PV Student

Q:

This chapter is pretty interesting. Brings me back to 8th grade, with all of that Algebra. It seems like during the mid 1800's there was a lot of advancement and development in electricity with all of those pioneers around. I wonder what the next 100 years will look like and who today's pioneers will end up being. We know so much more about electricity, and I predict the grid arrangement is going to change due to all the solar energy taking place.

A:

One of my hippies is thinking every day about how fast technology moves in the modern era, especially compared with older history.

1000 and 2000 years ago were relatively the same compared to 100 years ago an now. 1903 someone figured out how to fly a plane that looked like a kite made by bicycle mechanics. 65 years later Neil was walking on the moon.

With electricity and technology in the future, it is going to be something like we can't imagine. If I had to guess, we will have lots of interconnected devices that have a lot of intelligence. Some of us will be trying to merge with computers physically, energy will be in high demand, but we can easily meet that demand with our technology. The unknown, policy, politics and who is in control.

Thanks,
Sean

Q: 

I enjoyed learning more about the different applications of PV, such as the bi-facial modules in this lecture/ reading. It made me curious about the future of PV technology, especially in the area of Building Integrated PV. Any insights or predictions on what we may see in the near future?

A:

Some people are doing BIPV and Tesla is talking about it. It will be very hard to compete with mass produced generic rectangle modules in price.

In 2019, there will be NEC changes for module level rapid shutdown that may lead to BIPV in some cases, if there are no metal exposed parts on the BIPV.

People have been installing BIPV for decades, the problem is the price is not competitive.

Thanks,
Sean

Q:

I am trying to fully understand what is happening as it pertains to bypass diodes. When a single cell is shaded, the diode acts by decreasing the module voltage in exchange for preserving current. 
As an example, if the module current was 9.18A module Voc was 45.1 and one cell was shaded, then in theory the amperage would remain 9.18 and the approximate module voltage would decrease proportionally to 30.1 (one third/diode)? Or is the truth somewhere in the middle? Would the effect be the same if two cells were shaded in the same series wiring/diode? Three?

A:

Your example is correct if you were using Vmp and Imp in the example. With a typical 60 cell module, we have 3 groups of 20 cells. If 1 to 20 cells in that group gets shaded enough to kick in the bypass diode, then you will lose 1/3 of the voltage of that module.

The problem with using Voc, is that Voc is when the module is turned off (open circuited) and there is no current in that condition, so there will be no reason for the bypass to bypass current.

If you take a PV module into the sun and measure the open circuit voltage, then shade a cell, usually that shaded cell will have some degree of light in the shade and you will get close to full voltage. No bypass diodes kick in, because there is no current in the open circuit condition. A PV module or solar cell in the shade will have close to the same Voc as the module will in full hot sunlight.

Then if you short circuit the module and shade a cell, you will get full current, because the bypass diode will kick in. In the short circuited condition, you will have no voltage, because positive is connected to negative.

An inverter will maximum power point track the IV curve and will work at the voltage that is most efficient for making power. When there is a shadow on a cell that is slight, the inverter will decide to work at that voltage that makes the most power and the bypass diode will not kick in. If the shadow on the affected cell is progressively larger, then at some point the inverter will work at a lower voltage, which will cause the bypass diode to kick in. This all works well with a single string on a single MPP, however with multiple strings on a single MPPT, the inverter should be more efficient at making power working at the voltage of the multiple modules that are unshaded, leaving the shaded module voltage at the mercy of the unshaded strings. If we have a 1000V inverter and a string of 24 modules in series, then the voltage difference will be 24 modules x 3 diode segments per module = 72 segments, so we would only be talking about 1/72 of the voltage being missing, which is not a big deal for being on the peak of the IV curve. However if we have a charge controller with multiple strings that can only go up to a maximum system voltage of 150V, then we often have 3 modules in series. 3 modules x 3 bypass diode segments = 9, so that shaded cell would be taking out 1/9th of the voltage, which will have a much greater effect on the maximum power point for that string with the shaded cell, than in the 1000V inverter example.

One of the gray areas is the quality of the shadow and when the diode will kick in and bypass current. Shadows can be from near objects, far away objects, on hazy days with a lot of diffuse light, days with reflected light, etc. This is one of those aspects of PV that is difficult to model and the best PV software has trouble modeling bypass diodes.

The best idea.... No shade!

Thanks,

Sean

Q:

Sean: Thanks for a great course. Two questions: I think I would like some additional training for the PV Installation Professional Certification Exam. Do you have any recommendations?

