hello,
i currently have a topic on using excess power for heating water in the "classic" section, but i have some more specific questions about using PV to heat water and decided to start a new post so the other one is not so cluttered.
anyway my question is about using PV directly to the water heating element (no controllers/batteries).
would the same principals apply for DC direct PV hookups?
for example:
my 240VAC 4500W heating element will draw 1125 watts at 120VAC.
will the same heating element draw the same power at 120VDC?
I have 8 panels each rated at 33.0 Vmp and 4.25 Imp.
if i setup 2 strings of 4 paralleled together i would get 132V 8.5A = 1122 watts DC @ 132V
according to the calculation
4500W/240V = 18.75A
240V/18.75A = 12.8 ohm (R)
132V/12.8R = 10.31A
10.31A x 132V = 1361 watts
so if i run the 2 strings of 4 paralleled together they produce 1122 watts and if i hook that directly to the 240V 4500W heating element that element would be able to pull 1361 watts which is more than the power than the panels can produce which is what i am looking for correct?
i have these extra panels and they dont really match up well with others that i am running so instead of hooking 2 controllers in follow me i am just planning to run the panels directly to a small 30 gal water heater and pre heat water before my main 80 gal solar thermal water tank.
any inputs would be appreciated.
on my other forum discussion dgd posted about using panels direct to heat the water but he was using rectifiers/SSR's and the explanations were a little over my head so hopefully this new discussion can teach me/others about this direct PV to water heater process.
Contact Pete Gruendeman whose article on the subject of direct PV to water heater I attached in your other thread . He is an advocate for this approach and has a lot of experience with this.
Larry
I have a random thought running around in my head, and I suspect it won't work for some good reason... so please shoot it down if it is crazy... :o
Question : Is it possible to use PV with micro inverters (vs just DC output) to heat DHW if you could build a little rPi to shut off the output from each individual panel as the demand for heat lessens?
I hope this can stay on the topic of DHW heating only to support the OPs question(s)
I cant see any technical reason that would not work. I don't quite understand why rPi control to shut off outputs, I would just leave everything connected.
The BIG issue would be the cost effectiveness of this as micro inverters are not cheap. If they are free or low low cost then ok but my overall opinion would be to avoid them, just go the DC, and SSR route :)
dgd
What signal will the micro inverter sync to ? They generally want 240VAC 60.00Hz
the reason I thought of the rPi was to be able to shut down the array automatically when the DHW tank reaches max temp. Just a safety thing if left unattended.
Yes I had thought about the cost but the question is focused on attainable more than cost effectiveness... also a GT sytem could be tapped into under heavy weather times rather than idled because the Grid is down...
If using microinverters on pv panels then a normal ac element with integral thermostat could be used and that would disconnect when max temperature reached
whats the difference between this and a microinverter?
http://www.ebay.com/itm/302133928592?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Quote from: kauaisolarman on November 13, 2016, 04:38:17 AM
whats the difference between this and a microinverter?
http://www.ebay.com/itm/302133928592?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Well;
For one thing a microinverter has to sync to the grid voltage, phase and frequency and actually REQUIRE the grid to function properly. Built in safety controls.
Who knows what that unit you linked to really does?
Probably makes crude AC at an unknown frequency and voltage.
Not enough technical info I could find in the write up.
It probably "works" but don't be fooled by generalities like "Wow, I have 142 degrees at noon". Starting at what temp and how many watts in panels are important factors.
No matter what, you only get 250 watts from 250 watts of panels. Well, less with losses and efficiency issues.
I do like the idea of direct PV to water heating but I never trust Fee Bay seller hype.
Just saying.
Tom
Any gridtie inverter will shut down without the grid cycle 60hz.
From experience some of the chinese inverter do not island protect and they may work for what you are suggesting.
I would rather go with dc elements myself or use poly piping as a solar collector setup at a lower level than the tank.
There are plenty of basic plans for solar hot water systems on the web.
I've always like the idea of a SSR controlling the DC feed to a robust mod sine inverter that is wired to the heater, but the inverter has to be able to start with a load on it.
