Diversion ...

[RossW]

That's hardly left field thinking.  Most successful attempts at heating water from PV I see posted here involve sending it through the CC and battery and inverter as you suggest.

I too live completely off-grid, and faced this issue.
I hate to say it, but for hot water, PV really isn't the way to go IMHO.
With typical panel conversion efficiencies of what... around 15%? Then a few more losses in the charging process, and a few more going back to AC, and a few more in the cables, I reckon you get perhaps 10% of the suns energy as degrees rise in your water.

I purchased some evacuated tubes (60), a manifold and frame. It heats the water directly. A small circulating pump and a $12 temperature controller complete the setup. Close to 90% of the available sun goes in to heating the water.

I know, comparing apples to oranges - you can't use the hot water to charge your batteries, and this discussion is probably more about "using surplus PV", but sometimes its easy to focus too hard on one solution and miss another.

[gridloose]

If I can go direct from panels to water heater with $50 in electronics (and 6 months of screwing around, it's a hobby!) that's a big win.

And just to say how easy that is...
Use dc SSR, snubber diode and decent blocking diode. SSR control to aux1 set to Float High. Use SSR to switch in heater to output from pv array - or just part of array that has block diode between array and CC. Leave enough of array direct to CC  to maintain float with normal loads.
Aux1 has a few seconds off delay that stops SSR chattering.

That might work well enough, but is not ideal.
Those panels probably won't be operating anywhere near Vmp most of the time unless you vary the power to the heater.
Also, the CC is trying to do MPPT tracking while you are switching in and out that power to the heater.

[gridloose]

I hate to say it, but for hot water, PV really isn't the way to go IMHO.
With typical panel conversion efficiencies of what... around 15%? Then a few more losses in the charging process, and a few more going back to AC, and a few more in the cables, I reckon you get perhaps 10% of the suns energy as degrees rise in your water.

I purchased some evacuated tubes (60), a manifold and frame. It heats the water directly. A small circulating pump and a $12 temperature controller complete the setup. Close to 90% of the available sun goes in to heating the water.

I know, comparing apples to oranges - you can't use the hot water to charge your batteries, and this discussion is probably more about "using surplus PV", but sometimes its easy to focus too hard on one solution and miss another.

That 90% efficiency on the evacuated tubes seems rather high, can you point to a data sheet?
I'd guess the Y intercept is likely below 50%, and with a 100 F delta T (ambient to water temp) it drops another 10 or 15 %.
And then you still have all the plumbing losses.

The 15% efficiency of PV is only an issue if you lack the real estate.
What I care about most is dollars per watt.

PV efficiency goes up as ambient temps fall, so works best when you need it most.
Solar hot water efficiency falls with ambient temps.

When I investigated solar hot water a couple years ago, I concluded that PV was about as good on dollars per watt if you wanted it to work at ambient temps of 30 F or so.  And that's dedicated PV panels into an electric hot water heater, never mind that electricity can be put to many other uses.

If we have enough PV for lights and fridge and washer here during the winter, we will have plenty of PV hot water most days in spring summer and fall.  Most solar hot water systems don't do much good in winter.  (Your evacuated tubes might be an exception though).

[dgd]

If I can go direct from panels to water heater with $50 in electronics (and 6 months of screwing around, it's a hobby!) that's a big win.

And just to say how easy that is...
Use dc SSR, snubber diode and decent blocking diode. SSR control to aux1 set to Float High. Use SSR to switch in heater to output from pv array - or just part of array that has block diode between array and CC. Leave enough of array direct to CC  to maintain float...

That might work well enough, but is not ideal.
Those panels probably won't be operating anywhere near Vmp most of the time unless you vary the power to the heater.
Also, the CC is trying to do MPPT tracking while you are switching in and out that power to the heater.

i agree but you did say $50 
Surprisingly it does appear to work quite well... its takes some matching of heater wattage to available diverted strings wattage and CC seems to not get confused at it sees strings not diverted as better optimal V and A for continued battery charging.
dgd

[RossW]

That 90% efficiency on the evacuated tubes seems rather high, can you point to a data sheet?

Don't have the sheets of mine handy, but I spent a fair while comparing various tubes.

Here's the first one I found on the 'net.
http://endless-solar.com/evacuated-tube-efficiency.htm

Mine are wet tubes, which when I was doing all the numbers, were somewhat more efficient than the heat-pipe type, but had other downsides. Fortunately, the downsides that were mentioned were not an issue for my system because I was able to design around them.

