Mounting Systems

4.5.2.1 Mechanical Sizing of Mounting Systems

The factors that need to be taken into consideration when sizing a PV installation mounting system are the specific gravity of the modules or laminates (mass about 10 to 20kg/m2 and thus weight around 100 to 200 N/m2) and particularly the main sporadic snow and wind load forces that the installation is exposed to. As these loads vary from one region to another, only a general idea of these loads can be provided here. To calculate such loads precisely, the relevant national standards for the site in question should be consulted.

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Figure 4.60 Single-axis solar tracking system on a building roof. The modules are mounted in a north-south orientation on horizontal rods that rotate from east to west over the course of the day in accordance with the path of the Sun (Photo: Solon AG)

image446 Подпись: (4.14)

According to the applicable Swiss SIA standard, a minimum snow load amounting to pS = 900 N/m2 needs to be taken into account for the horizontal plane, although this value increases sharply with increased height above sea level (see Figure 4.63) and for very high elevations constitutes the predominant load. If a solar generator is mounted at tilt angle ( relative to the horizontal plane, snow load pressure is reduced accordingly, perpendicular to the module plane. The following snow pressures need to be taken into account for a solar generator at tilt angle (:

IEC 61215 [4.11] stipulates that solar generators must be rated for a snow load of 2400 N/m2. The value for heavy-duty models is 5400N/m2 for very snowy areas. However, the homogeneous test load that is used for tests in this regard often does not square with the real-world loads that inclined solar generators are subjected to (see Figure 4.64).

Подпись: Figure 4.62 SolarparkErlasee (12 MWp) near Wurzburg, Germany. This installation is composed of1500 dual-axis 6.5-9kWp (depending on the technology used) Solon Movers (Photo: Sunpower Corp.)

In the case of a solar generator with a tilt angle (to the horizontal plane, the uppermost snow on the generator often begins melting rapidly, whereupon some of it slides off the solar generator and piles up at

Подпись: Maximum Snow Load ps on Horizontal Surfaces (SIA 261) Figure 4.63 Maximum allowable snow load on horizontal surfaces according to the Swiss SIA 261 standard. The curve shown here roughly equates to snow load zone III in Germany in accordance with DIN 1055-5, Table 2 [DGS05] See [DGS05] for more detailed information concerning Germany
image450the bottom edge of the generator. As a result the solar module and frame loads are unequal (see Figure 4.64). The weight F = m ■ g exerted on the module consists of two components: normal force Fn which is at a right angle to the module surface and descending force FA on the solar module plane. At equilibrium, FN is offset by an equal and opposite reaction force FR and FA that stems from the opposing frictional force R, whereby R < ■ FN. Since between the snow and glass is relatively

small (particularly if the glass is wet), the snow can slide off during a thaw if the tilt angles are not abnormally steep.

image451 image452 Подпись: Snow Подпись: FN2 > F

For solar generators installed on pitched roofs, and particularly for laminates, the friction coefficient should be lower (for snow on a roof with the same pitch) than for pitched roofs with roof tiles. The energy yield is better the sooner snow slides off the roof, but also entails the risk of a roof avalanche that can cause personal injury or property damage if wet snow accumulates on a frozen roof surface and abruptly slides off. The risk of avalanches is high with roofs whose entire surface is covered with laminates, as in Figures 4.52 and 4.53. But roof avalanches can also occur with framed modules; in such cases the higher the building, the greater the risk. Hence precautionary measures aimed at preventing possible personal

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Figure 4.64 Typical snow load (shortly after snowfall) that a solar generator with tilt angle b is exposed to. If the adhesive friction coefficient mo < tan b (e. g. with an abnormally steep tilt angle b or on formation of a film of melted water) the snow can slide off the modules – although this can result in property damage or personal injury in the case of rooftop PV installations

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Figure 4.65 House roofs in Bavaria, Germany, covered with snow in spring 2006. Some rooftop PV plants incurred snow damage during this year (Photo: Schletter Solar-Montagesysteme GmbH)

injury and/or property damage should be taken for solar generators that are installed on pitched roofs. Such precautions include erecting barriers in the hazard zone after a snowfall and the use of snow collectors.

Figure 4.65 provides an impression of the possible mean snowfall in Central Europe (the figures given here are from the German state of Bavaria in the winter of 2005).

In addition to the risk of snow sliding off roofs, normal force FN can also induce module and/or rack damage, while descending force FA can damage the bottom edges of framed modules (see Figure 4.66). Modules can also be displaced by such forces.

image456

Figure 4.66 Solar module (in Germany) damaged by excessive snow load. One module was bent by an abnormally strong normal force, and in a number of modules the lower edge of the frame was torn off by descending force. This picture was kindly provided by the Schletter Company to illustrate the kind of snow damage that solar modules can incur (the system is not composed of Schletter products) (Photo: Schletter Solar-Montagesysteme GmbH)

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Figure 4.67 View of 109 arrays at the Mont Soleil PV system on 8 March 2009. The installation was almost buried under a deep snow covering, some of which had slipped downwards by the time this picture was taken. This snow (see Figure 4.64) increased the snow pressure on the installation, particularly at the very bottom. The storage capacity for the numerous snowfalls during the snowy winter of 2008-2009 was insufficient. Some of the arrays in the bottomrow of modules were still buried in snow at the end ofMarch 2009 (see Figure 4.44 forapicture of an array with little snow on it) (Photo: Pierre Berger, Mont Crosin)

These pictures show that the load tests called for by module testing standards [4.11] do not accurately simulate the actual snow loads that occur on pitched roofs, and that snow damage can occur even if a PV system passes such a test. A test involving unequally distributed loads and a test where module edges are subjected to shearing force with constant force per unit of length should also be conducted.

