Direct to water heating.

So, back on 5/1 I wrote:

Coincidentally, last week I ordered

PWM MPPT Heater Charger Regulator Solar Photovoltaic Solar Panels Hot Water PV | eBay
PWM regulator for heaters with the algorithm for searching the maximum MPPT power point from solar panels. A buffer module containing capacitors, a choke, diodes is added to the regulator, which allows greater efficiency (up to 30%) and reduction of interference caused by PWM regulation.

and that I bought a new 50-gal water heater off Craigslist to use as a preheat tank. I have installed the water heater, and connected the controller to the bottom (stock) 4500w heating element. It has been operating a couple of days with no issues. With six used 190w panels, I've got 800-900w going to the element in full sun (8 - 8.5a into the 12.8 ohm resistor at 100v). Over the last two days (about 5 hrs of mostly sun each day) I've heated the tank from 70 F to 140 F. I haven't made the last connection to the existing water heater yet, so I'm just heating a static 50-gal tank of water. The temperature disconnect works, the controller shut off at my test set point of 60 C (140 F).

I will write this up for other folks' benefit, and link to it when done. Also, I will order the boost converter that Bill Taylor mentioned (thanks Bill!), and come up with a circuit to use my main solar array to drive the top heater element after my batteries are full.
Want to make use of those idle panels in the afternoons.

The nice thing about that model is that besides MPPT on its own panels, it can work in an alternate control mode at a fixed power point voltage. That allows it to be used in parallel with existing MPPT charge controller off thos existing panels as a diversion controller. My concern is that it is not self powered and appears to need an isolated power supply. Is there a warning about connecting it to a battery? If 90 or more volts, a cheap 120V AC supply can be used on DC to supply the isolated power to the unit.

Yes it needs a 12vdc constant supply. I measured 150 mA on it, so it's only a constant 2 watt draw. I just used an old, but good quality, wall wart.

Missouri Wind & solar had a setup you miht find interesting. It was on an orange bucket, but it's an idea that can be scaled, I think. Incorporate a thermostat.

Fred S said:

Yes it needs a 12vdc constant supply. I measured 150 mA on it, so it's only a constant 2 watt draw. I just used an old, but good quality, wall wart.

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I've just been working on a techluck hit by lightning. Think you are much better off with the one you bought having an external supply. The techluck doesn't have a robust design to supply power to drop voltage to the micro. Any surge on the panels causes a transistor to short. That then puts full panel power onto the microprocessor. The one I have tested has EVERY semiconductor failed. Too bad, an extra nickle of parts would likely make these repairable to some extent. One installer told me he has several customers with failed units he can't get repaired. If your unit fails after warranty I would be happy to fix it cheap just to do some testing on it.

Don't know if it's been mentioned, but there is a good new electric water heater tank using multiple heat elements staggered in the tank so you only heat the amount of water needed. Or, starting from the top, they are switched on sequentially. Better efficiency, especially if you're not going to use the whole tank in one go.

Thanks efficientPV, I'll send it to you if it breaks!

Frankly, this thing was a risk, in that I don't know where to get support if I wanted it. It came with one sheet of paper, and the url
https://actii.pl/knowledge/?p=14
It is clearly well made though, and has been sold since 2007 I think. I also saw it for sale for around US$180 somewhere, unfortunately I can't recall where.

Silentpower, sounds like a good idea. But with home tanks being so short, it doesn't seem like heating only a specific amount of water using this technique would be very effective (compared to the usual 2 elements). However, maybe the idea is to turn on only enough elements for the amount of power available? For solar, this would be an alternative way to "match" the heating load to the panels' available output.

Random calculations for direct to water heating direct from panels.
Panels jinko 315watt 40.4voc 9.92lsc 32.8vmax 9.61imax
Element 24v 600watt (25amps 0.96ohms)
Use imax to work out number of panels required = 25amps/9.61imax= 2.6 panels @ 24 volts (from v=ir)

3 315watt panels in parallel needed for a 600w element to get close to the mpp under ideal conditions.

Just where are those ideal conditions? Try as you might, at 70% panel current you are down to 50% panel power. That is a decent day. By 50% of panel current better off having twice that ideal resistance. Electronic matching is the way to go or have three heating elements that can be switched in and out. I have two small tanks in series, each with a mppt control. Small tanks a pain but give you really fast recovery. My primary tank shuts off at about 10am.

Hi,
PHoganDive pointed me to this thread for a similar question. I was thinking about this solution for direct water heating from pv.

Added PV output monitoring in order to not activate relays when the sun is not out.

efficientPV, what's the voltage and array watts that you run on your tanks? I have 102 actual volts and maybe 900w (derated used panels) feeding my controller.

efficientPV said:

....Electronic matching is the way to go or have three heating elements that can be switched in and out.....

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That's exactly what we kept coming up against. I was looking at the Techluck unit, just seems like a lot of money for what you got. My DC-DC converter is working well (so far). I did run across this today on YouTube, it gives some insight into the design of the Techluck unit.

