CPV trackers

What are CPV trackers?

CPV trackers

Photovoltaics is here, and CPV trackers are gaining ground firmly!  Because cost is important. With CPV trackers, less of costly semiconductor and more of less expensive mechanical equipment is required. People do argue whether CPV trackers will be needed in the long run. They argue that the falling prices of photovoltaic modules will make the complexity of concentration systems less relevant, and maybe, totally irrelevant. But as long as concentration is there,  CPV trackers is a requirement.

Why the Tracker?

The reason lies in the narrowing of the acceptance angle of the system. A flat panel can receive a fraction of the sun’s rays even when looking away from the sun. Theoretically, in clear space, or placed on high ground a flat panel fixed horizontally, ie facing the vertical can receive some direct light (other than diffuse light) even when the sun is about to set. We say that the acceptance angle of the flat panel is 180ᵒ. However, the fraction of light intercepted by the panel is a function of the aiming error. In fact, if the sun is ϴᵒ offset from the line normal to the surface of the plate, the fraction of light received is given by cosine of this angle ϴ. We define an acceptance function A(ϴ) as the fraction of the intercepted irradiance to the maximum available direct irradiance if the plate were directly facing the sun. Then, A(ϴ)= Cos ϴ. The loss function is then, L(ϴ)=1-A(ϴ)= 1-Cos(ϴ) For under 1% loss in intercepted energy, the aiming error ϴ can be as large as 8.1ᵒ. That is a big allowance. And if we are ready to accept a loss of 10%, the aiming error can be up to 8.1ᵒ.  In fact, the loss function increases quite slowly with aiming error up till about 45ᵒ, when it starts increasing fast.

Aiming Error in Two Dimensions

For a static solar panel, the aiming error occurs due to both the east west daily motion of the sun as well as the apparent annual shift of 23.5ᵒ north in summer and 23.5ᵒ south in winter. In the absence of the north south error, the average energy fraction accepted by the flat panel will be about  or nearly 64% plus the diffuse light which will remain available at all angles. The loss of about 36% (due to east-west motion of the sun) could be either ignored or eliminated by using a single axis tracker. The decision will remain a matter of cost and simplicity. The north-south annual motion can cause up to (1-cos 23.5ᵒ)100% or 8.1% loss near the summer and winter solstices. The annual average of this will be just about 5.5%. That is why PV systems without concentrators may use no tracker or use one which is just a single axis tracker compensating the daily motion only.

Loss Function in Concentration Systems

Concentrators narrow the acceptance angle of the system by a factor of square root (N) where N is the concentration factor. A concentration of 100 times means the energy density incident after concentration will be 100 times as strong as from the sun. Or, we can say it is equal to that of 100 suns together. Hence, it is usual to give the concentration factor in ‘suns’. One 100 times concentration means a resultant irradiance of 100 suns. A concentration of 100 times can be achieved by narrowing the beam 10 times in each orthogonal dimension. Thus, a concentration of N times means the acceptance function is also narrowed by times. We can say: F_conc (ϴ) = Cos {ϴ x square root (N)} That is why the output of concentrated photovoltaics is so sensitive to tracking error. For what aiming error should we expect a loss of 1% in direct irradiance? To see that put: F_conc (ϴ) = Cos {ϴ x square root (N)} = Cos (10 ϴ) = 0.99 The result is ϴ = 0.81ᵒ. An offset of merely 0.81ᵒ with 100 suns concentration causes a power loss which in an unconcentrated PV system would tolerate an error of 8.1ᵒ. The aiming error for 10% loss in 100 suns concentration is given by ϴ= (1/10) Cos−¹ (0.9) = 2.6ᵒ.

What Can Affect CPV Tracker Accuracy?

Now that we know tracker accuracy is essential for concentrated photovoltaics, we should think of the factors that affect a tracking accuracy.

The Concentration Factor

The extent of concentration affects the loss function as defined above. For a fixed loss percentage, the tracking accuracy requirement tightens by square root (N). For example, increasing the concentration from 100 suns to 400 suns would demand that for 1% loss the tracking accuracy be under ϴ = (1/20) Cos−¹ (0.99) = 0.4ᵒ.  

The Environment

Tracking accuracy is subject to effects of the environmental changes. It is well-known to engineers that temperature changes can affect the position / angle sensors.  

Wind

High speed winds can affect tracking accuracy seriously. Wind affects the tracker two ways. Flexing of the structures supporting the optical systems can distort the acceptance cone thus reducing the interception of energy. It can also cause a pointing error in the CPV tracker.

Sand and Dust

While dust has a general degrading effect on electronic systems it can also settle on the optical surfaces and degrade their performance, in case of both mirrors and lenses. Sand, blown against these surfaces will also permanently damage the optical surfaces.

Temperature

Very low temperatures will cause problems in hydraulic actuators where used. High temperature can cause fluid leaks. High temperature will also affect the control system. Sensors feeding the control system will be mostly semiconductors and hence their performance is subject to variations due to variation of temperature.  

Layout and Layout Density-GCR

Layout does not affect CPV trackers performance directly. But in an effort to minimize costs, particularly where cost of land is high, GCR (Ground Coverage Ratio, the ratio of optical surfaces area to area of ground occupied by the entire systems) has to be increased. Adding systems close together can cause problems of shading especially in the higher latitudes and when the sun is low. At times of shading pre-programmed software can help reduce shadowing by intentionally offsetting the aim slightly. This is called Backtracking. The software decides when and how much backtracking is required. The decision is based on how much loss in output due to shading would be avoided at what loss caused by backtracking.  

