BP SOLAR SKIN AUTHOR(S) Øyvind Aschehoug, Dagfinn Bell. BP Solar, BP Norge CLASS. THIS PAGE ISBN PROJECT NO. NO. OF PAGES/APPENDICES

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1 TITLE SINTEF REPORT SINTEF Civil and Environmental Engineering Architecture and Building Technology Address: NO-7465 Trondheim NORWAY Location: Alfred Getz vei 3 Telephone: Fax Enteprise No.: NO MVA BP SOLAR SKIN - a façade concept for a sustainable future AUTHOR(S) Øyvind Aschehoug, Dagfinn Bell CLIENT(S) REPORT NO. CLASSIFICATION CLIENTS REF. STF22 A03510 Open BP Solar, BP Norge Ray Noble, Jan Geirmo CLASS. THIS PAGE ISBN PROJECT NO. NO. OF PAGES/APPENDICES Open H ELECTRONIC FILE CODE PROJECT MANAGER (NAME, SIGN.) CHECKED BY (NAME, SIGN.) BP SOLAR SKIN - FINAL REPORT.DOC Øyvind Aschehoug Anne Grete Hestnes FILE CODE DATE APPROVED BY (NAME, POSITION, SIGN.) Karin Høyland, Research Director ABSTRACT A prototype solar façade system, combining a double façade with a building-integrated photovoltaic (PV) system, has been constructed on an existing university building in Norway. It produces both electricity and heat, and is suitable for existing and new buildings. The prototype has been monitored for a year with satisfactory results. The PV system generated 7200 kwh this year (corrected for losses during a malfunction period), which is about 75% of theoretical maximum. This renewable energy would reduce CO2 emissions with 0.7 tons/year for fossil-fueled electricity generation. The heating demand for the building behind the new façade was reduced with 7-8%. The offices on the top floor of the building experienced summer periods of overheating. Revising the control strategy for double façade cavity venting will reduce this problem. KEYWORDS ENGLISH NORWEGIAN GROUP 1 Architecture Arkitektur GROUP 2 Energy Energi SELECTED BY AUTHOR Double façade Dobbel fasade Photovotaics Solseller Monitoring Måleprogram

2 2 Table of Contents PROJECT BACKGROUND...3 DOUBLE FACADES...4 BUILDING INTEGRATED PHOTOVOLTAIC CELLS...5 THE BP SOLAR SKIN CONCEPT...5 THE R&D STAGE...6 THE BP SOLAR SKIN PROTOTYPE...6 THE MONITORING PROGRAM...9 MONITORING RESULTS...11 ELECTRICITY GENERATION...11 THERMAL WINTER GAINS...12 SUMMER CONDITIONS...13 ACKNOWLEDGEMENTS...14 REFERENCES...14 PROJECT DETAILS...14 FOR FURTHER INFORMATION, PLEASE CONTACT:...14 MAP...15

3 3 Project background BP Norge and BP Solar commissioned Norwegian research units at NTNU and SINTEF to develop a building system for facades, based on photovoltaic solar cells. BP Norge is based in Stavanger and engaged in offshore petroleum exploration and production in Norway. BP Solar is among the world s leading producers of solar cells. The Norwegian University of Science and Technology, NTNU, and the affiliated contract research foundation SINTEF, came up with the idea of combining PV cells with a double skin façade. An appropriate existing building to mount the double skin façade on was chosen at the University Campus in Trondheim, Norway. The existing building, O.S. Bragstads plass 2, is shown in Figure 1, while Figure 2 shows a drawing of the double skin façade on the south facing façade of the building. Figure 1 O.S. Bragstads plass 2, the selected building for the double skin façade, before construction. Figure 2 Drawing of the new double skin façade on the existing building at O.S. Bragstads plass 2.

