Volume I
Chapter 2. Landfill Solar Rhode Island Renewable Energy Siting Partnership
Whereas irradiance refers to the rate at which power is delivered to a surface wattsm
2
, insolation
describes the total amount of power delivered to that surface over an interval of time watt-hoursm
2
Sargosis Solar Electric, no date. The amount of solar energy striking a surface is greatest when the surface directly faces the sun i.e., at a 90° angle Figure 2. As the
angle between the sun’s rays and the surface decreases to less than a 90° angle, as occurs during afternoon hours
, the amount of direct insolation also decreases. The earth’s orbit around the sun has a similar effect on insolation: during summer months, the suns’ rays hit the northern
hemisphere more directly closer to a 90° angle, resulting in a greater solar energy potential during that time of year. Lower latitudes receive a greater portion of their annual sunlight at
angles closer to 90°.
Ch. 2 Figure 2. Solar angles.
2.2 Climatic and Physical Siting Factors In addition to the raw solar resource available at a site, several other physical attributes of
a site determine its suitability as a location for solar PV systems. These considerations include rainfall, wind loading, snow loading, hail impacts, and temperature effects.
2.2.1 Rainfall
Rainfall has both negative and positive impacts on solar PV systems. The amount of rainfall is related to cloud cover, which decreases direct solar irradiance. Conversely, rainfall
helps to clean PV panels, which optimizes their efficiency. Average monthly rainfall at four locations throughout Rhode Island ranges from nearly 3 inchesmonth to over 5 inchesmonth
Weather Channel 2012. Rainfall in New England is sufficient to help mitigate the need for hand-cleaning of panels Stafford et al. 2011.
2.2.2 Wind Loading
Wind loading is the amount of force exerted per unit area by wind on a surface. It affects both the stability of the panel mounts and the panels themselves. Panels that are mounted higher
90° 90°
solar panel solar panel
more efficient less efficient
90° 90°
solar panel solar panel
more efficient less efficient
90° 90°
solar panel solar panel
more efficient less efficient
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Chapter 2. Landfill Solar Rhode Island Renewable Energy Siting Partnership
above ground to avoid shading from vegetation, or panels with higher tilt angles tend to be more exposed to winds, requiring stronger, heavier mounts to hold the modules in place Stafford et al.
2011; MassDOER 2012. These heavier loads can place greater strain on landfill covers and side slopes. PV panels themselves are typically certified to withstand maximum mechanical loads of
50 lbsft2, which equates to wind speeds of approximately 105 mph, or speeds typical of a Category 2 hurricane Sampson 2009;
NHC 2012.
2.2.3 Snow Loading
Snow loading on the PV array depends on a multitude of factors. These include the water content of the snow, depth of snow, cloud cover which prevents snow from melting, panel tilt
which, at steep angles, causes snow to slide off, and freeze-thaw cycles which can cause ice to build up on the modules and prevent snow from sliding off Stafford et al. 2011; MassDOER
2012. Snow loading generally has a more significant effect on the mounting structure than on the panels themselves. Typically, permitting for a solar project will require that a structural
principal engineer provide evidence that the supporting structure can handle a certain level of snow loads. Panels themselves are engineered to support maximum loads of 50 lbsft
2
, which is approximately equivalent to a 10-inch thick ice layer. Ground-level snow loads in Rhode Island
average between 30 and 40 lbsft
2
, within the range withstood by most PV systems ASCE, 2005. However, not all solar panel manufacturers cover snow damage in their warranties.
2.2.4 Hail Impacts
