Solar PV trackers are normally used with concentrators, especially dual axis trackers. Thus, they are required to move. Their movement causes a number of consequent problems.
- Load– they must carry a lot of load of the concentrator and arrays. They must be made strong enough to carry that load. This again adds to their own weight, which must be moved.
- Wind Resistance– The concentrator / arrays will be subject to wind forces. This demands strong structures, as well as control systems with loop gains tight enough to maintain the desired pointing accuracy in windy or gusty situations.
- Base Strength– Even when not tracking, the structure must be able to withstand any wind forces. Moving trackers and the allied systems invariably offer a much bigger wind profile than motion free trackers which could be made in a planar form and laid out horizontally.
- Power– Large masses demand electrical power for moving. Additionally, the control system will draw more power when facing a strong wind. The power consumed in the tracking and monitoring system subtracts from the solar PV output. Motion free trackers would avoid that.
Motion Free Visual Tracking
How can you track a moving object while standing motionless? Looks difficult. There are two possibilities to think about which could nearly fit the definition of motion free trackers.
- Keep your body and head fixed and move your eyeballs.
- Keep your body, head and eyeballs fixed. Somehow bend the rays from the object so that regardless of the direction of the tracked object the rays come into your eyes.
Motion Free Solar Tracking
Motion free trackers must also follow a scheme resembling one or more of these schemes. Depending on which scheme they follow, they can be absolutely motion free trackers or virtually motion free trackers.
- Holographic Motion Free Trackers
This type of motion free trackers is like the person who has the power to bend rays from all directions into his or her eyeballs. An optical structure made of an optically transparent material with stripes formed into it in a special pattern can do various things with rays of light. Among the various things such a structure can do to light rays passing through it is to bend them. This ability to bend light rays is used to focus light on to the active area of the photocell. A number of structures is placed such that light from the sun always falls on the cells regardless of the position of the sun.
Selective Focusing-Further, how much a given holographic structure will bend the rays depends on the wavelength of the light. The property to bend different frequencies (or wavelengths) differently can be used to advantage. Only rays in the wavelength band within the solar spectrum to which the cell is responsive can be focused on the cell. Energy from other wavelengths is directed away. This will avoid unnecessary heating of the cells.
Splitting For Multijunction Cells-Another scheme uses this property with multijunction cells. Multijunction cells have their individual junctions responding to different wavelengths. The holographic structure can be positioned such that light of the appropriate wavelength impinges on the relevant junction thus enhancing the efficiency. A discussion on the principle is available in the article: High and Low Concentrator Systems for Solar Electric Applications IV, edited by Lori E. Greene,Proc. of SPIE Vol. 7407, 74070E · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.829569.
- Electrically Steered Focusing
United States patent US6958868B1 was granted to John George Pender in 2005. The patent describes a one-dimensional beam deflector consisting of a pair of prism arrays made of a material whose refractive index can be altered by the application of an electric field. Varying the strength and distribution of the electrical field modifies refractive indices of the prism materials. The applied field is modified in such a manner that while the direction of the incident beam shifts with movement of the sun, the change of refractive index compensates the change and maintains the output beam in a fixed direction impinging on the target cell. Two such orthogonal systems can be made to control in two dimensions.
Partial Movement Concentrators
A near motion free concentrator has been described in the article: Innovative Solar Tracking Concept by Rotating Prism Array, published by International Journal of Photoenergy / 2014. It describes a horizontal array of parallel prisms which can be rotated together to keep the output rays vertical catering, say for the east-west motion of the sun. Below the array is another similar horizontal array but with prism axes oriented orthogonal to those in the first array. Thus, light deflection in two axes is possible. And the net output is a vertical parallel beam of sunlight spread over the area of the array. The prism axes are rotated to cater for the motion of the sun in two dimensions. The beam then passes through a Fresnel lens which concentrates and impinges it on the solar array.