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Solar Panels, Part II
Mar. 10th, 2010 11:13 pmPV cells
The photovoltaic (PV) cell is the basic unit of solar arrays. Most are made of silicon which is made as pure as possible and then doped with boron on one side and phosphorous on the other. The boron side faces the top where the sunlight (photons) enter. It has excess electrons which are given off when the photons hit it. The phosphorous side is starving for electrons and is attached to a back layer. A wire run from the front to the back can power something.
The first PV cells were made of a slice of a single silicon crystal. This crystal cannot be cut at right angles and still preserve its important properties, so it is typically shaped like a square with the corners cut off. These cells have an efficiency of 20%. These are still the most popular and have been seen to work for 50 years. These have the highest cost per watt.
PV cells can also be made from multiple slices fused together. These are cheaper to make but are a little less efficient (13 - 16%). They can be cut at right angles and are generally rectangular. These are newer but have still been seen to work for 20 years. These cost less per watt.
Panels
PV cells are put together in rows and columns into modules (panels). Monochrystalline silicon modules are dark blue with gridlines and white diamonds (where the corners were cut off). Polychrystalline silicon modules are a brighter blue with thicker gridlines.
Modules can also be made from a thin film of either amorphous silicon with no chrystalline structure (A-Si), or Cadmium-Telluride (Cd-Te) or CIGS among other things. These are still less efficient (6-7%). These have the lowest cost per watt but require more area and much more wiring. They are flexible and see-through so they can be used for windows, awnings, and skylights. And they work better in shade than the other two types. The Cd-Te and CIGS types have no silicone which was good during the period when refining capacity was maxed out, but more capacity has since been built.
Panels are strong. All can handle 1-inch hail at 60 mph; most, golfball-sized hail at 90 mph. They are stronger than a composition shingle roof. Baseball-sized hail can take it out, though. But this can be covered by homeowner's insurance. Modules do degrade by about 1/2% efficiency per year. Maintenance is easy and involves hosing them down occasionally.
Racking
Most (90%) systems are roof-mounted and of these, 90% are flat-mounted. They are attached to rails which, in turn, are attached to the roof. For composition roofs, bolts must go through the roof into the rafters (load-bearing joists) with a lag bolt to protect against the wind. Flashing or neoprene mastic is then used to protect the roof from leaks.
Solar panels can also go on Spanish tile roofs, but it's very labor intensive (takes three times as long) and involves pulling up most of the tiles. They are easiest to install on standing-seam (metal) roofs because they can be attached directly to the seams with no roof penetration at all.
They can also be pole mounted (expensive, but you get free shade) or ground mounted (cheap, but probably doesn't work in urban settings because of shade problems and because you probably have other uses for your ground). You can put them on carports and solar awnings.
Steel racking leads to expansion issues (solar panels are framed in aluminum) and rusting. It's much better to use extruded aluminum designed for solar panels. It costs a tiny bit more at first, but then it will be trouble-free.
Conduit
The wires must go through metal (not PVC) conduit and this must be on top of your roof (not in your attic). This is to protect firefighters who might need to climb on your roof and chop through it with an axe at which time they do not want to connect with your electricity.
DC disconnect
There's a place to turn off the power.
Inverter
The inverter converts the DC current from the panels to the AC current used in your house.
This is the weakest link in the system. Therefore, make sure you get one with at least a 10-year warranty. (SMA is best. Fronius is also great and a little cheaper.) Inverters hate the sun. Therefore, it's good to install yours in the garage and terrible to install it on a south-facing wall. They can actually handle heat up to 120 degrees, but they are not good with direct sunlight. At the very least, put an awning over it.
AC disconnect
There's another place to turn off the power which our instructor called expensive and pointless. The idea is that repair people can easily find this box and turn it off, but in reality they never actually use these boxes. That's probably because grid-tied systems are designed to stop producing power in blackouts and brownouts to protect repair workers.
