Sibilio, S.*, Rosato, A., Scorpio, M. Seconda Università degli Studi di Napoli Department of Architecture and Industrial Design "Luigi Vanvitelli" Via San Lorenzo, Aversa (CE), 81031, Italy *Corresponding author: sergio.sibilio@unina2.it ABSTRACT An efficient use of the energy that we can get for free from the nature, represents the first manner to achieve energy saving and to reduce our dependence from fossil fuels. In this context, daylight plays a fundamental role for both energy saving and improvement of the quality of life. On this track different tools are today available for researchers, designers and professionals to carry out the assessment of daylight influence at the early stage of design process both considering software simulation and/or other facilities such as real or artificial sky thus using physical simulation with scale-models. This paper deals with results of trials performed at the early stage of a design process referred to a low energy building located in south Italy where the abovementioned tools have been applied for natural light evaluation. To this aim a scale model has been investigated under an artificial sky of a mirror-box type and results compared with those obtained by computer simulation software Ecotect to determine the interior distribution of illuminance in terms of Daylight Factor. Later, simulation software has been used to evaluate the effect of of photovoltaic modules with different optical transparency (and number of PV modules integrated) integrated in large fenestration system, in terms of Daylight Autonomy, theoretical electrical energy deliverable and artificial lighting energy demand. Keywords: Daylighting, Artificial sky, Building-Integrated Photovoltaic Windows 1. INTRODUCTION An accurate study of the daylight behaviour inside a building represents the first step to realize a good daylighting project, that permits visual and climate comfort. To realize this, it is fundamental to use tools that can allow an accurate prediction of daylighting in all project steps. In the years, many tools have been developed to reproduce the variation of sky and sun condition throughout the day and the seasons. They can be divided into tools that use simulation algorithms and tool that use physical model of buildings posed under artificial or real sky to study the interior daylight distribution. Simulation programs use different lighting simulation algorithms to predict light behaviour inside the building [1] and allow investigation under different sky condition and with different accuracy level. With the physical approach a scale model of building is placed under artificial or real sky and appropriate micro-sensors to acquire photometric quantities are used. Several types of artificial sky can be used to reproduce sky and sun condition [2], [3]. In this paper are presented the results of a reaserch conducted on a building, located in south Italy (Casaldianni, Lat 41 21 N, Long 14 48 E), at the early stage of design process; the research aims to the definition of daylighting contribution to interior by the simplified assessment of "daylight factor" distribution as well as to evaluate the energy savings achievable with semi-transparent building integrated photovoltaic windows. - 1 -
A scale model of building has been realized and tested under an artificial mirror sky to measure interior daylight distribution; the artificial mirror sky has been realized at the Department of Architecture and Industrial Design of Seconda Università di Napoli and it has been designed and calibrated to reproduce standard CIE overcast sky. ECOTECT [4] software has been used for daylighting and energy calculation analysis, while DIALux [5] software has been used to determine the specific power of artificial lighting system required to comply with illuminance level imposed by national standard. Finally the potential energy savings achievable by the utilization of semi-transparent building integrated photovoltaic has been investigated too. 2. METHODOLOGY 2.1. Building The building considered for the present reaserch is the main part of a cultural center that will be realized in Casaldianni and has its main utilization as "library". It has been designed as an "open space" with an horizontal partition (floor) that separates ground and first floor. The south facade is composed of a 3.0 m height wall at ground floor (equipped with five windows) and a large fenestration system that can be considered made up of structural glass to create bright and highly attractive working environments, with more light and a greater feeling of space; in figure 1a is showed the scale model for the measurement under artificial sky and in figure 1b is showed the model for the simulation program. a) b) Figure 1 Scale model prepared for research into artificial sky (a) and Simulation model realized for research into simulation program (b). The 1:30 scale model of the building has been realized with boards of wood to ensure a solid model with internal surfaces covered using suitable cardboard; this selection permits to have surfaces with appropriate characteristics both for geometrical and photometrical terms. The photometric charateristic (reflectances) of cardboard has been measured with a spectrophotometer Konica Minolta 2600d (illuminant D65) with the results of measurements reported in Table 1.
