How to create a power limiter to get zero-on-the-meter to save taxes

If you are one of the many people exporting electricity to the grid, chances are you are familiar with the following…

When you export electricity you also get to pay energy taxes and or other grid fees. So even if your net-consumption is zero, you still own taxes. This might not be the case in your country or your specific utilities provider, but it will be the future for most of us.

With solar you are either importing or exporting power

The graph above is a simplification of typical grid usage for a solar house over the course of 24 hours. At night we consume power from the grid, and at daytime we are exporting. Either way there is power crossing the meter which can potentially cost us money as taxes.

Keep all your energy behind the meter

We need to keep the electricity meter at zero, most of the time. This means that we need to stop exporting en stop importing, or better, these need to be balanced!

Batteries

Many of the possible solutions for this require batteries to be used. When we are over-producing power, we can charge the batteries, and when we are short, we discharge them.

With a power limiter, we can steer towards zero-on-the-meter

During periods with more sun (or wind if that’s your angle), we can get through most if not all of the day without needing the grid at all. When we are producing less power, there might be a dip at the end of the day because of empty batteries and the sun not yet being up.

The blue line shows us near constant zero-on-the-meter when we can charge the batteries enough

Inverter

When the batteries are not empty, we can mirror our power consumption, by having an inverter produce the exact same amount of power.

An inverter takes DC power from a battery and changes it into 110v or 220v AC, this then flows back into the grid, or is cancelled out by consuming it in your house.

This specific type of inverter is called a grid-tie-inverter.

Limiter

Your typical grid-tie-inverter is normally connected directly to either solar panels, or batteries. Taking as much power as they can and convert is into grid power.

This is not what we want, we want it to be limited to the amount we are currently consuming. This is where the limiter comes in. It passed as much power as we allow it to.

Since this is going to be a DC limiter, we call these devices buck-converters.

Ebay buck converters

Ebay is full of buck converters, they are cheap and work fairly well. There is one problem however that prevents us from using them. They are not programmable. Every single one of them is controlled by a variable resistor which you can turn with a small screwdriver.

Ebay buck converter with screwdriver type potentiometer in blue

So that means we have to make one that does allow automatic adjustments.

Buck converters use coils

Typical buck design

If we were to switch power on to a device half of the time it would consume half the power over time. Then we want to do this really fast (many times per second) and that’s when we run into a problem. The inverter can’t start that fast.

Adding an inductor to the circuit fixes the problem. They resist changes in electricity. Now when we input a square wave by switching on and off really fast. Going through the inductor, on the other side the output power is more like a wave at roughly half the voltage going in.

A changing voltage through an inductor gets smoothed out

An inverter now would be happy to work with half this voltage. It might by a bit wrinkly but at least it is constantly available.

Prototyping

So I made a design, and started building and redesigning and so on.

The prototype power limiter

This is the device so far, I has a big inductor, a socket for a small Arduino to control all the switches and some status LED’s. It also has an I2C connector, this means it can talk to other devices, like a Raspberry Pi that is watching the electricity meter.

This device has more switches than a simple buck device should have. It is a bi-directional-buck-boost converter. That means that it can do more, but also, we can leave parts out if we only need to do buck-converting.

Some fixes

I made a few design mistakes, but was able to fix all of them.

It’s working!

This device can regulate 10 amps at 15 volts at present time, I have not tested any higher values as I discovered some improvements need to be made. I ordered new circuit boards and as soon as they arrive I’ll start assembling them for testing.

To be continued…

Designing a circuit board is a complex and lengthy task. Also I’m without solar panels for now, I bought new ones and some screws to fixate them to the roof are on back-order for month now.

I will start a short series of articles on this circuit board and as soon as I get my panels operational, I will continue this article.


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