Author Topic: PV to Heat Water-electrical resistance  (Read 3082 times)

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adobejoe

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PV to Heat Water-electrical resistance
« on: July 16, 2007, 04:05:08 AM »
I have two KYO 130 W panels and I want to try and connect them directly, no controller, to a standard hot water tank immersion element.  Basically, connect them in parallel for 24 V, and connect to lower element.  Use original thermostat switch to disconnect IF water gets to 150 deg F.  Maintain upper element on AC and set at 80 Deg F.


I approximate 150 w-hours will raise 1 gallon 60 Deg F.  Inlet water is 50 Deg F. Therefore 150 w-hours should raise 1 gall to 110 Deg F.  Approximate 8-9 good sun hours in a day (we are in Wyoming, over 300 clear days a year), so one panel will heat 8 gallons to shower temp.  Two panels should heat 16 gallons....does this make sense?


I am on grid--no batteries.


AdobeJoe

« Last Edit: July 16, 2007, 04:05:08 AM by (unknown) »

Nando

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Re: PV to Heat Water-electrical resistance
« Reply #1 on: July 16, 2007, 07:54:14 AM »
The message does not give all the details.


What is the heater resistance Ohms, ( both would be better to have)


You said 130 watts at what voltage, 17 or 34 Vmp , this to know the Amp at MP point.


If 12 volts ( 17 Vmp) then around 7.6 amps per panel.


If you are in a such solar insolation area, a much better project would be to make heating panels and the water moved by small pump if the placement precludes a gravity system.


The thermal harvesting would be superior by a factor of about 3 to 5, therefore heating 3 to 5 times more water volume and/ or increasing the temperature levels.


I have not tried to calculate the thermal rise of the water in either case.


Nando

« Last Edit: July 16, 2007, 07:54:14 AM by Nando »

adobejoe

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Re: PV to Heat Water-electrical resistance
« Reply #2 on: July 16, 2007, 09:51:47 PM »
Nando:


Resistance is 13 Ohms.  Panels will be in series for 24 V nominal.  I figure about 11 Amp current +/-.


Yeah, I know solar hot water is better and will look into that withing the next year or so.  Thing is I have all the materials, panels, copper wire, mounts for this PV to heat water thing and want to see what it will do. I am thinking 15+ gallons a day, which is plenty for two HOT showers.


I see these DC resistance heaters (screw into the tank)  advertised but wondering if I need them...what do they provide?


AdobeJoe

« Last Edit: July 16, 2007, 09:51:47 PM by adobejoe »

RP

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Re: PV to Heat Water-electrical resistance
« Reply #3 on: July 16, 2007, 10:50:26 PM »
24volts divided by 13 ohms is less than 2 amps.


Watts = volts squared / resistance which equals 44.3 watts.


What the low voltage elements provide is lower resistance which will allow you to use the power available from your solar panels.


Consider how dim a regular 120volt bulb is when connected to 24volts.  You have the same effect here with the standard heat elements.

« Last Edit: July 16, 2007, 10:50:26 PM by RP »

Nando

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Re: PV to Heat Water-electrical resistance
« Reply #4 on: July 17, 2007, 08:43:38 AM »
You are miss using the panels, you can not get the watts into the tank because the resistive element value limits the power to :


24 volts / 13 ohms = 1.85 amps  which represents 24 * 1.85 amps = 44.3 watts


To be able to feed 130 watts then the resistance of the element needs to have a value of:


24^2 / 130 watts = 4.43 ohms


You can solve the problem by getting 2 - 115 volts 1500 watts elements and placing them in parallel for the 4.43 Ohms value.


OR


Best solution; get a 115 volts 1500 watts element and place both panels in series for 48 volts, this way the power harvested will be :



  1. watt / 115 volts = 8,86 ohms
  2. ^2 / 8.86 ohms = 260 watts of two panels 130 watts each in series


NOW GET the 115 volts 1500 watts heater, those are hard to get.


Nando

« Last Edit: July 17, 2007, 08:43:38 AM by Nando »

Nando

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Re: PV to Heat Water-electrical resistance
« Reply #5 on: July 17, 2007, 09:19:16 AM »
Added correction:


Your panels are 24 volts units


I forgot that the Vmp = 34 volts, therefore 2 panels in series = 68 Volts


... 260 watt / 68 volts = 3.8 Amp ( MP).


Also 68^2 / 260 = 17.78 ohms


Then to get the proper element 115^2 / 18 ohms = 734 watts


You need a 750 to 800 watts element for 115 volts ( also 120 Volts)


This way the panels will run at MP with true 260 watts when the Vmp = 34 volts per panel.


Best efficiency in this case.


Nando


 

« Last Edit: July 17, 2007, 09:19:16 AM by Nando »

adobejoe

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Re: PV to Heat Water-electrical resistance
« Reply #6 on: July 17, 2007, 12:02:56 PM »
NANDO/RP:


Great comments. I should clarify, the panels are 12 V, but reach peak approx 17 V.

I understand by putting them in parrallel I should get approx 24 V, which is better to minimize line loss(I could probably go with 12 Gauge instead of 10 Gauge).  If I stuck with 12 V there would be more loss.


I do not what to replace both heater elements, because then I loose capacity to heat with AC. For example, we get three cloudy days, or we start having guests and increase hot water use. Brrrr.


So, I especially like RP's comment about why I need a low voltage element...dimming light bulb analogy...makes sense.


So my next step is to locate one of these elements, there are various places on line, in the neihgborhood of $50-$100, and install the system. I like that I should not need a controller, and other then a fused disconnect on the panels, and grounding, very little new gear. I have everything else except the DC element!


At some point, maybe this fall, I will look at installing some solar hot water with a heat exchanger for my radiant system...having the DC I can probably use a DC pump as well.


Thanks again,


AdobeJoe

« Last Edit: July 17, 2007, 12:02:56 PM by adobejoe »

iamdewayne

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Re: PV to Heat Water-electrical resistance
« Reply #7 on: October 31, 2007, 06:57:42 AM »
Your panels max power is 17.6 but the open voltage is 21.9. How do you regulate them down to 17.6 under different sunlite conditions?


Thanks, dewayne

« Last Edit: October 31, 2007, 06:57:42 AM by iamdewayne »