Matching Water Heater Elements to Solar PV Panels

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.

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

Because the MPP will change when the brightness changes. The given MPP (V_MPP and I_MPP) 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).

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.

LEDSchlucker said:

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

Click to expand...

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

https://electrodacus.com

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 20C to 55C so usable temperature delta is 55C-20C = 35C then
206 litre * 35C * 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

Click to expand...

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

I guess some are happy with just theentertainment, whereas others want plain simple facts and figures, and have the ability to comprehend.

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.

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