Season of year

The height of the sun’s travel on the horizon varies by season of the year. In the summer, it is more directly overhead; in the winter, it is much lower. Figure 2.4 shows the different paths* of the sun over the sky for Washington, DC. The maximum incidence angle during (solar) noon for the location in this example is 75 degrees on the 21st of June (the summer solstice). The minimum incidence angle occurs on the 21st of December (the winter solstice). And the average incidence angle occurs on the 21st of March and the 21st of September (the autumnal and vernal equinoxes). For locations further north, these curves will be flatter, and for locations further south, steeper.

Thus, the amount of solar energy that can be collected varies throughout the year. Monthly available solar radiation values are used to determine the size of the solar collector system required to satisfy the heating demand. Figure 2.5 shows the variation of beam and diffuse radiation over the year for three locations. (The values apply to a collector surface tilted 40 degrees south facing.)

The sum of beam and diffuse radiation falling on a horizontal surface is usually called “global solar radiation.” The annual total in the figure above, 40-degree tilted south facing, for Fayette NC is ~507,504 Btu/y*sq ft (~1600 kWh/y*m2); and for El Paso, TX, 697,818 Btu/y*sq ft (2200 kWh/y*m2); and for Anchorage, AK, 272,783 Btu/y*sq ft (860 kWh/y*m2). This results in an annual daily average of 1,396, 1,935, and 761 Btu/sq ft*day (4.4, 6.1, and 2.4 kWh/m2)*day respectively. The fraction of diffuse radiation is 44%in Fayette, 21% in El Paso, and 53% in Anchorage. Day and seasonal variation can be very high; a nice summer day may provide as much as 2,030.02 Btu/sq ft*day (6.4 kWh/m2)*day in Fayette and 2664 and 1459 Btu/sq ft*day (8.4 and 4.6 kWh/m2 *day) in El Paso and Anchorage and a cloudy winter day may have as little as 63.44 Btu/sq ft*day (0.2 kWh/m2*day) on average in the month of December in Anchorage (Figure 2.6).