[It is possible to build a hybrid system with a bypass to batters to run the fridge and other critical things--not the air conditioning, but it's expensive ($1000 - 3000 just for the batteries) so it would be better to get a whole-house generator ($400 - $500 for a 4 kW generator) if you want power during blackouts.]
PV meter
The inverter actually has a meter, but it can be 5% off. Therefore, grid-tied installations also have a revenue-grade meter.
Electric panel
The system is tied to your panel. It needs two slots at the bottom (with your main/grid power at the top). If you install a solar panel system, your electrical system will have to be brought up to code. (I can tell based on various things that people laughed about in class that our 55-year-old electrical system would require a fair amount of work in this area.)
Utility Meter
Grid-tied systems are connected to your meter.
Questions
I had a few additional questions when I went in that were also answered.
What happens with the solar panels outlast the roof? Solar installers are hired to disassemble the system, then roofers can re-roof the house, then the solar installers reassemble the system. (I put in 30-year composition tiles, expected to last 23 years in Austin, 14 years ago. I want my next roof to be metal so that it is also my last roof. I should probably wait until I do that before installing solar panels.)
Can you mix and match module types, say, to put film panels in the oft-shaded areas and more efficient panels in the other areas or to add on to your system? Not really. Being in series, they need to have the same current. However, you could put them on different inverters.
Any other questions? Ask me, I might actually know the answer.
Blog entry of the day especially for my vocabulary lovers: Manly Slang from the 19th Century - "Gullyfluff. The waste—coagulated dust, crumbs, and hair—which accumulates imperceptibly in the pockets of schoolboys." I think fraeuleinchen will like most of them, such as scandal-water: "Tea; from old maids’ tea-parties being generally a focus for scandal." Raaga123 might like follow-me-lads, "Curls hanging over a lady’s shoulder." Chikuru might like hobbadehoy, "A youth who has ceased to regard himself as a boy, and is not yet regarded as a man." I think I like bully trap, "A brave man with a mild or effeminate appearance, by whom the bullies are frequently taken in."
The photovoltaic (PV) cell is the basic unit of solar arrays. Most are made of silicon which is made as pure as possible and then doped with boron on one side and phosphorous on the other. The boron side faces the top where the sunlight (photons) enter. It has excess electrons which are given off when the photons hit it. The phosphorous side is starving for electrons and is attached to a back layer. A wire run from the front to the back can power something.
The first PV cells were made of a slice of a single silicon crystal. This crystal cannot be cut at right angles and still preserve its important properties, so it is typically shaped like a square with the corners cut off. These cells have an efficiency of 20%. These are still the most popular and have been seen to work for 50 years. These have the highest cost per watt.
PV cells can also be made from multiple slices fused together. These are cheaper to make but are a little less efficient (13 - 16%). They can be cut at right angles and are generally rectangular. These are newer but have still been seen to work for 20 years. These cost less per watt.
Panels
PV cells are put together in rows and columns into modules (panels). Monochrystalline silicon modules are dark blue with gridlines and white diamonds (where the corners were cut off). Polychrystalline silicon modules are a brighter blue with thicker gridlines.
Modules can also be made from a thin film of either amorphous silicon with no chrystalline structure (A-Si), or Cadmium-Telluride (Cd-Te) or CIGS among other things. These are still less efficient (6-7%). These have the lowest cost per watt but require more area and much more wiring. They are flexible and see-through so they can be used for windows, awnings, and skylights. And they work better in shade than the other two types. The Cd-Te and CIGS types have no silicone which was good during the period when refining capacity was maxed out, but more capacity has since been built.
Panels are strong. All can handle 1-inch hail at 60 mph; most, golfball-sized hail at 90 mph. They are stronger than a composition shingle roof. Baseball-sized hail can take it out, though. But this can be covered by homeowner's insurance. Modules do degrade by about 1/2% efficiency per year. Maintenance is easy and involves hosing them down occasionally.
