Receivers and cooling

The square primary reflector, 3.1 m (10 foot) on a side, with small central obscuration, focuses about 8.7 kW of concentrated sunlight onto a receiver (for 1 kW/m2 solar input). The net yield of electricity to the grid is 2.5 kW. Much of the remainder goes into heating the cells, which are actively cooled.

Sharing out the concentrated light equally

In the receiver, the concentrated light passes through a window into a small sealed chamber containing many cells. Optics in the receiver apportion the incoming light evenly among the cells in two steps. First, the window is made in the form of a ball lens, which maps out the light from the intense focus within the lens into a small concave image of the primary reflector. At this image, which is 6" square, the light is concentrated about 400x and is stabilized against any mispointing of the dish, caused by tracking errors. In the second step, the image is divided by secondary reflectors into areas of equal power, and the light in each area is relayed to one triple-junction cell. The relays increase the concentration to 1000x while preserving good uniformity of cell illumination. A powerful feature of the optical system is that all the light is directed toward the active areas of one cell or another. None is wasted on the light-insensitive cell edges or beyond.

Removing the heat

Liquid coolant is circulated to carry heat from behind the cells away from the receiver to a heat exchanger with forced-air cooling, in the same way that heat is removed from an automobile engine. The REhnu active cooling system is designed to be very efficient to improve the photovoltaic power output—a 2% increase results from each 10°C reduction in cell temperature. The extra power from efficient cooling more than offsets the electric power used to drive the coolant pump and air fan. During intervals when the sun is obscured by intermittent clouds, the coolant pump is switched off to minimize thermal cycling (to about 15°C), and hence any long-term thermal fatigue.

As a matter of good environmental stewardship, the cooling system uses very little aluminum. This minimizes the carbon dioxide emitted in aluminum manufacture, which is very energy intensive. The radiator fins, the only aluminum in the entire system, weigh only 10 kg/kW. The coolant circulates in a closed system, with no consumption of water.

End-to-end test at 1000x concentration

A prototype made by the University of Arizona demonstrates key features of the optical and cooling systems. A primary reflector made at the Mirror Lab was used with receiver optics (ball lens and secondary reflector), to illuminate a 15 mm square Spectrolab triple-junction cell at a geometric concentration of 980x. Curves of electrical current vs. voltage were recorded with an incoming solar flux of 925 W/m2 (direct flux at normal incidence). The secondary reflector collected incoming sunlight over a trapezoidal area of 0.22 m2, one of 15 segments of a 3-m diameter annular ring.

Under 980x concentration, the cell temperature was measured at 60°C for a coolant temperature of 41°C. The electrical power output under these conditions was 56W, indicating an end-to-end system conversion efficiency of 27.5%. This is consistent with Spectrolab’s measurements showing the cell efficiency of 33% at 60°C, and an overall reflection and transmission loss of 17% in the concentrating optical system.

The sensitivity of the output current of the UA prototype to tracking inaccuracy was measured by pointing away from the sun. For off-axis angles of up to ¾° the drop in current was no more than 10%. This is very good performance for a system with nearly 1000x concentration.

Ball lens at dish focus with single-cell receiver marked by arrow

Detail of triple-junction cell and secondary reflector on an actively cooled mount

Sensitivity of receiver to mispointing, measured at 980x concentration