4 4 Double facades A double façade, or double skin façade, consists of a glass skin mounted outside the normal insulated external wall of a building. The cavity formed between the outer skin and the main wall will provide extra thermal insulation and improved air tightness for the building. The cavity also offers a protected location for solar shading systems and other façade equipment, and platforms for maintenance, cleaning and fire escape can be introduced, as illustrated in Figure 3. With southerly exposure, the glazed cavity will trap solar radiation the same way as a greenhouse or an atrium. The energy captured can be used for preheating supply ventilation air, the raised cavity temperature will in any case reduce the heat loss from the building. In the cooling season, the cavity will have to be vented. The air movement and heat balance for the cavity is shown in Figure 3. cavity venting solar radiation window venting to cavity cavity venting Figure 3 Section of the new façade with platforms, vents and air movements (far right) with a closeup of an office, showing the air movements and heat balance for the double façade. Adding a double façade to an existing building will in many cases be a viable alternative to installing new windows and improving the thermal insulation, especially for historically important buildings where façade upgrading maybe not feasible. The double façade concept can also be applied to new buildings, often for the enhancement of a desired image of high tech high profile buildings.

5 5 Building Integrated Photovoltaic Cells Applying PV cells to the building envelope is often termed Building Integrated PV, BIPV. Better economy than for a standalone system is often realized with this concept, as the PV modules also have other functions. Figure 4 shows the BIPV cells used in the BP Solar Skin wall in Trondheim. At higher latitudes, the solar radiation on a vertical south facing wall is not much less than on a sloped surface oriented optimally for maximum radiation. It is therefore feasible to integrate PV cells in the cladding of facades. The PV output is dependent on temperature, the cells should be kept cool, and therefore in a BIPV application, the PV modules should be vented. Figure 4 Building integrated photovoltaic cells used in the BP Solar Skin wall in Trondheim. The BP Solar Skin concept The proposed building system is based on a combination of a double skin facade and building integrated PV façade cladding, where the PV cells are integrated in the outer glass skin. The cells are laminated in glass, and placed in façade sections that are not in front of the ordinary windows. The performance characteristics of the two combined façade concepts are complementary the systems help each other if controlled properly. Some important features of the BP Solar Skin concepts are: Both electricity and thermal energy are provided. The PV cells convert about 15% of the solar radiation to electricity. The thermal energy can be used to reduce the heating load of the building. The PV cells provide solar shading for the windows for summer high sun. The cavity can be implemented in a scheme for natural ventilation. The cavity can be vented to keep the PV cell temperature down for better performance. The cavity provides a protected location for solar shading systems. The skin and the cavity will provide improved noise protection. The cavity enables operable windows in high rise buildings. The cavity can be supplied with platforms for maintenance and evacuation. The concept is well suited for thermal upgrading and climate protection of existing buildings, but can also be applied in new buildings. The BP Solar Skin concept of course also implies some problem areas: important items are fire safety related to smoke propagation in the cavity, sound propagation likewise, reduced daylight levels, impairment of view in some configurations.

6 6 The R&D stage In order to assess the performance more closely and evaluate alternative designs, a comprehensive R&D project was carried out after the first conception stage. The energy performance of the façade and the thermal comfort in the building were assessed, using computer simulations, for a generic office building in various Norwegian climates. The thermal behavior of this rather complex system was difficult to simulate correctly; however, it was evident that optimal control of the cavity venting was a primary concern. The architectural and visual aspects of the concept were studied using scale models, full scale mock ups, photomontages and computer visualizations, as illustrated in Figure 5. An important consideration was the effect of PV cell spacing in the outer skin and the apparent transparency of the whole skin from both sides. The impacts on daylight levels and glare problems were also important aspects of the research. Figure 5 Computer visualizations of alternative designs using BIPV cells in the glass façade. The BP Solar Skin prototype Based on the results from this R&D and cost estimates, BP decided to construct and monitor a full scale prototype. An existing office building at the engineering campus of NTNU was chosen as the site; O.S. Bragstads plass 2. A wing of this office complex for the Electrical Engineering faculty has a slightly protruding four story façade, oriented 25 east of due south, and is shown as it was before construction of the BP Solar Skin in Figure 1. The main part of this wing was built around 1970, with fairly poor insulation and a large amount of air leakage.

7 7 The double façade prototype was constructed with glass panels mounted in a standard aluminium framing system. The outer skin is carried by vertical steel frames, these are bolted to the concrete floors of the building. Open grate steel platforms are installed at each floor level for monitoring, maintenance and cleaning purposes. The cavity can be vented by motor operated vents at the top and bottom, controlled by temperature sensors. Figure 6 shows some of the structural details of the double skin façade. Figure 6 Structural details. Clockwise from bottom left: welding the steel frames; new sprinklers for fire safety; PV cells/ laminated glass modules; motor operated vents at the top and bottom of the cavity.

