Solar PV and Phase Angle control of resistive heater - Page 1
Forum section: electronics / projects

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: September 05, 2025, 04:17:30 am
content: I have over 5kW of rooftop solar with low daytime use and pitiful feedin rate. No plan for battery for now.There is existing electric storage hot water and am thinking to use that as a "battery" - store the energy in the hot water and buy less/no grid power for hot water. I have lots of other automations for irrigation, HVAC, etc. I could do a simple direct switching of the 2.4kW resistive water heater but there are lots of times when there is only 500-2000W of solar. At those times the water heater would also draw power from the grid and this reduces the benefit, perhaps by quite a bit. Or the heater is not used at all and the solar is given away to the grid.So I'm wondering if phase angle control could be used to match the resistive heater to the available solar, allowing heating with lower solar input and no grid contribution. Use automation to read the solar output, set the controller to 100% when solar is comfortably over 2.5kW and ramp back to zero when solar is say 500W. Goal is to maximise storage of solar energy in water and minimise grid power for hot water. Also gives scope to vary hot water energy use if other loads may take priority eg room heat pump.So, comments/refinements on the idea?Am I likely to be creating an EMI headache? I feel I know enough to recognise a risk here, but not enough to assess or mitigate it.Thanks!

author: moffy
author_rank: Super Contributor
timestamp: September 05, 2025, 07:45:04 am
content: It sounds like a reasonable scheme, but there might be stability issues depending how dynamic the solar generation is and how often it is measured. A pity you can't prioritorze the solar to the heater only and when the heater is off then it gets sent to the grid but I guess that would require a different solar setup.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2025, 08:02:45 am
content: You need to figure out what is the integration period for netting consumption and production together for billing. If it's smaller than a mains cycle (so in milliseconds), then this will not work as you will get billed by consumption, and simultaneously get paid for export (usually at worse price). On the other hand, if the integration period is long enough - for example here it is 15 minutes - you don't need to do phase angle control, you can do it more slowly even with mechanical relay (e.g., a variable length cycle every 15 minutes).I have written basically this same reply tens of times and it always ends up being useless, because apparently there is no way to ask for any documentation how your energy is billed, anywhere on this planet. You just receive a bill you are supposed to pay, without given written or even spoken information on how that was actually measured and calculated. The only way I know the billing net period used in Finland is that I'm deeply enough involved to receive insider information from the industry. So you would need to find a specialist who knows how your transmission operator / country operates, to decide the suitable PWM frequency.

author: Marco
author_rank: Super Contributor
timestamp: September 05, 2025, 08:08:16 am
content: This seems like a very common use case. There are likely modules which take say 0-10V input to run a heating element with constant on time PWM with some small LC filters to attenuate the switching frequency for mains and load. Ideally that's how you want to modulate the power, not with phase angle.PS. Triac based phase cut circuits work fine too, but it's not polite to use them on high powered loads. Terrible power factor and all.

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: September 05, 2025, 12:13:43 pm
content: Siwastaja - that's a very useful insight, thank you!Based on that insight, I'm minded to try a zero crossing SSR that's fast enough to do PWM over some small number of mains cycles, aiming to get inside most/all integration periods. Fairly easy to do as an experiment and the EMI/power factor stuff just goes away. Since it's easy with minimal hardware, just do it and then look at the utility company consumption/export data to see if I am getting the desired result. If so, then happy days, if not then not much lost and have to decide if it's worth a more complicated or expensive Plan B.Further comments appreciated, otherwise will do my best to report back

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 05, 2025, 12:44:02 pm
content: This product does this, using a current transformer to minimise export - for reasons I'm not entirely sure of, it rectifies the AC, and switches using an IGBT, feeding pulsed DC to the heater. Don't recall offhand if it uses phase angle or cycle skipping.https://www.cef.co.uk/catalogue/products/4365857-solar-iboost-unit?gclsrc=aw.ds&gad_source=1&gad_campaignid=9938830706&gclid=CjwKCAjwlOrFBhBaEiwAw4bYDaeTbNYMIkFvIkAQmD-zuqB9aDJM7deEqK-kzoo1MoGqwoXhqKeQDhoC8bsQAvD_BwE

