PV Student

Q:

Ch 3 Question: Will we be asked to calculate voltage drop on the exam?

 

Ch 4 Question: Have you used optimizer/chips instead of bypass diodes? Jinko modules use them and they appear to be much better than bypass diodes in terms of losses.

 

A:

 

To answer your question 3 about voltage drop first:

 

I do not think you will be asked to calculate voltage drop on the NABCEP PV Technical Sales Exam, but you should understand what voltage drop is. I was surprised when I took the NABCEP PV Installation Professional Exam, that there was not one question that includes a voltage drop calculation. If there is not a voltage drop calculation on that exam, it is even more unlikely that there would be a voltage drop calculation on the NABCEP PV Technical Sales Exam. I think this may be because voltage drop is not a safety issue for PV systems, it is just an efficiency issue. The problem with a test question on voltage drop would be, what is the operating voltage and current that we would use for the calculation, since the voltage and current is constantly changing all day long as the cold wind blows, the clouds pass and the sun beats down. There are no solid numbers that we can use for the calculations. To keep it simple, I use Vmp and Imp for voltage drop calculations, but others use different numbers based on location, temperature, airflow behind the array and the quality of the atmosphere in a location.

 

What I think you should know about voltage drop, is that it exists. Especially when you have to transmit power over longer distances or with lower voltages. You should know that it is a better idea to have your inverter by the house if you have a ground mount array that works at 420Vdc and the house is 240Vac. The higher the voltage, the more efficient it is over a long distance. Also, you might not have to know this for the exam, but 3-phase power has a benefit over single-phase power by a factor of 0.866 (that is the square root of 3 divided by 2).

Also, since this is a sales exam, you should know that the longer the distance or the lower the voltage, the more money you will end up spending on wire. If the PV is cheap, then you might want to get an extra module to make up for the wire loss. You can also weigh the extra module cost against the wire cost.

I always like to point out the tricky thing about voltage drop, which is what the industry will refer to as “voltage rise”. Voltage rise is in reality still voltage drop, but it looks like voltage rise when the ac side of the inverter will have the voltage rise as the current increases. Since Vdrop = IR (ohms law) and when current rises on the ac side of an interactive inverter, then the voltage at the inverter must also rise to make up for the lost voltage on the inverter output circuit. If the voltage at the inverter is not higher than the voltage at the interconnection, then there would be no way for the inverter to push power to the interconnection. This is also true with large power plants on the grid. The nuclear power plant down the block will make more voltage at the source to push the power to where it is going. (This doesn’t account for transformers, which change voltage). If you have an interactive inverter and the wire between the inverter and the interconnection is too small, then the voltage gets higher at the inverter and if the voltage gets to be 10% higher than grid voltage, then the inverter will go offline. That is good to know.

 

 

Jinko Optimizer Chip Question 4:

 

I have not used the Jinko optimizer chips. It sounds good and I will wait until it has been around for a while and has withstood the test of time. Another thing I like to do is wait and see if the large companies use technology in mass. If the banks and the big companies will use it, then it has probably been researched a bit. Have you used the Jinko optimizer chip? I remember at a PV show in Shanghai a few years ago ET solar had something like that and I have not seen it take off.

One of the best arguments that string inverter people use when they are defending themselves against module level MPPT (microinverters and power optimizers) is “Why put the module in the shade in the first place?” That being said, I got a look at the 2017 NEC and I think when the 2017 NEC is in effect in full force, that rooftops will all have module level control (optimizers and microinverters). The 2017 NEC says that by 1/1/2019 we will have to be able to bring the voltage down to 80V inside the array (pretty much 1 module) or the array needs to be listed as rapid shutdown or there are no exposed conductors. I am guessing that most people will just default to module level electronics.

 

Thanks,

Sean White

Q:

Compounding Interest: I thought this was a simple formula(s) to memorize. Just remember to subtract from 1 for module degradation and inflation. Add to one for interest rates.

A:

I think a lot of textbooks make things as difficult as possible to understand, because the author is trying to impress the other instructors.

Glad you think it is simple to understand.

Einstein said:

"Everything Should Be Made as Simple as Possible, But Not Simpler"

Sean White

Q:

I have just finished Ch:1-6 of you book. It was very clear, informative, and funny. Here are my questions/comments from Ch 1 on the facts & Figures. FYI, I am a visual learner.

