Voltage, Current, Magnetic Declination, Power Factor, Tilt Angles and Busbars

From HeatSpring.com NABCEP PV Technical Sales Exam Prep Study Group

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:

https://eosweb.larc.nasa.gov/cgi-bin/sse/grid.cgi

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:

https://en.wikipedia.org/wiki/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

From HeatSpring.com NABCEP PV Technical Sales Exam Prep Study Group