My actual measured efficiency wasn't far off the mark either. I didn't have a flow-meter on my circuit, but the temperature difference between water in and out and the flow rate of the pump with the measured head came to about 86% conversion of the "available" power measured by my pyranometer at the time. Plenty of scope for errors though, because I was measuring the power available in the cosine-corrected pyro, comparing to the calculated sun angle, re-correcting that for the actual alignment of the evac tubes and then comparing to the temperature-delta*flow-rate method of determining input power.

[gridloose]

That 90% efficiency on the evacuated tubes seems rather high, can you point to a data sheet?

Don't have the sheets of mine handy, but I spent a fair while comparing various tubes.

Here's the first one I found on the 'net.
http://endless-solar.com/evacuated-tube-efficiency.htm

Mine are wet tubes, which when I was doing all the numbers, were somewhat more efficient than the heat-pipe type, but had other downsides. Fortunately, the downsides that were mentioned were not an issue for my system because I was able to design around them.

My actual measured efficiency wasn't far off the mark either. I didn't have a flow-meter on my circuit, but the temperature difference between water in and out and the flow rate of the pump with the measured head came to about 86% conversion of the "available" power measured by my pyranometer at the time. Plenty of scope for errors though, because I was measuring the power available in the cosine-corrected pyro, comparing to the calculated sun angle, re-correcting that for the actual alignment of the evac tubes and then comparing to the temperature-delta*flow-rate method of determining input power.

Choosing evacuated tubes is definitely better than flat plate collectors if you want solar hot water in winter.
Could well pencil out to be cheaper than PV for hot water in many cases.
But they pretty much have to be roof mounted on the house to avoid plumbing losses.  Worst case when offgrid is winter, I'd like collectors somewhere between 60 deg and vertical, and able to deal with high winds.  For me, it's a lot more convenient to park a whole bunch of PV panels out of sight 100 yards away, and run power back to the house over some 10 gauge wire at 200 volts or so. 

That datasheet you pointed to shows "normalized" efficiency with no explanation, which is to say it was probably put together by the marketing department.  An honest graph would not show the flat plate collector to have a lower efficiency than evacuated tube at small deltaT's, even if the curves are "normalized". 

The graph on the top right of page 4 here:
    http://www.homepower.com/view/?file=HP123_pg66_Marken
compares some flat plate with evacuated tube designs.  With collector water at ambient air temp the evacuated tube is shown to be around 50%, falling to about 25% when the difference in temperature gets to 100 F (say 20 F outside, and 120 F hot water).  There are plenty of yeah-buts here that could effectively raise those figures, such as the round tubes collecting heat for more hours in a day than a flat plate, or if you figure just the area of the tubes themselves rather than the entire assembly, but I suspect 86% is more than a bit high for a complete system including plumbing and such.  This webpage suggests around 80% transmission of the sun's heat energy through a single pane of uncoated glass:
    http://www.commercialwindows.org/shgc.php

[zoneblue]

i agree but you did say $50 
Surprisingly it does appear to work quite well... its takes some matching of heater wattage to available diverted strings wattage and CC seems to not get confused at it sees strings not diverted as better optimal V and A for continued battery charging.
dgd

Those big cyllinders have two elements right? If you used two, and sized them at a bit less than half the array, you you bring them on line in staged manner.

This is the one i had my eye on. 0.7 ohms, 1000W: http://ecoinnovation.co.nz/p-631-07-ohm-water-element.aspx .Gasp at the price if you want *everything* is expensive here.

[dgd]

Those big cyllinders have two elements right? If you used two, and sized them at a bit less than half the array, you you bring them on line in staged manner.

Yes they do - some even have 3 element  positions. One is usually at bottom of cyclinder and other nearer top. The top is to provide hot water in smaller volume for immediate use instead of waiting for lower element to heat whole tank of water.
I had considered enabling a top 1kw element first then the lower if excess power continued. I just use lower element position as 200 litre tank gets hot in a few hours.

This is the one i had my eye on. 0.7 ohms, 1000W: http://ecoinnovation.co.nz/p-631-07-ohm-water-element.aspx .Gasp at the price if you want *everything* is expensive here.

