Experimental and computational investigation of the thermal and electrical performance of a new building integrated photovoltaic concept
Authors: Ζώγου Ολυμπία 
Published: January 2014

A new concept of integrating PV modules to the south-facing walls of a building is developed. The feasibility of the concept is studied by means of experiment, CFD computations and building energy simulation. An air gap between the backsheet of the modules and the building wall is employed for circulating outdoor air to cool the modules, keeping their efficiency high. The heated air is exploited by the HVAC and service water system, thus improving building energy efficiency. Experimental data in real world operating conditions are critical to support the design optimization of the concept and layout. A testing device was designed for this purpose and subjected to indoor and outdoor testing. Three operating modes are tested: natural convection (no fan), and forced convection cooling by means of two different capacity axial fans. The transient electrical and thermal behavior of the device was recorded outdoors, for several hours during a number of days in summer and autumn. Further investigation of the air flow and velocity fluctuation field was carried out by means of indoor measurements in the above mentioned flow modes. The flow and heat transfer investigation of the device was supported by CFD modeling that also helped to the determination of more accurate wall heat transfer correlations. Based on reasonable assumptions and exploiting the indoor and outdoor measurements’ results, average convection coefficients were estimated to the order of 4 W/m2K for natural convection (no fan), 10 W/m2K for the low capacity viii Olympia Zogou fan and 10.6 W/m2K for the higher capacity fan. The existing average Nu/ Re or Nu/ Ra number correlations were improved after they have been found to underestimate Nu by a factor of 2 in the specific flow regime. The heat transfer correlations were used to improve the respective sub model of the TRNSYS building energy simulation, which was employed in the computation of the overall effect on the energy performance of the modules, by means of the electricity produced by the PV panels, the heating energy gains and their overall effects on the energy rating of the building. The annually produced electricity is calculated at the order of 750 kWh/ kWp. The respective thermal energy gain for space heating in winter amounts to 310 kWh/kWp. An additional annual thermal energy gain at the order of 210 kWh/ kWp was calculated for service water heating during the summer. For comparison, the respective electrical energy produced by rooftop installation of south-facing modules at optimal slope in the same location, amounts to 1200 kWh/ kWp approximately, with no further exploitable thermal gains. The results of this study show that the selection of flow rate and duct dimensions are affecting significant to the performance of the PV facade. According to the building energy simulation, measurable improvements in the thermal gains of the building can be succeeded by an improved design of the backsheet (enhancing heat transfer by micro-fins, modified backsheet material etc.). It should be noted that up to now, the design of the backsheet has not received the attention it deserves by the photovoltaic design engineers and manufacturers....