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Forum Index : Solar : Solar electric hot water conversion
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hi I've been running a thrown together hot water system for a few years. It used a 330 litre tank, a couple water heating solar panels, a pump and controller. It worked ok, but for a couple of months in winter it struggled to even warm the water, and I had to cover the panels each night to stop them freezing as it often goes below zero here. Fortunately I have a on demand gas hot water heater I can switch in when the solar water heater isn't up to the job. After some interest on the forum regarding heating water electrically, I decided to go down that path. Second hand solar panels are very abundant and cheap here in Australia, a side effect of people upgrading their grid connected solar plans. I bought 7 230watt 60 cell panels for $55 each from a guy on Facebook, and a 1000watt 48 volt heater element for about $84 delivered from https://ringhotwater.com.au/.  The existing 330litre tank was getting rusty, so swapped it for a near new 120litre tank I picked up at a clearing sale for $20. Mounted 6 of the solar panels in a series parallel setup, plumbed in the tank with the new element fitted and wired it up. I used a 100 amp relay, driven by 12 volts from a plug pack on a timer. This 12v was switched by the water tanks existing thermostat. All worked fine the first day, the tank was heater up to 70c from cold in about 6 hours. Next day, it wouldnt work. The relay had gone open circuit, on closer inspection, the contacts had vaporised.  This was due to the DC arc when the contacts were opened. So I tried new relay with capacitor resistor supression across the contacts. Same deal, zap. Dug up a power mosfet from a junked inverter welder, mounted it on a heat sink with a fan, added a few components and bingo, no more zap.  To keep an eye on things I wired in a amp/volt/watt meter.  So far so good. Be interesting to see how it goes in mid winter, at least I wont need to cover the panels at night anymore.   Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Old Seagull Man Regular Member  Joined: 21/12/2019 Location: AustraliaPosts: 55 |
Hi Gizmo; I love it. But two questions if i may? What do you use as the thermostat on the tank or are you using the existing 240v one. And please if your not planing to go into production, and become the next Solahart. Please publish the circuit, as many will e interested. This would be the perfect solution to power the water heater i have stuck under my 93 year old mother in laws caravan. Stay well. Old Seagull Man |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hi Seagull Yeah using the existing thermostat on the hot water tank to switch 12v to the MOSFET gate via a 47r resistor, and a 1k pull down resistor, plus a indicator LED with resistor. I'm using a big old MOSFET I found, but I would suggest using a 40 amp SSR on a heatsink with a fan. It would be easier and you can keep the 12v and solar panel volts separate. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Davo99 Guru  Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Have you monitored your Daily average energy input as yet? The things that will get you in winter are the shorter sunlight hours and the colder inlet temp of the water. I have a large tank But I calculated the difference between the inlet temps I measured in summer and the inlet temps in winter to be 3-4 Kw a day, just to bring the incoming winter water temp up to the same as it is in summer before you actually start any real heating. You won't have that much difference having a smaller tank but.... It will pull more power in winter. One thing with a larger tank is you have more reserve. We have 3 days even with the tank switched off before it's really too cold. Even if you only get half a days heating value per day, still stretches things out to about a weeks worth of water in bad weather. With a small tank one would probably be lucky to have one days reserve. I am looking at taking an amount of my panels down off the shed roof. One one side I have over 5 Kw worth of panels and struggling to get 3.5KW of power out of them. Got up and had a look the other day and a Lot of them have turned very yellow. I'll get up and check the outputs and swap out any bad ones. |
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Ralph2k6 Senior Member  Joined: 24/09/2017 Location: AustraliaPosts: 129 |
Very nice setup there Glenn.  Don't mean to he a stick in the mud but did you check the hot water cylinder anode before filling the tank? We got a cheap 2nd hand cylinder a few years back and I fitted a new anode to be safe, old anode was almost all gone and the previous heating element had open circuited due to the corrosion.. @Davo, Don't forget this time of year on your solar output can be dropping dramatically due to the sun angle etc etc, already down 20% on our non ideal system at home. Edited 2020-04-14 09:18 by Ralph2k6 Ralph |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah checked the anode, all good. Be interesting to see how it goes over winter. Yesterday afternoon the tank temp was 65c, and after 2 showers and washing the dishes, it was 50c this morning. See how long it takes to recover. If it struggles in the winter sun, I have a 48v 20amp battery charger I could use to give a boost in the middle of the day. I used that battery charger with my petrol generator to top up my main battery bank if needed, but havn't needed to do that since I added more panels a few years back. My main battery bank is finished its charge cycle just after lunch time in winter, so I could spare a couple hours solar power first thing in the morning to boost the hot water. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
50 to 70oC on 125L of water is only 3 KW. You should walk that in. With the panels you have and allowing a 4X generation rate, you should be able to take the tank down to about 35 and still have full temp at the end of the day. Of course on an overcast day you will only get enough energy to recover from about 55o but that's still plenty hot to have a shower! |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
