Residential photovoltaic (PV) power systems convert sunlight into electricity for use in the home. The residence remains connected to the electric utility at all times. Therefore, if power demand exceeds what the PV system can produce, it’s simply drawn from the utility.
Increased use of these PV systems, distributed throughout the utility’s operating area, will ultimately affect the utility. Though they lighten the utility’s load, distributed installations must be monitored to ensure that they don’t degrade utility power quality (see “SEGIS Pushes Photovoltaics Into The Grid”).
During a utility outage, an advanced PV system with a backup battery can provide the required power. The PV array (Fig. 1) supplies dc to trickle charge the backup battery as well as drive the dc-ac inverter that supplies ac for the residence. During the night and at times when there’s insufficient sunlight, the home relies on the utility’s power. PV systems without a backup battery remain powerless during a utility outage.
A battery-backup system can keep “critical-load” circuits in the house operating during a utility outage. When an outage occurs, the PV system disconnects from the utility and uses the battery and inverter to power the home’s critical loads.
These critical-load circuits are wired from a subpanel separate from the rest of the electrical circuits. If the outage occurs during daylight hours, the PV array assists the battery in supplying the house loads. If the outage occurs at night, the battery and inverter supply the load.
The amount of time critical loads can operate depends on how much power they consume and the energy stored in the battery system. A typical backup-battery system may provide about 8 kWh with an eight-hour discharge rate. Therefore, it can handle a 1-kW load for eight hours. Average usage for a home, when running lights, TV, and a refrigerator, is 1 kW.
Typical System Components
To explain the complexity of distributed PV-inverter systems, it’s best to start with the components. First, the PV array employs multiple cells that produce dc power when subjected to solar radiation (Fig. 2). Today, these photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide/sulfide.
Because PV arrays require protection from the environment, they’re usually laminated behind a glass sheet. PV cells are electrically connected together to form PV modules, or solar panels.
Under ideal conditions, all solar cells are equally irradiated and work at the same current. However, some cells may occasionally be partially shaded. These shaded cells limit the current generated from other fully irradiated cells.
In extreme cases, current flow may be blocked because the cells are completely obscured. The shaded cells then behave like a load. The current generated from the fully irradiated cells produces overvoltages that can reach a panel’s breakdown threshold. These “hotspots” can cause shaded cells to overheat and, in some cases, permanently damage them due to current leakage. Bypass diodes connected in parallel with the string of cells prevent these hotspots.
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