1. Do you know of a video/resource that reviews IV curves (Isc, Imp, Voc, Vmp)?

2. Do you know of a video/resource that explains optimal tilt angle . Any practice questions would also be helpful

3. Can you expand on the power factor? Why is it significant? I assumed when current increased, voltage decreased, and vice versa

4. Do you have additional magnetic declination questions?

5. When looking at residential and commercial SWGR, where can I find the busbar rating?

A:

Thanks for liking the book!

Let me answer the questions here:

 

1. Do you know of a video/resource that reviews IV curves (Isc, Imp, Voc, Vmp)?

 

I am uploading a part of my SOLAR PV INSTALLER BOOT CAMP TRAINING + NABCEP ENTRY LEVEL EXAM PREP course, so you can see some video on IV curves. Everything in that course is fair game for the NABCEP PV Technical Sales Exam.

 

Just uploaded now and you can watch in the assignments of week 2 of this course.

 

2. Do you know of a video/resource that explains optimal tilt angle? Any practice questions would also be helpful

 

There is no exact science on the optimal tilt angle. Historically, people were using what they called "rules of thumb" and were taking latitude tilt as optimal for annual production, latitude plus 15 degrees for winter production and latitude minus 15 for summer production. More recently for annual production, people have been using 30 degrees tilt throughout much of the latitudes in the US. It is more accurate to use software, such as www.pvwatts.nrel.gov

 

There are many practice questions in the book and here is another:

 

What would be the best tilt angle of the following for winter production at a latitude of 30 degrees?

a. 40 degrees

b. 30 degrees

c. 45 degrees

d. 15 degrees

 

The correct answer would be 45 degrees, since the sun will be lower and we will have to tilt up the PV higher to catch the winter sun.

 

What would be the best tilt for summer in the question above?

 

15 degrees is 30 - 15

 

This NASA tool also gives you an optimal tilt angle:

 

3. Can you expand on the power factor? Why is it significant? I assumed when current increased, voltage decreased, and vice versa

 

Power factor is something that can take years to fully understand and I would not worry too much about it for the NABCEP PV Technical Sales Exam. What you should know is that when current and voltage are in phase, then power factor is perfect, which is a PF of 1. When they are out of phase, then the power factor is less than one. When you have different kinds of loads, it will cause current and voltage to go out of phase. Motors are especially well known for causing power factor to decrease. We can fix power factor with the electronics commonly found in inverters if the inverter manufacturers want to spend more money making this happen. Many factories and utilities correct for power factor. Large utility scale PV plants are usually going to be required to do power factor corrections. In some cases reactive power is required to be made by inverters. If you see VA or kVA that is like power, but accounting for the current and voltage being out of phase. Some of the terms that are used are reactive power and apparent power.

 

Here is a good source for further information on the basics of power factor:

 

 

If you can consider Wikipedia a good source (don’t tell anyone).

 

The thing about current increasing when voltage decreases would have to do with transformers, dc to dc converters or other forms of converting power. If you are making power with PV, you can increase current and voltage by adding extra modules. When you are just converting power from one form to another, then you are swapping more current for less voltage or more voltage for less current.

 

4. Do you have additional magnetic declination questions?

 

If you are in San Francisco where the magnetic declination is 14 degrees east, what would be the true azimuth if your compass read 180 degrees?

a.     180 degrees

b.     194 degrees

c.      166 degrees

d.     346 degrees

 

The right answer is….

 

Scroll down some more

 

 

194 degrees. When you are on the west coast your declination is positive and you add to magnetic.

 

Other one:

 

If you are in New York where the magnetic declination is 14 degrees west, what would be the true azimuth if your compass read 180 degrees?

a.     180 degrees

b.     194 degrees

c.      166 degrees

d.     346 degrees

 

 

The right answer is….

 

 

Scroll down some more

 

 

166 degrees. When you are on the east coast your declination is negative and you subtract from magnetic.

 

 

 

 

5. When looking at residential and commercial SWGR, where can I find the busbar rating?

 

Usually you look for a label. Residential is going to be on the label of the service panel (aka panelboard). Sometimes the label falls off and you assume that since the busbar cannot be less than the main breaker that the busbar is the size of the mainbreaker. For example, if I see a main breaker that is 100A, I know that the busbar is at least 100A, so I go with 100A for the busbar unless I can prove otherwise.

 

I have a friend who goes door to door looking for other main service panels where the label did not fall off if he runs into this problem.

 

For commercial switchgear, you have to look for evidence and labels. You can also use engineering supervision for systems over 100kW and let the professional engineer make the call.

 

Hope this helps and you can also ask your questions one at a time when you have them if you like.

 

Thanks,

 

Sean White