Table 1 Photometric characteristic of cardboard used to cover internal surfaces of scale model Surfaces covered Reflectance L* a* b* Wall and ceiling 60.8 % 82.28 4.74 27.36 Floor 46.13 % 73.63-4.92 2.43 The virtual model of the building has been realized for the ECOTECT and DIALux simulation using the data reported in table 1 for internal surface; regarding the climate condition it was taken into account the climatic file of the location closest (Energy Plus data for Naples: latitude 40.8 - longitude 14.3 ) to the site. 2.2. Daylighting analysis The first step of the research covers the investigation of light distribution inside the scale model under the artificial sky of mirror-box type realized at Department of Architecture and Industrial Design. For the acquisition of illuminance distribution have been used miniaturized sensors [6] (V(λ)-matching lower than 3% and a directional response characteristic lower than 1.5%); in order to minimize insertion errors and in agreement with national standard, their sensible surface has been fixed at 29 mm height (0.85 m in the real scale) from the floor with an appropriate mounts. Before starting the measurement it has been checked that light sources within the artificial sky achieved electrical and thermal stabilization and it has been measured (on the supporting table) an illuminance mean value of 9500 lux (external horizontal unobstructed); this value was also set into ECOTECT for daylight simulation. Sixteen measurement points have been considered for each floor and using internal illuminance values acquired the Daylight Factor was calculated; the same daylight investigation has been carried out with ECOTECT. Figure 2a shows the reference system and the sixteen experimental measurement points, while figure 2b reports the percentage difference between simulated and experimental values of DF. According to the data highlighted in figure 2b, it can be noticed that the difference presented among the measured and predicted values of DF ranges from -26.2% to 3.2%; taking into account that the most part of the percentage differences reported in figure 2b are negative, it can be stated that the simulated results are generally lower than the measurements; the maximum difference has been obtained for the measurement points 9, 2, 14, 6 and 11. 2.3. Shading strategy and Semi-Transparent Photovoltaic Windows For an accurate building design, it is important not only the assessment of daylight availability, but also the control of glare as well as the reduction of overheating in summer season; to this aim the mean value of DF for a library should be higher than 3% with a ratio between minimum and maximum point values set at least 0.16 [7]. As described early, the building presents a large fenestrated upper wall that allows daylight utilization for many hours per day but it certainly faces with the abovementioned problems. A photovoltaic system has been considered with the utilization of semi-transparent photovoltaic windows [8] that allows the insertion of an increasing number of PV basic modules with a related optical transmittance from 10% to 70%. As basic PV module it
has been considered a poly-crystalline cell (dimension 156x156 mm - mean electrical efficiency of 16.5% - global efficiency of converting system of 85%). a) Figure 2 Reference system and experimental measurement points (a), percentage difference between experimental and simulation DF for the first floor (b). b) The simulation for the assessment of utilization of large fenestration with integrated PV modules has been accomplished using ECOTECT for the calculation of Daylight Factor (DF) and Daylight Autonomy (DA); these quantities have been correlated to the artificial lighting energy demand. The Daylight Autonomy has been evaluated considering a designed lighting level of 500 lux and the space was assumed to be occupied between 0800 1700hrs during the weekdays excluding the weekend from the simulation. For artificial lighting have been considered luminaires equipped with LED light source [9] (CCT: 4000 K, Luminous flux: 3942 lm and lamp wattage: 45 W); their distribution in the rooms has been designed by the simple CU factor approach using software DIALux; this leads to a specific power of 14.4 W/m 2. The variation of DF and DA has been investigated considering an optical transmittance of the photovoltaic system variable from 10 % to 70 % with step of 10 %, as reported in figure 3. The value of transmittance of 90 % has been considered as representative of the "standard glass" utilization. The figure 3 shows that the DF increases at increasing the optical transmittance for both the ground and first floor; in particular it can be noticed that the increment associated to the first floor is more relevant than that one related to the ground floor. In addition figure 3 highlights that the values of DF are always larger than 5%. In the figure 4 the annual theoretical electrical energy deliverable with photovoltaic system and the annual lighting electrical energy demand as a function of the optical transmission of photovoltaic system are showed. The annual lighting electrical energy demand reported in figure 4 represents the sum of energy required for both ground and first floor, while to evaluate the electrical energy deliverable with photovoltaic system,
