I am a bit fanatical about using surplus energy from wind and solar systems. I just had an article published in Home Power magazine about this subject.
I like using Tristar controllers in diversion mode and then triggering more usable diversion (AC water heating) from that using my Tristar Follower idea. That’s my latest trick anyway and I am proud of it. It’s done using hard-wired analog electronics.
However it turns out that there are folk out there who like things a lot more complicated, and I must say it looks like quite a lot of fun too. Will Eert wrote to me to tell me about his Arduino based PWM diversion control system that talks to his Midnite Classic controllers and uses surplus solar energy to heat his water tank based so as to limit maximum current into the battery, and limit its voltage to the prevailing charging setpoint.
You can learn a lot more about Will’s project on the Midnite discussion forum here.
Here is a basic summary of what it does:
The control uses a series of PID controllers to divert under various conditions. This means it modulates diversion at a variable rate based on the amount of power available. The control normally turns on when there 10w available of excess power anywhere in the system.
The control has a dynamic high amp limiter which uses the WBJr amps for an limit amp signal. This lets me “over array” the battery bank but not worry about putting too many amps into the the battery bank (when conditions permit) while still being able to utilize the full output capacity of the array at all times.
The control diverts power based on high amp flow to the batteries. This lets the control divert in Bulk if the excess power is available. I adjust this set point 5 amps lower than the dynamic amp limiter. At present I only divert into one HWT element which is two KW in size. If the amp diversion is at maximum then the amps to the battery increase and the dynamic amp limiter limits array output.
The control also diverts power based on voltage. It always maintains the batteries within .3V of the battery charge setpoints. The divert setpoints are set .3V lower than the Classic charge setpoints. When the divert setpoint is exceeded diversion starts automatically. It is possible to be amp and voltage diverting while dynamically amp limiting at the same time.
The control reads the Classic setpoints and adjusts its setpoints to match. This means that once the control is setup all the owner needs to do is adjust their Classic as they desire. The diversion control will make all needed adjustments to its setpoints to ensure diversion continues to happen optimally.
The control diverts at 10HZ. This is tunable however this frequency keeps the inverter very happy.
If the HWT tank thermostat opens the PIDs turn off and the Classic limits array output. Fully automatic.
Thanks for documenting all of this, Will! I love it.