DAYLIGHT MEASUREMENTS AND CALCULATIONS WITH AN A-SI PHOTOVOLTAIC SOLAR FACADE

Transcription

1 DAYLIGHT MEASUREMENTS AND CALCULATIONS WITH AN A-SI PHOTOVOLTAIC SOLAR FACADE Eero Vartiainen, Klaus Mäki-Petäys, and Peter D. Lund Advanced Energy Systems, Helsinki University of Technology, P.O. Box 2200, FIN HUT, Finland Tel , Fax , Abstract A multifunctional solar facade consisting of transparent glazing and amorphous silicon photovoltaic panels has been tested at Helsinki University of Technology, Finland (60ºN). The facade has a central window and PV panels below and beside the window, with gaps of clear glazing between the PV panels beside the window. The daylight illuminance inside a test room behind the facade was measured at three horizontal points on the interior desk level. Assuming a lighting requirement of 500 lx and continuous electric light dimming, the average daylight availability DA was about 55% during the office hours (9 am to 5 pm) for the test room during the measurement period from October to March. The DA for the summer months was almost 100%. However, no blinds were used at the test facade. For that reason, the DA was also calculated for a typical office room at four European locations with an optimized facade layout which has, as an addition to the test facade, Venetian blinds in the central section of the facade, and PV panels above the central clear-glazing window with gaps of diffusive glazing between these PV panels, so that no blinds are necessary at the top section of the facade. The yearly average DA for the optimized facade ranged from 60% in Sodankylä, Finland (67ºN) to almost 90% in Trapani, Sicily (38ºN). A continuous daylight-responsive electric light dimming and blind control system was used for the calculations. 1. INTRODUCTION A solar facade is a building component producing daylight, heat, and electricity. A part of the facade area is transparent to provide daylight into the building. The transparent area of the facade may include windows, transparent insulation, or other advanced daylighting devices. The opaque area of the solar facade consists of photovoltaic (PV) panels which produce electricity for the building. It is also possible to use semi-transparent PV elements to provide lighting and electricity simultaneously. An important design issue for a solar facade is the division between the transparent window and the opaque PV area of the facade. For example, the larger the window is, the more daylight and heat, but less electricity is produced. On the other hand, a larger window also means greater heat losses during the winter and increasing cooling load during the summer. Some examples of solar PV facades producing daylight and electricity are the Mataro public library in Spain (Lloret et al., 1995, 1997) and the Doxford Solar Office in England (Lloyd James et al., 1998) where the transparency has been obtained by gaps of clear glazing between the PV cells in the module. In this paper, the daylight illuminance measurements for the ASICOM (Amorphous-SI photovoltaics for COMmercial buildings) test facade in Helsinki, Finland are presented. The daylight availability at four European locations is also assessed for an optimized PV facade layout. 2. DAYLIGHT MEASUREMENTS 2.1 The ASICOM test facade In the ASICOM test facade, the amorphous silicon (a-si) PV plates are laminated between two sheets of glass (Leppänen et al., 1998) to produce large modular semitransparent PV modules. Although a-si PV has lower efficiency than crystalline silicon PV, a-si has been used in the ASICOM PV facade because it has much lower cost per area and is therefore more acceptable for building applications (Peippo et al., 1997). Moreover, the more uniform appearance of a-si may be an architectural benefit. The ASICOM test facade layout is presented in Fig. 1. At the centre of the facade, there is a clear glazing window. The rest of the facade area is covered with 30 cm x 90 cm a-si PV panels. To allow maximum amount of daylight into the test room behind the facade, there are transparent gaps between the PV panels in the modules beside the window. The gaps between the PV panels in the modules below the window are opaque because they are below the interior desk level and therefore, would not contribute significantly to the illuminance on the desk level (Vartiainen et al., 2000). The transparent gaps between the PV panels are about 30 mm wide.

