Advanced Photovoltaic Systems
AET 230-8204 Notes
This course is being taught on 8 Saturdays at Diablo Valley College from 3/21/09 through 5/16/09 (minus spring break 4/11).
These notes will be a work in progress during the course.
Here is the schedule (with a lot of great guest speakers):
Here's the syllabus:
Here are some notes with bits of trivia:
*disclaimer-these notes are only a portion of what we will be going over
& are being edited as we go. Please contact me with corrections.
Day 1, 3/21/09
Equinoxes, Solstices, Right Triangles, the Solar Window,
Solar Radiation/Radiance, PV Array Orientation, PV Review & Course Overview.
Equinox is usually close to 3/21/09, although due to variations, mainly leap years & time zones, true equinox this year was yesterday.
The earth is tilted & as it travels around the sun in a close to circular orbit, twice a year, the equator is pointing directly at the sun.
At the other extreme, the equator will point 23.45 degrees away from the sun at the solstices.
Since the earth does not orbit in an exact circle, it is closer to the sun in the winter and farther away in the summer.
According to the solar Energy Pocket Reference published by ISES on page 1:
On winter solstice, the sun is 89.8 million miles away and in the summer (N. hemisphere summer) it is 95.9 million miles away.
We usually take the approximate average and say it is 93 million miles away.
The intensity of the solar power that we get above the atmosphere is about 1366 watts per square meter.
The Inverse Square Law at stated on page 27 of Photovoltaic Systems, by Jim Dunlop:
"The inverse square law states that radiation energy is reduced in proportion to the inverse square of the distance from the source."
I wanted to see how much more radiation you would get when the earth is 89.8 versus 95.9 million miles away, so:
1/95.9 squared = .000124
1/89.8 squared = .000109
.000124/.000109 = 1.14
That means at winter solstice, there is 14% more solar energy at that distance.
Why is it colder? Angles.
Trigonometry sounds intimidating, but can be simple if you are just playing with a right triangle. It can also make you look smart and land you a good job.
We use trig to do more practical things, like:
- Figuring out how many solar modules you can fit on a sloped roof using an aerial or sattellite photo
- How far apart you can put rows of tilted modules on a roof or ground mounted solar farm.
- Determining shading, how tall a tree is from it's shadow & a lot more.
In the Book Photovoltaic Systems, they do some Trig, which at first look seems intimidating enough to skip over.
All you have to know are 3 simple equations.
Cosine = Adjacent/Hypotenuse
Sine = Opposite/Hypotenuse
Tangent = Opposite/Adjacent
You can read about it in the book PS on page 417.
here is a wikipedia link
On a scientific calculator (one that has sin, cos & tan buttons).
If a triangle is the same shape and a different size, the ratios of the sides and angles will be the same.
When you are trying to get an angle for an answer, you have to push the -1 button before you push the tan, cos or sin button. The -1 button in the book is represented by putting arc before tan, sin or cos. aka-arctan, etc.
The reason I am trying to tackle this now, is because that way, we have a few months to get it down.
There is a good article on inter-row spacing in SolarPro magazine. By the way, i suggest getting your free 6 month subscription to SolarPro at www.solarprofessional.com .
The article is in the Dec/Jan 2009 issue and is titled "Calculating Inter-Row Spacing".
After reading the article
I made a row spacing calculator on an excel spreadsheet which you can download by clicking on the rows below.
If it looks complicated, don't worry. This is just an example of how you can use triangles to figure things out, like shading.
Many people that do residential solar don't use trig. There are convenient rules of thumb and other tools, such as the Solar Pathfinder and the SolMetric SunEye.
Many of my roofer friends use charts to convert slopes & they are usually very impressed when you throw a trig function on a calculator.
Sun Path Charts
Here is a good place to find out what the sun is doing at any latitude: http://solardat.uoregon.edu/SunChartProgram.php
With a sun chart, a compass and a paper-clip, you can determine shading with a sun path chart, but you have to know what you are doing.
In reality, when you are on a roof, most obstacles are obvious and if something is on a roof, such as an A/C unit or a chimney, you can measure the object and apply the rule of thumb.
Many people say that you should try to have a 3:1 distance away from the shading object. The California Solar Initiative...
...considers a more forgiving 2:1 distance "minimal shading".