Mike, what about a <1 min (?) delay timer or relay(?), on the power-out side of the inverter? shouldn't take that long to bring the inverter up to full power
I remember about 15 years ago before mppt controller era I had a Trace dr2424e connected to a 1Kw water heater and after the Heliotrope controller went into float I powered on the Trace and the water heater worked good. Never had an issue with the Trace starting up under load, maybe it had something to do with the inverters soft start logic.
dgd
Quote from: ClassicCrazy on November 12, 2016, 09:24:46 AM
Contact Pete Gruendeman whose article on the subject of direct PV to water heater I attached in your other thread . He is an advocate for this approach and has a lot of experience with this.
Larry
thank you for the link to that article. this article also helped me understand a little more about correct sizing for element.
http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html
i cant seem to understand which is the better route to take when running direct PV to the heating element.
my heating element has a resistance of 12.8 ohms
if i run 6 panels; 2 sets of 3 panels in series paralleled together = 99V/8.5A = 11.64 ohms (140W x 6 = 840 watts)
if i run 8 panles; 2 sets of 4 panels in series paralleled together = 132V/8.5A = 15.52 ohms (140W x 8 = 1120 watts)
i have a total of 8 panels that i can use but not sure which route would be more optimal higher (8 panels) or lower (6 panels) resistance than the heating element (element = 240V 4500W 18.75A 12.8 ohms)
i wanted to run the 8 panels with the higher resistance than the element but it seems that the articles suggest using the lower resistance. am i understanding that properly?
thanks again for all input
Quote from: kauaisolarman on November 14, 2016, 03:32:22 AM
i cant seem to understand which is the better route to take when running direct PV to the heating element.
my heating element has a resistance of 12.8 ohms
if i run 6 panels; 2 sets of 3 panels in series paralleled together = 99V/8.5A = 11.64 ohms (140W x 6 = 840 watts)
if i run 8 panles; 2 sets of 4 panels in series paralleled together = 132V/8.5A = 15.52 ohms (140W x 8 = 1120 watts)
i have a total of 8 panels that i can use but not sure which route would be more optimal higher (8 panels) or lower (6 panels) resistance than the heating element (element = 240V 4500W 18.75A 12.8 ohms)
i wanted to run the 8 panels with the higher resistance than the element but it seems that the articles suggest using the lower resistance. am i understanding that properly?
OK, I will take a bite at this one... although I will probably regret it later (FFS)
Respectfully, can I suggest you find somewhere local to you that teaches an Electricity 101 course that includes basics such as Ohms law.
132 volts divided by 8.5 amps DOES NOT = 15.52 ohms.
However 132volts applied to a 15.52 ohm conductor will allow 8.5amps of current to flow.
Ohms is only a measure of the electrical resistance of a conductor.
Using Ohms law you can calculate what the current flow through that conductor will be for any given voltage
Connecting PVs to a circuit does not mean you get the nameplated power from that PV, nearly always it willl be a fraction of that power. Since the heating element has a fixed resistance it can, in accordance with Ohms law, only draw a current maximum that is easily calculated using Ohms law. So connect as many PVs as you want to the element, it makes no difference except more PVs mean you may increase the time you get closer to providing the maximum power through the element.
That Electricity 101 education would save you from the need to post so many newbie RTFM questions here
dgd (no offence intended!)
kauaisolarman;
Not to beat on you but DGD is on the mark.
Most of your questions are of the basic electrical knowledge type.
In my humble opinion, anyone who needs to ask these questions probably should hire a pro to do this stuff.
None of us mind helping at all but we need to feel that the questions are valid and will be understood correctly by someone who has the basic knowledge to apply the answers correctly and safely.
Otherwise it looks like you are just trolling for attention. Not saying you are, just what it reasonably appears to be.
None of us were born knowing this stuff but learned it over time. Some of us over decades of practical experience so we can miss fine details that are reasonably expected to be known by the questioner.
I don't know any other way to say it but blunt and to the point.