2 2 3 window Fig. 2. The horizontal position of the photometers in the illuminance measurements in the ASICOM test room: 1 - front, 2 - middle, 3 - back of the room. Fig. 1. The ASICOM test facade. The transparent area of the facade is in white, the PV panels in light grey, the frames in black, and other opaque area in dark grey. 2.2 The illuminance measurements The interior daylight illuminance was measured in Otaniemi, Finland (60ºN), from 1 st October, 1999, to 31 st March, 2000 in the ASICOM test facade room. The main parameters and dimensions for the test room are shown in Table 1. Three illuminance meters (Krochmann FETseries photometer heads) were installed at the table level (0.79 m from the floor) at one, two, and three quarter distances of the interior room depth from the window on the centre line of the test room (Fig. 2). Table 1. Main parameters and dimensions in the illuminance measurements. Facade orientation south Latitude 60º11'N Longitude 24º50'E Interior room depth m Interior room width 3.14 m Interior room height 2.37 m Interior facade area 7.44 m 2 Desk level height 0.79 m Window sill height m Window glazing height 1.43 m Window glazing width 1.00 m Window area 1.43 m 2 Total area of the transparent gaps 0.54 m 2 Total transparent area of the facade 1.97 m 2 Window normal light transmittance 0.67 Reflectance of the floor 0.30 Reflectance of the walls and the ceiling 0.70 Reflectance of the ground 0.15 Left edge of the window - the left wall 1.06 m Illuminance meters - the left wall 1.56 m 1 st illuminance meter - the window 0.76 m 2 nd illuminance meter - the window m 3 rd illuminance meter - the window 2.25 m The interior illuminances together with horizontal and vertical irradiances were measured continuously at ten second intervals, and the measurements were averaged and recorded in the memory of a PC every fifth minute by an HP75000B datalogger. Finally, the measurements were analysed and hourly averages calculated for the six month monitoring period. The monthly average vertical irradiance G v,ave and interior horizontal daylight illuminances E v during the office hours (9 am to 5 pm) for the three measurement points are shown in Table 2. The percentage of hours t % when the illuminance exceeded 500 lx and the daylight availability DA (in % of the 500 lx lighting requirement during the office hours) are also shown in the table. t % corresponds to an on/off electric light switching system and DA to a continuous dimming system. 500 lx is a typical illuminance level recommended for office work in various countries (CIBSE, 1984; IESNA 1993). Table 2. The monthly average vertical irradiance G v,ave on the test facade, the horizontal daylight illuminance E ave on the desk level, percentage of hours t % when the illuminance exceeded 500 lx, and the daylight availability DA (in % of the 500 lx lighting requirement) during the office hours (9 am to 5 pm) for the three measurement points in the ASICOM test facade room. Front Middle Back G v,ave E ave t % DA E ave t % DA E ave t % DA Month W/m 2 lx % % lx % % lx % % Oct ' Nov ' Dec ' Jan ' Feb ' Mar ' Average

3 It can be seen from the table that in March, about 90% of the lighting requirement during the office hours in the test room can be provided by daylight, whereas in December, the DA is only 20-40%, with continuous dimming. The average DA for the six month monitoring period was about 70% for the front, 60% for the middle, and 40% for the back part of the room. Similar measurements for the summer months (not reported here) showed that the DA was almost 100% for all measurement points, giving a yearly average DA around 75% for the whole room, with continuous dimming. With on/off electric light control (t % ), the daylight availability would be 10-15% less, on the average. In the ASICOM test facade, no blind system for the beam sunlight was used. This, however, would be unrealistic for a real south-oriented office room, since beam sunlight can cause overheating and discomfort glare for the occupants. Moreover, the dimensions of the test room are not representative of a real office space since the room was almost square. For that reason, the daylight availability at four European locations for a typical office room with a Venetian blind system and optimized PV facade layout is calculated in the next section. 3. DAYLIGHT AVAILABILITY CALCULATIONS 3.1 The optimized facade layout The ASICOM test facade is enlarged to a full-scale office facade by installing additional PV modules above the central clear glazing window and by widening the PV panels of the middle sections from 90 cm to 120 cm, giving an overall height of 3.0 m and width of 3.4 m for the facade (Fig. 3). The room depth is 5.0 m. Moreover, the window sill is placed higher because it gives a more uniform interior daylight distribution than a window with the sill at the desk level (Vartiainen et al., 2000). 3.4 m 1.3 m 1.1 m Desk 1.9 m 0.4 m level 0.7 m Fig. 3. A front view of an optimized solar facade with a mosaic PV structure with gaps of diffusive glazing (diagonal slashes) between PV panels at the top section. One solution to the glare problem could be a diffusing mosaic PV facade (Fig. 3) where the gaps between the PV elements at the top section of the facade are made of diffusive glazing, so that no blinds would have to be used there. However, diffusive glazing without blinds cannot be used at the middle section of the facade because the high luminance could still cause glare. If the desks and computer displays are placed in the front half of the room, the top section without blinds will be sufficiently high to be outside the field of vision and, therefore, the diffusive glazing at the top will not cause glare to the occupants. 