Storing the Sun


Trojan makes a typical flooded 6-volt deep discharge lead acid battery. Photo: Trojan

Most new solar installations are designed purely to slash energy bills. Grid-tie systems, meant to cut the amount of electric power bought from the utility company — or even to sell excess power back to the utility company — are a simple, economical solution for most homes.

But if you’re concerned about maintaining power for your home or business through a stormy power blackout or hot summer brown-out, consider a battery backup system.

Energy storage is a straightforward, mature technology, usually based on the simple, dependable lead-acid battery.

The first step in designing a battery backup system is to determine the size of the backup load — that is, the electrical devices you need to run during a blackout. This may be a mission-critical computer system for your home business, or simply the electric blower for the furnace that will keep the house warm through a long blizzard. You’ll probably want to run some lights and small appliances. add up the power demands of all these backup loads to find what your maximum power draw will be, in watts.

Then figure how long you may need to provide that power. if your local utility occasionally goes down for six or eight hours after a winter storm, you may only need a 12-hour reserve. if you need to be prepared for several days of independent power, your solar installer can help you figure out what proportion of the load can be met by your photovoltaic (PV) system while still charging the batteries for overnight support.

The batteries used in a home backup system are different from those used to start your car. They are deep-discharge batteries, which means they have thicker, heavier lead plates to withstand repeated depletion and recharge. Fewer plates means the batteries provide lower voltage.

Typical home systems depend on a series of 6-volt or even 2-volt batteries. to store a lot of energy (measured in amp-hours), the batteries are large and heavy. They come in three main types — flooded, sealed (SLA) and absorbed glass mat (AGM) — with different characteristics and prices; pricing for a typical battery runs between 50 cents and $2 per amp-hour of capacity. A 48-volt system consisting of eight high-quality 6-volt batteries providing 225 amp-hours — enough to run a small household for about 18 hours — might cost about $3,800.

The batteries, along with the specialized inverter, constitute most of the added cost of a backup system.

To charge the batteries, prevent over-charging and hold them at an optimal level of charge (the “float” charge), you’ll need a charge controller chosen to match the characteristics of your battery bank and power sources. A good charge controller measures both the state of charge and the temperature of the batteries and adjusts charging current accordingly.

SMA’s Sunny Island functions as both charge controller and grid-tie inverter switching automatically between power sources. Photo: SMA

In a grid-tie backup system, the charge controller is often built right into the inverter. The combined unit, called a back-up inverter, contains sophisticated circuitry that can switch automatically between power sources (alternating current from the grid or direct current from your PV array or wind turbine). It can also switch automatically to backup mode when the grid goes down. Because of the additional complexity, a backup inverter is more expensive than a standard grid-tie inverter.

Inserting a battery bank and charge controller into the circuit reduces the efficiency of the power flow from PV array to household and grid. Drawing less power from the array, and selling less power to the grid, is part of the price you pay for the guarantee of uninterrupted power.

Seth Masia is managing editor of SOLAR TODAY.

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