Matching the power...

Externet

Joined Nov 29, 2005
1,559
Hello.
If I have on the roof ten 250 Watt solar panels for a total of 2.5KW; and series/parallel wired to supply 115VDC output to a heating element inserted in my house HVAC duct (day only use)

The heating element is rated 5 KW at 230VAC (~22Ω); when fed by 115VDC from the panels will heat not glowing red.
If I split the heating element to two 115VAC paralleled elements instead (~6Ω), when fed by 115VDC from the panels, will glow red.

Which arrangement will heat the house more; the heating elements in series or in parallel ? Seems there is a flaw in my thinking ?

KeithWalker

Joined Jul 10, 2017
1,387
Use Ohm's law:
The supply has a fixed voltage of 115V
With a load of 11 ohms, the current will be 5.2A and the power will be 601 watts.
With a load of 5.5 ohms, the current will be 20.9A and the power will be 2,405 watts.
The parallel arrangement will give you four times more heat then the series one.
Regards,
Keith

Dodgydave

Joined Jun 22, 2012
9,837
Your heating elements are rated for 230V, so if you half the voltage then you will Quarter the wattage, so 115V supply with a 22 ohms heater is 601W.

2.4Kw on 230V.

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KeithWalker

Joined Jul 10, 2017
1,387
The figures I quoted in answer #2 are only approximate because the heating element will change its resistance with temperature.
Keith

Ian0

Joined Aug 7, 2020
1,113
The heating element is probably Nichrome, which won't change very much with temperature.
The solar panels will change voltage with temperature far more than the heating element will.
But did no-one ever think what happens when the panels are not in full sun?
What you have approximates to a constant current source, which varies with light level.
In full sun, you have a 22A constant current source, with a maximum output voltage of 115V, and the previous caculations are correct.
With the sun at an oblique angle or behind cloud, the current falls but the maximum voltage stays about the same
Now you might have a 10A constant current source, which would put 60V into the 6 ohm load = 600W, but switch to a single 11 ohm load and you would get 1100W.
If the light gets down to 25% of peak, and you get 5A, the you would get 550W into the two heaters in series (22 ohm), but only 150W into 6 ohms.
But you might live somewhere where the sun shines brightly all day, not in a cloudy, rainy little island off the coast of Europe! (although where I live there are 30 fewer rainy days a year than in Liverpool)

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Externet

Joined Nov 29, 2005
1,559
Thanks, gentlemen.
Then, simply, have to match the power of the solar panels assembly in use, to the heater elements rating.
If I add solar panels later, will have to add other matching heater elements to the added panels. Cannot choose the first heater elements with higher power rating foreseeing future panel additions. Goes by pairing them step by step...

Ian0

Joined Aug 7, 2020
1,113
Need to think about some sort of MPPT circuit.

What do you have against decibels?