RESIDENTIAL GRID TIE SOLAR SYSTEMS WITH BATTERY BACKUP
THE OPERATION BASICS
The grid tie solar panel systems with battery backup
are similar to batteryless systems, except they can additionally provide electricity for critical loads in case of a utility power failure.
When sun is shining, the solar panels generate power, which reduces the consumption of electricity from the grid and cuts electric bills accordingly. If this power is not enough for your home, the required balance is automatically provided by the utility. If the solar panel system generates more electricity than your house is using, it will feed the excess back to the grid after the batteries are fully charged, and may spin your electric meter backward. Under normal conditions, a small portion of the energy is also used to keep the storage batteries charged.
Conventional batteryless systems
do not provide any backup. An obvious advantage
of a PV-based generator backup is it provides a certain protection against short-term blackouts. Its disadvantages are larger up-front investment relative to a batteryless system and lower reliability due to the batteries limited lifetime. In addition, if "wet" batteries are selected, they need to be periodically checked for fluids.
Sealed batteries do not need any maintenance but may not last as long as the wet types. For a comparison, a typical warranty for
solar panels is 20-25 years, inverters- 3-10 years, batteries 1-3 years.
SOLAR PANEL WIRING AND PRINCIPLES OF OPERATION
This simplified solar panel wiring diagram illustrates the system's operation.
Note that earth ground for the DC circuit have to be provided in a single point. A DC ground fault interrupter disconnects the DC input when a certain leakage current from the ungrounded bus is detected.
The storage batteries should never be connected directly to solar arrays since it may result in frequent overcharging that reduces useful life of the batteries and can cause their damage. Also, with a direct connection, the battery will determine the voltage level at which the solar array will operate. This level most likely will not correspond to the peak power point of the cells (see characteristics of PV panels
). That's why a solar charge controller is normally put in between the PV array leads and the battery bank. If nevertheless you chose to not use a charger, you need to connect the batteries via a blocking diode that prevents their discharge at night due to reverse leakage current of the PV cells. Note that since practically the batteries never share loads equally, it is not recommended to run too many (probably more than four) parallel strings of the batteries unless you use special circuits for the current sharing.
In practice, a grid-interactive inverter for solar backup systems usually includes a DC ground fault interrupter, DC-AC inverter itself and a charger all in one package. It often also has several pairs of fused DC inputs, which can eliminate the need for an external combiner. For the basic principals of its operation see grid tie inverter
The above schematic diagram shows an example of a wiring configuration when the inverter has a low-frequency center-tapped output transformer for 120/240 VAC. Often a low-power residential-grade inverter provides only 120VAC. In this case, you need special "stackable" modules. You will need to stack in series two modules to get 120/240 output. Note that some models provide isolation in a high-frequency DC-DC converter stage and do not have a bulky low-frequency transformer. There are also transformerless
models which require ungrounded DC busses and additional protection devices.
The above system allows excess electricity that is generated by solar panels to be exported to the grid. Under normal conditions, the grid acts as an additional energy source to keep the system's batteries charged. If the grid fails, the inverter will automatically disconnects from the grid and supplies energy from the batteries to the critical loads wired to an auxiliary panel. UL1741 standard requires a grid tie inverter to disconnect from the grid within 0.1 second when input voltage goes off or drops below 60 VAC. When the line voltage returns, an internal AC contactor will automatically transfer the wiring system back to the utility. Note that in general, NEC® 2014 codes allow PV array voltages up to 600VDC. However, if the battery connections are above 60V, you need to make "live" conductors not accessible.