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Forum Index : Solar : Measuring and acting on solar export
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Haxby Guru  Joined: 07/07/2008 Location: AustraliaPosts: 418 |
Hi all, I have a single phase 10kw solar inverter and 3 phase power to my house, so the inverter sits on only one of the phases. I need to be able to turn on/off my 3.6kw electric hot water resistive storage heater, based on the solar production at the time. So far it's just been on a timer that turns on at 11am, but I think it's worthwhile to put it on a contactor that can be controlled by a magic Box that can measure excess solar generation. Ideally I'd be after a, say, 0 to 10v analog output corresponding to 0 to 10kw of export power. Is there an ali-express or other commercial box that can do this? It's a little tricky because the box would have to measure all 3 phases and calculate the total power being used/exported. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
Not exactly what you are asking for but I do the same thing slightly Differently. I use a voltage monitoring Relay to switch on my water heater when the solar is producing. IT have it set to measure the rise in the mains voltage and then switch the heater through another relay. I also have a PWM controller on the heater so I can turn it down to say 1500W -2KW so it can heat using less power particularly in cloudy weather. Voltage Relay These can DO AC and DC so you could set it up on either side. They have hysteresis and a timer function so you can set them so if the voltage drops say 3 V for 20 Sec, they kick out. I had Voltage rise problems initially and put one of these on so as to help pull the circuit voltage down to keep the inverter producing and it worked well. Don't have the problem not but still use the setup as I took the HW off the off peak as my own power is of course Cheaper. There is an over ride button which is handy if you get a lot of cloudy weather. I have 3-4 Days worth of reserve in my big tank so I have only had to kick it in manually a few times over the 2 years I have had this. I also have some current sensitive switches I got. Haven't used them yet but could work a similar way. They are adjustable and switch depending on the power running through the wire they surround. You could put one on an inverter and tune it so it switches on the heater when there is sufficient power. Current Switch You would have to calculate your other loads and set wither one to a point you know the solar would cover the heater and the rest of the house loads. I have seen meters that use CT clamps on all 3 phases to measure total production/ consumption but they were pricy and didn't do switching. The Voltage or Current relays are not perfect but probably a step up from a Timer. |
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RFburns Regular Member  Joined: 21/07/2017 Location: AustraliaPosts: 43 |
NOTE THESE ARE SINGLE PHASE UNITS. Yes there are various commercial product's available for this Iboost +, Watterson Optimmersion controller, WATTrouterEco (appears to be a programmed PLC unit), Solax, Gem, Immerson, Solic are a few that I have come across. A DIY unit HOT WATER DIVERTER Google does provide some 3 phase ones 3 phase Edited 2022-01-08 05:41 by RFburns Strong like horse smart like tractor! |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Back in ancient times, before smart meters were invented, we used to log three phase power by fitting a three phase spinning disc electricity meter, using an optical pickup on the disc. With two optical pickups arranged in quadrature, its possible to count both the turns and record the direction of rotation. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
There are a Bunch of those hot water diverters and all of them are expensive and not worth the price IMHO. One popular one has a bad reputation and fails quickly according to many accounts I have read. Looking at the thing I well suspect the reports to likely be true. The Power diverter is around $900 which is relatively cheap compared to some of the others which are blatant rip-offs. One would have to look at the ROI on these things. When I looked few years back and did some homework I wasn't satisfied with any of them hence going to the monitoring relay. I am sure the other things work better, when they work, However I only had to save $30 to be in front with the relay not $1800 like some of these things. One also has to remember that one has to save the Price over and above what a simple, cheap timer does the OP has now to be ahead and that's going to take a long time for most. The relay will not be perfect but I bet it's at least 80% effective of what the diverters are and the relay would be closer still. For me hot water is not THAT big a cost and for most it's on off peak so you would have to save the difference between what you get for your solar feed in and what you are paying for off peak which shrinks the margins even more. In other words, If a Timer or relay is 80% effective in saving what the diverters do, then you are making a 20% saving @ Maybe .10C Kwh. If that works out to be .50C Day, that's a LOT of days to get your money back..... and you are out that money up front lump sum. Certainly worth using your own power for the hot water but once you start spending $900-1800 to do it, the ROI becomes very negligible unless you are using massive amounts of hot water. If you are, then the simple solutions like Timers and voltage monitoring become even more effective as well. I see a lot of these things that supposedly save money priced such that any real savings are going to take a long time or never actualy happen. Things like this Seem to be a real good cash cow for some and a total and complete waste of money for others. Solar Diverter list |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 418 |