the total energy radiation has been evaluated by using the suitable routine available (ECOTECT) with the climatic data of Naples. 12% Daylight Factor (%) 11% 10% 9% 8% 7% Ground floor First floor 6% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% Optical transmittance (%) Figure 3 Variation of Daylight Factor with variation of visible transmittance of photovoltaic system, evaluated for ground and first floor. This figure highlights that both annual lighting energy demand and PV theoretical electrical production decrease at increasing the optical transmittance; in particular it can be noticed that, for an optical transmittance changing from 10% to 70%, annual lighting energy demand varies from 481 kwh to 145 kwh, while PV theoretical electrical production reduces from 7500 kwh to 2500 kwh, underlining that the annual lighting energy demand is always lower than PV theoretical electrical production. Annual lighting energy demand (kwh) 790 740 690 640 590 540 490 440 390 340 290 240 190 Lighting energy demand PV theoretical electrical production 140 10% 20% 30% 40% 50% 60% 70% Optical transmittance (%) Figure 4 Annual electrical energy required for artificial light (left axis) and annual electrical energy producible (right axis) as a function of the optical transmittance of photovoltaic system integrated into glass wall. 7900 7400 6900 6400 5900 5400 4900 4400 3900 3400 2900 2400 1900 1400 Annual theoretical electric energy production with PV system (kwh)
Finally this figure shows that the annual lighting energy decreases mostly when the optical transmittance increases from 10% to 20%. 3. CONCLUSIONS In this paper are presented the results of the early research step conducted on the main part of a cultural center that will be realized in Casaldianni and has its main utilization as "library". The inside DF distribution has been evaluated following two approaches using a scale model and using a virtual model and then the results were compared. It can be notice that the first floor DF ranges from -26.2% to 3.2%, and that the most part of the percentage differences are negative, showing that the simulated results are generally lower than the measurements. The variation of DF and DA considering an optical transmittance of the window integrated photovoltaic system variable from 10 % to 70 % with step of 10 % has been investigated showing that the DF increases at increasing the optical transmittance for both the ground and first floor. In order to evaluate the effect of the use of the semi-transparent photovoltaic windows, the annual lighting energy demand and the PV theoretical electrical production have been calculated as a function of optical transmittance of windows. The results highlight that both annual lighting energy demand and PV theoretical electrical production decrease at increasing the optical transmittance and that the annual lighting energy demand is always lower than PV theoretical electrical production. REFERENCES [1] OCHOA, C.E., ARIES, M.B., HENSEN, J.L. State of the art in lighting simulation for building science: a literature review. Journal of Building Performance Simulation 2012, 5, 209-233. [2] AGHEMO, C., PELLEGRINO, A., LO VERSO, V. The approach to daylighting by scale models and sun and sky simulators: A case study for different shading systems. Building and Environment 2008, 43, 917-927. [3] BODART, M., DENEYER, A., DE HERDE, A., WOUTERS, P. The new Belgian single-patch sky and sun simulator and its validation. In Proceeding of the Lux Europa Conference, Berlin, September 2005. [4] ECOTECT analysis 2010. [5] DIALux 4.10 ver. 4.10.0.2. DIAL GmbH, Germany [6] PRC Krochmann type MI-D8. http://www.prc-krochmann.de/. [7] UNI EN 10840-2007 [8] SCHUECO. http://www.schueco.com/web/it/architekten/solarstrom_und_waerme/ products/ fotovoltaico. Accessed 7 may 2013. [9] SITECO ARKTIKA. http://www.siteco.com/en/products/innovation/indoor-lighting/ arktika-p-led.html. Accessed 7 may 2013.