SIMULATION OF A PHOTOVOLTAIC HYBRID FACADE

The conception of complex buildings with innovative facade elements often demands dynamic building simulations. In some cases the required thermal parameters of the components must be obtained by experimental investigations. This paper describes this procedure exemplary for a photovoltaic facade, which allows the combined utilization of electrical and thermal energy. The parameters for a TRNSYS simulation could be obtained with the help of a PASSYS testcell. The results show that the total energy transmittance (gvalue) of such a facade strongly depends on the ambient air temperature. INTRODUCTION Double skin facades, consisting of a massive wall with a transparent or semitransparent curtain layer, are used increasingly, since they have a number of advantages. This technology can be used in new buildings as well as in the retrofitting of old buildings. By an intelligent control of ventilation flaps and use of additional fans the thermal behaviour of the facade can be adapted to the conditions of use and to the actual weather conditions. An overheating of the building during the summer can be prevented by opening a channel between the air gap and the ambient air. If in contrast the air from the gap is passed into the building solar gains can be used for heating purposes. Thus the control of the air flow causes a valve-effect, i.e. high solar gains can be combined with low heating losses. For an optimized design of the facade and of the HVAC-system of the building a quantitative knowledge about the thermal behaviour of the facade is necessary. In general, passive-solar components can be described by the U-value (heat loss coefficient) and the g-value (total heat or solar transmittance). The definition of the U-value can be applied to double skin facades without difficulties, whereas the definition and determination of the g-value causes some problems. In the case of a simple glazing the g-value is defined as the part of the radiation which is passed through the glazing. It can be derived only from the optical properties of the glasses. But in the case of an opaque facade the more general definition must be used: The g-value is this fraction of the incident radiation energy which can be used for the heating of the building. It is shown in this paper that the g-value of a double skin facade can strongly depend on the ambient air temperature and thus cannot be used in the usual way. The procedure to obtain the thermal parameters is shown exemplary for a photovoltaic (PV) hybrid facade. Such a facade allows besides the generation of electrical energy additionally the utilization of the thermal energy caused by the absorption of the solar radiation. To calculate the possible saving of heat energy by use of such a facade a TRNSYS simulation was performed. Since in this case only the thermal properties of the PV facade were of interest it was tried to use the TRNSYS type 37 („Attached sunspace“) for the model. Because of the differences between an attached sunspace and a PV facade at first some of the required parameters had to be determined by a preliminary outdoor experiment and following calculations. THE PHOTOVOLTAIC FACADE For the experimental investigations a photovoltaic facade was constructed, consisting of the PVmodules, an air gap and a massive brick wall. It is shown in fig. 1. Fig. 1: The photovoltaic facade The PV-area consists of 6 prefabricated modules with a total effective area of 4.9 m 2. The PV cells (monocrystalline silicon, t ype MIS-I) are embedded in glass panes (total thickness o f the modules 8.5 mm). During all of the experi ments the modules were MPP-tracked and grid-connected. The ventilation flaps above and belo w the PV modules allow an air exchange bet ween the air gap and the a mbient air during the summer time to prevent an overheating of the modules. During the described experiments these flaps stayed closed. The flaps above and belo w the massive wall are intended for the air exchange bet w en the air gap and the room. This air exchange is driven by three additional tube axial fans behind the upper flaps. The electrical power consumption of each fan was 18 watt. The massive wall had a thickness of 24 cm and was made of bricks with a density of 1.8 g⋅cm-3 and a thermal conductivity of 0.9 W/(m⋅K). In a real application of cours e an additional insulation layer must be added to the wall. However, during the experi ments the wall was used without insulation in order to achieve a better identifiabilit y of the thermal parameters of the wall. THE PASSYS TEST PROCEDURE The experi ments were done using a so-called PASSYS test cell [1]. These test cells were developed during an European research project about passive solar systems. They are appropriate to deter mine the heat loss coefficient U and the solar trans mittance g of passive solar co mponents under natural cli mate conditions. Every test cell consists of a well insulated test room and a small service room at the northern side. The temperature of the test roo m can be controlled b y a heating and cooling device. The test co mponent is mounted in an insulated fra me at the south oriented side of the test cell. The aperture of the fra me (2.7 m x 2.7 m) enables the installation of facade ele ments in original dimensions. The other walls of the test cell are well insulated (40 cm) to minimize the heat exchange with the surroundings. A cross section of a test cell is shown in fig. 2. Blower Ventilation flap PV-modules Air gap Massive wall Ventilation flaps 2,7 m Photovoltaic modules