Solar panel electrical calculations

[vyv_cox]

I am calculating solar panel sizing from theory, to predict sizing for various areas. Having found my daily amp.hour consumption, I next found the kWhr/m2 insolation figures for the areas I am interested in. My question is: What voltage do I take to divide the one into the other? Is it panel open output, somewhere around 20 volts or more, or controlled voltage, call it 14 volts?

Thanks for any assistance.

 

[sarabande]

I suggest it can only be useable voltage, so doesn't it depend on whether you are taking the output via a regulator ? In which case, e.g. my Steca regulator gives a range of 13.9 - 14.7 depending on battery type. Used without the regulator, the Kyocera panels go up to 18.5 v (southern UK June) with optimum orientation

Lots of maps available for theoretical solar radiation falling on Earth, e.g.
http://www.wirefreedirect.com/solar_panel_sizing.asp
so why not use this plus the efficiency figure for the type of panel under consideration to generate the kWhr/m2.

Or is this original research ?

 

[noelex]

I am calculating solar panel sizing from theory, to predict sizing for various areas. Having found my daily amp.hour consumption, I next found the kWhr/m2 insolation figures for the areas I am interested in. My question is: What voltage do I take to divide the one into the other? Is it panel open output, somewhere around 20 volts or more, or controlled voltage, call it 14 volts?

Thanks for any assistance.

If you use the Ipm (current at load) and the insolation figures you should get a good idea of the AHrs that will be produced.
with a MPPT regulator you could in theory use the Ipm figure and multiply this by the Vpm (voltage at load) (note this is not open circirit voltage) and divide this by the average battery voltage, but there are losses in voltage conversion and the losses in the battery (10AHrs in does not mean you can draw 10 AHrs out and end up even)

For example a Kyocera KD95 panel has
Ipm=5.31A
Vpm =17.9V
@ 1000W/m2 and 25 C

Given the solar cell temp is very unlikely to be 25C and there are losses in the battery, even with a MPPT regulator you are unlikely to get much above 5.3 AHrs out of a battery that has been fed from the 95W solar panel @ 1000W/m2 for 1 hour. (5.3A out @ say 12.6V=67W less than the 95W "promised", but battery and regulator conversion is not 100% and the solar cell temperature will be well above 25C. Higher irradience figures than 100W/m2 will however result in a higher output)
Shadows, which are unavoidable on a boat will make this worse.

 

[alan]

Solar Panel Calc

I think you should use the maximum power current "Impp". For instance Kyocera quote this for their 65W panel at the Standard Test conditions (1000W/m^2 at 25C): 3.75A and also for the 800W/m^2 condition:3.01A.
So, I would say, use this value along with the number of hours you have the 800W/m^2 condition in your area to calculate how many theoretical ampere/hours you can have. You say you have calcualted your daily consumption in ampere/hours, so I would stick with current and ampere/hours as being the easiest parameter to deal with.


Alan.

 

[vyv_cox]

Thanks to all. I have found plenty of insolation data, this one being particularly useful to me. As you see it gives the solar power generated in kWh/day, as do most of the others I found. As this expression matches what I am trying to do I would like to stick with this.

My consumption is easily reckoned up in Amphours/day. In order to achieve a panel requirement in square metres I need a voltage, which I assumed to be the battery/controller voltage but I wasn't certain. Thanks for the advice: it would seem that if I go for a nomonal 13.0 volts, with a 20% fudge factor as recommended in most of the sizing advice I have found to date, I won't be far out.

 

[noelex]

Thanks to all. I have found plenty of insolation data, this one being particularly useful to me. As you see it gives the solar power generated in kWh/day, as do most of the others I found. As this expression matches what I am trying to do I would like to stick with this.

My consumption is easily reckoned up in Amphours/day. In order to achieve a panel requirement in square metres I need a voltage, which I assumed to be the battery/controller voltage but I wasn't certain. Thanks for the advice: it would seem that if I go for a nomonal 13.0 volts, with a 20% fudge factor as recommended in most of the sizing advice I have found to date, I won't be far out.

How did you convert the suns Irradience to solar panel output?

 

[vyv_cox]

How did you convert the suns Irradience to solar panel output?

Yes, that's the problem! I was trying to do that, rather than use a manufacturer's information and work backwards. I've given up for the present, going with the Kyocera data instead. Thanks for the input.

 

[noelex]

As a starting guide for the med in summer if you divide the manufactures wattage for the panel by about 2.5, that gives you the number of AHrs the panel will produce.
So a 100w panel would produce about 100/2.5=40AHrs a day
This is only a starting point location, shading, angling the panel. MPPT regularor etc can all make significant differences.
Its always better to fit as many watts as you have the room for.

 

[vyv_cox]

Thanks. I'm not researching panels for my boat, I have had them for 15 years now. I'm looking for basic sizing information for an article I'm writing. My intention was not to look at panels and see if they might be big enough for the job, but to look at consumption and match panel areas to it in some fundamental way. But I'm grateful for your info and will incorporate it. (Although I have already written twice the number of words they wanted, so half of it may finish up spiked!)

 

[William_H]

Solar power

Hi Vyv you need to emphasise the variability of actual conditions to theoretical conditions.
As said the MPPT contoller can use more power out of the panel. Interesting that the solar power inverters that are common around here for producing 240VAC also use the MPPT concept.
If you use a simple regulator you are stuck with current that depends on current taken by the battery. ie depends on charge state. (as is different to the capabilities of the panel)
If you feed battery direct with no regulator then current will be pretty much as specified for the panel if sun is strong and panel is correctly angled. However of course feeding current into a fully charged battery does not mean it will later be available for discharge.
The watts figure for a solar panel is just a selling point and not really so useful for your calcs. I guess you know all this anyway. I might have found a point you havn't thought of however. good luck olewill