Solar Energy Systems Course Week 2
Previous entries detailing week 1 available at these links: Class 1 | Class 2
Week 2 was all about digging deep as we rolled up our sleeves and began to get our hands dirty, applying our rudimentary understanding of basic electricity and learned more about how solar PV actually produces power.
We wired up some small solar panels in series and parallel circuits. Solar systems consist of solar modules wired together in an array. Depending on how the modules are connected to each other (whether in series, parallel or a combination of the 2), total array voltage and current will shift. When wired together in a series circuit, the voltage of each module adds together while the amps (current) remain unchanged. So if you have 2 17 volt, 3 amp modules wired in series, together they produce 34 volts at 3 amps. Parallel circuits work the other way; volts remain unchanged while amps are additive. Using our example, the same 2 modules wired in parallel will produce 17 volts at 6 amps.
Sidebar: Mike, one of the instructors, had asked the class to bring in their most recent electric bill at the end of class 2. At the tail end of class 3, we each read out the number of kilowatt hours (kWh) used in the most recent ~30 day period. My GF and I live in a 2 bedroom apartment that is probably 850 square feet. Our most recent bill told us we used about 160 kWh. I was stunned as I listened to the usage stats from my classmates. 900 kWh ... 1,100 kWh ... the "winner" used about 1,400 kWh! That last guy is spending roughly $375 per month. Way to go! Now, admittedly, we walk around in the dark a little bit and we don't have a dishwasher - but that's a pretty big spread. Most of the other classmates own homes, so they have a much bigger AC load to power, but I was amazed to say the least. It actually makes me feel a bit better about our energy predicament - conservation and reduced use can obivously make a huge impact on the amount of electricity we consume in this country. If price continues to rise, expect people to start switching to CFL bulbs and turning the TV off when the room is empty.
We conducted a mock shading analysis. The "perfect" solar site is a rarity - trees and weird roof features or dormers cast shade, especially in winter when the sun is lower on the horizon. If one section of the array is shaded, it effectively reduces output of the entire array to near zero. Essentially, shade shuts down the electric circuit - and electrons stop flowing. No electrons, no power. The solar installer must know how much shade the panels will take at each daylight hour throughout the year. If you don't calculate the percentage loss in power resulting from shading, you end up with an undersized system and an unhappy home owner.
We delved into solar cell fundamentals. Most people know that cells are (for the most part) made from silicon, which is one of the most abundant elements on earth. Most people know that solar cells convert sunlight into electricity. But how that process actually works is a mystery to most, including myself prior to Tuesday night. The link above contains more details than I could ever hope to include here, but suffice it to say that most solar cells are "doped" in order to create a permanent electrical field. When sunlight hits the cell, it liberates electrons and those electrons are attracted to the positive side of the electrical field. The cell is wired with conductors and - as the freed electrons build pressure - they flow to the wire and ultimately power the load the circuit is connected to.
As you might imagine, there were a lot of questions about this process and the class bogged down quite a bit on this subject. Eventually, we began to discuss maximum power points, voltage open circuit amount and short circuit amounts (measured in amps). It's critical to know the boundaries of any circuit, in order to properly size inverters and wires.
Next up is a solar tour of homes on Saturday morning, which should be pretty neat. I'll be sure to take some photos.
