I want to see if I'm looking at this the right way and what I'm missing. I've looked through a lot about generating heat directly from panels to a heating element (no MPP). From research I see an MPP would improve output but I got the panels for free and all I want to do is heat one of my three barrels in a small greenhouse to increase the amount of radiant heat available at night to prevent the electric heater on grid power from kicking on for 4-6 hours on cold nights, I'll still have the heater available for colder nights. I'm in Colorado. My temperatures aren’t getting too high in the daytime so I have some room to allow the increase in daytime heat I’ll get from the heating barrel. I also want to start to learn about these panels to maybe use them for a different usage in the future and have more knowledge for setting up a system when I add a battery.

I’ve found found David Pop’s ‘matching heating element to load resistance’ vidoeo, I found an article ’56 issue 152 renew magazinerenew.org.au, DIY PV water heating’ that agrees with this resistance matching too. I don’t really understand how impedance affects this but I know it’s a factor.

3 panels = 840W

Pmpp=280W

Voc=38.97

Vmp=31.67

Imp=8.84

I was told the panels are about halfway through their advertised life.

Wired in parallel Vmpp=31.67V, Impp=26.52A, r= 1.194 (David Poz calculator, Vmpp/Vipp) 1.536-1.194=0.342

David Poz excel recommends 48V 1500W 1.536 Ohm heater (1500/24=31.25amps)

Wired in series Vmpp=95.01V, Impp=8.84A, r= 10.7477 (David Poz calculator, divide Vmpp/Vipp) 12.8-8.84=3.96

David Poz excel recommends 240V 4500W 12.8 Ohm element (4500/240=18.75amps)

From what I’ve read the panels are generating current and sending it to the heating element and the voltage and in turn wattage will drop off significantly if the resistance values do not match. It will be most efficient at max power and also drop off a lot around that, but I will have pretty steady direct sun on the panels for most of the day.

Does it matter that the parallel configuration will be able to send more of the current the element would draw, or will the higher voltage balance that out in the series connection? Is the smaller difference between the resistances in the parallel system going to make it a better choice?

How much power could the heating element generate?

I know this voltage won’t be Vmpp but how will series vs. parallel affect my results below?

Parallel: When connected to a 31.67 volt power supply (31.67*31.67)/1.536=668.66W

Series: (95.01V*95.01V)/12.8 Ohms=705.23W

To find out how much will this raise water temperature I found a chart that says I need 617.5W to heat 50gallons 5 degrees in an hour. If I have sun all day but maybe 4 hours when it’s striking the panels most directly that’s 20F in a day, which seems pretty good.

I didn’t find the right equation for raising temperature using power in watts and time so hopefully I can rely on this chart until I do.

I could also heat two barrels and I think running the heaters in parallel means I assume half the optimal load to each is the max?

I don’t see an element that’s available in the xcel that’s close to half the resistance for the series setup (6.4ohms) but for the parallel setup half the resistance would be 0.768ohms and there is a 24V 600W heater that’s 0.96ohms (0.96-0.768=0.19)

If the voltage in parallel is 31.67Vmpp half of that would be fine for a 24V heater.

Would each element ideally generate (15.84X15.84)/0.96=261.4W? Though that would still be only 522.8W total, less than the 1500W element above.

Doubling the surface area would double the rate of heat transfer so maybe two barrels radiating heat would be better than one hotter barrel.

If I have 261W per barrel this would heat a barrel about 2degrees F per hour, so if I’m using 4 hours it’s 8 degrees each and is less heat so would only be better if doubling the rate of heat exchange to the air was worth the loss.

If I use the proper size wires and ground the system is either setup safer, and what are advantages and disadvantages to each system?

I could also run two sets of panels in series (not connected) to two barrels/heaters with two 120V/2000W heaters, each with a resistance of 7.2 Ohms, only a 0.035 difference from the system resistance (Vmpp/Impp) of 7.165.

(63.34*63.34)/7.2 Ohms=557.22W*2=1114.43W

I didn't try two independent sets of two panels in series or parallel when I was looking at combinations before so if I'm looking at this right then this is the best combination.

Thanks to anyone who sees this and knows about these kind of systems or has worked on solar setups like this.

I’ve found found David Pop’s ‘matching heating element to load resistance’ vidoeo, I found an article ’56 issue 152 renew magazinerenew.org.au, DIY PV water heating’ that agrees with this resistance matching too. I don’t really understand how impedance affects this but I know it’s a factor.

3 panels = 840W

Pmpp=280W

Voc=38.97

Vmp=31.67

Imp=8.84

I was told the panels are about halfway through their advertised life.

Wired in parallel Vmpp=31.67V, Impp=26.52A, r= 1.194 (David Poz calculator, Vmpp/Vipp) 1.536-1.194=0.342

David Poz excel recommends 48V 1500W 1.536 Ohm heater (1500/24=31.25amps)

Wired in series Vmpp=95.01V, Impp=8.84A, r= 10.7477 (David Poz calculator, divide Vmpp/Vipp) 12.8-8.84=3.96

David Poz excel recommends 240V 4500W 12.8 Ohm element (4500/240=18.75amps)

From what I’ve read the panels are generating current and sending it to the heating element and the voltage and in turn wattage will drop off significantly if the resistance values do not match. It will be most efficient at max power and also drop off a lot around that, but I will have pretty steady direct sun on the panels for most of the day.

Does it matter that the parallel configuration will be able to send more of the current the element would draw, or will the higher voltage balance that out in the series connection? Is the smaller difference between the resistances in the parallel system going to make it a better choice?

How much power could the heating element generate?

I know this voltage won’t be Vmpp but how will series vs. parallel affect my results below?

Parallel: When connected to a 31.67 volt power supply (31.67*31.67)/1.536=668.66W

Series: (95.01V*95.01V)/12.8 Ohms=705.23W

To find out how much will this raise water temperature I found a chart that says I need 617.5W to heat 50gallons 5 degrees in an hour. If I have sun all day but maybe 4 hours when it’s striking the panels most directly that’s 20F in a day, which seems pretty good.

I didn’t find the right equation for raising temperature using power in watts and time so hopefully I can rely on this chart until I do.

I could also heat two barrels and I think running the heaters in parallel means I assume half the optimal load to each is the max?

I don’t see an element that’s available in the xcel that’s close to half the resistance for the series setup (6.4ohms) but for the parallel setup half the resistance would be 0.768ohms and there is a 24V 600W heater that’s 0.96ohms (0.96-0.768=0.19)

If the voltage in parallel is 31.67Vmpp half of that would be fine for a 24V heater.

Would each element ideally generate (15.84X15.84)/0.96=261.4W? Though that would still be only 522.8W total, less than the 1500W element above.

Doubling the surface area would double the rate of heat transfer so maybe two barrels radiating heat would be better than one hotter barrel.

If I have 261W per barrel this would heat a barrel about 2degrees F per hour, so if I’m using 4 hours it’s 8 degrees each and is less heat so would only be better if doubling the rate of heat exchange to the air was worth the loss.

If I use the proper size wires and ground the system is either setup safer, and what are advantages and disadvantages to each system?

I could also run two sets of panels in series (not connected) to two barrels/heaters with two 120V/2000W heaters, each with a resistance of 7.2 Ohms, only a 0.035 difference from the system resistance (Vmpp/Impp) of 7.165.

(63.34*63.34)/7.2 Ohms=557.22W*2=1114.43W

I didn't try two independent sets of two panels in series or parallel when I was looking at combinations before so if I'm looking at this right then this is the best combination.

Thanks to anyone who sees this and knows about these kind of systems or has worked on solar setups like this.

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