Also with the addition practice questions, I got really knotted up on questions 74 and 78. On question 78, when you state the temperature coefficient for the module, is this actually Voc x Temperature Coefficient Voc?

Again, thank you for two great classes. I look forward to meeting you in person someday, and perhaps attending hands-on training you might offer.

My best to you

A:

For 74 and 78 we have to work backwards.

We are asked to determine the cold temperature and we are given the number of modules in series, temperature coefficient Voc and the inverter maximum voltage.

We first divide the inverter voltage by the number of modules to get the cold temperature voltage.

78 has a temperature coefficient that is in mV per C, so it is a little confusing, but simple once you see it.

145mV = 0.145V

We then find out the difference between the Voc at STC and the Voc at cold temperature.

Then we will divide that difference in voltage by 0.145V to get our delta T and then figure out how far our delta T is from 25C.

1000V/20 modules = 50V per module

50V cold Voc - 44V STC Voc = 6V is the difference in voltage

6V/0.145V = 41.4 Delta T

41.4 less than 25 is -16.4C

Isn't that cool!

Sounds like you are about done with the class.

This is the best class for preparing for the NABCEP PVIP exam and I think the best thing to do is go over this material a bunch.

I also teach workshops at Intersolar US in SF in July with Bill Brooks and at Solar Power International in September (Vegas) with Bill Brooks right before the NABCEP Exam. That would be a good way for you to get a chance and heckle me from the back row!

Also, there are other classes here at HeatSpring by other instructors, such as Ryan Mayfield https://www.heatspring.com/courses/megawatt-design?aff_id=t3nlgw

I have a NABCEP PVIP Prep course here too that covers a lot of the same material as this course.

Thanks!!!

Sean White

Thanks for a really good prep class, Sean and Heatspring! I also got my certificate on the first try.

A:

That is excellent that you passed the NABCEP PVIP!! Glad to help!

Thanks for the feedback,

Sean White

Dr. White, I received my certificate today! Thanks so much for the great course, I will definitely recommend it to others who are prepping for the exam.

A:

That's great! Congratulations!!

Hello Sean,

I passed the exam. I got the certification yesterday. I am so happy I passed it first time trying. Your class was a great help.

Thank you so much!

A:

Congratulations!!

Submitted: 05/21/2016

Training: 40-Hour Advanced Solar PV Installer Training / Online / Anytime

How would you rate this course?
10/10

What did you like about the course?:
That it continuosly answers questions that I've had or I get in the workplace. I like that Sean uses a lot of examples and does not limit to the short explanation of an answer.

How effective was the instructor's communication during the course?:
Great.

What would you change about the course?:
I can't think of anything.

Thanks for the great review!!

Sean

Q:

I took the April exam for the first time and just found out I PASSED. After the completion of this course I felt really prepared and confident going into it . This course definitely covered enough information to pass the NABCEP exam and much more that I will be able to apply in my future career in solar. Sean White's book is the "Swiss army knife "of study material. Somewhat small but jammed packed with all sorts of useful information. Most definitely will recommend this course. Thanks Sean!

A:

Congratulations on being NABCEP PVIP Certified!!!

I like the swiss army analogy. When I was first in talks with the publisher, I asked them to make a book that was not too big to read in a week and something that is not too heavy to carry around. I also didn't want it to be expensive, so it would reach more people. The plan was to make the book have the information required to pass the exam. Glad it worked!!

Thanks,

Sean White

Q:

Dr.,

I believe it's difficult in Oman to get the most output from PV. This is because the temperature is very high especially in summer where the temperature reaches more than 50 degrees.

My question: Is there any means or technologies that can be used to cool down the cells even in summer which will help to increase the maximum voltage??

Regards

A:

The most cost effective way to deal with the heat is to mount with air spaces between the PV and the structure if it is on a building or if it is a ground mount, there are already air spaces. The other way to deal with this is plan on adding a little more PV to make up for the loss in production.

There are many PV systems in hot places and usually hot places are the best places for PV, since the reason there is hear is because of a lot of solar energy.

In many places where it is not so hot, the high summer temperatures are around 38C, which is 12C cooler than 50C. With a typical temperature coefficient of power of -0.45%/C the decrease in power due to heat will be:

0.45%/C x 12C = 5.4% decrease in power

The benefit that you have is the solar energy hitting the earth can be 20% greater than in the place where 38C is the high design temperature, so the penalty for the heat is worth the benefits of the extra solar energy.

I have heard of people contemplating fans, but it would be less expensive to just buy 5% more PV.

Thanks,

Sean White