Reliability

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Well proven silicon solar panels and permanent magnet D.C. motors are used in the tracker (neither exotic technologies nor hazardous materials like freones are used).

Heavy duty low speed (500 rpm) D.C. motor is completely protected against overload. During accelerated tests the driving unit performed 10,000 tracking cycles representing 30 years of field operation. Negligible wear only has been obseved on components of the drivig unit. Rated lifetime of brushes of the motor is 2000 hours at 5000 rpm i.e. 600 millions of revolutions while the motor will perform only 6 milioms of revolutions during 30 years lifetime. Unreliable and expensive components like batteries and driving electronics have been completely eliminated. It improves reliability substantially.

Nearly constant incident angles of solar radiation during working period (compared to fixed arrays) frequently enable to use tracked solar arrays without electronic Maximum Power Point Tracker (MPPT) because variation of the working point of the tracked solar arrays can be kept closer to optimum values.

Energy produced by tracked arrays fits much better to daily load diagram than that of fixed arrays. Tracked array delivers energy also at early morning and late afternoon when fixed array delivers negligible power. It enables more battery-less applications. Even when batteries are used their capacity can be lower. Because the efficiency of usual lead-acid batteries is <80% the efficiency of the system with limited battery use is higher compared to system where substantial amount of energy have to be stored [7].

Use of tracked solar arrays therefore enables more frequently their direct coupling to energy consuming devices without costly and unreliable batteries and MPPT’s.

Because batteries are, together with MPPT’s, the least reliable components in PV systems, the tracking PV systems without batteries and MPPT are more reliable and less expensive than fixed PV ones.

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Wind load

The solar trackers which are used more frequently than ever before now have to work for a minimum of 20 years with minimal maintenance if it should outperform fixed stands. Because dealing with a device which is mounted outside it has to meet international safety requirements, including wind speed. It raises reliability requirements strongly.

A wind of speed v in air of density r and acting perpendicularly upon rectangular area S, produces a force Fgiven by
F = cx S r v2 / 2 .
cx is the coefficient of the resistance of air for a rectangular planar area S, which is oriented perpendicular to the flow. The value cx is approximately cx = 1,2 for the common rectangular PV arrays.

The formula clearly shows that designing PV arrays local wind speed have to be carefully measured because aerodynamic forces on PV arrays are proportional to square root of the wind speed. At wind speed v = 160 km.h-1 the aerodynamic force on PV arrays is nearly double compared to the aerodynamic at wind speed v = 120 km.h-1. At wind speed v = 160 km.h-1 the aerodynamic force on PV arrays is also approximately an order of magnitude higher than the gravity force acting on typical solar arrays.

Wind flow is, however, never perpendicular to PV arrays and additional forces like aerodynamic drag influence resulting forces. Edge effects have to be also considered. It is the reason theoretical calculations are difficult and inaccurate. It is more effective to perform wind tunnel tests and tests in real environment.

The international standard ENV 1991-2-4 „The fundamentals of designing and loading of the constructions, part 2-4 Loading of the constructions by wind“ speaks about resistance of constructions to wind. The whole territory of Europe is never subject to speeds exceeding v = 160 km.h-1 near ground level. That is why the device was tested in a wind tunnel up to this particular wind speed.

The forces acting upon the solar arrays were to this date either theoretically calculated, or were submitted to static or dynamic testing by simulated weights. We have not yet found mention in any literature about testing any full size stand with mounted solar panels in an wind tunnel.

Tests of wind load durability were performed in June 1998 in a large wind tunnel with a diameter of 3 meters, which is located in the Aircraft Research and Testing Institute in Prague. This institute has the authorization to perform aerodynamic testing.

The tracking stand TRAXLETM for two 55W photovoltaic panels has protection against wind by a self-locking transmission with a maximum torque of M = 500 N.m, and is designed in a way that lets it withstand wind of more than v = 160km.h-1. The whole solar system was placed successively into four positions relative to the direction of wind. These directions were: perpendicular to the front, perpendicular to the back, perpendicular to the side, and sideways to the back with and angle of 45°. In each positions we slowly increased the speed of the wind up to v = 160km.h-1, and then let it affect the solar tracker for the time t = 3 min.

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The solar system during the tests in the wind tunnel

The whole structure was very stable and had minimal vibrations during the wind tunnel tests described above. The air flow around the solar systems also behaved calmly. The system kept it’s stability, even when we raised the wind speed to v = 180 km.h-1 for a short time. The tests did not damage the solar tracker in any way. We judge this from the fact of it’s working properly after the test.

At the very first step we have not been able to perform wind buffeting tests, however, we intend to test soon the TRAXLETM in the wind tunnel with simulated buffeting to make resulting forces on PV arrays close to that acting in natural windstorms. Reduced size models will be also tested to prove also our biggest TRAXLETM carrying up to 52 of 55W PV panels.

In December 1998 1kW tracking system TRAXLETM was installed at ITER test site (Tenerife, Canary Islands) nearby wind farm where strong buffeting winds are very frequent. One year test is just running. The tracking solar system at Tenerife is shown in the Picture Gallery.

Tracking PV structures have to be designed more carefully with respect to the wind load than fixed ones. By passive (freone) trackers there is usually no self-locking protection against wind buffeting. Driving units of active trackers are frequently protected by self-locking transmissions, however, even by these transmissions some backlash/free play always occurs.

Wrong design of self-locking transmission could lead to its damage due to vibrations caused by wind buffeting. Our practical experience show that transmissions of active solar trackers should be designed with safety factor at least 3 for given maximum wind speed.

The solar tracker TRAXLETM has withstood forces that are caused by stable winds of a speed of v = 160 km.h-1 with reserve. It therefore meets the international norm ENV 1991-2-4 „The fundamentals of designing and loading of the constructions, part 2-4 “Loading of the constructions by wind“. It also meets the safety criteria for mounting outdoors.

Designing PV arrays both fixed and especially tracking it is strongly recommended to measure carefully local wind speed because wind force represents major force acting on the PV array.