Solar panels are the hot new home retrofit on the block, and if you’re looking to save money on energy costs, they make an excellent addition to your home. If you’ve ever installed a solar panel on the roof of your house, you may have noticed that all the wires coming from your solar panel have to be routed through a little metal box before they can be routed into your house’s electrical grid. That box is called an inverter, and it is responsible for taking the DC current produced from the photovoltaic effect in your solar panels and converting it into the AC current used by your home. Choosing the right type of inverter for your solar panels can have a huge effect on installation costs, cabling, and energy savings. For a long time, solar panel users only had two types of inverters to choose from, micro inverters and string inverters. Now a new type of inverter is on the block, the micro parallel inverter could save homeowners up to 75% on solar installation, cabling and activation. To find out how, let us first dive into the basics of how inverters work.
Before we can dive into what an inverter is, you need to understand the two types of currents: alternating current (AC) and direct current (DC). Solar cells, batteries, and many modern devices work with direct current, or the unidirectional flow of electric charge. Charge flows in only one direction. In alternating current, the flow of electric charge alternates or periodically changes direction at a given frequency. The movement is often resolved with a sine wave, although other wave forms may be used depending on the application.
The War of Currents Our story begins in the late 1880’s with the War of Currents, a battle between Thomas Edison and George Westinghouse over who gets to power America and the rest of the world. In one corner we have the incumbent, Thomas Edison the Wizard of Menlo Park and a major supporter of using direct current to power America’s homes. In the other corner, you had George Westinghouse of Westinghouse Electric, who had acquired many of Nikola Tesla’s patents for alternating current, and had even hired the Tesla himself as his consultant. As the inventor of the incandescent light bulb, Edison had already established DC current as the standard for the United States electrical grid. He held all the patents, and did not want to lose his royalties. Edison initiated his infamous smear campaign, which culminated in the famous electrocution of an elephant using AC voltage. The DC power grid required larger cables and made transferring electricity over longer distances costly. With the development of a transformer AC power could be delivered over long distances in smaller wires for a much lower cost. The results were clear and AC current became the norm for delivering power from power plants to homes.
How Inverters Work.Since the photovoltaic process within your solar cells produces direct current, that is charge that flows in only one direction, this presents a bit of a problem when it comes to integrating that flow into your houses power grid, which uses alternating current. That’s where the inverter comes in; taking your unidirectional flow of electric charge and reverses polarity from positive to negative 50 – 60 times a second or 50-60 Hz. This is where the mains frequency rating. The easiest way to conceptualize how an inverter works is to imagine a simple circuit with a 9 volt battery hooked up to a volt meter. When you line up the positive lead of your volt meter with the positive terminal of the battery and the negative lead of your volt meter with the negative lead of the battery, your meter will read 9 volts on the display. If you reverse the leads, touching the positive lead to the negative terminal and vice versa, the volt meter will read -9 volts. If your hands were deft enough to switch the wires in this manner 50 times per second, you would effectively be serving as a crude mechanical inverter that takes the DC current of the 9 volt battery and transforms it into AC current at 50 Hz.
The simple mechanical inverter in our previous example would produce a square wave. Since the current is switching polarity drastically, it creates a large harmonic distortion or a “dirty” supply. While some devices like light bulbs wouldn’t notice the difference, a square wave can be damaging to many devices. It is far better to aim for a sine wave, where the polarity changes gradually into a nice smooth curve. Higher quality inverters produce waves that are closer to a true sine wave. Your mains power supply from the power plant is delivered to you as a sine wave, which is why you don’t have to worry about what devices you plug into your wall socket. Actual inverters use inductors and capacitors with pulse width modulation to switch currents more gradually and produce a sine wave.
Picking the Right Inverter for your Utility GridAs you might imagine, the type of inverter you use to integrate a solar panel array into your house’s electrical grid needs to be able to match the frequency and sine wave of your mains supply. If the crests and troughs of your inverter are out of phase with the mains supply you get destructive interference where the two waves cancel each other out. If the two waves are in phase you get constructive interference which results in a large sine wave. In order for solar power to enter your electrical grid, the two waves must be in phase. There for you need an inverter that is capable of sensing the phase of your mains supply and matching it. This phase matching capability is what differentiates grid tie inverters from regular inverters, and is also why they tend to be more expensive. Now let’s look at the two main types of grid tie inverters used in the home solar retrofit industry.
