I’ve just moved into a house that has a hot tub (lucky me!). It uses a lot of power to keep it warm so I’d like to try heating it from the sun. Although a solar water might be more normal, I’d like to try heating it with a solar PV system. I live in Berkeley, CA and the google calculator thing tells me I’ll get 1756 hours of usable Sun. I can get used panels locally for $0.60/W. So my idea is to connect a panel directly to hot water heating element and put that in my hot tub. At this stage I’m looking for some validation on the overall idea, my particular calculations and any safety tips.
Solar panel 1|35.68|8.15|290.792|44.9|8.94|4.37791411|
Using the Vmp and Imp I calculated the “ideal resistance”. I think it’s vital that the resistance of the heating element matches this pretty closely. I found a 600W 48V element which should have a resistance of 3.84 ohms which is pretty close (and I think gives me an efficiency of 87% based on (8.15 * 3.84 * 3.84) to get the power and dividing that by Pmax). I can do slightly better by getting a pair of cheaper and smaller mains heating elements 120V 1650W which have a resistance of 8.72 ohms or 4.36 for two in parallel. Which is now very close to my ideal!
Question1: For this ideal resistance calculation, I’m assuming the panel will be operating at Pmax, but presumably the panel never will be (unless it’s directly facing the sun on a clear day). Should I be calculating a different ideal resistance based on more realistic typical conditions?
Question2: What fuses and isolation switches should I have? I plan to have an isolation switch between the panel and the heating element (is something like this good enough). Do I need any fuses too? I can’t really get my head round how a fuse could help. I’m basically trying to short the panel out in the most efficient way possible, so a fuse will always be sized for more current than the panel could ever provide!
Question3: Any tips on cabling that’s OK if it comes into contact with the hot tub water (which contains some chlorine).
Thanks in advance for any tips, advice or warnings! Tom