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Forum Index : Solar : Hot water from solar PV
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Tim_the_bloke Senior Member  Joined: 15/11/2009 Location: AustraliaPosts: 105 |
I have a small grid connected set of photovoltaic panels on my roof. Currently power goes through the inverter to the grid paying 60c per kw. In 2017 the 60c will fall by a likely 90%. The panels are nominally rated at 1500W but rarely generate over 1100W. I have a spare Rheem hot water tank, 3 phase, 315 litres which has three 240V elements. Each element is 4kW, which could be a problem. I was thinking I could some how rig up my PV panels to heat water instead of selling to the grid for less than I would pay for the electricity to heat water. Surely heating water which gets used by the household is an effective method to sort energy from the PV panels. I wonder what happens if a 4kW heating element is connected to an insufficient power supply? I wonder if it is OK to connect DC power to an AC water heater. Have any of you done this or similar? Comments? |
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Madness Guru  Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Do you know what voltage the panels are producing? You will need to change how you control the temperature, you don't want it unregulated. The thermostat that is there will fail on HV DC, it will create a big arc first time it disconnects and most likely go up in flames. If you fit a solid state really that can handle 400 volts DC @ 10 amps and rig it so the thermostat turn it on or off it will work. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
I do not know the max DC voltage. I will think on how to safely measure that. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Be careful measuring the DC. DC is much more dangerous than AC, and as Madness points out, needs special consideration with switching. The regular AC thermostat will last years on 240v AC and about about 2 seconds on 240v DC. Elements can run at a lower voltage, they just generate less heat. If you feed 50 volts into those 4000 watt elements, you can expect about 800 watts. That will take longer to heat the water, but thats fine, you have 8 or so hours of sun so it will still heat the tank. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
From the meter box, the solar array has nominal rating of: Open Circuit Voltage: 260V DC Short Circuit Current: 7.6DC amps The SMA Sunny Boy inverter has a display which displayed 189V at midday of a sunny winter day (today). I will think on sourcing a solid state thermostat. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Glenn, sorry to correct you but it would only produce 174 watts at 50 volts and that element. volts squared over power = ohms 240 * 240 over 4000 = 14.4 ohms Voltage squared over resistance = power 50 * 50 over 14.4 = 173.61 watts Tim if you connected the solar directly to the element and assuming you still get 7.6 amps it would produce 832 Watts @ 110 volts. PS hope I don't get banned :) There are only 10 types of people in the world: those who understand binary, and those who don't. |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
If you series 2 of the elements, and let the voltage rise, you get 29 ohms to play with.... if you get the 7.6a, the 7.6 x 7.6 x 29=1675w.... not enough panel to drive it now..... I figure that the voltage will be beneficial, as the current won't be driving such a low impedance, and we get to use the next 80 volts or so in the panels.... ie every extra amp is 80watts.... 7.6a panels looks like 6x6 cells so probably mppt in the 32v range per panel, guessing 6 panels. 6 panels@32v is about 190v... your GTI is running very close to perfect I think. With only 190v into 29R, we get current of 6.5a...... ie. 1235 watts. So I would series two elements to get the best out of it, and it would be higher impedance rather than too low, which should do better in less than steller lighting, as it lets the voltage rise easier. Use your temp switch to drive a few fets... low rds on will require only a reasonable heat sink. Yes, Glen is a little optimistic ... leads me to wonder....... Queensland watts are closer to the equator... must be a fudge factor operating there..... ............oztules EDIT: because of the band gap in the silicon wafer, we can expect the voltage to be stable.... as it is cast in stone ( silicon anyway )... but because of the internal resistance, it does not work that way in practice... but by increasing the impedance, we get less internal losses at lower lighting, so in theory we should do better this way. The current is a function of numbers of photons captured, so is light dependent... we can't fiddle that part, but we can effect the voltage very very much by altering the impedance of the load..... thats my theory anyway... Panels are a current device Village idiot...or... just another hack out of his depth |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Steady on Oz I am a Queenslander too  There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