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 05, 2025, 01:27:59 pm
content: Good Morning HarryDoPECC, Here in the USA, most electric utilities will install a second meter for measuring any power you put back into the grid, but much of the wiring on your side has to be done by knowledgeable electricians, and pass a safety inspection, but in the long run, will save you money. Check with your utility.

author: kripton2035
author_rank: Super Contributor
timestamp: September 05, 2025, 01:49:54 pm
content: this one is in french, but explains all about water heaters and solar pv routing. open source.https://f1atb.fr/fr/routeur-photovoltaique-modes-de-regulation/

author: Marco
author_rank: Super Contributor
timestamp: September 05, 2025, 03:06:00 pm
content: Do low cost zero crossing switching mechanical relays exist BTW? Or is the only way to get zero crossing relays off the shelf with SSRs? I couldn't find them, but maybe just using the wrong terms.

author: kripton2035
author_rank: Super Contributor
timestamp: September 05, 2025, 03:24:35 pm
content: a mechanical relay is too slow to switch at zero cross. that's why they don't exist.

author: brucehoult
author_rank: Super Contributor
timestamp: September 05, 2025, 07:16:51 pm
content: Quote from: kripton2035 on September 05, 2025, 03:24:35 pmThe actual make/break should be fast enough, it would be more a matter of timing it correctly given an unknown delay in the mechanism working.Mains power is equivalent to 3000 RPM in an engine (3600 in the USA) and mechanical distributors have been solving this timing problem (reasonably) reliably for over a century. Just run something similar (but with a higher current capability) from a small synchronous motor and Robert is your mother's brother.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 05, 2025, 07:26:07 pm
content: i make SSR legacy products for my employer. Unless specified otherwise, AC SSR's are usually simple 3 quadrant TRIACs, triggered by an off the shelf zero cross optocoupler. Once triggered it stays conducting till within a few volts of zero in the cycle. 0->10V modules usually delay trigger in inverse proportion to the input on each half cycle.What you're paying for in an AC SSR is a usually ceramic SMD PCB, a bit of hand soldering, and protection and snubber networks. Don't forget the immense costs of certification to UL, IET, etc, standards. Liability insurance as well. Many of them on the market are copied straight out of TRIAC application notes.DC SSR however, have some really interesting waveform shaping and biasing circuits on the output side. My predecessor invented something I half jokingly call the 'Opto-Schmidt ' for shaping the drive pulse to the output device while debouncing the input. Specified risetime is less than 25 uS, less that 300 uS or Unspecifed, depending on the model, and fall times are typically 2-3 times that.Lossless rectifier and IGBT are what I thought of when i read your post. Be aware that home water heater elements are not purely a resistive load, some of them have considerable inductance. You may need snubbing. i've used them over the years as water cooled series pass resistors up to 7 Kilowatts. They have quite an inductive kick.So, choose your switch carefully. A commutating AC SSR is a different beast from the common catalog ones.No, we do not make one. Sorry. We make a decent living off legacy drop in modules for industrial plant control. My day often goes like this: "What do you mean we need a new UL approved T1-3/4 red led, what happened to the old supplier?", so not exactly state of the art.Steve

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 05, 2025, 08:22:59 pm
content: Quote from: HarryDoPECC on September 05, 2025, 12:13:43 pmInteresting idea, if a little brutal, you would not be allowed to do that commercially.The heater itself may not like the thermal cycling here either, so may need replacing more often.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 06, 2025, 02:44:07 am
content: Easiest would be to do zero crossing switching and essentially implement PDM with every cycle or half cycle being either fully on or off. In the half cycle case, it can be desirable to add a rule so that when turning on, always start with the polarity opposite the last on period to avoid a net DC bias.

