Design Considerations: The following discussion is geared towards off-grid living, but can be equally applied to utility-intertied homes as well. First and foremost for solar electric to be at all feasible, one must ask “is there sufficient sunlight at the intended locale for the solar panels to do their job (i.e. generate ample battery charging current for the job at hand)?” - and this is not always as obvious as it might seem. PV panels perform optimally in full sun. Even partial shade, such as bare winter branches, will dramatically reduce panel output. Shade basically turns the solar array underneath it into a big resistor, an electrical “bottleneck” which reduces the panel's current flow into the batteries. Minimally, the panels should be in full sun a minimum of 6 hours per day, preferably from 9 am to 3 pm daily. This time frame is called the “solar window”. Since the sun's position in the sky changes from minute-to-minute and season-to-season, a marginal site may receive full sun all day in the summer, but hardly any in the winter, due to shading from trees and other ground obstructions. Instruments are available to determine a fixed position's degree of solar access at any time of year. A fixed PV array must be installed facing true south. True south is found by compensating for the magnetic declination of the Earth. In Northern New England, true south is approximately 15° to the west of magnetic south. When the sun is at its highest elevation in the sky (termed solar noon or 0° azimuth) it is positioned at true south. The tilt angle of the array (i.e. its angle with respect to the plane
of the earth) will also effect array output. Daily performance is maximized
when the sun's rays strike perpendicular (a 90° angle) to the panel’s
face.
Since the sun's "altitude" in the sky changes from season to season, the array's output can be optimized by changing the tilt angle several times a year. But for seasonal use applications, fixing the array angle at the applicable season angle is sufficient. This means that the ideal summer angle would be (in northern New England) 30 degrees, wintertime 60 degrees. It can also be a good idea to fix the array at an angle of 60° if unattended, because the amount of power lost in summer won’t be noticed, and snow will slide off easier in winter. Another factor, and this enters directly into system sizing, is the concept of solar insolation. This is a gauge of the amount of sun reaching the earth. Insolation varies world-wide due to atmospheric conditions and the Earth's variable distance from the sun throughout the year. Government agencies have created a unit of measurement of solar insolation called a peak sun hour, and for years have compiled data for hundreds of locations nationwide. For instance Burlington, Vt. has a yearly average of 3.5 peak sun hrs/day; Denver, Co., 5.7. In order to generate the same amount of power, a Vermont system would require a 60% larger array than the Colorado system, at a significantly greater cost. Local micro-climate conditions and reflected light from snow, clouds and water also serve to affect available sunlight in any specific locale.  |