"TECHLUCK MPPT Water Heater Control,
Technical Look and Repair Part 1"
on YouTube

efficientPV said:

Just where are those ideal conditions? Try as you might, at 70% panel current you are down to 50% panel power. That is a decent day. By 50% of panel current better off having twice that ideal resistance. Electronic matching is the way to go or have three heating elements that can be switched in and out. .

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Wow- 2 of us have quoted at the same time the same part!!

There must be a starting point to match panels with elements just as we use a average setting for the incline of the panels. By determining where the ideal starting point is for whatever you have available or willling to obtain applies either way.
My element is sitting somewhere between here and China due to the virus so I'm going to have plenty of time to alter the plans. It is not going to have electronic controll, extra panels are free- only limited by the hassel of fitting and cable costs.
The extra tank is a good idea and probably the cheapest option when fitting a dc elemebt

In the very beginning, my original thought was to put a 500w element in the bottom of my water heater. And power it with a couple of surplus 250+w modules, each with a used Enhase micro inverter connected directly to the element. So when the thermostat kicked on, the Enphase inverters would see AC and come on a few minutes later. With a timer on the water heater that only allowed it to operate between, say, 10 a.m. and 3 p.m., sounded like a good plan. Total price would have been about $200. On cloudy days, the grid would heat my water. On sunny days, the solar would cover it. Then I realized, the panels would be sitting doing nothing for most of the day. So I started exploring alternatives. And I found a lot of them. In hindsight, "Plan A" may have been the smartest way to go. I have since found out my local utility allows grid-tie of less than 1 KW with no inter-tie agreement or contract. I have spent countless hours toying with this idea, and in the end, I'm not sure if I am any further ahead than if I had just gone with my original plan. I could have just grid-tied 1000w worth of panels. No permit, no hassles. And moved on to the next project. But sometimes we need "projects" to keep ourselves busy, right?

Bill Taylor said:

in the end, I'm not sure if I am any further ahead than if I had just gone with my original plan. I could have just grid-tied 1000w worth of panels. No permit, no hassles. And moved on to the next project. But sometimes we need "projects" to keep ourselves busy, right?

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Ha, seems to be a common disease Yeah, if it was exclusively about saving money, we'd just stop with a grid-tie and move on.
I and undoubtedly many others have been going through the same thought process as you describe for PV heating water. I liked the simplicity of the boost converter idea, and also the one letting the Enphases' see the grid when the water heater thermostat turns on, and start sending power.

I purchased the boost converter you mentioned and tied it temporarily to the upper heating element in my preheater tank. The ACTii AC7391 controller remains connected to the bottom element. The Actii is fed by 900w of old panels, but I tied the boost board to my 2kw array and manually engage it when the batteries are full. A few observations so far:

-The actii functions well, reaching full array 900w into the 13 ohm element in good sunlight. With clouds, 75-200w. (The array's equivalent ohms was matched to the stock 240v 4500w element (13 ohms) to maximize the power delivered.)
-The boost converter is set for the max possible 120v, and attempts to step up the 55-65v from the array (which is a slight overvoltage). It can only produce a max of 66v and 5a into that 13 ohm load. Even so, that is a free additional 300w heating the tank.
-I operate the main mppt scc and the boost converter on the same array in parallel, with a breaker keeping the boost normally disconnected. Because I am overpaneled, in sunlight the scc doesn't know the boost converter is there when I engage it, and they both crank happily. With clouds, the scc drops to zero output and the boost converter pushes out what it can.

We have had very little sun the last 2 weeks, so only a moderate benefit so far. I put totalizing meters on the grid-powered water heater and the new preheat tank just for interest's sake. I will monitor the accumulating kw-hrs and operating hours both for grid and for solar, and we'll see who wins after a month or so.

Easiest way to have a two or three element system is to have a small pilot panel to measure sun intensity. This will relate to how much current the panels can produce. This is loaded down with a resistor small enough so that pilot panel can never reach anywhere near power point voltage. That makes it a linear indicator of current. Cheap voltage switch module can switch in more elements with FET or SSR. You can not just measure panel voltage to switch. You can monitor panel current to switch, but this would be too small a voltage for anything off shelf. Pretty simple circuit if you build electronics. Still, MPPT electronics works better and isn't much harder to build.

I see, cool. So you mean measure the voltage across the resistor, to use as a trigger to engage the next element? (When you said "you cannot just measure panel voltage", you meant just measuring the panel w/o a load on it - i.e. you need the load resistor?)

I haven't built much in electronic ckts since dabbling in college many years ago. But this looks doable. I do want to stay on the panels/DC side to accomplish water heating, instead of putting the load on my 240v inverter (which frankly would be much easier).

Also, the Actii controller is limited to 10 amps, and even though it says it can take 400 volts, I'm reluctant to push much more power than the 900w I'm at. So I have to use a 2nd source for the other element anyway. A 2nd Actii would be good, but I don't want to sink more money into this project.