3.2 Modelling approach The daylight availability in a typical south-facing office room at four locations was calculated with a simulation tool DeLight (Vartiainen, 1996; Vartiainen et al., 2000) developed at the Helsinki University of Technology. The simulation model uses hourly horizontal beam and diffuse irradiance measurements as input data. The irradiance measurements are converted to illuminance values by a luminous efficacy model, and the hourly horizontal illuminance values are used to generate a sky luminance distribution by a sky luminance model. The Perez luminous efficacy model (Perez et al., 1990) and the Perez all-weather sky luminance model (Perez et al., 1993a, 1993b) have been used here, as they were found to be the best models in comparisons between various models with experimental data (Vartiainen, 2000a, 2000b). From the sky luminance distribution, it is possible to calculate the interior diffuse daylight illuminance. The calculation of the interior beam illuminance is quite straightforward, given the position of the sun and the room geometry. However, no beam sunlight is admitted into the building in this study. The modelling of the reflected component of daylight is explained by Vartiainen (1996). Only exterior and firstorder interior reflections are included in the model. In DeLight, the room is divided into two daylighting zones, so that tasks requiring accurate vision (e.g., reading and writing) would be performed in that half of the room closest to the window (the front half). The recommended illuminance requirement is 500 lx for the front half and 300 lx for the back half (CIBSE, 1984; IESNA 1993). Both room halves have their own independently controlled luminaire. For each office hour of the year (9 am to 5 pm, 5 days per week), the interior horizontal daylight illuminance for the room halves is calculated from the Test Reference Year beam and diffuse irradiance measurements at each location. If the daylight illuminance falls under the required level during the lighting demand, the shortage must be provided by electric lighting. A continuous electric light dimming strategy maximizing the daylight utilization has been assumed in this study. If

4 only an on/off control were used, the effect of daylight on the energy savings would be much smaller (Vartiainen and Lund, 1998). To avoid overheating and discomfort glare, the incoming beam radiation is reflected back by Venetian blinds which are controlled automatically. A prototype of this kind of system has been tested in the USA (Lee et al., 1998). When the blinds are closed to block the beam sunlight, 75% of the diffuse daylight is also assumed to be lost. DeLight has been validated with continuous year-round outdoor irradiance and simultaneous test room illuminance measurements at Helsinki University of Technology, Finland (60 11 N, E) (Vartiainen et al., 2000). As was expected, the model underestimated slightly the measured illuminance at the back half of the room, mostly due to the simplification in the treatment of the internally reflected beam light, since the reflected daylight component is relatively more important further away from the window. However, the annual mean bias error at all points of measurements in the room was within the accuracy of the photometers (± 4%) if only the hours when the measured illuminance was less than 300 lx are taken into account. It was concluded (Vartiainen et al., 2000) that the error on the annual lighting electricity requirement would be less than 2% if two lighting zones of 500 lx (front half) and 300 lx (back half) were used. 3.3 Daylight availability The yearly average daylight availability (in % of the lighting requirement) DA during the office hours (9 am to 5 pm) for a south-facing facade similar to Fig. 3 is presented in Table 3 at four European locations: Trapani, Sicily (37 55'N, 12 30'E), Paris, France (48 46'N, 2 1'E), and Helsinki (60 19'N, 24 58'E) and Sodankylä (67 22'N, 26 39'E), Finland. For comparison, the DA is also given for a facade with a minimum window area which has only a central clear-glazing window but no other light-transmitting glazing, and for a facade with a maximum window area (no PV panels above the window sill). The light-transmitting area of the total facade area is 14% for the minimum window facade, 24% for the mosaic facade of Fig. 3, and 60% for the maximum window facade. It can be seen from the Table that the mosaic facade has a 23-39% points higher DA than the minimum window facade. In addition to having a greater light-transmitting area, the mosaic facade benefits from the diffuse glazing at the top section of the facade where blinds are not necessary and therefore, valuable diffuse daylight is not lost. The maximum window facade has only a 8-16% points higher DA than the mosaic facade. This difference is relatively small, considering that the light-transmitting area of the maximum window facade is 2.5 times that of the mosaic facade. Therefore, the maximum window facade is unlikely to be optimal energy-wise, since it has greater heat losses during the winter and greater cooling load during the summer. Moreover, the maximum window area facade has less than 50% of the PV area of the optimized facade for the electricity production. Table 3. The yearly average daylight availability (in % of the lighting requirement) during the office hours (9 am to 5 pm) at four locations for a south-facing minimum window facade, the mosaic facade of Fig. 3, and a maximum window facade, with the total lighttransmitting window area of the facade (in %). Window Daylight availability area Trapani Paris Helsinki Sodank. Facade layout % % % % % Minimum w Mosaic facade Maximum w CONCLUSIONS The daylight illuminance with a south-oriented multifunctional PV test facade was measured during a half year winter period in Otaniemi, Finland (60ºN). The facade consists of a central window, PV panels below and beside the window, with gaps of clear glazing between the PV panels beside the window. The average daylight availability DA (in % of the lighting requirement) for the square test room during the office hours (9 am to 5 pm) with continuous dimming was about 55% in October- March. For the summer months, the DA was almost 100%. With on/off electric light control, the DA is about 15% less. However, no blinds were applied to the facade and beam sunlight was allowed to enter the test room during the measurements. Therefore, the daylight availability in a realistic office space with Venetian blinds was assessed for four European locations for an optimized facade layout. The optimized facade layout has, as an addition to the test facade, PV panels above the central clear-glazing window with gaps of diffusive glazing between these PV panels. That way, no blinds are necessary for the top section of the facade and valuable diffuse and beam daylight can be fully utilized. This optimized facade layout has a total light-transmitting area of 24% of the total facade area. The yearly average DA for the optimized facade ranged from 60% in Sodankylä (67ºN) to almost 90% in Trapani (38ºN). A continuous daylight-responsive electric light dimming and blind control system was used for the calculations because with on/off switching and manually adjusted blinds, a large part of the valuable daylight is lost. Intelligent light control systems are essential for full daylight utilization and such systems are currently coming to the market.

