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Old 12-22-11, 04:56 PM   #1
Lex Parsimoniae
 
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Default DHW from PV still working



I'm using four 200w panels connected to a 10 ohm load, trying to heat my hotwater storage up a few degrees, when it's sunny.
www.skyfireenergy.com/downloads/CS6P.pdf

But, the Max power point voltage for the PV series array is 115.6 Vdc.
So the 10 ohm load is too low & drops the voltage down to 40-60Vdc.
Typically, I've been seeing 400-450 watts from the panels.
(Higher when conditions are perfect. Real cold and very sunny).
So, it's been working, but not working very well..


But today, I installed a new 6.3 ohm (300w) resistor in series with the 10 ohm load.
Ohmite C300K6R3 6.3 Ohm 300 Watt Power Resistor | eBay
(7 amps squared is 49 x 6.3 Ohm = 309 watts, for the new resistor. So it might not last very long).

When it's all warmed up, the actual total resistance is now about 16.5 ohms.

115.6v / 16.5 ohms = 7 amps! (the rated current for my panels is 6.93A)!


So, next year when the sun comes back, we should see about 115v x 7a = 800 watts!!!!
800w = 2729.713 BTUs per hour!!

When I checked the power before making the mod, it was under 400w.
For a short time today, the PV was up to 100 volts @ 6.72 amps.
Making 672 watts with hazy sunshine is a good sign.

16.5 Ohms seems like the near prefect match for the series array.
These are good panels. We are hoping to see 600-700 watts on the average sunny day.
I'm keeping my fingers crossed!

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Old 12-22-11, 09:42 PM   #2
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Default Still Puzzled...

Xringer,

I'm still puzzled as to why you chose PV panels to heat domestic water?

I'm pretty sure that two or three 4' x 8' solar thermal panels would supply all the hot water you and your wife would need, just about year-round. Evacuated tubes would be even better.

Looks like you guys get a fair amount of sunny days where you live, even in the winter. Wish I did.

If you were using a 2.5 gal/min shower head (that is assuming you'd want to use a slightly 'green' shower head), and assuming your incoming water temp is 50F and your shower water temp is 104, a ten minute shower would consume about 11,246 BTUs.

If your max PV power production was averaging 650 watts max, and your solar exposure was 6 hours, and since the sun shining on the panel is sinusoidal in nature, I applied a .707 fade factor... you'd be able to generate something like 9,408 BTUs/day. Even if that resistor that you have bolted onto your water heater transferred all of its heat at 100% efficiency, your entire daily harvest of PV energy would only give you an 8 minute, 22 second shower.

If you searched your local craig's list you wouldn't be able to find cheap PV panels, no way. But hot water collectors, now that's a different story.

I bet that if you spent a couple of weeks looking for solar hot water collectors, you'd find loads, for $100 at the most, $50 if you were motivated, and for free if you have lived a good and virtuous life up to this point.

Daox somehow came up with a whole corral of hot water collectors. I don't know anything about the virtuousness of his life, but I don't think he spent a fortune on them.

I really think PV panels are terrific, absolutely a modern miracle, and that they are best used for what they are best at...

Why, with all the energy you are generating, you could probably run your plasma TV for a few hours each day, and help heat your living room at the same time.

I'm just not convinced that dumping all those furious electrons into a load resistor is the best application. Especially when heat from the sun can be accomplished with so much less technology involved... and so much more efficiently, too.

-AC_Hacker
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Old 12-23-11, 11:58 AM   #3
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You Sir, are correctomodo.. PV is the worse way to make hot water..

But, the 500w tracking array is keeping my battery bank charged,
and for the moment, the 800w array has nothing else to do.
Making AC power for the Plasma TV (in the den) would be nice,
but we mostly watch it at night..

I was wrong about waiting until 2012 for more sun..
Woburn, MA 01801 Month Weather Report - AccuWeather.com
We're going to have one day of Mostly Sunny this month..


I have been thinking of getting one of those Evacuated tube units.
But, I have no place close to the house to install it. (can't use roof).
The backyard PV is feeding indoors on 100 foot cables.
It would be a long and down hill run for hot water.

