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Matching Water Heater Elements to Solar PV Panels
#1
Since this has been the topic of discussion a few times, I figured I'd share this. This David explains this issue pretty straight forward and simple, with added humor.


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#2
Wouldn't it be more efficient with a MPPT buck converter?

Because the MPP will change when the brightness changes. The given MPP (VMPP and IMPP) represents the MPP at 1000W/m2 and not at any brightness.

A chart from Wikipedia showing the different MPP's at different brightnesses pretty well:


The different colors are different brightnesses, the inclining grey line represents the Maximum Poer Point (MPP).
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#3
Of course an MPPT would be better. But that's not what the video was about. It's about sizing heating to your panels. I know that the charge controller is to be factored in, but that doesn't negate the math to get the right elements.
Regardless if using straight connection or pwm or mppt, the numbers and math required to size correctly is the same. It's based on the Vmpp and Impp of the panels.

Granted, if he'd used mppt controller, the water would of heated faster as the usable wattage would be available far longer.
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#4
(02-17-2019, 01:22 PM)LEDSchlucker Wrote: Wouldn't it be more efficient with a MPPT buck converter?

Yes, direct connection of a resistive load to PV is a wasted resource, you might gain more enlightening and factual correctness by viewing Daciens content, specifically his MPPT thermal controller presentation.

https://electrodacus.com
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#5
Quote:Water can usually be considered free so the cost in this case is related to storage and I selected in this example a very common 206 Litres (55 Gallon) HDPE barrel than can be had
new for about 60 USD or used/recycled for much less.
The water has the highest heat capacity of any liquid and is 4185.5 J/(kg⋅K) where 1 Joule is actually 1Watt-second to transform this in Watt-hour you just need to divide this by 3600
the number of seconds in one hour. So heat capacity of water is about 1.16Wh/(kg⋅K) and since water density is about 1 then 1kg of water is the same as 1 litre of water and a delta of 1
Kelvin is the same as a delta of 1 Celsius the calculation of amount of energy stored in that barrel of water is quite simple.
Say that usable temperature range in that HDPE barrel is 20°C to 55°C so usable temperature delta is 55°C-20°C = 35°C then
206 litre * 35°C * 1.16Wh/(litre⋅°C) = 8363.6Wh = 8.364 kWh this is a large storage capacity for a small cost and almost infinite cycle life so I hope this shows why thermal storage is
so important in a solar setup

I think the guy in the video explained it a lot better, imho. He explained it in such a way that "anyone" can understand it. Not saying that the guy from Electrodacus didn't do a good job, just not in everyday language that most people could understand.
Plus, the guy in video presented it in an entertaining manner instead of reading a pdf Tongue
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#6
I guess some are happy with just the entertainment, whereas others want plain simple facts and figures, and have the ability to comprehend.
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#7
I do believe that the math the guy in the video presented was actual facts and figures. He explained the delta Temps and what to look for. Then he presented a spreadsheet that would allow you to input your solar panel figures to find out which heating element would be best matched with the panels. I think that calls for Facts, Figures, AND ability to comprehend.
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#8
PSEUDO MPPT  Often we learn a generality and hold on to that like Grim Death. Ideal resistance isn't that for long.  I went back
to an example I had made to see where that improvement would begin. This comparison is between MPPT with direct connect of parallel and single heater element and two elements in series elements. For simplicity, a 60V 10A array and a 6 ohm parallel element. Solar panels are current sources and the voltage of them is based on the load resistance. The 10A can be looked at as 100% of panel rated current and can be applied to any array. Many heater elements are two loops in parallel with shorting bars and one of these is used for the comparison. 2R is using only one of the pair and it can be seen that 70% of the panels current range improves dramatically by switching to  a single element. 4R is the two elements in series and requires an additional SSR.  It would be a simple microprocessor program to look at the panel voltage and choose which produced more power. That could be done even every 5 minutes and only requires an additional SSR to that which is used in a simple thermostat.

MPPT    10A  60V  600W    EVEN    Pure MPPT is this much better than fixed resistor
DIRECT  10A  60V  600W

MPPT      9A  60V  540W    1.1X
DIRECT    9A  54V  486W

MPPT      8A  60V  480W    1.3
DIRECT    8A  48V  384W

MPPT      7A  60V  420W    1.4X
DIRECT    7A  42V  294W
DIRECT 2R 5.4A 65V  350W    1.2X estimate

MPPT      6A  60V  360W    1.7X
DIRECT    6A  36V  216W
DIRECT 2R 5.2A 62V  324W    1.1X estimate

MPPT      5A  60V  300W    2.0X
DIRECT    5A  30V  150W
DIRECT 2R 5A  60V  300W    EVEN

MPPT      4A  60V  240W    2.5X
DIRECT    4A  24V    96W
DIRECT 2R 4A  48V  192W    1.25

MPPT      3A  60V  180W    3.3X
DIRECT    3A  18V    54W
DIRECT 2R 3A  36V  108W    1.6X
DIRECT 4R 2.7A 65V  175W    EVEN estimate

MPPT      2A  60V  120W    5.0X
DIRECT    2A  12V    24W
DIRECT 2R 2A  24V    48W    2.5X
DIRECT 4R 2A  48V    96W    1.2X

MPPT      1A  60V    60W      10X
DIRECT    1A    6V    6W
DIRECT 2R 1A  12V    12W      5X
DIRECT 4R 1A  24V    24W    2.5X
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