Assessment of shading devices with integrated PV for efficient energy use

Assessment of shading devices with integrated PV for efficient energy use Maria Mandalaki, Konstantinos Zervas and Theocharis Tsoutsos Technical University of Crete, Department of Environmental Engineering, Chania, Crete, Creece. Alexandros Vazakas Technical University of Crete, Department of Architecture, Chania, Crete, Creece. ABSTRACT This paper is part of a research for the characteristics of the optimum shading device: one that reduces the energy needs for heating and cooling and at the same time, maximizes the daylight availability and produces the maximum energy through its integrated Photovoltaic panels. The final goal of the research is the design (geometry and materials) of the optimum fixed shading device for an office building. The use of external fixed shading devices for solar influx radiation control and for energy saving issues is a well - known strategy in building sciences. On the other hand it has been proved that fixed shading devices can reduce daylight availability, increase artificial light needs and block the beneficial winter solar radiation. In order to investigate the balance between the above mentioned parameters, thirteen types of fixed shading devices have been studied and categorized according to their energy performance, for a single occupant office room, in Chania, Crete. The energy performance and the daylight analysis are assessed through computer simulations with the programs Autodesk Ecotect v5.6 and Energy Plus v3.1. For the analysis, constant parameters were the internal loads in the office room, the south orientation of the façade, the size of the window and the type of glazing. Variable parameter was the type of the fixed shading device. The study shows that there is a big deviation between the optimum shading device in terms of thermal comfort and in terms of daylight availability and energy production. The study is setting up the geometrical parameters that will be incorporated to the overall characteristics of the optimum fixed shading device. 1. INTRODUCTION The use of shading devices (S.D.) is essential for south oriented facades, especially in Mediterranean climates. Fixed S.D. can reduce increasing thermal loads in summer and at the same time can control daylight availability, visual comfort and glare. The choice of the S.D. for office buildings is an issue of high importance, due to the very specific standards of visual and thermal comfort and the need for low energy consumption. The use of fixed S.D. reduces the incident solar radiation. This fact, poses a problem of balance between the positive reduction of cooling loads in the summer and the negative increasing of heating loads in the winter. The aim of the project is the assessment of S.D. according to their energy performance, for office buildings, in Mediterranean climate. This will give a guide for introducing the appropriate S.D. according to the sustainability approach of the building and will introduce a manual of best practice guidelines. 1

2. METHODOLOGY In order to evaluate fixed external south facing S.D. according to their energy consuming factor, we examine thirteen types that have been used in office buildings. The S.D. s are being conceived as machines of energy production and machines of energy reduction, that contribute to the building s energy balance. The types are: 1.Horizontal canopy single, 2.Horizontal canopy double, 3.Inclined canopy single, 4.Inclined canopy double, 5.Louvers horizontal, 6.Louvers horizontal inwards inclined, 7.Louvers horizontal outwards inclined, 8.Vertical louvers, 9.Brise - soleil full façade, 10.Brise soleil semi façade, 11.Brise soleil semi façade with louvers, 12.Canopy with louvers, 13. Surrounding shading, as shown in figure 1. These are compared to the single window type without shading. Figure 1: Types of shading devices Three are the basic criteria for the evaluation of the efficiency of the shading devices: A. the cooling and heating loads of the inner space, B. the electric light needed to ensure visual comfort and C. the energy production of the integrated PV panels. In this project we examine the sustainability of an office room and its energy consumption in order to ensure temperatures between 18 to 26⁰ C and lighting levels over 500 lux for the most of the space and working times. (08.00 to 20.00). Additionally in order to minimize the energy consumption form non renewable sources we exploit the sunlight surfaces of the shading devises and we cover them with PV solar cells. These can produce energy that can supply the electricity needed for each office room. In order to evaluate the shading device according to lighting performance and energy consumption we introduce the electricity independence factor. This is a coefficient described by the ratio of the electricity produced from the PV panels, to the electricity used for supporting the electric lighting. A typical room of 3.5m x 5.4m x 2.9m (width x depth x high) is used as a reference, with a window facing south, 2.4m x 1.9m (width x high). This is making a space of 18.9 m² with a 4.56 m² south window, so the ratio of the window s surface to the floor area is 24%. The position of the occupants as well as the position of the luminaries can be seen in the plan. 4 x 50 W, type: HF tubes, mirror-luminaries are being used for the electricity needs (Fig.2). The reflectance of the material used is 0.85 for the ceiling, 0.65 for the walls and 0.20 for the floor. All the dimensions, provision of 2

