CHP EFFICIENCY OF A 2000 × CPV SYSTEM WITH REFLECTIVE OPTICS

A combined heat and power (CHP) solar system gives both the electricity generated by photovoltaic (PV) cells and the thermal (T) energy in the form of hot fluid (typically water) by heat exchangers behind the cells. Capturing the waste heat increases the overall efficiency of the system and justifies the efforts to develop the PV/T hybrid technology. Actually, many commercial PV/T systems use flat Silicon plate collectors to obtain an electrical output with low efficiency (10-20%) and extract heat suitable for domestic water heating but inadequate for industrial uses or to drive absorption chillers for polygenerative applications. In this context, the combination of a concentrating photovoltaic (CPV) system based on multijunction solar cells (with electrical efficiency above 40% ) and an active heat exchange circuit can significantly increase the total solar power conversion efficiency [1]. 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 realized a CHP prototype system with the following features (for the details see ref. [2]):  A reflective optics based on a point-focus rectangular off-axis parabolic mirror with aperture area of 2116 cm2 and a secondary optics in BK7 glass attached in front of a triple junction InGaP/InGaAs/Ge solar cell with active area of 108 mm2 , providing a concentration ratio of about 2000 suns; A liquid cooling system based on de-ionized water (flow rate in the range 0.2 ÷ 1 liters/min) passing across an aluminum heat sink with a slot nozzle and a planar output. When the water comes out of the slot nozzle, a sheet of water flows on the rear of the triple junction solar cell, with an effective heat transfer. The innovative heat exchanger combines a high performance and reliability with a low cost of production. Our test bench allows to record temperature, pressure and flow rate of the cooling liquid together with the electrical performance of the cells. We have studied the CHP efficiency and the partitioning between the electrical and thermal power; the experiments have been carried out in sunny days when the DNI measured in the laboratory was about 600 W/m2 using the equipment described in ref. [3]. In particular, to simulate a common thermal application, we have changed the water input temperature (Tin) in the range 35÷60 °C, and we have investigated the coolant-flow-rate dependence of the CHP efficiency. Fig. 1 shows the Tin dependence of the CHP efficiency and its distribution between the electrical and thermal outputs under a flow rate of 0.5 liters/min that makes negligible the parasitic power consumption due to the coolant pump. The maximum CHP output power obtained at 35 °C is 105 W (67 Wthermal and 38 Welectrical ) with a CHP efficiency of about 82%; we observe that the small decrease with increasing Tin agrees with previous studies [4], and is due to the increase of convective heat transfer to the environment. In conclusion, our results appear to be promising to design efficient and cost-effective solar CHP systems (ηCHP>80%).