String Inverters String inverters treat your entire solar panel array as a single giant solar panel. Some of the benefits of string inverters include a lower initial cost per peak watt price and easier install. Since string inverters accept DC current from multiple panels, wiring tends to be easier on initial install. However since string inverters treat the entire system as a single large panel, and performance problems experienced by one panel are extended to the other panels in the array.
Micro inverters take a more modular approach, and must be installed on each solar panel in an array. Since multiple smaller units must be installed in tandem with each solar panel, installation costs tend to be higher. The advantage of using micro inverters is that if one solar panel fails or experiences more shade, it does not affect the rest of the panels in the array. Micro inverters use maximum power point tracking (MPPT) to monitor the environmental condition of a cell and apply the proper resistance or load to ensure maximum power is obtained from the panel. The modular design also makes it easier to locate points of failure.
Micro Parallel Inverters Technology Research Corporation TRC designed the micro parallel inverter with the goal of combining the ease of installation of string inverters with the ease of maintenance and efficiency of micro inverters. A single micro parallel inverter contains four separate channels that can each be hooked up to a separate solar panel. Each channel is like a modular micro inverter, capable of using MPPT to maximize power output from the attached solar panel. Unlike the string inverter, the micro parallel inverter is aware of the status of each individual solar panel, and if performance drops from one panel due to shade, performance of the rest of the panels will remain unaffected.
A study conducted by the U.S. Department of Energy revealed that installation labor is the largest expense associated with solar power installations. The costs of installing a solar power system often outweighed the price of the panels, framing, and wiring themselves. Recognizing this issue, TRC designed the micro parallel inverter with the installation technician in mind. The micro parallel inverter gets its name from its ability to invert four panels in parallel. What this means for the technician is that they can save the time spent installing four separate micro inverters by routing all the cables to a single micro parallel inverter. Each channel on the micro parallel inverter can be removed for maintenance. The micro parallel inverter takes a plug-n-play approach to installation and can easily be configured for grid tied, emergency backup, or off grid applications. It can be mounted in 4.8 kW clusters in rows of four and may be attached to the panel frame, panel array or directly to the roof. Quick disconnects make routing solar panels to each of the four channels a snap. Multiple micro parallel inverters can be connected to a single disconnect at the breaker panel. Up to 16 solar panels maybe tied to a single disconnect, as opposed to 16 separate disconnects for a conventional micro inverter system. With such a drastic reduction in the number of installations it is clear how the micro parallel inverter can save someone up to 75% on cost in certain situations.
The micro parallel inverter is a smart system that allows it to control four 300-W solar panels at once via four separate channels. The modular architecture supports individual fault knowledge, power sharing, health status and other communications. The system can tell you when an individual panel needs to be cleaned, if an inverter has failed, or if a channel requires a service visit. Even if three out of the four panels fail, the micro parallel inverter will continue using MPPT to invert the maximum power output from the remaining solar panel.
The Electricity Monitor and Control (EMC) can provide self-mapping and automated control to the micro parallel inverter. The EMC can be hooked up to the breaker panel and may connect directly to the site’s internet router or a computer via an Ethernet cable. The EMC uses Power Line Carrier Communication (PLCC) to communicate directly with the individual micro parallel inverters which in turn relay information from the attached solar panels. As a result micro parallel inverter system can map the physical locations of its components automatically. The owner or installation technician can log in to the EMC website and follow the instructions to complete installation, startup, checkout and commissioning. The web interface also provides some of the logistical data mentioned in the previous section, and can alert the user via email in the event of a failure. TRC’s micro parallel inverters have been ruggedized with an internal thermal failsafe that protects the hardware against heat related failures via a power output throttling circuit. When a panel reaches its limit, the circuit can be throttled to prevent generation overages from occurring. Instead of immediately cutting the circuit, throttling allows the panels to operate optimally at higher temperatures with a reduced risk of failure. Finally the EMC offers remote access to the owner, allowing them to activate or deactivate the system remotely.
It is clear that the micro parallel inverter effectively offers the best of both worlds, but is it really a new class of inverter? The micro parallel inverter effectively combines the benefits of string inverters and micro inverters into one assembly. Whether you consider the micro parallel inverter to be four micro inverters on a single panel, a “smart” string inverter with multiple channel functionality, or an entirely new class of inverter, only one thing is for certain. With enhanced ease of installation, modular design, and powerful logistics and communications capability, the micro parallel inverter is sure to make your solar panel installation technician’s job go a lot quicker.