Respectfully.
Tom
thanks for the input again dgd and tom.
sorry for the stupid questions but i figure every one has to start somewhere.
2 months ago i knew very little about solar but through reading forum posts and asking questions i have learned a lot.
i know the questions are basic electric 101 but unfortunately I don't personally have that basic 101 experience (hence the questions , im trying to learn)
i know i could go to the local community college and pay to take electricity course and gain knowledge through that route, but, since i had some free time at night i figured i would post my newbie questions here. thought the forum could help me with a little free knowledge.
I didn't know that i was offending people by asking these simple questions, i just figured that there must be someone out there with the same level of knowledge as me and can possibly learn from the layman questions that i ask. (every other forum post i have read about the subject matter is a little too complex for my current level of knowledge)
I am a certified flight instructor and if you need any knowledge about aviation i would be glad to share any information that i have.
The 1 thing i make clear to all me new flight students are "there are no stupid questions, if something doesn't make sense ask, and if the way i explained didn't make sense ask again and i will try to explain in another way you can understand"
I cant imagine if someone came to me to learn to fly and after they asked a few questions i told them they should just save money to hire a pilot because their current level of knowledge is not sufficient. (wasn't that why they are trying to "learn".)
maybe the guidelines for the forum should say only for experienced electricians & solar installers with technical jargon terminology only. If not you will be shown how little you know about things.
weird thing is i though that that is what forums were for to start discussions and get input for questions that you don't necessarily know the answers to.
so again sorry for my newbie questions.
maybe instead of getting technical you could explain that if the element has a resistance of 12.8 ohms and you apply 132V the conductor will allow 10.31A of current to flow through or 1361 watts maximum (132V/12.8R = 10.31A) (10.31A x 132V = 1361watts).
or
if the element has a resistance of 12.8 ohms and you apply 99V the conductor will allow 7.7A of current to flow through or 765 watts maximum. (99V/12.8R = 7.7A) (7.7A x 99V = 762W)
this may help someone with the same level of knowledge as me as they could actually learn something.
@TomW
"None of us were born knowing this stuff but learned it over time"
Imagine if when you were going through the process of learning you were told that the questions you had were to stupid and instead of asking you should just get a professional to do it for you. would you have ever learned anything?
Respectfully
Quote from: dgd on November 14, 2016, 03:18:03 PM
Quote from: kauaisolarman on November 14, 2016, 03:32:22 AM
i cant seem to understand which is the better route to take when running direct PV to the heating element.
my heating element has a resistance of 12.8 ohms
if i run 6 panels; 2 sets of 3 panels in series paralleled together = 99V/8.5A = 11.64 ohms (140W x 6 = 840 watts)
if i run 8 panles; 2 sets of 4 panels in series paralleled together = 132V/8.5A = 15.52 ohms (140W x 8 = 1120 watts)
i have a total of 8 panels that i can use but not sure which route would be more optimal higher (8 panels) or lower (6 panels) resistance than the heating element (element = 240V 4500W 18.75A 12.8 ohms)
i wanted to run the 8 panels with the higher resistance than the element but it seems that the articles suggest using the lower resistance. am i understanding that properly?
OK, I will take a bite at this one... although I will probably regret it later (FFS)
Respectfully, can I suggest you find somewhere local to you that teaches an Electricity 101 course that includes basics such as Ohms law.
132 volts divided by 8.5 amps DOES NOT = 15.52 ohms.
