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Forum Index : Microcontroller and PC projects : PV and solar hot water-feed in?
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| domwild Guru  Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Hi, Saw an ad with what looks like a good idea: Rather than feeding the mains for four cents/kWh, to look after your wallet by sending the surplus to the hot water system instead if it needs it. As I do not understand the feed-in regime, is that a good idea, or does the system ALWAYS give preference to your consumption first anyway? Thanks. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 2006 |
I think it gives preference to your needs first. I have a friend with PV solar and when installed they put a timer on the hot water so it only heats during the day. Mind you we live in far north Queensland it may be a different story in cold climates. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2971 |
Hi Dom, We are lucky enough, in Vic, to have got in on the PFIT (Premium Feed In Tariff) and we get 68c per kw we feed the grid.. (roughly double our peak buy in) so I want to use less during the day to feed the grid as much as possible.. But to answer your question what you generate feeds your house before it feeds the grid, or at least that is how my setup works. Kind Regards Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1584 |
There are more than one of these ( overpriced) units around. It would depend on which one you are talking about and if it is programmable, what it is set to. Most of these things are feelgood ideas but the price makes them a waste of money. they will not offset their own cost in power savings over their life time. Unless they do that, you'd be better off without them. People forget they aren't eliminating the cost, they are just saving the DIFFERENCE between having them and not which is a lot less than the full price of the power. Usually it's just cents so you need a lot of cents over a long time to recoup the often $600+ these non money savers cost. I went a different way which has worked well for me. Having solar and a high grid voltage to start with, in the middle of the day my line voltage gets to the point where the inverters are throttling back or switching off all together. I got a Devise called a voltage monitoring relay. I can set it to switch on when the voltage gets to a certain level for a certain time and switch off again after a certain time if the Voltage falls below another set level. As I have it, When the line voltage gets High, the how water ( and septic pump out) Kick in. This drops the line voltage and lets the inverters feed the hot water with any excess going back to the grid within the inverters working parameters. If a cloud Comes over, the relay drops out and if the sun comes back and the voltage goes high, it kicks back in again. it has hysterisis so If I switch on another load which may be momentary like the fridge Kicking in, the relay will hold for 15 Sec before dropping out. I found a description of these on the net from someone and since installing my own in jan this year there has only been 2 occasions I have had to hit the over ride button to manually kick the tank in. I have disconnected the off peak all together and at least 95% of my hot water is free now. I did lower the voltage setting a bit for winter as the panels obviously don't make as much power and although the water isn't always up to full temp, it's been hot enough for what we need. The devise cost me $40 plus $30 for a water proof enclosure to put it in and some conduit to wire it up with a relay as the contacts on teh relay are not rated high enough for my element. I think the DPDT realy I used was about $7 and I put in another Breaker at the tank for good measure. Return on investment was probably a month - 6 weeks so was infinitely better than the years it takes for a lot of these things to still not justify themselves. If you have solar, Even a simple timer that kicked the heater in from say 11 am till 2 PM would save you more over all than any of these other devises which cost many hundreds. They would be far from perfect but they would still repay back where these other things are proven they can't. |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
As the ad in the papers was not explicit enough, I do not know if his scheme switches DC in directly from the panels to the DC heating element in the tank. This may be just an idea of mine, but does it save any money as DC heating elements may not match the voltage of the PV system or they may not even have the wattage to heat a larger tank? Davo99's idea is great as inverters switch off at 240VAC +/- 7%, which brings it up to about 256VAC. Then the investment in the panels is not wasted. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1584 |
