PV Student

Q:

Good Morning Dr,

To which extent we can consider the stand alone systems. I mean is there any maximum capacity for stand alone systems that we can't go further capacity.

The charge controller is very important device that will help to save the batteries longer. Is there any other function other than preventing the batteries over/under charging? And what are the protection schemes related to this device if any.

Thanks

A:

If a stand alone system is big enough, it can be more of a micro-grid and if a micro-grid is big enough it can approach being a utility.

There are no size limitations, but as the system becomes larger, there are different criteria to take into consideration.

With battery selection, it is best to avoid multiple parallel connections, so if you wanted to have one large battery bank, you would often have to use parallel connections. With small systems, this is not a problem, but with larger systems there are different ways of splitting up the battery banks into smaller units in a micro-grid system. There are also large container sized battery banks.

If you would be working on one of these larger systems, you would most likely be working with the manufacturer to make sure all of the products properly communicate with each other.

Mostly the charge controller will function to prevent overcharging and undercharging, however the charge controller can also do maintenance on a battery. Many flooded lead-acid batteries can be equalized, which is a controlled overcharge that will help prolong the batteries life.

Equalizing is overcharging a battery, which will split H20 molecules into hydrogen and oxygen gas bubbles. These bubbles will stir up the heavy stationary batteries to prevent stratification. Stratification is the heavier acidic electrolyte settling to the bottom and the less acidic lighter electrolyte settling on the top.

Also equalization will help remove led sulfate buildup on the lead plates. We call this scraping the places. Lead sulfate buildup can occur when the battery does not receive a full charge often.

Charge controllers may have overcurrent devices built in and they may require separate overcurrent devices. I recommend reading the detailed manufacturers installation manual.

Thanks for the good questions!

Sean White

Q:

Hi Dr.sean,

I used to be in project that works with PV but the drop is reaching 20% of the rated power. The type of PV (polycrystalline)

At which condition the PV will be more efficient regard the production of power.
suppose we have two PV panels at the same intensity of sun rays for example 1000 w/m2 but each PV have different ambient temp for example PV1 (25c) and PV2 (50c) now which one of these will produce more power?

A:

If polycrystalline PV has lost 20% of its rated power under Standard Test Conditions, then there would have to be a manufacturing defect.

If PV produced 20% of its rated power in the field, that would be good performance, because Standard Test Conditions are very unusual and because there are other losses before the electricity is converted to alternating current.

Also, Standard Test Conditions is for the solar cell temperature and usually the cell temperature is about 30C hotter than ambient temperature on a sunny day.

If the cell temperature is 25C different, then to calculate the difference in power, we would use the temperature coefficient of power for the calculation, which is usually about -0.45%/C

25C x -0.45%/C = 11.25% decrease in power

Other factors that would decrease power would be irradiance less than 1000W per square meter at the angle the light hits the solar modules and other derating, such as inverter efficiency, soiling, voltage drop, etc.

It is common on a sunny day for a PV array to be putting out 50% of the solar array power due to these factors.

Thanks,

Sean White

Q:

Article 690 - Part II - 690.7(C): Maximum System Voltage. In your lecture you mentioned that the NEC distinguishes between 3 family dwellings (up to 1000V PV max system voltage), versus 1-2 family dwellings (up to 6000V PV max system voltage). Two questions: First, I proudly bought the book, but I do not see this distinction in 690.7. From what I read, it addresses 600V and 1000V. Second, it seems arbitrary to distinguish to maximum inverter size between 3 family dwellings vs. 1-2 family dwellings. Do you think this will change? Wouldn't you want ideally to have one string on one inverter versus breaking strings out onto multiple inverters, or to run the system in parallel?

A:

The rules are the rules. The 2014 NEC changed to allowing 1000V PV systems, however 1 and 2 family dwellings were still limited to 600V.

Personally, I would feel safe with a 1000V system on my roof. If you start looking at buildings that are more than 1 and 2 family dwellings, it is more commercial property.

