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
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
Proposal/sizing/estimating software:
California Solar Initiative (rebate level $1.55/watt and falling)
PV Information:
- 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
Associations:
