Photovoltaic Trackers

To increase solar yield and electricity production solar trackers and concentrator systems are sometimes used. Large portion of the text published on this site is published with permission of Traxle Solar. Further information about solar trackers can also be found on Traxle Solar web pages. A combination of trackers and plain concentrating mirrors as presented on the pictures below is the simplest possibility and can be used at small photovoltaic systems as well at large scale PV power plants.

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.

TABLE 1: One-axis trackers, basic construction features

Polar axis trackers in large scale PV power plants in Spain and China
(courtesy: Traxle Solar)

One-axis azimuth trackers (courtesy: Titan Tracker)

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.

Two-axis trackers: right pedestal mounted tracker, left elevation-hour mounted tracker
(credit: pvresources)

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.

Polar axis trackers with concentrators (courtesy: Traxle Solar)

One-axis azimuth trackers, with mirrors
(courtesy: Centro Ricerche ISCAT)

Bifacial tracking concentrator

Bifacial PV modules are recently delivered by several manufacturers. Because there is either no or low price difference in the price of such bifacial and single sided modules it is reasonable to use bifacial modules which could produce 5-20 % more energy (in comparison with monofacial PV module with the same nominal output power). Polar axis solar tracker and/or tracking concentrator is always mounted on high support structures (to avoid contact of the rotating PV array with the ground). It improves back side energy collection in comparison with fixed PV arrays or even roof integrated PV modules. The energy gain can be in the range 10-15 % (for typical albedo 0.3). Air-flow around the modules and corresponding cooling is improved as well (especially in comparison with roof integrated modules).

Bifacial concentrators (courtesy: Traxle Solar)

Web Sites

DEGEREnergie - Single-axis active tracking systems for flat roofs, facades, dumps, and open spaces Dual-axis, active tracking systems for free-standing applications, on buildings and for fitting on different mast heights.

Traxle Solar - the company was established in 1994 by group of solar energy professionals with long term engineering skil.

TITAN TRACKER - TITAN TRACKER breaks from the usual trend of mounted-pole systems and offers remarkable advantages in the complete life-cycle of the installation

Sources and Additional Information - Books

	Libra, M., Poulek, V. (2001), Bifacial PV Modules in Solar Trackers and Concentrators Physical Methods in Agriculture; Proceedings of the International Conference, Prague, August 27-30, 2001; Kluwer Academic/Plenum Publishers, New York.
	Libra, M., Poulek, V. 82006), Solar Energy; Czech University of Agriculture in Prague, Prague, ISBN 80-213-1489-3.

Reports

Shingleton, J. (2008), One-Axis Trackers – Improved Reliability, Durability, Performance, and Cost Reduction; Final Subcontract Technical Status Report, Shingleton Design LLC, NREL. Subcontract Report, NREL/SR-520-42769.

Papers

	Rubio, F.R. et al. (2007), Application of new contol strategy for sun tracking, Energy Conversion and Management 48, pp.2174-2184.
	Rauth, H.U. et al. (1995), Annually generated electricity of one and two axes solar tracking systems, Proc. 13th European PV Solar Energy Conference, Nice, 23-27 October 1995, pp.1015-1018.
	Poulek,V., Libra,M. (1998), New solar tracker, Solar Energy Materials and Solar Cells, 51, 2, 1998, pp.113-120.
	Poulek,V., Libra,M. (2000), A New Low Cost Tracking Ridge Concentrator, Solar Energy Materials and Solar Cells, 61, 2, 2000, pp.199-202.
	Klotz, F.H. (1995), PV systems with V-trough concentration and passive tracking concept and economic potential in Europe, Proc.13th European PV Solar Energy Conference, Nice 23-27 October 1995, pp.1060-1063.
	Nann, S. (1991), Potentials for tracking photovoltaic systems and V-troughs in moderate climates, Solar Energy, Volume 45, 1991, pp.385-393.
	Nostell, P. et al. (1998), Ageing of solar booster reflector materials, Solar Energy Materials & Solar Cells, 54, 1998, pp.235-246.
	Perers, B. et.al. (1994), Intensity Distribution in the Plane From Structured Booster Reflectors With Rolling Grooves and Corrugations, Solar Energy, Volume 53, Issue 2, pp.215-226.
	Schissel, P. et al. (1994), Silvered PMMA reflectors, Solar Energy Materials & Solar Cells, 33, pp.183-197.
	Stacey, R.W., McCormick, P.G. (1984), Effect of Concentration on the Performance of Flat Plate Photovoltaic Modules, Solar Energy, Volume 33, Issue 6, 1984, pp.565-569.

Other

	Erik Rossen's personal pages. Some interesting information about solar concentrators and heliostats.
	Red Rock Energy, Heliostats.
	Red Rock Energy, solar trackers, electronic cirquits and design instructions.