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: September 07, 2025, 01:41:36 am
content: Thanks again all,Taking all of the above it looks better for many reasons to use longer period modulation of power to the heater - a zero crossing SSR (mindful of load characteristics - thanks) with modulation period of many seconds or even minutes, based on any more info I can get about the import/export metering integration periods here (Victoria, Australia - anybody know anything?). Or if no info available, experiment with timing, maybe starting with a period of a minute, see what effect on meter.Will report back

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 07, 2025, 04:11:06 am
content: Quote from: HarryDoPECC on September 07, 2025, 01:41:36 amNo reason to do that with a SSR, modulating on a cycle by cycle basis would in fact mean less thermal cycling on the components involved. Only real downside is the need for a zero cross detect circuit for clocking the logic, but that's trivial to implement.

author: rteodor
author_rank: Frequent Contributor
timestamp: September 07, 2025, 04:17:59 am
content: Quote from: NiHaoMike on September 07, 2025, 04:11:06 amWouldn't this cycle-by-cycle modulation move the stress from the heater to the inverter and MPPT stabilization loop and the capacitors in them ?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 07, 2025, 04:52:48 am
content: Quote from: NiHaoMike on September 06, 2025, 02:44:07 amI think that rule would need to apply to within each burst, (ie whole cycles only) as there is an indeterminate time between bursts.You could modulate the time between bursts to half cycle precision, if it is long enough (some seconds) but I'm not sure a water heater needs that level of precision.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 07, 2025, 05:00:40 am
content: Quote from: rteodor on September 07, 2025, 04:17:59 amThe method in #5 of faster modulation of an IGBT would give a more constituent load to the MPPT, but that has more loss and is more complex.The caps and MPPT inverter may not have been designed or tested with either type of repetitive pulse loads.What fraction of the system, rating is the hot water load?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 07, 2025, 04:00:32 pm
content: Quote from: rteodor on September 07, 2025, 04:17:59 amNot for grid tie. The inverter just puts out more or less constant power while that's going on.Quote from: PCB.Wiz on September 07, 2025, 04:52:48 amIt avoids the condition where it repeatedly switches on the same polarity. A simple logic would be to check on every zero crossing if it should be on or off in order to try to zero out the running average, with accumulated error being put towards the next check. (That's essentially what Delta Sigma is.) The modified logic just forces a wait until the polarity is opposite of the last one that's on, so that it just adds a little bit to the accumulated error. There would not be long runs of the output being on or off except when it needs 0% or 100%.

author: Marco
author_rank: Super Contributor
timestamp: September 07, 2025, 05:52:39 pm
content: For the chopper type circuit ... I can't think of an easy way to feedback power back into the grid for ill behaved inductive loads.The straightforward chopper circuit is first a full wave rectifier with a single transistor behind (creating a rectified chopped voltage), can also use two rectifiers with two transistors (creating an AC chopped voltage). Either way the rectifiers prevent power flowing back.That leaves full bidirectional AC-AC converters, but those arequite complex.. Still much cheaper material wise than a motorized autotransformer, but quite complex.PS. significant inductance would also not be great for a SSR.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2025, 07:04:47 pm
content: Quote from: rteodor on September 07, 2025, 04:17:59 amThe capacitors already have to handle the ripple current from the full power output of the grid-tie inverter, and I do not think the pulse-density modulation using whole and half cycles will be greater than this.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 08, 2025, 01:26:15 am
content: Quote from: HarryDoPECC on September 07, 2025, 01:41:36 amFrom the dusty past, I recall a TDA1023 triac controller that was designed for proportional control of domestic heaters.IIRC it uses a small triangle wave to give a long period PWM on the heater.Looking at the data I see their default period was ~ 40s and they give cap values for down to ~6s.I recall app notes of the time talking about flicker regulations?, meaning faster than 6s was discouraged.I do not know if the part had full-cycle operation to ensure many parts could not cause DC creep.