sorry, the only reason i had this understanding was because of what i got off this website: http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html
"THIS IS WHERE I GOT THAT INFO" (Vmp 33V Ipm 4.25A) {33/4.25 = 7.7 ohms (per panel)} (2 panels parallel = 15.5 ohms)
Plan below uses solar panels only, and requires no kit
1000 watt system: four 250 watt panels
3000 watt system: twelve 250 watt panels
Specifications of the 250 Watt RENOGY panels
Maximum Power: 250W
Optimum Operating Voltage (Vmp): 30.1V
Optimum Operating Current (Imp): 8.32A
Panel Ideal resistance at 1000W/m^2 solar isolation = Vmp / 1mp 30.1V / 8.32A = 3.617 Ohms (per panel)
Element Wattage = 4500W
Element Resistance: 240 * 240 / 4500 = 12.8 Ohms
Ideal number of panels per single string assuming 1000W/m^2 solar isolation:
12.8 Ohms / 3.617 Ohms = 3.538 panels, round down to 3 panels per string.
I just wanted to know if it was OK to run the higher resistance as this article suggest rounding "down" to the nearest resistance load.
i guess to answer my own question
132 V setup with 8 panels = panels can produce 1120watts (140W x 8 ) and element can use up to 1361watts @ 132V (more than all power that the panels can produce)
99V setup with 6 panels = panels can produce 840 watts (140W x 6) and element can use up to 762 watts
(less watts than panels can provide, so in peak sun panels can potentially produce 840 watts but element will only allow for 762 watts of power to come through losing 78 watts of power that the panels can produce that cannot be used)
again this is just for the people that don't have all the electrical knowledge and need some know how.
the only reason i now know this information is because of the answers to my questions on this forum. It allows me to understand a little more and that clears my understanding of the "bigger picture" of things when i read other articles/forum discussions about the subject matter.
so thanks again.
yesterday i could not wrap my head around it and now it makes perfect sense.
kauaisolarman;
Going up in resistance will simply reduce the power it draws for a given voltage. More resistance means less current [all else equal].
This is called an inverse relationship. "Current is inversely proportional to resistance" less resistance means more current.
The thing there is this only applies if the source has sufficient voltage and current to drive that load. In the example of solar panels under load the voltage will likely vary a lot as will the available current. You will likely never see name plate power from any solar modules!
Watts [power] is amperes multiplied by voltage and that is why it is less power with higher resistance.
So, in answer to your question, increasing the resistance will lower the power the element can produce for a given voltage. It should cause no problems just lower power consumption.
Not rocket science but essential information for anyone working with electric circuits.
My main concern is someone applying information obtained here incorrectly and causing harm to people or property.
Electricity is inherently dangerous and DC even more so once you get up above 50 or so volts.
When things go bad it happens very fast. It is commonly understood it can take as little as one amp of current to kill a human with sufficient voltage behind it.
It is not just understanding formulas. There are hundreds of little "gotchas" that can trip you up if you are unaware.
Best of luck with it all.
Tom
Quote from: kauaisolarman on November 15, 2016, 01:10:46 AM
thanks for the input again dgd and tom.
sorry for the stupid questions but i figure every one has to start somewhere.
2 months ago i knew very little about solar but through reading forum posts and asking questions i have learned a lot.
i know the questions are basic electric 101 but unfortunately I don't personally have that basic 101 experience (hence the questions , im trying to learn)
i know i could go to the local community college and pay to take electricity course and gain knowledge through that route, but, since i had some free time at night i figured i would post my newbie questions here. thought the forum could help me with a little free knowledge.
I would suggest finding an online tutorial - like on youtube that explains Ohms Law - and one that explains basic series and parallel circuits. When I wanted to learn electronics ( way before internet !) I found that getting my ham radio ( amateur radio ) license was the best way. Every book on electronics I had picked up was way over my head . But with ham radio study guides they started with no knowledge - just the basics and took time to explain it. So check out ham radio and get your own FCC callsign at the same time. Or just take a look at the numerous free ham radio online courses available and look for the basic electronics sections in them.
With your questions about the various hot water elements - basically the thing to understand is that if you put current through the element it will heat up and raise the temperature of the water. The more you can force through it the faster it will heat the water but also the more power it will take to do that. So if you put 60 volts through an element it will use half the power as if you put 120 volts through it . And it will heat the water half as fast.
As Dgd mentioned you don't always get the rated power out of solar panels. It depends on the amount of light and temperature . In the winter with humidity low and when it gets well below freezing my panels will put out a lot more power than in the summer.
Larry