Simplified Solar Hot Water

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azhaque Senior Member Joined: 21/02/2017 Location: Pakistan Posts: 131	Posted: 04:56am 17 Jan 2025         Davo99 said   I realised that the timing was not really important nor was the voltage levels. As long as the panels weren't dragged off their VMP or at least more than 10 % off it, the rest was pretty small potatoes. Agreed 100%. As long as the circuit can 'PWM' the mosfets to keep them at the Max PP, all should be well. This can easily be done with a 555 with variable duty cycle. I am also prototyping the pvdirect water heater using the same concept that you have used. Initially however I am not even going to pulse it, letting the load draw the power. Infact the main problem is not the PWM freq or others but how to match the available solar panels to the existing mains designed water heater resistance, for max power point. Here is a photo of breadboarded prototype. Top left is the breaker. Below it is the clamped down 12v isolated power supply. Next to it is a cheap DC ammeter-voltmeter. The voltmeter is only rated 100VDC so I cannot risk putting it to 240VC coming from my panels. Current measuring capability 10AMPS. My panels have an I-MPP of 8 Amps so this should work. It uses a shunt to measure the current making it suitable for low side measurement by connecting it between GND and the mosfet SOURCE(S). The MOSFETs (IRFP460)are mounted on an ALUMINIUM heatsink, go in the space visible between the Breaker and the lower row of instruments. These are soldered to a TO247 based PCB for a square wave UPS of yesteryear. Half of the PCB has be used. The other half is for my other geyser. A fan will be mounted next to the Breaker, and will blow onto the heatsink. Finally on the extreme right on the Breadboard the famous W1209 temp controller is visible. The acts as the thermostat by connecting/disconnecting 12 VDC to the MOSFET gates. Also conveniently displays the hot water temp.   Quote   Also Realised as long as the caps weren't allowed to get Full and therefor waste solar power, voltage and timing wasn't real critical either. Agreed. However the main solar-time wasted isn't what you have mentioned. It would infact be the period when the water temp rises to the set point and the circuit remains off. This could vary from minutes to hours depending upon consumption/heat leakage from the geyser. The on-off cycle time is insignificant in comparison . Also true. When the mosfets are not conducting, the panels are charging the caps. When they are conducting panel current PLUS capacitor current goes to the load.   Quote   And that was it. The thing runs all the time, initially from a small USB power bank thing which lasted days anyway. I'll use a power coming from the panels as the power supply can take upto 250Volts. It is designed for mains use and therefore should have no qualms about powering from a 250VDC solar array. Another advantage is that it can be connected to the AC mains supply and thus keep the circuit and temp display running all the time.   Quote   Always felt that using an arduino was a waste so I bought a pre-built timer that went down to Hz for $12 I think it was and used that as the driver instead.   Good idea. $12 is too much for this . I'll use a veroboard mounted 555 circuit which should cost less than $0 since I have all the parts available .   Quote   All it does is turn the fet on and off and that's all that's needed.  The caps pose virtually zero impedence on the panels so they work efficently. In my case the W1209 will perform that function. I don't think even the 555 would be needed.   Quote   The power in the caps does not really matter as long as they don't fully charge and leave no more room for the power the panels are generating.  Even if they are 1/20th Full, all the power gets dumped into the load so there is no more loss than if they were 98% full. As I submitted above, wasted panel-power is the power not consumed when the water temp is at its setpoint.   Quote   The aim of the game is not to drag the panels  to 7V or whatever where they become highly inefficent and keep them in their happy place. Crucial point. Matching up V-MPP to the existing resistance of the hot water geyser is the main issue. Here is a pic of my dummy load using a 2KW electric heater element. It sagged when I connected it directly to the 240VDC solar array. Therefore the heat insulating tile.     Quote   This very crude and basic setup seems to do that.  Also don't matter what size the cap bank is.   Proof of the pudding is in eating it. I disagree that the size of the cap-bank doesn't matter. It may not do so technically but it certainly does $$$$. As the Egyptian President of the not so recent past said and I quote. SIZE DOES MATTER.   Quote   I would have liked to get this circuit working but it was beyond me but it did give me a more through understanding of what needed to be done to make the idea work and it seems my even more simplified soloution works just fine. This I learnt from your thread about this topic. I was going to use Tony's circuit but then I saw your idea and now I am prototyping this one. Azhaque Edited 2025-01-17 16:07 by azhaque