author: Seekonk
author_rank: Super Contributor
timestamp: September 08, 2025, 10:20:54 pm
content: In the solar world many inverters have trouble with phase triggered power reduction. Primarily lights flickering because control loops can't respond fast enough. Sending complete cycles in variable numbers is better for inverter but still bound to have some flickering. What seems to be popular, you will see it in some YT Iboost controller repair videos is full wave rectifier in series with heating element. The DC section going to a FET which chops at higher frequency varying duty cycle. This has some filtering to get rid of noise. I encourage this approach because it is better and eliminates all timing concerns.I heat water with PV off grid and divert excess power right from the panels. This always leaves batteries with the best charge at the end of the day and doesn't use any inverter or charge controller capability. I look at PV voltage and when that voltage exceeds Vmpp, there is excess to send to the water heater. This system works in complete harmony with a parallel charge controller. I actually have multiple water heater controls that operate on a priority basis based on slight differences in set voltage. Power point voltage tracks approximate panel temperature with an outside temperature sensor. Arc interrupt is included o turn off power every 10ms for about 0.6ms. This allows mechanical thermostats to be used to daisy chain heating elements. Basically just a PWM capacitor bank of 6,000uf or more with a delta V of 5%. This is low speed and I purposely slow down FET switching speeds to eliminate EMI, transition heating is not an issue.I don't see why this isn't more popular other than the solar world lacks even minimal technical talent. This can easily be adapted to zero grid export current sensors. For the lazy in all of us, An old MSW inverter can easily turn into a controller by modifying a couple pins on the SG3525 they all use. All the hard work is done for you. H bridge can easily handle double the inverters rating.

author: brucehoult
author_rank: Super Contributor
timestamp: September 09, 2025, 02:47:30 am
content: Quote from: Seekonk on September 08, 2025, 10:20:54 pmThat seems like a lot of work to gain a little bit of efficiency (it is even 20%?) vs just running the hot water heater from the inverter. Assuming your inverter is sized to be able to handle that in addition to other loads.I manage to get 100% of my hot water heating done in my "Free Hour of Power" each day. Unless I have guests, in which case I turn it on at 11 PM (night rates) and turn it off if/when I get up to pee (the switchboard is in the hall opposite the doors to the toilet and bathroom, so it's not hard).

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: September 09, 2025, 03:38:29 am
content: Thanks again for further interesting and stimulating dialog. Seekonk: your setup is clever, and the rectifier/PWM DC method looks interesting.Seems I should not have specified phase angle control up front, it introduces even more complexities than I'd first thought, AND it may well be unnecessary if the metering characteristics are considered. So next iteration of the plan is:Try long-period modulation, perhaps 0-5 repeats of 20s bursts over 100sec periods. It's a coarse control of average power but may suit the water heater scenario, is quick, cheap, easy and may provide the majority of the benefit with little complexity and few(er) side effects.If the metering sees through something like the above, then perhaps rectifier/PWM DC is overall the better way to go if I want to take the next step? Seems to get around a lot of the secondary effects that can arise from trying to regulate at the level of cycles/half-cycles/fractions of half-cycles

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: September 09, 2025, 04:11:11 am
content: Quote from: HarryDoPECC on September 09, 2025, 03:38:29 amThere are 2 devices on the market that divert power to restive water heaters already: Catch Power and Powerdiverter. Catch works via whole-cycle SSR triggering, Powerdiverter works via phase angle control. Catch had to modify their firmware at one stage because they were too slow and people were getting hit for simultaneous cheap exports and expensive consumption.Quote from: Siwastaja on September 05, 2025, 08:02:45 amThere's no information here either, only heresay. I suspect the meters are sampling many times per cycle, then reporting net production/consumption over 'N' cycles (likely fewer cycles than you have appendages). Each reported data point is then tallied into separate "consumption" and "production" buckets and then billed at 15- or 30-minute increments.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 09, 2025, 10:33:27 pm
content: Quote from: Seekonk on September 08, 2025, 10:20:54 pmA good quality LED driver should have no problem rejecting the flicker.