With the amount of panels I have to the size of the inverter, I should still be getting full output. This inverter wasn't really performing well over summer either but because I had so much power I wasn't really worried and had it turned off most of the time anyway. No intention to do anything with it till the weather cooled down. Bad enough being on roofs now let alone on hot days. I'll get up with my power meter and see what they are doing. Would be good to make up some sort of variable load so I could pull the things down to power point and see what they are making. Don't really have to look for max power but rather those that are significantly different to what the others of the same size are doing under the same conditions. Winter always makes me short of power so I'll give them a check just to satisfy myself anyway. I have heard that yellowing panels are bad, would be interesting to see if that holds up or not. If It does, I have plenty to replace them with anyway. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I went and checked on it just after lunch, it had finished heating and shut down by then. Its funny watching the temperature. I have a thermister sitting against the bolt head attached to the anode, then covered in a rag to keep it close to the bolt temperature. So in effect I'm measuring the temperature of the anode, which extends down into the tank most of the way. When the heater element is turned on, the temperature drops about 5c in about 10 minutes and then sits there for an hour or so before starting to rise. Must be some sort of convection going on. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Convection will be pretty powerful, and keep the water mixed while the element is working. There will be ice cold water right at the bottom where the entering water is, but through mixing having the whole tank at a fairly even temperature makes the most efficient use of the insulation. Later on, you might like to try wrapping the whole thing in some additional insulation. You won't gain hugely, but its pretty much a way to gain something for nothing. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
I covered all the exposed Piping and fittings on my HWS. Being copper, the pipes are warm for some length and must wick away some substantial amount of heat. If it were only half a kw a day, that's plenty over the course of a year. I reckon it would have been a good KW or above on mine. In winter when the temp difference is greater so is the heat loss. The smart/ efficient thing to do would be have some sort of plastic pipe connected to the tank or at very least something PROPERLY insulated. In my roof there is some thin green plastic on the hot water pipe. Not sure if that is for insulation or identification. Probably far more effective for the latter than the former. I was talking about painting my tank black as it gets the full afternoon sun and I thought that would have to help decrease the temp difference and may even add some temp in summer. A bunch of people reckoned that would allow the heat to radiate back out faster as well. Can't see it Myself and if it did, I think the gains would outweigh the losses anyhow. Was also thinking of covering it in a big black plastic bag but I thought the condensation would destroy the tank in no time. |
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| rosspeel Newbie  Joined: 27/03/2020 Location: AustraliaPosts: 17 |
I am starting to play around with a similar setup using a 36v element and changing the bridging link on the element to turn it into a 72v element, my big problem is killing SSR relays, I don't know why this is happening they are rated at 40amp and 250VDC (SSR_40DD-H), I would like to know about your mosfet setup as these SSR relays only last a few day. I got it on a trial 80L tank but as soon as i get the bugs out I am going the 400L tank. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I'll draw up the circuit later today, its pretty simple. Where did you get your SSR's? I bought a 60A SSR of ebay once, it got very hot with just a few amps. So I pulled it apart and discovered a 30 amp NPN transistor instead of a much higher rated MOSFET. It was a fake. Also, SSR's need lots of heat sinking and fan cooling when you push the amps. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| rosspeel Newbie Joined: 27/03/2020 Location: AustraliaPosts: 17 |
I got them from ebay, I will have to see how to pull a dead one apart and check the transistor |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Here's the circuit, from memory.  The mosfet is something I found in a dead inverter welder. Its probably expensive, so look for something cheaper, 40 amps 100 volts N channel mosfet. I'm using a 12v plug pack on a timer as the 12 supply. Once day I'll use a little 12v solar panel instead. Glenn Edited 2020-05-05 11:53 by Gizmo The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| rosspeel Newbie Joined: 27/03/2020 Location: AustraliaPosts: 17 |
 Had a spare heatsink so as temporary measure i have mounted 3 of these 40a cheap ebay SSR, I will look into mosfets Edited 2020-05-05 13:41 by rosspeel |
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| rosspeel Newbie Joined: 27/03/2020 Location: AustraliaPosts: 17 |
Well those 3 SSR ganged together didn't last long, killed all 3 of them, only lasted one thermostat switch cycle |
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| rosspeel Newbie Joined: 27/03/2020 Location: AustraliaPosts: 17 |
Thanks for the drawing Gizmo, I have ordered some Mosfets online, and now the wait for the postman... |
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| rosspeel Newbie Joined: 27/03/2020 Location: AustraliaPosts: 17 |
I have late this arvo made up a mosfet switch, bit late in day to give it a full test, I am using two FB31N20D rate at 200 volts, 31amps, tomorrow will be the test   |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Looks good. Watch the heat closely the first few days, mine needed a fan. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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