2:1 would mean that PV should be placed twice as far away from the object as it is higher than the object.
Radiation v. Irradiance
The trick I used to remember this is that radiation is what can be dangerous. It is something that is accumulated over time. It can cause cancer (remember sunscreen in your solar tool kit).
Radiation is irradiance over time, just like energy is power over time. (The way to remember poWer is measured in watts is the W in poWer).
In your house you are charged for kWh's, but you are probably not using exactly 1 kW for one hour.
For the same reason, solar radiation data is measured in kWh's per sq meter per day.
You get your data at the National Renewable Energy Lab (NREL) by clicking on the famous Red Book below.
Just remember, radiation takes time, just like energy.
Let's go surfing now, surfing AET
California Solar Initiative (rebate level $1.55/watt and falling)
- www.nabcep.org Breaking news-eligibility requirements made easier for PV Installer (hard) exam/certification. 2 years experience installing PV no longer required. Look for the Study Guide for a lot of information.
- http://brooksolar.com/services.html Pay special attention to the recommended reading material on the lower right side of the page. This the website of Bill Brooks, who trains the inspectors & the installers. If you are lucky, you sign up early enough to get into one of his free classes at the Pacific Energy Center. He also, helps write the National Electric Code.
- www.homepower.com The original.
- www.solarprofessional.com Brand new by Home Power
Advanced Photovoltaic Systems
AET 230-8204 Notes-Week 2
The highlight of this week is our Guest:
Tony Diaz of Century Roof & Solar
You can learn more about Tony from his websites:
Tony is a C10 electrical contractor, a C46 solar contractor, C39 roofing contractor and probably a few more.
He just took the "hard NABCEP" PV installers exam & will tell us how it went and his path to taking the exam.
He will go over wire sizing and the NEC.
We should also get to see some photos of his project at the the California Academy of Sciences in San Francisco.
Topics for this week:
Site Assessment/Surveys, Design & Proposal Software, Shading, Rebates, Taxes, Stimulus, Financial.
There are many different ways to asses a situation for PV.
Benefits for the Solar Pathfinder:
- Like a bicycle it is human powered.
- You will not show up with dead batteries or an error message.
- It takes no time to boot up.
- You can take pictures of it and convert to digital.
Benefits for the Solmetric SunEye
- High-tech reports.
- It does all of the thinking for you.
- It calculates for magnetic declination.
- You can use it with a GPS and you won't have to enter the nearest city.
- You can download the data into proposal generating software, such as OnGrid and Clean Power Finance.
Benefits of the compass, protractor, paperclip & sunchart method
- The benefit to this method is that you have to really understand what you are doing. You will be correcting for magnetic declination & determining altitude angles for different bearings.
- It is very inexpensive and there is a good chance you already have what you need at home.
Many times you will go to a roof and not need to take these measurements. When you are on a flat commercial roof in an industrial area, there is a good chance that the only objects that will make shade are on the roof, such as air conditioners. What I do is take an aerial image with me that shows the roof and the structures on it. Typically the resolution of the best online aerial images is 6 inches per pixel.
I have my favorite place to get images from, which costs
thousands of dollars per year. www.pictometry.com
They do not have everything, so I still use the others, which also are a great resource, such as:
This website is also very interesting and besides maps, it has real estate values (estimates) and lot lines:
People used to rent helicopters for views that you can now get for free.
Once I get on the roof with the images, I measure the height of the potential shading objects. I also draw on the image things I might not have seen, such as conduit and vents.
Ideally, we would get the building owners to work with us and move everything to the north side of the roof, where it will not cause a shading problem. I assume that as solar gets more mainstream that people will design buildings with all of the objects on the north side of the roof.
Many people use rules of thumb for how far away from a shading object you need to be. The closest you can get by any standard I have seen is 2:1 or twice as far away from the object as it is above the PV.
Once you start laying out the modules with a CAD program or with a pencil, you will get a better idea of how things will fit and at some point there will be a module that you will probably contemplate over. Should you place it there or not. One thing that will help you make your decision is how it fits with a series-string. If you add one more string, it might be overly shaded and if you left a string off, you might have more empty space than you wanted. In this case, you will probably err on the conservative side and leave some extra space on the roof.
