roofs and not the internal (other side) pressure appearing on open platforms. External pressure coefficients are only covering tilt angles up to 75°.

Another structures, closer to the case of tracking systems are the free-standing monopitch canopy roofs. In this case, instead of the pressure coefficients c_{p}, force coefficients c_{f }are presented (fig. 2) and they are considering wind pressure on both sides. The use of these force coefficients is limited by the range values (α = 0° ... 30°) of the tilt angle (angle between roof surface and wind direction) which is not covering all the functional positions of the tracking system’s platforms. Table I presents the maximal absolute values of the force c o e f f i c i e n t s c f , p r e s e n t e d i n [ 4 ] f o r t h e c o v e r e d t i l t a n g l e α = 0° ... 30°. s

c_{f }> 0

## Wind

α

F_{w }

## Fig. 2. Diagram of wind load on monopitch canopy [4]

α

0°

10°

20°

30°

90°

c_{f }

0.5

0.9

1.3

1.8

1.8

Table I – Force coefficients for the case of monopitch canopy

and signboards [4]

Another similar case with the case of tracking systems can be considered the case of vertical sign boards, perpendicular towards the wind direction (α = 90°), and the value recommended by [4] is presented in Table I. This value of the force coefficient can be used on tracking systems only for two specific functional positions – sunshine and sunset.

The Russian standard for wind actions on buildings [5] presents same cases and similar values with the ones presented by [4].

As a conclusion none of the standards are entirely covering the case of wind loads on the platforms of tracking systems.

Based on the rules set by the Spanish standard NBE-AE 88 Titan Tracker company presents the specific case of wind action on platforms with the whole range of tilt angles [6]. It gives recommendations for calculus of wind load based on distribution of pressure coefficients, like the one presented in Fig. 3, for angles between platform and wind direction in the range 0° - 90°.

Table II presents values of the resulted force coefficient resulted using the pressure distribution from [6]. The force coefficient is calculated, in this case of linear distribution, as the average of the minimum and

maximum pressure coefficients acting on the surface (fig. 3) with relation

c_{f }

c _{p1 }

c

_{p2 }

2

.

(2)

Fig. 3. Diagram of pressure coefficients distribution on tilted platforms [6]

## Table II – Force coefficients for the case of tilted platforms [6]

α

0°

10°

20°

30°

40°

50°

60°-90°

c_{f }

0

0.4

0.8

1.2

1.2

1.2

1.2

An experimental study and a comparison with a FEM analyze has been developed for wind action dual axes tracking systems and presented in [7]. The experimental work has been developed in a wind tunnel with a section of 1200x1200 mm, on an experimental model of panel scaled 1/3 towards the real model. Pressure coefficients and forces have been determined for tilt angle of the panel in the range 0° - 50°.

The differences between the values presented in Tables I and II can lead to the conclusion that further research must be developed based on experimental study. These results should help development of standard recommendations for the specific case of tracking systems platforms.

# 3. Experimental setup

This paper is presenting the results of an experimental study developed in the Renewable Energy Laboratory from Transilvania University of Braşov. A wind tunnel type HM 170 [8], known as „Eiffel” (fig. 4, a), has been used. This wind tunnel is educational, with open circuit, air being taken from the atmosphere and blown back out into the open.

A reduced scale model of an azimuthal tracker has been developed (fig. 4, b) in order to simulate the functional positions of the tracker in the wind tunnel.

The experimental model 3 is placed in the measurement section 1 (see. fig. 4, a), where a linear wind flow is obtained and where the wind velocity can reach up to 100 km/h.