Food for thought. Or in my case, food that kept me from sleeping last night.
A 3kW immersion on 110V will be about 750W. To raise the temperature of a 150l cylinder from 10 to 60C needs about 9kW of energy. If we ignore that it loses 1kW/day, with 750W, this is about 12 hours of heating. As you have a large array, you probably will have a 750W excess, but for how long? I currently have 500w surplus on a 1kw array, with another 5 hours of (potential) sunlight. I would have more, but someones got the router and a PC plugged in. So I suspect you'll have a greater surplus. You could think about a large dimmer switch to control the power and hand tweak it.
I think your circuit is fine. The comment about the diode is a good one. The AC values you gave are most likely RMS so the lowest figure of 0.2 to 0.8RMS is 0.28 to 1.2V peak. If you use a germanium diode, that's 0.2V gone, so a large error at 5A, which is roughly where a 1kw heater starts. It might work in your favour, as the error will mean (I think) you are firmly into surplus energy. (you could look here
http://sound.westhost.com/appnotes/an001.htm at precision OPAMP rectifiers, but dont look too carefully) Equally, the out of phase load I & V should work in your favour as the current vector (bear with me) will be longer and hence will make you think that you are using more power than you are. Hence your system will be pessimistic. Better that way. Comments anyone?
My brain hurts. Time for a cuppa. Can you please just build this so we can all go back to reading the paper?
Sorry I have kept you awake Artful (I bet it’s kept me awake more though).
A bit of background.
I have only had my system for a month. Not the best time of year to harvest the sun for whatever reason. As you say, I have a system that will be quite useful, especially in the summer. In fact in the summer the immersion will be on a timer and will come on irrespective – working on the principle I will generally be exporting more than I need. It’s the winter time that’s the problem – low sun, unreliable conditions and short days. At present I have the 2 Owl monitors, one measuring PV output and the other house load with their displays sat in the kitchen. When I go PV positive by enough I use a remote controlled switch which turns on the immersion. Fun at first to think you are heating the water FOC, but after a month the novelty has worn off. Automation is required ! Hence trying to come up with my own solution without costing a fortune.
Thanks for the maths etc regarding heating the water up. The thing is, it’s only a short reach stat, so it tends to only heat up the top 1/3 of the tank. It’s adequate for me and her indoors for hand-washing and a shower etc each day.
I have been to Maplins today and bought myself a breadboard, so construction starts soon (thanks again for the tip).
Sleep well.
