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Advancement and Development of Electricity and Technology
Building Integrated PV
Bypass diodes
Determining low temperature that caused the inverter to go over voltage
Another NABCEP PVIP Certificate in the mail!

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Feeder PV Connections 705.12(D)(2)(1)(b)
Crimping MC4 Connectors without the tool
PV Temperature coeficcients in V/C, mV/C and %/C with conversion examples
NEC 310.15(A)(2) Exception, 10% or 10 feet or less for sizing conductors
Converting kWh to pounds of CO2

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PV Boot Camp and NABCEP Entry Level Exam Prep Course

When to clean PV

From HeatSpring.com PV Boot Camp Discussion Board

Q:

To keep modules operating at their peak efficiency, is there a recommended cleaning solution or is just plain old water the best thing to use? In SoCal I've seen many modules that are so dirty they could probably grow plants on them.

A:

I like using a window analogy. Many people never wash their windows but still get the sunshine.

If you have plants growing out of your PV, that would be shading and time to do something.

I would say that most residential PV systems are primarily cleaned by the rain.

Solar in Germany, the FIT, Hermann Scheer and his Solar Revolution


Q:

I'm currently traveling in Germany and am amazed at how much solar is in use here. Many of the installs look like they have been in use for well over 20 years. Lots of module mismatch in physical size. I have to wonder how well they perform. Some serious slope installs as well.

A:

If it wasn't For Hermann Scheer and the German solar feed-in-tariff, solar in the world would not be as mature as it is today.

http://www.hermannscheer.de/en/

Hermann was a German politician who promoted the policy of requiring the utility to pay 20 year contracts for a price that was a decent return of investment for the solar industry.

Monocrystalline vs. Polycrystalline Solar, which is better?


Q:

For the average consumer is there any advantage to installing a Mono versus a Poly module system?

A:

I would prefer the most efficient module and hopefully one that I could trust. That being said, usually mono is most efficient, but if you were to look at a 260W polycrystalline module made in 2017 vs. a 250W monocrystalline module made way back in early 2016, and the brand was the same, I would go with the poly. I think there are some out there who would disagree with me and 250 vs.

STC Voltage Higher than PTC, CEC and NOCT because ambient vs. cell temperature


Q:

Hi Sean,
I have just taken the test and question 25 seems to be off. Perhaps I require further clarification. Anyways, the question is:
Which test conditions have higher voltage?
a. STC, b. PTC, c. CEC, d. NOCT
According the the textbook STC (option 'a') is the correct answer and it explains that the reason is because the other test conditions have higher cell temperatures and therefore lower voltage.
Now, STC (option 'a') temperature is 25C, whilst the other are 20C.

Derating factors, accuracy and precision


Q:

Hi Sean,
This question follows a previous one on comparing the output in a 'irradiation rich' and 'rather hot' location (South Western US desert) versus an irradiation 'not as rich' but 'rather cold' location (South Western prairies in Canada).
I noted that when calculating the energy output of a system, irradiation and its derating (by a number of factors) is accounted. However, I can't see that temperature is taken into consideration for the calculus (it is not seen as any of the derating factors).

When does the PV bypass diode kick in with a partially shaded solar cell?


Q:

Can you go into more detail on what happens when a solar cell is partially shaded? At what point does the bypass diode trigger?

A:

That is a good tough question and there are many variables.

What it comes down to in an interactive (grid-tied) system, the inverter will MPPT the PV. If the array will be more efficient using the group of PV cells that include the partially shaded cell, then that bypass diode will not kick in, because the inverter will be working at the voltage that includes those cells.

PV Permitting and Finding Out What the AHJ Wants


Q:

Thanks for this answer. I have a follow-up about AHJ's in Massachusetts. It seems to be the most regulated of all the states. How do you check which AHJ is the right one and make sure of compliance in Massachusetts?

A:

The only real way to find out what the AHJ wants is to contact them.

