REhnu dish based CPV: Performance and reliability improvements based on a year of field experience

Abstract

REhnu has now built up a year of experience operating two M-8 CPV generators with dish-receiver architecture described in Stalcup et al. 2017 [1]. The M-8 generators use 8 primary collector mirrors made of backsilvered low-iron glass, 1.65 m square with a 1.5 m focal length. Small sealed receivers at each dish focus house 36 Solar Junction 3J cells operating at 950x concentration. Over the year of operation, a good measure of output power vs DNI and atmospheric conditions has been obtained. The efficiency varies through the day depending on air mass. The system is currently best matched to the solar spectrum at air mass 2. The daily peak CSTC efficiency averages 31%, with seasonal variation of ±1% through the year, peaking at 32% in August. Several improvements were made over the year to ensure reliability and to improve optical throughput. A dry air purging system has mitigated damage to the cells from moisture. Material upgrades to secondary reflectors in the receiver have resolved a runaway failure mode that went previously undetected due to inadequate testing time.

CPV generator with dish reflector and fly’s eye receiver

“…Multijunction PV cells, used at high concentration, are more efficient and already less expensive in terms of $/watt than silicon cells. For example, for cells at a 2015 cost of $35,000/m 2 [2], working at 950x in a system with 31% overall efficiency [1], the per watt cost is $0.12. Improved manufacturing methods, including 5x reuse of substrates [2], are projected to reduce cell cost to as low as $6,000/m 2 , or $0.02/watt.…”

“…We differ from other CPV dish or heliostat systems in that we use imaging optics in the receiver to make equal division of the concentrated light between the cells, in a way that gives good tolerance to mispointing in order to help keep down tracker cost. This path has already been adopted in the dish-based system used by REhnu [1], but in a departure from this and other previous dish-based CPV, our new configuration provides air cooling of the cells, avoiding the need for heat transfer by pumped liquid. There may be little value to such thermal energy when generated in utility scale electricity plants.…”