Evenly distributing the energy
Within REhnu’s receiver, or power conversion unit (PCU), are unique optics at the dish focus that take concentrated sunlight energy and apportion it evenly among the triple-junction cells. The optics comprise a ball lens and a concave array of optical funnels. Sunlight from the dish reflector enters the ball lens and comes to a highly concentrated focus near its center. On emerging from the ball, the light forms a concave image of the dish at ~400x concentration. Here, the array of optical funnels constructed to match the size and shape of the image captures all of the light. The light is further concentrated as it passes through the funnels, emerging at 1200x geometric concentration. Immediately behind the funnels are the triple-junction photovoltaic cells.
The funnels are sized so that all the cells receive the same power. This ensures that all cells generate the same electrical current, as required for efficient electrical connection. Because the funnel entrances lie at the dish image, the uniformity of illumination is preserved even when there are tracking errors, as caused by wind gusts. Another powerful feature of the optical system is that all the collected light is directed toward the active areas of one cell or another. No sunlight is wasted on the light-insensitive cell edges or gaps.
Triple-junction photovoltaic cells
Regular photovoltaic cells—such as the silicon cells that make up most conventional PV panels—have a single photoelectric junction and can only capture a small fraction of the energy of solar photons to make electricity. In contrast, triple-junction cells have three different junctions, which separately capture the energy of different wavelengths (colors) of light. The result is more than double the conversion efficiency of single-junction cells.
Triple-junction cells with close to 40% conversion efficiency under field-operating conditions are manufactured commercially by technology that is already mature. More advanced cells with conversion efficiency 44% have been demonstrated in the lab and will shortly be commercially available. It is anticipated that by 2020 cells will be made with 50% conversion efficiency.
Triple-junction cells are more difficult to manufacture than silicon cells and cost much more per unit area, but, when used with highly concentrated light, cost much less per unit power generated. When sold in the quantity needed to generate several megawatts, the cost is ~$5/cm2, which at 1200x concentration and 30% module conversion efficiency, works out to $0.16/watt. This is one sixth the power conversion cost of photovoltaic panels with single-junction cells, which operate without concentration.
Thermal management—Active cooling
To illustrate the power of concentrated light at the dish focus, we replaced the ball lens with a ¼-inch-thick steel plate and melted a quarter-sized hole in seconds. Even with highest efficiency cells available today, most of the sunlight energy still goes to heating the cells. This heat is removed by mounting the cells on a thin copper substrate which is actively cooled from behind by a closed-loop system. Liquid coolant is circulated behind the cells and through a fan-cooled radiator, as in an automobile. While this process is more complex than the simple aluminum heat sinks used in conventional CPV, it uses much less aluminum, about 10 kg per kW of electrical output power. The sealed cooling system consumes no water. The pumps and radiator fans within the power conversion unit are designed to be very mechanically efficient, so their power consumption is more than offset by the extra cell power that results from efficient cooling.