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Off grid water heating project
eidolon:
I would like to start out saying that I think boost converters are generally a bad idea. I had someone from Europe inquiring with an array of 120V. Their water heater had a 2,000W 230V element and there was no lower resistance replacement for it. In that case using a 48 power supply to increase the voltage was justified. It was not an issue of efficiency, but accomplishing a goal.
A boost converter can not be totally turned off. Input voltage always flows from the input to the output thru the diode and inductor. Only the amount of boost is controlled. The lower panel power is just passed thru. One has to accept that the lower wattages are just lost. I give this example which can control a boost or buck converter only for educational purposes.
Every switching converter has an IC pin which is a reference which is connected to the output to regulate voltage. The reference is typically 1.2 or 2.5 volts. That connection is easiest to find by either measuring the lowest voltage or resistance. Pot pins are easiest to solder to. When the reference pin goes above the reference voltage the switching regulator will turn off. A 12K resistor can usually supply enough current thru the diode to that pin. Less than 1mv increase is more than enough. The diode is there only to affect the circuit in turn off mode. When the FET, TL431, or opamp pulls the 12K resistor to - common, the added control circuit has no effect, and the switching regulator operates normally. A FET is used in this example for simplicity, and it is a power FET because these are common. It is also not linear having a sharp turn on between 2.5 and 4V with a high impedance that will not load down a voltage divider. There is some drift with temperature. Everything to the right of the diode are switching module components. The 12V can be obtained from the regulator of the switching module.
As PV voltage drops below the power point voltage, added current feeds to the reference pin. This adds to the current from the output voltage divider. This action does not completely turn off the switching controller. Rather it reduces the amount of boost till the PV voltage stabilizes near power point.
This circuit helps a little preventing a boost converter from putting the panels in a death spiral by drawing more and more panel current. It turns off the boost converter and makes it the same as direct connect. Boost converters are just a bad bad bad idea. To be efficient the panel should feed into a capacitor bank and be pulsed from that maintain a constant voltage from the panel near power point.
eidolon:
Here is a version of one of my designs in northern Europe. I'm quite impressed as he had no prior electronics experience and even etched his own circuit board. He states he is getting over 5KWH a day in winter from four panels. Please respect his privacy and do not comment.
//www.youtube.com/watch?v=3TY_u6m9JXM Lithuania video
This next pre shipment video is of a board I sent to NM operating on four 255W panels facing east. This is on the 40 gallon tank in the garage I just use for laundry hot water. It is
connected to only the lower 5500W element for the 120V test. I normally connect upper and lower in parallel with my 60V house array. Setting priority on the basis of PV voltage works well at only diverting excess house power to the garage only when house water heater finally turns off. The garage has a transfer switch so I can run the washer off 120V DC garage panels without using a battery. Totally free laundry now. The generator is only used now for the wood saws.
//www.youtube.com/watch?v=iaLbLYhPLtw 120V first test
eidolon:
Here is an example of constant voltage as represented by the green line. The small higher peaks are when the panel goes over power point due to heating element being higher resistance than optimal. Power is the yellow line.
This other graph is when the board is operated in diversion mode. This scan is only a half hour and demonstrates how varied power can be. Besides clouds, as devices in the house turns on diversion drops to zero. At the very end the voltage drops to 44V, likely refrigerator turning on. Manel panels are shaded reflecting the lower voltage of the charge controller.
Pete:
You guys take a pretty high tech approach, but it seems to be working out fine.
I have a evacuated tube solar hot water system. It is backed up in winter with a wet back on our wood heater.
Then when the wood heater is not running and the sun not quite strong enough, I use an element in the tank and power it by my inverter.
It is a low tech approach where I look at a guage I have to decide if the water temp is high enough. Then I switch the element on. The power is regulated by both a thermal switch on the gauge and also by the battery level.
The Victron Battery monitor I have has a settable relay, so i have the relay switching a higher power relay to turn the water heater element on. It allows the element to run only when the batteries are fully charged and turns the element off if the batteries go below 97%.
I know it is a low tech approach but it suits me.
I am guessing that your systems boost DC up and use DC to run the elements, Does this damage the over temp thermostats in the tanks?
Cheers
Pete
eidolon:
Many focus on a single issue. Heating water can be complex having many issues. My board has arc interrupt so that high voltage DC can be used with standard mechanical thermostats. Water heating should be done with the highest PV voltage directly from the panels eliminating conversion losses and avoiding buying more expensive equipment for the increased load. My method allows using existing array to heat water and supply charging needs which have priority. These methods will be common in another 10-15 years. At this time almost no one understands the science. For now the solar world goes for easy.
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