Solar hot water, me too

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Murphy's friend


Joined: 04/10/2019
Location: Australia
Posts: 612
Posted: 02:48pm 04 Mar 2023      

The recent post of a solar hot water system has inspired me to build one too.
Hot water here comes from an instant gas heater, it works well (no pilot light used) but its getting old and might require replacement in the near future.

So I installed 12 x 190W (2.2KW) second hand mono solar panels which are connected as two 6 panel strings.
These 6 panel strings give me a 210V MPPV, close enough to power a 230V water heater.

The controller was build from information posted here with the addition of the solar power being re directed to a 2KW GTI once the hot water element thermostat turns off.
That GTI is connected to my mini grid (NOT the street power - I'm off grid here) to be available for general AC power or battery charging via my inverter.

Another feature was the addition of an analog 10A panel meter, monitoring the solar current, beside the usual flashing neon light.

Here are some pictures of this build:

For the capacitors I re cycled 16 off 470uF/450V, giving me around 7500uF in total.

Both were enclosed in metal cases to contain possible high Volt DC fireworks .

All is bolted to the wall next to my off grid power equipment.

Testing was done initially on the bench with a variable high volt DC supply.
First I set the low and high volt trip points. With the optimal MPPV being 210V I choose a smaller than usual range to cycle around that voltage but stay below the open circuit voltage of the string.
So it was set to 195V for the low and 225V for the high point.

I then adjusted the DC supply to 210V, connected a 150W flood light as load, simulated an ON thermostat and switched the DC to the controller's solar input on.

Nothing happened, the floodlight stayed off.

After a while of head scratching the penny eventually dropped when I realised the DC voltage has to be at the upper trip point to turn the IGBT on .
Cranking the supply up to 225V, the lamp started  cycling on and off as expected.
My DC supply voltage is rectified AC via an isolated Variac of only 500W capacity.

Next everything was installed on the wall and connected to my solar panels. This time I used an older type whistling kettle (1500W) as load as its thermostat only cuts out when it boils dry.
I inserted the NTC sensor of my 12V thermostat and a thermometer in the kettle's spout and turned the solar power on.
This time no cycling, the Amp meter showed 6A, with obviously plenty of solar power available just then, and the kettle soon got hot enough to make the thermostat turn the controller off.

So its working (with a kettle anyway), but I still have to complete the power wiring to the hot water heater and install that as well.

An interesting project, I enjoyed building it.
BTW, I used a low Ohm resistor to completely discharge the lethal potential of the capacitors each time before I worked with them, something to consider for anybody planning to build this controller.