1-Go down there
2-Give them a call
3-Check their website
4-Check www.solarpermit.org

Large solar installers have an advantage, because they keep a database and know which AHJs want what.

Solar PV Snow Removal... Is it worth the investment?

From HeatSpring.com PV Boot Camp Discussion Board

Q:

Would it ever be worth it to remove snow from a large solar field in New England? Or would the price of the snow removal be more than the energy lost?

A:

Probably not in California with the new $15/hr minimum wage. Even with $5/hr labor it would be difficult.
There are 100 names for snow in some cultures and I am sure there is an instance somewhere in the world where cleaning a module before the snow froze and stuck for a month would make good sense, but with those 99 other types of snow I think it is going to be cost effective to wait for it to slip off.

Easiest way to pass NABCEP PV Installation Professional Exam


Q:

Sean,
I have completed my OSHA 10 course and I am contemplating taking the OSAH 30 as it is recommended. What would you do if you were setting for the NABCEP PV Installer exam?

A:

I recommend taking as many courses as you can, including every OSHA class offered. If your only goal is to pass the NABCEP PVIP exam, and you have limited time, lots of studying, the 10-hour OSHA requirement, common sense and the advanced class here atHeatSpring.comwould get you through it with the least amount of effort, which is a heck of a lot of effort!

How often to equalize batteries (flooded lead-acid)


Q:

How often should batteries be equalized? I installed a few systems on a missionary compound in Haiti last year and we can initiate/schedule equalization of the batteries but have not done it just yet. All three systems are identical but the houses have different usage profiles, some loads have been modified since we left. They use big flooded lead-acid forklift batteries that the customer purchased. The customer has some one top off all of the batteries every month with distilled water.

Tesla Powerwalls in the cold


Q:

Are power wall battery systems affected by cold in northeastern areas or are they insulated in some way, since they will be on the outside of the structure and exposed to cold?

A:

Since batteries store energy chemically and chemical process slow down when it gets cold, there will be some loss in capacity.

Here is what Elon Musk says:

"Musk said that the batteries will have thermal management systems to allow them to power houses in hot and cold climates, too—the batteries have an operating temperature range of -20C (-4F) to 43C (110F).

Calgary Solar Energy


A:

Only after going over this chapter, I realized how significant is the temperature gradient / variation in the power / energy generation in PV.
In the last few weeks there has been a lot of talking in the province of Alberta - Canada about an upcoming 'solar boom' given the new provincial government commitment for phasing-out generation of electricity from coal (which accounts for about 55% of the electricity generation in Alberta) by means of replacing it with Solar and Wind.

A BIPV future?


Q:

It seems that even-though BIPV is more expensive than conventional silicon modules, in addition of presenting some disadvantages (lack of airflow being one) will likely become the preferred option sometime in the future. Would you agree on this assumption? 
I believe that alongside with energy storage, there is a lot of room for improvement on both, which is just a matter of time.

A:

My thoughts are that someday our grandchildren are going to say,

Estimating production for different tilt angles with PV Watts


Q:

For Solar Energy Fundamentals, do you have any good resources on energy optimization for different tilt angles and pitch? Often times I find myself constrained to a certain area or thinking ahead to a potential second phase for a client. Any good materials related to this topic would be appreciated.

A:

The simplest tool is PV Watts http://pvwatts.nrel.gov/
In the past when I have had consulting jobs for students setting up residential installation companies, I would make a table with the common roof tilts and annual production percentages as compared to optimal tilt and azimuth.

Battery inverter connected to battery vs. charge controller


Q:

In an off-grid system, do the batteries connect directly to the inverter? or do they need to pass by the charge controller before connecting to the inverter? (as suggested in the figure shown in the Chapter 1). Thanks!

A:

It can be either way. If the connection is made through the charge controller, which is usually the case, it is really just a pathway for the battery to be connected to the inverter on a typical system. Even when connecting to the battery through the charge controller, the conductors are sized based on the battery voltage and the inverter power.

