Grid Connected PV System
A grid-tied photovoltaic system is a system in which photovoltaic panels or arrays are connected to the utility grid through power inverter units, allowing them to operate in parallel with the utility grid.
Freestanding solar systems are self-contained fixed or portable solar photovoltaic systems that are not connected to any local utility or mains grid as they are typically used in remote and rural areas. This often means that the appliance is far from the nearest fixed power source, or that extending power lines from the local grid can be prohibitively expensive.
Grid-connected photovoltaic system
Grid-connected tower
In recent years, however, the number of solar-powered homes connected to the local grid has increased dramatically. These grid-connected PV systems feature solar panels that provide some or even most of the electricity needs during the day, while still being connected to the local grid at night.
Solar PV systems can sometimes generate or consume more electricity than is actually needed, especially during the long, hot summer months. This extra or excess power is either stored in batteries or, like most grid-connected PV systems, fed back directly to the grid.
In other words, homes and buildings that use grid-connected PV systems can use some or all of their energy needs through solar energy and still use electricity from the normal grid at night or on cloudy and rainy days, giving the best of both worlds. Then in a grid-connected PV system, electricity flows back and forth into and out of the main grid depending on the sunshine conditions and the actual electricity demand at that time.
In a grid-tied photovoltaic system (also known as a “grid-tied” or “on-tied” solar system), photovoltaic solar panels or arrays are electrically connected or “connected” to the local main grid that feeds back electrical energy into the grid.
The main advantages of grid-connected PV systems are their simplicity, relatively low operating and maintenance costs, and reduced electricity bills. The downside, however, is that a sufficient number of solar panels need to be installed to generate the desired amount of excess electricity.
Because grid-tied systems feed their solar energy directly back into the grid, expensive backup batteries are not required and can be omitted from most grid-tied designs. Additionally, since this type of PV system is permanently connected to the grid, no solar energy consumption and solar panel sizing calculations are required, providing a variety of options that allow systems as small as 1.0kWh to be installed on the roof to help reduce electricity usage bills, or a larger floor-standing array that’s large enough to virtually wipe out your electricity bill.
Grid-connected metering
Connecting solar panels together to form larger arrays for direct connection to the local grid enables you to participate in one of the most beneficial parts of your own electricity generation: net metering or net billing. If on a sunny day your solar PV system produces more electricity than you use or consume, the excess solar energy will be sent back to the utility grid, spinning your meter backwards. When this happens, the local power company will usually give you points based on the amount of electricity your grid-connected PV system produces.
If you use or consume more electricity than you generate during the billing period, you will be billed as “net electricity” as normal. However, if you generate more solar energy than you consume, you will receive a “net” of your electricity generation, which may be a reduction in your monthly electricity bill or a positive repayment directly to you or the account holder.
When installing a PV system, if your local utility can do net metering, you may need to install a new second meter instead of using one that rotates in both directions. This new meter allows the measurement of net energy consumption entering and leaving the system and will be used to reduce your electricity bill. However, each power company has its own policy for buying back the energy produced by your own small solar power plant.
While net metering is an ideal way to resell excess electricity generated by solar, some companies buy back energy at wholesale prices that are lower than the electricity you consume from the same utility. This means you may need to generate more solar energy than you normally consume to break even.
Simplifying grid-connected PV systems
Typical grid-connected photovoltaic system
Grid-tied PV systems are always connected to the public grid via a suitable inverter, as the PV panels or arrays (multiple PV panels) only provide DC power. In addition to solar panels, additional components that make up a grid-connected PV system compared to a stand-alone PV system include:
Inverter – The inverter is the most important part of any grid-tied system. The inverter extracts as much DC (direct current) electricity as possible from the PV array and converts it into clean power AC (alternating current) electricity with the correct voltage and frequency to feed into the grid or power household loads.
It is important to choose the highest quality inverter within the budget allowed, as the main considerations for choosing a grid-tied inverter are: Power – The maximum high voltage and low voltage power that the inverter can handle and efficiency – The inverter will How efficient is the power to convert to alternating current.
Electricity meters – Electricity meters, also known as kilowatt-hour (kWh) meters, record the flow of electricity into and out of the grid. Dual kWh meters are available, one indicating the power consumed and the other recording the solar power sent to the grid.
A single bi-directional kWh meter can also be used to indicate net electricity drawn from the grid. A grid-tied PV system will slow or stop the aluminum disk in the meter and may cause it to spin backwards. This is often called net metering.
AC Breaker Panels and Fuses – A circuit breaker panel or fuse box is a common type of fuse box that provides household power and installation, with the exception of additional circuit breakers for inverter and/or filter connections.
Safety Switches and Wiring – A PV array will always produce voltage output in sunlight, so it must be possible to disconnect it from the inverter for maintenance or testing. The isolation switch is rated for the maximum DC voltage and current of the array, and the inverter safety switch must be provided separately to facilitate disconnecting the system.
Other safety features required by electrical companies may include grounding and fuses. The cables used to connect the various components must also be of the correct rating and size.
Finally, the grid itself needs to be connected as well, because without a utility grid it is not a Grid-connected PV system.
Grid-tied systems without batteries are the simplest and cheapest solar installations available, and they are also more efficient by not having to charge and maintain batteries. It is important to note that, unlike stand-alone systems, grid-tied solar systems are not stand-alone power sources. If the mains supply to the grid is interrupted, the lights may go out even when the sun is shining. One way to overcome this problem is to have some form of short-term energy storage built into the design.
Grid-connected system with battery
A small photovoltaic solar system designed with batteries is also operated in partnership with the local power company. Batteries meet short-term peak demand without taking electricity from the grid and paying extra.
When used in grid-connected PV systems, batteries can be divided into short-term storage of hours or days for severe weather and long-term storage of several weeks to compensate for seasonal changes in solar radiation between summer and winter months.
Integrating batteries into grid-connected systems requires more components, costs more, and reduces the overall efficiency of the system. But for many homeowners in remote areas, who regularly experience grid supply interruptions during severe weather conditions or have critical power loads that cannot be interrupted, having some form of backup energy storage in their grid-connected systems can be a great benefit.
Grid-connected PV system with battery storage
Grid-connected photovoltaic systems with batteries
So we can see from the above that the PV system with battery storage is basically the same as the previous Grid-connected PV system, just with the addition of batteries and a charge controller. The battery charge controller determines whether the electricity generated by the solar panel is needed for home use, running low-voltage equipment and lighting, or whether to charge a deep-cycle backup battery for later use.
The DC current leaving the controller goes through a DC-to-AC inverter, which converts it into electrical energy usable by common household appliances. Any excess electricity that is not consumed or used by households can be sent to the power company’s grid. It is best to run DC rated lighting and appliances directly from the solar system before the current is converted from the inverter to AC. This will achieve maximum efficiency.
Using a grid-tied solar PV system is no different than just using the regular grid, except that some or all of the electricity consumed comes from the sun. Photovoltaic solar systems designed for grid connection are typically designed to meet at least half of a homeowner’s electricity needs.
Purchasing a home solar photovoltaic panel array large enough to meet the full power needs of a home will be very expensive because solar panels take up a lot of space. Therefore, the solar energy generated by the grid-connected system is only partially generated, and the rest of the energy is supplemented by the power company.
Advantages of a grid-tied PV system, with or without batteries, on a clear blue sunny day, when the PV system is producing a lot of current and the home is consuming low energy levels, for example, if you don’t have your home to work all day, your solar system is always on generate electricity.
The excess electricity generated is not wasted, but fed back into the grid for use by households near you who unknowingly end up using clean, renewable energy themselves while earning money for you through your “net metering” arrangement.
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