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Solar Thermal Photovoltaics
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How does panel orientation affect their output? This FAQ applies equally to solar thermal and photovoltaic collectors. Any solar collector will perform best when perpendicular to solar radiation (ie it should point at the sun). The problem is that the sun moves about, both across the sky through each day, and in elevation (height) through the year. Evacuated tube collectors are very efficient at dealing with daily movement, see here for more on this. An automatic tracking mount can make financial sense for PV collectors, typically showing yield benefits of 20-30%. However, for solar thermal it is usually much cheaper to increase panel area. The annual variation in the elevation of the sun is usually dealt with by choosing a mounting angle which is a compromise. The most common reason for non optimal mounting is lack of a more suitable roof. The following chart illustrates how energy collection will fall off as the mounting angle moves away from the ideal. The contour lines in the chart show boundaries of areas on the chart representing possible orientations where annual irradiation will be equal or greater than the figure shown. Figures on the left side show the angle of the collector from the horizontal, and those along the base show the deviation from due South in degrees. So a collector facing south at 30 degrees will have 1050 kWhr/m2 pa fall on it, whereas one facing East at 50 degrees will benefit from 800 kWhr/m2 pa. The good news, as can be seen from this chart, is that reasonable deviations from the optimum position do not degrade performance too badly. Notice that these are annual totals. Altering the elevation (mounting angle) of the panel will change the proportion of energy collected at different times of the year. Mounting closer to the horizontal will increase summer collection at the expense of winter, and more vertical mounting vice versa. For a large solar thermal array which often dumps heat in the summer, mounting at a steeper than normal angle might make sense as it would dump less heat in the summer and provide more in the winter, at the cost of a lower total annual figure.
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