New Off-Grid Solar Inverter Design Reaches 99.5% Peak Efficiency
"I don't know for sure if this is the most efficient in the world, but it might very well be."
Solar panels, while they can last for decades, can suffer from loss of efficiency when routed into your home. Instructables user MPaulHolmes has a fairly uncomplicated solution for off-grid solar systems to eliminate pulse width modulation, have bare minimum switching of the transistor, and maintain small total harmonic distortion.
The hardware requirements for this high-efficiency off-grid solar inverter are fairly simple — you will need to build a circuit that can independently control three separate H-bridges. The bill of materials for the prototype, as well as the software and schematic, are freely available should you contact the designer via email, or you can wait to purchase a newer version of the PCB that will soon be made public.
The circuit operates so efficiently because the switching frequency is incredibly low; the setup creates three isolated islands of batteries, one with nine 12V batteries, the next with three 12V batteries, and the last with a single 12V battery. In a solar setup, this means three separate MPPTs. For the H-bridge switching between the nine batteries in series, that’s four state changes in 1/60 second. For the island with three batteries in series, you have sixteen state changes in the same timeframe and fifty-two for the final H-bridge. In a standard inverter, MOSFETs are switching at 100kHz or more. You will need MOSFETs rated for their respective batteries: 40V for the single battery H-bridge, 60V for the three batteries, and for the nine battery H-bridge, 150V.
The device employs a dsPIC30F4011 microcontroller to toggle the ports that control the H-bridges at the appropriate time and demonstrates no lag for generating a given voltage. You are also able to regulate voltage, for instance, if running from -13 to +13 results in too large an AC waveform. In the above picture of the prototype, the six heavy wires go to the three different battery islands, while two heavy wires are for the 120V AC power.
The full system is sensitive to power factor, which may affect how many watts should be set aside for each of the islands. The ideal situation is three islands of infinite capacity. At peak efficiency, the prototype reached 99.5%, which should be a vast improvement from many other methods of off-grid power management.