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Solar Panels, Part I
Mar. 8th, 2010 10:00 pmRobin and I went to an informal class on designing and installing solar panel systems this weekend. We are not now prepared to actually install a system, but we learned a fabulously huge amount of stuff that will help us make good decisions when/if we hire someone to do this.
The first presenter, Doug Soester (pronounced SOY-ster) gave us an amazing amount of information at all levels from basic physics to what he thinks about state-of-the-art microinverters.
Shade
If you're interested in getting a solar system, you will need accessible shade-free areas. Generally, you are looking for a portion of south-facing roof that is free of shade from 9 am to 3 pm (or at least six solid hours) every day of the year. Ruinous shade may be cast by nearby buildings, trees, your chimney, or even your vent pipes.
You can get a quick and dirty measure by looking at your roof at 9 am, noon, and 3 pm on a clear winter day (even as late in the year as right now). You can get a perfect measure on any day during daylight hours using a Solar Pathfinder at all potential points of your rooftop. Its domed lid reflects all potentially shade-causing items. The chart beneath shows you which times of day during which parts of the year these obstructions will be causing shade at the spot where you're standing. It's totally cool and totally $200.
Solar panels are typically connected in series, like Christmas tree lights. If even one part of one panel is shaded, your total output could be reduced by over 80%.
Orientation
Ideally, your solar panels are always facing the sun. In real life, look for the best fixed location. Facing due south is a compromise between morning and afternoon sun positions that produces the maximum daily energy yield. But if you are selling excess electricity to your local power company and if they charge more during peak periods (as Austin might start doing in about 2 years), a more westerly direction will maximize your profits. Any orientation within 45 degrees of due south is acceptable by incentive programs.
The sun's path is lower in the sky in winter than in summer. Facing the same angle as your latitude (30 degrees in Austin) is the compromise that maximizes total energy production. But since energy costs tend to be higher (here, anyway) in summer than in winter, you might want your angle to be a bit flatter to maximize profit. In summer, 15 degrees is ideal in Austin. In practice, most systems are flush-mounted, and you will go with whatever your actual roof angle is. Your panels can be installed at a different angle, but a structural engineer will have to get involved to make sure that wind uplift doesn't become a problem. Also, you don't want the uplifted ends of some panels shading any of your other panels. Anything from 0 to 50 degrees can work.
So long as your orientation and tilt lead to energy production within 90% of optimal (or 80% in some places), your project can qualify for incentives.
Panels
How many panels can you fit in your space? Panels are generally installed in rows along a single surface, each panel having a portrait rather than landscape orientation. Panels are typically 80 watts and about 2 feet by 4 feet.
Yield
Now that you know these things, you can estimate what your cost savings would be using PV Watts calculator (use version 1). You need to enter the size of your system (typically 3-4 kW), your orientation, your roof's pitch, your location, and the amount your utility charges per kWh.
Incentives
Our city has a rebate program, our state is about to have some kind of program, and I think there may be federal tax incentives. In my town, the payback period is down to about 10 years. Our city will let us run the meter backward, but for any month one produces more energy than one consumes, we get paid only the fuel charge rate for the energy. Different parts of the country have vastly different economic factors.
My house
I have no idea what part of the roof is shaded--the two closest trees have come down, so it's less than before. We have extremely little south-facing roof, probably not enough for a single panel, so we'd probably go with a west-facing roof (if there aren't too many vent pipes). I think the instructor said that the roof angle of a typical ranch house has a pitch of 20 degrees. My preliminary guess is we could get a system with an efficiency of 89% of ideal. We get lots of sun here, but prices are low. I need to do more research.
The first presenter, Doug Soester (pronounced SOY-ster) gave us an amazing amount of information at all levels from basic physics to what he thinks about state-of-the-art microinverters.
Shade
If you're interested in getting a solar system, you will need accessible shade-free areas. Generally, you are looking for a portion of south-facing roof that is free of shade from 9 am to 3 pm (or at least six solid hours) every day of the year. Ruinous shade may be cast by nearby buildings, trees, your chimney, or even your vent pipes.
You can get a quick and dirty measure by looking at your roof at 9 am, noon, and 3 pm on a clear winter day (even as late in the year as right now). You can get a perfect measure on any day during daylight hours using a Solar Pathfinder at all potential points of your rooftop. Its domed lid reflects all potentially shade-causing items. The chart beneath shows you which times of day during which parts of the year these obstructions will be causing shade at the spot where you're standing. It's totally cool and totally $200.
Solar panels are typically connected in series, like Christmas tree lights. If even one part of one panel is shaded, your total output could be reduced by over 80%.
Orientation
Ideally, your solar panels are always facing the sun. In real life, look for the best fixed location. Facing due south is a compromise between morning and afternoon sun positions that produces the maximum daily energy yield. But if you are selling excess electricity to your local power company and if they charge more during peak periods (as Austin might start doing in about 2 years), a more westerly direction will maximize your profits. Any orientation within 45 degrees of due south is acceptable by incentive programs.
The sun's path is lower in the sky in winter than in summer. Facing the same angle as your latitude (30 degrees in Austin) is the compromise that maximizes total energy production. But since energy costs tend to be higher (here, anyway) in summer than in winter, you might want your angle to be a bit flatter to maximize profit. In summer, 15 degrees is ideal in Austin. In practice, most systems are flush-mounted, and you will go with whatever your actual roof angle is. Your panels can be installed at a different angle, but a structural engineer will have to get involved to make sure that wind uplift doesn't become a problem. Also, you don't want the uplifted ends of some panels shading any of your other panels. Anything from 0 to 50 degrees can work.
So long as your orientation and tilt lead to energy production within 90% of optimal (or 80% in some places), your project can qualify for incentives.
Panels
How many panels can you fit in your space? Panels are generally installed in rows along a single surface, each panel having a portrait rather than landscape orientation. Panels are typically 80 watts and about 2 feet by 4 feet.
Yield
Now that you know these things, you can estimate what your cost savings would be using PV Watts calculator (use version 1). You need to enter the size of your system (typically 3-4 kW), your orientation, your roof's pitch, your location, and the amount your utility charges per kWh.
Incentives
Our city has a rebate program, our state is about to have some kind of program, and I think there may be federal tax incentives. In my town, the payback period is down to about 10 years. Our city will let us run the meter backward, but for any month one produces more energy than one consumes, we get paid only the fuel charge rate for the energy. Different parts of the country have vastly different economic factors.
My house
I have no idea what part of the roof is shaded--the two closest trees have come down, so it's less than before. We have extremely little south-facing roof, probably not enough for a single panel, so we'd probably go with a west-facing roof (if there aren't too many vent pipes). I think the instructor said that the roof angle of a typical ranch house has a pitch of 20 degrees. My preliminary guess is we could get a system with an efficiency of 89% of ideal. We get lots of sun here, but prices are low. I need to do more research.