Racking
Most (90%) systems are roof-mounted and of these, 90% are flat-mounted. They are attached to rails which, in turn, are attached to the roof. For composition roofs, bolts must go through the roof into the rafters (load-bearing joists) with a lag bolt to protect against the wind. Flashing or neoprene mastic is then used to protect the roof from leaks.
Solar panels can also go on Spanish tile roofs, but it's very labor intensive (takes three times as long) and involves pulling up most of the tiles. They are easiest to install on standing-seam (metal) roofs because they can be attached directly to the seams with no roof penetration at all.
They can also be pole mounted (expensive, but you get free shade) or ground mounted (cheap, but probably doesn't work in urban settings because of shade problems and because you probably have other uses for your ground). You can put them on carports and solar awnings.
Steel racking leads to expansion issues (solar panels are framed in aluminum) and rusting. It's much better to use extruded aluminum designed for solar panels. It costs a tiny bit more at first, but then it will be trouble-free.
Conduit
The wires must go through metal (not PVC) conduit and this must be on top of your roof (not in your attic). This is to protect firefighters who might need to climb on your roof and chop through it with an axe at which time they do not want to connect with your electricity.
DC disconnect
There's a place to turn off the power.
Inverter
The inverter converts the DC current from the panels to the AC current used in your house.
This is the weakest link in the system. Therefore, make sure you get one with at least a 10-year warranty. (SMA is best. Fronius is also great and a little cheaper.) Inverters hate the sun. Therefore, it's good to install yours in the garage and terrible to install it on a south-facing wall. They can actually handle heat up to 120 degrees, but they are not good with direct sunlight. At the very least, put an awning over it.
AC disconnect
There's another place to turn off the power which our instructor called expensive and pointless. The idea is that repair people can easily find this box and turn it off, but in reality they never actually use these boxes. That's probably because grid-tied systems are designed to stop producing power in blackouts and brownouts to protect repair workers.
[It is possible to build a hybrid system with a bypass to batters to run the fridge and other critical things--not the air conditioning, but it's expensive ($1000 - 3000 just for the batteries) so it would be better to get a whole-house generator ($400 - $500 for a 4 kW generator) if you want power during blackouts.]
PV meter
The inverter actually has a meter, but it can be 5% off. Therefore, grid-tied installations also have a revenue-grade meter.
Electric panel
The system is tied to your panel. It needs two slots at the bottom (with your main/grid power at the top). If you install a solar panel system, your electrical system will have to be brought up to code. (I can tell based on various things that people laughed about in class that our 55-year-old electrical system would require a fair amount of work in this area.)
Utility Meter
Grid-tied systems are connected to your meter.
Questions
I had a few additional questions when I went in that were also answered.
What happens with the solar panels outlast the roof? Solar installers are hired to disassemble the system, then roofers can re-roof the house, then the solar installers reassemble the system. (I put in 30-year composition tiles, expected to last 23 years in Austin, 14 years ago. I want my next roof to be metal so that it is also my last roof. I should probably wait until I do that before installing solar panels.)
Can you mix and match module types, say, to put film panels in the oft-shaded areas and more efficient panels in the other areas or to add on to your system? Not really. Being in series, they need to have the same current. However, you could put them on different inverters.
Any other questions? Ask me, I might actually know the answer.
Blog entry of the day especially for my vocabulary lovers: Manly Slang from the 19th Century - "Gullyfluff. The waste—coagulated dust, crumbs, and hair—which accumulates imperceptibly in the pockets of schoolboys." I think fraeuleinchen will like most of them, such as scandal-water: "Tea; from old maids’ tea-parties being generally a focus for scandal." Raaga123 might like follow-me-lads, "Curls hanging over a lady’s shoulder." Chikuru might like hobbadehoy, "A youth who has ceased to regard himself as a boy, and is not yet regarded as a man." I think I like bully trap, "A brave man with a mild or effeminate appearance, by whom the bullies are frequently taken in."