space and material properties are based on the project Project SWIFT, Switchable Facade Technology, Reference office for thermal, solar and lighting calculations, terminated on 30.4.2003. (Van Dijk, 2001) Figure 2: Configuration and electric lights in the office module The PV panels used are constructed form a photovoltaic polycrystalline material. They are integrated to the SD following the geometry of the exterior face of each plane of it, as shown in fig.3. We take into account the repetition of the module office room, in order to construct a multi storey office building and to avoid overshadowing from SD of adjacent floors. Figure 3: Faces of integrated P.V. panels The energy needs for thermal comfort in the space have been measured with computer simulation software Energy Plus v. 3.1. The daylight levels that each shading device ensures in the space and the electricity production of the solar cells have been measured with Autodesk Ecotect v.5.6. 3

For the analysis, invariant were the thermal loads of the office room, the south orientation of the facades, the geometry and dimensions of the office room and window, and the types of the materials used for the glazing the interior of the room. The only variant was the type of the shading system. The simulation is been held in a Mediterranean island, Crete where the use of fixed shading devices in office buildings is a common practice, due to the extreme hot weather conditions. The weather data for the city of Chania, Crete and their incorporation to the software is a work that has been done by the Laboratory of Sustainable and Renewable Energy, Department of Environmental Engineering of Technical University of Crete. 3. RESULTS We conceive the S.D. s as lighting control machines, ones that reduce daylight availability and at the same time produce through their integrated PV the electricity needed for supporting electric lighting, the period of time that the daylight is not sufficient. We introduce the lighting independence factor that is the ratio, of the electricity needs for lighting the space in order to ensure at least 500 lux on the desk level during all the working hours (08.00 20.00), to the electricity produced from the integrated PV panel. This factor is positive for four types of shading devices in row (as shown in fig 4): the surrounding shading, the full façade brise-soleil, the single inclined canopy and the single canopy. At the same time these devices are the most energy producing. Figure 4: Lighting independence factor 4

The positive lighting independence factor means that for these S.D. the electricity production is sufficient for supporting the electric light needed. It is important to note that, even if the SD of horizontal louvers has a very good performance in terms of electricity production through its integrated PV, it has negative lighting independence factor, due to the fact that reduction of the daylight is much higher than almost all the other SD. One could provide this spare energy for lighting into the inner common spaces. The three more darkening SD are: horizontal louvers outwards inclined, the horizontal louvers and the surrounding shading. It is important to note that the electricity produced by each device is not relevant to the surface of the PV installed in the system. That is happening due to the geometry of each shading device and the position of each PV panel according to this geometry, that allows the overshadowing between the elements. We conclude that the type surrounding shading is producing more power than both types of Brise-soleil (type full façade semi facade) even though the latter has more surface of PV installed (Fig. 5). In order to categorize the SD in terms of economical construction we introduce the term efficiency that is describing the ratio of the amount of electricity produced from each device to the square meters of PV installed. As shown in figure 6, the systems of canopy inclined single and the horizontal canopy double and single are the most efficient from the economical point of view, because with less PV material use the electricity is increased. Figure 5: Electricity production according to surface of PV material 5

Figure 6: Electricity production according to m 2 of PV installed In general, one would expect that systems which are blocking most of the direct sun rays would produce the most of electricity, due to the fact that incident solar radiation is higher in these cases. Systems with very low daylight autonomy, produce electricity, but not as sufficient as one would expect. Only the type surrounding shading which is the optimum system in terms of blocking direct sun rays is at the same time optimum in terms of exploitation of the solar radiation and electricity production. On the contrary the system of louvers horizontal inwards inclined is performing very low in terms of electricity production and in terms of daylight availability in the space (Figure 7). 6

