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Old 08-05-18, 12:59 PM   #1
CrankyDoug
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Default PV direct to water heat

I have an unused area in the middle of my heated basement that should be ideal for four IBC's. I also have a place near the house to put about 3KW in PV. I do not, at present, have the time or money to build a complete off-grid system with heat pumps.

Since most of my winter electric bill is for heating and DHW I was wondering about the possibilities of using the PV's to provide DHW and thermal storage in an IBC array. I have used IBC's for mineral oils at 100-110F without any problems.

Are there any known complications in using the DC output from the PV's to directly power a DHW tank or resistive elements?

This system would help me through the winter, by which time I will be in a better position to install an inverter for next summer's air conditioning.

Grid tie is not an option due to local policies.

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Old 08-05-18, 11:00 PM   #2
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The only real issue in this type of system is the switch. Common relay contactors are rated for alternating current and won't live long trying to switch a DC power source. Make sure that you get power contactors rated for direct current.
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Old 08-06-18, 04:22 PM   #3
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Switching DC did occur to me as a problem. I got around this in the past by using substantially oversized force guided contactors according to their de-rating schedule. It may not work in this case. The contactors I have here don't even specify a de-rating for DC applications.
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Old 08-06-18, 06:14 PM   #4
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Use MOSFETs or IGBTs for switching. Or for the simplest temporary solution for space heating, leave the heaters connected at all times during the winter and open a window as needed to vent excess heat.
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Old 08-07-18, 07:39 AM   #5
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If you're running low voltage (24vdc or less), contact arcing is less of a problem and regular AC rated contactors can work. Once the voltage goes up above about 50vdc, the contactors with magnets inside must be used. When the switch contacts open, the magnets repel the arc and blow it out. AC contacts tend to either weld to each other or vaporize in short order trying to do their job with high voltage DC. The more current run through the switch, the faster the arc welding process occurs. I try to get a magnet contactor rated about 50 percent over the circuit breaker amps.

Then again you could use a solid state device. They operate silently and for many more cycles than a mechanical contactor. I try to get double the voltage and current ratings in the specs on the label. The things tend to run cooler and handle blips and spikes without failing immediately.
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Old 08-25-18, 09:15 PM   #6
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Quote:
Originally Posted by jeff5may View Post
If you're running low voltage (24vdc or less), contact arcing is less of a problem and regular AC rated contactors can work. Once the voltage goes up above about 50vdc, the contactors with magnets inside must be used. When the switch contacts open, the magnets repel the arc and blow it out.
I was involved in the design/build of Chicago's 3200 series transit cars which operate on 600VDC. We wanted to redesign a disconnect due to space considerations. GE said sorry, all the engineers that did that sort of thing were dead. We got a 50 year old design and made it fit.

Your comments immediately made sense. Those disconnects had huge blast chutes.

You and Mike are probably right about using solid state devices. As for Mike's suggestion to vent any excess heat through the window, it just occurred to me the garage could always take the unused heat. That would keep the condensation down.

Thank you both for the help.
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Old 10-03-18, 06:34 PM   #7
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I found a MPPT water heater control that has been on the market about five years.

Solar Hybrid Hot Water Solution - No Plumbing Mods Required - At TechLuck

The manufacturer is based in Las Vegas. He sells them on ebay and has good feedback. I've only seen one independent review on the device but it was positive. I'm probably going to order one. The price seems a little steep but if it works as advertized I don't mind someone making a buck off a good idea.

Comments?
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Old 10-08-18, 03:08 PM   #8
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I'm curious about this myself. Please post anything you learn. Thanks.
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Old 11-20-18, 02:13 PM   #9
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Quote:
Originally Posted by CrankyDoug View Post
I found a MPPT water heater control that has been on the market about five years.

Solar Hybrid Hot Water Solution - No Plumbing Mods Required - At TechLuck
Here is a follow up on the above mentioned device. First note that I haven't had the chance to test it yet.

It came well packaged but without documentation. I was about to contact the seller when an email arrived with the PDF manual. It is well written and clearly explains how to use the board. I was a little disappointed that it did not contain the technical data I was after. Since none of this data would be useful to the average buyer I can't blame the seller for omitting it.

I was able to extract enough details from the seller's website to extrapolate the rest. A PIC16F690 provides the MPPT algorithm. There is a MIC 10 blocking diode and a rather large zinc oxide varistor for spike protection. The double sided PCB is thoughtfully layed out and all components are screenprinted on the top side.

Being insensibly curious I wanted to know how this thing worked. So I started following traces and looking up component datasheets. Nothing made sense because because I really didn't know the first thing about an IGBT. It also didn't seem to have any means of monitoring current.

Re-reading the website I finally got it. This device doesn't need to monitor current to perform MPPT because the load is a simple resistance.

P=V**2/R.

The maximum power point therefore occurs at maximum voltage. There is no requirement for a buck/boost circuit. The PV voltage simply passes through the IGBT to the water tank heating element. The PIC modulates the on-time and a few electrolytics act as a buffer.

At 100% duty cycle the standard 240V, 4400W element would run at about 1100W if three PV panels in series were operating at a knee voltage of 120V and a little over 9A.

Purists might quibble over the use of the term MPPT. The end result is the same. It's just a special case made possible by the fact that the load is entirely predictable.

There is what appears to be a MOSFET next to the IGBT, both of which are heat-sinked to the aluminum mounting plate. I'm guessing it is used to drive the IGBT - a wild guess as I still don't know much about IGBT fundamentals.

Best of all, no batteries!
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Old 11-22-18, 08:52 AM   #10
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Thanks for those details Doug. I've got one on the way as well.

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