5 REFERENCES CIBSE (1984) Code for Interior Lighting The Chartered Institution of Building Services Engineers, London, UK. IEA (1994) Passive solar commercial and institutional buildings. Ed. by Hastings S.R., John Wiley & Sons, Chichester, UK. IESNA (1993) Lighting Handbook, 8 th edition. Illuminating Engineering Society of North America, New York, NY, USA, Lee E.S., DiBartolomeo D.L. and Selkowitz S.E. (1998) Thermal and daylighting performance of an automated venetian blind and lighting system in a full-scale private office. Energy and Buildings 29, Leppänen J., Farkh S., Morcant K., Loyer W., Lund P.D., Peippo K. (1998) Manufacturing options for large a-si PV facade elements. In proceedings of the 2 nd World Conference on Photovoltaic Solar Energy Conversion, 6-10 July, Vienna, Austria, pp Lloret A., Andreu J., Merten J., Chantant M., Servant J.M., Aceves O., Sabata L., Sen F., Puigdollers J., Person C. and Eicker U. (1995) The Mataro public library: a 53 kw p grid connected building with integrated PV-thermal multifunctional modules. In proceedings of the 13th European Photovoltaic Solar Energy Conference, October, Nice, France, pp Lloret A., Aceves O., Andreu J., Merten J., Puigdollers J., Chantant M., Eicker U. and Sabata L. (1997) Lessons learned in the electrical system design, installation and operation of the Mataro public library. In proceedings of the 14th European Photovoltaic Solar Energy Conference, 30 June - 4 July, Barcelona, Spain, pp Lloyd James D., Matson C. and Pearsall N.M. (1998) The solar office: a solar powered building with a comprehensive energy strategy. In proceedings of the 2 nd World Conference on Photovoltaic Solar Energy Conversion, 6-10 July, Vienna, Austria, pp Peippo K., Lund P.D., Leppänen J., Nieminen J.-P., Morcant K., Siino S., Farkh S., Caccavelli D., Laret L., Abraham B., Loyer W., Fabre E., Hestnes A.G., Andresen I., Skarstein Ø., Wathne E. and Jager W. (1997) JOULE/ASICOM: Amorphous SIlicon photovoltaics for COMmercial buildings. In proceedings of the 14th European Photovoltaic Solar Energy Conference, 30 June - 4 July, Barcelona, Spain, pp Perez R., Ineichen P., Seals R., Michalsky J. and Stewart R. (1990) Modeling daylight availability and irradiance components from direct and global irradiance. Solar Energy 44, Perez R., Seals R. and Michalsky J. (1993a) All-weather model for sky luminance distribution - preliminary configuration and validation. Solar Energy 50, Perez R., Seals R. and Michalsky J. (1993b) Erratum to All-weather model for sky luminance distribution - preliminary configuration and validation. Solar Energy 51, 423. Vartiainen E. (1996) Utilization of daylight in interior lighting. Licentiate thesis, Helsinki University of Technology, Espoo, Finland, in Finnish. Vartiainen E. and Lund P. (1998) Daylighting strategies for advanced solar facades. In proceedings of the 2nd ISES Europe Solar Congress, EuroSun '98, September, Portorož, Slovenia, pp. II Vartiainen E. (2000a) A new approach to estimating the diffuse irradiance on inclined surfaces. Renewable Energy 20, Vartiainen E. (2000b) A comparison of luminous efficacy models with illuminance and irradiance measurements. Renewable Energy 20, Vartiainen E., Peippo K. and Lund P.D. (2000) Daylight optimization of multifunctional solar facades. Solar Energy 68,