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:

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

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

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

How would you rate this course?
10/10

What did you like about the course?:
Instructor was available for questions even if question strayed from entry level content. Instructor is industry leader

How effective was the instructor's communication during the course?:
Excellent - his videos contained recent events

What would you change about the course?:
nothing

If you enjoyed the course and are interested in providing a testimonial, please enter one here.:
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

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