..too expensive for me. I bought one similar to this one  from Ebay

http://www.ebay.com/itm/Camco-2000-Watt-120V-High-Watt-Density-Water-Heater-Screw-In-Element-/400581805927?pt=LH_DefaultDomain_0&hash=item5d44894767
Note element only no thermostat.

and used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

Placed it direct to top of tank with heat sink compund and used plastic wire ties to hold in place. It is in wire path from Aux1 to SSR.

and these as blocking diode 
http://www.ebay.com/itm/261260112924?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

The Crydom 20A ssr i use also from ebay. I seem to remember the lot was less than USD50 no including postage..

I didn't have an HWC so trademe was were I found good price mains pressure one and valve set. My HWC now feeds Rinnai LPG water heater. I checked that Rinnai monitored incoming 'cold' water feed so it did not activate gas heater if incoming water temp was already high enough. It did. Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months. 

dgd

[Westbranch]

Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months. 

that's quite a reduction!  looks like somewhere between 15% to 20% of previous consumption...

[dgd]

that's quite a reduction!  looks like somewhere between 15% to 20% of previous consumption...

It was also saving $ because the LPG supplier would not fill 'user owned' 45Kg bottles so I had to pay rent each month for use of their bottles. I terminated that deal  a year ago (they didn't even bother to pick up the bottles - yet!)  and bought a 13Kg that I could get filled almost anywhere.

[gridloose]

...  used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

One thing to keep in mind.  The contacts on that thermostat are rated for 240 VAC, nothing said about DC.
With AC, the voltage goes to zero once each half cycle, so with 60 Hz line freq that's 120 times per second.
The voltage is at zero long enough for any arcing to break and the ozone disperse a bit to where when it comes back
there is no more spark.  With DC, the contacts might continue arcing indefinitely.  Circuit breakers such as the SquareD QO series that are rated for both AC and DC generally are considered good at DC for only a fraction of the AC voltage.
    http://static.schneider-electric.us/docs/Circuit%20Protection/Miniature%20Circuit%20Breakers/QO-QOB%20Circuit%20Breakers/0730CT9801R108.pdf

So maybe best to wire the thermostat to the control side of that SSR?
One would hope that they would fail burned open, but I could imagine them welding closed.

Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months. 

Very cool.  That does indeed work well enough.  I'm tempted.

But I think the scheme I outlined a few posts back with PWM to the heater and monitoring the PV voltage to check for a pattern indicating the CC is performing an MPPT scan and recording the Vmp, that could be built into a nice cute box that mounts at the water heater, wired to the PV panels with blocking diode into the CC, and just work with no tuning of any sort (assuming the panel voltage is up high enough to get enough power into the heater element).  And operate at close to optimum efficiency in bulk, absorb, float and equalize.
So a turn-key solution with no head scratching, possibly work with other CC's as well with a firmware change to match the MPPT pattern.

[dgd]

...  used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

One thing to keep in mind.  The contacts on that thermostat are rated for 240 VAC, nothing said about DC.
With AC, the voltage goes to zero once each half cycle, so with 60 Hz line freq that's 120 times per second.
...
So maybe best to wire the thermostat to the control side of that SSR?
One would hope that they would fail burned open, but I could imagine them welding closed.

Yes that is what I do, this switch interrupts the control from the Classic AUX1 to the SSR. So its only dealing with 12v about 10ma max.
I checked with the supplier tech dept to see if this was ok for their AC switch and they assured me it was and if it ever failed to bring it back for replacement.

But I think the scheme I outlined a few posts back with PWM to the heater and monitoring the PV voltage to check for a pattern indicating the CC is performing an MPPT scan and recording the Vmp, that could be built into a nice cute box that mounts at the water heater, wired to the PV panels with blocking diode into the CC, and just work with no tuning of any sort (assuming the panel voltage is up high enough to get enough power into the heater element).  And operate at close to optimum efficiency in bulk, absorb, float and equalize.
So a turn-key solution with no head scratching, possibly work with other CC's as well with a firmware change to match the MPPT pattern.

I had thought about your scheme and maybe all it would take is the Classic to signal via an AUX1 output or perhaps setting a modbus register when it is doing an mppt scan. The register may not be the way as it would need to be continually polled as no interrupt processing would be possible, BUT the aux1 could prompt a BB to read the voltage over next 20 or so seconds to get the high Pv reading then the BB could go from there..

dgd

[gridloose]

I had thought about your scheme and maybe all it would take is the Classic to signal via an AUX1 output or perhaps setting a modbus register when it is doing an mppt scan. The register may not be the way as it would need to be continually polled as no interrupt processing would be possible, BUT the aux1 could prompt a BB to read the voltage over next 20 or so seconds to get the high Pv reading then the BB could go from there..