I agree that there are diminishing returns as well as increasing complexities with this sort of stuff. Maybe for a simpler solution I could use one of my 2 kw aerosharp 93v transformers to drop the 240v feed to 147v into the water heater. This will make the load around 1.4kw instead of 3.6kw It will run for longer but maybe catch more excess solar throughout the day. Any forseeable safety issues with this arrangement? I can't think of any. Ultimately that might be a better idea instead of attempting to monitor everything. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
I think this overall it's better to lower the power of the element. Much easier for a system in not so great weather to generate say a couple of kw that may meet all the home needs than 4 Kw+. I have asked someone To do a program for an arduino for me that is basically a voltage controlled switch. I want to try a couple of things with it including a staged output depending on voltage. To this end I could have multiple elements and switch them in according to the rise I got from the solar. Still waiting for them to get back to me, beggars can't be demanding. Once I get the base program I think I should be able to copy the main part and attach that to different voltage dividers and assign different pin outs to control relays or mosfets. Just not able to work out the code from scratch. Before Xmas I bought a 6 Kw element which is 3x 2Kw elements. It's supposedly for 3 phase but with removable links, is easily made into 3x single phase elements. The beauty of this is I could also direct connect it to panels with Ohm matching which would give a lot of combined power. Better to use a controller though so the ohm matching is not required. This may also be something for you to consider. Take the hot water off your solar all together and set it up on it's own system. You could get some used panels and just hook them up with a DC relay though the thermostat. With a DPDT relay, you could also have a manual over ride so if the weather was bad you could top it up from the mains. Being a 3.6 element, 4x350/ 270W panels would supply around a KW so might be enough for your needs over a day. You could add more panels using the controllers that have been published here a while back. Edited 2022-01-11 02:32 by Davo99 |
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Technophiliac Regular Member Joined: 18/12/2020 Location: New ZealandPosts: 89 |
Other "3 phase" 3 element elements are available for example 3 x 1KW = 3 KW total. I've been pondering the possibilities while trying to use one thermostat, for example running one element on night power, and doing green things with the other two elements, for example green power into one element or 2 elements or even 2 elements in series with switching between these options possible. I've been a bit stuck on the thermostat bit though (to ensure no mains or green overheating) and pondering if one could use the thermostat to switch a common neutral, or maybe use the thermostat to drive contactors.... As controlled hot water is the cheapest power the savings are the least available I was concentrating on eliminating / reducing my expensive power as the priority (noting the very real ROI issues pointed out) Davo, Wellington. You can have it perfect, on time, and at the best price. Choose any two. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1577 |
I have been mulling over the Exact same thing. Saw it suggested somewhere, maybe here and got me thinking. In theory it sounds good but trying to do the mental arithmetic for me seems to come up short. I can run 2 elements off panels, no problem so we have hot water at night when we shower. Thing is if the 3rd element is then connected to mains, we shower at night, the thermo kicks in and we pay for the water to be heated and the solar powered elements do bugger all, just maintain the temp and make up for any of he minimal use through the day. Ideally would want to be using the mains in offpeak times which is not when I want the heating. We shower at say 10-11 PM, the off peak comes in at 11 and again does all the heating when we don't need it. I'd really want it to kick in about 8 PM so it makes up any shortfall in the tank temp and is right for when we want to have the main use. Only thing would be to have the mains power switched manually so it only kicked in during bad weather when the solar was lagging. Be