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Old 08-05-18, 12:59 PM   #1
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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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