You can also play with changing the amount of modules in a string, but on a large commercial roof, you will probably not sacrifice the voltage drop or increase in copper wire needed associated with the lower voltage you would get from a smaller string.
If you are using the pencil method, make sure you have a good eraser, because to make a good fit, you will be moving things around a lot.
Inter-row spacing exercise
Go to the second issue of Solarpro Magazine www.solarprofessional.com & read the article on calculating inter-row spacing.
We can also calculate the area under the module and get the north to south repeating distance of a row. The repeating distance is important, because it lets you know how many modules you can fit on the roof.
I like to do my layouts with simple rectangles.
Here is a link to my excel spreadsheet that calculates the repeating distances, so you can get a rectangle.
Another thing to keep in mind is that there is a space between the modules in a row, which varies depending upon the type of racking system you use. It could be an inch or more, so add that to the dimension of your rectangle.
It can get a lot more complicated than this.
What would you do if the modules were not facing due south?
What if you were above the arctic circle?
Many times, in my experience, designers end up using rules of thumb, such as a 3:1 or a 2:1 distance from shading objects. Rules of thumb are not very scientific, but the amount of money you might spend figuring out how to become a fraction of a percent more efficient could be better spent on adding another module for more than a fraction of a percent production.
Computer-Aided Design (CAD) Programs
There are CAD programs that can be used for this.
There is a SketchUp class coming up at the Pacific Energy Center that is about calculating shading.
SketchUp www.sketchup.com is a free 3D CAD program by Google.
If you want to develop a skill that will impress employers, learn AutoCAD. You can take classes at DVC, UCB Extension and from the makers of AutoCAD, www.autodesk.com
On tip is that since AutoCAD software is very expensive, don't do the 30 day free trial until the class starts. Wait until the last minute. You can download it once per computer. You can also purchase a student version that is more affordable for a student.
I have learned more about AutoCAD from a private tutor, than from a class. With a private tutor, I can tell them what I want to do & then they show me.
I have seen people do single line diagrams (SLD) of PV systems with Microsoft Word.
A co-worker of mine uses Microsoft Visio.
Here is a list of CAD software companies:
Rebates, Taxes, Stimulus & Financial
The portal website to the California Solar Initiative (CSI) is:
Once a month, the Pacific Energy Center (PEC) has a CSI class which you should attend. Having this knowledge will help you get a job.
The CSI paperwork is a bit complicated and there are many people in the industry that would like to put that paperwork on your desk.
It is not that bad, because there are friendly & helpful people on the other end processing the rebate paperwork who will help walk you through it. You will meet some of them at the CSI events at the PEC. It can be helpful to introduce yourself, so that they have a face to associate your phone calls with.
The rebates step down as more and more megawatts get used up.
You can see what level we are at right now at:
The reason that the rebate amount keeps falling is to give people an incentive to act now.
There are too many people afraid to sign contracts, because of all the hype you hear about solar getting cheaper.
Here are some of the reasons people give for waiting:
- Thin film
- An article that cited $1/watt
- 40% efficient module article
- Stimulus package
- Cap & trade
- Carbon credits
It can be frustrating reading every new article about some technology that is going to revolutionize the industry in a year.
I think that a lot of this hype is from companies looking for venture capital funding.
Here are some reasons to get solar now:
(good to know for Solar Salespeople especially)
- CSI rebate is dropping
- Demand is low and supply is up
- Approximately $3/watt all time low PV wholesale prices
- Tax credits probably will not get better
- If they wait, installers will be busy & prices will go up
- If they wait a year, that is a year worth of PV production that they did not have. That is worth $$$.
- How many pounds of greenhouse gasses are they putting in the atmosphere for each day of delay?
Here is where to go as soon as you know you are going to file for a CSI rebate:
When you submit an online reservation, it does not mean much. When you submit your application with "wet" signatures and a copy of a contract, then you will get a place in line for a particular rebate level.
If you are installing your own system, in place of a contract, you need proof that you have paid at least 10% down on the equipment. An invoice from a distributor will do.
You have to mail in the application and can not carry it to PG&E.
Towards the end of a rebate level, there is a rush of people trying to get a reservation.
If you are in a different utility district, this does not apply.