Often times with smaller PV systems, there is no need to go over 600V since we are always trying to fit PV on a rooftop and it is often difficult to fit 20 modules on a normal house facing all in the same direction.

Also, with the 2014 NEC we are more likely to see electronics under modules on rooftops, so that is another reason we will not be needing to go over 1000V.

I dare you to put a 1001V system on your roof (don't tell anyone).

Thanks,
Sean

Thanks for making my day with the review!!!

Submitted: 05/16/2016

Training: Solar PV Installer Boot Camp + NABCEP Entry Level Exam Prep / Online / Anytime

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Instructor was available for questions even if question strayed from entry level content. Instructor is industry leader

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Dr. White being an industry leader has developed an excellent course for NABCEP Entry Level. I will be registering for the more advanced course once hands-on experience is obtained

Q:

Sean

My question is about determining the minimum number of modules required to turn the inverter on. All modules I have reviewed have "temp coefficient for Voc" but none have "temp coefficient for Vmp"

Example from Canadian Solar module

Temperature Coefficient (Pmax) -0.43% / °C

Temperature Coefficient (Voc) -0.34 % / °C

Temperature Coefficient (Isc) 0.065 % / °C

Is Temperature Coefficient (Pmax) -0.43% / °C used when calculating minimum number of modules required?

Thanks

A:

Most module datasheets do not come with the temperature coefficient of Vmp, however whenever I have seen the manufacturer list the temperature coefficient of Vmp, it has been the same as the temperature coefficient of power, so I just use the temperature coefficient of power.

Thanks,

Sean White

Q:

Thanks Sean,

The Enphase AC battery could be a big thing but they don't seem to promoting it for grid-tied battery backup or off-grid.

From their web site - https://enphase.com/en-us/products-and-services/storage

What’s your primary energy goal?

Thanks

A:

Since the Enphase battery has not been released yet, it is difficult to know exactly how it will work. Enphase might not know exactly how it will work.

They are promoting their battery to work with the grid, so it may not work when the grid is down. When you have a battery that will only work when the grid is working, it will make a lot of sense for systems that are not allowed to spin the meter backwards or if there is a big difference between the credit for spinning the meter backwards and buying from the grid.

These types of systems that work with the grid will make no sense in an area, where we have net-metering policies where we get 100% credit/retail value for the energy that we export. Perhaps they can make some sense if there is a time-of-use rate schedule, but it still might not be worth the hassle and the price of replacing the batteries.

In many places in the world, there are not net-metering policies where you can get a good incentive for exporting to the grid. In these places, it makes sense to have this kind of battery system.

Since Enphase is not promoting their battery and are recommending a "legacy" system for people that would like off-grid capabilities, then I would assume that their battery will not work off-grid. I don't blame them, because the off-grid market is not their focus, it is a very small percentage of the industry and it would make their systems much more complicated and expensive.

If you want a more cost effective grid-tie battery backup system, get a dc coupled multi-modal inverter to get the best bang for your buck. If you want to use microinverters, then get an ac coupled system.

Thanks,

Sean White

Q:

Dear Doc,

A little late question ,concerning article 310 and 110.14 (c),110.14(c)(1)(a) or (C)(1)(b)

When you talk about terminal ratings,I am aware of three areas in solar where wires terminate ,combiner box ,inverter and main panel ocpd.

Are we talking here about, adjustment and correction factors ,dealing with ocpd of the solar breaker in main panel or the equipment shutoff breaker ,I am assuming these articles are referring to the main panel termination or equipment shut off termination.

This is the missing link I need to know to understand.

Appreciate the help

A:

When we are doing any wire sizing, we have to take the terminal temperature limits into consideration, whenever a wire terminates at a terminal.

This does include at a termination at a combiner box, inverter, main panel OCPD, even a wire nut.