Figure 7: Relation of electricity production to electricity needs for lighting If one would look at the average daylight autonomy of each space he would see that the type of horizontal canopy with louvers is being in the first place, providing for most of the year (83% ) sufficient daylight above 500 lux. On the contrary the worst performing system in terms of providing sufficient daylight is the louvers horizontal outwards inclined (Fig. 8). Figure 8: Daylight autonomy for different shading systems 7

In terms of providing thermal comfort in the space, the systems of horizontal louvers and horizontal louvers outwards inclined are the most energy consuming. On the other hand the systems of canopy horizontal with louvers and single canopy inclined are the most energy saving systems as well as the horizontal louver inwards inclined. It is very important to note, that in the later, changing the inclination of the louvers is dramatically affecting the temperature of the interior space. Additionally, it is important to see that in almost all of the SD most of the energy consumption is due to heating reasons. Only in the systems of full and semi façade brise soleil and the surrounding shading the cooling loads are more than the heating ones. This is pointing out an important issue that the fixed SD present: the reduction of cooling loads and the increasing of the heating ones. (fig.9) Figure 9: Energy needs for thermal comfort We are trying to balance all the above mentioned parameters, in order to categorize the SD according to the energy consumption they lead to. 4. CONCLUSIONS In order to assess the SD a balance between the above mentioned parameters needs to be done. It has been proven that the best performing SD according to energy needs for heating and cooling the space is not corresponding to the best performance in terms of daylight availability and use of less electric light. For example the SD of horizontal canopy with louvers, canopy inclined and horizontal louvers inwards inclined are the best in terms of energy consumption to ensure thermal comfort. At the same time above these systems only the SD of canopy inclined can sufficiently support the electric light needed for visual comfort. The SD of surrounding shading is performing the best in terms of energy consumption for visual comfort and at the same time it presents very high energy needs in terms of thermal comfort. In general the SD of horizontal or inclined canopy with louvers, is performing the best in terms of less energy consumption for thermal and for visual comfort in the interior of the building. (Figure 10) 8

Figure 10: Energy needs for thermal and visual comfort It is important to note, as well, that the most commonly used shading system for office buildings, the horizontal louvers, is performing very low. This is a factor that should be taken into account for future developments or energy renovations for these types of buildings and is introducing a path for rethinking the shading devices in office buildings. In order of course to arrive to a most massive conclusion concerning the use of SD as valuable machines of improvement of the quality of the interior space in office buildings with less energy consumption, a further research needs to be done, that will include not only the quantitative effects of the SD but the quality ones. These are dealing with the glare problems and the visual contact with the outside as well as the infiltration rate through fenestration REFERENCES Dubois M.C. (1997). Solar shading and building energy use. A litterature review. Part I. Lund Sweden Eiffert P. and Kiss J. G., (2000). Building-Integrated Photovoltaic. Designs for Commercial and Institutional Structures. A Sourcebook for Architects. Springfield: Us Department of Commerce Grandjean E. (1973). Ergonomics of the home. London: Taylor & Francis. Hascher R., Jeska S., Klauck B.(2002). Office buildings: A Design manual. Basel: Birkhauser. Hermstad K. ( 2006). Architectural Integration of PV in Norwegian Office Buildings. Research made for Norwegian Research Council. Mehrotra M, (2005) Solar control devices; balance between thermal performance and daylight. Palenc Conference proceedings Munari Probst M. C. and Roecker C. (2005). Integration and Formal Developments of Solar collectors. In: Proceedings of PLEA The 22 nd Conference on Passive and Low Energy architecture. Beirut, Lebanon Papantoniou Sotiris, Tsoutsos Theocharis. (2008). Building integrated PV application in the island of Crete. 23 rd European Photovoltaic Solar Energy Conference Proceedings Papantoniou Sotiris, Tsoutsos Theocharis, Gouskos Aris, (2008). Builidng Integrated PV. Application in the Technical University of Crete. Technica Chronika, September October 2008. 9

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