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Old 11-22-18, 01:13 PM   #11
pinballlooking
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Very cool product. I bet you could get decent payback on this.

How is it working for you?

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Old 11-22-18, 03:18 PM   #12
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Hi Pinball;

The controller is still sitting on my desk. I've got the panels, wire, lightning arrestors, breaker, switches, conduit, etc. Just waiting for a minor health setback to go away so I can put it all together. The final installation with two 340W panels should be about $800.

That is a lot less money than a HPWH and it has no mechanical components to fail. Whether or not this system will outlast a HPWH is yet to be determined. Having read so many negative reviews on box store HPWH systems I'm betting in favor of this device. If it does go poof I can probably repair/improve it at very little expense.
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Old 11-23-18, 11:39 AM   #13
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Quote:
Originally Posted by CrankyDoug View Post
... Just waiting for a minor health setback to go away so I can put it all together. The final installation with two 340W panels should be about $800.

That is a lot less money than a HPWH and it has no mechanical components to fail...
Hope that health issue resolves quickly. I've been down that road before.

Looking forward to how this device works for you and Sunspot. I've seen how well a solar thermal works in South Florida, but this is much less complicated. I've got a segment of roof I could devote to a series of PV panels, if it works out as planned. I've already reduced the wattage of the elements in my 50 gallon water heater, to allow me to heat water on a generator if needed.
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Old 11-23-18, 01:26 PM   #14
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Where2 - Thank you for your concern.

The manufacturer recommends using the standard 4500W element. This provides optimal power transfer since the circuit does not have a buck/boost stage like a normal PV controller. The controller provides power to the lower element. Leaving the upper element connected to mains power allows partial heating on cloudy days. The panels are wired in series. As the sum voltage goes up so does the power. Maximum working values are as follows:

current - 10A
Voltage - 200V
Panel nameplate watts - 1280W

He lists various approved combinations without going into detail as to why, though again, I think he is trying to prevent "Tim the Toolman" mentality and the resulting failures. The mounting plate doubles as a heat sink and is inadequate for much more than 700W. The manufacturer recommends a bigger box for higher wattages. He is adamant about what box to use, probably the result of bad prior experience with unknowing customers. I think a real heat sink or even a bigger slab of aluminum would be a better approach. He certainly deserves credit for avoiding cooling fans.

I was discussing this device with a tinkerer friend and fellow dumpster diver. He offered me two large solar collectors as an alternative. After considering the plumbing and various safety mechanisms that would be required I decided the PV approach would be cheaper and simpler. We get a lot of freezing weather here.
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Old 11-23-18, 10:54 PM   #15
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The igbt devices aren't a mystical mystery device at all. If you know the characteristics of the old school bipolar power transistors, that's how igbt transistors handle the high power switching side. High maximum voltage and current handling, they have a minimum "on-state" impedance, and have a negative temperature coefficient. They can experience thermal runaway and self destruct. They're pretty impervious to voltage spikes on the power lines as well.

On the control side, they're pretty much exactly the same as a n-channel mosfet. Super high impedance, defined and narrow threshold voltage for switching, really stable over a wide temperature and power range. That's because an igbt is basically the same as a mosfet on the control side. In fact, an igbt is basically a power mosfet with an extra slice of material bonded to the substrate.

The only real significant difference kicks in at high switching frequency. The extra slice of material is p-type material, which creates a drain-collector pn junction. At high frequencies, the junction adds some turn-off lag to the switching action that a mosfet does not. This "collector current tail off" affects switching losses. The higher the switching frequency, the higher the losses.

Recent improvements in manufacturing have blurred the selection process as far as which device is better for doing the job at hand. Below about 25 khz, either one is fine. Above that, it becomes an exercise in choosing between conduction losses versus switching losses. Above about 100 khz, the switch time lag of the igbt is too slow, and the switching losses pile up against it.
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Old 11-24-18, 11:29 AM   #16
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Hmm negative thermal coefficient, That probably explains why IGBT's can be put in parallel without instabilities. This water heater board was designed for two in parallel but they were too close together for proper heat dissipation. I think that is why he left the second one out of the final product.

The vast majority of the parts on this board are for the MPPT. It derives its logic voltage from the PV voltage so there are also some parts for the low voltage power supply. The high side is just an IGBT, nothing else. There isn't even a dedicated gate driver, just a resistor fed by two transistors (in push-pull I think) for turning the gate off and on. The collector goes straight to the output lug. The seller advises keeping the wire between the board and heater under five feet. I'm guessing this is to reduce radiated noise.

Looks like a perfect application for an Arduino. I think I would splurge an extra three bucks for an optically coupled gate driver though, like the HCPL3120. Logic and high voltage/power should live in separate rooms.
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Old 11-24-18, 11:42 AM   #17
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Depends on the design. A lot of igbt stuff uses the igbt connected like a relay. A bipolar drive is inherently less noisy than a mosfet due to the lower capacitance and the pn junction lag, so lots of circuits don't have any emi suppression components. Maybe a damper diode to help with inductive loads. They are more forgiving than a power mosfet.
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Old 11-24-18, 12:50 PM   #18
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Some of the literature I read says IGBT turn-off time (and thus the frequency) depends on the driver's ability to pull the gate down quickly. This is because of gate-emitter (I think it was the emitter) capacitance.

My PV's are stored 30 miles from here so I haven't been able to fire this thing up. I have no idea what the switching frequency is.

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