Hi, i'm planning on using a couple of PV panels to heat domestic hot water to reduce the bills. I don't plan on installing a full solar array in the short term and thermal solar (tubes with liquid in them) Isn't an option. The cheap option is using a DC rated immersion heating element directly, sized so the panels generate roughly mpp power. Since this is high voltage DC, maybe 120V 10A, I'm trying to do it the safe way.
My big concern is switching the load on and off based on water temp. I already have a board, temp sensor and relay installed, but I don't want to risk arcing etc. I think the best solution is an appropriately rated power mosfet controlled through an optocoupler, that way the PV current is totally isolated from the board. Is this a good approach?
For redundancy, i intend to add another mosfet driven through a temp fuse, the idea being if the board or other mosfet fails, and in-tank water reaches near boiling point, fuse pops and mosfet opens.
The other aspect that has me worried is corrosion of the heating element. As i understand, normally, the resistance sits in a high temp isolator wrapped in a stainless shell. If the outer shell corrodes away, the resistance touches water, electrolysis may happen in the tank, big problem. I'd like to detect that condition to shut my control mosfet off asap. How can i use the arduino to check for a condition like that? I.e. outer shell isn't isolated from high voltage high current DC lead anymore?
Look into PTC heaters. The PTC material inherently controls its own temperature with no extra internal or external components. It is considered "fail safe".
The PTC material abruptly increases resistance at some temperature defined by the material mix (during mfg). In doing so the heater will reduce input power when above that temperature.
You could use the arduino to control the heaters depending on the temperature by using the PWM function and a MOSFET. This can also monitor the PV voltage and compensate for that. If the heater is rated for the job it should work just fine. You could put it in another tube with a heat transfer material in it then if it fails your system will not be contaminated.
Usally for domestic water heating the temp never rech the 100°C, so you can use thermistor but i recommend you a LM35.
For the proteccion of the heating element, in stores of heating resistors or internet you can buy heating elements in stainless steel or ceramics.
For the control of the heating, if you plan is use DC (120VCD i see or i read wrong) you need MOSFETS or IGBT with transistors to gates for proteccion and rapid switching and high voltage in DC. If the control is with AC current you can use solid state or mecanical relays with an adecuate delay.
I use solid state relays to control 80vcd heating devices without problem