Plus, my backyard PV location is marginal at best..
My roof gets about twice the sun as the PV out back!

The other factor is the oil burner. It's our back-up heating system.
Running it for 20-30 minutes a day is our security blanket.
Knowing we can heat the house when it's near zero outside, is good.
We know it's there and can be cranked up in a matter of minutes.
Plus, it's providing us with DHW..


Having a few extra BTUhs from PV going into the boiler can't hurt anything.
And, if it keeps the boiler from running the full 30 minutes, it's going to
save some money on oil. (I think it's about 6.5 cents a minute).

IIRC, the government studies stated that DHW from PV wouldn't be feasible unless PV got down to $1 a watt.
But, what if you are using PV to replace Oil? And the price of oil keeps going up?
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Old 12-24-11, 11:35 AM   #4
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Default Sweet Christmas Eve Day!! 800 watts!!

It's hazy sunshine, due to high thin clouds, but I decided to check the power.

Seven (7) amps!

It's 28.8 deg F out there, so we getting good PV performance today.
The current is drifting between 6.8 to 7.0 amps.. (763 to 808 watts).

So far, I'm really pleased with the mod.
I hope the new resistor can handle the power!
A 300w resistor shouldn't be running 308.7 watts!
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Old 01-29-12, 11:53 AM   #5
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It's been over a month now and the 6.3 Ohm resistor is still working.
Today it's warmer out (38F) so the power is down to about 725 watts.
(That's 90% of the 800w PV array's rating).

And I think that little 2,474 BTUh is helping, because the water temperature
is staying up around it's 140F setpoint on most days..
(The oil burner's timer turns it on for 30 minutes at 6AM each morning).

Late last evening, we wanted to take two showers. So, I went down to the
boiler to give it a 10 minute burn cycle and found it wouldn't come on..
The burner controller was on standby.. Since the water temp was just a tad under 140F..
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Old 01-30-12, 10:42 AM   #6
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Default More hot water in the evening..?.

At about 6:20AM, I woke up and heard the oil burner running..
I started wondering if it might be starting it's half-hour run, too early.?.

I'm not working much these days.. And I can always shower the night before a work day..
So, what do I gain by resetting the timer?
Well, the burner does pull a lot of outside air into the basement,
and maybe some cooling occurs in the upstairs living space too.?.
(Meaning the Sanyos have to work harder).



Humm, at 06:20 it was 30.4 F & later at 08:30 it was 35.0 F

So, if delayed until 8:30, the intake air would have been 5 degs warmer..

Less chill in the house, and the heat in the tank will last longer into the evening.
Which is better for those mediocre solar days.
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Old 01-07-13, 04:21 PM   #7
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I just bought a new Hot water heater ELECTRIC only need 30 gallon (just 2 of us) it has dual 4500 watt elements.
I also use a timer that comes on about 4 hours a day

Has anyone tried one of these 12-24 volt DC water heating elements ? say like to replace the bottom 4500 element ?
I have a few extra 235 watt PV panels I could try , I wonder if it would work to prevent the top element in the heater from coming on - we use low flow shower heads..
panels cost me .63 cents per watt

24 Volt 400 Watt DC Low Voltage Submersible Water Heating Element 4 Wind Solar | eBay


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Old 01-07-13, 04:36 PM   #8
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Default

You might want to try running low-heat mode, using 120 instead of 230 vac.
I checked out the heating elements with my new water heater and at 120Vac they use 1.130 kW. (3856 BTUh).

Or, if you have enough PV you can put in series to get up about 120 to 150 vdc,
that might pump in a few BTUh.. But, the AC grid would be a lot more reliable.
Nice thing about using a 120vac hotwater heater, timers are cheap!!
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Old 01-07-13, 04:42 PM   #9
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electric water heaters have a 120 volt setting(low heat)?? Hmm Il have to check - my heat has dual 4500 watt its a low boy heater since my space is limited - or i can switch out the top 4500 for for a 1.1k watt 120 volt ? Hmm - I still haven't installed the HWH yet but my old Bradford white is 20 + years and sure about to go

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