Most Efficient Off-Grid Solar Inverter in the World
Introduction: Most Efficient Off-Grid Solar Inverter in the World
Solar power is the future. Panels can last for many decades. Let's say you have an off-grid solar system. You have a refrigerator/freezer, and a bunch of other stuff to run at your beautiful remote cabin. You can't afford to throw away energy! So, it's a shame when your 6000 watts of solar panels end up as, say, 5200 watts at the AC outlet for the next 40 years. What if you could eliminate all transformers, so a 6000 Watt pure sine wave solar inverter would weigh only a few pounds? What if you could eliminate all pulse width modulation, and have absolutely bare minimum switching of the transistors, and still have an extremely small total harmonic distortion?
The hardware is not very complicated for this. You just need a circuit that can independently control 3 separate H-bridges. I have a bill of materials for my circuit, as well as the software and schematic/pcb for my first prototype. Those are freely available if you email me at <EMAIL_ADDRESS>. I am not able to attach them here since they are not in the required data format. In order to read the .sch and .pcb files, you will need to download Designspark PCB, which is free.
This instructable is mainly going to explain the theory of operation, so you can make this too as long as you can switch those H-bridges in the necessary sequences.
Note: I don't know for sure if this is the most efficient in the world, but it might very well be (99.5% peak is pretty dang good), and it does work.
Supplies
13, or 13*2, or 13*3, or 13*4, ... 552
12v deep cycle batteries
A very basic electronic circuit that can independently control 3 H-bridges. I made a prototype, and am happy to share the PCB and Schematic, but you can certainly do it differently than how I did it. I also am making a new version of the PCB that will be for sale if anyone wants it.
Step 1: Theory of Operation
Have you ever noticed that you can generate the integers -13, -12, -11, ..., 11, 12, 13 from
A*1 + B*3 + C*9
where A, B, and C can be -1, 0, or +1? For example, if A = +1, B = -1, C = 1, you get
+1*1 + -1*3 + 1*9 = 1 - 3 + 9 = +7
So, what we need to do is make 3 isolated islands of batteries. In the first island, you have 9 12v batteries. In the next island you have 3 12v batteries. In the final island you have 1 12v battery. In a solar setup, that means also having 3 separate MPPTs. (I will have an instructable on a cheap MPPT for any voltage very soon). That is a tradeoff of this method.
To make +1 on a full bridge, you turn off 1L, turn on 1H, turn off 2H, and turn on 2L.
To make 0 on a full bridge, you turn off 1L, turn on 1H, turn off 2L, and turn on 2H.
To make -1 on a full bridge, you turn off 1H, turn on 1L, turn off 2L, and turn on 2H.
By 1H, I mean the first high side mosfet, 1L is the first low side mosfet, etc...
Now, to make a sine wave, you just switch your H-bridges from -13 up to +13, and back down to -13, up to +13, over and over and over. All you have to do is make sure that the timing of the switching is done so that you go from -13, -12, ..., +12, +13, +12, +11, ..., -11, -12, -13 in 1/60 second (1/50 second in europe!), and you just have to make the changes of states so that it actually conforms to the shape of a sine wave. You are basically building a sine wave out of legos of size 1.
This process can actually be extended so that you can generate the integers -40, -39, ..., +39, +40 from
A*1 + B*3 + C*9 + D*27
where A, B, C, and D can be -1, 0, or +1. 552