PV system installed prices as low as $1.50 to $1.00 per watt


Q:

Do you know the price break down of how utility projects are getting installed for $1.50/watt?

A:

It can be less than $1.50/Watt outside of the US with PV available for less than $0.50/Watt. I know people installing complete residential grid-tied systems in the Philippines for $1/W.

First Solar modules are often used in utility scale projects in the US and since they build their own projects and First Solar modules are not sold to the public, it might be that First Solar modules are less than $0.

MPPT is just for things connected to PV


Q:

Are MPP Tracking systems are features built-in in the Grid-Tied inverters? In Off-Grid systems are these features (MPPT's) in the charge controllers or in the inverters?

A:

MPPT is only built into equipment that is connected to PV.

All interactive (grid-tied) inverters have MPPT.
Many charge controllers have MPPT
All power optimizers have MPPT

What MPPT or MPP (same thing) does is control the parameters (voltage and current on the IV curve) so that the PV is working at the optimal voltage for maximum power production.

Grounded Conductors, White, Red and Black Wires


Q:

When it comes to color coding wires. Are white (or gray) wires / conductors grounded and neutral, or negative? This confuses me as I tend to relate white to negative, however in the text white appears to be neutral and grounded. If that is the case (white wires being neutral and grounded), what would be the color code for negative?
I am totally new to electrical design... sorry if the question sounds redundant? Thanks!

A:

That is a very good question.

North Facing Modules


Q:

I do not have any particular question on your second lecture. Super clear, and thank you.
One technology question: I heard a few years ago of researchers trying to create modules designed for the north side of a roof. I do not know if there is any developments in this area, but it seems like it could be an important development.
Frankly, using PV Watts (great NREL product showing that government can work), I discovered that my 15 degree north facing roof could produce energy cheaper than what I can buy it from in CT.

The Difference Between Voltage Drop and Voltage with Ohms Law and Voltage Drop Percentage Calculations


Q:

Hi Sean: I guess I am getting a little knotted up by this too. If we have a 100V system, I take it that this would be considered the "nominal" voltage of the system. However, if we have a 2A, 3Ohm wire in the system, wouldn't the voltage of the wire be calculated in determining the nominal voltage of the system?
Intuitively I understand why V drops (I think): As resistance increases, volts must drop. But, I am not sure how V in Ohms Law relates to V-system.

Lead-Acid Battery Ventillation and Debris under the PV


Q:

Hi Sean:
A couple of questions: If lead-acid batteries need to be adequately ventilated, how should this be done in super airtight structures? Can the batteries be placed outside the envelop? Is there an optimal temperature they should be stored at?
Also, what is the best way to keep debris from collecting under the modules. This seems like it could be a big problem.
Thanks

A:

In the NEC, it just says that battery banks need "sufficient ventilation" and does not give instructions.

Ac-coupled Systems


Q:

Sean:
This is a great class and thank you! Your lecture suggested that ac coupled systems are becoming more commonplace. It seems this requires finding the right balance of energy storage to feeding the grid over the life of the array and other components, noting that batteries (maybe hydrogen?) will become more cost effective over the next 20 years. How should one plan for sizing battery systems and infrastructure needs in a market that is changing so quickly?

Educational Enthusiasm


Q:

Happy to be able to help, your educational enthusiasm is contagious. By the way I once heard you mention GRID Alternatives, I was a construction manager for them for them since 2007 and recently retired from roof top applications.

A:

Educational enthusiasm! Thanks for the accusation!
Grid Alternatives has a lot of EE going on. I taught a class for them at the Solar Living Institute last year and that was the most enthusiastic class I ever had. Great group!

Dc to ac ratio (PV to inverter ratio)


Q:

I am having a hard time figuring out the math for DC to AC size ratio. Maybe your covering it further on in the course or not. I used a online calculator and came up with a number, that seems right but who knows. Hoping someone can help me out. Lets just say I am pairing 285 Watt Modules with Enphase M250 Invertors. Typically we see the inverters topping out at about 265 watts. How would I figure out the DC to AC Ratio for this combination?

