I've been wanting to post something on the use of boost converters so that standard water heater elements coud be used for a
dump load at 12V. The power point controller circuit can also be used with higher voltage panels. I bought a single 100W 12V
panel just to do this experiment before heading off to camp in a couple of months. I had added a heat pump water heater to my
tank and the resistive heating element was disconnected. Adding some solar would reduce some of the daytime running of the
compressor. I have very low water use and the HPWH is fairly efficient. My daily electric use for water heating is less than
$.40 a day on average.
The boost converter is set to the maximum voltage of ab 34-35V. Multiple boost converters in parallel should be set to exactly
the same voltage so they load share. A FET controls the output to the heating element. Simple boost converters always have a
diode and inductor connected from the input to the output and output can never drop below input voltage. A large capacitor at
the input is required to store energy at the input. This voltage is monitored by the voltage divider and fed into analog input
A0. The PWM output at pin 3 drives two 80NF55 5V FET in parallel. I usually like to drive these with at least 10V but they seem
quite happy and cool at this drive level.
RESULTS
PEAK READINGS AND DAILY TOTAL (It was a bad week for testing)
1.9A 23.55W 231WH. NO BOOSTER nice sunny day about the same the next day
3.73A 64,1W 255WH. BOOSTER ADDED partly cloudy day
3.68A 63.2W 126WH VERY OVERCAST
4.22A 72.6W 191WH VERY OVERCAST
4.08A 70.7W 155WH OVERCAST & RAIN
2.87A 50.4W 285WH WISPY CLOUDS
2.92A 51.2W 307WH HAZY, PTLY CLOUDY
This is about as high as can be expected using a 16GA extension cord ftom the roof. Measured voltage the loss from the roof
using a third wire which gave a resistance value of 1ohm total. 70W is about as good as can be expected. With better wire
that could get to the low 80's.
These results are with the 3500W heater element measuring 14 ohms. A 2000W 120V (7 ohm)heater available for about $10 at most big
box stores would double the current capability and result in a four time increase in power (with more panels). A 5500W 240V element
is 10 ohms.These $5 ebay boost converters are rated at 150W but I wouldn't use them at more than 70W each. Multiple converters can
be placed in parallel for higher power levels of 12V nominal panels or dumping from a 12V battery.
The boosters allow use of this circuit as a dump load for a 12V battery with a change in power point voltage. If placed on the
panel side of a typical PWM controller, a diode must be used to isolate the capacitor from the controller. Solar panels are
current limited. Placing a capacitor on the panels would cause a many times increase in current blowing out the controller
if a diode is not used. This dump load would use any excess panel when the PWM controller was in the off cycle. With a MPPT
controller, by setting the power point high, this would act as a diversion controller as battery load decreased.
If panels are placed in series, there is no need for the boost converter. Just change the power point to the appropriate voltage..
A 36V string would be the limit for common 55V FETs.