DC requires some extra precautions as you just can't run it to the thermostat in the tank as the second it kicks out it will fry the thermo easily to the point where it could catch fire. It would be a one shot deal normally that's for sure. There are ways around it but in any case, it's going to be less efficient than feeding the DC into an inverter and controlling the AC input such as can be done with a timer or Relay. There are a couple of threads going ATM where the boffins are trying to work out a control for direct DC from panels that gets decent efficiency. One could get some panels, an off grid inverter, a battery and a PWM controller and do it that way. Panels charge battery, inverter takes DC to AC, PWM limits current to element to as not to overtax inverter/ charge controller and all is good. Of course by the time you purchase all that, you are going to be pretty much back to the scenario of it costing more than it will save over the life of the battery or other components. A simpler way of Doing this would be to use a Low voltage element and a preheat tank. If one had say a 300 or 500W element in a tank that was fed cold water and the use was known and relatively consistent, then the element may never get the water too hot and therefore need to be switched at all. The preheated water feeds into the main tank which of course saves the mains power. Instead of heating the water from maybe 10oC in winter, the incoming water may be say 40o which although still relatively cold, would save a lot of power use in the main heater. Switching low power DC is a lot easier than High power. A standard DPDT relay could be used if the other side went to a dummy load which could be something like a 500W halogen tube. DC wants to in effect keep going which is why it arcs. If one uses a double relay so the power goes from the element to another load, the arc will quench and will not destroy the relay contacts. If the power is low enough and the rating of the components is sufficient, you can just do a dead short which kills the panels output and reduces their output dramatically. That's the Crux of the system. The nominal voltage now is in fact 230V not 240 but 256 is the max allowable in current standards and where inverters HAVE to cut off. Some I believe are set lower still. I have seen regularly voltages of 245V at midnight and 3am due to the flakey power settings on the transformers here so with a few people with panels around me, it's not hard to push the voltage too high when your panels are producing even now in winter. Actually summer was better I noticed probably because people were running AC and pool pumps and even though the generation would have been at it's highest, I surmise loads were too. I have been using one of these relays on a heater fan in the house as well. I have several arrays on 2 Phases so even if the HWS is kicked in, the voltage on the other phase may be still high. The relay senses that and kicks in the fan heater which I use not only to warm the air in the house which may be OK, but rather to try and store some of that energy by heat soaking the Furniture, walls and floors so they may re radiate that heat at night. I was thinking to use a tank in the house which has an element kick in but the amount of energy I am getting isn't worth while.... Yet. I have another 35x 250w Panels to hang so when they are all up this may be viable.... or not. There has been a LOT of cloudy days here this last 6 weeks and no matter how many panels you have, overcast weather where it's near rain but does not, absolutely destroy output. I'm looking at a generator setup for those times. |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Thanks Davo for all that. Very interesting! As you ever only had to switch TWICE over to mains in that 10 AM to 2 PM time slot means all the other times the inverter had switched off, which is very interesting and shows that without tricks like yours the pay-back time for panels blows out. I read of a German remote village off the grid, where there is a roster who can do the laundry and on which day. My friend still gets his 40c/kWh as the politicians were unlucky in WA to change that back to 4c/kWh due to protests. I thought 240 VAC +/- 7% and not 230 VAC was an Aussie wide standard or would WA be different because of longer distances? Noticed I can buy a GEYA GRV8-02 single-phase voltage monitoring relay for about $20 ex Hong Kong with its 10 Amp NO/NC contacts. It allows the switching at up to 260 VAC. Is the "action time setting" as shown on the photo of the unit what you call "Hysteresis" as it allows a time gap to act up to 10 seconds? Mind you, the electricians of my builder in the city may or may not want to install this unit as it may not be AS/NZ approved? I hope 10A is good enough for an element in a tank. Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1584 |
Just to clarify, I did not meant to say MY relay is on 10-2, I was suggesting a TIMER that turned the water heater on at that time would save more money than one of those other diversion devices. Timers can be had cheap where those other things I have seen start at about $400. By using a timer to turn the heater on through the day, for most people it would be cheaper to use their own power they might get 10c Kwh to feed back to teh grid but be paying 15C kwh to buy it back as off peak at night. I know there are some deals now ( generaly limited bait deals) where the FIT is more than the off peak but generally it's more to buy it even on off peak than what you can sell it for. With the voltage relay, I could still have High voltage at 4pm in summer so if we are say going out and have showers early, the heater might kick in straight away. I have been thinking of adding a timer to the relay as well so it kicks in at the point of max solar production but all I need for that is to up the threshold accordingly and will get the same