You can find out about different incentives for different areas at: www.dsireusa.org
Federal Tax Credits
The 3 letters that were most important to everyone in the PV industry last year were ITC, which stands for Investment Tax Credit.
The ITC was finally thrown into the Emergency Economic Stabilization Act of 2008 and extended for 8 years. This was very important. For the second half of last year, large commercial projects were put on hold, because if they were not completed & inter-connected to the grid by 12/31/2008, they had no assurances that they would be eligible for the 30% tax credit. On a megawatt project, that would amount to millions of dollars.
The 8 year extension of the ITC also removed the $2000 cap on residential PV projects. Now commercial and residential projects are eligible for a 30% tax credit, that may be carried forward, i.e., they can spread it out over the years until 2016.
At this point, like everyone else, I will tell you to talk to a tax professional to know for sure.
Note: Non-profit organizations & public institutions such as schools, who do not pay taxes can not benefit directly from a tax credit. One way around this is to have someone else own the PV system and this is called a Power Purchase Agreement (PPA).
A problem people are having now with the downturn of the economy is that they are not making money and therefore do not owe taxes. To counteract this problem, included in the American Recovery & Reinvestment Act of 2009 is a provision to turn the tax credit into a grant. This is for commercial projects and once the project is completed, a check for 30% of the project cost is supposed to be issued in 60 days.
This is all very new. To get the best information, look to the Solar Energy Industries Association at www.seia.org .
Taxes on Materials
You have to pay the sales tax rate of where the PV is shipped. If you are in Alameda County and install in Contra Costa County, the higher sales tax of Alameda County applies. If you have it shipped to Contra Costa County, then that sales tax rate could apply, however the extra paperwork associated makes that extra savings not worth the money saved.
Advanced Photovoltaic Systems
AET 230-8204 Notes-Week 3
The highlight of this week is our Guest:
James Caling, Electrical Engineer
James has experience with PV design, large commercial, project management, site supervision & site inspections.
James is due to arrive at 12:30pm.
Johan Alfsen, QuickmountPV
Johan is the Dealer Trainer for QuickmountPV
You can see some good videos at www.quickmountpv.com
Johan will do a demonstration and give us some hands on time with the flashed mounting system.
They are located in Concord, CA and ship their product everywhere.
Johan is due to arrive at 10:30am.
Ryan Jones, Electrical Engineer (our classmate)
Will do a Power Point presentation on the p-n junction and how the semi-conductor properties of silicon make electricity from light.
Think of doing your own presentation for extra credit.
Just released was the World PV Industry Report Summary. You can see it here at:
Here you can see how Spain in 2008 passed Germany as the number 1 PV installer and that 82% of the installations in the world were in Europe in 2008.
Stimulus money is here
Grid Alternatives with AmeriCorps is looking for employees.
Field Trip on May 2 with Tony Diaz
Topics for this week:
System Components & Configurations
PV cells, Types, Characteristics & Inverters
(Chapters 4, 5 & 8 in the book Photovoltaic Systems)
Components & Configurations
The main focus of this class will be battery-less, grid-connected, utility-interactive systems, although we will go over some of the other components briefly.
Inverter-convert DC current to AC current
Battery-electrochemical energy storage
Flywheel-kinetic energy storage
Capacitor-store charge (supercapacitors store more)
PCU-power conditioning unit (often in the in the inverter housing) often called the inverter for simplicity.
Charge controller-regulates battery charging
Rectifier-converts AC to DC
Charger-A rectifier with filters, transformers, etc. to charge batteries with.
DC to DC converter-change voltage
Maximum Power Point Tracker (MPPT)-changes input voltage in order to maximize power from PV on IV curve
Utility-interactive-grid tied-PV/inverter/utility-parallel to the grid-our main focus of study
Stand-alone-operates independent of the grid
Bimodal-can act alone or with the grid, charging batteries if the grid is down with PV
Battery backup-has batteries that will work when the grid is down until the batteries go dead. Will not charge batteries when the grid is down.
PV cell types (typical)
Monocrystalline-from one crystal-solid color and square with rounded edges usually. More efficient.
Here is a link to where you can see where they are selling silicon:
Polycrystalline-Multiple crystals-usually square shaped with rainbow hues. Almost as efficient.
String Ribbon-Made like bubbles with a string. Here is a link to the process:
Thin film-There are various types of thin film PV. Half as efficient. Has better thermal characteristics.