I'm warming to the thought of inspecting the PV voltage for the MPPT scan pattern.  Then the only connections needed into this PWM box are power from the panels, power to the heater element, heater tank temp sensor, and perhaps a circulation pump SSR if heating a larger cistern.  This box needs to watch the PV voltage anyway, as it is trying to send only enough power to the heater to hold the panel voltage at Vmp.  A TI MSP430 Launchpad at $12 would give all the processing power and the ADC as well.  I distrust solutions involving more than a billion or so transistors, often having another billion little flash cells of electrostatic charge slowly leaking off toward senility.

Edit: That billion transistor thing does not necessarily apply to the BB, and the BB could well do a fine job of this and perform other tasks as well.  But an MSP430 in a DIP20 costing around $1US (single unit, from Mouser or Digikey!)  should have all we need here for the water heater PWM.  At $12, the Launchpad gives a complete development system including the USB cable to your laptop.  Unfortunately TI dropped the ball a bit on packaging the software tools, it's non-trivial to get that first blinky light going even after you get through the 50 megabyte download and install.  Silly.

[zoneblue]

..too expensive for me. I bought one similar to this one  from Ebay
http://www.ebay.com/itm/Camco-2000-Watt-120V-High-Watt-Density-Water-Heater-Screw-In-Element-/400581805927?pt=LH_DefaultDomain_0&hash=item5d44894767
Note element only no thermostat.

Are you sure that one for 27v dgd? 2000W at 120V is  7 ohms. At 27v thatll be about 100W.

and used this for water temp  control
http://www.jaycar.co.nz/productView.asp?ID=ST3821&form=CAT2&SUBCATID=969#1

Placed it direct to top of tank with heat sink compund and used plastic wire ties to hold in place. It is in wire path from Aux1 to SSR.

Quite the diode, do you have a wire diagram of all this? Sounds interesting.

I didn't have an HWC so trademe was were I found good price mains pressure one and valve set. My HWC now feeds Rinnai LPG water heater. I checked that Rinnai monitored incoming 'cold' water feed so it did not activate gas heater if incoming water temp was already high enough. It did.

Ive asked several califont reps this question and all i get is blank stares or fudged answers. It seesm clear that the cheaper ones dont regulate gas pressure, are either on or off, so not good for this.  One guy told me that the bosch or better grade do regulate gas pressure, but only in a band. if you put in 30C water it boosts it to temp nicely. But if you put in 45C water it gets confused and heats it to 80. Gas cant go low enough. SO then you have to put in a temper valve to cut the top off. What model rinnai is yours?

Now I use less than a 13kg bottle of LPG in 12 months instead of two 45Kg bottles worth every 8 months. 

Nice job. Our gas usage is the last bastion.

[dgd]

Are you sure that one for 27v dgd? 2000W at 120V is  7 ohms. At 27v thatll be about 100W.

Its no good for 24v but then I'm connecting it to the output of PV's, 5 to a string = about 88V mpv.  That is about 1Kw with this element

Quite the diode, do you have a wire diagram of all this? Sounds interesting.

I will draw up a diagram and get it posted here...

Ive asked several califont reps this question and all i get is blank stares or fudged answers. It seesm clear that the cheaper ones dont regulate gas pressure, are either on or off, so not good for this.  One guy told me that the bosch or better grade do regulate gas pressure, but only in a band. if you put in 30C water it boosts it to temp nicely. But if you put in 45C water it gets confused and heats it to 80. Gas cant go low enough. SO then you have to put in a temper valve to cut the top off. What model rinnai is yours?

I'm using an old Infinity 22. Seems to be ok with taking water from a tank that is already hot enough although it does start up the gas burner but turns it off almost immediately. I still use a tempering valve out of the HWC which keeps the water to 55c even though the HWC can get to 60c.
The Rinnai pdf online manual recommends in a solar system where a tank feeds the continuous flow heater that the heater is set to 75c
and tempering valve then drops it to 50c. Its for bacteria control of water from the tank (eg legionaires). I might revisit this sometime with my setup but it would probably mean more LPG use. Also I think an HWC at 60c may be sufficient for basteria control.

Nice job. Our gas usage is the last bastion.

Same here.