A:

There are many factors in determining the dc to ac ratio.

690.31(G) Dc PV Source and Output Circuits on or in a building

From HeatSpring.com PV Boot Camp Discussion Board

Q:

thanks for the great info and pictures, and just to clarify all our dc into building are in metal conduit complying with NEC.

A:

Yes!

690.31(G) Dc PV Source and Output Circuits on or in a building tells us that when we have dc PV source and output circuits touching a building that they shall be inside metal (typically EMT) up until the first readily accessible dc disconnect.

Thanks,
Sean White

Fast changing state of PV technology


Q:

This was a very interesting section for me, especially since I am unfamiliar with a lot of the DC systems being almost strictly an enphase installer.

A:

Enphase microinverters certainly make solar simple. It is amazing how fast the technology and Code changes. What I was teaching 5 years ago is much different than what I am teaching now. We did have Enphase since 2008, but now we are having Rapid Shutdown requirements, grounded inverters are now rare, multiple MPPT inverters are normal, power optimizers are very common and 3-phase 480Vac/1000Vdc systems with string inverters are the commercial PV system of choice.

Pole Mounts, more energy and other benefits


Q:
 
In Liberia the subject on temperature and radiance effects on voltage and current is very interesting and confirm my believe that pole mount is better and lots of breeze. (especially not knowing how old is the roof and the construction) 
Another thing I observed are the wire size and quality. Always buy AWG made in the USA and not elsewhere. It's a big difference!
 
A:
 
A pole mount will make more energy because of the increased voltage due to lower temperatures than a roof mount with less airflow, that is for sure!

Stringing batteries for PV Systems in Africa


Q:

When I size batteries for systems in Liberia, I always double the quantity of the batteries for days of autonomy. My last system was 48 volts. two sets of 8/6 volts batteries hookup in series and tie together in parallel for higher amps. I also used two XW MPPT.
The information on the type of panel materials are very knowledgeable for me. Thanks.

A:
 
It is best to have a single string of batteries if you can get the right equipment, but there are many good systems with 2 strings of batteries.

Batteries-All the Rage and Nevada killing Net-Metering comments


Q:
 
I am very impressed at how fast the technology within the solar industry has improved over the past couple decades. The panels continue to become more and more powerful, and now with the new battery technology coming out I'm excited to see what the future holds.
 
A:
 
If you go to a solar show, it seems that batteries are all the rage for the last few years. If you are in a place with net-metering, it is hard to financially justify batteries, since net-metering works like a 100% efficient battery that you can store energy in for up to a year in most parts of the US.

Ac-coupled PV system evolution and multimodal inverters


Q:
 
The textbook considers 'AC coupled systems' within the category of Off-grid systems. I think that there is an error here as 'AC coupled systems' are actually tied to the grid, even-though they have batteries for storage. Am I missing something here?
In the same chapter another category of system is 'Multimodal or bimodal PV systems', aren't we talking about the same here than 'AC coupled systems'?
Thanks in advance for clarifying the above.

Why low PV System Voltages in the 1990s


Q:
 
Why were residential systems in the 1990s at such low voltages? Was this due to low PV module efficiencies? Or simply because the niche applications (rural telephone systems etc.) did not require a high voltage?
 
A:
 
In the 1990s, most PV systems were battery based and battery based systems usually have lower voltages to match battery voltages. Our old system (which is still working) is a 12V battery charging system. It has a charge controller that is not an MPPT charge controller.

Voltage Drop and Ohm's Law

From HeatSpring.com PV Boot Camp Discussion Board

Q:

I understand that resistance is a constant since it is tied to a material's properties, but you said that Ohms Law measures voltage drop or loss and I don't see that in the law.

A:

Ohm's law states:
V = I x R

The V is going to be the voltage drop and not the system voltage.

Example:

If we have 2A, a 3 Ohm wire and a 100V system, then:

Vdrop = I x R
Vdrop = 2A x 3 Ohms = 6 Volts

Then if we want to figure out voltage drop percentage:

6 volts of voltage drop / 100V system = 6% voltage drop.

