I'm in Aus, we're 240v & we're talking a household water heater setup. Its a full size electric hot water heater with 2 elements - the top one is a "boost" which is always on, the lower element is wired as off-peak or "controlled load" - which means the power companies turn it on-off when power is cheap, mostly at night. Both elements are 4.8kw elements (12Ohm).
So i've got 7.5kw rooftop solar, which during the day will have up to 5kw spare capacity available during the middle of the day, but mostly 2kw for a good 8hrs a day (depending on the season).
I get 6c/kwh for feeding into the grid, and during the night i'm using off-peak power at 9-15c to heat up the water in the heater. So wasting 3-6c heating water every evening. So wanting to see if I can use the excess solar to heat the water, the issue is the heater has 4.8kw elements - if i turn it on during the day i'll be paying 16-20c/kwh for any power from the grid not covered by solar. So i must be able to throttle it down.
Looked into all sorts of power control to bring down that 4.8kw - SCR's, PWM... but trying to throttle/control 5kw of power was a hassle.
So i decided to keep it simple - just use a transformer to drop the power, and run the heater element on the transformer secondary! Super simple & efficient, no heat sinks or other stuff to worry about.
Its 240v here in Aus, and luckily we can get cheap step down transformers for USA (110v), or Japan (100v) that are pretty high capacity. A 1000VA transformer was $100AU. Its a 12ohm heater coil, at 100v its 8.3A, so 830W - a 1000VA transformer gives the right headroom & putting 8.3A through the 10A relay (which isnt actually switched - as i switch the primary side before the secondary)
I've got the main element on a 30A relay (snubbers on all relays) which will enable/disable Off-peak. Off peak starts at midnight every night & it'll heat for 2hrs, then again at 5am for 30min. The output of that relay then goes to the secondary winding (100v) of the transformer, I did this so i'm not putting 240v through the 100v winding & generating 400v on the primary.
I then also switch the primary side of the coil.
I switch the relays in specific order so i'm switching the minimal current. I never switch the off-peak relay during its use (20A switch), I then switch the secondary (no current as primary is off), then i switch the primary coil - as its lower current than the secondary. (As above - all relays have snubbers)
Everything is controlled by a ESP8266 board with 2 relay boards, & a 5v Vigortronix supply.
Havent had a sunny day yet, but the plan is to run it as much as possible during the sun (can voice activate/web page as its WiFi connectred), I should get 8kwh or so - so that at night it only runs for 30min or so. I could adjust the switching so i can turn it off so it wont operate at night - but there's a risk if its not sunny enough. So i'll just get it as hot as i can during the day & then allow it to top up on the cheapest grid power.
It basically switches/replaces the off peak line with "throttled" peak power into the heater unit - so all the safety stuff remains & the thermostats are still the primary controls etc.
Circuit diagram & photo attached below.
Hopefully this helps some others.
So i've got 7.5kw rooftop solar, which during the day will have up to 5kw spare capacity available during the middle of the day, but mostly 2kw for a good 8hrs a day (depending on the season).
I get 6c/kwh for feeding into the grid, and during the night i'm using off-peak power at 9-15c to heat up the water in the heater. So wasting 3-6c heating water every evening. So wanting to see if I can use the excess solar to heat the water, the issue is the heater has 4.8kw elements - if i turn it on during the day i'll be paying 16-20c/kwh for any power from the grid not covered by solar. So i must be able to throttle it down.
Looked into all sorts of power control to bring down that 4.8kw - SCR's, PWM... but trying to throttle/control 5kw of power was a hassle.
So i decided to keep it simple - just use a transformer to drop the power, and run the heater element on the transformer secondary! Super simple & efficient, no heat sinks or other stuff to worry about.
Its 240v here in Aus, and luckily we can get cheap step down transformers for USA (110v), or Japan (100v) that are pretty high capacity. A 1000VA transformer was $100AU. Its a 12ohm heater coil, at 100v its 8.3A, so 830W - a 1000VA transformer gives the right headroom & putting 8.3A through the 10A relay (which isnt actually switched - as i switch the primary side before the secondary)
I've got the main element on a 30A relay (snubbers on all relays) which will enable/disable Off-peak. Off peak starts at midnight every night & it'll heat for 2hrs, then again at 5am for 30min. The output of that relay then goes to the secondary winding (100v) of the transformer, I did this so i'm not putting 240v through the 100v winding & generating 400v on the primary.
I then also switch the primary side of the coil.
I switch the relays in specific order so i'm switching the minimal current. I never switch the off-peak relay during its use (20A switch), I then switch the secondary (no current as primary is off), then i switch the primary coil - as its lower current than the secondary. (As above - all relays have snubbers)
Everything is controlled by a ESP8266 board with 2 relay boards, & a 5v Vigortronix supply.
Havent had a sunny day yet, but the plan is to run it as much as possible during the sun (can voice activate/web page as its WiFi connectred), I should get 8kwh or so - so that at night it only runs for 30min or so. I could adjust the switching so i can turn it off so it wont operate at night - but there's a risk if its not sunny enough. So i'll just get it as hot as i can during the day & then allow it to top up on the cheapest grid power.
It basically switches/replaces the off peak line with "throttled" peak power into the heater unit - so all the safety stuff remains & the thermostats are still the primary controls etc.
Circuit diagram & photo attached below.
Hopefully this helps some others.
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