Here is a link to the latest on thin film:
p-n junction-where the conversion of light to electricity happens
n-type semiconductor-the top of the cell facing the light which is doped with phosphorous. This is the negative charge side of the cell where the electrons come from
p-type semiconductor-the bottom of the cell where the boron doping is. The boron is melted with the silicon before the crystal is made.
High frequency inverter-lighter without iron core transformer-less surge capacity and by some considered lighter duty. Fronius inverters are very light.
DC to AC (20,000 Hz) to DC to AC
Low frequency inverter-Has an iron core transformer and is heavy. SMA inverters are low frequency.
DC to AC (60 Hz) to transformer
Transformerless not common in the US. Ungrounded without transformer. Can be bipolar. Common in Europe & may become more common here in the future.
Power factor has to do with the current and the voltage being in phase. When everything is in phase, it is called a resistive load and the power factor is 1. When it is out of phase, it is called a reactive load and the power factor is less than 1.
Inverters only generate a power factor of 1.
Your electric bill is for reactive power.
You can measure PF (Power Factor) with a Kill A Watt
According to Marvin Hamon, EE, the other instructor of AET 230
"Large inverters over 100kW almost never work out of the box & you need a factory representative to make them work."
AC modules: they are a thing of the future. We have not seen one yet.
An Enphase inverter is not factory wired to a module.
Enphase is a microinverter and there is a lot of interest, especialy from high-tech consumers.
Enphase has said that they will come to our class for a talk. You can see their product at www.enphaseenergy.com.
Remember, next week is spring break, so don't come to class.
In 2 weeks, the subjects we will cover are system sizing & mechanical integration, which you can read about in chapters 9 & 10 of the book Photovoltaic Systems. We will have a presentation by Brian Swanson of Fidelity Roof Company.
Advanced Photovoltaic Systems
AET 230-8204 Notes-Week 4
Our guest today is:
Brian Swanson of Fidelity Roof Company
You can learn more about Brian's company from their website:
Brian has spent most of his life in the roofing business, where he specializes as a commercial roofing and solar salesperson. There is no roofing job to big for Brian to tackle.
Today we had a presentation by student Scott Cornell who shared with us his PG&E solar "bill". It is not really a bill because as you can see by the documents below, he is an overproducer. Thanks Scott!
Next Monday April 20, NorCal Solar is having a networking Event. Read about events at their website on the right margin: www.norcalsolar.org
Next Friday April 24th at 6:30pm, Vote Solar is having a volunteer appreciation party at 2360 Polk Street in SF. Sign up to be a Vote Solar volunteer, so you can go at www.votesolar.org
Next month May 11 through May 16 is the ASES (American Solar Energy Society) Annual Convention/Trade Show/Training Event, etc. I will be there, but make it back on time for you final on May 16th. They call it "Solar 2009" in Buffalo NY and if you can, sign up to be a volunteer at "Solar 2010". You can find out more at www.ases.org
Next Month on May 20, 2009 John Wiles (the main person responsible for Article 690 of the NEC) will be speaking in Palo Alto. Deadline to sign up is May 1 and this class is expected to fill up fast. You can click on the NEC book below for a link to John Wiles presentation schedule on the right side of the page. There are also many other useful links on this page, such as the Code Corner articles, which you should look at. Also, if you sign up to be a member of NorCal Solar, you get $5 off at www.norcalsolar.org.
This weeks topics are System Sizing & Mechanical Integration, which you can read about in the book, Photovoltaic Systems in chapters 9 & 10.
System Sizing in PG&E Territory-
System sizing in the textbook and system sizing in PG&E territory can be a different proposition with a unique set of rules.
Residential customers in PG&E territory have what are called tiered rate structures. What that means is if you conserve energy, your rates are not very high. If you use a lot of electricity, your rates are sometimes 4 times what the low level (baseline) rates are. In sizing a system here, we try to offset the more expensive electricity for a faster return on investment. This can vary on a case by case basis, since some people do want to offset even their cheap electricity, because they care about the environment. Also, another factor to consider is that if you oversize a system &/or cut back on your usage to a point where you would make more electricity than you use, you end up giving it for free to PG&E. There are lawmakers trying to change this, but that is the way it is now.
