Author Topic: Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!  (Read 8804 times)

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Offline ghurd

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Re: Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!
« Reply #15 on: March 09, 2012, 10:45:15 pm »
The simplest things can mess up the calculations.

Or, in my case - the photographs :)

Or in my case - the pencil on the paper.
I can not write as fast as a camera!

I think a lot of 'published' calculations are faulty due to when and how the original numbers were obtained and recorded.
G-

Offline tomw

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Re: Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!
« Reply #16 on: March 10, 2012, 12:05:30 am »
I used to write test and calibration procedures and I can tell you "how" it is done matters. Everything from connections to equipment settings should be laid out to ensure accuracy and repeatability.

Long ago in another life when the details mattered. Nobody wants the autopilot or glide slope receiver to fail on an airliner  because a master oscillator is out a percent or two from design specification due to a shift in capacitance from a test probe.  An unusually picky case but illustrates the point.

Basically "how you do it" matters.

I digress.

Tom
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I thought that they were angels, but much to my surprise, We climbed aboard their starship and headed for the skies

Offline GW@PE

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Re: Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!
« Reply #17 on: March 10, 2012, 08:09:36 am »
Hi Readers,

back to the topic of testing PV.

I have dabbled with PV for many years.  Since early mono days using ARCO panels made in WA, Solarex poly cells and panels in the 1980's and even Green cells from the one and only.  Have not had the funds to go the GaAs, or hybrid cell route yet.

Solar panel performance data is only as good as the incident solar radiation measuring equipment used to compare panel output with. 

I made a solar radiation [pyranometer] sensor from a single silicon solar cell.  I was given some space grade cells [75mm round], but any silicon solar cell will do, with suitable current output.  I connected an Alegro Hall effect current sensor, directly across the cell, with the shortest leads.  This measures cell current, that happens to be directly proportional to incident solar radiation.  Temperature effects are minimal, as this effects the cell voltage, and in this instance this is close to "0V".  I used a current sensor with a 0-5A range, and this worked well with my solar cell that produced 3A at 1050W/m^2 solar radiation.  This was calibrated against my Davis weather station incident solar radiation sensor, and gives better than 1% correlation.

Visual human interpretation of how much solar is available is a guess at best.  Probably have more luck winning Lotto. 

It is surprising the effects of high cloud, [glare] and haze have on solar energy harvest, compared to cloud free conditions.

I have incident solar records going back years, logged at 10s intervals at my location.  I have test records for many comparison solar systems including Amorphous panels, Poly-Crystalline, Mono-Crystalline [Std] and Sanyo [HIP] and mono-Crystalline [Sunpower buried conductor] types.  I have found that most panels produce their rated power.  Deficiencies occur usually with MPPT equipment connected to them.  In certain weather conditions I have measured up to 20% difference in energy harvest from the same panels in the same orientation, but with different inverters. 

It is very difficult to measure objectively and conclusively say one particular system is better than another, or a system is not performing properly, based on spot measurements. 

I have spent a lot of time identifying the interactions of weather and MPPT equipment on solar panel energy yields.  I am no expert yet.

I have to conclude that even from an informed position, removing the confusion is difficult.

The one thing that has happened over the years is that solar panels are far cheaper now than even 6 years ago.  The low price makes the confusion dated.  When I first looked at home solar, I had to really work the numbers to justify a system.  The honeymoon period has passed with free systems, but systems are still about 1/10 the price I paid in 2006, for a 1.5kW installed.  I will not go off grid, but continue with a hybrid system.  I have made the grid my friend, and look forward to the checks they pay me.

At the end of the day, as long as the panel voltage is reasonably matched to the batteries [inverter], and the regulator can handle the maximum power, and the load demands are met, then you are most of the way there.  The OFF grid systems I have seen that have the most problems are related to insufficient, battery capacity and recharging sources, to meet the demands of the loading.  Typically battery loss of capacity, due to prolonged periods of low SOC.  It really takes a typical lead acid battery many more hours than are left in a solar day to fully replace the energy that is drawn out.  This is especially noticeable if the battery has been drawn down to the 50%SOC level.  The best system I have monitored has 1300Ah @ 48Vnom battery, with 7kW of solar RE, and a 2.5kW windmill, with an auto start Genset, supplying a house with 10-14kWh daily loading.  There are times during the year when the genset is called due to low battery SOC [<50%].   When there is plenty of solar, the battery is effectively charged by 10AM, and the bulk of the day is holding the battery at float voltage.  You really can't beat a surplus of RE, in reducing the worry of maintaining an OFF-grid system.   

Goood luck deConfusing.

Gordon.


Offline tomw

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Re: Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!
« Reply #18 on: March 10, 2012, 08:49:08 am »
Gordon;

Nicely put. Should help clear some confusion.

I had not even thought about the inability of the human eye to gage solar values. I "knew" it but neglected it as a testing factor.  :o I like your sensing & evaluation rig.

Thanks.

Tom
Do NOT mistake me for any kind of "expert".

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24 Trina 310 watt modules, SMA SunnyBoy 7.7 KW Grid Tie inverter.

I thought that they were angels, but much to my surprise, We climbed aboard their starship and headed for the skies

Offline GW@PE

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Re: Testing PV. I(sc) I(mp), V(oc), V(mp) !! It's all so confusing!
« Reply #19 on: March 10, 2012, 04:25:54 pm »
Hi Tom,

Attached here is some test graphs of solar energy yesterday.  The power is normalized to give kW per kW of incident solar, for each system.  This is a feature I have added to my CC128 Professor program. 

Not sure of how to show an image in a post.

734-0

I think this works.

The white trace is the plot of the pyranometer.  This is orientated horizontally.  The other traces are as per the colour codes on the LHS check boxes.

The green trace is from a west orientated system, 5.24kWh/kW.  The red is of an east/south orientated system, 5.36.  The blue and orange is north orientated, 6.0kWh/kW.

The fattening of the output graphs, compared to the incident solar, is due to the fixed panel angles relative to the sun during the day.

The spikey component near the middle of the graphs is an effect of cloud.

This will add to the confusion.

Gordon.