handy to have a temp read out to know when it needed boosting or switch it to the mains. If I have 2 elements on solar and with the element I bought being 6 KW ( although not fitting any standard tank here which is a problem) If I'm getting about 1.5KW out of them each or even half that in crappy weather, then the times I should need to heat from the mains will be minimal here. If I put a controller in between the panels and element, Would be easy to overclock the element so there was good power in any but the worst weather and I should only need a single element if I have plenty of panels on it. even 1500W will raise 400L of water from 50 to 70oC in 6 hours so that would add up. I don't know what the tank drops to in the morning. Might be an idea to start taking temps and see. I thought I can do that with a Simple DPDT relay. Have the thing wired on the resting side to the Solar and have the Mains Pull it in when powered. That way the 2 sources will never be connected at the same time. The solar side will need to go through a DC relay from the thermostat but that's no issue either and that won't be closed when the mains is in so again the 2 would be isolated and no problem. My 3rd idea is having a small generator for when the weather is bad. Use one of the little diesels I have just driving an IMAG. Perfect job for one of these. Only thing they are good for is constant non switched loads so the thing could sit there for a few hours putting away and I wouldn't need a mains connection at all. My hot water is 400L so we get about 3 days out of it before it gets too cold for a shower which makes me think the water temp isn't dropping a lot and I might get 2 days worth of heating, certainly 1 in every 3 days of bad weather which would give us a window of 4 days minimum. Being a grub, I don't always shower every day in winter anyway. If I have been inside and not out working, I don't stink so will pass that night. That would reduce the load a bit as well and might give a bit more breathing space before outside power is needed. Thinking next heater I get will be a twin element but not really sure how advantageous that would be in reality. Make it easier to put 2 Solar feeds I guess. |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 418 |
I'd be reluctant to do any sort of switching with DC. Whether with relays or igbts, either way it gets a bit complicated and needs a lot of thought put into all the ways it can go wrong. I'm also not really keen on changing elements in the heater, nor keen on adding more elements. So I think I'll add the transformer to drop the voltage and call it a day. The existing timer will be set from 10am to 4pm. That's 6 hours at 1.4kw. should be plenty. I could have a manual switch to revert back to 3.6kw on the occasional day we have very heavy hot water demand. If I had off-peak electricity, I'd consider having 2 thermostats on the tank, set to 2 different cutoff temperatures. The higher cutoff temp thermostat would be connected to daytime solar production, and the lower temp thermostat connected to the off-peak electricity. So the sun does as much as it can during the day, and the off-peak only turns on when the water has dropped to close to an unusable temperature. Interestingly here in Victoria, we have a government scheme that will install a free heat pump in exchange for your old resistive tank. I hear that the free one is small and not good quality, but you can pay a nominal fee to upgrade to a bigger and better quality unit. I absolutely love heat pumps, but if I went with this approach, I'd have 3 heat pumps on the side of my house. The noise might become an issue for the neighbours. With the pool heat pump, pool water pump, air conditioning and now a Hot water heat pump, there would always be an industrial hum coming from our house  |
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| Technophiliac Regular Member Joined: 18/12/2020 Location: New ZealandPosts: 89 |
Exactly, I've been pondering how to do this too. One can buy 3 phase elements that also have a central temperature probe provision. I understand a temperature controlled probe / relay could be used this way for the 2nd thermostat. Some SSR's may be best avoided, however I find it difficult to believe they are all untrustworthy. I am "currently" testing a heat sink mounted one, with no problems to date although "currently" its only switching / breaking <=25A @ 13 volts DC. From a green energy perspective I expect Heat Pumps require bigger inverters compared to simple resistive Hot Water elements. Even with slow start motors I anticipate higher peak power requirements than pure resistive elements will minimally have. Plainly there will be variance in peak power load with different models and manufacturers. Slow start motors may well have lower peak power demand, I doubt it will match a hot water element. This may or may not be a consideration depending on each case. Plainly efficiency is in their favor. Are there any that run at lower voltages? Davo, Wellington. You can have it perfect, on time, and at the best price. Choose any two. |
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