result. I believe 230V is the national standard and has been for over 10 years now. Most people still say 240 including myself most times in casual conversation but it is in fact 230. I'm lucky to see 230 here at night with the AC, the oven and the stove going thanks to the commonly over high voltage coming from the badly tapped transformer across the street. having it at 230 would make things much better for my solar production. That is a different relay to what I have. Mine has an LCD screen and you can program in the exact voltages and times for on and off " Tripping" of up to 30 Min I think it is. under voltage can also be programmed but not a feature I have had occasion to use. I would think the 10 sec delay is the same thing. It prevents the relay Cycling machine gun style at slight drops in power. Fortunately not a problem I have. perhaps they could do like I have and put it on it's own fused circuit. will depend how you wire it up. You could keep the off peak if you wanted so the water would always be automatically hot no matter what the weather and the relay was a daytime boost. The wife and daughter asked me when I set the relay up if I had turned the water temp up? They thought the water was hotter. As well all shower at night, I figured as off peak cuts out at 4 am normally at the latest, if we were showering at 10-11 which would be normal, the water may have been sitting there cooling off with losses and hot water used for washing up etc through the day for up to 19 hours. In summer it could easily still be on if needed at 4Pm or later so the temp loss would be much less hence the hotter water at shower time with it not ever being heated hotter at all. I severely Doubt it. Most HWS I have seen are either 3600W, 15A or 4800W, 20A. Even if you did have a 2400W element, I'd still be reluctant to run a 10 A relay flat out. You could do what I have and use that relay to drive another relay with a 20A rating or a pair to give you some margin. I tend to over wire and under Fuse. IE, I might have a 20A rated cable or relay but put the bare minimum breaker size in so any fault will quickly shut the breaker off. You can get elements down to 1800W for the main water heaters here, ( Rheem/ Dux etc) but that will take longer to heat your water. Mind you, that may not be a bad thing. If you have a 4800W element but you only have say 2000W surplus on your panels, you will be drawing from the grid on a timer or the relay will be cycling as the voltage pulls down below threshold. If you have an 1800W element, thing may be able to kick in and sustain at 10 am and run through the rest of the day. May also be able to stay in the threshold when there is some cloud cover where a bigger element will pull down the voltage too much and it will be switching every 10 sec and drawing from the grid partially when it does. It really depends on the size of your system and how much you have to spare. I have 20Kw worth of panels and adding so it's not a problem for me most of the year. Glad this month of lowest solar output is over and it will be getting better from here to about feb where it will go into noticeable decline again. I built myself a Box with a PWM controller and a Volt/amp/watt/frequency meter in it. I can run a 10,000W load from a 2400W outlet... but not at full tilt. One thing I can do with this is to put it in the heater circuit and see how much power it takes to pull the voltage down to a certain level. This allowed me to get a better idea of where the relay needed to be set in order for the voltage to be high enough to support the water heater load and still stay engaged. You will get voltage drop when a load like that kicks in, that's the premise the idea works on. I'm effectively using the excess power the panels produce and determining that by the voltage. You could also measure the voltage drop just with a multimeter on teh same phase if you have 3 phase and determine where the relay needs to be set that way.If you only have a smaller solar setup or a large base load, going to a smaller element may be a helpful thing. You don't need to have the water at full temp every day, as long as it gets there once a week you are good and it only has to be about 45 max for a shower. If it does not come up to full heat every day, that's not a problem. My water stays hot enough for 3 days with Zero power input which is why I have only had to kick it in manually twice so far. Even if it only got say 50% he power needed to get it to full temp, its still going to offset the temp dropping too low by probably another 2 days so now we are good for 5 Plus whatever lower input we get next day which may keep the water at the same temp so we get another day even if it is 20 below max. There is a lot of mental arithmetic and thinking things through with something as simple as hot water but I find it all practical and basic which makes it fun! :0) |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
Thanks for help, Davo. Looks like a voltage monitor, relay to fit heating coil in tank and a voltage display to watch what happens during lunch time should do the trick. You are correct, such units are being sold for $400 to $600 and one advertiser has not even replied to my emailed questions, they were perhaps too close to the mark! Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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