Modules destined for use in snowy areas should integrate sufficiently thick glass, as well as frames that can withstand the force exerted by snow. A sufficiently robust mounting system is also indispensable.

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Figure 4.68 A snow-free Mont Soleil installation array in late March 2009. Two bottom-row modules were damaged (snapped off) and one module in the centre was unable to withstand the snow pressure

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Figure 4.69 Close-up of a Mont Soleil laminate module that was damaged by snow pressure. Like most of the damaged modules, this one was on the bottom row, where the snow pressure is highest (see Figure 4.64). The laminate is bent in the middle and the two outer elements above the rack bars simply snapped off

Modules whose edges are not robust enough can be reinforced by adding an additional support element to the bottom edge of the solar generator.

SomePV plants incurred damage during the snowy winter of2008-2009, when, at the 1270 m elevation of the Mont Soleil installation (see Sections 4.5.1.1 and 10.1.2), 35 laminates incurred snow pressure damage that also induced a solar generator grounding fault (see Figures 4.67-4.70). It should also be borne in mind that, snowfall aside, wind can carry deep snow from higher to lower elevations and thus provoke local drifting.

Evidently the glass in these laminates was insufficiently robust for the peak snow pressure that the Mont Soleil installation was subjected to at this tilt angle (b = 50°), thus resulting in damage during the snowy

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Figure 4.70 Close-up of a Mont Soleil laminate module that was damaged by snow pressure that in turn induced a grounding fault (the damaged area is at the lower right). This module was also on the bottom row, where the snow pressure is strongest. Both outer segments of the laminate, as well as its middle portion, were pushed back and snapped off

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Figure 4.71 Alpine PV system module damaged by snow load (tilt angle 50°, elevation 1800 m). In this installation, both the lower modules and upper modules were damaged, i. e. apparently the entire solar generator was covered with snow for a time (Photo: Alpha Real AG/IUB)

winter of 2008-2009. Until such time as better snow load tests become available, modules installed in snowy areas should integrate glass that is thick and robust enough to withstand 5400 N/m2.

Figures 4.71 and 4.72 illustrate the kind of damage that massive snow pressure can cause at still higher mountain sites. These pictures show the damage induced by snow pressure at a ground-based solar generator (b = 50°, around 1800 m above sea level) that is used for lighting in the Sommeregg Tunnel on the Grimsel Pass and has been in operation since 1987.

For tilt angles of greater than 60° it is safe to assume that snow will slide off the modules, which means that snow load need not be factored into the static sizing for such installations. For installations in high

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Figure 4.72 Metal rack element (from the PV system in Figure 4.71) deformed by snow load (Photo: Alpha Real AG/IUB)

mountain areas, upwards of 60° tilt angles should be realized whenever possible so as to avoid the effects of snow pressure and so that snow load can be disregarded for sizing purposes. But it is essential to leave sufficient clearance for the snow to slide off modules, i. e. the modules should be mounted at a sufficient height above the ground since otherwise snow pressure damage could occur. However, at sites exposed to high winds a tilt angle of upwards of 60° does not guarantee that snow will immediately slide off the modules.

Tilt angles ranging from 75° to 90° have been shown to work well at extremely high Alpine locations. For example, as at December 2009 the installation realized in October 1993 on the Jungfraujoch (3454 m above sea level; see Section 10.1.3) had incurred no damage despite its extremely high altitude, owing to the 90° tilt angle of the system modules.

Wind load also needs to be taken into account. Wind forces, which rise four-fold as wind speed increases, are determined by the form, size and arrangement of nearby structures and by the direction of flow. Depending on the solar generator tilt angle and form, the windward side of the installation is exposed to wind pressure while the leeward side is exposed to an equal measure of wind suction, which should particularly be taken into account for roof installations.

Подпись: Dynamic pressure q —/ ■ dL ■ v2 Подпись: (4.15)

Using Bernoulli’s law, dynamic pressure can be determined as follows for a specific wind velocity v:

where dL is air density (which at 0 °C at sea level is1.29kg/m3).

According to the SIA 160 standard, a maximum of q — 900 N/m2 can normally be presupposed for dynamic pressure q, which equates to a wind velocity of about 135 km/h at 0 °C at sea level. The q value can be more than twice as high as this at sites that are exposed to strong winds (e. g. in high mountains, in coastal areas, or on very tall buildings), where higher wind velocities are observed. See [DGS05] for the relevant information in this regard concerning Germany.

Подпись: Wind force FW — cW ■ q ■ AG Подпись: (4.16)

Wind force can be estimated as follows using the value for dynamic pressure q, solar generator area AG and a direction of flow coefficient cW:

Depending on the direction of flow and the form of nearby structures, the solar generator cW ranges from about 0.4 to 1.6. The lowest cW values apply to factors such as wind suction at roof-integrated solar generators that are either on the leeward side at the centre of a pitched roof or completely in the wind shade of other buildings or generator elements. The highest cW values occur with ground-based PV installations that are inclined into the wind and that may be exposed to strong lifting forces. In such cases, the wind forces are at a right angle to the solar generator surface. Lifting wind forces are particularly dangerous for solar generators, and counteracting these forces usually entails greater expense than is the case with pressure forces.

The above values allow for a rough estimation of the forces that a solar generator will be exposed to. Hence the sizing of a solar generator mounting system should be mainly based not on gravity but rather on the wind and snow forces that the solar generator will be exposed to. More detailed information concerning solar generator wind load can be found in [Her92] and [DGS05].

Updated: August 6, 2015 — 2:09 pm