Positioning system
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The positioning system can be viewed as a chain of referencing elements that starts at the sun position and ends at the collector receiver. In fixed mirror concentrators this chain is different from moving reflector systems such as parabolic dishes or parabolic troughs. In those systems the receiver is connected directly to the reflector through a fixed structure. Therefore, using closed loop control systems, the chain can be reduced to:
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The precision of the relative position of the sun to the reflector is mainly determined by the precision of the sensor used. Thus, in the manufacturing process it is only necessary to reference accurately the reflector to the receiver, two elements that have no relative motion. In fixed mirror concentrators the former reference chain is slightly different:
In this case, the reflector and the receiver have relative motion and therefore should be linked through the tracking mechanism. Then, it is necessary to precisely reference the tracking mechanism to the reflector and the receiver to the tracking mechanism.
Another particular characteristic of the fixed mirror concentrators is that the average focus distance for a given reflector width is higher than in moving reflector systems. This fact increases the precision requirements of both the tracking mechanism and the mechanical referencing system. Furthermore, this large focus distance could be a handicap to building integration in those cases where the visibility of the collector system is not desired.
Given those special characteristics two main design choices were made:
1. The width of each reflector was reduced as much as possible (546 mm in the current prototype)
2. In order to reduce the cost of the tracking mechanism the reflectors and the receivers were arranged in arrays of eight lines of evacuated tubes sharing the same tracking and positioning structure.
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The figure 5 shows a drawing of the final design of the current prototype. The tracking mechanism consists of four rotating arms that support and reference eight receiver rows. The receivers are standard “Sydney” evacuated tubes of 1.5 m in length and an absorber 47 mm in diameter.
