heating water with PV

electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Just a thought experiment here. Say you have a lot of cheap/free/extra modules and you want to heat water. Lets say also that you have grid power but cant backfeed or have already maxed out your backfeed. What is the most economical way to do it? Directly connecting modules in series to a 240V (or whatever) heating element would seem to be simplest, but of course you take a big hit by having no optimization of the PV power curve to the heating element. I guess you would size things to get peak output on sort of an average sunny day. An improved option might be to make a up a controller and contactor that will give you two "Settings', sun and shade. Is there a cheap (LOL) MPPT controller that would do this? They make "linear current boosters" for panel direct pumping applications, but they are pretty lower power. A MPPT off grid charge controller (such as the xantrex or morningstar 600V) connected to a small battery with a diversion controller is a nice setup that I have used before, but gets kinda pricey if we are talking about a dozen KW or more. I just keep running into any sort of MPPT controller not being worth it. Ideas?
 
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pv_n00b

Senior Member
Location
CA, USA
Occupation
Professional Electrical Engineer
I've seen systems with modified microinverters connected directly to water heater elements. The utility is not involved, no interconnection requirements. They are dead simple. First time I saw it was in Hawaii many years ago.
I've also seen some designs that combine a PV module with a heat exchanger on the backside that water is pumped through to provide low temperature preheating for water.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
How much of a hit do you actually take by not having MPPT tracking? I'm not going to fall down the rabbit hole of doing the calculation :)

I agree that the simplest/cheapest approach is to simply connect the panel DC to the heating element.

Cheap MPPT would probably be something along the lines of switching in different resistance heating elements to closely match the panel output.

Of course if you want the opposite of cheap:

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G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
pv_n00b said:
I've seen systems with modified microinverters connected directly to water heater elements. The utility is not involved, no interconnection requirements. They are dead simple. First time I saw it was in Hawaii many years ago.
Click to expand...
What supplies the voltage and frequency sync to the microinverters?
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
winnie said:
How much of a hit do you actually take by not having MPPT tracking? I'm not going to fall down the rabbit hole of doing the calculation :)
Click to expand...
For the simple model of I vs V for the PV system being a step function (constant non-zero I for V < Vmax ; I=0 for V > Vmax), where the current is proportional to irradiance, you can pick a load resistance that is provides maximal power for any single irradiance level. Then when the irradiance is higher, you get no extra power (you would need a lower load resistance to get more power), and when the irradiance is lower, you get quadratically lower power (you would need a linearly higher load resistance to get linear power behavior with decreasing irradiance).

Cheers, Wayne
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
wwhitney said:
I infer nothing, which is why modified microinverters were required.
Click to expand...
How would one modify a microinverter to enable it to run while islanding without batteries, and how could the output of an array of them be configured to match the load?
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I don't think microinverters, even in island mode would work for this application. They would need to be modified to reduce their output voltage in response to changing insolation. Though I suppose with suitable control modification they would have all the necessary power electronics to get the job done.

The 'CyboInverters' linked in post 10 look like they have variable output voltage for the water heater...

-Jonathan
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If the modules are free hook them all up thru a DC contactor to the heating elements and switch the contactor on off via the thermostat.

No need to worry about efficiency if they are free.

A better option is to use solar directly to heat the water. No need to convert to electricity first.
 
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pv_n00b

Senior Member
Location
CA, USA
Occupation
Professional Electrical Engineer
In the olden days of off grid PV it was pretty common to have a diversion load controller to shift the PV output to water heating when the batteries were full. These came out of small wind and hydropower where a diversion load was necessary when power from these types of systems could not be curtailed as easily as PV. Most people in grid tied PV have not been exposed to these systems so the expertise is being lost.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Such diversion systems have the benefit that the voltage applied to the heating element is pretty well regulated to the battery voltage.

Back to the original question: if the heating element were itself non-linear with the right I/V curve, then you could get pretty close to MPPT tracking in a completely passive system.

If you use a long series string of diodes as your heating element, or a mix of diodes and resistors, you could get pretty close to MPPT, missing only because the IV curve of the heating element wouldn't change with the temperature of the panels.

Something like a long chain of these, mounted in an insulating tube filled with insulating oil would give you about 14W per $0.35 diode:

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-Jonathan
 
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Birken Vogt

Senior Member
Location
Grass Valley, Ca
If you run your panel strings in the right configuration, you can get close enough to MPPT. That is how they did it in the old days of off grid solar, just engineered the strings correctly and used PWM to finish the charge cycle, during the main charge run time everything just ran wide open and was close to MPP.

MPPT charge controllers were invented to take advantage of panels that did not match battery voltage, also allowed higher DC voltage and smaller wires, we thought we were hot stuff when the wires coming in from the array ran 110 volts instead of the previous 24 or 48 volts and big thick copper that was required.

Anyway the heating elements would work fine on 240 DC but the thermostats might not be able to interrupt DC because of the lack of zero crossings.
 
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