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slewing drive for solar tracker photovoltaic positioner


Slewing drive for solar tracker photovoltaic positioner 

Photovoltaic (PV) devices generate electricity directly from sunlight via an electronic process that occurs naturally in certain types of material, called semiconductors. Electrons in these materials are freed by solar energy and can be induced to travel through an electrical circuit, powering electrical devices or sending electricity to the grid.

Photovoltaics is the science behind the most popular form of harnessing solar energy. It is the process of converting sunlight directly into electricity. The photovoltaic (PV) effect was first observed in 1839. However, it wasn’t until 1954 that scientists were able to discover exactly how it works.

Historically, space programs were the largest supporters of PV technology, since the system was the best energy source for their satellites. The industry has since grown and you have probably seen PV systems used to power electronics, cars, houses, commercial buildings, and to supplement power grids. Due to increased efficiency, decreasing cost and increased environmental concern, photovoltaic installations have increased dramatically in recent years.

For solar tracking in plants, see Heliotropism. For solar telescope tracking, see Solar telescope.

Largest single axis tracker project in Asia- 172MW- Arctech Solar 8-megawatt PV plant using horizontal single axis tracker, Greec A solar tracker is a device that orients a payload toward the Sun. Payloads are usually solar panels, parabolic troughs, fresnel reflectors, lenses or the mirrors of a heliostat.

For flat-panel photovoltaic systems, trackers are used to minimize the angle of incidence between the incoming sunlight and a photovoltaic panel. This increases the amount of energy produced from a fixed amount of installed power generating capacity. In standard photovoltaic applications, it was predicted in 2008-2009 that trackers could be used in at least 85% of commercial installations greater than one megawatt from 2009 to 2012.

As the pricing, reliability and performance of single-axis trackers have improved, the systems have been installed in an increasing percentage of utility-scale projects. According to data from WoodMackenzie/GTM Research, global solar tracker shipments hit a record 14.5 gigawatts in 2017. This represents growth of 32 percent year-over-year, with similar or greater growth projected as large-scale solar deployment accelerates.

In concentrator photovoltaics (CPV) and concentrated solar power (CSP) applications, trackers are used to enable the optical components in the CPV and CSP systems. The optics in concentrated solar applications accept the direct component of sunlight light and therefore must be oriented appropriately to collect energy. Tracking systems are found in all concentrator applications because such systems collect the sun's energy with maximum efficiency when the optical axis is aligned with incident solar radiation.

Active solar trackers

Considering basic construction principles trackers can be devided into active and passive solar trackers. Electrooptical solar trackers are usually composed of at least one pair of antiparallel connected photoresistors or photovoltaic solar cells which are, by equal intensity of illumination of both elements, electrically balanced so that there is either no or negligible control signal on a driving motor. By differential illumination of electrooptical sensors a differential control signal occurs which is used to drive the motor to orient the apparatus in such a direction where illumination of the electrooptical sensors is equal and balance is restored. Such high accuracy trackers are intended mainly for concentrator solar systems. These trackers are complex and, therefore, expensive and unreliable. Active solar trackers based on clockworks or combining both principles exist.

Passive solar trackers

Passive solar trackers are based on thermal expansion of matter (gases) or on shape memory alloys (SMA). They are usually composed of a couple of actuators working against each other which are balanced by equal illumination. By differential illumination of actuators, the resulting unbalance of forces is used for orientation of the apparatus in such a direction where equal illumination of actuators and balance of forces is restored. Passive solar trackers, compared to active trackers, are less complex but they are working with lower efficiency and at low temperatures are not working at all.

One axis trackers

Different one-axis trackers solutions are available on the market. In table below different basic features of different one-axis tracker designs are presented.

Tilted N-S axis trackerRotation axis is tilted.

Polar axis trackersAxis tilt equals local latitude, incidence angle equals declination

Horizontal N-S axis trackerNo shadows in the N-S direction, low wind profile, suitable for flat land

Azimuth axis trackerAdapt well to uneven land

Two axis trackers

Two-axis tracker products include azimuth-elevation trackers and declination-hour angle trackers. Azimuth trackers can be pedestal mounted or with multiple support or wheel type support. In case of multiple support points wind resistance is better than for pedestal type trackers.

Azimuth-elevation trackerPedestal type: Pile or slab single point foundation

Azimuth-elevation trackerMore supporting points: lower wind profile, increased number of supporting points

Declination-hour angleLow wind profile

TABLE 2: Two-axis trackers, basic construction features

Photovoltaic trackers with mirrors

A combination of solar trackers and concentrators is the best possibility and can be used at small and at large scale photovoltaic systems. The simplest tracker/mirror design combines solar tracker with flat booster mirrors. Most common solutions include V-trough mirrors or "upside down turned" V-trough mirrors like presented on the pictures below. It is advantageous that soft concentrators for photovoltaics do not need highly specular expensive mirrors. Weather resistant mirrors with high total reflectance are required. The mirror can be made of rolled stainless steel sheet with special surface finish, of rolled aluminium alloy sheet (plated with pure aluminium) protected by a weather resistant polymer (PVF) film, of silver coated polymer (acrylic) film or sheet, of aluminium coated polymer (acrylic) film sheet, or silver coated hardened glass.



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