.....Oops... I'm out numbered.... Lemme see.... fudge factor.....aahhh got it. Electrons move very very slowly in a wire, we are talking snails speed in most cases.... hehehe .... If your closer to the equator, your travelling about 2-300kph faster than me, so your electrons are moving thousands of times faster than my electrons are ( well if they are heading east at least) Taking this into account, and that fact that antimatter is being annihilated in the upper atmosphere... closer to ground level than our deeper angle of inclination through the atmosphere down here, and creating high energy gamma radiation etc... Those antimatter reactions affect your electrons more than they do ours.. you have more positrons up there I'm sure... thats the running fast in the rain effect of travelling much faster than me down here. Not only that, but you have more thunder storms up there that will create ... wait for it..... terrestrial gamma-ray flash (TGF)... yes... ah ha... these TGF create antimatter beams... which of course interact and annihilate any normal matter they contact with, generating gamma radiation...... which somehow speeds up/slows down/confuses ( strike out which ever one you feel needs it here) the electrons in the QLD state. I submit that the laws of the universe are different up there than down here, I am not even sure if the neutrons up there have two down quarks and an up quark like they do here either..... as a quark has charge measured in thirds, I submit that the electrom has not one unit of charge, but three, and in fact the quark has one not -1/3 or +2/3... so an up quarks will now have 2 charges, and the down quarks will have -1 charge... so the electron will have -3, and the proton 3+... hence three times the power for the same number of electrons now the charge is greater than before I messed it up.. I mean remodelled it. Now, I'm so suitably confused, I have reached a state of normality..... for a village idiot any way... 
..........oztules Village idiot...or... just another hack out of his depth |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
LOL I thought you where joking about being the village idiot. Easier solution for you Tim would be replace the 4000 w element with a 2400 w, then you will have 24 ohms and at 7.6 amps that will be 1386 watts at 182 volts. Should be pretty close to the sweetest spot for the panels and a 2400 watt element should be available off the shelf. Even better it will just screw in palce of the existing element, so no mods at all to fit addition elements. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
What have I started??? |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Bit like feeding time at the zoo. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Tim_the_bloke Senior Member Joined: 15/11/2009 Location: AustraliaPosts: 105 |
It would be possible to power the hot water element with AC after the grid tie inverter. This would resolve the issue with the thermostat. However... 1: The inverter is an expensive lump of equipment and would presumably last longer if it was used less by not pulling power through it to the hot water tank. 2: There would have to be significant losses in the DC to AC conversion in the inverter. Better to heat water in the tank than heat the inverter on the wall. Is there an economical way to still sell power into the grid once the hot water is hot and the thermostat goes open circuit? I expect there will soon be a market for such conversions. |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi Tim, there are some energy diverters around. https://www.immersun.co.uk/ the immersun makes a AC sine wave with variable voltage to regulate the power going to the HWS openenergymonitor.org have some advanced projects that do something similar, beyond my skill level. or I recall that Silicon Chip done a circuit notebook (JULY12) on using capacitors to throttle the Watts on an urn element. i dont recall the calculations it should be possible to restrict an element to 500-700 watts or so and run a timer for 5 hours a day. there is a few more options  I'm confused, no wait... maybe I'm not... |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
A simple solution may be to have a light activated relay that turns on the AC to the HWS via the thermostat. If you can adjust the light sensitivity to where you are making plenty of solar power then you will not be drawing too much from the grid. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 224 |
This thread might shed some light on it over at Whirlpool. https://forums.whirlpool.net.au/forum-replies.cfm?t=2530161 "Creativity is detirmined by the way you hold your tounge".My Father "Your generation will have to correct the problems made by mine".My Grandfather. |
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| Jarbar Senior Member Joined: 03/02/2008 Location: AustraliaPosts: 224 |
This also looks interesting. http://gswitch.com.au/ "Creativity is detirmined by the way you hold your tounge".My Father "Your generation will have to correct the problems made by mine".My Grandfather. |
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