Thermal Characteristics of BIPV Module (U-Value, g-Value)
Abstract
One third of the worldwide total energy demand is consumed by building applications. In order to supply this demand, reduce this considerable consumption and still maintain the comfort of the building, Building Integrated Photovoltaic (BIPV) is one of the most well-disposed elements for building applications, thanks photovoltaic energy generating effect, thermal insulation and solar gain, and its multifunctional characteristics, respectively. BIPV as an element integrated on building envelops have to comply the requirements of building products as defined in Construction Products Directive [CPD-89/106/EEC]. The thermal characteristics of BIPV are considered as one of the most important functions in total energy efficiency in building as defined in European Performance of Building Directive (EPBD); for instances: thermal insulation (U-value), sun control (g-value and Fc-value), etc. To determine these thermal characteristics of BIPV, most manufacturers define their products specification based so far on conventional building products, since no specific research on PV in building products exist. With same module configuration like laminated glass, the U-value of PV-laminated glass is the same as conventional laminated glass. With respect to higher radiation absorption and representing higher operating temperature of BIPV compared to conventional building products, however, changes of the thermal characteristics have to be taken into account. The operating temperature of roof-integrated PV can reach 90°C. [Mei et al. 2009] In this paper, the influences of PV-specific characteristics will be investigated on these relevant building functions in comparison with conventional building products; heat transmission coefficient (U-value), solar heat gain (g-value) and solar reduction ratio (Fc-value) in both summer and winter periods. The most significant coefficients related to operating temperature and corresponding surface temperatures are external and internal heat transfer coefficients (he and hi) and heat transmission coefficient in cavity (hs). In the BMU research project "MULTIELEMENT, the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) together with 15 industrial partners study on the development of manufacturing, testing and installation methods of multifunctional photovoltaic devices / modules in buildings