Some people also want to size their system for the future. They might want to add extra capacity for an electric car. They might want to just wire it and leave the space for the future extra modules. Others might think the future will be more efficient, with energy saving devices that would make them under-size their system.
Another thing to take into consideration when sizing a system is time of use (TOU) rates. When you have net metering, your net is for the price of electricity and not the net KWHs. This means that if you are on a TOU rate structure and when you are making electricity during the day in the summer when it is 3 times more expensive than in the night, that you are getting 3 times the credit. Theoretically, if you only produced energy at peak times and only used it during off peak times, you could offset your whole bill by only making one-third of the electricity that you use. This is an extreme example which probably never happens, but it makes a point.
System Sizing off the Grid-
Sizing an off-grid system can be an art form. There are many variables. One thing to consider is how much of an inconvenience it would be to be without electricity part of the time on a bad day in January.
Another thing to consider is if you are willing to have a hybrid system. Many people have back up fossil fueled generators. A lot of my off-grid friends in Alaska also have wind turbines hybridizing their systems.
Unless you are only using your PV in the summer, you will probably increase your tilt angle to latitude plus 15 degrees to maximize production for the short days when the sun is low. You could also have a way to seasonally tilt your modules, which means you change the tilt angle to go with the seasons.
The text book gets into more detail about how to estimate your load and size your system from there.
System Sizing with Monthly Net Metering-
Apparently in some states they have monthly net metering, which means that you zero out your bill every month. Probably in those places, you would want to increase your tilt to get a little more production in the winter, unless you were using a lot of air conditioning. If you overproduce in some of these places, you would be compensated very little to nothing for anything over what you use.
System Sizing with a Feed in Tariff-
What makes a feed in Tariff different is that your system makes money for what it produces and does not have to offset you usage. You could have a large field, with no house on it and produce as much as you can.
Feed in Tariffs would really simplify everything. You would get paid for what you produce and would not have to deal with rebates, offsetting your usage, etc. The rates are guaranteed for a long time, so it also takes the risk out of the investment.
There are many ways to put PV on a roof, field, etc.
Here are some links to some of the most common ways to go with "mechanical integration" of PV:
"Flat" v. Sloped roofs:
In practice, if someone says commercial roof, you think flat and if they say residential, you think sloped.
A flat roof is actually a low-slope roof in practice, since some drainage is needed. Typical low-slope roofs start at a 1/8 (one eighth):12 slope and a steep slope roof starts at 2.5:12 slope according to the book "Roofing Handbook" by Robert Scharff and the editors of Roofer Magazine.
In general, you design for a sloped roof with racking going across the roof in an east to west direction. You usually use lag screws and screw them into rafters. Your attachments should be properly flashed if you want it to not leak.
There are many different types of flat roof racking systems. Some penetrate the roof, some don't and are ballasted with "pavers" (like bricks). Some are ballasted with a few penetrations. They can be in rows. They can be flat.
Thin film is not the most typical installation in the US, but its use is growing. There is the UniSolar peel and stick solar, which is like a sticker. This is typical for a standing seam metal roof. If it is put on a single-ply roof, the manufacturer should tell you that it is compatible.
Single-ply roofs are usually white and rolled on in sheets. Some types of single-ply roofs are TPO (Thermoplastic Polyolefin) and PVC.
The standard low slope (flat) roof over the years is built up or hot mopped. You can smell when they are putting one of those roofs on (tar). Sometimes they top it off with gravel or a cap sheet. A cap sheet goes over the surface and can be different colors. The color you usually want in this area is white or a lighter color that reflects the solar energy back to space.
Another type of mounting system, that is a product in it's own category, is the Solyndra, "thin film" cylindrical product, which is made here in the Bay Area. www.solyndra.com
You should be concerned about voiding roof warranties. It is best to work with whoever installed the roof, or someone that is certified to work on the brand of roof you are working on by the manufacturer.
When a roofer installs a particular brand of roofing product, they are commonly inspected and signed off by the manufacturer.
Probably the most critical piece of the PV installation puzzle that is commonly overlooked by the inexperienced person is proper mechanical integration into the roof.
Post class follow up...
Also, Brian Swanson recommended roofer training and here is the National Roofing Contractors Association website, where you can find out about online and other training from the experts.