COMBINED HEAT AND POWER GENERATION WITH A HCPV SYSTEM AT 2000 SUNS

Abstract

In the framework of the FAE “Fotovoltaico ad Alta Efficienza” (“High Efficiency Photovoltaic”) Research Project funded by the Sicilian Region under the program PO FESR Sicilia 2007/2013 4.1.1.1, we have developed an innovative solar CHP system for the combined production of heat and power at the high concentration level of 2000 suns [1]. This work shows the experimental results obtained on FAE-HCPV modules and analyses the behaviour of the system. The solar radiation is concentrated on commercial InGaP/InGaAs/Ge triple-junction solar cells designed for intensive work. The primary optics is a rectangular off-axis parabolic mirror (with a size of 46x46 = 2116 cm2 in a projection normal to the sun), providing a nominal concentration greater than 2000 suns per cell. As secondary optics at the focus of the parabolic mirror we use a frustum made of BK7 glass glued in optical contact with a TaiCrystal cell (108 mm2 ). The total optical transmission in the relevant spectral range (320-1800 nm) is about 90% and the field of view on sky of the cell is a square with a full width at half maximum (FWHM) side of 2.6 degrees. The 1 kWe module prototypes consist of 20 multijunction cells, each one is integrated with the secondary optic and the active heat sink. Primary reflective optics and integrated receivers are mounted on a 2 axis tracker (Alt-Azimuth type) composed by a N-S primary axis supporting 10 E-W secondary rotation supports, each one is moving a couple of mirrors and receivers. The secondary axes are driven by a linear actuator via a parallelogram transmission. The system is highly scalable and it can be installed on roofs, urban areas or industrial spaces. The tracking algorithms are managed by a double control loop: an “open loop” algorithm (providing an accuracy of a fraction of a degree, limited by installation inaccuracies and by mechanical tolerances) and a “closed loop” system (based on a four quadrant sun sensor) providing a high tracking accuracy. The cell is connected to an active heat transfer system properly designed in order to reduce the thermal resistance of each substrate of connection material. The particular fluid dynamic geometry of the heat sink allows to keep the cell at a high level of electrical efficiency (ηel ≈ 35%), while bringing the heat transfer fluid (water and glycol) up to an output temperature of 90°C, suitable for civil and industrial low temperature applications. Accordingly with the experimental data collected from the first 1 kWe prototypes, the total amount of extracted thermal energy is more than 50% of the harvested solar radiation, that, in addition to a system electrical efficiency of about 30% [2], contributes to reach an overall CHP efficiency of 80%. By a variation of the flow rate from 0.2 l/min to 1.0 l/min of the cooling fluid on the heat sink it is possible to modulate both the thermal response and the electric efficiency of the system. The on field results confirmed a maximum electric power generated of 50 We per cell at a DNI of 850 W/m2 and 90 Wth at 85°C of the output cooling fluid temperature.