Damage to inverter: Using excess solar photovoltaic to power water heater like trailing edge dimmer

Question

Motivation

I know that a similar question has been asked before (control power to water heater,) but prices and technology have changed, and my situation is slightly different so I am asking again.

My situation is that I have about 5kW peak solar panels, about 11kWh Li batteries and an 8kW inverter (Deye.) I am prepared to add a couple more panels if necessary.

Our electricity supply is intermittent (South Africa) and there is currently no feed-in tariff for small home systems so I have no motivation to supply power back to the grid. On sunny days, my battery (which I let discharge in the night) is usually charged up by noon, and then the rest of the solar power during the day goes to waste. I would like to add an intelligent controller to save this excess energy as heat in my hot water system.

My initial thoughts were a Raspberry Pi/Arduino based system that interrogated the inverter, determined the load, and PV generation, then (based on the currently set control algorithm) would funnel excess power to a hot water system. My current element is about 3kW, 250V.

I am a semi-retired scientist/software developer, but not an electronic engineer. I have programmed Arduinos before for semi real time applications.

I was wondering about using a SSR capable of switching 15 or 25 amperes at 250V, and setting it up to act like a trailing edge (or leading edge) dimmer, i.e. wave form as shown below:

Proposed solution

With an Arduino (or Raspberry Pi) it should be easy enough to switch within millisecond accuracy to get the required percentage of the full element power.

Obviously we would also have a thermostat in the loop (either being intelligently read by the control system, or just the old simple type on/off thermostat as used in normal electrical heaters.)

Questions

Is this a practical idea?
Would it be better to just switch off a whole cycle (or half cycle) at a time?
Will the SSR be able to switch fast enough? (The ones I have seen are 0.1msec switch on time and ~ 11msec switch off time.) Presumably you get faster switching ones.
Could this damage the inverter, or place some kind of weird inductance/power factor/ (name your own bad thing here) load on the inverter?
The SSRs I have seen for sale mention that they have a TRIAC inside (with built in snubber on the output). Is this good or would it be better to use something else (MOSFET maybe)?

I know that one needs adequate heat sinks, wiring thicknesses etc. for this kind of power, and would get help from electrical design people/ colleagues if I decided to go ahead with this.

Why do you want to have fast switching on the SSR ? Can't you just switch once the battery is full and switch back if the battery gets discharged? You are using SSR outside their designed purpose which is never great. — Damien, Jul 04 '22 at 11:16
Features like that are implemented in some systems already (PV ready is a trademark for this), so it's entirely doable and practical. I'd also rethink the choice of not supplying energy to the grid - I don't know the South African grid, but if you'd supply energy to the grid it could help reduce the conventional power needed and thus reduce the emissions. But if it is like in Germany bureaucratic hurdles prevent a simple solution... — Arsenal, Jul 04 '22 at 12:17
If you need to heat water, solar thermal panels may be cheaper, less complicated and safer. — Solar Mike, Jul 04 '22 at 13:12
@Damien I want a way of applying continuously variable power to the thermal element in the water heater. For example as angle of the sun changes and the panels can no longer supply the full 3kW. If not an SSR, what other switching component would you recommend? — Paulus, Jul 04 '22 at 16:07
@Paulus MOSFET based on the DC side would be simpler, but it might cause issues with the solar inverter. — Damien, Jul 05 '22 at 04:45

Harper - Reinstate Monica · Answer 1 · 2022-07-05T19:27:06.420

2

Your triac solution ignores the fact that electrons do not store.

OK, so there you are with a 2000 watt solar surplus. You decide to divert 2000 VA into your 4000 VA water heter, by running the triac at 50%.

During the first half of the half-wave, the triac is off. The solar is pumping the 2000 VA surplus onto the grid.

During the second half of the half-wave, the triac is on. An additional 4000 VA of load is now on. 2000 VA of that comes from the surplus coming off the solar panels. The other 2000 VA comes from the grid in the normal fashion.

Nice job!

You have electron storage. Use it.

Since you already have a battery, you are better off running loads in the normal manner to keep the battery from quite reaching 100%. At 100% turn on a load, and leave it run until battery is 99%. Then turn it off and let the solar array top up the battery.

And since you're running loads on normal sine-waves instead of doing wackadoodle PWM, you can run almost any load! HVAC, dehumidification, a pecking order of loads based on demand.

By which I mean, you need to watch this video.

https://www.youtube.com/watch?v=0f9GpMWdvWI

"I use a roughly 16 kilowatt-hour battery that I just happen to have lying around".

Rather than use your boiler to store heat energy, use your whole (efficient) house!

And that way, your storage water heater is not bouncing around at random dangerous temperatures. You need to keep the water heater at 60C to kill legionella and other bacteria, and your plan requires keeping it much lower so you have thermal headroom to store solar energy. I bet the legionella thing is news to you... it is relatively recent science and was exposed due to the Flint, Michigan water crisis.

Why not make the grid one of the dump loads?

You point out quite correctly that your power company won't give you net metering. Well then! Why back-feed the grid as a default? I would think about rerouting your solar flow so that grid back-feeding is NOT the system default, but is a low priority dump load, behind other priorities such as HVAC, dehumidification, water heater etc.

