DC (direct current) electricity flows in one direction. It is an inefficient way to deliver electricity for more than short distances because of the transmission loss as it moves from point A to point B. AC (alternating current), just like its description, alternates between positive and negative directions. AC power doesn’t suffer from critical power losses over long distances like DC power. That’s why utility supplied power is AC.
The basic function of the inverter in a photovoltaic solar power system array is to convert the DC electricity generated by the solar panels into standard AC power. Any photovoltaic solar power system that supplies power to an AC load must use an inverter.
The earliest inverters were electromechanical devices that coupled a DC motor with an AC generator. Both noisy and inefficient, these types of inverters are no longer manufactured. All inverters produced today are Static (solid-state). They employ sophisticated circuitry using MOSFET (semiconductor field effect transistors) and have no moving parts. Many of these inverters now boast efficiencies of 96% or more.
Today, many inverters are also multi-tasking electronic devices, not only converting DC power to AC, they are also power conditioning units (PCU). Performing tasks like rectification, maximum power point tracking and transformation.
Another feature of some modern inverters is the ability to eliminate external connections like AC and DC disconnect boxes. Newer inverters incorporate disconnects right into the unit. This not only simplifies the system installation and wiring, it also provides a cost savings from the elimination of external disconnect devices.
Yet another innovation in modern inverters is the ability to use internal switching to have a single device operate as a grid-tie unit or in stand alone mode when a battery storage system is connected to the same array.
Below are the descriptions of four types of photovoltaic solar power inverters.
Stand Alone Inverters Stand-alone inverters connect to battery storage systems as the DC electricity source and are independent of the solar panel array and the utility grid. The panels charge the batteries and do not interact with the inverter. Only the electrical load connected to the AC output affects the operation of the inverter.
It’s important to carefully size stand-alone inverters to meet the steady state and surge capacity of the connection’s AC load requirements. Stand alone inverters automatically shut down when overloading the unit causes overheating.
Grid Tie Inverters Also known as utility-interactive inverters, grid-tie inverters are connected to and work in tandem with the local utility grid. Power from the array is first directed to the point of consumption (on site) load demand. Any excess power from the solar array not absorbed on site is fed back into the utility grid via the power company’s electrical meter. The swapping of power with the local utility is called net-metering. Simply put, net metering offsets the building electricity load demand and cost by netting out the imported power from the grid and the exported power from the building’s solar array.
These inverters incorporate safety features like anti-islanding. Anti-islanding shuts down transmission of power from the solar array to the utility during a blackout or other serious fluctuation in voltage or frequency. This prevents the hazardous condition of unexpected energizing of utility lines while workers are repairing power lines on the grid.
Bimodal Inverters Bimodal inverters are dual operating in that they can either operate in utility interactive (grid-tie) mode or as a stand alone system, but not at the same time.
Bimodal inverters are connected to the battery storage supply like a stand alone inverter as the solar array charges the batteries. After the critical charging of the batteries is complete, the inverter then processes the solar generated power to the building’s load. Excesses over the battery requirements and the building load are exported back to the utility grid via net-metering.
Unlike a grid tie inverter which cannot supply power during a blackout coupled with zero output from the solar array (such as during the night), bimodal inverters are popular when critical loads need to be energized 24/7. For homeowners, these may be appliances like refrigerators, sump pumps and emergency lighting. Hospitals and other emergency response municipalities as well as businesses like web hosting companies may also use a bimodal inverter powered system.
Again, just like stand alone only systems, the inverter and the battery storage system must be correctly sized to handle the critical loads required of the system during a blackout or other interruptions of power flowing in from the utility grid. Anti-islanding switching is required for grid-tied bimodal inverters.
AC Module Inverters AC module inverters simplify system design and installation because fewer components are needed. These units output power through an interactive inverter attached to individual solar panels which eliminates the use of DC junction boxes. There is no accessible DC wiring so they are not subject to normal DC circuit requirements.
For modular PV systems, this makes changes and expansion of the system relatively easy. The system is connected in parallel to form AC branch circuits. One drawback is that the modules must be built to withstand weather and temperature extremes making them somewhat pricey. In some cases, this may totally offset the savings of lower DC equipment purchase and installation costs.
Although smaller, AC module inverters require many features of their larger cousins such as anti-islanding and maximum power point tracking. They can also perform detailed system output performance analysis and reporting right down to individual solar panels.
Inverter Installation Tips
Inverters are generally installed indoors as this obviously provides a controlled environment and protection from the elements. However, they can also be mounted outdoors provided there is protection against moisture, direct sunlight and other factors that can raise operating temperatures. Like all other electronic devices, exposure to heat is a sure way to shorten the life of an inverter. A cool, dry and ventilated installation area is the optimal environment.
Another factor to consider is the weight of the units, which can be very heavy. Plan your installation beforehand, making sure the mounting area can support the weight. This may require heavy duty brackets and reinforcement of the vertical wall mounting area. The install area should also be free of obstructions to allow easy access for monitoring and possibly repairs. Installations directly attached to drywall (sheetrock) should be avoided.
Inverter Market Analysis
The inverter market is extremely fluid with ongoing innovations in technology, design and functionality raising efficiency and lowering costs. Competition with other Industries for raw materials like semiconductors and other components can also affect pricing and availability.
As the photovoltaic market continues to grow, new manufacturers will surely enter the market or attempt to consolidate existing companies. Manufacturing capabilities and quality control has shown to make a huge difference in reliability. In some instances, where your inverter is manufactured is more important than when it was manufactured.
By: Rick Contrata
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