Author Topic: PV Direct to Resistance Heating  (Read 4491 times)

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adobejoe

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PV Direct to Resistance Heating
« on: May 08, 2016, 09:06:34 AM »
I know it is more efficient to space heat via evacuated tube or flate plate solar. But I am curious, I have a garage with plumbing and I just want to introduce a little heat. How practical is it to direct wire a resistive heating element to PV panels? Say, two 250 watt panels for 500 watts at 24VDC 16 amp (I know it will be higher) and a 24 volt 15 amp Load resistor. OF course include an inline fuse. Second option a simple load controller and two 12 VDC batteries in series? Thoughts?

DamonHD

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Re: PV Direct to Resistance Heating
« Reply #1 on: May 08, 2016, 10:00:59 AM »
It's not necessarily lots more expensive to use PV (though arguably not the best use of its high-exergy output), but providing the correct loading to match the panel output as light levels change is non-trivial.

Putting batteries in there will wear them out fast, IMHO.

But let's hope some better-informed opinions come along shortly...

Rgds

Damon

OperaHouse

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Re: PV Direct to Resistance Heating
« Reply #2 on: May 08, 2016, 11:48:06 AM »
That is how I heat all my hot water at the camp with excess PV power.  The panels would be best placed in series and operated at power point voltage.   Direct connection is absolutely the wost thing to do ad would result in power loss of more than 60%.  The controller you would have to build but it is fairly  simple.

adobejoe

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Re: PV Direct to Resistance Heating
« Reply #3 on: May 08, 2016, 03:16:46 PM »
Okay, so I am in Montana and there is low light in the winter, critical time. I have access to Poly silicon, or thin film (100 watt schott). I think the thin film may be better choice? The critical months are Dec-Feb of course, and so what if I just set up two batteries and use a monrningstar with low voltage disconnect, as the feed for the element. Doable?

OperaHouse

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Re: PV Direct to Resistance Heating
« Reply #4 on: May 08, 2016, 03:42:19 PM »
No batteries, no standard charge controller.  Panels have to operate at power point.  I believe costs are comparable with direct solar methods, but 500W is a joke.

richhagen

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Re: PV Direct to Resistance Heating
« Reply #5 on: May 08, 2016, 05:35:51 PM »
I have used resistance heating as a dump load for quite a while.  Presuming you only match the panels full output to the heating element with no controls, I think you will find it to be very inefficient in less than full sunlight.  One solution would be to use the panels to charge up a capacitor bank and build a simple switching circuit to turn the heating element on at around the optimum voltage for the panels and off as the voltage drops too far below.  In this way you could keep the panels operating nearer their optimum, which will give you more heat for your garage. 

To echo OperaHouse, you will barely notice 500 watts in a 300 square foot or larger garage.  Obviously insulating the structure would be a good idea as in general (thermal energy in - thermal energy out = accumulation in thermal energy) but of course in general, as the temperature increases inside relative to the outside, the heat transfer will also increase.  Heat transfers by convection, conduction, and radiation only, but all three methods can increase with increased temperature differential between the inside and the outside.
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richhagen

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Re: PV Direct to Resistance Heating
« Reply #6 on: May 08, 2016, 05:59:49 PM »
To further illuminate my response above, here is a chart of power point vs solar irradiation for a typical solar panel.
9738-0


















If you notice the optimum voltage only increases slightly with increasing irradiation, so if you pick a turn on voltage at say 16.5 volts per panel connected in series and off at say 16V would generate much more power in low light conditions relative to directly connecting the panels to the the heating element.  Obviously those set voltages will depend upon the characteristics of your panels and how many you connect in series. 
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adobejoe

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Re: PV Direct to Resistance Heating
« Reply #7 on: May 08, 2016, 08:21:29 PM »
Forgot to mention, roof is R-70 Walls R-50....we could go 1 KW.

OperaHouse

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Re: PV Direct to Resistance Heating
« Reply #8 on: May 08, 2016, 09:09:05 PM »
It feels like I've explained this a million times and still nobody gets it.  http://www.fieldlines.com/index.php/topic,148385.0.html  The cheapest and easiest way is to use a UNO.  This is an absolute beginner level program.   Panel voltage varies with temperature not light level, a capacitor stores that energy, PWM acts as the transmission matching power level to the resistance heater.  All good ideas are basically very simple.

richhagen

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Re: PV Direct to Resistance Heating
« Reply #9 on: May 09, 2016, 02:59:25 AM »
I believe that the correct answer would be that it varies with both, imperfections in the diodes of the panel cause peak voltage to vary with light intensity as the above curve illustrates that it actually does.  That said, sure you could program up an Arduino and have it turn on and off according to an algorithm which accounts for temperature, voltage, and even other variables, however, with a transistor a few resistors and a few FET's you can get ninety plus percent of the way there by keeping just to the left of the peak of the curves.  You could optimize this better with an Arduino a temperature sensor or sensors and voltage sensor its just a matter of how comfortable you are with building circuitry and programing. It's kind of like optimizing blades on a wind turbine, you can get a few more percent by having a more perfect airfoil, but you can get 90 plus percent of the way there with a few approximations that greatly simplify construction.
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Eraser3000

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Re: PV Direct to Resistance Heating
« Reply #10 on: October 08, 2016, 10:25:40 PM »
Just some info for you before you get to far.

1 Kwh =  3412 BTUs

So if you install 1000 watts of solar and you get 6 hours of sunlight in the winter, then you could possible get around 20,000 BTUs.

Or 15 % of a gallon of kerosene from a kero heater.

 :-[  I sure wish it wasn't so. 

I'm not trying to discourage anyone, just make sure you know going in.  I only get 4 hrs of sunlight per day in the winter.

 

OperaHouse

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Re: PV Direct to Resistance Heating
« Reply #11 on: October 11, 2016, 10:35:26 AM »
Last year I was just able to build the new power shed and move the electronics into it.  This season was short due to medical issues, but working with ontfarmer on his heating project inspired me to clean up the wiring of the control panel and really tune up the system software wise.  As a result I am getting more hot water than I need from excess panel power not used for the basics.  Have little more than a week left in the season and it has gotten chilly.  Seriously thinking of heating one bedroom with solar just to raise it a couple of degrees.  That would make a big difference in comfort.  Maybe just heat up the 10 gallon water tank for dishes.  I can take showers at the YMCA.  It would only be a few KWH, but it is worth a try to make the last two weeks a little better.

keithturtle

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Re: PV Direct to Resistance Heating
« Reply #12 on: October 30, 2016, 11:30:26 PM »
This approach does not use PV, but claims to provide good heat in higher latitudes during winter. http://www.iedu.com/Solar/Panels/#Box
I'm building a small one to accelerate drying firewood, not done yet

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