Snow on the PV Array


Q:

I have been trying to find a chart that alows us to take into consideration snow that sits on the modules. Nrel dose have a spot for this but im not sure witch percentage to put into this spot. We have always taken off January and Febuarys production amounts and half of March since those are our snowest months and we have had months when it never slides in some elevations and locations.

A:

They say Eskimos have hundreds of words for snow. There are so many different types of snow specific to different areas, that you have to know your area and take an educated guess or look at data from similar systems in your region.

PV efficiency


Q:

Understanding the development of the PV module, the size, configuration and production, I wonder what the next generation will be? Reduced footprint with similar or more generation? What is possible?

A:

They are always making PV more efficient and you can buy 20% efficient PV from SunPower and a few other manufacturers. The thing about 20% efficient PV is it is more expensive than 15% efficient PV and most jobs use about 15% efficient PV as the cost effective compromise in order to make a reasonable return on investment.

Another Great Review Today!!


Submitted: 03/07/2016
Training: Solar PV Installer Boot Camp Training + NABCEP Entry Level Exam Prep / Online / March 7 - April 17, 2016

How would you rate this course?
10/10
What did you like about the course?:
Dr. Sean White articulated very well, now it’s just up to us students to process the information and get it right…:)
How effective was the instructor's communication during the course?:
Very effective.
What would you change about the course?:
Not sure at this stage of my understanding.

Latitude Plus 15 for Winter PV Production


Q:

Question about correct winter tilt, which is latitude +15°. Let’s say April for my location is latitude 57°, would the correct angle be 42° or 72°? I would think 42° is correct with the sun less over head in winter?

A:

The latitude +15 tilt angle for winter optimization is not an exact science.
If we were setting a PV system up on the North Pole, then we would have to tilt the system facing the ground. If we were at the equator, what 15 would we go towards, north or south.

Ac coupled system inside the Grand Canyon


Q:

I am looking forward to learning from professionals in the PV industry, as sometimes the way universities teach can be a bit dry. Hopefully this course will help me take the next step in becoming a more polished PV professional. (works for Ameresco)

A:

Welcome to the course!!

I did a 2-week PV and wind teaching job for the Hualapai tribe on the rim of the Grand Canyon in AZ and inside of the canyon they have an ac coupled SMA Sunny Island PV system that was put together by Ameresco, which was very impressive.

Acceptable voltage drop for short distances close to zero percent

Conversation fromHeatSpring.comNABCEP Entry Level Course

Q:

Good explanations of all the details per wire sizing. The altE store and many others have their calculators to sell us wire, combiner boxes, and OCPD’s. 
I see most of them default with 2-3% voltage drops max for wire sizing per distance, and inverter wattage.

A:

If your distance is short, you should be very close to 0% voltage drop. It is all relative to the price of the wire and the price of the rest of the PV system. The increased price on 5 feet of wire is much less than the increased price of 500 feet of wire.

Making your numbers behave for Battery Bank Sizing


Q:

I am paying attention to the important stuff, but also like your great tip-bits, like “pointing to the Sun = location latitude + 23°” .
Sorry your battery sizing math depiction got away from me for a moment, (slide 104 minutes into assignment 2/7 PV system sizing)
Your explanations were good to divide the CL after Wh x DOA / DOD, but the depiction show it in multiplication?
Agreed kWh or Wh is a better way to view the process than Ah.

New anti-island requirements (joking around)


Q:

Sean, you are the first person to make these presentations enjoyable. Informative and entertaining at the same time. I liked the bridge between energy, distance, light and space.

A:

Thansk! Hopefully I don't get arrested for messing with the forces that keep us subdued.

Did you know it takes 500 seconds from the sun to make it from the sun to earth? I think that instead of the 5 minute anti-islanding requirement of UL 1741 inverters, it should correlate with the time it takes to get light from the sun.
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