Why is grid-tie battery backup not an easy addition to a grid-tie solar system?
The standard grid-tie solar system will not work when the power grid is down. Grid-tie inverters shut down as soon as they lose the AC signal from the energy grid and hence grid-tie inverters do not accommodate grid-tie battery backup. That is because government regulations and electrical codes mandate that solar systems never put power onto an energy grid if the grid is down. Those laws are meant to protect power line workers from electrocution. Hence, grid-tie inverters have always complied with these laws and it has caused problems for solar system owners.
So, owners of solar installations suffer the same bad effects of losing grid power as non-solar users. The sun might be shining away, but it doesn’t matter if the grid-tie inverter shuts down when the grid does! Many people who buy solar systems are surprised to find that their lights go out when their neighbors do with a widespread blackout. The fault all lies with the design of the grid-tie inverters that meet the government regulations.
Grid-tie battery backup ideas that work around government rules legally.
I am not nearly so worried about regulations as I am about the very real risk of electrocutions. Thus, I assure you that anything I am going to try in the near future and will describe to you violates neither laws or lives.
Let us discuss the simplest system first as I have it all in place waiting for my electrician friend to wire it up to the grid. Though I am an electrical engineer, I do not understand or know all the electrical codes. I rely on my friend to get me over those hurdles. This is all going to be fairly technical, but I will try to make it as easy as possible for the layperson.
Phase I of my grid-tie battery backup system will provide about two days power off grid.
I designed and built my Phase I grid-tie battery backup system to keep emergency loads going two days. At least I will be ahead of all the other neighbors who do not have backup generators. But I will be in no better shape than those who do have the backup generators. Those with the generators can purchase extra gasoline to stay energized for a few more days. I will not be able to purchase more solar energy, nor will I be able to use my solar panels to re-charge my batteries. That is not until I test and complete Phase II plans.
We have already purchased and installed all the Phase I equipment in the shop. A battery of two volt AGM (Absorbent Glass Mat) cells makes up a 48 volt 30 kWh energy storage system. I placed all the cells on two shelves of a substantial industrial storage rack along with an 8kW inverter/charger unit. I like the several advantages the AGM batteries offer. Several years ago I started using them in my fishing boat because normal lead-acid batteries were lasting only about a year for me. They designed AGM batteries specifically for deep cycle service which fits a backup application perfectly. You can store the cells in any position because they are sealed and do not leak. Most lead acid batteries do leak and they cause a lot of corrosion problems. Battery corrosion creates performance degradation besides just being annoying.
I plan to configure my 48 volt battery backup system with the grid soon.
All the actual equipment for Phase I is waiting to be wired into the emergency breaker panel and the grid. I plan to do that soon with the help of my electrician friend Mike Egbert. I am inserting a diagram of how I plan to connect this backup system to the grid. RV readers might recognize this scheme as being similar to what their motor coach has. My coach provides us with an almost identical system.
The block diagram below will give you a picture of how the Phase I system will operate. First let me explain the normal operation of the solar system when the grid is up (properly functioning). In that mode, the solar panels feed 350 VDC energy to the three power inverters. Those inverters detect the 240 VAC energy at the main breaker panel and start producing 240 VAC energy they feed to the main panel. The main panel supplies 240 VAC energy as needed by the house and the other two panel loads. Any surplus is fed to the grid and the smart meter runs backwards. If the total loads consume more energy than the three inverters supply then the meter will run forwards . The forward running meter subtracts any energy credits we have stored up. The backwards running meter stores up energy credits we can use later.
The Rest of Phase I grid-tie battery back up system 48 VDC battery bank for a total of 24 two volt cells.
Phase II of the grid-tie battery backup system will follow after experimentation.
I have worked up a paper design that utilizes the same equipment as the Phase I plan uses. This new plan might work out great if the three solar people (from different companies) I have talked with are correct in their affirmative statements. Once I have applied the changes I plan to do an update following this sentence on this page!