8 8 The PV cells and the conduits are embedded in a resin layer in laminated glass modules. These modules are located outside the façade sections without windows. The cells used are of the high efficiency, mono crystalline BP Saturn type, with an efficiency of about 16%. The PV cells generate low voltage direct current that is coupled to the building s electricity supply via an inverter and a transformer. The key figures relating to the prototype are shown in Table 1, whilst the completed BP Solar Skin is shown in Figure 7 and Figure 8. Table 1 Key data for the BP Solar Skin Total façade area 455 m 2 PV module area 192 m 2 PV cell net area 102 m 2 Cavity width 0.8 m The prototype design went through much the same analysis as was conducted in the feasibility study. A major concern was fire safety: the fire code and available fire research have little bearing on the problem of smoke propagation through the stack effect (e.g. air movement upwards due to hot air rising) in a double skin cavity. The designers, therefore, decided to install a dry sprinkler system to cool down window glazing on the existing façade in the event of fire. They also improved the fire detecting system. Figure 7 O.S. Bragstads plass 2, with the finished BP Solar Skin double façade.

9 9 Figure 8 Details of the completed BP Solar Skin double façade. The monitoring program The prototype is instrumented with automated monitoring equipment that registers both the electricity delivered to the building s grid, and how the solar energy facade affects the heating demand and indoor climate of the rooms that face the new facade. A weather station is located on the roof of the building. In addition to electrical energy delivered to the building, the monitoring includes surface and air temperatures, air velocity in the cavity, and position of the vents. One years worth of data from May 2001 to May 2002 was collected and verified by SINTEF Energy Research, and will be made available for system evaluation. The monitoring equipment will be maintained in operation after the project monitoring is completed. The data will be made available for student projects in the new NTNU engineering study program Energy and Environment. The occupants in the rooms facing the new façade have been subjected to three rounds of questionnaire surveys: before construction, during a winter period, and during a summer period, the occupant satisfaction with the indoor climate is shown in Figure 9. Of special interest is the impact of the new façade on the thermal comfort and perceived air in the summer season. The venting through the cavity under clear sky summer conditions has therefore been the subject of special investigations as to the performance of the cavity under cooling demand conditions (Aschehoug & al., 2000).

10 10 satisfied neutral dissatisfied 100 % 80 % 60 % 40 % 20 % 0 % pre-constr post-constr (winter) post-constr (summer) Figure 9 Indoor climate occupant satisfaction in the offices on the south facing facade, before construction started and with the BP Solar Skin facade during winter and summer. The daylight losses due to the new glass wall was also studied before construction, included was also the impact of the PV cell spacing on the transparency of the outer skin. The daylight reduction is quite large, but the levels inside are still well within the building code requirements. Figure 10 shows the PV cell placement compared to the windows, and how they affect the view from the windows. Figure 10 The placement and spacing of the PV cells do not obstruct the view, and although the daylight reduction in the offices is noticeable, the daylight level is still well within the building code requirements.

11 11 Monitoring results Electricity generation The PV cell electricity generation is much as anticipated, with a general efficiency very close to the manufacturerʹs specification, as shown in Figure 11. For some situations, nearby buildings and trees shade part of the system, this reduces the PV output considerably. A different PV module subdivision and inverter strategy would have been less vulnerable to shading. For a full year, May 2001 to May 2002, the PV system delivered about 7200 kwh to the building, this result is corrected for lost energy during a short period of system malfunction. The theoretically maximum generation should have been about 9400 kwh with the solar radiation registered for the monitoring period. The system overall efficiency, or performance ratio, was therefore about 75%. The losses are due to the shading at the site, the inverter configuration which is sensitive to shading, and lower efficiency at low solar intensity, which at this latitude is more pronounced. The efficiency compares well with results from other measured grid connected systems (Ransome and Wohlgemuth, 2001). The highest irradiation on the façade was measured at near 800 W/m2 in March. Later in the year, the higher sun altitude results in a more oblique incidence angle on the PV cells. The measured data and produced power on June 23rd 2001 is shown in Figure 11 and Figure Produced power [kwh] Solar radiation on south facing surface [W/m2] Solar radiation on horizontal surface Power [kw] Solar radiation [W/m 2 ] :00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00 Time of day [hrs] Figure 11 Measured data for the BP Solar Skin on June 23rd One can see that the solar radiation on the vertical south facing surface was about 550 W/m2 at 12 noon. Knowing that there is 102 m2 of PV cells the total available solar radiation was about 56.1 kw. The measured power from the PV cells was about 8 kw, resulting in a 14.3% efficiency, very close to the manufacturerʹs specification of 16%.

