plug it into wall power, and then provides backup power during an outage. A UPS most often powers a computer. The battery in a UPS lasts a finite amount of time. If the power outage is long, you may only have time to save your computer data before the UPS runs out of power. Most UPS units used with computers are supplied with a cable and interface software to shut down the computer before the UPS battery runs out of power.

Inverter systems

If a computer-style UPS isn't quite powerful enough to suit your needs, consider stepping up to an inverter system. A UPS is a self-contained system. It stores direct current (DC) power in internally housed batteries, and then a component called the inverter converts that to the alternating current (AC) power most household electrical devices use. The inverter is a component of an inverter system. It uses the same technology as the UPS, but the external battery banks are much larger than those inside a UPS. They both convert battery power into alternating current (AC) when needed. Figure 1 shows a typical inverter.

Most UPS units don't deliver enough current to power refrigerators and other big-load systems, but you can install a battery bank and use an inverter to connect those batteries to your electrical system when necessary. A battery bank and inverter are basically just a large-scale UPS for your home. The battery bank provides instant power when an outage occurs; you don't have to wait for a generator to start up. When the outage ends, the inverter turns into a battery charger and recharges the batteries.

Batteries are designed for two primary types of service:

• Lead-acid batteries: Batteries such as the one in your car supply a large amount of current over a very short time for powering the starter motor. Once the car starts, the alternator handles the loads.

• Deep-cycle batteries: Batteries such as the one used for a boat's electric trolling motors or in electric golf carts supply a steady amount of current over a long period of time.

Green power systems

Green power systems don't cause conventional pollution. Wind and water systems use their respective mechanical forces to spin generators and generate DC voltage to charge batteries or use directly.

So-called green power systems include

• Wind: Wind power is very costly. Older systems with batteries used DC wiring systems within the home. But with the development of high-efficiency inverters, these DC systems don't make much sense.

• Solar: Solar panels can produce DC power. These panels need no maintenance other than an occasional cleaning and are relatively inexpensive.

• Water (hydroelectric): Water-wheel motion can very effectively turn a generator if your home and property include a stream or creek with a sufficient waterfall.

From a purely financial standpoint, green power systems only make sense if the cost of bringing conventional utility power to the home is excessive.

Power from two or more environmentally friendly sources can be combined. Figure 2 shows a general diagram for charging batteries with solar and wind power, as well as with a backup generator. You can control the generator automatically to start when the battery voltage is below 11 volts on a 12-volt battery bank. If the wind and solar sources can supply the load and keep the battery voltage above 11 volts, you need to start the generator only to test reliability and for maintenance.

External combustion steam systems

Another method of producing power is by steam. Most utilities use steam to produce their power. Their power plants may use nuclear power, oil, coal, or wood to produce steam, which then drives steam turbines. For now, home steam power systems are a thing of the future.

Internal combustion systems

The most common type of backup system is a generator running on an internal combustion engine. For homes, these generators typically range in size from 4 kWto 25 kW. Some are portable units so you can drag it out of the garage and hook it up temporarily during an outage. Others may be permanent and start automatically during an outage, although you usually have a lag time between when the power goes out and the generator starts. The most efficient internal combustion generators run on diesel fuel, although propane or natural gas-fueled units are more common.