Low Cost Camp PV Hot Water System Concept Using Excess Solar Power

NANOcontrol
NANOcontrol Registered Users Posts: 260 ✭✭✭
This is the efficient PV hot water system I use at my camp. It works surprisingly well and is low cost. The tank
used is an ECO Smart 6 gallon 1400W 120V, normally for a point of use applications. These are also available in
4 and 2 gallon sizes from other manufacturers too. It may not sound like much, but this is enough for two people
 to take a quick shower. It works off the existing 60V array of cheap grid tie panels in series. These panels
also supply the charge controller which works in parallel with the hot water controller. Any excess power not
going to the batteries, whether it is 5W or 500W,is sent to heat water. Additional panels are not necessarily
needed. The only major expense is to buy the tank itself, in the $200 range. The water has to be drained at the
end of the season and there is no drain for the tank. It is plumbed with hoses because it must be turned upside down
to drain. Tank has about one inch of foam for insulation.



The controller circuit is based on a $5 300W inverter board found on ebay. With small modifications, this board
holds the panels at the panel power point voltage and can be used in parallel with either a PWM or MPPT controller.
At the half voltage of 60V, any 120V heating element is capable of making 1/4 of its rated power. The controller
 utilizes a little known property of half bridge oscillator drivers. When the oscillator capacitor is shorted,
both outputs will turn off. This is sort of a wham bam PWM, but is capable of some variable duty cycle. The
circuit is capable of holding the panel voltage to less than a tenth of a volt. Energy storage is in a capacitor
 bank of about 6,000uf. A TL431 amplified zener controls the voltage. This inverter board has a limit of 75V
due to the installed FET. Similar design can be any voltage.



The water heater box stated that the recovery time to raise 6 gallons of water 60F is 37 minutes, looks close enough.
That is only about 900WH and is easy to harvest on most PV systems in only a few hours. The temperature control
system uses the mechanical switch supplied with the tank. Besides being free and not requiring any power, this has
the advantage of the unit still capable of being plugged into a 120V generator on those rare bad days to heat water.
I have a jumper cord that accomplishes this. If the tank is at 80F, only 15 minutes of generator time is needed
to reach full temperature. The temperature control range in the manual says 50-140F. However, it shuts off at
124F. Likely to avoid possible burn lawsuits in the US.

What about DC arcing contacts of the contacts?  This is not a problem at all. The water heater is modified to
use a 4 contact plug. The additional contact is a tap between the heating element and the thermostat. About
half the capacitor bank is connected between the switch and element.  When the contacts open, the voltage
differential is very low due to the capacitor continuing to supply power to the heater element for a short time.
The FET also will rapidly turn off as a lower voltage at this location is sensed. When the contacts close again,
there is no great current surge because the FET are turned off. The voltage across the contacts is also small
requiring little charge current. A small pre charge resistor of several hundred ohms keeps the capacitor bank
at near the panels power point voltage.  There is a cost of about 1/4W to keep the pre charge system alive.
A very minimal drain.



The internal picture is the only one I have and is from when the circuit was just being developed. Enough to show the
board and simplicity. This circuit also powers my dishwasher heating element now. This allows the dishwasher to only
need 100W from the 120V inverter making it a rather minimal load! Additional wash heat and drying for the dishwasher
are switched automatically to the controller when needed.

This nearly free cheap water heating that can be added to most small summer camps. The control system only
costs about $25 to build. Any well designed system should have an excess of panel power. Harvest that wasted
power! Panels are far cheaper than batteries. This is only to demonstrate the concept of what can be done at
a summer camp to make life a little more civilized at very little cost. This should not be atemped by those
with limited skills. For your protection, no circuit will be supplied.