Simple Water Heater Diversion Based On PV Voltage Using AC Brick & Zeners

[efficientPV]

Everyone wants simple diversion and this uses cheap zeners, diodes and an AC solid state relay. This is a way to divert or turn on a water heater only when there is excess power based on array voltage going over expected power point. Instead of timers, use this as a way to prevent water heater from turning on while batteries are still charging.

I used 1N756 500mw 8.2V zeners as a way to build up offset voltage. 20 of these were only $3 shipped from US. Additional voltage offset is using the forward voltage of 0.6V to fine tune. Four zeners prevent the voltage from exceeding the 32V input voltage of AC brick.

The solid state relay is a very non linear load which doesn't conduct till the LED reaches on voltage. Then an internal circuit makes the load constant current. That will be about 2ma from 3V to 32V. That makes it difficult to use a potentiometer to adjust the voltage over a wide range. That is a bonus because the resistors will always see limited current and not have to dissipate a lot of heat. This is very simple to correct if a FET or TL431 is added to circuit making it more complex. Stack as many zeners as you want to get near array voltage then use any silicon diode forward voltage drop to make finer adjustments . Later, I will show will show how an old MSW inverter can be easily modified to operate on any voltage array and divert directly from panels. On a 48V system the turn on and off is close enough to allow diversion based on battery voltage.

These SSR have a fairly sharp turn on at about 2.2 to about 2.4V. This is for positive half cycle turn on. You have to get to 2.7V to about 2.9V before both positive and negative cycles turn on. This may irritate some lower powered sine wave inverters. A MSW inverter won't care. Again, this is simple and cheap

 

[FilterGuy]

Is this a correct re-draw of the design:

I am struggling to understand how the current through the Zenors is only 2mA. When the PV is at 260V it seems to me that it would be 169.4V across R1 and that results in 169.4/3000 = 56.5mA. For component sizing, that will be nearly 10W across R1 and nearly 1/2W across each Zenor.


I understand that when the dump load the PV voltage will drop and all of that drop will be across R1, but 1) if the Hot Water tank is at temp, there will be no dump load and 2) I don't think it would drop enough to get the current down to 2ma.

 

[efficientPV]

The idea of the zeners is not to have the SSR turn on until the voltage exceeds the stack of zeners. The SSR input is constant current once there is 3V across it. That lower resistance only adds about 1/2ma. That exact design is just for approximately a 60V array. Reduce the number of zeners and it would work with a 48V battery. The number of zeners is

top 3K dropping resistor 3,000 ohms X 2ma = 6V

SSR = 3V turn on 3V to 32V is the operating range. The lower 4 zeners cap the voltage at that.

For 120V array current does not increase much till array is over 150V

( PV array power point -6V - 3V) / 8.2V example: (120V -6V -3V) /8.2V = 13 .5 zeners or 13

13 X 8.2 = 106.6V 106.6 + 9 = 115.6V That 4.4V could be made up with silicon diodes or just add an extra zener for turn on a couple
volts higher.

I used a high quality CRYDOM 75A zero crossing brick. These are not TRIAC, but back to back SCR inside. They obviously triggered by an optical triac since turn on in the positive cycle is lower than the negative cycle. I would have thought they would go the extra effort to make turn on symmetrical. In the case of AC motors, any analog signal driving this SSR should avoid this 2-3V range or the motor will over heat.

In another oddity, I have some of these which are 120V AC signal input. I could run a DC motor with this SSR by applying only Positive cycle to input or negative. That would cause the motor to go clockwise or counter clockwise. There are many things a data sheet doesn't tell you and are only found out with testing.

If willing to go with a TL431 design, the zeners can be eliminated and the asymetrical turn on is eliminated.