12 % 500 PV cell efficeincy 25 % Solar radiation [W/m2] Solar radiation on south facing surface [W/m2] 20 % 15 % 10 % PV cell efficiency % 0 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 0 % Time of day [hrs] Figure 12 The PV cell efficiency for the whole day of June 23rd As one can see the efficiency varies between 10 15% during business hours, and drops to 5% just before 9:00 pm. Thermal winter gains The increased cavity temperature over the external temperature in the heating season gives an indication of the heating demand reduction achieved by the Solar Skin buffering and solar gains. The total heating energy saved was calculated to be 7 8 %, using a computer simulation program that takes into consideration the overall thermal balance of the whole building. The prototype building was very leaky for infiltration of outside air during windy periods. The savings here due to the new façade could not be taken into account in the calculations, as the improvement in air tightness could not be measured. Figure 13 shows the mean diurnal temperature difference between the cavity and the ambient air for the whole monitoring period. Especially during the heating season, which in Trondheim typically runs from mid September through April, there is a positive contribution from the cavity heat.

13 Diurnal temperature difference [C] apr 03-jun 08-jul 12-aug 16-sep 21-okt 25-nov 30-des 03-feb 10-mar 14-apr Date (May May 2002) Figure 13 Mean diurnal temperature difference between the cavity and the ambient air for the monitoring period. Summer conditions A pre construction study of the cavity performance (Aschehoug & al., 2000) showed that the best cooling strategy for summer conditions is to keep office doors open to the north side corridor and draw cold air from the corridor windows, using the solar façade cavity as an exhaust stack. The occupant survey shows, however, that some of the occupants at the top (4th) floor do experience periods of overheating in the summer. A closer investigation has revealed that under unfavorable combinations of high solar radiation, high outdoor temperature, and certain wind directions, the air flow in the cavity is reversed, resulting in the top floor offices receiving hot cavity air through the windows. A revised venting control strategy will be implemented to improve the summer conditions.

14 14 Acknowledgements The BP Solar Skin project was primarily financed by BP Norge and BP Solar, but also by a grant from the Royal Research Council of Norway under the NYTEK program for new and renewable energy technology. Materials for the prototype at NTNU were supplied by Hydro Aluminium and Pilkington Floatglass. References Aschehoug, Ø., Hestnes, A.G., Matusiak, B., Lien, A.G., Stang, J., & D. Bell: BP Amoco Solar Skin A Double Façade With PV. Eurosun 2000, Copenhagen June Ransome, S. & J. Wohlgemuth: Analysis of Measured kwh/kwp From Grid Ties Systems Modelling Different Technologies Worldwide With Real Data. 17th European Photovoltaic Solar Energy Conference, Munich October Project Details Project leaders: Øyvind Aschehoug and Anne Grete Hestnes, Norwegian University of Science and Technology, Trondheim, Norway. Architect: Anne Gunnarshaug Lien, SINTEF Civil and Environmental Engineering, Trondheim, Norway. For further information, please contact: Jan Erik Geirmo, Press spokesman BP Norway Phone: Fax: E mail: geirmojane@bp.com Øyvind Aschehoug, Professor NTNU Phone: Fax: E mail: Oyvind.Aschehoug@ark.ntnu.no Anne Grete Hestnes, Professor NTNU Phone: Fax: E mail: Annegrete.Hestnes@ark.ntnu.no

15 15 Map The BP Solar Skin prototype is on the south facade of O.S. Bragstads plass 2, a building located at the engineering campus of the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway. The BP Solar Skin Figure 14 Map showing the engineering campus of NTNU; Gløshaugen in Trondheim, Norway