When we do the checks for corrections factors for derating of ampacity with tables 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(3)(c) (conditions of use derating) we do not take the terminal temperature limits into consideration for that calculation.

We do take the terminal temperature limits into consideration when we are looking at the required ampacity for continuous current, which is 125% of maximum circuit current.

This means that when we check for continuous current, we take the terminal temperature into consideration.

When we check for conditions of use, we do not take the terminals or continuous current into consideration.

Let me say that one more time:

When we check for continuous current, we take the terminal temperature into consideration.

When we check for conditions of use, we do not take the terminals or continuous current into consideration.

Remember that conditions of use are the 310 tables 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(3)(c) where we reduce our ampacity due to high temperatures and greater than 3 current carrying conductors in conduit.

Good questions!

Sean White

Sean, a further question. In the video you made the point that batteries are still expensive but costs are coming down. There was also a description of a grid tied, AC coupled system. If you are designing or purchasing a residential or small commercial system that may not require batteries now (perhaps you have net metering), but may require batteries in the near future, say a few years time when battery costs come down and net metering is removed, what should you specify so that your system is able to incorporate batteries without having to make costly change out of equipment,

A:

There are a few different ways at looking at a solution to adding energy storage in the future to a PV system that you are installing now.

I would try and use an inverter that is made by a company that is likely to have a battery inverter in the future, which is probably a company that already has a battery solution. This way, we will be likely to have equipment that is more compatible. Companies can include SMA, Schneider, ABB and others.

Will you require the system to work when the grid is down? If your grid is stable and rarely goes down, it may not be worth the extra costs to have the PV system charge the batteries when the utility is down. In this case, you can get a battery system independent of your PV system. Your battery system will be a load when you want to hang on to some of your solar energy and it can send out electricity when you need to use more.

If you require your battery system to charge the batteries when the grid is down, it can be more complicated. You will want to plan having the solar and the backed up loads to go to a common subpanel (usually battery systems do not backup all of the loads).

There are many different ways that batteries can help and it will depend on the incentives, net-metering standards, time-of-use electricity rates, stability of the grid, generators on premises and utility rules.

Any grid-tied system can be converted to an ac-coupled battery backup system. It will take careful planning to determine your needs.

Here is a Schneider video with a battery solution that can work in many different ways depending on your needs:

https://www.youtube.com/watch?v=VmlS_YBqLEs

I am sure in a few years, there will be something better and less expensive.

Thanks,

Sean White

Q:

What is the best angle to put solar panels at for removing snow loads while still gaining solar energy?

A:

There is no exact answer for snow, since sometimes snow will stick vertical and sometimes it will slip off at a 5 degree tilt. There are hundreds of different types of snow.

From my experience, I rarely see snow stick to PV for more than a day. It often melts, slips or blows off.

I do not often see installers and designers worry about snow.

Here is an array that was designed for deep snow in Truckee CA:

They are using a 35 degree tilt.

I have seen zero tilt in Canada. When there is snow on it, it will not work well, but when there is snow on it, it is that dark time of the year.

Thanks,

Sean White

Q:

If there are wires in a conduit on a roof, what is the best way to prevent them from over heating? Let's take out the variables of wire size & wire insulation, & focus on good techniques to prevent general overheating from good old Mother Nature.

A:

The Code will err on the safe side, so if you are following the Code, you probably will not have to deal with other types of cooling, however I have thought of this:

1. You can paint your conduit white to reflect sunlight

2. You can keep wiring in the shade

3. You can put your conduit where there is good airflow.

4. You can put the conduit over a cooler roof, such as a white roof or a living roof.

5. You can keep your white roof clean and reflective.

The Code calls for accounting for greater than 3 current carrying conductors in conduit, wires in sunlight over a roof and high ambient temperatures.

We can find these Code variables in 310.15(B)(2)(a), 310.15(B)(3)(a) and 310.15(B)(3)(c).

Thanks,

Sean White