Plans for my off-grid photovoltaic (PV) system have been pretty much the same for the past two years; enlarge the system and use more of the available power. I’ll continue to do both of these things in 2008, but I’ll also make a major change in the way I use energy from the system. In the past I’ve carefully monitored battery voltage and manually switched loads on and off. By doing this manually I not only miss opportunities to use energy from the sun, I risk damaging the batteries by over discharging them. To resolve these problems, and to relieve myself of the chore of manually switching between power sources, I plan to automate this task.
I’ll automate the switching on and off of loads using ideas from a recent post, “Getting the Most from an Off-grid System”. I’ve decided to use a transfer switch, and a circuit of my own design, to accomplish this task. Refer to the drawing below:
How it works:
The Transfer Switch is configured to use power from the inverter as the primary source of AC power for the load, only switching to grid-supplied AC power when the inverter is switched off. The inverter will be automatically switched on when the battery state of charge (SOC) is high, and switched off when the battery SOC has dropped to a predetermined value.
Thumbwheel switches, precision resistors, and a regulated reference voltage allow precise settings of the “Low Voltage Threshold” and the “High Voltage Threshold”. The thumbwheel switches and resistors create two voltage dividers. The output of the low voltage threshold voltage divider is equal to the low voltage threshold thumbwheel setting, and the output from the high voltage threshold divider is equal to the high voltage threshold thumbwheel setting. These two reference voltages are fed into two comparators.
When the battery voltage falls below the “Low Voltage Threshold” setting, comparator 1 changes state, triggering the Flip Flop, and the inverter is turned off. The Transfer Switch senses the loss of AC voltage from the inverter and switches the load to grid-supplied AC power.
When the battery voltage rises above the “High Voltage Threshold” setting, comparator 2 changes state, resetting the Flip Flop, and the inverter is switched on. The Transfer Switch senses the AC voltage from the inverter, and connects the load to the inverter.
Initially, I’ll set the low voltage threshold voltage to 12.25 volts. That voltage represents an approximate 75% state of charge (SOC). I’ll set the high voltage threshold at 14.75 volts, ensuring that the batteries are fully charged before allowing them to power the load.
It’s interesting to note that the battery voltage will not reach the high voltage threshold setting unless the sun is shining and the batteries have been fully charged. I’ll experiment with other settings in an attempt to improve system efficiency without endangering the batteries.
Since I’m not able to constantly monitor battery voltage, I’ve missed opportunities to use as much of the free power that my solar electric system is capable of providing. Instead, I disconnect the load when I think that the battery SOC may fall below 80%. Once I’ve implemented this plan, I’ll be able to use more of the available power from the system without the fear of damaging the batteries. Additionally, this automation will help to keep the batteries at a high enough SOC to ensure the availability of power in the event of a grid power failure.
With a Twist:
Because of a utility company plan that results in low rates at night, I’m thinking about storing energy in a larger battery bank at night when rates are low, and using that energy to power loads during the day when utility rates are high. And because my PV array is still small, I’m thinking about using a battery charger to supplement the charging that now comes from my PV array. The charger will be turned on via a timer in the early morning hours when rates are lowest, and turned off later in the morning before rates go up. When cloudy conditions limit the amount of charging my PV panels are able to provide, the battery charger will take up the slack and the batteries should be fully charged each morning. Confident that I’ll have plenty of stored energy in the morning, I’ll add more to the daytime battery load, and therefore save money on my electric bill.
With the charger switched on during the early morning hours, it’s likely that the batteries will quickly become fully charged. When that happens, the inverter will once again be used to power the load. Because the charger is still connected, and switched on via the timer, it too will provide power to the load. However, this will occur in the early morning hours when electric rates are low, and I’ll be taking advantage of the lowest rates, cutting my electric bill.
The graph below shows the expected results over a 24 hour period. As a result of charging from the PV panels during the day, and charging from the battery charger at night, the load will be powered by the batteries most of the time.