water heating questions

[kauaisolarman]

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.

[ClassicCrazy]

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

[Westbranch]

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...   

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)

[dgd]

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

[mike90045]

What signal will the micro inverter sync to  ?  They generally want 240VAC 60.00Hz

[Westbranch]

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...

[dgd]

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

[kauaisolarman]

whats the difference between this and a microinverter?

http://www.ebay.com/itm/302133928592?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[TomW]

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

[russ_drinkwater]

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.

[mike90045]

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.

[Westbranch]

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

[dgd]

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

[kauaisolarman]

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

[dgd]

 
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!)