Honestly, it sounds like if your solar system never fed the grid at all, that would be fine by you. So why not just arrange the system that way in the first place? Have all these loads driven off the battery/inverter (or switched to battery/inverter as directed). Have an algorithm that shaves the top of the battery by calling for storage loads like HVAC or water heat, and the algorithm also looks at when battery is getting too low, and allows the grid to recharge the battery at those times.

If the algorithm was internet connected, it could also pull public-source data that would tell you the expected solarization in your area for the coming hours and days, and use that information for battery management. Like that video says, weatherpeople have the next day pretty well figured out.

So many design possibilities!

edited Jul 05 '22 at 19:27

answered Jul 05 '22 at 07:30

Harper - Reinstate Monica

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Harper: what is it about Legionella that was revealed in the Flint water crisis? Lukewarm water harbouring legionella has been a thing since legionella. Especailly when a lot of that water is going to end up aerosolised, but even in hottubs etc. – Dan Sheppard Jul 05 '22 at 07:55
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_”During the first half of the half-wave, the triac is off. The solar is pumping the 2000 VA surplus onto the grid.”_ The inverter will have at least one quarter cycle worth of capacitors so that’s unlikely to happen. – winny Jul 05 '22 at 09:05
@winny How would the inverter **even know how to do that?** That would be an exceedingly challenging and highly customized design problem. I'm not saying it's un-do-able if bespoke is no object, I'm saying there are *many* better ways to do that goal. Doing it on a scale of seconds or minutes by top-shaving the battery is a better way to use those "capacitors". – Harper - Reinstate Monica Jul 05 '22 at 19:11
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@DanSheppard Lansing's order to "make it happen" forced Flint treatment engineers to work outside the realm of their experience, and they messed up the water treatment that normally keeps legionella and other bacteria in check. And much like Chernobyl, there were "X-factors" they just didn't know about. (the condition of legacy pipes and mineralization on those pipes). – Harper - Reinstate Monica Jul 05 '22 at 19:30
It wouldn’t “know” anything intra-cycle. It will simply be low-passed away by filters. Do you think it it will determine export or local consumption in the kHz range? – winny Jul 05 '22 at 19:37
@Winny Perhaps I envision a different design than you do. I'm assuming a simple grid-tie inverter, and a water heater being intermittently connected to the same panel using normal AC mains panels/consumer units. – Harper - Reinstate Monica Jul 05 '22 at 19:43
Perhaps I can ask a leading question, if you have 1 kW of DC solar incoming, in order to export 1 kW AC, you need 2 kW peak at the top of the sine wave. With no filtering and inter-cycle import-export decision by the inverter as you suggest, export a sine wave would not be possible? In reality, there is energy storage to low-pass away all these things inside the inverter. – winny Jul 05 '22 at 20:03
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@winny yeah we are focusing on different things. Making the sine wave isn't the problem. The problem is how the inverter is connected to the water heater and to the grid. OP seems to have the usual/normal grid-tie arrangement of them all paralleled onto one big happy bus, and once the inverter exports the sinewave, it has no control over where it goes - back to the grid or the heater. The only way the heater can prevent grid export is by jumping in front of it and absorbing the electrons. But it must do so all the time, not some. – Harper - Reinstate Monica Jul 05 '22 at 20:10
@DanSheppard so solar water storage tanks can get above 80 deg C and ours has got to 95 deg C so legionella : dead. Second point if the rate of flow through through the tank is high then the legionella don’t get to breed quickly enough to get a sufficient concentration - as there is just about always trace amounts. – Solar Mike Jul 05 '22 at 20:15
Ah! In this case it will be down to if OPs grid meter samples fast enough to capture part of the sine wave or integrates over a longer time. Perhaps one of those 4/2/1 kW water heaters with binary control of the 7 steps available would be better to match the solar input. – winny Jul 05 '22 at 20:42
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@winny Since a triac solution would involve adding an *abysmal* power factor load to the grid, it would depend muchly on how meters treat poor power factor. "hostile to the customer" is certainly the answer; utilities Do Not Like low power factor. – Harper - Reinstate Monica Jul 05 '22 at 20:56
Many countries grids have no requirements on private homes and just charge kWh of active power, regardless of PF. Perhaps just a different inverter with such a load option built in and a no export setting would be best for OP. – winny Jul 05 '22 at 22:46
@winny i quite agree OP may be making it harder than it is by insisting on backfeeding the grid. Seems like a lack of thoughtful full-system design, just iteration of "add 1 more thing and now I am here, what works from here". – Harper - Reinstate Monica Jul 06 '22 at 01:03
This answer completely ignores the ability of the inverter and battery to store energy. – user253751 Aug 30 '22 at 02:02
@user253751 Cute, but I am addressing OP's proposed solution which doesn't aim to use the battery. I think we agree, since in my answer I propose using the battery to smooth things out. As far as "the ability of the inverter", you have to be careful in this area of practice not to ascribe magical abilities to inverters. Most of them are much dumber than we would hope. I read through the manual of OP's. The "prevent the battery from topping" solution really beats any triac setup. – Harper - Reinstate Monica Aug 30 '22 at 03:56

score 0 · Answer 2 · answered Jul 05 '22 at 01:43

A quick search reveals a lot of commercial solutions that do that: heat water directly with PV power. Also seem to be references for running heat pumps as well. Example: https://www.fronius.com/en/solar-energy/home-owners/products-and-solutions/heating-with-pv