Grid tied solar pv
Solar energy is not very complicated. This short ebook is meant to give you a general overview of what makes up a solar pv system, types of pv systems, and how to calculate what size system you would need, all in the shortest amount of time possible.
We will start with what a grid tied pv system is actually made of. Most grid tied pv systems are composed of the following:
A racking system, to either make a stand alone structure or to fasten to the support structure of a roof and permanently secure the solar panels.
Solar panels, which are connected together with pluggable connectors called mc4’s
There is usually kind of junction box, to provide a waterproof way to transition from solar wire to conduit, and to provide a safe place to house overcorrect protection, a rapid shutdown system, or combiner panel, if necessary.
Next is the conduit. The conduit provides protection for the wires that connect the array to the inverters, as well as connecting the inverter to the service power.
The inverter is the heart of the system. It has a very important job of constantly turning DC solar power into clean, usable , AC grid power. Sometimes there will be one single inverter, and other times each solar panel will have its own inverter. Most of the time you will have a DC disconnect built into the inverter.
The AC disconnect is simply a way to shut the system down by breaking the connection between the inverter and the grid. Where ever there is electricity, there will have to be an easy and convenient way to shut it down.
And lastly, there has to be some means of interconnection to the grid. Common forms are a dedicated solar breaker in the main service panel, or a line side tap.
That is basically what a grid tied pv system is made of, there are a lot of details involved with each step, but it is good to start with a general idea of the whole subject then start learning details, so you’re not wondering how something applies to what you’re learning.
How solar actually interacts with our grid.
On a small scale, grid tied solar is very simple. Think of it as having your own mini power plant on your roof.
The solar power is fed into the houses power, powers the loads that are on, and sends the rest out to the grid.
The power that is sent out to the grid is tracked by a bi-directional meter, which is a meter that can track the power that you use and the power that you send back.
Different places have different policies, so you will have to ask your local power company or go to their website and look for something called the “net metering agreement”.
Common types of solar pv.
Grid tied: Just solar panels, inverter, and grid power. No need for batteries. Usually has the best return on Investment, but will not work if the grid power turns off.
Grid tied battery backup:
Sort of like a backup generator, you will have a separate, protected load panel for all of the loads that you want to work when the grid power goes out.
The solar power comes in and charges a battery bank, which is regulated by a charge controller. The battery inverter uses the battery power to make usable grid power, which will then supply power to your protected load panel, then send the rest back to the grid.
In the event that the solar can’t charge up the batteries, the inverter can act as a charge controller and use grid power to charge the batteries.
Off grid: Sort of the same as a battery backup system, except, you guessed it, no grid power.
Instead of sending excess power back to the grid, the charge controllers will just not allow the solar to make extra power.
When the state of charge on the batteries gets too low, a generator will automatically kick on and bring the batteries back up to a specified state of charge.
Simplest off grid solar system
The solar panel collects solar energy to charge the battery, which is regulated by the charge controller, then the battery inverter uses the power from the battery to create usable AC power.
What helped me learn this is if you were to disconnect the solar and charge controller, and only have the battery and inverter, the inverter will still make power.
This is called a “DC coupled” system, the solar energy does not directly power through the inverter, but it goes to the batteries first.
There are also “AC coupled” systems, and hybrids of all of the above. For now we will focus on just getting the fundamentals of solar down, so you can build a solid understanding of grid tied solar design.
What to remember
The dc electricity from the solar panels goes through an inverter to convert it to usable AC power.
The inverter feeds the converted solar power to your houses power supply, either in the main service panel, via a breaker, or through a line side tap.
You are still 100% on the grid when you have grid tied solar, No need for batteries, unless you want battery backup.
Off grid solar is almost completely different in design when compared to grid tied solar.
Grid tied system size calculation
The first things that you will need to know are your local net metering policies and average hours of solar irradiance.
For you net metering policy, you will need to go to your local power company website and search for something called "net metering agreement".
If the net metering policy does something besides give dollar for dollar credit on power that you send back and use you will want to get a thorough understanding on what the policy is and factor that in to your equation.
If you want to simplify this even more you can skip this step.
For the solar irradiance hours you can go to the NREL interactive map and find your location.
https://www.nrel.gov/gis/solar.html
Now that you have that info you can do this simple equation and get a good idea of how big of a solar system you would need.
Get the average KWH usage from the past 12 months and average them together to get the average monthly kWh usage.
Divide the monthly kWh by 30.42 to get the daily usage, if you already have the avg daily usage you can skip these steps
Find 82% of your solar irradiance hours
Divide your daily average kWh usage by the number you just got from your irradiance hours.
That number will be what you can use for rough estimations for DC kilowatts.
Example:
Average monthly kWh usage is 1,438
Divided by 30.42 is 47.27 kWh per day
Solar irradiance hours are 5.78 hours per day multiplied by 0.82 = 4.74
47.27 / 4.74 = 9.97
9.97 kilowatts DC
So now I know that including regular solar inefficiencies such as DC to AC conversion, cloudy days, rainy days etc. That this systems will need to be around 10 kilowatts.
This wont include factors such as shading, abnormal system inefficiencies, suboptimal azimuth or tilt, or how much solar you can actually fit on the roof. This isn’t a final design its just a base line to start with.
The formula:
Daily kWh usage / solar irradiance hours X 0.82 = DC kilowatts needed
For more information go to https://www.solarenergysimplified.com/
