Author Topic: Power Point Hot Water Heating With Boost Converter  (Read 3662 times)

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OperaHouse

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Power Point Hot Water Heating With Boost Converter
« on: April 19, 2015, 09:41:16 AM »
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.

OperaHouse

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Re: Power Point Hot Water Heating With Boost Converter
« Reply #1 on: July 14, 2015, 08:17:26 AM »
The latest improvement to the hot water heating system has been implemented at the camp. Prior to this the control switched from tank #1 to tank #2 when it reached temperature.  Now there is a PWM controller on each tank.  This system works on sensing the power point voltage. When panel voltage exceeds the power point, power will also go to the second tank. Not a lot of data yet, but total daily power into heating water increased from 2KWH a day to 2.4KWH.  Peak wattage readings increased from 400W to 690W.  I had planned to go to a 48V panel array this year, but with the purchase of an additional panel I had 900W (9 identical) in a 36V array. That required me parallel up the load in order to utilize all the available power.  Going to 48V also gets into a nasty voltage to deal with.  Here is the control box with the power monitor.  Note the bubble wrap used as the TP56 enclosure, it works quite well.  One form or another of control electronics has be under this for a couple years.  With luck the shed for the hot water tanks will be built this year. Heat loss is a major problem and the tanks need extra insulation.  A picture of the control module,  hot melt glue works wonders.    There are two if these built from an old UPS. The two FET in parallel are driven by an opto isolator into a resistor.  There is no FET heating and the signal comes from 30 feet away.