Proven Results

The University of Arizona team proved the technology in an end-to-end system test in which 8 small cells generated 500W of solar power, with high tolerance to wind buffeting.

End-to-end test in which a reflector with 4 rectangular segments powers a receiver with 8 triple junction cells. The receiver’s ball lens glows with concentrated sunlight in this first test, which yielded 500W of DC power.

Power Generation

An end-to-end test was made using a partial paraboloidal reflector with 4 rectangular, back-silvered segments, as illustrated. The reflector area is 22% of a full 3.1 m square paraboloidal dish.

In the test receiver, the image of the 4 segments formed by the ball lens falls onto four pairs of adjacent optical funnels, instead of the full set of 36. Behind the 8 funnels are eight 15 mm square cells, totaling 3 square inches, operated at the full geometric concentration of 1200x.

The DC power output of the 8 cells connected in series was measured at 511 watts. This is at the optimum power point of 24.4 A and 20.9 volts shown in the I-V curve. The same curve indicates an open circuit cell voltage 3.05V, and hence a cell operating temperature of 60°C, only 20°C above the high ambient air temperature. From the measured DNI of 940 W/m2, the end-to-end efficiency is calculated at 25%.

These results scale up for a full receiver with 36 instead of 8 cells, powered by a full, single-piece dish, to an output close to the projected 2.5 kW at 1000 W/m2 DNI solar flux. Tests of such a receiver and dish are projected for early 2011. Efficiency improvements made later in 2011, from higher efficiency optical coatings and more efficient cells optimized for the 1200x concentration, will ensure the 2.5 kW target is met or exceeded.

Tolerance to wind buffeting

The power-stabilizing property of the optical concentrator system was measured by switching off the tracking drive. The output measured as the sun moved across the sky, passing through the system axis, is shown in the figure at right. We find an angle of acceptance for power >90% of its on-axis peak of ±0.6°. Such a wide tolerance is enough to maintain full power in all but the strongest wind gusts, because the spaceframe module and its support has shown to be very stiff, despite its lightweight construction. During a month of daytime solar tracking, the prototype spaceframe module loaded with tarps was measured to point within 0.1° of the sun for over 99% of the time. Thus even in a less sheltered location on a solar farm, we can expect very small loss to wind buffeting.

The first power-generating prototype is installed, with mirror segments at left and receiver at right.