Using solar energy, for example, I am told that excess energy can be sold back to the grid. How does this exactly work? Maybe I am a bit confused due to thinking about the analogy of water pressure / flow rate in a pipe network being similar to voltage / current in an electrical network. If your home generated excess energy, how would it overcome the high voltage / current of the grid?
In: Engineering
What you have in an electricity grid that you do not have in a water grid is transformers. A transformer will convert between different voltages. So you might for example have a 50kV line to your neighborhood which gets transformed down to 240V which is what goes to each house. If your solar panels produce more then 240V the power will go back into the line to the transformer and get transformed up to 50kV. A transformer is fully symmetrical and will convert the power up and down just the same.
Imagine the power grid is a giant wheel spinning at a constant speed. You can connect smaller gears (your appliances) to this wheel and that will cause power to be transferred from the wheel to your appliance. By doing this, the wheel will slow down ever so slightly.
The power company will constantly monitor the speed of this wheel and if it slows down, then they need to add generators to speed up the wheel. The same thing happens if the wheel is spinning too fast. They’ll turn off generators.
Your solar panels are just motors that turn in the same direction as the giant wheel, so it makes it spin faster.
The same way your solar panels can overcome the voltage in your house and feed your appliances. Your solar panels create DC voltage which must be converted to AC through the use of an inverter. The inverter is connected to your breaker box and senses the voltage coming into your house. It converts that DC voltage into AC voltage that’s 1 volt higher than what’s being fed into the house.
The relationship that transfers energy to the grid has more to do with the idea of frequency and “torque” than voltage. Yes, the voltage needs to be matched, but that isn’t what is adding power to the grid.
Solar converts energy to DC power, which is converted by an inverter into AC power which can be added to the grid.
For any kind of electricity generator, before it can be attached to the grid, you must sync up the frequency of AC current between the generator and the grid (as well as the voltage). Once in sync, this is like a bike rolling down a hill at a steady pace. It doesn’t speed up, and no power is added to the system.
After the two are in sync, you can increase the speed of the inverter frequency, which “pulls” the grid speed. Since the grid speed has a much higher “inertia”, it doesn’t change, but that energy has to go somewhere, so it is transferred to the grid as power.
Depending on your location and your utility, the term excess energy is a misnomer. All your solar energy is actually going to the grid in most cases, and whatever you are using in your house is coming from the grid, not your panels. With net metering, if you produce more than you are consuming off the grid, then you get credit for the excess, reducing or eliminating your bill. As for how the electricity from your panels actually makes it onto the grid, I think u/roylennigan explains it best.
Instead of water coming out of the faucet, you pump water into the faucet and get paid for doing it.
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