Other Factors

Like all other engineering equipment, CPV trackers will be affected by all those other minor factors which are known to affect other equipment. For example, operation of any equipment will gradually degrade its performance, especially when operated under field conditions.  

Photovoltaic Technologies Used

Multiple types of conventional and emerging PV technologies are being used. Monocrystalline silicon photovoltaic cells have been in use for a long time. Newer costlier materials like Gallium Arsenide single junction cells are also used for higher yield. But the extra cost of trackers does encourage us to use the even costlier but more efficient multijunction solar cells. Multijunction solar cells extract energy from a broader part of the solar spectrum and hence, are more efficient. On the other hand, because of their cost, they are better justified with concentrated photovoltaic systems.  

Types of CPV Trackers :

Single-axis Trackers

Single-axis solar trackers have only one axis of rotation which is normally aligned to the true north. In this way, the concentrator tracks the east-west motion of the sun only. Obviously, these are suitable only for low concentration systems as is obvious from the calculations earlier in this article. Several implementations are possible for single axis trackers. Examples are HSAT (Horizontal Single Axis Tracker), HTSAT (Horizontal Single Axis Tracker With Tilted Modules),VSAT (vertical single-axis trackers), TSAT (tilted single-axis trackers), and PSAT (polar aligned single-axis trackers).  

Horizontal Single-Axis Tracker (HSAT)

The rotation axis of HSAT (horizontal single-axis tracker) is, as the name suggests, horizontal with respect to the ground. With all end posts being at the same height, these can be shared between axially adjacent trackers and thus cost of installation gets reduced. In fact, a single long tube can carry many trackers mounted on bearings. Additionally, rows of coaxial trackers can be made parallel thus reducing the area requirement, again reducing overall cost. The simple geometry of layout permits better ground coverage ratio. However, too dense a packing can cause shadowing during hours when the sun is low. Backtracking under computer control can help reduce the overall energy loss. The use of horizontal single axis trackers is limited to equatorial latitudes only.  

Horizontal Single-Axis Tracker with Tilted Modules (HTSAT)

The HSAT is not suitable for higher latitudes because the mean north-south position during the year is not zero, while the modules are mounted at 0°. This means an unacceptable loss of energy. The HTSAT compensates for this by mounting the modules tilted up by an angle equal to the latitude. Thus, at a latitude of 20ᵒ North, the polar (north) end of the module must be tilted up by 20ᵒ. Since the modules are tilted with respect to the horizontal, they cover less space on the ground improving the Ground Coverage ratio.  

Vertical Single-Axis Tracker (VSAT)

The axis of rotation for a vertical single-axis tracker (VSAT) is along the local vertical with respect to ground. They rotate east to west morning till evening, all the time with respect to local the ground thus facing the sun throughout the day (except for the error due to annual north south shift of the sun). Mutual shading must be considered in the layout plan.  

Tilted Single-Axis Tracker (TSAT)

These trackers have their axes neither horizontal nor vertical but tilted from the vertical according to the latitude of the location. The modules face parallel to the axis of the tracker. Thus, the modules face the sun. While they track the daily east west motion of the sun according to the latitude perfectly, the annual north south motion of the sun relative to the normal to the surface of the concentrator remains limited to 23.5ᵒ north and south. However, tilt angles may be limited to reduce the wind profile and limit the height of the elevated end. Backtracking may be employed to reduce shading effect of close packing.  

Dual-axis Trackers

Dual-axis trackers have two degrees of freedom, that is, they can rotate about two orthogonal (perpendicular to each other) axes. Thus, they can track the east west motion as well as the north south motion. One axis is fixed with respect to the ground. It is the primary axis while the other axis that is normal to the primary axis is considered as the secondary axis. Most often, dual-axis trackers have their modules parallel to the secondary axis of rotation. Because they can follow the sun both vertically and horizontally, these trackers will point to the sun wherever it may be in the sky. Thus, they allow for optimum solar energy extraction. Several common implementations of these dual-axis trackers exist. Two of the common implementations are (1) tip-tilt dual-axis trackers (TTDAT) and (2) azimuth-altitude dual-axis trackers (AADAT).  

Tip-Tilt Dual-Axis Tracker (TTDAT)

The tip-tilt dual-axis trackers (TTDA) have the panels mounted on the top of a pole. On top of the rotating bearing is a T- or H-shaped mechanism that provides vertical rotation of the payload and provides the main mounting points for the array. The posts at either end of the primary axis of rotation of a tip-tilt dual-axis tracker can be shared between trackers to lower installation costs. Tip-tilt dual-axis trackers make field layout very flexible. For proper positioning of the trackers with respect to each other all we require is to keep the rotation axes parallel to one another. Mutual shadowing when the sun is low in the sky would demand the trackers a fairly low positioning density. A balance between loss due to shadowing and optimal tracking can be made by reducing slightly the tip-tilt angle  

Azimuth-Altitude Dual-Axis Tracker

In the case of an azimuth-altitude (or altitude-azimuth) dual-axis tracker (AADAT) the primary axis (or the azimuth axis) is vertical to the ground. Thus, the secondary axis, (the elevation axis), is usually normal to the vertical primary axis. AADATs operate somewhat similar to tip-tilt systems, but there is a difference in the way the array is rotated for tracking of daily motion of the sun. The array is not rotated around the top of the vertical pole, but a large ground mounted ring supports the array mounted on a number of rollers. Thus, the loading on the vertical pole is shifted to the ground mounted ring. Hence, much larger arrays can be supported.  Diameter of the ground ring will, of course, affect the minimum spacing of adjacent trackers.